Late Palaeozoic fault-controlled hydrothermal Cu–Zn mineralisation on Vanna island, West Troms Basement Complex, northern Norway

Abstract

The Vannareid-Burøysund fault is a major, brittle, normal fault in northern Norway, with cohesive fault rocks (cataclasites) that host Cu-Zn-bearing quartz-carbonate veins. The fault is exposed on the island of Vanna in the Neoarchaean to Palaeoproterozoic West Troms Basement Complex, separating variably deformed tonalitic gneisses in the footwall from mylonitised metasedimentary rocks and tonalites in the hanging wall. Radiometric dating (K-Ar illite) of normal fault movement along the Vannareid-Burøysund fault yielded a late Permian age, concurrent with incipient post-Caledonian continental rifting. The fault evolution and internal architecture of the Vannareid-Burøysund fault largely controlled the spatial distribution of mineralisation, and two main phases of the Cu-Zn mineralisation have been discerned. Early quartzsphalerite veins are deposited in the cataclastic fault core zone, where initial movement along the fault created a fluid conduit that allowed for fluid flow and sphalerite deposition. With subsequent movement and widening of the fault zone, a later and spatially more extensive generation of quartz-chalcopyrite veins were deposited in both the fault core and the damage zones. Fluid inclusion micro-thermometry revealed that the ore-forming fluids were highly saline aqueous solutions (20-37 wt.% NaCl + CaCl 2 ) that carried base metals and sulphur. Further, the isotopic composition of hydrothermal carbonates indicates a magmatic source for the CO 2 . The structural data and obtained geochemical results indicate that the Cu-Zn mineralisation in the Vannareid-Burøysund fault was epigenetic and strongly controlled by extensional brittle faulting and cataclasis during early stages of post-Caledonian (Permian) continental rifting, thus providing a new model for exploration of post-Caledonian hydrothermal ore deposits in basement rocks of northern Norway.