THE ENASEN GOLD DEPOSIT, CENTRAL SWEDEN .1. A PALAEOPROTEROZOIC HIGH-SULFIDATION EPITHERMAL GOLD MINERALIZATION

Results from geochemical and mineralogical studies indicate that the Enasen gold deposit is a metamorphosed, Palaeoproterozoic analogue to recent epithermal Au deposits associated with acid-sulphate alteration in a high sulphidation environment. The deposit occurs in supracrustal rocks within the Enstern-Enasen region, central Sweden. Mineralization consists of disseminated chalcopyrite and gold in a topaz bearing quartz-sillimanite gneiss hosted by quartz-feldspar and quartz-mica gneisses. The gold bearing quartz-sillimanite gneiss has an unusual composition with quartz and sillimanite constituting more than 95% of the bulk mineralogy. Geological setting, geochemistry, gold and sulphide mineralization, and the occurrence of topaz, suggest that hydrothermal activity with strong leaching and intense silicification produced the protolith to the quartz-sillimanite gneiss. Absence of potassium-bearing silicates in the rock distinguish it from other hydrothermally altered rocks in the Palaeoproterozoic Svecofennian province and suggests acid-sulphate alteration. Geochemistry, mineralogy, and lead isotope composition of the host rocks suggest a mixed volcano-sedimentary origin for the quartz-feldspar gneisses in the hanging wall, while the protoliths to the quartz-mica gneisses in the footwall were probably hydrothermally altered igneous rocks of intermediate composition. Subsequent deformation and metamorphism to upper amphibolite-granulite facies produced the present day shape and mineralogy of the deposit. Similarities with Tertiary age high-sulphidation epithermal deposits, including grade, size and the distribution of base and precious metals and alteration suggest that the deposit acted as a closed system with respect to metals during metamorphism.