The potential for gold mineralisation in the British Permian and Triassic red beds and their contacts with underlying rocks

A review of the potential for gold and related mineralisation in Permian and Triassic red beds and their contacts with other sequences has been carried out for the whole of Britain. This follows on from the MRP discoveries of gold in Devon and the formulation of a model to account for the association of gold-bearing mineralising solutions with red-bed basins and the sites of gold deposition with interfaces with more reduced rocks and fault zones carrying more reduced fluids. Attention has been focused on areas close to the contacts between Permian or Triassic rocks and underlying rocks, on sequences containing alkali b,asaltic rocks of Permian age and on areas of extensive reddening of rocks beneath the Permian land surface. Drainage sampling in the Mauchline Basin of red sandstones, sedimentary breccias and alkali basalts of Permian age revealed that gold was widespread (present in 38 out of 76 panned concentrates - collected) and locally abundant. The gold is relatively poor in silver (median 3.0% Ag) and rich in copper (26% of grains with >0.6% Cu) and there are also several grains of palladian gold (maximum 6.1% Pd). Inclusions in the gold are mostly selenides of Hg and Cu, with smaller numbers of other selenides and isolated examples of palladium telluride, cobaltite and chalcopyrite. Apart from the last two minerals, this is an inclusion assemblage very similar to that seen in the gold from the Crediton Trough in Devon. The distribution of Au in drainage sediment shows some positive correlation with that of Fe, Ba and Hg, but not As. Limited drainage sampling was also carried out at isolated sites in the Vale of Eden, Ingleton Coalfield, Charnwood Forest, West Midlands, Malverns, Mendips, Quantocks and Brendon Hills. Gold was found in drainage sediment in Charnwood, in the Weatheroak area north of Redditch, at the south end of the Malvern Hills, south of Radstock, near Wells, to the east of Wiveliscombe and west of Tiverton. The auriferous drainage sites represent 65% of those sampled. Limited rock sampling was carried out from carbonate horizons in the reddened coal measures of Ayrshire. These rocks are considered to be the oxidised remnants of coal seams and therefore the potential sites of metal deposition from oxidised mineralised solutions, but no evidence of Au or other metal enrichment was found by chemical analysis. Two grains of gold were extracted by panning from an exposure of soft altered alkali basalt and some of the eight analysed samples of alkali basalt, though not enriched in Au, show evidence of hydrothermal alteration. Two samples of a sandstone raft within the alkali basalt were found to be impregnated with native copper (with 0.36% Cu). This orientation and reconnaissance exploration has demonstrated that gold is associated with the Permian red beds of Britain, especially where alkali basalts are present within the sequence, and to a lesser extent with Triassic rocks. The findings support the mineralisation model, developed from previous MRP work in Devon, which envisages that significant amounts of gold can be carried in oxidised solutions which typically circulate within red-bed basins and that deposition is favoured where these solutions are reduced by reaction with unoxidised rocks or fluids of different composition. Further exploration is recommended, particularly in the Mauchline Basin.