MULTIPLE ORIGINS FOR NATIVE COPPER IN CONTINENTAL THOLEIITIC BASALTS, COPPERMINE RIVER GROUP, NUNAVUT, CANADA: CRYSTALLIZATION IN MAGMAS, AMYGDULES AND HYDROTHERMAL REMOBILIZATION

Native copper has long been known to occur in tholeiitic basalts of the Coppermine River Group. Earlier workers have interpreted native Cu in the groundmass of basalt flows to have a magmatic origin. Native Cu is also found in amygdules and in hydrothermal veins cutting the lavas. The Coppermine River Group comprises a 2 km thick section of flood basalts of the Copper Creek Formation overlain by red sandstones and intercalated basalt flows of the Husky Creek Formation. The lower half of the Copper Creek Formation includes picrites, high MgO basalt and basalt that show trace element and radiogenic isotopic (Nd) evidence of crustal assimilation. This lower member is contemporaneous with the Muskox layered intrusion, and both are cut by Mackenzie dykes that feed the upper tholeiitic basalts. Native Cu occurs in the upper half of the Copper Creek Formation. Magmatic sulphides are absent in both the Muskox intrusion and Coppermine River Group. This is reflected in Cu and Pd abundances, both of which share a strong affinity for immiscible sulphide liquid, and yet show an increase in abundance with increasing differentiation in the volcanic pile. In differentiated basalt with 300 ppm Cu, native magmatic copper (100-300 micron) is disseminated in a groundmass of plagioclase, clinopyroxene and titanomagnetite, and is in contact with quartz and K-feldspar interpreted as interstitial melt. Laser ablation ICP-MS analyses reveals that the magmatic Cu is rich in As (up to 2770 ppm) and Ag (up to 94 ppm) relative to native Cu in amygdules (up to 800 ppm As) or veins (less than 500 ppm As). Burial metamorphism of the Coppermine River Group, as documented in amygdules ranges from prehnite-pumpellyite to lower greenschist. In prehnite-pumpellyite facies assemblages, native Cu is associated with calcite, quartz and chlorite. Calcite and quartz react to form wollastonite in amygdules at greenschist facies, with some recrystallization of native Cu. Hydrothermal quartz and calcite veins formed in fractured basalt and within brittle fault zones, contain native Cu, or chalcocite with minor bornite and trace chalcopyrite. Copper isotopic analyses will be used to determine whether: i) disseminated Cu has a magmatic signature; ii) the source of Cu in chalcocite mineralisation was derived by fluids circulating and leaching native Cu from the basalts.