Platinum-group element mineralisation in the Loch Ailsh alkaline igneous complex, north-west Scotland

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 cumuIus or intercumulus phases and hydrothermal, related to post-cumulus low-temperature PGE concentration and precipitation in structurally-controlled settings. Reconnaissance sampling of drainage sediments and bedrock indicated localised PGE enrichment in mafic and uhramafic rocks in the south-east and northern parts of the Complex. Concentrations were generally higher in fine-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 south-east 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 skams 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. Full data listings, together with detailed logs of samples, are available from the Mineral Recomraissance Programme Database, BGS, Keyworth, on request.