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

By D.G. Cameron, D.C. Cooper, P.M. Allen and H.W. Haslam


A geochemical drainage survey at a density of 1 sample\ud per km2 was carried out across the Preseli Hills, southwest\ud Dyfed. Stream sediment, water and panned concentrate\ud samples were collected from each of 358 sites,\ud and Cu, Pb, Zn, Ba, Fe, Mn, Co, Ni, V, Cr, B, Zr, As, MO\ud and Sn were determined in sediment, Cu, Pb, Zn, Ba, Fe,\ud Mn, Ti, Ni, Sn, As, Ca, Ce, Sr, Sb, Zr, U and MO in\ud panned concentrate and Cu and Zn in water.\ud From a study of regional variation patterns and\ud multivariate statistical analysis, the main sources of\ud geochemical variation in the data were found to be\ud bedrock lithology, mineralisation, contamination and\ud hydromorphic processes. Strong geochemical signatures\ud were shown by dolerite intrusions and by acid volcanics\ud of the Fishguard Volcanic Group. Dark mudstones of the\ud D. murchisoni Beds and Sealyham Volcanic Series also\ud showed characteristic geochemical features whose\ud impact was limited by their restricted and sinuous outcrops.\ud A feature of most of the area is the presence of\ud monazite nodules, generating high levels of rare earth\ud elements and uranium in the panned concentrates. These\ud are particularly high over the Llandeilo-Ashgill sedimentary\ud rocks.\ud Geological thresholds were established using cumulative\ud frequency plots and percentile division. Anomalies\ud due to contamination could be discriminated satisfactorily\ud from those arising from mineralisation only by field\ud observation and mineralogical examination of panned\ud concentrates. In major streams crossing lowland areas\ud and in the vicinity of roads, widespread contamination\ud proved to be the source of most large base metal\ud anomalies.\ud Anomalies related to known mineralisation are\ud located in the southeast of the area, around Llanfyrnach\ud in the Taf valley. Anomalies reflecting hitherto unrecorded\ud bedrock mineralisation may be present in the\ud following areas:\ud a1\ud b)\ud cl\ud d)\ud Llanfyrnach; anomalies for Cu, Pb, Zn and Ba are\ud more extensive than can be accounted for by the\ud known vein mineralisation. High levels of other elements\ud suggest an association with shales.\ud Crosswell-Crymmych; barium and base metal anomalies\ud are associated with the Fishguard Volcanic Group\ud and overlying pyritiferous dark mudstones of the D.\ud murchisoni Beds. The geological setting of these\ud rocks, deposited in a sulphurous environment on the\ud flanks of an acid volcanic pile, suggests that massive\ud sulphide deposits are a possible target here. Locally,\ud weak arsenic anomalies, old trials and ancient mining\ud records suggest that there is also some potential for\ud Au mineralisation associated with pyrite in veins and\ud mudstones.\ud Minas Dinas and Pentre Ifan; small amounts of gold\ud and chalcopyrite were found in panned concentrates.\ud Very small amounts of cassiterite were found in\ud panned concentrates from several localities, but no\ud source was readily identifiable.\ud The anomalies in the Llanfyrnach and Crosswell areas\ud were considered to. merit more de tailed\ud are the subject of further work by BGS.\ud investigatio

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 1984
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