Polymetallic mineralisation in Carboniferous rocks at Hilderston, near Bathgate, central Scotland

Abstract

Five boreholes in the vicinity of the ancient Ag-Ni-Pb mine at Hilderston, near Bathgate have yielded new stratigraphic, mineralogical and geochemical information. These results, together with a critical reexamination of old records, are interpreted in relation to a palaeo-environment profde across a volcanic island with coastal lagoon and fringing reef deposits, as proposed by Jameson (1980). Stratabound Zn-Pb mineralisation occurs in the lower, argillaceous part of the Petershill Limestone, which was deposited in an anaerobic lagoon on the edge of a volcanic landmass during the Lower Carboniferous Epoch (Lower Limestone Group, Vi&an Stage). The best intersection shows 8 m of mineralised limestone, with underlying carbonaceous mudstone (1 m) and tuffaceous seat rock (2 m), having an average concentration of 0.14 % Pb and 0.66% Zn and maximum values of 0.6% Pb and 3.1% Zn in the carbonaceous mudstone. Further drilling was subsequently carried out in order to investigate possible lateral extensions of the stratabound mineralisation and to test for mineralisation in similar lithologies and geological environments at other stratigraphic levels, and a report on the results will be available at Edinburgh. Late-Carboniferous hydrothermal veins occur within the Petershill Limestone and in immediately overlying elastic sediments, where they are cut by E-W faults and quartz-dolerite dykes. At Hilderston Mine two assemblages are recognised in the vein: Ba-Fe- Ni-Co-Ag-As on a dyke margin adjacent to the elastic sediments and Fe-Pb-Zn-S at lower levels adjacent to the limestone. Zones of alteration in the dolerite dykes carry hydrocarbons and weak Ba-Fe-Cu-F mineralisation. No potentially-valuable vein deposits were discovered in the present investigation