Exploration for gold in the Thornhill Basin, Southern Scotland

Alluvial gold has been discovered in association with Permian red beds of the Thornhill basin some 20 km north of Dumfries in southern Scotland. The gold occurs mostly in streams on the eastern side of the Thornhill basin where the Permian rocks are in faulted contact with the Lower Palaeozoic rocks. The higher values of gold are situated on the outcrop of the Permian red sandstones, usually with the outcrop of the Permian alkali basalts just upstream or, in one case, on the Lower Palaeozoic rocks just east of their contact with the Permian volcanics. Automated electron microprobe grain characterisation shows the gold to be similar to that from the Crediton Trough and the South Hams areas of Devon with some Pd-rich compositions and inclusions of several selenide minerals. The discovery strongly supports the model developed for the controls of gold transport and deposition within the red bed environment. This model envisages the leaching of gold from a dispersed large volume source such as alkali basalt, by breakdown of sulphide minerals due to the activity of saline oxidising fluids, which typically circulate within a red bed basin. Under these conditions, the gold is carried as a chloride complex. Such a solution is likely to precipitate all, or most, of the gold if it meets a more reduced environment so that the contact zones between red beds and more reduced underlying rocks, whether unconformable or faulted, comprise the most favourable environment for gold precipitation. There is sufficient evidence from the distribution of gold in drainage samples around Thornhill to suggest that mineralised structures carrying gold occur at contacts between the Permian strata and the underlying Lower Palaeozoic rocks and, possibly, also in structures cutting the Permian rocks. Follow-up exploration is recommended to locate and assess the economic potential of the mineralisation. Most of the anomalies occur in streams which cut the boundary faults at high angles. Because of the limited dispersion of coarser particles of gold in streams traversing bedrock in this area, detailed drainage sampling for gold may serve 10 locate cut-offs. In addition, overburden sampling by pitting could be used to establish if gold is concentrated in boundary structures. Drill holes through the contact zones will be the only means of establishing the grade and the extent of gold mineralisation arising from the interaction between mineralising fluids circulating in the Permian red bed sequence and the underlying greywacke-shale sequence.