The nature and age of Mesoproterozoic strike-slip faulting based on Re–Os geochronology of syntectonic copper mineralization, Assynt Terrane, NW Scotland

Abstract

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained due to a lack of datable fills. An array of NW-SE sinistral and antithetic E-W dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchaean gneisses and c. 2.25 Ga quartz-pyrite veins close to the trace of the unexposed, regional-scale NW-SE fault. Copper-iron mineralisation occurs at an intersection between an antithetic dextral fault and an older c. 2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper-iron sulphides, hematite and barite. Complex mm-thick zoned alteration rims rich in epidote occur at contacts between the sulphides and gneisses. Rhenium-Osmium copper-iron sulphide geochronology yields an age of c. 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralisation based on the asymmetric fibrous growth forms of these minerals within local dextral shears which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognised strike slip faulting, previously termed the ‘Late Laxfordian’, should be referred to as the ‘Assyntian’ in order to distinguish it from kinematically distinct Laxfordian events.