Chapter 8 Scottish mineral Geological Conservation Review sites – Replacement minerals

Abstract

This is, the eighth and final paper of a series that describes the Scottish mineral Geological Conservation Review (GCR) sites within both mainland Scotland and the associated Isles. It features locations where the mineralogy of the original rock unit has been either largely or completely replaced as a consequence of their interaction with either fluids or gaseous components derived from external sources. At both Lunda Wick and the Queyhouse Quarry, Unst, deformation of ultrabasic rocks located within or close to Cambrian-age obduction-related shear zones has resulted in the formation of new mineral species including talc, actinolite, chlorite and biotite. Within the former mine site at Craignure, copper and nickel mineralisation is a consequence of the metasomatic alteration of metabasic igneous intrusions within the youngest part of the Dalradian Supergroup sequence resulting in the formation of pyrrhotite, pentlandite and chalcopyrite. The Lecht Mine site involves Dalradian and younger rocks, where iron and manganese oxides, derived from the metamorphic host rocks, have replaced both clasts and matrix within Devonian-age intrusive breccia pipes. The predominant replacement mineral within this setting is cryptomelane, but also includes ramsdellite, pyrolusite, chalcophanite, lithiophorite, woodruffite and caconexite. These replacement minerals also contain enhanced concentrations of barium, zinc, thallium, molybdenum and silver.