The Enåsen gold deposit, central Sweden

Abstract

Stable isotope analyses of quartz, sulphides, and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the Palaeoproterozoic Enasen gold deposit. Reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks. However, oxygen isotope geothermometry on a coexisting quartz-magnetite pair gave a minimum temperature for peak metamorphism of around 650 °C which agrees with Fe-Mg geothermometry. This suggests that grain-scale equilibrium is achieved. The variation in oxygen isotope ratios (δ18O = 7.3 – 10.5‰) on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher δ18O-values in more superficial parts. Temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit. It is suggested that alteration, silicification, and mineralization at the Enasen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200–250 °C and that the hydrothermal fluids consisted of meteoric and magmatic water. A tentative reconstruction of the fossil hydrothermal system is presented. Sulphur isotope ratios of sulphides from the fold-bearing quartz-sillimanite gneiss gave δ34S-values close to zero indicating a magmatic source of the sulphur.