Petrography, mineral chemistry, geochemistry and sulphur isotope studies of the Abhainn Strathain copper mineralisation, Meall Mór, South Knapdale, Scotland
The Abhainn Srathain copper mineralisation with at least 10 million tonnes of rock containing copper was worked during the Eighteenth Century and is situated 1-2km to the south of Meall Mór, South Knapdale. The mineralisation is hosted by epidiorites, quartzites and schists of the Upper Erins Quartzite Formation in which the levels of copper reach up to 2%, 1.3% and 0.8% respectively. The main suiphide phases, pyrite and chalcopyrite, occur in disseminations, in layers and as large crystals in quartz and/or calcite cross-cutting veins. The observed opaque mineral textures are due to recrystallisation, deformation and limited mobilisation indicating a premetamorphic origin for the mineralisatlon. Microscopic compositional variation of the minerals and isotopic geothermometry of analysed pyrite-chalcopyrite pairs suggest disequilibriun conditions during the regional metamorphism. Sulphides contain low minor element concentrations with a high Co:Ni ratio in pyrite (12.5:1). The mineralisation is associated with the local development of epidote, Mn-rich garnet, chlorite, muscovite and calcite and/or quartz cross-cutting veins which all resulted from premetamorphic alteration during ore formation. During this alteration CaO, Fe₂0₃, CO₂, MnO, Cu, S and some trace elements were added, Al₂0₃ was diluted and MgO, FeO, alkalis and some trace elements were removed. The isotopic composition of bacteriogenically reduced sulphur from sulphides throughout the Knapdale Pyrite Horizon ranges between o³⁴s=+4.5 and 12.8 per mil. The consistent isotopic values of the suiphides from the Abhainn Srathain copper mineralisation with an average of around +7 per mil regardless of location, depth, lithology and style of mineralisation suggest that the source of the hydrothermal sulphur is a mixture of inorganically reduced downward percolating Dalradlan seawater sulphate and sulphur leached from interbedded basic igneous rocks. Weak exhalative activity caused by the shallow intrusion of sill bodies into the wet unlithified sediments of the Lower and Upper Erins tiartzite accompanied the deposition of the Upper Erins Quartzite and is expressed by weak disseminated and stratiform pyrite with traces of chalcopyrite and sphalerite (Knapdale Pyrite Horizon). Increasing intensity of this exhalation was due to the creation of a geothermal system centred at the site of the present copper mineralisation. During this stage the hot ascending water reacted with the rocks causing local alteration and precipitation of pyrite and chalcopyrite as disseminations, layers and cross-cutting veins. At the same time cold water descended into the hot intrusives and altered the rocks by dissolving silica and precipitating calcite and oxides.