Stratabound mineralisation in Dalradian rocks near Tyndrum, Perthshire

Abstract

Synopsis Two horizons of low-grade stratabound sulphide mineralisation occur east of Tyndrum in the Ben Challum Quartzite, a newly recognised formation in the Argyll Group (Middle Dalradian). The lower horizon exceeds 9 km along strike and averages 1250 ppm Zn and 340 ppm Cu over an 80 m intersection. The upper extends for 4 km and locally contains up to 3.6% Zn over 1 m. Both comprise stratiform pyrite with minor pyrrhotite, sphalerite and chalcopyrite, in quartzite with possible meta-chert and subordinate schists, and contain manganese-enriched garnet. Sphalerite and galena in the upper horizon are associated with microcrystalline quartz and siliceous breccia interpreted as relict hydrothermal rocks. The Ben Challum Quartzite is in part highly albitic and includes bands of hornblende-schist with tholeiitic composition, some of which probably formed as basic tuff or volcanic sediment. The basal part of the overlying Ben Lui Schist includes sediment carrying talc, magnesite, chromite and Ni-sulphides which probably derived from altered ultramafic rocks. The Zn–Cu–Pb mineralisation is considered to be of the sedimentary-exhalative type, deposition occurring during a regime of fault-bounded subsidence prior to the turbiditic sedimentation of the Ben Lui Schist.