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

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