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Mineral Reconnaissance Programme (MRP) reports

Copper-lead-zinc-pyrrhotite stratiform ore body, Vidlin Ness, Shetland. BGS © UKRI.

The Mineral Reconnaissance Programme (MRP) provided geological, geochemical, geophysical, mineralogical and metallogenic information on prospective areas in Britain. Work was carried out at various scales, from regional reconnaissance surveys or appraisal to the drilling of a geochemical or geophysical anomaly. Projects were multidisciplinary and used a combination of tried-and-tested methods alongside innovative techniques arising from research and development programmes. By the end of the programme in 1997, 146 MRP reports had been issued covering localities across the UK, which are now available to download for free. Sixteen data releases are also available.

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MRP summaries 1 to 20
1 The concealed granite roof in south-west Cornwall

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This report outlines the results of gravity surveys and drilling to locate cusps in the concealed granite roof north and west of the Carnmenellis granite outcrop. The results are plotted as Bouguer anomaly maps. Three other maps are presented that show the results of similar surveys to the north of the St Austell granite. The surveys around the Carnmenellis granite show that shallower-depth prolongations of outcropping granites can be defined, but that smaller or more deeply buried rises in the roof are not easily recognisable. The relationships between mineralisation, porphyry intrusion and granite eminences are considerably more complex than had been postulated previously. A borehole at Bosworgy penetrated greisenised granite. A hole at Parbola failed to reach a postulated granite cusp but found cassiterite in some narrow quartz-chlorite veins and a complex copper-lead-zinc lode. It is concluded that the location of concealed cupolas is likely to define areas of future mineral potential.

2 Geochemical and geophysical investigations around Garras Mine near Truro, Cornwall

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Detailed geochemical and geophysical surveys indicated that only very limited extensions exist to the lead-silver lodes in Devonian sediments previously worked at Garras.

3 Molybdenite mineralisation in Precambrian rocks, near Lairg, Scotland

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Low-grade molybdenite mineralisation is intermittently exposed in Moinian and Lewisian rocks intruded by Caledonian granites to the west of Lairg in Sutherland. Molybdenite occurs with pyrite in thin, post-foliation quartz veins and as coatings to joints and foliation planes in the schists and gneisses. Chalcopyrite, fluorite and bismuth minerals sometimes occur as accessories. Galena, barytes and sphalerite are also present in narrow, brecciated zones and small veins. Sampling of sparse rock exposures and of cores from a series of shallow boreholes drilled through peat and glacial deposits shows the molybdenum and copper concentrations to average less than 100 ppm. The regional distribution of mineralisation appears to be related to the presence of small, late-Caledonian granites that contain traces of molybdenite, but the local controls are mainly structural and include zones of thrusting associated with the Moine/Lewisian boundary.

4 Investigation of copper mineralisation at Vidlin, Shetland

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The mineralisation occurs in an amphibolitic belt (possibly metamorphosed tholeiitic lavas) and consists of a strata-bound sulphide horizon outcropping at four localities at Vidlin Ness, within a Dalradian succession of dominant calc-silicate granulites, minor marbles and semi-pelitic gneisses. The massive sulphides comprise mainly pyrrhotite and interesting amounts of chalcopyrite, sphalerite and galena, associated with sulphide-bearing quartz rock and tremolite rock. A southern extension of the mineralisation is indicated by linear geophysical anomalies and occasional outcrops of sulphide-bearing amphibolite. At the northern end, well-defined electromagnetic and magnetic anomalies suggest that the belt of massive sulphides at Vidlin Ness has a strike length of at least 1000 m. Six drill holes penetrated the sulphide horizon at Vidlin Ness, confirming that it persists laterally for at least 500 m and in depth to probably at least 100 m. The horizon increases in thickness from just under 2 m in the southern drill holes to about 10 m in the most northerly holes. Average values across the sulphide intersection range from 0.46 per cent copper and 0.12 per cent zinc in the south to 1.19 per cent copper and 1.27 per cent zinc in the north.

5 Preliminary mineral reconnaissance of central Wales

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Numerous lodes in the lower Palaeozoic mudstones, shales and sandstones of central Wales have been worked, principally for copper, lead, zinc and silver. This report presents 1:100 000 geochemical maps for copper, zinc and lead in stream waters; manganese, copper, zinc, tin, barium and lead in stream sediments, and zinc, barium, cerium and lead in panning concentrates for the central Wales mining field, together with a geological map and a list of named mines. It is concluded that future potential lies in the extension, under cover and at depths greater than 130 m, of known base-metal lodes.

6 Report on geophysical surveys at Struy, Invernesshire

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Geophysical surveys carried out over two veins containing lead and zinc mineralisation in Moine schists and granulites in Strath Glass showed that resistivity lows coincide with both veins and that the adits are probably located in the areas of most concentrated mineralisation. There was an indication that one of the veins may extend to the west, but the size and ore content of the exposed deposits are insufficient to make them of economic importance.

7 Investigation of tungsten and other mineralisation associated with the Skiddaw Granite near Carrock mine, Cumbria

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The tungsten mineralisation at Carrock mine is closely associated with a greisenised cupola of the Caledonian Skiddaw granite and, in the investigation described in this report, similar cupolas were tested for tungsten veins. Analyses of stream sediments, panned concentrates and rock samples indicate anomalous tungsten and arsenic values from an area west of the mine and further exploration is suggested. Analytical data are presented for arsenic, barium, copper, gold, iron, lead, manganese, molybdenum, tin, titanium, tungsten, uranium and zinc.

8 Investigation of stratiform sulphide mineralisation in parts of central Perthshire

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Detailed geological mapping in the area north and west of Loch Tay confirmed the presence of a weak but persistent zone of pyritic mineralisation and subsequent reconnaissance surveys showed it to have considerable lateral extent. It was, however, found to contain only minor amounts of chalcopyrite and no other base-metal sulphides. Chemical analyses of rock samples showed that copper, the most abundant base metal, seldom averages more than 60 ppm, though locally it exceeds 1000 ppm. A limited number of gold analyses failed to confirm a previous report of anomalously high concentration.

9 Investigation of disseminated copper mineralisation near Kilmelford, Argyllshire, Scotland

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Geological and geophysical surveys and geochemical sampling (drainage, soil, deep till and rock) showed the presence of low-grade copper and molybdenum mineralisation over an area of approximately 1500 × 800 m, associated with a Caledonian dacitic porphyry. The sulphide mineralisation shows features characteristic of porphyry-style mineralisation. It is in the form of blebs and veinlets associated with quartz-carbonate vein stockworks and is accompanied by pervasive hydrothermal alteration. Two boreholes were drilled and maximum assays of 0.34 per cent copper over 2.15 m and 0.04 per cent molybdenum over 1.80 m were obtained. Further drilling is recommended.

10 Geophysical surveys around Talnotry mine, Kirkcudbrightshire, Scotland

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Magnetic field and very low frequency electromagnetic surveys were performed to investigate the nickel-cobalt lode at the contact between a diorite and Silurian greywackes in the aureole of the Cairnsmore of Fleet intrusion. No significant anomalies were found in the area, except over the small amount of mineralisation already known.

11 A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall

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A 3D computer model of the Cornubian granite batholith, defined by polygonal contours, was created so that its calculated gravity field matched the observed Bouguer anomaly field both onshore and offshore. The model was used to define a background ('regional') field in three areas where detailed gravity surveys had been undertaken in the search for shallow granite. Maps of depth to granite were produced from the residual field by an iterative technique. Geological interpretations of the batholith model and of the depth maps are included in the report.

12 Mineral investigations in the Teign Valley, Devon. Part 1: barytes

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Geochemical studies in the Teign Valley, carried out on stream sediment and soil samples, have indicated extensions of barytes mineralisation some 2.5 km north and 1.2 km south of the formerly mined strike length in upper Palaeozoic sediments. Barium-lead-zinc mineralisation is now known to extend over a total length of 12.3 km and over a width rarely less than 0.5 km, though ore bodies large enough to be worked occur only intermittently within this belt. Part of the southern extension was investigated by deeper sampling, a percussive drill being used to obtain powdered rock samples for assay. To the south of Hennock village, a barytes-rich zone has been defined, some 1.2 km long and 260 m wide.

13 Investigation of stratiform sulphide mineralisation at McPhun's Cairn, Argyllshire

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A small stratiform occurrence of massive sulphide, apparently over 2 m thick, occurs in the Dalradian schists at McPhun's Cairn beside Loch Fyne. At outcrop, it contains 3.5 per cent zinc, 3.0 per cent lead, 6 ppm silver and 0.75 ppm gold. Geological, geophysical (induced polarisation; magnetic; resistivity) and geochemical (soil; stream sediment; panned concentrate) surveys and shallow drilling indicate the presence of only sporadic, weak mineralisation. Regional structural analysis and examination of the mineralisation suggest that local concentration of sulphides occurred at the nose of a small, steeply plunging fold system. A borehole to investigate the outcrop occurrence proved the extension of mineralisation down the fold axis.

14 Mineral investigations at Woodhall and Longlands in north Cumbria

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Geological mapping, soil and stream sediment sampling and induced polarisation and electromagnetic traverses over Lower Carboniferous limestones identified several geochemical and geophysical anomalies. Auger sampling of the former showed them to be due to surface redistribution of metals from disused lead-zinc-copper-barytes mines, whereas the weak geophysical anomalies in themselves do not constitute viable drilling targets.

15 Investigation of stratiform sulphide mineralisation at Meall Mor, South Knapdale, Argyll

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A geochemical, geophysical and geological investigation of copper mineralisation in the Meall Mor area was followed by the drilling of six shallow holes. The mineralisation occurs in a zone of weak stratiform sulphide mineralisation (the pyrite zone) with a strike length of 10 km in the Upper Erins Quartzite of the Middle Dalradian. A geochemical drainage survey showed the existence of a strongly anomalous distribution of copper and antimony in the Abhainn Srathain, which drains south from Meall Mor. Detailed soil and basal till sampling over the pyrite zone outlined a broad area enriched in copper and a coincident induced polarisation anomaly was found that stretched from Meall Mor south to the old mine workings on Abhainn Srathain and had probably been caused by a local enrichment of pyrite and chalcopyrite within the pyrite zone. Copper values range up to 0.24 per cent copper over 4.27 m in the first two holes and up to 1.06 per cent copper over 2.67 m in the third. This enrichment may be related to a later remobilisation of the disseminated chalcopyrite.

16 Report on geophysical and geological surveys at Blackmount, Argyllshire

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The underlying Precambrian Moine psammite at Blackmount, on the southern fringe of Rannoch Moor, contains granitic veins that probably stem from the adjacent Moor of Rannoch granite. These veins are generally pyritiferous and, at one locality, carry small amounts of molybdenite. Blackmount is also traversed by the Ericht–Laidon Fault, which, in theory and by analogy with the Tyndrum Fault, could be a site of significant sulphide mineralisation. Magnetic, very low frequency electromagnetic, Slingram electromagnetic and induced polarisation measurements, which were carried out in the area of the veins, suggest that the mineralisation has little or no lateral or depth continuation. Similar surveys were successful in locating the Ericht–Laidon Fault beneath superficial deposits, but suggest that, to the greatest depth that was investigated, no associated mineralisation is present.

17 Lead, zinc and copper mineralisation in basal Carboniferous rocks at Westwater, south Scotland

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A zone of lead, zinc and copper mineralisation is developed over a minimum of 4 km of strike of basal Carboniferous cementstone group sediments and immediately underlying Birrenswark Lavas at Westwater, near Langholm. Grades in sparse rock exposures and shallow boreholes are usually 0.1 to 0.3 per cent of combined metals over 1 to 2 m of thickness, but a fissure vein of higher grade and a relatively thick zone of disseminated sulphides were also located. Galena, sphalerite, chalcopyrite and barytes occur mainly in thin dolomite veins, but disseminations of galena are also present in sandstone units. The mineralisation is of low-temperature type and was emplaced along north-easterly trending normal faults and cross-faults regarded as late Carboniferous in age. Mineralisation was controlled by faulting, regional facies variation and local lithological variation, as well as by stratigraphic position. The heavy mineral fraction of stream sediment is the optimum sampling type in reconnaissance exploration of areas of calcareous rocks, such as the early Carboniferous of south Scotland, and basal till sampling is the most effective method of follow-up exploration in those areas where glacial deposits are widespread and often thick.

18 A mineral reconnaissance survey of the Doon–Glenkens area, south-west Scotland

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Geological, geophysical and geochemical surveys were carried out over the Loch Doon and Carsphairn granites and the surrounding lower Palaeozoic sediments. Geochemical drainage surveys led to the discovery of several occurrences of copper, gold, lead, molybdenum, tungsten and zinc. Airborne electromagnetic and magnetic surveys showed prominent anomalies associated with belts of black shales.

19 A reconnaissance geochemical drainage survey of the Criffel–Dalbeattie granodiorite complex and its environs

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Regional geochemical maps, compiled from multi-element analysis of stream sediments and panned concentrates, show patterns of element distribution related to geology and mineralisation of the region. Broad-scale patterns in the distribution of some elements reflect compositional variations in both the lower Palaeozoic turbidite sequence and the Criffel–Dalbeattie granodiorite complex, both of which may be subdivided into specific units on the basis of the geochemical data. The follow-up of copper anomalies in drainage samples from the Black Stockarton Moor area led to the discovery of porphyry-style copper mineralisation (Report No. 30) and related disseminated copper mineralisation at Screel Burn. The area to the west of the Criffel–Dalbeattie plutonic complex is also characterised by relatively high boron levels in stream sediments, which reflect the widespread occurrence of tourmaline both in association with and peripheral to the copper mineralisation. Vein mineralisation, usually containing barytes in addition to base metals, is identifiable from the drainage survey at the eastern margin of the Criffel–Dalbeattie granodiorite, in association with the early Carboniferous rocks along the Solway coast and within the lower Palaeozoic turbidites in the west of the area.

20 Geophysical field techniques for mineral exploration

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Details are given of the geophysical equipment and methods used in airborne and ground surveys for the mineral reconnaissance programme. Airborne surveys employ magnetic, electromagnetic and radiometric equipment. Ground surveys use electromagnetic (Slingram, Turam and very low frequency), resistivity and induced polarisation, magnetic and gravity methods. Borehole logging methods are also described and six case histories are given.

MRP summaries 21 to 40
21 A geochemical drainage survey of the Fleet granitic complex and its environs

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Data from multi-element analyses of stream sediments and heavy mineral concentrates are presented in geochemical maps. Broad-scale patterns exhibited by some elements make possible the division of the sedimentary rocks into eight distinct geochemical units, each characterised by different element distibution patterns. The Fleet and Loch Doon plutons are subdivided on the same basis. Follow-up investigations of drainage anomalies led to the discovery of both structure-controlled and disseminated base-metal mineralisation in the Penkiln drainage basin within the southern aureole of the Loch Doon granite. The distribution of copper, lead and zinc to the south and south-west of the Fleet granite suggests a zonation of vein mineralisation, with copper prominent adjacent to the granite contact and lead and zinc having a wider dispersion away from the granite. Other anomalies delineate a mineralised lineament following the regional strike of the lower Palaeozoic sediments, south-east of the Fleet granite.

22 Geochemical and geophysical investigations north-west of Llanrwst, North Wales

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Anomalous zinc values had been found in previous regional-scale stream sediment surveys over Ordovician volcanic and sedimentary rocks north-west of the former Llanrwst mining field. To find the source of these anomalies, stream sediment, panned concentrate and stream water samples were collected from 28 sites in the Afon Dulyn and Llyn Eigiau areas and analysed for copper, lead and zinc (in all three sample types); iron, manganese and barium (in sediments and concentrates) and cerium, calcium, nickel, titantium and tin (in concentrates only). Statistical treatment of the results led to the conclusion that most of the high zinc in stream sediment was related to hydrous oxide precipitates. It is apparent that a source of zinc exists in the general area, perhaps on the wide interfluves, but the nature of this source is not known. Ground magnetic traverses indicate that an airborne anomaly over Foel Fras is a complex anomaly caused by the Foel Fras volcanic complex and Conwy Rhyolite Formation, and that an anomaly near Drosgl was related to a diorite body.

23 Disseminated sulphide mineralisation at Garbh Achadh, Argyllshire, Scotland

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A small stock of biotite-feldspar porphyry, 0.25 km2 in area, crops out in a sequence of Dalradian schists and quartzites with interbedded epidiorites. Disseminated sulphides occur within the porphyry and the hornfelsed epidiorite, but do not normally exceed 3 per cent of the rock by volume. Assays of both rock types showed a maximum level of 0.24 per cent copper. Hydrothermal alteration is prominent within the porphyry, with the widespread development of sericite and kaolinite. The results of geological, petrographic, geochemical and geophysical studies demonstrate the presence of several features characteristic of porphyry-style mineralisation, but the small surface area and low grade of the deposit, combined with a lack of encouraging geophysical responses at depth, suggest that there is little chance of discovering an economic orebody. Several small strata-bound lenses of massive sulphide within the metasediments were recorded, but were not investigated in detail.

24 Geophysical investigations along parts of the Dent and Augill faults

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Airborne electromagnetic (AEM), radiometric and magnetic surveys were conducted over selected parts of the Dent and Augill faults, followed at the most promising anomalies by ground electromagnetic surveys employing very low frequency (VLF), Turam and Slingram methods and by detailed geological mapping. A gravity survey was undertaken to provide Bouguer anomaly data typical of the margins of the Lower Carboniferous blocks in the northern Pennines. Five AEM anomalies were attributed to conductive shale or mudstone horizons in the Carboniferous sequence. The VLF and Turam methods produced anomalies at Kitchen Gill and Birkett Common, which correlate with faults and mudstone outcrops. Anomalies at Long Rigg and Dowgill are thought to be due to conductive boulder clay. The regional gravity survey indicated that the main faults are characterised by weak Bouguer anomaly highs. Other anomalies probably reflect variations in the basement. Such changes may be generally useful in defining the boundaries of uplifted blocks of basement rocks.

25 Mineral investigations near Bodmin, Cornwall. Part 1: airborne and ground geophysical surveys

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Airborne magnetic and electromagnetic surveys and ground follow-up surveys showed that airborne geophysical methods are unsuited to this area. No significant new mineralised areas were located.

26 Strata-bound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: preliminary report

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The presence of an extensive zone of mineralisation in the area was recognised initially by a geochemical drainage survey and reconnaissance geological mapping. Subsequently, more detailed mapping and drainage sampling were supplemented by very low frequency electromagnetic (VLF-EM) geophysical surveys, overburden and rock sampling, and shallow drilling. The VLF-EM technique successfully delineated resistive rocks within the generally conductive graphitic schists and proved a valuable mapping aid in areas of poor exposure. The mineralised zone is defined by the presence of bedded barytes, sulphide concentrations, quartz-celsian rock and micaceous schists in which the muscovite is barium-rich. It varies in thickness from about 60 to 110 m and extends, at least intermittently, over 7 km of strike and through a vertical interval of 370 m. Individual barytes bands are 2.3 to 15.5 m thick and may extend along strike for up to 1.8 km. The greatest sulphide concentration is in carbonate rock and assays 8.5 per cent zinc and 3.6 per cent lead over 4.3 m. Other unusual constituents of the mineralised zone are hyalophane and cymrite (bairum silicates) and fuchsite (chromian muscovite). It is concluded that the mineralisation is of synsedimentary origin and involved the introduction of metal-rich hydrothermal brine into an euxinic basin.

27 Airborne geophysical survey of part of Anglesey, North Wales

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An airborne magnetic, electromagnetic and radiometric survey was carried out over most of the north-western half of Anglesey. The electromagnetic and radiometric results show little of interest, but the magnetic results, presented as an aeromagnetic map, show prominent anomalies associated with the Carmel Head Thrust, with Tertiary dykes and various hornfels bodies. Possible extensions of known ultrabasic bodies and areas where other such potentially mineralised bodies may be concealed are indicated, in particular near Llandyfrydog and near the coast south of Valley.

28 A mineral reconnaissance survey of the Abington–Biggar–Moffat area, south-central Scotland

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Panned heavy mineral concentrates from an area underlain mostly by lower Palaeozoic sedimentary rocks were examined for heavy detrital minerals and analysed for antimony, arsenic, barium, calcium, cerium, chromium, copper, iron, lead, manganese, molybdenum, nickel, tinm titanium and zinc. Numerous new occurrences of lead, zinc, copper and barium minerals were found and nine areas are recommended for further investigation. Minor amounts of barytes and traces of cupriferous pyrite were identified in the basal breccia of the New Red Sandstone deposits in Annandale. The mercury mineral cinnabar was identified for the first time in Scotland. Chromiferous spinel is a major constituent in the majority of panned samples. Corundum is widely dispersed in trace amounts. Historical references to a wide distribution of particle gold were confirmed and many new occurrences found. Some placer concentrations of gold and chromiferous spinel are likely in the alluvium of the valleys of the River Tweed and the Megget Water. Six greywacke formations were mapped in the project area, each distinguished by a characteristic lithology and heavy mineral content.

29 Mineral exploration in the Harlech Dome, North Wales

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This report presents the results of an airborne geophysical (electromagnetic, magnetic and radiometric) survey of the eastern part of the Harlech Dome and the consequent geological, geochemical and geophysical investigations carried out on the ground and in the laboratory. The area is underlain predominantly by lower Palaeozoic sedimentary and volcanic rocks. The airborne magnetic and electromagnetic results show a complex pattern of anomalies, but little variation is apparent from the aeroradiometric data. Ground studies of aeromagnetic anomalies demonstrated that they are related either to magnetite present in some sandstones and igneous rocks, or, more importantly, to pyrrhotite enrichments in several lithologies. Mineralogical studies showed that, whereas some enrichments in siltstones and mudstones are probably syngenetic in origin, others are possibly related to epigenetic concentrations produced during vein mineralisation. A study of the electromagnetic anomalies showed that many of them are related to comparatively low concentrations (less than 3 per cent) of carbonaceous material or non-commercial quantities of sulphides in the dark mudstones of the Dolgellau Member and Clogau Formation. An interpretation of the radiometric data confirmed that the only indications of uraniferous enrichment occur in the black mudstones of the late Cambrian Dolgellau Member. A rock geochemistry study indicated that statistical treatment of rock analysis could be used to distinguish intrusions associated with prophyry-style mineralisation. It also showed that the porphyry-style mineralisation at Coed-y-Brenin is probably cogenetic with the end-Tremadoc magmatism that gave rise to both the volcanic pile on Rhobell Fawr and intermediate intrusions in the Cambrian, and therefore is quite distinct from the end-Silurian quartz-sulphide vein mineralisation. Intrusion breccias, which are also linked to the Rhobell Fawr volcanism, were recognised in the area for the first time. One of them contains the worked-out Glasdir copper deposit. Fluid inclusion studies showed differences between quartz veins associated with the vein and disseminated styles of mineralisation, which may be useful in exploration.

Ground follow-up work showed indications of mineralisation in many of the areas examined. Among these, the possibility of vein and strata-bound lead and zinc mineralisation was found at Hengwrt Uchaf and Benglog. At Bryn Coch, Tyddyn Gwladys and Hafod Fraith, possible associated but separate bodies or extensions to the proved porphyry-type copper deposit at Coed-y-Brenin were identified, as was modest vein mineralisation. In three areas — Mynydd Foel Uchaf, Hafod-y-fedw and Y-Gors — dispersed epigenetic sulphide mineralisation in bedrock was found, mainly pyrrhotite with subeconomic base-metal sulphides. Similar metalliferous concentrations were tentatively identified in a number of other areas, which included Garth Gell, where pilot studies were carried out on coincident electromagnetic and magnetic anomalies. At Mynydd Bach, Craiglaseithin and Dol Haidd, there are indications of either new veins or extensions to known veins containing copper, lead and zinc and, at Craiglaseithin and Dol Haidd, feeble disseminated copper mineralisation; here, too, the metal concentrations are considered subeconomic. Ffridd Dol-y-moch and Waun Hir are both covered by superficical deposits that may conceal mineralisation, but exploration is hampered by contamination problems. At Nannau, slight copper enrichment was found in volcanic rocks that are believed to be cogenetic with the Coed-y-Brenin porphyry copper deposit. Recommendations are made for further work at Glasdir, in the Coed-y-Brenin area, at Hengwrt Uchaf and Benglog, at Ffridd Dol-y-moch and Nannau. Further investigations are also recommended on the nature and extent of the sulphide concentrations in the Clogau Formation.

30 Porphyry-style copper mineralisation at Black Stockarton Moor, south-west Scotland

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A Caledonian multiphase subvolcanic complex intruding lower Palaeozoic turbidites to the west of the Criffel granodioritic plutonic complex has been mapped. An induced polarisation survey delineated an arcuate anomaly about 6 km long and a geochemical soil survey showed that there is a zone with anomalous levels of copper (from 140 to 5500 ppm) in the southern part of the area and that it is essentially parallel to the induced polarisation (IP) anomaly but partially displaced to the east. Three deep and nine shallow drill holes confirmed the widespread presence of both veinlet and disseminated pyrite and copper mineralisation of the porphyry type. Regular zonation can be observed in the style and intensity of mineralisation and hydrothermal alteration. An outer, propylitic alteration zone occurs to the west, passing eastwards into a sericitic zone. Pyrite is most conspicuous within rocks of the outer sericite zone, the outcrop of which coincides roughly with the axis of the IP anomaly. Further east, pyrite decreases but chalcopyrite and bornite with some chalcocite become relatively conspicuous and copper levels are among the highest attained (in the 400 to 1100 ppm range). Manganese, zinc, arsenic and lead are enriched in the outer propylitic zone; barium in the sericitic zone and copper in the inner sericitic zone, whereas arsenic, antimony and gold are concentrated with copper and molybdenum in isolated brecciated sections.

31 Geophysical investigations in the Closehouse–Lunedale area

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The area around the Lunedale Fault and the Closehouse barytes mine, at the northern edge of a deep Carboniferous sedimentary basin, was investigated by an airborne magnetic, electromagnetic and radiometric survey. The Whin dolerite gives rise to pronounced magnetic features, some of which indicate previously unknown intrusions. The Closehouse barytes deposit is related to the distribution and alteration of the Whin dykes and ground investigation of the aeromagnetic anomalies is recommended. Ground follow-up of electromagnetic anomalies was incomplete, but the four areas that were studied show no indications of mineralisation.

32 Investigations at Polyphant, near Launceston, Cornwall

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Geological examination, shallow boreholes and geochemical soil sampling over the Polyphant ultrabasic igneous mass indicate a marked variation in composition, with at least two types of peridotite and two of gabbro. The distribution of nickel, cobalt and chromium in overlying soil gives an impression of macrolayering within the peridotite, but the concentration levels of these metals and of copper are normal or low for peridotite and gabbro and offer little prospect of layered base-metal orebodies being found. No significant mineralisation was indicated by the investigations, but high induced polarisation values at the southern margin of the complex may be due to minor sulphide concentrations.

33 Mineral investigations at Carrock Fell, Cumbria. Part 1: geophysical survey

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A geophysical survey was conducted in the vicinity of the Carrock tungsten mine, to establish an optimum geophysical exploration procedure for the location of the style of mineralisation known at Carrock. The very low frequency electromagnetic (VLF-EM) method proved to be the best tool for this environment. It recorded only weak or indistinct anomalies over much of the known mineralisation, but a weak anomaly coincident with the Emerson vein was traced northwards for 1 km. Several similar linear features were recorded in the area on trends favourable for mineralisation, and two, at Poddy Gill in the east and Arm o'Grain in the west, are coincident for part of their strike with exposed mineralisation. These three anomalies appear to warrant investigation by drilling. Resistivity measurements indicated that most fault structures have coincident low-resistivity zones; a detailed traverse across the Emerson vein showed a minor high-resistivity peak within the low zone. Induced polarisation, magnetic and self-potential anomalies were recorded only within the gabbro on the extrapolated positions of the Smith and Wilson lodes. It is concluded that these anomalies are caused by discontinuous near-surface lenses of pyrrhotite (which have little VLF response).

34 Results of a gravity survey of the south-west margin of Dartmoor, Devon

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A gravity survey (station density: four to six per square kilometre) of the south-western margin of Dartmoor, including the Hemerdon stockwork, was interpreted with the aid of computer techniques to indicate the depth to buried granite. The results show that the Hemerdon ball granite is an isolated block that does not extend to depth and that no vertically continuous, shallow granite occurs at any distance from the known outcrop. Various computer graphics presentations of the data are given.

35 Geophysical investigation of chromite-bearing ultrabasic rocks in the Baltasound–Hagdale area, Unst, Shetland

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Economic deposits of chromite in Unst were worked intermittently until exhaustion of the known near-surface deposits in 1945. Since it is likely that further comparable deposits exist at shallow depth, detailed geophysical surveys employing gravity, magnetic and electrical methods were carried out over 1 km2 of the area of known mineralisation to test the feasibility of detecting and delineating them. Seven of 16 small positive gravity anomalies were tested by shallow boreholes, but only two were attributed to chromite concentrations and the efficacy of geophysical techniques in the detection of chromite concentrations is unproven.

36 An appraisal of the VLF ground resistivity technique as an aid to mineral exploration

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A study of the very low frequency (VLF) ground-resistivity method confirmed that it is well suited to the mapping of broad mineralised zones, flat-lying conductors of limited lateral extent, or abrupt changes in conductivity associated with geological contacts. In resistive terrains, the method offers distinct operational advantages over galvanic resistivity methods. The principal disadvantages of the technique relate to interpretational ambiguities associated with the complex behaviour of surface impedance at VLF and the fact that the operator has no effective control over the depth of investigation.

37 Compilation of strata-bound mineralisation in the Scottish Caledonides

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This report forms part of the UK contribution to the International Geological Correlation Programme Project 60, 'Correlation of Caledonian strata-bound sulphides'. Details of seven deposits are presented in tabular form, accompanied by a geological map of Scotland.

38 Geophysical evidence for a concealed eastern extension of the Tanygrisiau microgranite and its possible relationship to mineralisation

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A Bouguer anomaly low near Blaenau Ffestiniog in North Wales, in an area of lower Palaeozoic sedimentary and igneous rocks, is interpreted as being due to a concealed eastward extension of the Tanygrisiau microgranite. The magnetite-bearing granite is also thought to be responsible for a pronounced aeromagnetic anomaly, which has a form that supports the gravity evidence for the eastward extension of the granite body but requires the extension of a magnetic body down to a depth of 15 km. The mineralisation in the area consists of sulphide-bearing quartz veins. The veins occupy faults trending chiefly to the north-east, their distribution appearing to be mainly coincident with the southern flank of the concealed granite.

39 Copper-bearing intrusive rocks at Cairngarroch Bay, south-west Scotland

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Two intrusion complexes, the Bay and the Glen, which probably represent an early phase of the Devonian magmatic episode, and a number of dykes are emplaced within a folded succession of Silurian sedimentary rocks at Cairngarroch Bay. The Bay Complex consists of microtonalite and granodiorite. The Glen Complex comprises quartz porphyry, porphyritic quartz microdiorite and quartz microdiorite. Local high-chargeability zones were identified along three geophysical traverse lines. Soil samples were collected on a 50 m grid over an area of induced polarisation anomalies. In addition, water, base-of-stope talus and rock samples were chemically analysed. Both the intrusion complexes and some of the sedimentary rocks show locally intense hydrothermal alteration. In the Bay Complex, narrow zones of bleached rock are rich in calcite, chlorite and pyrite and contain minor chalcopyrite and pyrrhotite. The Glen Complex displays network fracturing, brecciation and locally intense alteration to sericite or calcite. There is locally abundant pyrite in veins and disseminated and rare chalcopyrite. Arsenopyrite is present in wallrock adjacent to the Bay Complex. Rock geochemistry indicates a pervasive but patchy copper-iron-arsenic-molybdenum mineralisation in all rock types, with copper values up to 600 ppm. The mineralisation is accompanied by irregular barium, potassium and strontium enrichment. The potassium/rubidium ratios suggest that the hydrothermal liquors were not entirely late magmatic. The mineralisation and alteration have some characteristics of a porphyry system and it is conceivable that copper enrichment might increase with depth.

40 Strata-bound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: final report

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The presence of an extensive zone of mineralisation in the area was recognised initially by a geochemical drainage survey and reconnaissance geological mapping. Subsequently, more detailed mapping and drainage sampling were supplemented by very low frequency electromagnetic (VLF-EM) geophysical surveys, overburden and rock sampling, and shallow drilling. The VLF-EM technique successfully delineated resistive rocks within the generally conductive graphitic schists and proved a valuable mapping aid in areas of poor exposure. The mineralised zone is defined by the presence of bedded barytes, sulphide concentrations, quartz-celsian rock and micaceous schists in which the muscovite is barium-rich. It varies in thickness from about 60 to 110 m and extends, at least intermittently, over 7 km of strike and through a vertical interval of 370 m. Individual barytes bands are 2.3 to 15.5 m thick and may extend along strike for up to 1.8 km. The greatest sulphide concentration is in carbonate rock and assays 8.5 per cent zinc and 3.6 per cent lead over 4.3 m. Other unusual constituents of the mineralised zone are hyalophane and cymrite (bairum silicates) and fuchsite (chromian muscovite). It is concluded that the mineralisation is of synsedimentary origin and involved the introduction of metal-rich hydrothermal brine into an euxinic basin.

MRP summaries 41 to 60
41 Metalliferous mineralisation near Lutton, Ivybridge, Devon

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Localised uranium and base-metal mineralisation in Devonian slates and volcanics was traced by radiometric and geochemical soil surveys. The mineralisation is confined to two narrow structures in a fault zone trending north-west to south-east, and at surface it has a strike length of no more than 200 m. Percussive drilling down to the shallow water table indicated persistence of the secondary metalliferous minerals, but cored drilling failed to intersect any recognisable, well-mineralised structure. It remains uncertain whether a small ore shoot exists below the surface anomalies; if so, it must be presumed to pitch south-eastwards. Only oxidised and possibly enriched mineralisation was sampled; this yielded a little cassiterite, sphalerite, pyrite, pyrrhotite and covellite, abundant hydrated iron and manganese oxides with adsorbed arsenic, bismuth, copper, lead, uranium and zinc, and flakes of secondary uranium and silver minerals. Radiometry confirmed gross uranium disequilibrium.

42 Mineral exploration in the area around Culvennan Fell, Kirkcowan, south-western Scotland

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Reconnaissance geochemical and geophysical surveys were concentrated on the west of the Culvennan diorites, where numerous dykes, mainly of intermediate composition, and three small bodies of intrusion breccia intrude folded greywacke, quartz wacke, silty mudstone, siltstone and calcareous mudstone of the Silurian Gala Group. A zone of high chargeability was defined, within which there are areas of low resistivity and narrow magnetic anomalies. The cause of these anomalies is most likely to be strata-bound concentrations of sulphides within the sedimentary succession and the dykes and there is no evidence to show that the high chargeability is associated with porphyry-style mineralisation. The results of the geochemical survey substantiate this, though minor secondary concentrations of metals and weak, local copper-arsenic-iron-lead mineralisation were indicated.

43 Disseminated copper-molybdenum mineralisation near Ballachulish, Highland Region

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Chalcopyrite-pyrite-molybdenite mineralisation, with minor scheelite, occurs in disseminated, veinlet and fracture-filling forms in microadamellite and surrounding adamellite in the Ballachulish igneous complex. The mineralisation is best developed in and around the eastern part of the microadamellite over an area of about 250 × 450 m, where it was observed over a vertical interval of 250 m from the highest exposure to the base of a borehole. An induced polarisation survey showed that chargeability values are slightly higher in this area. In 3 m lengths of core, the maximum copper content was 260 ppm and the maxiumum molybdenum content 500 ppm, but the average tenor over the mineralised area is not more than 50 to 100 ppm copper and 10 to 30 ppm molybdenum. Selected mineralised outcrop samples gave values of up to 2400 ppm copper, 9200 ppm molybdenum, 2400 ppm yungsten, 0.31 ppm gold and 8 ppm silver. It is thought that the ore minerals were introduced by a hydrothermal system, which, compared with those of classic porphyry models, was small in extent and weak in intensity. Sericitic alteration is generally associated with the mineralisation, but no potassic alteration is evident and the standard zonation of porphyry copper deposits is absent. There is very little potassium or rubidium metasomatism, the best defined chemical change being a loss of strontium in altered rocks. Anomalously low rubidium and high potassium/rubidium values in the unaltered microadamellite are attributed to the separation of a rubidium-rich, aqueous fluid from the microadamellite before or at the time of consolidation of the rock.

44 Reconnaissance geochemical maps of parts of south Devon and Cornwall

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These geochemical maps cover, in four sheets at a scale of 1:50 000, the area bounded by National Grid lines 170 and 290E and 040 and 100N. The data presented are:

  • stream sediments:
    • barium
    • chromium
    • cobalt
    • copper
    • iron
    • lead
    • manganese
    • molybdenum
    • nickel
    • tin
    • uranium
    • zinc
    • zirconium
  • panned concentrates:
    • antimony
    • barium
    • cerium
    • copper
    • iron
    • manganese
    • nickel
    • tin
    • titanium
    • zinc
45 Mineral investigations near Bodmin, Cornwall. Part 2: new uranium, tin and copper occurrences in the Tremayne area of St Columb Major

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Investigations of radiometric anomalies in slates and calc-silicate rocks of Devonian age 2 km north-east of St Columb Major indicate that at the main (Tremayne) occurrence a body containing 600 to 1000 t of uranium at a grade of 0.2 per cent uranium may exist within 60 m of the surface. A tin stockwork, similar to those of the nearby Mulberry and Prosper mines, may be indicated, but tin values are erratic. Radiometric reconnaissance and gridding outlined three main groups of surface anomalies greater than 15 µR/h in an area 1.2 × 0.7 km. They probably represent separate but related mineralised zones located where northerly orientated structures intersect the easterly trending boundary between calc-flintas and slates. Two inclined boreholes were drilled to examine the largest radioactive structure. Meta-autunite and meta-torbernite occur at true depths of 34 to 38 m in the first borehole and at 18 to 29 m and 34 to 36 m in the second. Analytical and gamma log data indicate broad zones of moderate-grade uranium, which reach a grade of at least 0.2 per cent uranium over 3 m in the second borehole. Copper, zinc, cobalt and tin values are also high and further drilling is recommended.

46 Gold mineralisation at the southern margin of the Loch Doon granitoid complex, south-west Scotland

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Following the identification of native gold and arsenopyrite in the area, geochemical soil sampling of the southern margin and aureole of the Loch Doon plutonic complex found several arsenic anomalies with levels in excess of 1000 ppm within the pluton and its aureole. Seven shallow boreholes were drilled to test their source. Geophysical surveys by magnetic, Slingram electromagnetic,very low frequency and induced polarisation methods were carried out, but none showed anomalies that correlated with zones of high arsenic in soil. The Loch Doon plutonic complex in this area is intrusive into a sequence of graded turbidites of probable Caradoc age. Swarms of concordant, minor intrusive rocks of quartz monzonite and granodiorite, which predate the pluton, have been encountered in its aureole. Major differences in chemistry exist between these minor intrusions and the composition of the margin of the plutonic complex. Pervasive metasomatism has affected the sedimentary rocks throughout the area. Two phases of gold-bearing, arsenic-rich mineralisation have been recognised. The earlier comprises disseminations of pyrrhotite, arsenopyrite and pyrite in the margins of monzonitic minor intrusions and disseminations of arsenopyrite in the adjacent metasediments. This mineralisation occurs in zones of thickness up to at least 18 m, within which arsenic levels reach 3000 ppm and gold levels 0.16 ppm in samples of around 1 m of core. It is probable that the majority of soil arsenic anomalies originate from this type of mineralisation. Superimposed upon this is a series of discordant quartz veins and stringers, which trend roughly south, cutting all rock types, and which may be richly mineralised with arsenopyrite and some pyrite and may also contain minute grains of native gold. Individual veins range up to 30 cm in thickness, but thicker stockwork zones also exist. Arsenic levels in 200 to 300 g samples of veined material exceed 3.5 per cent and gold assays up to 8.8 ppm have been obtained. A separate, minor phase of sphalerite and galena mineralisation also occurs within the area, usually in association with carbonate veinlets.

47 An airborne geophysical survey of the Whin Sill between Haltwhistle and Scots' Gap, south Northumberland

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The survey, conducted with magnetic, very low frequency electromagnetic (VLF-EM) and radiometric equipment, covered the outcrop of the Whin Sill (a dolerite sill in early Carboniferous limestones and associated detrital sediments), its downdip extension and the Haydon Bridge mining district. Maps are presented of the magnetic and VLF-EM results. The aeromagnetic map shows a clear correlation between the distribution of anomalies and the mapped outcrops of the sill and, in areas covered by superficial deposits, allows more accurate delineation of the subcrop of the sill. The magnetic data also indicates that the outcrop pattern consists of a series of linear segments and it is suggested that the form of the sill was subject to control by the pre-existing joint or fault system during intrusion, as well as to extensive modification by later faulting. In the Settlingstones mine area, the magnetic anomalies show a clear spatial relationship to the known veins and have been used to guide the search for vein extensions; elsewhere, comparable anomalies suggest new sites to be considered for detailed exploration.

48 Mineral investigations near Bodmin, Cornwall. Part 3: the Mulberry and Wheal Prosper area

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The results of geochemical soil sampling and geophysical surveys (gravity, very low frequency electromagnetic (VLF-EM) and induced polarisation (IP) in an area of Lower Devonian sediments north of the St Austell granite, between the former opencast tin workings of Mulberry and Wheal Prosper, suggest that the most promising ground for future mineral exploration lies to the south of the latter workings. Geochemical and geophysical evidence points to the presence of a previously unrecognised mineralised zone subparallel to the Prosper vein sheets and some 200 m south of them. The presence of tin, copper, zinc and a little tungsten is indicated. Traced westwards, the Prosper mineralisation becomes more tenuous and, as seen in core from shallow boreholes, uneconomic.

49 Seismic and gravity surveys over the concealed granite ridge at Bosworgy, Cornwall

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An attempt to define the form of the concealed granite ridge by seismic surveys carried out at a station separation of 20 m was unsuccessful and it was concluded that, if the seismic method were to be effective, a more intensive coverage would be needed, possibly with a 2 m station separation. The detailed gravity survey was useful in providing an approximate shape for the granite ridge and an order of magnitude to the actual depths.

50 Geochemical drainage survey of central Argyll, Scotland

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A reconnaissance geochemical drainage survey of 720 km2 of Dalradian outcrop in central Argyll is described and geochemical maps are presented to show the distribution of copper, lead, molybdenum, nickel, uranium and zinc in stream sediments and of antimony, barium, calcium, cerium, copper, iron, lead, manganese, nickel, tin, titanium and zinc in panned concentrates. The survey identified base-metal anomalies in the pyrite zone, Ardrishaig Phyllites, Loch Tay Limestone and the Green Beds. The distribution of metal content within these formations has been modified by faulting and igneous intrusion. Resampling and the investigation of anomalous stream courses defined parts of Glen Fyne, the Garabal Hill–Glen Fyne igneous complex, the southerly outcrop of the pyrite zone and the Loch Tay Limestone as zones of base-metal mineralisation in which further investigation is recommended.

51 A reconnaissance geochemical survey of Anglesey

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A reconnaissance field survey identified three groups of mineral occurrences: (a) copper; (b) copper (lead, zinc), and (c) barytes (lead), of which (b) is the most important. Drainage samples were collected from 440 sites and soil samples were collected from three areas of poor drainage and thin superficial cover. Barium, cobalt, copper, iron, lead, manganese, molybdenum, nickel and zinc were determined in sediment samples and antimony, barium, calcium, copper, iron, lead, manganese, nickel, tin, titanoum and zinc in panned concentrates. Copper, lead and zinc were determined in soil and water samples. The geochemical drainage survey encountered major difficulties from the lack of surface drainage, contamination, subdued topography, variable background geology and extensive superficial deposits. The comparison of statistical analyses and mineralogical observations indicated that all high tin and antimony levels were related to contamination and that factor analysis was an effective means of discrimination between anomalies caused by contamination and those due to mineralisation. Eighteen anomalous areas related to sulphide or barytes mineralisation were delineated. Six of these — Carmel Head, Llandyfrydog, City Dulas, Llanbadrig, Cerrigceinwen and Lligwy — were the subject of further study.

52 Geochemical reconnaissance in the Cheshire Basin (pages 1 to 13)

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Geochemical maps are presented to show the distribution of anomalous values of copper, lead and cobalt in soil samples from five areas on the outcrop of the Triassic Helsby Sandstone and Tarporley Siltstone formations. In each area, the soil geochemistry identified the known area of mineralisation. Several other small areas with anomalously high soil values are presumed to reflect local, hitherto unknown mineralisation, but none of them appear to have the areal extent necessary for economic deposits. Suitable porous host rocks and fault structures appear to be common factors in the final location of the mineralisation.

52 Titanium dioxide in the Ayrshire Bauxitic Clay (pages 14 to 18)

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Titatium oxide (contents range from 1.2 per cent to 14.17 per cent, with a mean of 4.68 per cent. The titanium oxide is thought to come from the Carboniferous Passage Group lavas from which the clay is derived, the process of residual concentration being responsible for upgrading it. Extraction of titanium from bauxite has not yet proved possible, even at much higher grades.

52 The Marl Slate (Kupferschiefer) in the Southern North Sea Basin (pages 19 to 26)

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Samples of the Permian Marl Slate from borehole cores in the Southern North Sea Basin were analysed for antimony, arsenic, chromium, cobalt, copper, lead, manganese, molybdenum, nickel, silver, tin, vanadium and zinc. The geochemical characteristics are similar to those described in Germany and north-east England. Maximum values of 7000 ppm copper, 1.3 ppm zinc and 340 ppm lead were recorded.

53 Investigation of polymetallic mineralisation in Lower Devonian volcanics near Alva, central Scotland

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Boreholes drilled on three known occurrences of fracture-controlled mineralisation in the Lower Devonian Ochil Volcanic Formation intersected mineralised structures that contain only minor amounts of barytes and geochemical enrichments of copper, arsenic and uranium. Earlier silver-cobalt mineralisation appears to have been followed by copper-barium mineralisation. Differential fracturing within the volcanic sequence and increased brecciation at fault intersections are the principal controls.

54 Copper mineralisation near Middleton Tyas, North Yorkshire

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Geochemical soil sampling outlined an area of 6 km2 of anomalous copper values to the south of the mined area at Middleton Tyas. The supfericial cover in this area is thick and the cause of the anomalies is not known. Examination of the known mineralisation in the area, which occurs in lower Carboniferous limestones, suggest that it originated from metalliferous brines migrating from the Stainmore Trough or a similar Lower Carboniferous sedimentary basin to the east. The possibility that the mineralisation is syngenetic is now discounted, and shallow holes into the Permian succession showed that no Kupferschiefer facies is present, which eliminates the Permian as a source for the Cu. The primary Cu sulphides were enhanced in grade by supergene enrichment under arid conditions in early Permian times.

55 Mineral exploration in the area of the Fore Burn igneous complex, south-western Scotland

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The Fore Burn igneous complex consists mainly of quartz microdiorite, tonalite and feldspar porphyry forming semiconcordant or concordant bodies in early Devonian volcanic and sedimentary rocks, just north of the Southern Upland Fault, 24 km east of Girvan. Several small bodies of intrusion breccia occur within the complex and the country rock and there is a zone of monolithological breccias along a fault followed by the Fore Burn itself. Alteration to sericite, carbonate and chlorite is widespread, and tourmaline is also widely distributed. The most intensely altered rocks occur in the breccia zone along the Fore Burn. There, the breccias contain locally abundant disseminated sulphides and are cut by veins rich in sulphides. Drainage and rock geochemistry and detailed mineralogical study showed that arsenopyrite, pyrite and chalcopyrite occur in the breccia zone, with smaller quantities of tennantite, tetrahedrite and cobaltite. Antimony, arsenic, bismuth, cobalt, copper, gold, lead, molybdenum, nickel and zinc were all enriched in mineralised rock. Native gold was identified in a quartz-tourmaline vein, with chalcopyrite and other sulphides. Geophysical surveys located three small areas of low resistivity, one of which is in the breccia zone. The induced polarisation chargeability levels do not indicate any widespread, significant, near-surface disseminated sulphide mineralisation.

56 Geophysical and geochemical investigations over the Long Rake, Haddon Fields, Derbyshire

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Geophysical and geochemical investigations were undertaken over the Long Rake, a major mineralised vein in Dinantian limestones, to establish which methods show the best response to the mineralisation. The mineralised structure carries high concentrations of fluorite with associated lead and zinc minerals and the gangue minerals barytes and calcite. Gravity and magnetic anomalies such as those obtained over the Long Rake could have limited applications for the indirect location of veins whose approximate position is known. Induced polarisation, resistivity and electromagnetic measurements failed to produce anomalies that could be directly attributed to the mineralisation or its host structure. However, reconnaissance mapping with very low frequency electromagnetic (VLF-EM) and Radiohm methods showed that, over a large section of the survey area, the fluorspar vein could be mapped by its association with the sub-superficial cover shale-limestone contact. The determination of a wide range of elements in soils and tills showed that the more mobile elements, such as fluorine and zinc, are particularly useful in detecting mineralisation over broad areas. Less mobile elements tend to exhibit localised dispersion patterns, which have applications for the precise location of an orebody. Barium, calcium, lead, rubidium, strontium, thorium and zinc are enriched in soils above the Long Rake in areas of thin overburden, but only barium, strontium and lead maintain significant contrast in thickening cover towards the west. The use of basal till samples was found to have no advantage over subsurface soil samples, as geochemical contrast was not improved.

57 Mineral exploration in the Ravenstonedale area, Cumbria

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Following stream sediment and soil sampling, four areas of anomalous copper, lead and zinc in soils were recognised in an area of Lower Carboniferous rocks close to the Silurian unconformity: Stennerskeugh Clouds, Birkett Common, Crosby Garrett Fell and Windy Hill. Very low frequency electromagnetic (VLF-EM) and induced polarisation (IP) surveys at Stennerskeugh Clouds and Crosby Garrett Fell showed anomalies attributed to the presence of shale bands in the Carboniferous succession. Previous geophysical surveys at Birkett Common had suggested the presence of small mineralised structures, but the present geochemical and geophysical results do not indicate the presence of mineralisation of economic significance in the area.

58 Investigations of small intrusions in southern Scotland

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Sixteen small Caledonian intrusions, ranging from granite to diorite, were examined for indications of disseminated mineralisation. Stream sediment and panned concentrate samples were collected from streams crossing some of the intrusions and reconnaissance geophysical surveys were conducted over Priestlaw, Cockburn Law and Lamberton Moor. Rock samples from some of the intrusions were analysed for major and trace elements. Hydrothermal alteration in places with associated pyrite was recorded at Mains of Dhuloch, Mochrum, Priesthope, Lamberton Moor, Broad Law, Glenluce, Priestlaw, Cockburn Law and Mull of Galloway. Copper enrichments were recorded locally, but it was concluded that in no case is there a likelihood of appreciable porphyry copper mineralisation at or near the surface.

59 Strata-bound arsenic and vein antimony mineralisation in Silurian greywackes at Glendinning, south Scotland

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Stratiform and disseminated pyrite-arsenopyrite concentrations are overprinted by fracture-controlled polymetallic mineralisation, including stibnite, in Silurian sediments at Glendinning, near Langholm. Three shallow boreholes were drilled on an anomaly defined by very low frequency electromagnetic (VLF-EM) and induced polarisation (IP) surveys and by antimony values over 20 ppm in thin, B-C horizon soils. A parallel conductive zone with an accompanying soil anomaly but lacking an IP response was investigated by a fourth hole. The strata-bound sulphides form disseminations and bands parallel to the bedding and are particularly concentrated in intraformational breccia units regarded as debris flows, which, together with the presence of small-scale slump folds in the greywackes, testify to the existence of an unstable slope during sedimentation. The thickest such unit has a true thickness of 4 m and, together with 8 m of adjoining greywackes, grades 0.7 per cent arsenic and 0.07 ppm gold. Phases of fracture-controlled iron-arsenic-antimony-lead-zinc-copper-(?)mercury mineralisation associated with widespread dolomite and quartz veinlets and narrow breccia veins are superimposed on the strata-bound mineralisation. Their spatial association with the strata-bound mineralisation and the presence of up to 0.33 per cent antimony in the stratiform arsenopyrite and as much as 5 per cent arsenic in the stratiform pyrite favour a common source for the arsenic and antimony. This source was probably a synsedimentary metal accumulation in a mid or lower fan environment in which euxinic conditions developed periodically. (After publication of this report most of the drill cores were analysed for gold by BP Minerals International Ltd: maximum values were 0.4 ppm gold over 3.7 m true thickness of breccia vein and fractured siltstone.)

60 Mineral investigations at Carrock Fell, Cumbria. Part 2: geochemical investigations

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Soil samples were collected from an area of 5 km2 around the tungsten veins at Carrock. New areas of mineral potential to the north-east of the mine were identified from the coincidence of geochemical anomalies with strong linear very low frequency (VLF) features. High levels of arsenic, copper, lead, rubidium, tungsten and zinc and low levels of strontium are evident in the soils above the worked veins. These elements, with the exception of arsenic, constitute a broad zone of anomalously high values to the north and east of the vein system, accompanied by low strintium values extending up to 1 km east of the worked veins over a distance of 1.2 km north–south. A broad arsenic halo is apparent for some 800 m west of the main vein system. The distribution patterns of some elements unrelated to mineralisation reflect bedrock lithological variation. Gamma spectrometry demonstrates a distinct increase in the potassium/thorium ratios over the main veins and delineates a potential area of mineralisation to the west of the mine, coincident with a strong VLF anomaly. Qualitative, contoured maps of the count rate for the uranium and potassium channels delineate the suboutcrop of the granite. The geochemical results and previously reported geophysical results (Report No. 33) enabled three areas to be recommended for further, detailed exploration.

MRP summaries 61 to 80
61 Mineral reconnaissance at the Highland Boundary with special reference to the Loch Lomond and Aberfoyle areas

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The chromite geochemistry and hornblende-schist mineralogy of the serpentinite-spilite-black shale-chert assemblage tends to confirm its ophiolitic character. Relict textures indicate that the serpentinites were derived from peridotitic precursors, but one unaltered ultrabasic sample comprises mainly chromian diopside. Magnetic and very low frequency traverses across the Highland Boundary Fault near Helensburgh identified several anomalous zones. One may be due to a concealed serpentinite sheet. The most mineralised serpentinite body showed chromium values in the range 1000 to 3035 ppm. Such concentrations, though not economically significant, may indicate that larger, unaltered serpentinites elsewhere at the Highland Boundary merit investigation.

62 Mineral reconnaissance in the Northumberland Trough

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A reconnaissance geochemical drainage survey across Lower Carboniferous sediments identified not only the known mining areas but also a number of other areas with anomalously high metal values. Barium in stream sediments and panned concentrates is a reliable indicator of mineralisation and identifies the Whinnetley–Settlingstones–Fallowfield area. High barium values in concentrates were obtained from an area to the south of Rothbury (Ewesley Farm); subsequent soil sampling also revealed high levels of barium and it is considered that unexposed barium mineralisation exists in the area, probably associated with a fracture-cutting dolerite of the Whin Sill. A number of linear magnetic anomalies were identified from the results of a detailed low-level airborne geophysical survey over the part of the basin underlain by the Whin Sill at shallow depth. Several of these can be equated with known fault structures or their probable extensions, some of which have carried significant mineralisation. Selected anomalies were further examined by geochemical (soil sampling) and ground geophysical techniques, but the soil samples contained generally low values of ore elements. Four boreholes were drilled on a magnetic anomaly that indicates an eastward extension of the Sun Vein near Newbrough, to test the fault structure affecting the Whin Sill as interpreted from the magnetic data. Considerable variations in the texture and alteration of the quartz dolerite sill were evident from the cores. Base-metal mineralisation was identified associated with this alteration and in some of the carbonate sediments. Chemical analyses of samples from the Whin Sill quantify the changes in composition effected by the hydrothermal alteration. Magnetic susceptibility values determined on the Whin Sill core show great variability, consistent with the variation in the degree of alteration.

63 Exploration for volcanogenic sulphide mineralisation at Benglog, North Wales

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The Ordovician Aran Volcanic Group south-east of the Harlech Dome was explored for volcanogenic sulphide mineralisation and work is described for one of three areas that were selected for detailed investigation. The volcanic rocks of the Benglog area are acid and basic in composition; the acid rocks are mostly ash-flow tuffs derived from outside the area, whereas the basic rocks have a local derivation. They are all interbedded with dark grey or black silty mudstone and were probably erupted in a submarine environment. Contemporaneous dolerite sills were intruded into the wet sediment. This environment was suitable for the formation of volcanogenic exhalative sulphide deposits, and indications of a metallogenic horizon were found at the top of the Y Fron Formation in the form of abundant pyrite, minor pyrrhotite and minor base-metal enrichment. Soil samples were collected for copper, lead and zinc determination, and geophysical surveys were conducted along 11 east–west traverse lines 300 m apart across the succession. Indications were found of minor vein mineralisation at dolerite intrusion margins and locally along faults. Very high chargeability and low resistivity anomalies over mudstones do not coincide spatially with geochemical anomalies in soil, which in many cases may be transported. Geochemical drainage data, in conjunction with rock analyses, show strong barium enrichment in mudstones, which could be volcanogenic in origin and related to two separate eruptive episodes.

64 A mineral reconnaissance of the Dent-Ingleton area of the Askrigg Block, northern England

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The western margin of the Askrigg Block of Lower Carboniferous sediments was investigated by reconnaissance geochemical drainage survey. In addition, following identification of a moss-like plant with tolerance to high levels of base metals, a soil survey was carried out on Tow Scar, 3 km north-west of Ingleton. The drainage survey revealed no new major mineralisation, though minor mineral shows were located following investigation of some anomalous sites. The soil survey led to the discovery of minor mineralisation, which suggests that similar occurrences might exist adjacent to the Craven Fault system east of Ingleton.

65 Geophysical investigations in Swaledale, North Yorkshire

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An airborne geophysical survey was carried out over part of upper Swaledale and the adjacent moorland, where the Lower Carboniferous geology and the style of mineralisation are representative of the Northern Pennine orefield. Magnetic, electromagnetic (EM) and radiometric methods were employed to assess their applicability in this environment. Eleven airborne EM anomalies were followed up with detailed ground surveys. Drilling targeted on anomalies at Oxnop Gill revealed no evidence of significant mineralisation. Further promising anomalies at Whirley Gill were not drilled and remain unexplained. Of the other airborne anomalies, some were not detectable on the ground and others were considered to reflect stratigraphical or artificial conductors. It is concluded that the airborne EM system employed is not effective in exploration for Pennine-type mineral veins.

66 Mineral reconnaissance surveys in the Craven Basin

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A geochemical drainage survey indicated the presence of anomalously high copper, lead and zinc in stream sediments and panned concentrates over Lower Carboniferous limestones and mudstones in various parts of the Craven Basin. An airborne geophysical survey (magnetic, electromagnetic (EM) and radiometric) over the Craven faults at the northern margin of the basin, identified 25 localities that gave an anomalous EM response; ground EM surveys showed that five of these merited more detailed examination. High radiometric readings were obtained over several limestone reefs. Seismic traverses over the South Craven Fault provided information about the stratigraphy on either side of it and regional gravity data provided information about the major structures.

Detailed geophysical, geochemical and geological investigations were carried out in 16 areas where the geological environment or the results of reconnaissance work suggested that mineralisation might be present. Sulphide mineralisation associated with limestones of reef facies was proved, notably at How Hill and Cow Ark, and evidence was found of a continuation of the mined Bycliffe vein, but on present evidence none of the areas appears to contain deposits of ore grade. Many of the minerals appear to have been emplaced by the concentration of brines in structural or stratigraphical traps in which limestones have acted as host rocks. Comparison with the important sulphide deposits in the Lower Carboniferous of Ireland suggests that the most promising area for mineralisation is near the northern boundary of the Craven Basin, possibly at depths of 300 to 400 m.

67 Barytes and copper mineralisation in the Renfrewshire Hills, central Scotland

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Barytes veins are concentrated in massive, open-jointed rocks of the Misty Law trachytic complex, where they occupy a variety of fracture directions within the limits of a north-west to south-east swarm of Tertiary dolerite dykes. Outside the trachytic complex, in the less massive basaltic sequence, barytes veins are confined to major east-south-east to east-north-east trending fault zones and the margins of the east-north-east trending late Carboniferous quartz dolerite dykes, with which they are probably contemporaneous. Several new discoveries of isolated, wide veins of pure barytes could be economic if worked on a small scale and areas are recommended in which follow-up geochemical work may reveal more extensive deposits. It is suggested that barytes mineralisation occurred at intervals from the late Carboniferous onwards during tensional stress regimes, when increased heat flow circulated low-temperature, barium-rich brines, which combined with sulphurous groundwaters in near-surface, oxidising conditions. Copper mineralisation occurs in a wide variety of environments, which range from replacement of plant debris by malachite in sandstones to veins of chalcocite, chalcopyrite and malachite on the margins of quartz dolerite dykes. Mineralised rocks include basal Carboniferous to Lower Limestone Group sediments and volcanics and late Carboniferous dykes. Some of the copper has a direct, late-stage hydrothermal association with the basaltic magmas and it is suggested that cupriferous veins were deposited by later, possibly late Carboniferous, hydrothermal fluids which leached copper from the basalt pile.

68 Polymetallic mineralisation in Carboniferous rocks at Hilderston, near Bathgate, central Scotland

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Five boreholes in the vicinity of the ancient silver-nickel-lead mine at Hilderston yielded new stratigraphical, mineralogical and geochemical information. Strata-bound zinc-lead mineralisation occurs in the lower, argillaceous part of the Petershill Limestone, which was deposited in an anaerobic lagoon on the edge of a volcanic landmass during the Lower Carboniferous (Lower Limestone Group, Viséan stage). The best intersection shows 8 m of mineralised limestone, with underlying carbonaceous mudstone (1 m) and tuffaceous seat rock (2 m), which has an average concentration of 0.14 per cent lead and 0.66 per cent zinc and maximum values of 0.6 per cent lead and 3.1 per cent zinc in the carbonaceous mudstone. Late Carboniferous hydrothermal veins occur within the Petershill Limestone and in immediately overlying clastic sediments, where they are cut by east–west faults and quartz dolerite dykes. At Hilderston Mine, two assemblages are recognised in the vein: barium-iron-nickel-cobalt-silver-gold on a dyke margin adjacent to the clastic sediments and iron-lead-zinc-sulphur at lower levels adjacent to the limestone. Zones of alteration in the dolerite dykes carry hydrocarbons and weak barium-iron-copper-fluorine mineralisation. No potentially valuable vein deposits were discovered.

69 Base metal mineralisation associated with Ordovician shales in south-west Scotland

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Base metal anomalies in drainage and overburden are associated with a belt of hornfelsed black cherty mudstone and siltstone (the Moffat Shales) in the Penkiln Burn area, 13 km north-north-east of Newton Stewart, which is host to weakly disseminated and epigenetic lead-zinc-copper mineralisation. Three varieties of mineralisation have been recognised, the earliest of which is probably syngenetic, whereas the latter two are structurally controlled. The first phase consists of fine disseminations, chiefly of sphalerite and pyrite, in mudstone and chert or stratiform pyrite laminae in mudstone. It is characterised by zinc levels averaging 500 to 1000 ppm over several metres of drill core; lead levels rarely exceed 300 ppm. The second phase of the mineralisation occurs in thin quartz veinlets, which contain accessory sphalerite, galena and pyrite. Where the veining is intense, lead reaches 7000 ppm and zinc 1500 ppm, but these values persist over only a few tens of centimetres of core. Finally, a low-temperature mineral assemblage in which plumbogummite is dominant is associated with the altered margins of dykes and gossan-like zones occupying a north–south fault system. Lead levels in the dyke margins range up to 1.5 per cent in zones that are generally less than 0.5 m thick, but 4.5 per cent lead has been recorded in one specimen from the exposed gossan.

70 Regional geochemical and geophysical surveys in the Berwyn Dome and adjacent areas, North Wales

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A gravity survey across the Berwyn Dome confirmed a broad, regional Bouguer anomaly low in the central part of the dome with smaller, irregular highs and lows, some of which may reflect small igneous bodies. The Bryneglwys Fault coincides with a 4.5 mGal anomaly, but, southwards, the two features diverge, suggesting that the density interface is related either to a splay fault or to the eastern margin of the lower Palaeozoic Montgomery Trough. The Bouguer anomalies probably reflect such factors as variation in the Precambrian basement and changes in the lithology and thickness of lower Palaeozoic sedimentary rocks. There is no evidence for a large granitic body in lower Palaeozoic rocks underlying the mineralisation at Llangynog. A drainage geochemical survey provided evidence of:

  • hydrous oxide precipitation processes
  • contamination from human activities
  • base-metal and barytes mineralisation
  • the presence of monazite nodules
  • hitherto unrecorded gold mineralisation
  • lithological variations

The lithological variations were related principally to shale-sandstone variation, but geochemical signatures attributable to basic intrusions, phosphatic rocks, coal measures, sandstones, limestones and volcanics were also discerned. A few geochemical anomalies deserve further investigation, notably those associated with:

  • gold mineralisation in the north-west of the area
  • barytes, perhaps accompanied by base-metal mineralisation associated with Caradocian volcanics and phosphatic rocks
  • mineralisation associated with Llandeilian limestones and volcanic rocks north of Llanrhaeadr
  • copper mineralisation associated with intrusives near the eastern margin of the dome
71 A regional geochemical soil investigation of the Carboniferous Limestone areas south of Kendal (south Cumbria and north Lancashire)

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The soils of four areas were sampled and analysed for barium, copper, lead, manganese and zinc. The few significant soil geochemical anomalies can be related to contamination, association with minor iron mineralisation or the reefal limestones that occur in the south of the area. The last are the most significant in view of the similarities with areas along the Craven Fault to the east and the association of economic mineralisation with Carboniferous reefs in Ireland.

72 A geochemical drainage survey of the Preseli Hills, south-west Dyfed, Wales

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The main sources of variation in the geochemical drainage data were found to be bedrock lithology, mineralisation, contamination and hydromorphic processes. Strong geochemical signatures are shown by dolerite intrusions and by acid volcanics of the Ordovician Fishguard Volcanic Group. Dark mudstone of the Didymograptus Murchisoni Beds and Sealyham Volcanic Series also show characteristic geochemical features. Over most of the area, abundant monazite nodules give rise to high background levels of rare-earth elements and uranium in the panned concentrates, particularly over Llandeilo to Ashgill sedimentary rocks. Anomalies detected in three areas may be due to hitherto unrevealed mineralisation:

  • barium, copper, lead and zinc near Llanfyrnach, which may represent an extension of the known lead-silver mineralisation of the area
  • barium and base metals near Crosswell-Crymmych, associated with the Fishguard Volcanic Group and overlying pyritiferous dark mudstones of the D. Murchisoni Beds, with potential for massive sulphide deposits
  • minor native gold and chalcopyrite occurrences near Minas Dinas and Pentre Ifan
73 Platinum-group element mineralisation in the Unst ophiolite, Shetland

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The ophiolitic basic and ultrabasic rocks of Unst comprise a sequence of harzburgite, dunite, clinopyroxene-rich cumulates and gabbro. Concentrations of chromite are found in the harzburgite and dunite and, to a small extent, in the pyroxene cumulate rocks. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. Exploration for platinum-group element (PGE) mineralisation was conducted by drainage, overburden and rock sampling and up to 20 other elements were determined in the samples. Low-amplitude iridium anomalies are present in drainage samples from three discrete areas in the harzburgite, but the maximum level of 210 ppb iridium is derived from a prominent north–south zone of faulting and hydrothermal activity. This discordant zone, which extends for at least 7 km, is also marked by enrichments in arsenic, cobalt, copper, iron and nickel. The highest chromium levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromite float have been discovered. Systematic collection of panned heavy-mineral concentrates from overburden samples in the Cliff area outlined a zone of coincident palladium, platium and rhodium enrichment near to, but separate from, the chromite workings known to be enriched in PGEs. The distribution of ruthenium was entirely different, with scattered, low-amplitude anomalous zones and a maximum anomaly 300 m from the chromite-rich zone. Low-amplitude palladium and platinum anomalies were detected at other locations within the dunite unit, especially in a traverse at Helliers Water across the trace of the prominent north–south fault zone adjacent to the outcrop of the cumulate unit.

Very high levels of all PGEs occur in rock samples from chromitite, chromite-rich dunite and dunite in the Cliff area. The proportions of the PGE with strong relative enrichment in palladium and platinum are similar to those in deposits in major layered basic/ultrabasic complexes and completely different from the ruthenium-iridium-osmium-dominant assemblage typical of ophiolitic rocks. There is no correlation with chromium and some samples of chromitite from the Cliff area contain only background levels of PGEs. High to moderate levels of PGEs with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit. In contrast, PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave have proportions of PGEs that closely resemble the pattern found in typical ophiolites. In samples from the Cliff area, the platinum-group minerals sperrylite, stibiopalladinite, hollingworthite, laurite and, possibly, irarsite have been identified, mostly as grains less than 10 μm in size. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. Pre-existing concentrations of chromite may have acted as a precipitation barrier, causing rich platinum-group mineral deposition in the alteration haloes around chromite grains. The high levels of PGEs and the evidence of widespread occurrence of the Cliff-type PGE enrichment are favourable indications of economic mineralisation. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Targets of a larger tonnage may, therefore, be present in this unit by comparison with the likely size of structurally controlled mineralisation elsewhere in the complex.

74 A reconnaissance geochemical drainage survey of the Harlech Dome, North Wales

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The results of a geochemical drainage survey show that the Harlech Dome area is metalliferous, containing large anomalies for a wide range of metals. Strong regional patterns are caused by bedrock lithology, hydromorphic processes, mineralisation and contamination. Geochemical signatures characteristic of the following metalliferous concentrations were identified:

  • disseminated copper 'porphyry-style' mineralisation
  • 'gold-belt' vein-style mineralisation in Cambrian rocks
  • mineralisation in Ordovician volcanic and sedimentary rocks
  • manganese vein-style mineralisation in Ordovician volcanic rocks
  • bedded manganese deposits in the Cambrian
  • granite-related mineralisation
  • dark mudstones

Eleven areas or styles of mineralisation were identified where it was considered that further work might lead to the recognition of deposits of economic or supply significance. These targets include:

  • base-metal anomalies in Ordovician volcanic rocks where there is some potential for volcanogenic stratiform mineralisation
  • copper and gold anomalies in Cambrian rocks indicating the presence of further gold-belt, vein-style mineralisation
  • arsenic anomalies over Ordovician acid volcanic rocks whose gold potential merits investigation
  • maganese and barium anomalies related to maganese-barium vein mineralisation in Arenig volcanic rocks
  • metalliferous concentrations in dark mudstones marginal to the Rhobell volcanic centre
75 Geophysical surveys in part of the Halkyn–Minera mining district, north-east Wales

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Induced polarisation (IP) surveys were carried out over Carboniferous limestones and sandstones at four localities in the Halkyn–Minera area. The first covered the Llandegla Moor area, but failed to produce any evidence for a north-westward continuation of the rich Minera lode system. Gravity data subsequently demonstrated a Bouguer anomaly low over the western margin of the Cefn-y-Fedw Sandstone Group, including the Llandegla Moor area, suggesting that these rocks thicken rapidly eastwards, perhaps along a concealed northerly trending fault. Trial IP surveys were conducted at three sites in an attempt to locate this fault and test its mineral potential, but with negative results. The Bouguer low is interpreted as probably due to the presence of high porosity and rapidly thickening sandstones in the Cefn-y-Fedw Sandstone Group. For these reasons, the area where the low was discovered is not strongly recommended for any future exploration for extensions of the mineral veins.

76 Disseminated molybdenum mineralisation in the Etive plutonic complex in the western Highlands of Scotland

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A drainage geochemical survey covering most of the Etive plutonic complex, of Caledonian age, found the highest concentrations of molybdenum in stream sediment (40 to 120  ppm) to be in streams draining an area about 5 km in diameter within the Central Starav granite. Within this area, molybdenite occurs sporadically, mainly in quartz veinlets. Although selected samples of mineralised rock have been shown to contain up to 0.9 per cent molybdenum, the incidence of sulphide mineralisation is too sparse for a meaningful estimate of tenor to be given. Molybdenite is usually accompanied by pyrite. Chalcopyrite and scheelite are also widespread, though less common. Mild hydrothermal alteration accompanies the mineralisation, but there is no pervasive or zoned alteration, nor is there any potassium or rubidium metasomatism. It is suggested that the ore minerals were deposited from hydrothermal fluids, which, in the absence of any structural or physico-chemical constraints, circulated freely throughout a large volume of rock, with the consequence that the ore minerals are widely dispersed.

77 Follow-up mineral reconnaissance investigations in the Northumberland Trough

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A regional mineral reconnaissance of the Carboniferous sediments of the Northumberland Trough (Report No. 62), including detailed airborne geophysical coverage, enabled several areas to be defined for further investigation, largely on the basis of magnetic data. At Todridge Fell, Wheathill and Brown Moor, soil geochemical data indicate that faulting in the Whin Sill is accompanied by mineralisation. The amount and type of such mineralisation are, however, not determinable except at Brown Moor, where a short drilling programme indicated alteration of the Whin Sill accompanied by barium, lead and manganese mineralisation. At Ewesley, a linear zone of barium enrichment in soils was identified, with associated lead and zinc. A weak linear magnetic feature does not coincide with the surface geochemical 'highs'. The source of the geochemical anomalies can only be determined by drilling; mineralisation associated with a fault structure or a strata-bound concentration is possible. The work in these four areas provides further evidence of the value of magnetic methods in identifying fault structures affecting the Whin Sill, with which alteration and mineralisation may be associated.

78 Exploration for porphyry-style copper mineralisation near Llandeloy, south-west Dyfed

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Geological, geochemical and geophysical surveys followed by drilling in the area around Llandeloy have located disseminated copper mineralisation of porphyry type associated with intermediate intrusive rocks masked by thick overburden. Geological examination combined with a stream-sediment survey revealed the presence of weak polymetallic sulphide mineralisation associated with the margin of a tonalitic intrusion at Middle Mill. Six traverse lines, totalling 10.5 km in length, were surveyed by induced polariatsion (IP), very low frequequency electromagnetic (VLF-EM) and magnetic methods. Soil samples were collected at 25 m intervals along the same lines and analysed for copper, lead and zinc. Few anomalies were located and it was concluded that no substantial body of disseminated copper mineralisation is present at or near the surface in this area. The mineralisation found in Middle Mill quarry is thought to be minor epigenetic mineralisation associated with the intrusion.

At Llandeloy, traverses totalling 37 line kilometres in length were surveyed by IP, VLF-EM, magnetic and radiometric methods. Soil samples were collected at 50 m intervals along all traverses and analysed for copper, lead and zinc. Gravity data were also collected from some traverses and sites. Several strong soil copper and geophysical anomalies were identified. Nine boreholes were drilled to investigate their cause. Disseminated copper mineralisation was intersected, occurring principally within a concordant or semi-concordant sheeted complex of dioritic and tonalitic rocks, believed to be latest Cambrian or early Arenig in age, whose composition is consistent with emplacement within a volcanic-arc setting. The intrusions and their host rocks have suffered a two-phase, pervasive, hydrothermal alteration that is inseparable from the sulphide mineralisation and was recorded in boreholes over an area of 1 km2. The alteration shows features common to porphyry copper systems, consisting of an early, patchy and irregularly developed, porphyritic and potassic alteration overprinted by widespread and locally intense, late propylitic alteration. Copper-iron-sulphur mineralisation accompanied the alteration. Copper levels are generally modest, the best intersection being 0.1 wt% over 3.4 m. Copper, and particularly the copper/sulpur ratio, is generally highest in the most altered rocks, but locally high levels of copper may be found in weakly altered rocks. Molybdenum enrichment is weak and erratic and high levels of copper and molybdenum show only a weak correlation. There are localised, very weak enrichments of arsenic, lead and zinc. It is suggested that the present erosion level cuts a deep section through a copper porphyry deposit, thus explaining the imperfectly developed zonation, low copper content and abundant magnetite. The richest material has probably been removed by erosion and some of it may be represented in the overlying lacustrine sediments, which contain abundant magnetite, clay, feldspar and up to 640 ppm copper.

79 Volcanogenic and exhalative mineralisation within Devonian rocks of the South Hams district of Devon

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Soil samples collected across the outcrop of the Devonian volcanic rocks between the River Yealm and Totnes in the South Hams district display several geochemical anomalies, the most extensive and highest-amplitude of which comprise:

  • barium with smaller amounts of other elements in the Burraton area
  • barium and other elements in the Higher Ludbrook area and further north-east
  • antimony in the Ladywell area
  • arsenic in the extreme west of the area
  • copper in association with a diabase body near Weeke
  • lead and zinc around Willing Cross

In the Burraton and Higher Ludbrook areas, resistivity/induced polarisation (IP), very low frequency electromagnetic (VLF-EM), very low frequency resistivity (VLF-R) and some detailed gravity surveys were conducted. Around Burraton, resistivity anomalies were generally coincident with soil barium anomalies, but there was no coincident gravity anomaly. In the Higher Ludbrook area, a massive carbonate horizon found by drilling is responsible for a zone of high apparent resistivity and a residual Bouguer anomaly high; IP anomalies indicate that disseminated pyrite-rich mineralisation may be extensive, although the results of EM and resistivity surveys suggest that the massive pyrite intersected in one of the boreholes is of limited lateral extent. Geophysical surveys were also carried out near barium anomalies around Whetcombe Cross and near Fursdon in an area of diffuse geochemical anomalies. A small-amplitude IP anomaly in the Fursdon area indicates a possible zone of disseminated, pyrite-rich mineralisation.

In the Higher Ludbrook area, the drilling proved a sequence of massive ankeritic-carbonate-quartz rock about 25 m thick, underlain by massive pyrite up to 7 m thick resting on highly altered tuffaceous volcanic rocks; the sequence is interpreted to be of exhalative origin. Associated with the carbonate rock are high zinc, minor barytes and veinlets containing pyrite, tetrahedrite and chalcopyrite. The carbonate rock also contains some inclusions of highly altered schistose tuff with more than 5 wt% barium. In this rock and in similar volcanic rocks beneath the pyrite, barium appears to be accommodated chiefly in muscovite. The massive pyrite is lensoid in shape, with very minor chalcopyrite. Pyrite in layers up to 0.25 m thick and as rich disseminations also occurs in the upper part of the volcanic rocks beneath the pyrite rock. The tuffaceous volcanics are highly altered basic rocks enriched in potassium. They contain minor amounts of discordant tetrahedrite, chalcopyrite and cobalt-nickel mineralisation.

In the Burraton area, argillaceous sedimentary rocks were the dominant rock type in the drill holes, a 10 m volcanic horizon also being present. Quartzite and barytes occur within a 2 m zone, which may be similar to the massive, layered barytes float seen in the vicinity.

The Ladywell borehole intersected an inverted sequence of volcanic rocks similar to those from the Higher Ludbrook area. No significant antimony mineralisation was intersected and the source of the surface anomalies remains uncertain. The extensive exhalative mineralisation and disseminated sulphide in altered volcanic rocks in South Hams suggest the activity of large, convective hydrothermal cells. The geochemistry suggests that enrichments of gold may also occur in the region.

80 Mineral investigations in the Ben Nevis and Ballachulish areas of the Scottish Highlands

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A drainage geochemical survey of the Ben Nevis area revealed above-background levels of molybdenum that are associated with the outer margins of the porphyritic outer granite and probably related to small-scale vein-type mineralisation, such as that exposed in the Allt Daim. Disseminated pyrite, sometimes with pyrrhotite and/or chalcopyrite, is quite common in the dioritic, appinitic, trondhjemitic and ultrabasic rocks near the Ballachulish igneous complex. Copper values are low and the sulphide disseminations are probably primary.

MRP summaries 81 to 100
81 Investigations for tin around Wheal Reeth, Godolphin, Cornwall

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Recognition of greisenisation associated with worked tin lodes in granite near Wheal Reeth suggests the possibility of unrecorded mineralisation of stockwork or vein-sheet type. Geophysical methods were unable to define either greisenised or mineralised ground and a line of shallow, percussive boreholes was drilled to examine the distribution of tin, associated base metals and fluorine in solid rock below surface soils, which may have been highly contaminated by former mining. No economic mineralisation was revealed by the investigation nor was any broadly disseminated metallisation indicated. Not all of the worked tin-bearing structures could be identified from vertical percussive holes, but one new vein was located by vertical and inclined drilling and trenching. The drill holes outlined at least four more stanniferous veins or vein zones south of the Lady Gwendolen workings. Heavy-mineral concentrates from the drilling samples revealed the ubiquitous enrichment of cassiterite at the base of the regolith cover.

82 Mineral investigations near Bodmin, Cornwall. Part 4: drilling at Royalton Farm

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Fourteen percussive drill holes showed that weak tin mineralisation persists for a distance of at least 50 m into the Devonian slates that form the hanging wall of the Royalton elvan. Anomalous tin values are recorded along the full strike length of the former opencast workings of Old Castle-an-Dinas mine, but the richest concentrations are found near the circular, western pit. Even there, the grades rarely reach economically interesting levels and then only over intervals of 1.5 m. Immediately east of the open works there is a marked enrichment in copper and nickel, but at levels well below those of economic significance.

83 Mineral investigations near Bodmin, Cornwall. Part 5: the Castle-an-Dinas Wolfram Lode

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A soil survey over Devonian slates to the south of Castle-an-Dinas wolfram mine produced anomalies indicative of at least two subparallel zones of tungsten veining and a broad area of anomalously high tin values. Percussive drilling confirmed widespread tin mineralisation beneath the soil anomaly, but the in situ tungsten mineralisation was confined almost entirely to one zone, which can be correlated with the Wolfram Lode in the mine. To the north of the former workings, three sets of traverses were also sampled from percussive drill holes. Two zones of tungsten-tin mineralisation, sometimes with copper, were located, one correlatable with the Wolfram Lode and the other subparallel and some 90 m to the west. Close to the surface, these lode extensions are subeconomic, but it appears that viable ore grades are located in the metamorphosed slates within about 200 m of the contact with the small granite outcrop at Castle-an-Dinas. The ore potential south of the old workings can be estimated at about 1000 tonnes of recoverable tungsten. To the north, the strike length of possible mineralisation is less predictable, but there is little doubt that this area offers the better target for exploration.

84 An airborne geophysical survey of part of west Dyfed, South Wales, and some related ground surveys

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A detailed airborne geophysical survey was made of part of west Dyfed with magnetic, very low frequence electromagnetic (VLF-EM) and radiometric equipment mounted in a helicopter. The 670 km2 area includes the Precambrian anticlines of St David's and Hayscastle; the Ordovician Fishguard, Sealyham and Treffgarne volcanic groups, and the adjacent lower Palaeozoic sediments and basic intrusions. Ground geophysical surveys were carried out at 33 localities to confirm the nature and the sources of the airborne anomalies and a geological examination was also made at selected localities. Rock samples were collected for petrographical examination and the determination of physical properties. A regional gravity survey was also conducted. The aeromagnetic data show clearly the distribution of the Precambrian rocks, the numerous dolerite intrusions and some of the pillow lavas associated with the Fishguard Volcanic Group. This distribution generally confirms the outcrop pattern observed in geological mapping. The magnetic data are likely to be more reliable for mapping on a more detailed scale on account of the extensive superficial cover that hinders geological mapping in many places and they have revealed some large-scale structures, including a previously unrecorded dyke at least 40 km long. The VLF-EM data indicate the presence of many conductive horizons, mostly within lower Palaeozoic sediments.

85 Geophysical surveys near Strontian, Highland Region

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Reconnaissance very low frequency electromagnetic (VLF-EM) and magnetic surveys were carried out over barium-lead-zinc prospects near Strontian. Rather than attempting to detect the economic minerals directly, which is unlikely to be practicable by geophysical methods, the trials concentrated on exploration for the crush zones and associated Permo-Carboniferous basic dykes that act as hosts to mineralisation. The VLF-EM method proved effective in delineating crush zones, whereas magnetic traverses detected the basic dykes. To the east of Bellsgrove mine, a crush zone and dyke extend eastwards along the strike of the Strontian Main Vein. Several crush zones and associated dykes were identified in the Corrantee–Whitesmith area. Probable extensions are indicated to a number of known veins near Fee Donald mine.

86 Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales

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An integrated programme of geological, geochemical and geophysical investigations in the Treffgarne area identified a zone of intense hydrothermal alteration associated with disseminated and vein pyrite within acid volcanic and sedimentary rocks of the Roch Rhyolite Group. Reconnaissance geophysical surveys revealed a 6 km zone of high chargeability coincident with rocks of the Roch Rhyolite Group. Following geological mapping and reconnaissance soil sampling, three boreholes were sited to investigate the geophysical anomalies. The acid volcanic rocks are all highly altered, characterised by exceptionally low total sodium oxide/calcium oxide/potassium oxide and high aluminium oxide, iron, sulphur and, locally, strontium and barium. This is reflected mineralogically by the presence of corundum, barytes and abundant pyrite. The associated altered sedimentary and pyroclastic rocks are intensely sericitised and contain numerous veins and stringers of quartz and pyrite. Some samples of highly pyritiferous dark mudstone contain enhanced levels of gold. Evidence from geological mapping, lithogeochemistry and palaeontological studies suggests that the rocks of the Roch Rhyolite Group are of Lower Ordovician (Arenig) age and not Precambrian as previously documented. An Ordovician age enhances the mineral potential of the Roch Rhyolite Group because of the known association of Ordovician igneous activity with volcanogenic mineralisation in the southern Caledonides.

87 Exploration for strata-bound mineralisation in Middle Dalradian rocks near Huntly, Grampian Region, Scotland*

Test drilling on geochemical and geophysical anomalies associated with very poorly exposed Middle Dalradian metasedimentary rocks of the Portsoy Group at Wellheads Farm, 4 km west-south-west of Huntly, has revealed intraformational breccias, sulphidic graphitic chert and strata-bound pyrite in graphitic quartzite-features that are favourable for the occurrence of strata-bound base metals in the area. Adjacent basic-ultrabasic rocks, although not examined, are favourable for platinum and chromite investigation. Initial zinc anomalies in stream sediment were followed up by shallow and deep overburden sampling and by geophysical surveys (very low frequency electromagnetic, induced polarisation and magnetic) along 5.2 km of across-strike traverses over an area of 2.5 km2. Metal dispersion is hydromorphically controlled in overburden (up to 30 m thick) and probably also in biotite-muscovite schist — the principal bedrock type in boreholes 1-5, which is decomposed and leached to at least 30 m below surface. A final, southernmost, borehole (No. 7) proved fresher rocks, including breccias and pyritic quartzites, chert and limestones. Further geochemical and geophysical surveys, followed by drilling with improved core recovery, are needed in this area, and deeper drilling is required further north to intersect unaltered mica schists.

* Available only in the form of a data package.

88 Mineral exploration for zinc, lead and barytes in Middle Dalradian rocks of the Glenshee area, Grampian Highlands

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Drainage surveys and airborne geophysical surveys of a 600 km2 area from Blair Atholl to Braemar identified several targets near Glenshee within the Ben Eagach Schist, the host formation of the Aberfeldy deposits 30 km along strike to the south-west. Integrated geological, geochemical and geophysical surveys were carried out over these targets. The extensive cover of peat and glacial overburden, particularly over the softer lithologies of the formation, hinders geological mapping and near-surface leaching has destroyed most of the sulphide. The presence of base metals and barytes is best shown by detailed drainage sampling and the sulphide-bearing graphitic schist can be traced through ground covered by superficial deposits by very low frequency electromagnetic, induced polarisation and self potential surveys. Six shallow boreholes were drilled on the basis of the geochemical and geophysical anomalies and mapping of the available outcrop. Zinc-lead mineralisation was found in the clastic lower member of the Ben Eagach Schist as well as in the upper member of graphitic schist. Vein barytes with minor base metals is present in the Ben Lui Schist, a higher Middle Dalradian formation, in southern Glenshee.

89 Geochemical and geophysical investigations of the Permian (Littleham Mudstone) sediments of part of Devon

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New geophysical and geochemical data are presented from three deep, cored boreholes drilled through the Littleham Mudstone sequence. Also included are chemical data for a number of the uraniferous nodules that are scattered throughout the mudstones and which have long been known to be enriched in a variety of metals besides uranium and vanadium. The borehole geophysical data provide new information on the character of the sediments and the distribution of the nodules in an area where there are few alternative sources of this information.

90 Geochemical and geophysical investigations in Exmoor and the Brendon Hills

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Drainage geochemical surveys over Exmoor and the Brendon Hills indicated areas of anomalous metal concentrations in stream sediments which call for further investigation. Some of these anomalies undoubtedly relate to vein-style mineralisation, but others probably reflect a stratiform distribution of ore metals. Barium anomalies were also recognised and indicate previously unrecorded veins of barytes. Investigation of an aeromagnetic anomaly trending west-north-west to east-south-east over the upland areas indicates that it comprises components of both deep and shallow origin. The source of the more deep-seated magnetic anomaly remains uncertain, but two drill holes showed the shallow source to be pyrrhotite mineralisation in the form of disseminations and veinlets. Detailed soil geochemical studies were conducted over some of the aeromagnetic anomalies.

91 A geochemical survey of part of the Cheviot Hills and investigations of drainage anomalies in the Kingsseat area

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A reconnaissance geochemical drainage survey in the Cheviot Hills of northern England identified anomalous concentrations of metals, which are, in general, related to:

  • known mineral occurrences
  • hydrous oxide precipitation and scavenging processes
  • the relatively widespread occurrence of barytes
  • contamination
  • tourmalinisation and other hydrothermal activity
  • high background levels in unmineralised rocks

Some anomalies may reflect hitherto unknown mineralisation. Anomaly groupings and regional variation patterns in the data are influenced by major structures such as the Gyle–Harthope fracture zone. In the Kingsseat area, rocks previously mapped as extrusive mica felsites are reinterpreted as a high-level intrusion complex, named the Cock Law Complex, which contains five distinct types of porphyry. Many of the intrusive and extrusive rocks are highly altered. Metal enrichments were recorded in many of the analysed rocks. The greatest enrichments, for the widest range of elements, occur in samples taken from a gossanous structure, where the mineralisation has features in common with the epithermal precious-metal style of mineralisation associated with subaerial volcanism.

92 A mineral reconnaissance survey of the Llandrindod Wells/Builth Wells Ordovician inlier, Powys

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A reconnaissance drainage geochemical survey, supplemented by soil sampling, located several lead and zinc anomalies, many of which appear to be associated with the outcrop of the main volcanic unit of the area. Samples collected from an area of old lead workings in the west of the inlier failed to provide evidence of further vein mineralisation beyond the limits of the workings. Elsewhere, the biggest concentration of lead and zinc anomalies and the greatest amplitude of anomaly (2000 ppm lead) were located over the northern outcrop of the main unit of tuffaceous volcanic rocks, particularly in the vicinity of the farm Pen Rhiw Frank. A detailed geological, geochemical and geophysical survey of this area was carried out. The geophysical work revealed a zone of high resistivity with roughly coincident low amplitude chargeability maxima and a very low frequency crossover over part of the zone. There was a general association of this zone with lead in soil anomalies and the presence of significant amounts of weathered pyrite in the limited outcrops. The lead in soil anomalies broadly follows the local strike of the rocks. Four boreholes were drilled to test the down-dip extension of apparently strata-bound surface soil anomalies. Secondary lead minerals occur within a soft clay-rich section 5.8 m thick near the top of one of the holes. Lead levels up to 0.52 per cent over 3.4 m were found in this zone, which is located at the interface between a dacitic tuff and an andesitic lava.

93 Strata-bound base-metal mineralisation in Dalradian rocks near Tyndrum, Scotland

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Strata-bound zones of base-metal enrichment (sphalerite, with subordinate galena and chalcopyrite) occur in the Middle Dalradian Ben Challum Quartzite, a newly recognised horizon occcurring between the Ben Lawers Schist, at the top of which there is a horizon of cupriferous pyrite, and the Ben Lui Schist, at the base of which a chromiferous horizon is developed. The distribution of mineralisation over some 9 km of strike length between Tyndrum and the upper Glen Lochay has been mapped at a scale of 1:10 000 and integrated geophysical, geochemical and mineralogical studies carried out. Four boreholes were sited on selected anomalies and the resultant core analysed.

94 Geochemistry of some heavy mineral concentrates from the island of Arran

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Chemical analysis of panned concentrates has indentified areas of possible mineralisation that merit further study. Moderate values of tin and tungsten in the Tertiary Goatfell granite suggest the possibility of disseminated low-grade mineralisation. A cluster of anomalous values near the southern margin of the northern granite holds out some hope of zones of quartz-molybdenite veining associated with the marginal areas of the granite. Anomalous values for silver are quite widely spread in the southern half of the island and appear to be associated with some of the basic minor intrusives. There are high barium values in samples from the environs of the northern granite and over the Palaeozoic sediments east of Machrie Bay. Occasional very high values for copper and lead occur in lithologies that might well host sulphide mineralisation.

95 Mineral reconnaissance at Menear, St Austell, Cornwall

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Geochemical soil sampling shows no evidence for a continuation of the Menear stockwork tin mineralisation beyond the eastern rim of the old openwork, though there may be some extension westwards below areas of recent housing development. Elevated levels of tin in soils are indicated to the north of Wheal Eliza, with above average values of copper and zinc immediately to the south, over the site of that mine; very low frequency electromagnetic anomalies delineate several possible mineral veins in this area. None of these metals are present at concentrations likely to be of economic interest.

96 Geochemistry of sediments from the Lui drainage, Braemar, Grampian

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Stream sediments from the western headwaters of the Lui Water all contain markedly high amounts of niobium and some bear anomalous levels of cerium, yttrium, thorium or zirconium. Most of this compostion is attributable to the presence of a refractory mineral suite presumed to be derived from nearby granitic rocks. The tenor of these elements, however, offers little prospect of any significant concentrations of industrial or ore minerals.

97 Magnetic and geochemical surveys in the area between Geltsdale, Cumbria, and Glendue Fell, Northumberland

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A magnetic survey over the north-west corner of the Lower Carboniferous Alston Block revealed anomalies en echelon along the line of an aeromagnetic feature. These may represent part of a major north-east to south-west structural feature extending to the north-east across Northumberland. The magnetic data indicate that the Whin Sill may be more extensive than suggested on the geological map and additional evidence for this is provided by the geochemical data from the Thinhope Burn and Glendue Burn catchments immediately west of the River South Tyne. Geochemical stream and soil sampling confirm lead-zinc mineralisation in the area.

98 Exploration for gold between the lower valleys of the Erme and Avon in the South Hams district of Devon

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A geochemical drainage survey within a poorly understood belt of Lower Devonian rocks, using panned concentrates, revealed gold anomalies that were subsequently followed up by overburden sampling, geophysical surveys and drilling. The geophysical data suggest that the upper crustal structure of the area and its relationship with the Start Complex to the south require re-interpretation. The multivariate statistical procedures of principal component analysis and cluster analysis, applied to the drainage geochemical data, facilitated recognition of geochemical patterns reflecting major geological boundaries and mineralisation of different types. Comparison of the analyses of different size fractions of the concentrate samples proved useful in detecting anomalies likely to be derived from contamination and in classifying anomaly types. The drainage data strongly suggest the presence of a major fault, trending around west-north-west, separating two entirely different sequences in the north-east of the area. The use of panned overburden samples from shallow pits in exploration for gold is described and compared with the results of conventional sieved soil samples. Mineralogical examination and chemical analysis of mineral grains from overburden concentrate samples led to the discovery of the very rare mineral potarite (palladium + mercury) and the recognition of several types of iron oxide, casserite and some secondary base-metal minerals. On average, the diameter of gold grains in drainage is three times that in overburden grains, whether from background or anomalous sites. To account for this, it is suggested that gold grain growth has occurred in head and perhaps in more recent alluvium in valley bottoms. The chemical composition of overburden gold grains from a number of sites has been determined by electron microprobe. Many grains have relatively silver-rich rims while the bulk of the grain is silver-poor, frequently containing a few percent palladium. Other grains are pure gold to the detection limit of the analytical procedure employed.

99 Base metal and gold mineralisation in north-west Anglesey, North Wales

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Mineral exploration in north-west Anglesey, an area of complex geology with rocks ranging from Precambrian to Ordovician, has indicated the presence of hitherto unknown base-metal mineralisation, accompanied locally by gold. Very low frequency electromagnetic ground survey data contains strong features related to mapped fault lines and steeply dipping geological boundaries. Induced polarisation and soil survey data indicate the presence of anomalies related to mineralisation and more detailed surveys were carried out in the area between Carmel Head and Llanfairynghornwy. The sources of seven of the geophysical anomalies were investigated by fifteen boreholes ranging in depth between 32 and 122 m. These boreholes showed that the anomalies were related to buried base-metal sulphide (copper; lead; zinc) mineralisation, locally accompanied by gold. The mineralisation is polyphase: syngenetic/diagenetic pyrite and disseminated pyritisation (possibly associated with sericite alteration) events are followed by hydrothermal mineralisation characterised by quartz ± carbonate ± chalcopyrite ± sphalerite ± galena ± pyrite veins and impregnations. This mineralisation occurs in at least five separate structures. Geochemical and geophysical anomalies not investigated by drilling suggest the presence of further mineralised structures and extensions to those intersected by drill holes.

100 Molybdenum mineralisation near Chapel of Garioch, Inverurie, Aberdeenshire

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Molybdenum and tungsten mineralisation in quartz veins is associated with the Caledonian Middleton granite, a small stock apparently rooted in the buried roof of the large Bennachie pluton. The granite is emplaced in Dalradian schists. The veins are usually in granite, which is intensely sericitised (greisenised?); the alteration and mineralisation are believed to be coeval. Geophysical surveys were used to define the granite stock, an epidiorite body (a sill?) that caps the hill and a late (Tertiary?) east–west basic dyke. Four short inclined boreholes were drilled in the Dalradian schists to the east of the granite stock and three into the granite itself. The drilling intersected minor quartz-molybdenite mineralisation in both schist and granite. Exposure is poor and neither the drilling nor base-of-superficial geochemical sampling were effective in defining the distribution of the mineralisation.

MRP summaries 101 to 120
101 Skarn-type copper mineralisation in the vicinity of Belstone Consols Mine, Okehampton, Devon

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Significant concentrations of metalliferous sulphides occur in the Lower Carboniferous Meldon Chert Formation, mainly in calc-silicate rocks. Although examined only within a limited area around the former Belstone Consols Mine, similar strata occur as a narrow belt with a strike length of some 22 km between Sourton Tors and Drewsteignton. Stream sediment samples from streams crossing this belt are dominated by minerals derived from the Dartmoor granite to the south and provide little evidence of the location, nature or richness of any sulphide ores. Soil geochemical surveys, however, do indicate clearly the presence and general composition of near-surface mineralisation, even when sited on steep valley slopes or over rather narrow ore beds. Surface geophysical surveys immediately west of Belstone Consols Mine detected and traced horizons of contrasting resistivity and chargeability and provide a new insight into the geological structure. Most of the geophysical markers do not relate directly to potentially economic mineralisation, although higher chargeability values were observed over the principal mineralised zones revealed by subsequent drilling. Magnetic surveys indicate that pyrrhotite is no more than a minor constituent of the mineralisation in the vicinity of the mine. Drilling proved the presence of significant copper and arsenic mineralisation, with little zinc. Cobalt is not important as an accessory metal, but high values of bismuth are quite common. Tin is well developed in most calc-silicate lithologies but is present mainly in the garnets. Metal values locally exceed 3 per cent. The worked ore beds were not identified with certainty, but it seems that a previously unknown mineralised horizon can be recognised higher in the Meldon Chert Formation.

102 Geophysical and geochemical investigations of the manganese deposits of Rhiw, western Llyn, North Wales

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Detailed magnetic surveys were carried out south-west from the old Benallt manganese mine as far as the old Nant mine, and northwards from Benallt towards Sarn. Rocks in the area are of Arenig and Llanvirn age (Lower Ordovician) and consist of mudstones, siltstones and sandstones with interbedded basic lavas and sills. The manganese deposits occur in a structurally complex setting in Arenig sediments, between a basic sill and a dolerite or basalt lava. That part of the manganese mineralisation, which is of ore grade, is unique within the British Isles because of its strong magnetisation, caused by the presence of the iron-manganese oxide jacobsite. Soil samples from across-strike traverses proved of limited value for exploration purposes, mainly because of the variable depth of superficial cover. Titanium and vanadium were useful in areas of limited superficial cover as an aid to mapping the subcrop of the basic igneous rocks. The more extensive magnetic anomalies mark the subcrop of a basic sill (the Footwall Sill), which occurs below the sediments that host the manganese mineralisation. In addition, several very localised magnetic anomalies were identified, three of which were investigated by drilling. Two of them were found to be due to strata-bound ironstones of very limited lateral extent and of particularly high magnetic susceptibility. These ironstones contain up to around 70 per cent iron oxide (Fe2O3) and show a marked depletion in manganese compared with the enclosing sediments and basic igneous rocks. It is thought that other anomalies may be due to discrete bodies of manganese ore.

103 Exploration for volcanogenic mineralisation in Devonian rocks north of Wadebridge, Cornwall

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Reconnaissance overburden sampling across the main outcrops of Middle Devonian volcanic rocks clearly showed the position of the contacts between volcanic and sedimentary rocks, either as sharp increases in elements like titanium or in the value of principal component 1 derived from a principal component analysis of the geochemical data. Follow-up overburden sampling delineated several types of anomaly, some of which were investigated with ground geophysical surveys. Finally, eight diamond-drill holes were collared to test the source of five overburden anomalies. Forty horizons of basic igneous rock were intersected in the drill holes, some clearly volcanic and others clearly intrusive, varying in inclined thickness from a few centimetres to over 50 m. Four compositional groups of basic igneous rock were recognised on the basis of relative concentrations of the immobile elements niobium, titanium, yttrium and zirconium. Two varieties of quartz vein were found as loose blocks during the overburden sampling, one containing boulangerite + galena and the other with arsenopyrite + pyrite. A significant amount of gold (up to 1.0 ppm) is associated with the arsenopyrite-bearing veins. No veins corresponding exactly to these two varieties were intersected in the drill holes, though quartz veins and veinlets with either manganoan siderite or ankerite are common. Associated with some of these veins and with chloritic veins are pyrite, arsenopyrite, chalcopyrite, sphalerite and galena in varying proportions, and minor amounts of tetrahedrite, some of which is richly argentiferous. A second variety of mineralisation, consisting of minor amounts of bournonite, jamesonite and stibnite, is closely associated with intrusive greenstone bodies and their immediate aureoles. Stibnite and secondary products of its alteration in association with siderite is a third type of mineralisation.

104 Strata-bound barium and base-metal mineralisation in Middle Dalradian metasediments near Braemar, Scotland

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Strata-bound mineralisation comprising barytes, barium silicates, sphalerite, galena and other sulphides has been discovered in the upper part of the Ben Eagach Schist Formation, 11 to 13 km south-south-east of Braemar. This formation forms part of the Middle Dalradian (late Precambrian to Cambrian) sequence of metasedimentary and meta-igneous rocks. The new mineralisation occurs at the same stratigraphical position as the Aberfeldy and Loch Lyon deposits, 45 km and 90 km south-west respectively along the regional strike in the Grampian Highlands. Up to 10 per cent zinc + lead is present in Coire Loch Kander in quartzite that also contains hyalophane and armenite, the rare hydrated barium-calcium aluminosilicate.

Bedded quartz-barytes rock, some 5 m thick, is exposed in the headwaters of Allt an Loch, 1 to 2 km south of Loch Kander. A barium anomaly in the overburden extends over 1.6 km along the strike of the bed. The mineralisation was found as a result of integrated geochemical-geophysical-geological investigations north-eastwards along the presumed strike of the Ben Eagach Schist from the Glenshee district. Overburden sampling and geophysical (very low frequency electromagnetic, magnetics, induced polarisation and self potential in part) measurements were conducted along 40 km of across-strike lines running for 11 km from Glen Brighty in the south, across the mountains of Glas Maol and Cairn of Claise to the Allt an Loch district and Coire Loch Kander.

Host rocks are graphitic schists and quartzites, regionally metamorphosed to amphibolite grade, lying at or within a few tens of metres of the top of the Ben Eagach Schist against a thick, sill-like amphibolite body incorporated into the Ben Lawers Schist Formation. Sharp variation in the thickness of the Graphitic Schist Member from 0 to 300 m may be partly attributable to folding but along-strike facies variation is probably of greater significance. Younger igneous rocks include a stock-like diorite that has contact metamorphosed both the bedded sulphide mineralisation and a thin barytes-galena vein unaffected by the regional metamorphism.

The geochemical and geophysical information provide an excellent guide to the bedrock geology, which is very poorly exposed except in Coire Loch Kander. The zinc-lead sulphide enrichment in quartzite is accompanied by pyrite, actinolite and diopside as well as by armenite, hyalophane and traces of barytes, while the massive fine-grained quartz component of this rock is rich in fluid inclusions. The precursor assemblage may have been a hydrothermally altered sediment or a chemical exhalite. The sulphidic quartzite is interdigitated with 15 m of highly pyritic graphitic quartz schist, regarded as a distal exhalative iron-sulphur concentration in carbonaceous mud, and banded calc-silicate schist. In contrast, the bedded, quartz-barytes rock occurs in a gossanous clastic quartzitic sequence containing very little graphitic schist 1 km to the south along strike.

105 Investigations at Lambriggan Mine, near St Agnes, Cornwall

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Ground geophysical and soil geochemical surveys were employed in an attempt to define the extensions of lead-zinc mineralised veins, in Devonian slates formerly exploited in Lambriggan Mine. The geochemical surveys did not indicate any extensions to known mineralisation nor any new mineral veins. Of the geophysical methods used, only the induced polarisation method offered any promise and even that yielded a somewhat speculative interpretaion. It seems unlikely that a significant body of lead or zinc ore awaits discovery in the immediate area of Lambriggan Mine.

106 Marine deposits of chromite and olivine, Inner Hebrides, of Scotland

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A reconnaissance survey was carried out of near-shore marine deposits considered to have been derived from Tertiary ultrabasic rocks in south-west Skye and southern Rhum, rocks known to be enriched in chrome spinel and forsteritic olivine. Dive sampling close to the rocky coastlines supplemented grab sampling at surveyed locations in water depths of 50 m or less. In the bay off Harris, southern Rhum, a heavy mineral sand deposit 3 km2 in area occurs within 2 km of the coast in waters of 20 to 25 m average depth. A 1 km2 deposit is present up to 1 km off Dibidil, also in southern Rhum, in an average water depth of about 20 m. Using a wet density of 2.2 g/cm-3, some 9 million tonnes of sand are calculated to be present in the topmost 1 m of the deltas. Shell calcite, forming about 20 percent, was removed prior to chemical analysis. The analytical results indicate that the surficial 1 m of sand contains some 70 000 tonnes of chrome spinel averaging 32 per cent chormium oxide (Cr2O3) at a grade of nearly 1 per cent. Also present are 1.5 to 2 million tonnes of olivine averaging 47 per cent magnesium oxide (MgO) at 25 per cent grade. Accompanying minerals are ilmenite and vanadiferous magnetite, and traces of platinum-group elements have been detected. The minerals occur in sand-size fractions (125 to 500 μm), from which concentrates of 86 per cent chromite and 78 per cent olivine at recoveries of 60 per cent and 50 per cent respectively have been achieved in the laboratory. Most of the 63 seabed samples from Loch Scavaig and the Soay Sound, south-west Skye, are grey glacial sandy clays averaging only 0.5 per cent Cr2O3 and 2 per cent MgO after carbonate dissolution. Heavy mineral sands derived from the Cuillins igneous centre may nevertheless underly the glacial deposits. Geophysical surveys and profile sampling are required to determine the thickness and grade of the heavy mineral sands.

107 Mineral investigations near Bodmin, Cornwall. Part 6: the Belowda area

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A series of soil sampling traverses were made across the Belowda Beacon granite and across slates of the Devonian Meadfoot Beds, for a distance of 1.7 km east of the granite outcrop, to test the extensions of known and formerly worked mineralisation. In two areas, gridded auger sampling was undertaken. All samples were analysed for tin and its common associates, copper, zinc, uranium, arsenic and boron. Trenching was used to investigate the geochemistry of the area around Brynn Tye mine and Wheal Tregoss: the bedrock was examined for tin using a hand-held portable radioisotope fluorescence analyser and significant mineralisation was channel sampled for laboratory assay. Anomalous tin values in one of the two gridded areas indicate extensions to two veins that may have been previously worked. Trenching at Brynn Tye mine demonstrated that tin mineralisation is associated most commonly with quartz-tourmaline veining and alteration of the slates. Two wide zones of low-grade ore were defined by the sampling, one of which was recognised in two trenches, thus defining a limited strike length, but two inclined diamond drill holes failed to find depth continuation to the veining or the cassiterite distribution.

108 Geochemical investigations around Trewalder, near Camelford, Cornwall

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Stream sediment geochemistry over Devonian slates to the north of St Teath revealed two zinc anomalies, suggestive of an apparently unexplored extension to the lead lode of Trewalder Mine. More detailed stream sampling confirmed these findings, with a significant length of the River Allen reporting anomalous zinc levels in sediments and panned concentrates. There is no correlation with either lead or silver. Some 150 soil samples were collected for analysis from six lines across the strike of the Trewalder Lode and its possible extensions to the north. Log-probability plots failed to show any significant correlation between the distribution populations and the mapped lithology and mineralisation. Traverse profiles for lead, zinc and barium, on the other hand, can be interpreted as showing both particulate and hydromorphic anomalies, suggesting that there may be more than one mesothermal lode in the Trewalder area, extending north of that hamlet for some distance, and that to the north of the River Allen, near Helland, the lode may change in character to become essentially barytes. It appears from the low tenor of the anomalies that the prospects for significant economic ore concentrations must be extremely small. Some previously unexplored lead-zinc-barium mineralisation seems to be indicated on the slopes to the west of the River Allen.

109 Copper and molybdenum distribution at Shap, Cumbria

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Examination of rock outcrops in and around the Shap granite and percussion drilling behind the Pink Granite Quarry confirmed that copper and molybdenum are present over a wide area, though in amounts that are subeconomic everywhere. Induced polarisation surveys did not provide any evidence of anomalously high concentrations of sulphide mineralisation, although a very low frequence electromagnetic anomaly indicates a possible northward extension of a fracture zone that is mineralised where exposed in the quarry face. A model suggesting a more deeply buried porphyry-type deposit is not wholly disproved, but the evidence obtained from drill holes and geophysical surveys is not encouraging.

110 Mineral investigations near Bodmin, Cornwall. Part 7: new occurrences at Quoit and Higher Trenoweth

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Radiometric anomalies over Devonian slates near Quoit and Higher Trenoweth were investigated to determine their subsurface continuity and their potential for associated concentrations of elements such as cobalt and nickel. Surface geochemical investigations and drilling indicate that the individual surface anomalies at Quoit are too isolated to offer significant targets for further exploration. The low levels of copper, lead and zinc suggest that there is little likelihood of this structure being of any significance for mineralisation. Although the anomaly at Higher Trenoweth is traceable for a strike length of at least 300 m it still presents too small a structure to merit further attention. Some high surface tin and boron values are indicated at the southern end of the structure, where kaolinised elvan is known to occur, but although the values reflect the mineralisation they are considered too low to warrant further investigation.

111 Gold and platinum group elements in drainage between the River Erme and Plymouth Sound, South Devon

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Significant amounts of gold have been found in drainage sediment at several localities in the area of south Devon bounded on the east by the estuary and the lower part of the River Erme and on the west by Plymouth Sound. Gold is particularly abundant at a site about 2 km to the south-west of Holbeton, where many of the grains are dendritic or otherwise extremely irregular in shape. The dendritic gold shows internal zonation in palladium abundance very similar to that seen in material in carbonate veins in limestone at Hope's Nose, Torquay. Other grains show incomplete rims rich in palladium (up to 13 per cent) or are close to pure gold in composition. Grains with silver-rich rims are much less common than in drainage and overburden samples from east of the River Erme. Associated with the dendritic gold from south-west of Holbeton are dendritic, elongate knobbly and subrounded grains rich in either platinum or palladium or both, forming around 5 per cent of the total. Though some grains are either pure potarite (palladium = mercury) or gold-bearing potarite without significant internal compositional variation, the majority, including all those containing a significant amount of platinum, show very complex compositional zonation. Six platinum group element (PGE)-bearing compositional types have been distinguished. The cores of grains are generally gold-rich: either gold with only small contents of other metals, or gold with significant palladium and mercury. Rims are generally platinum-rich, often with a significant gold or copper content, which may be recognisable as a distinct thin marginal zone. Other compositions include platinum with a significant mercury content and a combination of roughly equal amounts of palladium, mercury, gold and platinum. Some grains show clear concentric zonation and there may be very large compositional differences between zones only a few microns thick. Cinnabar occurs in small amounts in several drainage samples from the area but is particularly abundant (up to 2000 ppm mercury) in samples derived from an area underlain by Middle Devonian rocks, chiefly mafic volcanics, in the north-east of the area.

The distributions of antimony, arsenic, barium, bismuth, calcium, copper, iron, lead, magnesium, manganese, nickel, rubidium, sulphur, tin, titanium, tungsten, zinc and zirconium in drainage samples are also discussed. Gold concentrations show greatest positive correlation with manganese and iron but a correlation with tin is also significant.

112 Geophysical and geochemical investigations on Anglesey, North Wales

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This report describes a number of surveys carried out on Anglesey and not covered by previous reports in the series. A gravity survey of the island identified two large-amplitude lows: one is associated with volcanic rocks and granite cropping out south-east of the Menai Strait Fault; the other is centred off the north-west coast and is possibly caused by a concealed granite. If of Caledonian age, such a granite would have influenced the distribution of base-metal mineralisation on the island.

Positive anomalies are associated with metabasic rocks in the south-east of the island whilst Carboniferous sedimentary rocks give rise to gravity lows between Malltraeth and Dulas. Geophysical orientation studies of the Ordovician volcanogenic massive sulphide copper-lead-zinc-silver mineralisation at Parys Mountain showed that this style of mineralisation generates strong chargeability anomalies but only weak electromagentic (EM) anomalies, prone to interference from artificial sources. Very low frequency (VLF) EM proved useful for detecting steeply dipping conductors and magnetic anomalies are produced by some basic rocks. A gravity survey detected Bouguer anomalies, which two seismic refraction lines showed may be caused by concealed acid volcanic rocks. Induced polaisation (IP) traversing indicated that no substantial mineralisation was associated with the Bouguer anomalies.

Ground geophysical surveys confirmed airborne EM and magnetic anomalies at Bodewryd, Rhosbeirio, Treferwydd and Tyntywyn. At Rhosbeirio and Tyntywyn, the cause of the EM ground anomalies remains uncertain whilst at Bodewryd and Treferwydd, basic dykes are the probable source of magnetic and EM anomalies.

Soil sampling was carried out around Cerrigceinwen, City Dulas, Llanbadrig, Llandyfrydog and Lligwy to investigate promising indications of mineralisation arising from earlier regional surveys. In addition, geochemical groundwater surveys were carried out around Cerrigceinwen and Llanbadrig, geophysical traversing at Llanbadrig and City Dulas, and rock sampling at Llandyfrydog.

Anomalous results related to mineralisation, possibly of similar style to that found at Parys Mountain or Carmel Head, were recorded at Llanbadrig. Geochemical and geophysical anomalies probably caused by hitherto undiscovered mineralisation were also found at City Dulas. At Llandyfrydog, large base-metal anomalies in soils were ascribed to metal-rich water derived from the Parys Mountain Mine flooding across and percolating into superficial deposits. Some smaller anomalies are probably derived from weak base-metal vein mineralisation. In the Cerrigceinwen area, stream sediment and groundwater survey data suggest that mineralisation might be associated with spilitic rocks within the Mona Complex and the basal Carboniferous succession, but limited soil sampling across these lithologies only located a few isolated base-metal anomalies. The single soil traverse sampled across the basal Carboniferous at Lligwy produced similar results.

113 Mineral investigations at Tredaule, near Launceston, Cornwall

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Five sediment samples from the two small streams east of Tredaule yielded panned concentrates with anomalous contents of tin and tungsten, suggestive of local mineralisation. A single soil-sampling traverse was sited parallel to the main stream and, in the analyses of 34 soils from this line, a small group of coincident tin and tungsten anomalies were reported, as well as a marked pair of silver anomalies farther south. In an endeavour to determine the source of these anomalies, a gridded pattern of soil samples was collected. A total of 379 samples were analysed for a range of ore metals and associated elements. For some elements the results were combined with those from the adjacent traverse line prior to statistical treatment. From these results it is possible to recognise several soils anomalous in tin, usually with associated elevated levels of tungsten, and a different set anomalous in silver. The latter are sometimes associated with anomalous levels of copper, but there is a separate grouping of copper anomalies that may have a closer relationship either to the tin anomalies or to the volcanic rocks over which they are located. It remains an open question as to whether the anomalies have been fully defined in this restricted geochemical programme or whether they continue to the east of the Tredaule stream. The correlation between tungsten and tin and the location of their anomalies relative to those of copper and to the mapped geology suggests the presence of an east–west hypothermal vein.

114 The Mineral Reconnaissance Programme 1990

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Summaries of MRP reports 1 to 113 adapted from compilations first published in the Transactions of the Institution of Mining and Metallurgy. The text of this document has been used in the preparation of this electronic copy (author's note).

115 Platinum group elements in ultramafic rocks of the Upper Deveron Valley, near Huntly, Aberdeenshire

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The results of reconnaissance drainage surveys over an area of mafic and ultramafic Caledonian intrusives showed a good correlation with the known geology. Following the identification of a grain of sperrylite (platinum arsenide, PtAs2) during panning at a site within a poorly-exposed serpentinite near Bridgend, detailed overburden and rock sampling guided by a ground magnetic survey defined a zone of platinum enrichment in basal overburden close to the margin of the serpentinite. The reconnaissance survey was supported by drilling at Red Burn in the Succoth-Brown Hill intrusion. Elevated platinum-group element (PGE) contents (up to about 270 ppb platinum+palladium, with many values in excess of 100 ppb) were recorded in ultramafic lithologies. The highest levels were often accompanied by an increase in palladium relative to platinum and sometimes by elevated gold values. Automated searching on the microprobe revealed several grains of gold, together with platinum-copper and palladium-antimony minerals, in association with base-metal sulphides in sheared and altered host rocks. A hydrothermal origin for the mineralisation is proposed.

Drilling was conducted to investigate a zone in a similar setting near Kelman Hill. One borehole intersected a zone of serpentinite enriched in PGEs, up to 280 ppb platinum+palladium. The host rock has elevated chromium levels with sporadic enrichment in arsenic. A new technique of automated searching for rare phases using the electron microprobe was applied to this drill core, resulting in the detection of several complex PGE-bearing grains intergrown with nickel arsenide.

116 Gold in the Ochil Hills, Scotland

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Reconnaissance drainage sampling in the late 1970s identified gold in heavy-mineral concentrates in the Ochil Hills. Subsequent detailed sampling showed that alluvial gold is present over a large area of the central Ochils and eastwards to the Firth of Tay. The most anomalous catchment, Borland Glen, was the focus for further integrated geological, geochemical and geophysical studies, over Lower Devonian andesitic lavas and pyroclastics intruded by a diorite body and porphyry dykes. A large induced ploarisation (IP) anomaly between Borland Glen and Coul Burn was interpreted as a steep-sided zone of disseminated pyrite with associated hydrothermal alteration. Overburden sampling proved gold and mercury anomalies. Seven boreholes, drilled to a maximum depth of 102 m, revealed intense hydrothermal alteration and brecciation affecting the lavas and pyroclastics in the central, IP-anomalous zone, accompanied by pyritisation with associated minor base-metal sulphides. Gold values in the drill core reach a maximum of 505 ppb gold and it is concluded that the bedrock source of the alluvial gold has not been proved.

However, the intense hydrothermal alteration in the setting of an evolved calc-alkaline volcanic complex is indicative of a large epithermal system, and a more fertile source may yet be discovered in the area. Other gold sources are indicated in the central Ochils and further detailed investigations are thought to be warranted.

117 Exploration for vanadiferous magnetite and ilmenite in the Lizard complex, Cornwall

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Drilling near magnetic anomalies over the Trelan gabbro proved extensive oxide-rich gabbro with 10 to 15 per cent combined ilmenite and magnetite. Ilmenite is the predominant oxide and only rarely is magnetite more than a minor constituent. The vanadium content of the magnetite is commonly 2 to 3 per cent, reaching up to 5 per cent V2O3. The oxides occur in clusters and vein-like growths, suggesting that a filter-pressing mechanism may have caused local coalescence of oxide-rich liquid prior to crystallisation. No economic concentrations of oxide were found in the solid rock, but the friable near-surface weathered oxide-rich gabbro is a potential ilmenite resource. Part of the Crousa gravel is a channel deposit up to 9m deep, cut through clay-rich material interpreted as glacial till, with boulders of gabbro from the Crousa gabbro on the coast to the east. At its base there is concentration of heavy minerals of exotic origin, including cassiterite and tourmaline. The till is thought to have been deposited from sea ice pushed up against the coast during an early glacial event. The results of limited power-auger sampling from the north-east of the complex suggests potential for exhalative mineralisation. Siliceous sediments with anomalously high Ba occur in association with probable mafic volcanics. A provisional revised geological map of the eastern part of the Lizard based on geochemical mapping of the overburden samples and showing the extent of the Trelan gabbro is included in the report.

118 Mineral exploration in the Cockermouth area, Cumbria. Part 1: Regional surveys

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Revision geological mapping over Carboniferous rocks between Caldbeck and the coast at Maryport discovered many new occurrences of barytes, particularly in the Dinantian and Namurian rocks of the Tallentire-Bothel area. The mineralisation usually comprises epigenetic fracture fillings of barytes with, in some places, minor chalcopyrite or malachite. Barytes also occurs in disseminated and veinlet form in the Hensingham Grit. Lead-zinc mineralisation is less common; it occurs rarely as epigenetic fracture fillings and locally as syngenetic or diagenetic concentrations in mudstones and shales of the Coal Measures. A geochemical drainage survey confirmed the widespread occurrence of barytes and suggested that hitherto undetected mineralisation may be present around Ruthwaite (Ba), Tallentire Hill (Ba, Cu), Broughton Moor (Ba, base metals), near Binsey (polymetallic) and south of Stockdale (Zn, Pb). Gold was observed in 21 panned concentrates, mostly over Upper Carboniferous rocks in the west of the area. It probably comes from glacial deposits derived from mineralised lower Palaeozoic rocks in southern Scotland and the north-east Lake District. Cinnabar was identified in 22 concentrates and is believed to be derived locally from the epigenetic mineralisation, as some samples of mineral veins and altered wallrocks contain appreciable Hg (up to 40 ppm). It was concluded that the barytes mineralisation may be present locally in sufficient quantities to be of economic interest. The epigenetic mineralisation is considered to be Upper Carboniferous to Lower Permian in age, the product of fluid flow through open fractures at the margin of the Solway Basin.

119 Investigations for Cu-Ni and PGE in the Hill of Barra area, near Oldmeldrum, Aberdeenshire

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Following promising indications from commercial surveys, limited investigations for copper-nickel mineralisation were conducted at the eastern end of the Insch intrusion over a sheared and disrupted zone of olivine-bearing cumulates regarded as potentially favourable for PGE enrichment associated with the base-metal mineralisation. Magnetic data delineated the southern contact of the intrusion and highlighted structural discontinuities within the cumulates. Enhanced levels of copper and copper/nickel ratios in overburden above these structures indicated potential for hydrothermal base metal enrichment. Precious metal concentrations were generally low, but one site produced values of 88 ppm arsenic and 12 ppb gold. A small suite of rock samples showed no significant enrichment in base or precious metals.

120 A gravity investigation of the Middleton granite, near Inverurie, Aberdeenshire

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An 8km2 detailed gravity survey was conducted around the Middleton granite (which is associated with Mo and W mineralisation) at a station density of 30 per km2, and a surrounding area of 120 km2 was surveyed at about two stations per km2. A residual Bouguer anomaly low over the granite indicates a granite subcrop centred to the north-west of its previously mapped location and a shallow granite ridge extending about 1.5km to the south-south-west. This structure enlarges the target area for further mineral exploration. Modelling indicates that the granite is a cupola projecting from the buried roof of the Bennachie component of the Eastern Highlands batholith and delineates a north-east trending feature interpreted as the concealed margin between the Bennachie granite and the more dense and/or thinner components of the batholith to the south-east.

MRP summaries 121 to 140
121 Exploration for gold in the South Hams district of Devon

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Gold in panned drainage samples is widely distributed over the area south of a line between Plymouth and Brixham, with values over 0.5 ppm at 44 out of 450 sites. Anomalies are present over the Lower Devonian sequence and the Start Complex but are less frequent over the Middle Devonian. There is no simple pathfinder for gold and the factors influencing its concentration are complex. Many of the grains are very intricate in shape, with projections that could not survive if transported more than trivial distances from source. The compositions of the gold grains show great variety and complexity, with local concentrations of palladium and silver. Follow-up overburden sampling showed that gold is present in head and weathered bedrock, as well as in near-surface overburden. Four holes drilled to test the source of one east–west zone of anomalous gold in overburden intersected black slate, pyritiferous in part, and widespread lensoid vein quartz, often with minor carbonate. Samples from a 5 m-wide zone of oxidation alteration, brecciation and carbonate veining contained minor levels of gold, reaching a maximum of 380 ppb. Two phases of mineralisation are thought to be responsible for the gold anomalies in drainage and soil. The first comprises polymetallic mineralisation associated with hydrothermal alteration, predating the main deformation of the rocks. In the second phase, which accounts for most of the gold grains in drainage, saline oxidising solutions carrying precious metals circulated within and beneath the Permo-Triassic red-bed sequence, which had been deposited on the eroded Devonian surface. Deposition of gold occurred where conditions became more reducing, particularly within Devonian rocks by reaction with pyritiferous slates.

122 Mineral exploration in the Cockermouth area, Cumbria

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This report describes the results of geochemical, geological and geophysical surveys across three small areas of Carboniferous and lower Palaeozoic rocks along the northern margin of the English Lake District. The areas were chosen from the appraisal of regional-scale survey data described by Cooper et al. (1991). In two of the areas, Ruthwaite and Tallentire, the objective was to provide more information on the extent and magnitude of fracture-controlled epigenetic barytes and base metal mineralisation. In the third area, at Whitrigg, brief surveys were carried out to aid the interpretation of unexplained geochemical and geophysical anomalies found during two projects carried out under the Mineral Exploration and Investment Grants Act (MEIGA).

At Ruthwaite, where a mine formerly worked barytes from a fault separating lower Palaeozoic and Carboniferous rocks, surface indications of further barytes mineralisation were found. Soil analyses indicated that mineralisation may be present along the continuation of the faultiine worked at Ruthwaite and in the Eycott Volcanic Group rocks to the south of it. In this area, relatively small, but in some circumstances perhaps economically attractive, deposits of barytes may be present under superficial cover.

In the Tallentire Hill area, geological mapping followed by traverse-based soil sampling showed that fracture-controlled mineralisation is widespread in the Carboniferous rocks. The fracture fillings consist dominantly of barytes, often accompanied by carbonate, with traces of copper and mercury. Where seen at surface, the fracture fillings are too small, patchy and low-grade to be of any economic importance. Barytes mineralisation also occurs locally as patchy impregnations in sandstones. These are considered to be epigenetic deposits related to the fracture-controlled mineralisation. Trial geophysical surveys suggested that electrical methods may be useful in determining the extent of the mineralised sandstone. There is a possibility that more extensive barytes deposits may be present in the limestone succession underlying the mineralised sandstones.

In the Whitrigg area, Carboniferous rocks are separated from lower Palaeozoic rocks of the Eycott Volcanic Group by the easterly-trending Boundary Fault and north-westerly-trending Bothel Fault. Evidence from an old mineral working and the results of a soil survey indicate that patchy, epigenetic, fracture-controlled barytes and base metal mineralisation occurs along the Bothel Fault and, locally, in the adjacent rocks. A feature of this mineralisation is the presence of mercury, which is most abundant in a sample of brecciated and altered rock from the Eycott Volcanic Group. Prominent base metal in soil anomalies discovered by MEIGA-funded projects near Stangerhill are not associated with barium anomalies. It was concluded that these soil anomalies are most likely to be caused by secondary concentration in overburden and that the source of metals may be a subcropping metalliferous horizon within the Carboniferous succession or, more probably, fracture-controlled mineralisation.

Trial geophysical surveys carried out in all three areas indicated that, in ground free of artificial sources, very low frequency electromagnetic and conductivity mapping methods could be useful for tracing faults beneath superficial geology and providing information on its thickness. Closely spaced soil sampling proved effective for detecting mineralisation in areas where the superficial cover is thin and a trial soil-gas survey showed that this technique could also be useful for tracing faults beneath superficial geology.

123 Mineral investigations in the Teign Valley, Devon. Part 2: base metals

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This report describes the search for new base metal reserves in the Teign Valley between Dunsford and Chudleigh, an area worked for lead and zinc ores, with associated silver and copper, in the late 19th century. A programme of geochemical drainage and soil surveys was followed by geophysical surveys and diamond drilling.

Chemical analyses were carried out on waters, stream sediments and panned concentrates collected from secondary drainage. The water samples, including effluences from old mine workings, were only rarely anomalous in base metals. However, stream sediment and panned concentrate analyses revealed copper, lead, zinc and arsenic anomalies caused by the Teign Valley lode zone and manganese anomalies that reflected areas of former opencast mining. One cluster of anomalies suggested possible lead-zinc-copper-arsenic-barium mineralisation to the east of the River Teign.

Soil sampling was carried out mainly across interfluve ridges to the west of the river. Interpretation of the soil analyses confirmed the common occurrence of anomalous lead, zinc and copper within the Teign Valley lode zone and indicated that a few parallel mineralised structures may also be present. Some anomalies suggested the presence of disseminated mineralisation within the bedded succession of shales, cherts and tuffs.

Induced polarisation (IP) geophysical surveys were carried out in four separate areas containing geochemical anomalies using the dipole-dipole array. Locally, more detailed measurements were made using the gradient array. Anomalies believed to be related to concealed sulphide mineralisation were recorded in all four areas. In the Dunsford area, chargeabillty anomalies coincident with lead anomalies in soil may be caused by disseminated mineralisation. Near Bridford, anomalies with different characteristics were attributed to disseminated and vein-style mineralisation. The presence of a small high-grade galena vein was suggested by anomalies to the east of the main vein at Wheal Exmouth. Sixteen interlinked traverses north-east of Bovey Tracey defined two significant anomalies compatible with the presence of sulphide mineralisation; soil geochemistry indicated significant lead with copper but only minor zinc enrichment. Four short inclined diamond drill holes were sited north-east of Bovey Tracey, between Lower and Higher Coombe, to investigate the clusters of geochemical anomalies which IP data suggested were caused by sulphide mineralisation. The mineralisation was found to comprise disseminated and thin, discontinuous strata-bound veinlets of sulphides within shales, cherts and tuffs close to the Lower/Upper Carboniferous boundary. Galena and sphalerite with a little chalcopyrite, arsenopyrite and loellingite are associated with pyrite, quartz and siderite.

Chemical analysis of drillcore revealed appreciable zinc concentrations in some sections, one containing 2 per cent zinc over 3 m. Lead values are lower, with a maximum of 0.2 per cent over 1 m; several 1 to 3 m lengths containing 0.1 per cent lead are present. Copper concentrations are very variable; the best intersection contained 0.14 per cent over 1 m.

Finely disseminated galena and sphalerite have not been reported previously from the Teign Valley and their discovery opens up the potential for this type of deposit concealed within the Carboniferous condensed sequence both here and in other areas of south-west England.

124 Platinum group elements in the Huntly intrusion, Aberdeenshire, north-east Scotland

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This report describes a programme of exploration for the platinum group elements (PGE) carried out over the Caledonian mafic/ultramafic Huntly intrusion, located in west Aberdeenshire, Grampian Region, Scotland. The area was considered as having potential for two principal classes of PGE-bearing mineralisation, namely magmatic reef style in cumulate rocks and hydrothermal type in structurally controlled settings.

Recent re-mapping of the poorly exposed Huntly intrusion by BGS and Aberdeen University has revealed a complex internal structure and clarified the nature of its external contacts. Several discrete bodies of cumulate rocks have been recognised, while much of the intrusion comprises granular or contaminated gabbroic rocks without cumulate characteristics.

A comprehensive programme of lithogeochemical sampling was carried out over the intrusion and adjacent country rocks, in order to detect any indications of PGE-bearing mineralisation. Olivine cumulate rocks have the highest background concentrations of platinum and palladium. Maximum values of 50 ppb platinum and 25 ppb palladium were recorded in a peridotite from the West Huntly cumulate body. Deformed ultramafic rocks in a tectonically complex zone near the northern margin of the intrusion around Whitehill are locally enriched in precious metals: up to 28 ppb platinum, 63 ppb palladium and 30 ppb gold.

The highest levels of enrichment in precious metals (up to 462 ppb platinum+palladium+gold) were found in discordant mineralised pegmatitic pyroxenites in the West Huntly cumulate body, best exposed in the Bin Quarry. Drilling undertaken in the Bin Quarry showed that these bodies are narrow and impersistent at depth.

The West Huntly cumulate block was investigated for reef-style PGE mineralisation using a variety of techniques. In the Dunbennan Wood area, detailed ground magnetic surveys were used to guide basal overburden sampling along reconnaissance traverses. Sporadic minor PGE enrichment was detected, with levels generally enhanced in the fine fraction (-100 mesh/-150 micron) samples, relative to panned material. A biogeochemical orientation survey was conducted over the same area using various sample media from the four principal conifer species present, namely larch, Scots pine, Norway spruce and sitka spruce. The twigs of all species sampled showed a positive response to both platinum and gold. Although no conclusive association with bedrock geology was established, a general enhancement of precious metals is noted in larch twigs over ultramafic cumulates in the Dunbennan zone.

Reconnaissance surveys were conducted over the Central Huntly Shear Zone, a branch of the Portsoy lineament that transects the Huntly intrusion and is associated with PGE-bearing Cu-Ni sulphide mineralisation on the flank of the Knock intrusion a few kilometres to the north. Magnetic and very low frequency surveys failed to detect any indications of structurally controlled sulphide mineralisation in this zone. Soil-gas surveys, involving the measurement of oxygen and carbon dioxide, were conducted along reconnaissance traverses across this structure in order to detect any indications of oxidising sulphide mineralisation. Significant anomalies, with C02 content locally in excess of 10 per cent, were revealed in several areas. In most cases, these anomalies were attributed to pedogenic or biogenic causes. At Cumrie North, however, soil-gas anomalies investigated by pitting are ascribed to the tectonic juxtaposition of contrasting lithologies in a complex shear zone in the northern part of the structure. Only traces of sulphide mineralisation are present within this anomalous zone, but deformed ultramafic lithologies with minor enrichment in PGE were revealed by pitting. Soil sampling has indicated that additional bodies of ultramafic rock are also present in this sector of the shear zone.

125 Geochemistry database: data analysis and proposed design

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The results of the data analysis for a geochemistry relational database to hold the land-based UK datasets currently managed by the Minerals and Geochemical Surveys Division, plus some other geochemical datasets held by BGS, are presented in full in the form of a geochemistry data model. Recommendations for the formal acceptance of this model by the management of the Minerals and Geochemical Surveys Division are also produced. Financial support for this project came from the Department of Trade and Industry Mineral Reconnaissance Programme.

The minimum requirement for all potential datasets is defined. The same standard sample numbering system must be used for all samples. All batches of analysed samples must have the correct and complete index information available. An entity relationship diagram showing all relationships between the 43 identified entities is provided and 35 of the entities, which map directly onto the relations identified by data normalisation, are defined in detail and all 79 of their associated domains are also defined.

Defects identified during the data analysis and subsequent quality assurance review are listed and discussed. The report only covers BGS onshore data and offshore and overseas data are specifically excluded from this data analysis. Gas and vegetation entities are not adequately covered by this report and require further work. Few samples of these types have been collected in the past so this is not a serious deficiency.

126 Mineral exploration in the Pitlochry to Glen Clova area, Tayside Region, Scotland

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Reconnaissance drainage sampling of the area between Pitlochry and Glen Clova in the Highlands of Scotland was undertaken in 1987 and 1988. Three hundred and nine stream sediment and 347 panned concentrate samples were collected and analysed for a variety of major and trace elements.

Four areas were identified with potential for economic gold mineralisation: Glen Clova, Glen Uig, east of Dunkeld and Bridge of Cally. Detailed sampling and follow-up investigations in Glen Clova identified a gold-bearing fault zone in the Burn of Fleurs. Levels up to 7 ppm gold were detected in clay fault gouge and the fault can be traced by geophysical methods for 1.6 km. Features with a similar trend, such as the orientation of stream courses, can be identified extending for a further 10 km to the south-east. Recommendations for further work to investigate this and other prospective areas are presented.

The potential of the area for base metal deposits is probably low and the few sulphide-bearing veins are of little economic significance when compared to the strata-bound deposits in the Middle Dalradian. The economic potential of the Highland Boundary fault zone was, however, unexplored.

127 The metalliferous mineral potential of the basic rocks of the Penmynydd Zone, south-east Anglesey

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Geophysical and geochemical investigations have been carried out to assess the mineral potential of an area in south-east Anglesey containing positive gravity anomalies and coincident aeromagnetic anomalies. The anomalous area contains outcrops of dense, basic hornblende schist of the Precambrian Penmynydd Zone, which is a complex, fault-bounded zone of tectonised metasediments and basic schists.

The existing regional gravity survey coverage has been augmented by over 150 additional gravity stations and three detailed traverses made on lines normal to the regional strike. Over 250 soil samples have been collected on seven traverse lines that, together with over 30 rock samples, have been analysed for a suite of up to 13 elements (soils) and up to 28 elements (rocks). Some of the rocks have also been analysed for gold and platinum.

The geophysical data suggests that the gravity and aeromagnetic anomalies over the Penmynydd Zone can best be explained by the presence of a near-surface, relatively dense body of low magnetic susceptibility underlain at around 3 km depth by a body with a much higher magnetic susceptibility. The anomalies could therefore be due to a layered basic intrusion, fault-bounded on both its north-west and south-east sides. Along-strike modelling of this body indicates that it is disrupted by block faulting along a north-west trend, with segments becoming progressively deeper towards the north-east.

The geochemistry of the basic hornblende schists indicates that they have an oceanic tholeiitic basalt parentage. The soil sampling results show several barium anomalies, probably associated with thin barytes veins, and one Pb/Zn anomaly over Carboniferous limestone. There are no immediate geological or geochemical indications of potentially economic, near-surface mineralisation in the area.

128 Mineral exploration in the lower Palaeozoic rocks of south-west Cumbria. Part 1: regional surveys

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The results of geochemical, geological and geophysical surveys over lower Palaeozoic rocks in the south-western part of Cumbria are given in two reports. This report (part 1) describes the results of a geochemical drainage survey and an examination of mineralised sites, and relates them to information from new geological mapping and an assessment of regional geophysical data. Part 2 contains details of follow-up surveys in the Black Combe inlier.

The geochemical drainage survey, involving the collection and analysis of heavy mineral concentrates and stream sediment samples from 119 sites, found substantial antimony, arsenic, barium, bismuth, copper, iron, lead, tin, tungsten and zinc anomalies. Gold was reported for the first time from this part of the Lake District: small amounts were noted in panned concentrates from five sites. Other minerals identified in panned concentrates included arsenopyrite, barytes, bismutite, bismuthinite, cassiterite, chalcopyrite, cerussite, pyrite, pyromorphite, scheelite, sphalerite, stolzite and wolframite.

The examination of old workings and outcrops revealed many undocumented occurrences of quartz-sulphide vein-style mineralisation. The chemical analysis of samples taken from old workings and other occurrences confirmed field observations that locally, particularly in the Black Combe area, this mineralisation is polymetallic with variable amounts of arsenic, gold, bismuth, copper, lead, zinc and in a few cases antimony, barium, cobalt, nickel, tungsten and tin. Iron mineralisation occurs both as oxide (haematite) and sulphide deposits. Mercury was present in appreciable amounts in samples from the High Brow pyrite mine.

The distribution of panned concentrate anomalies suggests that the vein-style mineralisation is polyphase and that individual phases may be zoned. Highest zinc anomalies occur near Torver and the highest lead on the west side of Black Combe. Tin and tungsten are restricted largely to the central part of Black Combe and the most prominent arsenic and bismuth anomalies are found in the same area. Copper anomalies are widespread over the Skiddaw Group and the Borrowdale Volcanic Group. Barium anomalies indicate that barytes mineralisation is weak and localised, occurring principally within the Black Combe area and close to the Windermere Supergroup basal unconformity. Iron oxides from host rocks and haematite mineralisation are responsible for local enrichments of iron, antimony, arsenic and molybdenum in panned concentrates.

The results indicate the presence of a hitherto undetected episode of tin-tungsten mineralisation in Black Combe, where it is associated with tourmalinites and bleached (metasomatised) rocks of the Skiddaw Group. Gold, associated with arsenic-bismuth-copper-cobalt mineralisation, is also present in Black Combe and to the north-west in the Borrowdale Volcanic Group. This mineralisation is believed to be Devonian in age and associated closely with a putative buried, evolved, end-Caledonian granitic intrusion on the southern margin of the Lake District batholith. Lead-zinc mineralisation may either accompany this event or form a separate episode of mineralisation. Barytes occurrences may be related to later Carboniferous to Mesozoic events, including haematite mineralisation. The regional controls on the location of mineralisation include the Lake District batholith, the Ulpha basin, the Westmorland Monocline/Southern Borrowdales Lineament and host rock lithology.

129 Mineralisation in the Middle Devonian volcanic belt and associated rocks of south Devon

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This report describes the results of further exploration within the belt of Middle Devonian volcanic rocks between Plymouth and Totnes in south Devon. Soil sampling was carried out to augment the coverage of the volcanic belt described in Mineral Reconnaissance Programme Report No. 79 (Leake et al., 1985) and to extend it into adjacent sedimentary rocks. The 4815 overburden samples indicate that the area as a whole is highly enriched in antimony and, to a lesser extent, arsenic.

No evidence was found of further stratiform exhalative mineralisation in addition to the massive pyrite and ferruginous carbonate at Higher Ludbrook and the barytes at Lower Burraton described in MRP Report No. 79. However, four main areas showing evidence of metal-enriched sedimentary rocks have been outlined. In three of the areas, enrichment in manganese in the soils derived from the sedimentary rocks is accompanied by low amplitude enrichment in zinc and lead, reaching around 200 ppm zinc and 170 ppm lead. The fourth area, adjacent to the separate belt of volcanic rocks north-west of Totnes, is more extensive and of higher amplitude (reaching over 700 ppm zinc and 600 ppm lead). The soil and drill hole data indicate that extensive hydrothermal systems were associated with the alkali basaltic volcanism in the area and that submarine hydrothermal activity took place. The close similarity in geology between the area and the Rhenish basin in Germany, which hosts the Meggen sedimentary exhalative deposit, indicates that south Devon and east Cornwall remain prospective for submarine exhalative mineralisation.

Overburden samples indicate that polymetallic mineralisation occurs within a zone about 3 km long in the west of the area. The zone is enriched in arsenic, copper, lead, manganese and zinc and is similar to polymetallic mineralisation carrying gold, which occurs further south in Devon and is described in Mineral Reconnaissance Programme Report No. 121 (Leake et al., 1992). Evidence for further polymetallic mineralisation is present in the north-east of the area but this differs geochemically from the other areas in having a higher proportion of zinc to lead and in the presence of anomalous concentrations of tin. Proximity to the Dartmoor granite suggests that this anomalous zone could be related to the contact aureole of the granite.

Two further boreholes were drilled to investigate the source of the zone of anomalous antimony in soil at Ladywell, as the earlier hole described in MRP report No. 79 did not intersect sufficient mineralisation to account for the surface anomaly. One hole intersected a zone of oxidised rock containing 120 ppm antimony over 6.4 m within a wider zone showing lower amplitude enrichment in antimony (75 ppm over 21 m) and containing minor amounts of bournonite, tetrahedrite and stibnite. This enrichment in antimony may be primary, in association with one episode of volcanicity. No evidence of an association of precious metals with this mineralisation was found, though there was some enrichment in mercury (up to 11 ppm). The second hole showed no enrichment in antimony but contained minor amounts of base metal sulphides in association with carbonate veinlets and sections of dark slate enriched in zinc (up to 1600 ppm zinc over 1 m).

130 The occurrence and economic potential of nodular monazite in south-central Wales

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Review of all Mineral Reconnaissance Programme panned concentrate data confirmed that very high (over 5000 ppm) levels of cerium, caused by the presence of nodular monazite, are characteristic of samples collected from catchments containing sedimentary rocks of Upper Cambrian to Silurian age deposited in the Welsh Basin. The largest panned concentrate anomalies (more than 1 per cent cerium) are associated with rocks of Upper Ordovician age and the most extensive area containing these very high values is in south-central Wales, near Newcastle Emlyn.

Follow-up panned concentrate sampling in this area showed that cerium anomalies caused by nodular monazite extend from tributary drainage into the main river systems and, in the Afon Teifi, persist for over 20 km into the estuary near Cardigan. Levels of monazite in stream sediments locally exceed minimum grades exploited in placer deposits, reaching 1.65 per cent in the below 2 mm (sand, silt and clay) fraction of the samples collected. Consequently, it is recommended that further work is undertaken to assess the concentrations of monazite and other heavy minerals in river and estuarine sediments, dune sands and beach deposits associated with the Afon Teifi and other rivers draining Upper Ordovician and Lower Silurian sedimentary rocks deposited in the Welsh Basin. Few other heavy resistate minerals were recorded in the sediments collected from the Afon Teifi, but the placer deposit potential of some other monazite-bearing estuarine sediments in Wales is likely to be enhanced by the presence of additional economic minerals, notably gold.

Mineralogical studies of nodular monazites found in the rocks of the Newcastle Emlyn area showed that they have very similar properties to those described from central Wales, Belgium, France and Spain. They occur in mudrocks subjected to low-grade metamorphism, are less than 2 mm in size, ovoid to discoid in shape, dark grey in colour, and have a prominent inclusion fabric indistinguishable from the host rock. The nodules are characterised by a low thorium and high europium content compared with monazites of igneous origin and are compositionally zoned with light rare earth element-rich rims. However, the monazites from the Newcastle Emlyn area have some distinctive properties: they are notably smaller and more ragged, and commonly contain more inclusions than those from central Wales. They contain lower thorium and higher europium levels than many other nodular monazites, and display complex chemical zonation with up to seven concentric zones, 10 to 100 μm wide, distinguishable within a single nodule.

In contrast to central Wales, where nodules are concentrated in Llandovery-age, hemipelagic mudstone horizons showing considerable enrichment in rare eath elements (REEs), nodules appeared to be dispersed in the mudstone-dominated succession of the Newcastle Emlyn area. Bulk analysis of these nodule-bearing rocks showed that they do not contain unusually high overall levels of REEs and no evidence of strata-bound REE enrichment was found in the survey area. The concentrations of nodules recorded both here and in the hemipelagic mudstones of central Wales suggest that an economic deposit of nodular monazite in bedrock is unlikely to exist in Wales.

It is believed that the nodules formed by post-depositional, pre-metamorphic, diagenetic growth under physico-chemical conditions that are poorly understood, but which involved at least local saturation of REE with respect to REE phosphate coprecipitation in the pore fluids. Under the anoxic conditions likely to have prevailed during early diagenesis, REEs may have been released from iron-manganese hydrous oxides and other phases, and fixed by phosphate released during the decomposition of dispersed organic matter.

131 Platinum group element mineralisation in the Loch Ailsh alkaline igneous complex, north-west Scotland

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A programme of exploration for the platinum group elements (PGE) carried out over the Loch Ailsh alkaline intrusive complex in north-west Scotland is described. The area was considered to have potential for two styles of PGE-bearing mineralisation, namely magmatic, associated with cumulus or intercumulus phases, and hydrothermal, related to postcumulus low-temperature PGE concentration and precipitation in structurally controlled settings.

Reconnaissance sampling of drainage sediments and bedrock indicated localised PGE enrichment in mafic and ultramafic rocks in the south-eastern and northern parts of the complex. Concentrations were generally higher in line-fraction drainage samples than in corresponding panned coarser fractions. Maximum values in stream sediments are 859 ppb platinum and 43 ppb palladium. Drainage sampling also identified enhanced gold levels in panned concentrates in the northern sector of the complex and markedly enhanced gold concentrations in stream sediments at several, widely spaced localities.

Detailed basal overburden sampling over the south-east part of the complex indicated widespread PGE enrichment in pyroxenites. The highest PGE concentrations in overburden, up to 110 ppb platinum and 70 ppb palladium, occur within the central section Allt Cathair Bhan catchment.

Lithogeochemical sampling revealed a widespread, low-tenor PGE enrichment in the Allt Cathair Bhan valley in pyroxenites, pyroxenite mylonites and pyroxenite skarns consistent with a magmatic origin. Higher PGE contents, up to a maximum of 300 ppb platinum+palladium, occur sporadically in the lower section of the Allt Cathair Bhan valley. In these PGE-enriched rocks, sperrylite (PtAs) and isomertieite (palladium+antimony+arsenic) were identified by automated microchemical mapping. Complex tellurides of bismuth, lead, palladium and silver were also found in these rocks.

A detailed ground magnetic survey was used to define the extent and structure of the concealed parts of the mafic and ultramafic components of the complex. These investigations indicated that the pyroxenite exposed in the south-east of the complex underlies a large area of lower Palaeozoic and Moinian cover. Additionally, very low frequency resistivity techniques were employed to search for potentially PGE-enriched sulphide mineralisation without success.

Geochemical and petrological investigations suggest that the Ailsh pyroxenites are of magmatic origin and that PGE enrichment is associated with late-magmatic, volatile-rich fluids deficient in sulphur. Further localised upgrading of the PGE and gold may have occurred at lower temperatures, together with base-metal enrichment.

132 Reconnaissance drainage survey for base metal mineralisation in the Lleyn Peninsula, North Wales

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A geochemical drainage survey has been carried out over the Precambrian and lower Palaeozoic sedimentary and volcanic rocks of the Lleyn Peninsula using a combination of -100 BSI mesh stream sediment samples and panned concentrates collected from the same site. There is widespread contamination of drainage sediment with metallic material of domestic and, locally, industrial origin. Accordingly, 39 concentrates containing high levels of base metals were examined mineralogically to help distinguish anomalous samples containing natural minerals from those containing metallic contaminants. Chalcopyrite, sphalerite, galena, secondary lead minerals and barytes were the main minerals of hydrothermal origin detected. In some samples, weathering of contaminants was seen to have produced secondary copper and lead minerals similar to those of natural origin.

Samples containing high levels of base metals are widespread throughout the area and those with the largest contents contain contaminants. Anomalies largely of natural origin, which probably reflect mineralisation upstream, are concentrated within the outcrop of Lower Ordovician shales towards the western end of the peninsula, mostly between Llanbedrog and Botwnnog. From the spatial distribution of anomalies in this area, both structurally-controlled mineralisation and strata-bound sulphide enrichment are possible types of source. In addition, there are several copper and barium anomalies along the outcrop of the Precambrian Gwna Group, an olistostome containing a variety of igneous and sedimentary rock fragments, which probably reflect minor vein mineralisation.

133 Exploration for gold in the Crediton Trough, Devon. Part 1: regional surveys

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The results of geochemical surveys in the Crediton Trough of Devon, an area of Permian and Carboniferous rocks north of Dartmoor, are given in two reports. This report (part 1) describes the broad-scale drainage and lithogeochemical survey carried out mainly in the area from Hatherleigh in the west to the valley of the River Exe in the east, over the outcrop of Permian red-bed sediments, minor alkaline basalts and lamprophyric lavas and the surrounding Carboniferous sediments. The Permian outliers at Hollacombe (near Holsworthy), Peppercombe (near Clovelly) and Holcombe Rogus (south-west of Wellington), together with parts of the Permian outcrop of the Tiverton Basin and west of Cullompton, were also sampled. In addition, the results of an interpretation of Landsat Thematic Mapper (TM) imagery over the survey area are presented. Part 2 contains details of follow-up overburden surveys at Deckport, Solland and Smallbrook.

The area was selected for gold exploration on the basis of the model of precious metal transport developed to account for the widespread gold in south Devon, which suggested that gold mineralisation might be present in the Permian sequence and the contact with underlying Carboniferous rocks.

Drainage surveying confirmed the presence of gold for the first time at numerous localities on the Permian outcrop and subsequent microchemical mapping of grains demonstrated a number of close similarities with gold from south Devon, strongly suggesting a similar origin.

The analysis of rock samples from the Crediton Trough also showed gold to be locally enriched, up to 1.8 ppm in alkaline basalts and up to 42 ppb in samples of Permian sedimentary breccias.

Extensive manganese and zinc drainage anomalies at the southern boundary of the Crediton Trough can be related to mineralisation within the Permian and Carboniferous, some of which was worked in the vicinity of Newton St Cyres. Cinnabar was reported for the first time from this area, and detrital tin, copper and lead anomalies, thought to variously reflect ore minerals or contamination, were also recorded by the drainage survey.

The distribution of gold anomalies in the drainage samples indicates that the source is probably associated with the early Permian sediments, the boundary faults between the Permian and Carboniferous sequences, and structures in the Permian, especially where they are underlain by volcanic rocks. Further overburden sampling at three sites is reported in part 2.

Satellite imagery interpretation showed that all the gold anomalies lie on or near lineaments, usually a set trending north-east, which may be more closely associated with mineralising pathways than other directions.

134 Exploration for gold in the Crediton Trough, Devon. Part 2: detailed surveys

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This report describes the results of geochemical sampling to trace the source of three groups of high-amplitude gold anomalies in panned drainage sediment within the outcrop of the Permian New Red Sandstone sedimentary rocks of the Crediton Trough. The sites for follow-up were chosen after appraisal of regional-scale survey data described in the previous report (Cameron et al., 1994). They comprised the Deckport and Solland areas at the western end of the Crediton Trough and the Smallbrook area adjacent to the faulted southern margin of the Permian rocks some 20 km further east. This work consisted of more detailed drainage sampling and reconnaissance overburden sampling at all three sites, augmented by detailed overburden sampling and the mechanical excavation of a series of pits and trenches at Smallbrook. Microchemical mapping of gold grains from drainage sediment and from excavations in overburden aided the interpretation of the origin of the gold.

At Deckport, where the Bow Breccia (early Permian) is in faulted contact with the Crackington Formation (late Carboniferous), follow-up sampling indicated strongly that the major source of gold was the Bow Breccia. Telluride inclusions were more frequent in the gold grains from the southern part of the Permian outcrop than in gold from most other sites in the Crediton Trough. This may indicate that the source is related to a nearby centre of igneous activity, the root of which may be marked by the lamprophyric dykes and vent agglomerate south and south-west of Hatherleigh.

The Solland area is immediately east of the trace of a component of the Sticklepath–Lustleigh Fault. Gold persists in drainage sediment towards the southern, faulted contact of the Bow Breccia with Bude Formation (late Carboniferous) strata to the south. Overburden sampling across the trace of a fault to the east of Solland, parallel to the Sticklepath–Lustleigh Fault, indicated that the gold was not associated with this fault but occurred in alluvial terrace material derived from further south. However, the analysis of overburden samples indicated high values of uranium associated with this fault.

At Smallbrook, where the highest-amplitude drainage enrichments in gold had been found, further sampling showed a sharp cut-off for gold just north of the boundary fault with the Crackington Formation. The gold grains from the Small Brook differ from grains from other locations in the Crediton Trough in being finer grained, generally rounded, not enriched in palladium and with fewer and smaller inclusions. Gold was found physically and by analysis in panned overburden pit samples at several sites to the south-east of the Small Brook, particularly in the residual overburden derived from the Newton St Cyres Breccia (late Permian), to a maximum of fourteen grains from one site. Trenching and pitting confirmed that the shallow overburden samples closely reflected the weathered bedrock beneath. Gold was found (12 to 35 ppb gold) in several unpanned below 0.5 mm fraction samples, but no highly anomalous levels were detected. Microchemical mapping of a gold grain extracted from Newton St Cyres Breccia showed internal chemical characteristics and inclusions identical to grains from the alluvium of the Small Brook, and may indicate an igneous association. The horizontal and vertical distribution of gold in the overburden and weathered bedrock indicate that it is widely dispersed in the Newton St Cyres Breccia in the form of a fossil placer. The source of the gold is probably the older Permian sequence, within which a rich source of mineralisation may exist to the west of Smallbrook. Excavations in alluvium adjacent to the Small Brook indicate the widespread presence of gold (maximum 1130 ppb gold in panned material) from above 1 m.

Three grains of gold extracted from core from the faulted contact between Permian and Carboniferous rocks in the Upton Pyne Borehole, 5 km east of the Small Brook, were similar in chemistry but not in shape to grains from the Small Brook. This discovery reinforces the potential for gold mineralisation close to the contacts, both faulted and unconformable, of the Permian red-bed sequence in Devon.

Further work, including drilling, is recommended to determine the concentration of gold in the basal Permian rocks and to determine the potential and controls exerted by Permian igneous rocks on the mineralisation.

135 The potential for diamonds in Britain

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This report reviews existing geological and geophysical data for Britain in the light of current models for diamond formation and deposit signatures.

Most diamonds at the Earth's surface are the product of two unusual processes: the lowering of the geothermal gradient deep in the mantle, which allows the diamond stability field to be entered, and the formation of a magma at or below these depths, within which diamond-bearing xenoliths can be transported rapidly to the surface. As most diamonds seem to have formed in subcratonic roots during the early history of the Earth, the Lewisian terrane of north-west Scotland is the most favourable environment in Britain. No kimberlites are known in this terrane, but exposure is very poor over most of the Hebrides and, although existing aeromagnetic data is widely spaced, several interesting features are identified in the Highlands. The Proterozoic Torridonian rocks of north-west Scotland represent the most likely location of a palaeoplacer in Britain.

Recent thinking suggests that diamond could also crystallise in a cold subducted slab, where it causes a depression of the geothermal gradient. Such a slab may have been left behind after subduction in the Southern Uplands of Scotland at the end of the Silurian, forming a transient source of diamonds that may have been tapped by lamprophyres, alkali basalts and nephelinites. A subducted slab could also have been stagnant beneath south-west England and tapped by the Permian lamprophyric and lamproitic intrusions or alkaline lavas.

Further work is recommended, particularly in south-west England and north-west Scotland, to determine whether potentially favourable intrusions and palaeoplacers are diamondiferous.

136 A review of detailed airborne geophysical surveys in Great Britain

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Detailed airborne geophysical surveys, with flight line spacings of a few hundred metres, low terrain clearance and a combination of magnetic, electromagnetic (EM) and radiometric instruments, provide high-resolution data that is particularly valuable for mineral exploration purposes. BGS holds data for 17 of these airborne mineral exploration surveys, covering a combined area of approximately 16 000 km2, carried out in Britain since 1957.

The Mineral Reconnaissance Programme has funded a number of detailed airborne geophysical surveys flown in Britain between 1972 and 1978, covering a combined area of approximately 3000 km2. Flying heights were 60 m or 75 m with flight-line spacings of 100 m, 200 m or 250 m. Methods used were magnetics, radiometrics and EM or very low frequency (VLF) EM. The earlier surveys were flown using analogue data recorders and many of these datasets have now been digitised and incorporated with the later, digitally recorded surveys. BGS also holds datasets from an extensive detailed survey flown over south-west England for the UKAEA/BGS in 1957 to 1959, and for several flown in Scotland by commercial exploration companies. Details of all these surveys, including a brief description of the geology, results, ground follow-up and availability of data, are contained in the report, which also lists other detailed airborne geophysical surveys known to BGS.

137 Exploration for volcanogenic mineralisation in south-west Wales

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The voluminous Ordovician volcanic rocks of south-west Wales are interbedded with black shales and other sedimentary rocks, an association that hosts economic base-metal deposits elsewhere in the Caledonides of Europe (for example, Parys Mountain, Anglesey; Avoca, Ireland) and North America (for example, Buchans, Newfoundland). This report reviews the mineral potential of all the volcanic rocks exposed in south-west Wales using the results of a geological reconnaissance and lithogeochemical studies, as well as previously published work and BGS regional datasets.

A number of rock groups and areas meriting further investigation were identified, including the Roche Rhyolite Group at Treffgarne, the Sealyham Volcanic Formation and the Fishguard Volcanic Group, near Crosswell. Geochemical and geophysical surveys in the Treffgarne area aimed at supplementing previous MRP work in the area that confirmed the presence of pyritic and highly altered (silicification and loss of alkalis) volcanic rocks, locally containing barytes. Brief investigations in the Crosswell area revealed anomalous levels of barium in soil (more than 2000 ppm) and coincident strike-parallel electromagnetic conductors associated with black shales overlying the Fishguard Volcanic Group. The poorly exposed Sealyham Volcanic Group locally contains acid volcanic rocks with similar alteration to that observed in the Treffgarne area. Further work is recommended in these areas to search for concealed volcanogenic sulphide mineralisation.

138 Gold exploration in the Duns area, Southern Uplands, Scotland

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Follow-up to previous BGS surveys involving panned stream sediment sampling and soil sampling led to the recognition of markedly anomalous gold concentrations, sometimes associated with enrichment in arsenic. Excavations to bedrock in two areas revealed localised gold mineralisation (up to 5 ppm) in hydrothermally altered greywacke, with enrichment in adjacent, altered, minor intrusives. Rock sampling of surface outcrops revealed widespread low-tenor (10 to 100 ppb) gold enrichment in hydrothermally altered Silurian greywackes, siltstones and subordinate porphyritic intrusives, some of which show pervasive propyllitic alteration. Gold values in excess of 100 ppb are associated with fracture zones, hydrothermal breccias and localised quartz veining in the greywacke. Mineralogical studies indicate that early-phase sulphide mineralisation occurring as disseminations and in fracture veinlets was followed by a later event, associated with fracturing and involving the precipitation of iron oxyhydroxide minerals.

On a regional scale, mineralisation and alteration of the greywackes is focused within a previously unrecognised east-south-east trending regional structure, within which a series of late Caledonian calc-alkaline intrusives has been emplaced. The establishment of major structural controls on the mineralisation provides a number of potential targets for further exploration aimed at the discovery of mesothermal mineralisation associated with regional strike-slip shear zones. The style may be comparable to gold mineralisation developed in the Sierra Nevada foothills, the Canadian Cordillera and the Hercynian of Northern Spain.

139 Exploration for carbonate-hosted base-metal mineralisation near Ashbourne, Derbyshire

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A search for carbonate-hosted base-metal deposits was directed at an area south-west of Ashbourne, where two small mineralised inliers of early Carboniferous limestone were thought, on gravity evidence, to be connected by a shallow ridge of limestone buried under thin Permo-Triassic cover. In early Carboniferous times, this area was situated near the boundary between the Widmerpool Gulf, to the north-east, and the stable area of the Staffordshire Shelf. Following geophysical surveys, the limestone ridge was proved at shallow (60 m) depth by a borehole between the inliers. A further borehole intersected over 10 m of replacement lead-zinc-barytes mineralisation, including a short interval containing 3 per cent zinc and more than 10 per cent lead. This style of mineralisation contrasts with the characteristic vein deposits of the South Pennine Orefield to the north. The mineralised host rock is a dolomitised Waulsortian knoll-reef of Chadian age, similar to the Courceyan knoll-reefs that host important lead-zinc mineralisation in Ireland.

Further work is recommended along the 7 km-long ridge to search for similar mineralised structures in Carboniferous rocks concealed by thin Permo-Triassic cover.

140 Mineral exploration for gold and base metals in the Lewisian and associated rocks of the Glenelg area, north-west Scotland

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The Glenelg area in the north-west Highlands of Scotland is underlain by Lewisian rocks of the Glenelg-Attadale inlier, which is the largest outcrop of basement gneiss within the Caledonian orogenic belt and formed the target for a mineral exploration programme looking for gold and base metals. Stream sediments and panned concentrates were collected from over 180 km2 and analysed for base metals, gold and pathfinder elements. The area covered Lewisian rocks of both the eastern and western facies and Moinian rocks in the Caledonian orogenic belt and the late-Caledonian Ratagain igneous complex.

Detailed investigations were carried out at Carr Brae near Loch Duich on iron-rich rocks similar to those associated with the copper-gold deposit at Gairloch hosted by the supracrustal Loch Maree Series. The iron-rich rocks were traced along strike from Carr Brae for 14 km to the south-west but, despite chemical evidence that the rocks are metamorphosed chemical exhalites, little significant gold or base metal mineralisation was found. Calc-silicate gneisses within the Eastern Lewisian assemblage of metamorphosed sediments and volcanics show copper enrichment. Graphitic gneisses have potential as a source of crystalline graphite, with the carbon content of the rocks reaching 16 per cent. Further work is recommended to assess the economic potential of this occurrence.

The Ratagain igneous complex, despite the recorded occurrence of veins carrying electrum, is not considered to be a good target for further mineral exploration as the complex is well exposed and the mineralisation is widely scattered and of relatively low grade. Veins within the Strathconon fault system are a more favourable target, given its long (100 km) strike length and the occurrence of gold at two localities within this area and at Scardroy 40 km to the north-east.

MRP summaries 141 to 146
141 Assessment of the potential for gold mineralisation in the Southern Uplands of Scotland using multiple geological, geophysical and geochemical datasets

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Digital geological, geophysical and geochemical datasets for the Southern Uplands of Scotland have been combined and evaluated using computer-based visualisation methods to enable selection of areas potentially favourable for the occurrence of gold mineralisation. Datasets utilised include:

  • residual polar aeromagnetic anomalies
  • lineations derived from images of aeromagnetic and gravity data
  • significant deviations from the regional strike of the lower Palaeozoic rocks
  • distribution of antimony, arsenic, bismuth, copper, lead, nickel and silver in -150 microns stream sediment samples
  • locations of known gold occurrences

Particular attention was paid to:

  • areas where widespread strike deviations were possible indications of anomalous relative tensional stress fields, which could favour mineralisation
  • intersections and foci of geophysical lineations
  • small groups of arsenic and antimony anomalies, which could indicate centres of minor intrusive igneous activity

Three primary target areas and 19 smaller secondary target areas were identified containing variable combinations of favourable geological, geophysical and geochemical features. Reconnaissance drainage and rock samples were collected from the three primary target areas and, on a smaller scale, from eight of the secondary target areas. In one of the primary target areas, south-east of Moffat, widespread gold was found in drainage and 2.23 ppm gold was recorded in a sample of brecciated and altered red greywacke siltstone. Gold was also found in drainage in the other primary target areas to the south of Peebles and west of Castle Douglas, though amounts were small in the latter area. Significant amounts of alluvial gold were found in three of the secondary target areas and lesser amounts in two others. The results of the test sampling validate the methodology and are sufficiently promising in several areas to merit follow-up investigations to define and assess the source of alluvial and bedrock gold.

142 Industrial mineral potential of andalusite and garnet in the Scottish Highlands

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A desk review of andalusite and garnet occurrences in the Scottish Highlands was followed by a programme of field sampling, laboratory characterisation and mineral processing to identify their potential for use as industrial raw materials. Bulk samples were collected from selected occurrences of andalusite schists in the Dalradian metasedimentary rocks of north-east Scotland and from garnetiferous Lewisian, Moine and Dalradian rocks and beach sands at sites throughout the Highlands.

Following petrographical analysis, five andalusite samples and nine garnet samples were chosen for laboratory mineral processing trials whose aim was to produce concentrates of the target minerals with high grades and recoveries. The most promising samples were then subject to pilot-scale mineral processing trials.

The andalusite concentrates produced by both laboratory and pilot-scale mineral processing do not match the chemical specifications required for industrial grade andalusite. The Al2O3 and Fe2O3 contents, which are particularly critical, are respectively too low and too high.

The garnet samples, however, yielded concentrates with high garnet contents and recoveries. Concentrates gave densities of 4 g/cm3, indicating their suitability as industrial-grade garnet products. The more promising samples were collected from garnet-mica schists of Dalradian age at locations from near Huntly in north-east Scotland and near Loch Fyne in the south-west Highlands. Further investigations are recommended to prove the scale of resources and to carry out more detailed mineral processing and quality assessments.

143 Gold mineralisation in the Dalradian rocks of Knapdale–Kintyre, south-west Highlands, Scotland

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Following reappraisal of regional geochemical and geophysical data for the northern part of the Kintyre Peninsula, a geochemical survey was conducted to search for the presence of gold in the Dalradian rocks of the area.

Drainage sampling identified gold enrichment in several areas underlain by Argyll Subgroup Dalradian metasedimentary rocks. Associated lithogeochemical sampling confirmed the presence of local elevated gold concentrations in bedrock. The highest levels, up to 22 ppm gold, were found in base metal-bearing quartz veins at the disused Stronchullin lead mine in the north-east of the project area. Similar veins carrying high levels of gold have been found elsewhere in the Stronchullin valley, in the Inverneil area in the north, and in the south of the project area near the shore of West Loch Tarbert. Gold enrichment also occurs in association with discordant copper veins in the Meall Mor area.

Analysis of geophysical and remotely sensed lineations indicates the importance of structural control on the distribution of gold in the area. The drainage geochemical data identifies several additional prospective zones possibly associated with east–west lineations.

In the light of recent discoveries of potentially economic gold mineralisation elsewhere in the Dalradian belt in Scotland and Northern Ireland, further investigations in the south Knapdale area of the Kintyre Peninsula are recommended. Priority should be given to the Stronchullin area and the other known base-metal vein occurrences.

144 The potential for gold mineralisation in the British Permian and Triassic red beds and their contacts with underlying rocks

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A review of the potential for gold and related mineralisation in Permian and Triassic red beds and their contacts with other sequences has been carried out for the whole of Britain, using the mineralisation model of gold transport within oxidising saline solutions and deposition at interfaces with more reduced rocks, and in fault zones carrying more reduced fluids. Drainage and rock sampling was carried out in several areas considered prospective, to search for direct evidence of gold mineralisation.

In Scotland, gold compositionally similar to that found in the Crediton Trough in Devon was found to be widespread and locally abundant in alluvial sediment from the Mauchline Basin of red beds and alkali basalts of Permian age. Two samples of a sandstone raft within the alkali basalt were found to be impregnated with native copper and two grains of gold were extracted from an outcrop of hydrothermally altered basalt. Streams draining the nearby Thornhill Basin were also auriferous.

Gold was also found in stream alluvium at sites in north-west England, Charnwood Forest, near Redditch, the southern Malvern Hills, adjacent to the Mendip Hills and north of Tiverton in south-west England.

This desk study and reconnaissance exploration has demonstrated that gold is associated with the Permian red beds of Britain, especially where alkali basalts are present within the sequence, and to a lesser extent with Triassic rocks. The findings support the red bed mineralisation model, developed from previous MRP work in Devon.

Further exploration is recommended in several areas, particularly the Mauchline Basin.

145 Exploration for strata-bound mineralisation in the Argyll Group (Dalradian) of north-east Scotland

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The Mineral Reconnaissance Programme identified several horizons of strata-bound mineralisation within the Argyll Group during investigations of the Dalradian Supergroup in Scotland between 1973 and 1987. These included the multimillion tonne Aberfeldy barytes deposits. Most of the discoveries were made in the central and south-western Scottish Highlands, where the Dalradian succession is well recognised. Because of its poorer exposure and greater complexity, the Dalradian of north-east Scotland did not receive the same level of attention.

The project reported here was set up to assess the potential of the Argyll Group in north-east Scotland for strata-bound mineralisation, based on the geological models developed in the central Highlands. An initial desk study involved the digitisation (where necessary), integration and review of the following datasets:

  • geological mapping, including the results of new BGS mapping
  • BGS airborne aeromagnetic data
  • BGS geophysical ground survey results
  • BGS Geochemical Baseline Survey of the Environment drainage data
  • existing MRP data
  • traverse-based BGS soil sampling results
  • datasets collected by Exploration Ventures Ltd, principally detailed airborne and ground geophysical information

Assessment of these datasets identified three broad areas that contained geochemical, geophysical and geological features meriting examination to determine their sources and indicate the potential for economic strata-bound mineralisation. These areas were Ballater–Strathdon, Upper Deveron and Huntly–Portsoy. Geochemical and geophysical ground surveys were carried out in parts of all three of these areas and followed up locally by pitting and trenching to clarify the sources of individual anomalies.

The studies indicated that the areas with the most potential for strata-bound mineralisation occur to the east of the Portsoy Lineament or in rocks that cannot be easily correlated with Argyll Group rocks elsewhere. These rocks tend to be lithologically different from the typical Argyll Group sequence and are generally more fractured, sheared, brecciated and altered.

The most promising area for the discovery of strata-bound mineralisation is considered to be in the Upper Deveron area at Wellheads. Here, high levels of lead and zinc in overburden are probably enhanced by hydromorphic processes, but the source of the lead has been traced to quartzites and the zinc to adjacent pelites within the Corinacy Pelite Member of the Blackwater Formation. The predominantly lead-zinc mineralisation is similar to that at Glenshee and Dericambus. Detailed, very low frequency electromagnetic (VLF EM) and induced polarisation (IP) surveys are recommended to assist in defining trenching and drilling targets. Lead and coincident IP anomalies at Succoth-Gouls also merit follow-up investigation.

In the Ballater-Strathdon area a large EM and magnetic anomaly on Creagan Riabhach, although apparently low in base-metal content, merits further investigation on the grounds that:

  • a significant amount of sulphide mineralisation is present at shallow depth
  • there is good evidence for strata-bound mineralisation
  • the rocks are thought to be similar to the Ben Lawers Pyrite, where copper mineralisation occurs sporadically

In the Upper Donside part of the Ballater-Strathdon area, several lead-zinc anomalies have been identified from drainage sampling, but very little mineral exploration work has been carried out over the approximately 10 km strike length of the Argyll Group metasedimentary rocks. The Glenbuchat Graphitic Schist Formation has a volcanic component here, providing the potential for hydrothermal mineralisation. In the Glen Avon part of the area, lead and zinc drainage anomalies have been identified and their potential is enhanced by proximity to the Lecht mineralisation.

146 Mineral exploration in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands

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Supracrustal rocks in the Lewisian of north-west Scotland have several of the characteristics of 'greenstone belts' in basement gneiss terrain, which host many of the world's gold deposits. Often these deposits are closely associated with banded iron formations (BIFs) and iron-rich exhalites are present in the Loch Maree Group of mixed volcano-sedimentary origin. The exhalites include massive and microbanded iron oxide facies, together with various banded silicate and sulphide facies rocks, which form well-defined and laterally extensive suites.

A subeconomic, Besshi-style copper-zinc-gold deposit was discovered by Consolidated Goldfields in the Loch Maree Group supracrustals near Gairloch and an extension to this mineralisation has been found 11 km to the south-east at Flowerdale Forest. Sulphide-bearing and banded iron formations have been located with significant gold values up to 4 g/t. A significant very low frequency electromagnetic (VLF EM) and magnetic anomaly is found in the area and further exploration should be concentrated along strike under a thin cover of Torridonian sedimentary rocks. A separate outcrop of the supracrustals, north-east of Loch Maree, contains two BIF horizons and chlorite schists similar to those forming the footwall to the Gairloch deposit. The BIF horizons were traced over 6 km by measuring their magnetic susceptibilities and contain up to 0.8 g/t gold. The tectono-stratigraphy of the Loch Maree and Flowerdale areas has been clarified and can now be compared to the better-known Gairloch succession.

Reconnaissance lithogeochemical sampling of other supracrustal successions in Coll and Tiree, North and South Uist, and Benbecula was less promising. Investigations in other areas of the Lewisian, east of the Caledonian front at Glenelg and Scardroy, have been described in MRP Report 140 and MRP Open File Report 12. The Central Mainland belt of the Lewisian contains a number of dismembered relicts of mafic and ultramafic parentage and limited sampling of these layered complexes for platinum group elements and copper-nickel was inconclusive.

Data releases 1 to 23
Releases 1 to 7: largely superseded by later work and reports

No summaries available.

Release No. 8: data arising from drilling investigations in the Loch Borralan intrusion, Sutherland

Release No. 8 Vol 1 - not available digitally

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No summary available.

Release No. 9: data arising from investigations of the Knock intrusion at Claymires, Aberdeenshire

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Data arising from detailed geophysical surveys followed by the drilling of two induced polarisation (IP) anomalies on the eastern margin of this intrusion are presented. The work, based on a model of platinum-bearing shear zone-hosted copper-nickel-sulphide mineralisation, did not identify any significant sulphide mineralisation or attendant precious-metal enrichments.

Release No. 10: data arising from investigations into the distribution of platinum group elements, South Harris, Isle of Lewis, Scotland

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Geochemical (stream sediment, panned stream sediment and surface rock sample) data is presented. The data shows minor palladium enrichment (maximum 210 ppb) in ultramafic rocks within the Langavat Metamorphic Belt, on the eastern margin of the South Harris igneous complex, and in metasediments within the same belt. Some palladium-enriched rocks also show minor enrichment in platinum and gold.

Release No. 11: rare earth elements in alkaline intrusions, north-west Scotland

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Geochemical data on rare earth element (lanthanum, cerium and yttrium) concentrations in rocks from the Loch Borralan, Loch Ailsh and Ben Loyal alkaline intrusive complexes is presented. In drill core from the Loch Borralan Complex, the analyses indicate only localised minor concentrations in excess of 1000 ppm lanthanum+cerium. At Loch Ailsh, localised minor enrichment of rare earth elements occurs in syenites and pyroxenites near the south-east margin of the complex. Data from a limited programme of sampling at Ben Loyal resulted in the discovery of veins rich in rare earth elements (maxima 5667 ppm lanthanum and 19785 ppm cerium) in syenites.

Release No. 12: Mineral investigations in the Scardroy area, Highland Region, Scotland

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Data arising from a reconnaissance geochemical survey in the Scardroy area of north-west Scotland is presented. Stream sediment and panned concentrate results indicate that the majority of gold anomalies in the area are associated with the Strathconon Fault. The survey also led to the discovery of strongly deformed, chalcopyrite-bearing pyrrhotite bodies up to 4 m thick in Lewisian amphibolite.

Release No. 13: Exploration for gold in central Wales

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The results of a panned concentrate drainage survey across selected areas of central Wales are contained in this release. The objective of the survey was to search for evidence of mesothermal 'slate-belt type' gold mineralisation similar to that worked at Ogofau, near Pumpsaint. Two anomalous areas were identified: the Rhiwnant Dome near Rhayader and a catchment close to the Ogofau mine at Caeo. Both areas have structural and other geological features in common with the site of the Ogofau mine.

Release No. 14: Geochemical surveys for gold in the Berwyn Hills

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The results of a geochemical survey centred on the Afon Trystion in the western Berwyn Hills are reported. The presence of coarse gold in the Afon Trystion, first reported by an earlier regional-scale survey, was confirmed, but its source was not identified. Only minor indications of mineralisation were found in the lower Palaeozoic clastic sedimentary rocks that form this area.

Release No. 15: An appraisal of the gold potential of mine dumps in the North Molton area, north Devon

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Values of gold are reported for twelve samples taken from dump material at the old Bampfylde iron-copper mine. These values suggest that there may be sufficient gold present to support a small-scale commercial extraction enterprise. Samples from tips at six other iron ± copper mines in the area were found not to be auriferous.

Release No. 16: Exploration for strata-bound mineralisation around Chillaton, Devon

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Results of reconnaissance overburden sampling and geophysical surveys over an area of lower Carboniferous sediments, mafic volcanics and basic intrusions with associated manganese mineralisation are presented. Drilling showed the source of prominent conductivity, chargeability and self-potential anomalies, coincident with a positive gravity anomaly, to be black, pyrite-rich shales. Minor manganese mineralisation was intersected in the envelopes of doleritic intrusions and weak base-metal enrichments were recorded locally in the volcanic and sedimentary succession.

Release No. 17: Regional appraisal of potential for strata-bound base-metal mineralisation in the Solway Basin

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The basin, which is up to 7 km thick, developed due to north–south extension with major fault-controlled subsidence during the Courceyan–Holkerian (synextension phase), followed by more gradual post-extension regional subsidence during the later Carboniferous. It straddles the trace of the Iapetus Suture on whose extension major Irish lead-zinc deposits are located. The basinal succession is poorly exposed away from the margins where it is covered by later Permo-Triassic sediments and extensive superficial deposits. Minor base-metal and barytes veins and replacement mineralisation are recorded close to the northern and southern margins. Investigations of the regional geochemistry, geophysics and deep geology, together with Landsat imagery, indicated that the basin could be prospective for syndiagenetic mineralisation of sedimentary exhalative style found in Ireland adjacent to basin-controlling faults, but that the most prospective rocks in chronostratigraphical terms are likely to be at considerable depth on the southern margin. Close to the northern margin, prospective areas were identified and limited follow-up investigations carried out. These are described in data releases 18 and 20 to 23.

Release No. 18: Mineral investigations in the Northumberland Trough Part 1: Arnton Fell area, Borders, Scotland

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In this area, at the northern margin of the Northumberland Trough, stream sediment anomalies for barium and base metals recorded by regional-scale surveys were followed up by panned stream sediment and deep overburden sampling and geophysical (magnetic and very low frequency) surveys. Strong anomalies for barium, copper, lead and zinc were recorded in panned stream sediments over much of the area. Mineralogical examination of the samples revealed the presence of coarse, untarnished sphalerite, more locally galena, chalcopyrite and gold, and rare cinnabar and mimetite. Outcrop sampling revealed weak, vein-style mineralisation and weakly disseminated galena in a sandstone unit. The small amount of deep overburden sampling undertaken revealed few high values but some coincidence of these with the Coomb Edge Fault was evident. Geophysical surveys failed to detect pronounced anomalies associated with the suboutcropping Birenswark Lavas or the Coomb Edge Fault. It was concluded that vein-style mineralisation was the most probable source of most anomalies and that this might include polymetallic (barium; copper; gold; lead; mercury) veins associated with lower Palaeozoic rocks, and sphalerite-barytes mineralisation in the Carboniferous cementstone succession. Most mineralisation is concealed by the extensive superficial deposits.

Release No. 19: Exploration for gold in the Thornhill Basin, southern Scotland

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The presence of alluvial gold associated with the Permian red beds of the Thornhill Basin is reported. The gold occurs mostly in streams on the eastern side of the basin where Permian rocks are in faulted contact with the lower Palaeozoic succession. Electron microprobe studies of the gold grains show that they are similar to those discovered in Devon (South Hams and Crediton Trough) and that the mineralised source is therefore likely to be metallogenetically similar. There is sufficient evidence to suggest that mineralised structures carrying gold occur at contacts between the lower Palaeozoic and Permian rocks and, possibly, in structures cutting the Permian rocks.

Release No. 20: Mineral investigations in the Northumberland Trough Part 2: Newcastleton area, Borders, Scotland

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In this area, close to the northern margin of the Solway-Northumberland Trough, stream sediment anomalies for zinc with lesser amounts of lead, barium and copper recorded by regional-scale surveys were followed up by panned stream sediment, outcrop and traverse-based deep overburden sampling. Mineralogical examination of the panned concentrates showed the anomalies to be caused by sphalerite often accompanied by variable amounts of barytes, chalcopyrite, galena, pyromorphite and secondary lead minerals. Deep overburden sampling revealed base-metal anomalies in a well-defined zone close to the Bedda fault. Most anomalies are attributed to fault-controlled, vein-style base-metal and barytes mineralisation, but evidence of disseminated syngenetic sphalerite-pyrite mineralisation was discovered in a carbonate-cemented sandstone unit in the Harden beds. Geophysical orientation studies indicated that magnetic surveys are useful locally for tracing faults beneath the thick overburden.

Release No. 21: Mineral investigations in the Northumberland Trough Part 3: Ecclefechan-Waterbeck area

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In this area, at the northern margin of the Solway-Northumberland Trough, stream sediment anomalies for barium and base metals recorded by regional-scale surveys were followed up by panned stream sediment, outcrop and deep overburden sampling. Mineralogical examination of the panned concentrates indicated that the anomalies were the product of both mineralisation and contamination. Nevertheless, a substantial zone of coherent copper-lead-barium enrichment over the Birenswark Volcanic Formation and a cluster of anomalies close to the Silurian-Upper Old Red Sandstone boundary were identified. Deep overburden sampling revealed base-metal anomalies presumed to be derived from concealed mineralisation in the vicinity. Gold grains, seen in panned concentrates at 13 sites, may be derived from Silurian rocks at the basin margin. Examination of the core from a previously drilled borehole (Hoddam No. 2) sited close to this area revealed the presence of gypsum-anhydrite in veins, breccias and massive beds, increasing the prospectivity of the area according to some metallogenetic models.

Release No. 22: Mineral investigations in the Northumberland Trough Part 4: the Bewcastle area

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Stream sediment anomalies for zinc with lesser amounts of lead, barium and copper recorded by regional-scale surveys were followed up by panned stream sediment and outcrop sampling. In the lower parts of the area, streams are contaminated, giving rise to anomalies from both natural and anthropogenic sources. The large zinc anomalies are caused by the presence of coarse sphalerite; coarse barytes is also common. Widespread but weak vein-style sphalerite-barytes-pyrite mineralisation was found in anomalous catchments, but no strata-bound mineralisation apart from pyrite was seen. Gold was panned from ten sites and appears to be associated with the presence of Permo-Carboniferous quartz dolerite dykes, but its source is uncertain.

Release No. 23: Mineral investigations in the Northumberland Trough Part 5: the Kirkbean area, south-west Scotland

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Drainage geochemical anomalies recorded by previous surveys in this area, close to the northern margin of the Solway-Northumberland Basin, were followed up by traverse-based soil sampling from pits, outcrop sampling and geophysical surveys (magnetic, electromagnetic and gravity). The work was designed to provide information on the geology and structure beneath the thick superficial cover and outline base-metal and barytes drilling targets. Deep overburden sampling using a power auger was then carried out in part of the area where the earlier work suggested the presence of subsurface mineralisation coincident with intersecting faults. The results suggested that mineralised (vein-style) structures carrying barytes and/or base metals may extend over a strike length of 2.5 km or more in this area. Both base metal and barytes mineralisation is exposed at surface in a few places, but most of the area is covered by thick superficial deposits. Much of the mineralisation may comprise small, epigenetic occurrences but there are good indications of a relatively large, steeply dipping, barytes-bearing vein structure and the possibility of more substantial strata-bound or breccia-infill deposits.