Mineral Reconnaissance Programme (MRP) reports 121-130
121 Exploration for gold in the South Hams district of Devon
R C Leake and others (1992)
Gold in panned drainage samples is widely distributed over the area south of a line between Plymouth and Brixham, with values >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 Au, and the factors influencing its concentration are complex. Many of the grains are very intricate in shape, with projections which 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 Pd and Ag. 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 Au 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 Au, reaching a maximum of 380 ppb. Two phases of mineralisation are thought to be responsible for the Au 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
D C Cooper and others (1992)
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 baryte 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 baryte from a fault separating Lower Palaeozoic and Carboniferous rocks, surface indications of further baryte 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 baryte may be present under drift cover.
In the Tallentire Hill area, geological mapping followed by traverse-based soil sampling showed that fracture-controlled mineralisation is widespread in the Carboniferous (Dinantian and Namurian) rocks. The fracture fillings consist dominantly of baryte, 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. Baryte 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 baryte 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 baryte 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 sub-cropping 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 the VLF(EM) and conductivity mapping methods could be useful for tracing faults beneath drift and providing information on drift thickness. Closely-spaced soil sampling proved effective for detecting mineralisation in areas where the drift cover is thin, and a trial soil-gas survey showed that this technique could also be useful for tracing faults beneath drift.
123 Mineral investigations in the Teign Valley, Devon. Part 2: base metals
K E Beer and others (1992)
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 which reflected areas of former open-cast 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 drillholes 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% zinc over 3m. Lead values are lower, with a maximum of 0.2% over 1m; several 1-3m lengths containing 0.1% lead are present. Copper concentrations are very variable; the best intersection contained 0.14% 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
A G Gunn and others (1992)
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 Pt and Pd. Maximum values of 50 ppb Pt and 25 ppb Pd 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 Pt, 63 ppb Pd and 30 ppb Au.
The highest levels of enrichment in precious metals (up to 462 ppb Pt+Pd+Au) 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 utilised 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 Pt and Au. 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 which transects the Huntly intrusion and which is associated with PGE-bearing Cu-Ni sulphide mineralisation on the flank of the Knock intrusion a few kilometres to the north. Magnetic and VLF 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%, 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
J R Harris and J S Coats (1992)
The results of the data analysis for a geochemistry relational database to hold the UK, land-based 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 has come 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 specifically excluded from this data analysis are offshore and overseas data. 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 that this is not a serious deficiency.
126 Mineral exploration in the Pitlochry to Glen Clova area, Tayside Region, Scotland
J S Coats and others (1993)
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 Au were detected in clay fault gouge and the fault can be traced by geophysical methods for 1.6km. Features with a similar trend, such as the orientation of stream courses, can be identified extending for a further 10km to the southeast. 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 stratabound 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
T B Colman and R J Peart (1993)
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 homblende 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 7 traverse lines which, 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 been analysed also for gold and platinum.
The geophysical data suggest 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 3km 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 homblende schists indicates that they have an oceanic tholeitic basalt parentage. The soil sampling results show several barium anomalies, probably associated with thin baryte 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
D G Cameron and others (1993)
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, baryte, 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 (hematite) 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 baryte 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 hematite 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. Baryte occurrences may be related to later Carboniferous to Mesozoic events, including hematite 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
R C Leake and G E Norton (1993)
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 baryte 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 Mn in the soils derived from the sedimentary rocks is accompanied by low amplitude enrichment in Zn and Pb, reaching around 200 ppm Zn and 170 ppm Pb. The fourth area, adjacent to the separate belt of volcanic rocks northwest of Totnes, is more extensive and of higher amplitude (reaching over 700 ppm Zn and 600 ppm Pb). The soil and drillhole 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 SEDEX 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 3km long in the west of the area. The zone is enriched in As, Pb, Zn, Mn and Cu and is similar to polymetallic mineralisation carrying gold which occurs further south in Devon, described in Mineral Reconnaissance Programme Report No 121 (Leake et al., 1992). Evidence for further polymetallic mineralisation is present in the northeast of the area but this differs geochemically from the other areas in having a higher proportion of Zn to Pb and in the presence of anomalous concentrations of Sn. 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 Sb over 6.4 m within a wider zone showing lower amplitude enrichment in antimony (75 ppm over 21m) 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 Zn (up to 1600 ppm Zn over 1 m).
130 The occurrence and economic potential of nodular monazite in south-central Wales
R T Smith and others (1994)
Review of all Mineral Reconnaissance Programme panned concentrate data confirmed that very high (>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 (>1% Ce) 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 sediment locally exceed minimum grades exploited in placer deposits, reaching 1.65% in the <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 2mm 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 LREE-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, 10-100 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 REE, 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 stratabound 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, REE may have been released from iron-manganese hydrous oxides and other phases, and fixed by phosphate released during the decomposition of dispersed organic matter.
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