Metallic ores, Geology and man, Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
In the Northern Highlands metallic ores (P915506) have been worked Extensively only at Strontian, where veins containing galena associated with sphalerite were mined between 1722 and 1872. This region has since been the subject of several mineral surveys, including prospect- drilling. Between 1984 and early 1986 the Strontian Main Vein was worked for baryte, galena and sphalerite; subsequently small amounts of high grade calcite have been produced. Strontianite (strontium carbonate) from which the element strontium was first derived, is a collector’s item and other rare minerals (such as brewsterite and harmotome) can also be found.
Lead and zinc vein ores were once worked on a small scale at Lurga in Morvern, at Struy in Strath Glass (IGS Mineral Reconnaissance Programme Report No.6) and at Achanarras near Halkirk in Caithness, the Achanarras area being one over which intensive investigations were carried out by IGS in the late 1960s.
Bands of magnetite are found in the Lewisian of Iona and Tiree, but extensive trials in Tiree did not lead to exploitation. No similar occurrences are known in the Lewisian gneiss of the Outer Isles or the mainland. Ilmenite was found in concealed portions of the Borralan Complex (p.103) during boring intended to assess the apatite content of the rocks. No apatite has been exploited, but the area must be considered as having potential.
Small quantities of molybdenite were found around the Grudie Granite, Loch Shin. Although mainly concentrated in shear-zones in the schists the primary source of the molybdenite was probably in the granite itself (IGS Mineral Reconnaissance Programme Report No.3).
Uranium-bearing minerals occur in certain shale bands in the Middle Old Red Sandstone Caithness Flags, and another notable uranium anomaly Was found in the Lower Old Red Sandstone Ousdale Arkose, which overlies the Helmsdale Granite in southern Caithness. The occurrence in the Ousdale Arkose gives rise to the speculation that uranium-bearing deposits may lie concealed at the base of the Old Red Sandstone elsewhere in the area. On the whole, Caithness is still considered to be a minor uranium province, with a mining potential worth further study should economic and political circumstances justify it (Gallagher and others, 1971).
Alluvial gold was worked in the Strath of Kildonan in Sutherland between 1868 and 1870. At that time unofficial estimates of the value of metal abstracted were around £12 000, a considerable sum at that period. Gold is still found as a placer deposit in the streams of the area. It appears to be derived from the hydrothermally altered schists and granites of the hinterland of the Helmsdale River, in which the metal is sparsely disseminated. No main ‘lode’ has been found.
Other deposits of metallic minerals which have given rise to small-scale workings, trials, or investigations are copper ores at Rassal near Kishorn and at the Castle of Old Wick in Caithness; cassiterite in the aegirine gneiss found in the Carn Chuinneag intrusion (P915478); lead, silver and gold in the Loch Maree area; and hematite near Reay in Caithness, and near the head of Loch Kishorn.