A FLUID INCLUSION AND LIGHT-ELEMENT STABLE-ISOTOPE STUDY OF THE GOLD-BEARING QUARTZ VEIN SYSTEM, FALUN, SWEDEN

The Falun gold quartz vein mineralization is located ca 230 km NW of Stockholm, Sweden, within the Early Proterozoic volcano-sedimentary sequence of Bergslagen. The mineralization consists of a system with subparallel quartz veins that crosscut the alteration zone to the Falun massive sulphide deposit. Early barren and late gold-bearing quartz veins follow tectonic structures postdating the formation of the massive sulphide ore. Both generations of veins are epigenetic to the massive sulphide ore and were formed by hydrothermal processes. Fluid inclusion study of the gold-bearing quartz veins indicates a low-moderately saline fluid (0.3 to 17.4 equiv wt% NaCl). Heterogeneous trapping is indicated by coexisting inclusions showing a variable CO2 content from 100% CO2 +/- CH4 to 100% aqueous fluid. Temperatures of total homogenization also show a wide spread from 116-350 degrees C with a slightly bimodal distribution with peaks at ca 180 degrees C and 280 degrees C. Measured delta D values -69 to -63 parts per thousand (SMOW), of inclusion fluid and calculated delta(18)O values of hydrothermal fluids -7.5 to -1.4 parts per thousand (SMOW), strongly suggest a meteoric origin for the fluids. The quite consistent delta D values and the range in delta(18)O values indicate that major water-rock interaction led to the evolution in delta(18)O of the hydrothermal fluids.