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Mineral Reconnaissance Programme (MRP) reports 131–140

131 Platinum–group element mineralisation in the Loch Ailsh alkaline igneous complex, north–west Scotland (9.35 Mb)

M H Shaw, A G Gunn, K E Rollin and M T Styles (1994)

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 southeast 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 Pt and 43 ppb Pd. 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 southeast part of the Complex indicated widespread PGE enrichment in pyroxenites. The highest PGE concentrations in overburden, up to 110 ppb Pt and 70 ppb Pd, 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 Pt+Pd, occur sporadically in the lower section of the Allt Cathair Bhan valley. In these PGE–enriched rocks sperrylite (PtAs) and isomertieite (Pd + Sb + As) were identified by automated microchemical mapping. Complex tellurides of Pd, Ag, Bi and Pb 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, VLF–resistivity techniques were employed to search for potentially PGE–enriched sulphide mineralisation but 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 (2.76 Mb)

R C Leake and T R Marshall (1994)

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 minus 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 baryte 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 stratabound sulphide enrichment are possible types of source. In addition, there are several Cu and Ba 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 (6.52 Mb)

D G Cameron and others (1994)

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 (3.86 Mb)

R C Leake and others (1994)

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 alter 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 20km 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 (12–35 ppb Au) in several unpanned <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 Au in panned material) from above 1.0 m.

Three grains of gold extracted from core from the faulted contact between Permian and Carboniferous rocks in the Upton Pyne Borehole, 5km 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 (4.11 Mb)

R C Leake and others (1995)

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 sub–cratonic 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 are 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 which 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 (9.77 Mb)

J D Cornwell, S F Kimbell, A D Evans and D C Cooper(1995)

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

The Mineral Reconnaissance Programme (MRP) has funded a number of detailed airborne geophysical surveys flown in Britain between 1972 and 1978, covering a combined area of approximately 3000km2. Flying heights were 60m or 75m with flight–line spacings of 100m, 200m or 250m. Methods used were magnetics, radiometrics and EM or 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. The BGS also holds datasets from an extensive detailed survey flown over south–west England for the UKAEA/BGS in 1957–59, 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 the BGS.

137 Exploration for volcanogenic mineralisation in south–west Wales (15.9 Mb)

T B Colman and others (1995)

The voluminous Ordovician volcanic rocks of south–west Wales are interbedded with black shales and other sedimentary rocks, an association which hosts economic base–metal deposits elsewhere in the Caledonides of Europe (e.g. Parys Mountain, Anglesey; Avoca, Ireland) and North America (e.g. 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 confirmed the presence of pyritic and highly altered (silicification and loss of alkalis) volcanic rocks, locally containing baryte. Brief investigations in the Crosswell area revealed anomalous levels of barium in soil (> 2000 ppm) and coincident strike–parallel EM 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 (7.03 Mb)

M H Shaw and others (1995)

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 (10–100 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 (5.86 Mb)

J D Cornwell and others (1995)

A search for carbonate–hosted base–metal deposits was directed at an area south–west of Ashbourne, where two small mineralised inliers of Dinantian limestone were thought, on gravity evidence, to be connected by a shallow ridge of limestone buried under thin Permo–Triassic cover. In Dinantian 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 (60m) depth by a borehole between the inliers. A further borehole intersected over 10m of replacement lead–zinc–baryte mineralisation, including a short interval containing 3% Zn and more than 10% Pb. 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 which host important lead–zinc mineralisation in Ireland.

Further work is recommended along the 7km 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 (7.16 Mb)

J S Coats and others (1996)

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 180km2 and analysed for base metals, gold and pathfinder elements. The area covered Lewisian rocks of both the Eastern and Western facies and, also, 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 14km 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%. 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 (100km) strike length and the occurrence of gold at two localities within this area and at Scardroy 40km to the north–east.