Fluid evolution and AuCu genesis along a shear zone: a regional fluid inclusion study of shear zone-hosted alteration and gold and copper mineralization in the Kautokeino greenstone belt, Finnmark, Norway

Gold and copper mineralization of Early Proterozoic age occur along shear zones cutting metamorphosed sedimentary and volcanic rocks, within the Kautokeino greenstone belt, Finnmark, northern Norway. A fluid inclusion study of quartz veins at the Bidjovagge Au-Cu mine, Ucca-vuovdas, Masi River and Object 43, and of pegmatitic quartz at Dælljadas, has been conducted in order to model fluid evolution along the shear zone at different crustal levels. Fluid inclusions associated with the pegmatitic intrusions in amphibolite facies metamorphic rocks are composed of a low to moderately saline H₂O + CO₂ solution. Within the upper to middle greenschist facies, fluid inclusions contain immiscible highly saline H₂O and CO₂. At low greenschist facies, fluid inclusions contain immiscible highly saline H₂O and N₂. The highly saline H₂O + CO₂ fluids were at temperatures between 250 and 400°C and resulted in the albitization, carbonatization and scapolitization of the wall rocks along the shear zones. High temperatures, high salinities, and high f{hook}O₂ of the fluid resulted in transport of gold and base metals as chloride complexes. Mineralization, such as at the Bidjovagge Au-Cu mine, formed within the ductile to brittle transition in the shear zone as a result of phase separation and reaction of the mineralizing fluids with graphitic schist. Reduced solubility of metal chloride complexes resulted from the phase separation of CO₂ from H₂O-rich fluids which increased pH, and from interaction with graphitic schist that drastically reduced the oxygen fugacity of the fluid. Reaction with the graphitic schist also resulted in oxidation of the schist, and addition of CH₄ and higher hydrocarbons to the fluid. A metal zonation exists, with copper occurring at each crustal level examined and gold restricted to the brittle-ductile transition. The tendency of gold to be precipitated predominantly in the brittle-ductile transition may reflect the dependence of gold transport on temperature and pressure. Methane in fluid inclusions may serve as a guide to areas where fluid reaction with graphitic schist occurred, such as in the Bidjovagge Au-Cu deposit.