Chapter 3 Scottish mineral Geological Conservation Review sites – Late magmatic minerals

Abstract

Late-magmatic granitic fluids are typically enriched in Si, K and rare lithophile elements, including Be, Li, Sn, W, Rb, Cs, Nb, Ta, REE and U, and are also characterised by volatile components such as OH, F and B. Consequently the resultant pegmatite bodies are very coarse-grained and contain accessory minerals with unusual mineral chemistries. Pegmatites, usually present as veins, can occur as emanations from granite bodies or be produced by granite fronts generated by migmatisation during high-grade metamorphism. The former process is represented in the sites by Loch Avon; located within the Main Granite of the Cairngorms, this is the area where large topaz crystals, renowned for their blue colour, are found along with cairngorm and citrine varieties of quartz. Rare beryllium minerals, genthelvite and bertrandite, also occur. The latter process was responsible for the generation of the South Harris and Loch Nevis pegmatite suites which collectively encompass a range of mineral assemblages as follows. The Chaipaval Pegmatite site, some 2 km long, and previously worked for potassium feldspar, contains an unusual suite of uranium-bearing minerals together with spessartine, columbite and gahnite. By contrast the Sletteval Pegmatite site, also previously worked, is more thorium-rich and again contains the rare mineral thorogummite. The more mafic biotite-magnetite-bearing Loch a’ Sgurr Pegmatite site presents an intimate association of uraninite with magnetite, monazite and feldspar. The mica pegmatites at the Loch Nevis site, which formed a series of concordant lenses and veins, were rich in extractable muscovite. Additionally garnet and minor beryl are present in some pegmatites.