Mineral Reconnaissance Programme (MRP) reports 81-90
81 Investigations for tin around Wheal Reeth, Godolphin, Cornwall
K E Beer and others (1986)
Recognition of greisenisation associated with worked Sn 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 Sn, associated base metals and F 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
K E Beer, K Turton and T K Ball (1986)
Fourteen percussive drill holes showed that weak Sn mineralisation persists for a distance of at least 50m into the Devonian slates that form the hanging wall of the Royalton elvan. Anomalous Sn 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 1.5m. Immediately east of the open works there is a marked enrichment in Cu and Ni, but at levels well below those of economic significance.
83 Mineral investigations near Bodmin, Cornwall. Part 5—The Castle-an-Dinas Wolfram Lode
K E Beer, T K Ball and M J Bennett(1986)
A soil survey over Devonian slates to the south of Castle-an-Dinas wolfram mine produced anomalies indicative of at least two sub-parallel zones of W veining and a broad area of anomalously high Sn values. Percussive drilling confirmed widespread Sn mineralisation beneath the soil anomaly, but the in situ W 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 W-Sn mineralisation—sometimes with Cu—were located, one correlatable with the Wolfram Lode and the other sub-parallel and some 90m to the west. Close to the surface these lode extensions are sub-economic, but it appears that viable ore grades are located in the metamorphosed slates within about 200m 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
J D Cornwell and R Cave (1985)
A detailed airborne geophysical survey was made of part of west Dyfed with magnetic, 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 petrographic 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 drift cover that hinders geological mapping in many places, and they have also revealed some large-scale structures, including a previously unrecorded dyke at least 40km long. The VLF data indicate the presence of many conductive horizons, mostly within Lower Palaeozoic sediments.
85 Geophysical surveys near Strontian, Highland Region
G S Kimbell (1986)
Reconnaissance VLF-EM and magnetic surveys were carried out over Ba-Pb-Zn 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
M J Brown and others (1987)
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 6km 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 Na2O + CaO + K2O and high Al2O3, Fe, S and, locally, Sr and Ba. This is reflected mineralogically by the presence of corundum, baryte 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—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 stratabound mineralisation in Middle Dalradian rocks near Huntly, Grampian Region, Scotland*
J S Coats and others (1987)
Test drilling on geochemical and geophysical anomalies associated with very poorly exposed Middle Dalradian metasedimentary rocks of the Portsoy Group at Wellheads Farm, 4km WSW of Huntly, has revealed intraformational breccias, sulphidic graphitic chert and stratabound pyrite in graphitic quartzite—features that are favourable for the occurrence of stratabound base metals in the area. Adjacent basic-ultrabasic rocks, although not examined, are favourable for platinum and chromite investigation. Initial Zn anomalies in stream sediment were followed up by shallow and deep overburden sampling and by geophysical surveys (VLF-EM, IP and magnetic) along 5.2km of across-strike traverses over an area of 2.5km2. Metal dispersion is hydromorphically controlled in overburden (up to 30m thick) and probably also in biotite-muscovite schist—the principal bedrock type in boreholes 1-5, which is decomposed and leached to at least 30m 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 baryte in Middle Dalradian rocks of the Glenshee area, Grampian Highlands
J S Coats and others (1987)
Drainage surveys and airborne geophysical surveys of a 600km2 area from Blair Atholl to Braemar identified several targets near Glenshee within the Ben Eagach Schist, the host formation of the Aberfeldy deposits 30km along strike to the southwest. 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 baryte is best shown by detailed drainage sampling, and the sulphide-bearing graphitic schist can be traced through drift-covered ground by VLF-EM, IP and SP 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 baryte 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
J H Bateson, C C Johnson and A D Evans (1987)
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 U and V. 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
R C Jones, K E Beer and J M C Tombs (1987)
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. Ba anomalies were also recognised and indicate previously unrecorded veins of baryte. Investigation of an aeromagnetic anomaly trending west-north-west-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.
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