Fluid chemistry of the Palaeoproterozoic Faboliden hypozonal orogenic gold deposit, northern Sweden: evidence from fluid inclusions

A new ore province, the Gold Line, southwest of the Skellefte District, northern Sweden, is currently under exploration. The largest known deposit in the Gold Line is the hypozonal Faboliden orogenic gold deposit. The mineralization is hosted by arsenopyrite-bearing quartz veins, within a steep shear zone in amphibolite facies metagreywacke host rocks. Gold occur in fractures and as intergrowths in arsenopyrite-lollingite, and as free grains in the silicate matrix of the host rock. The hydrothermal mineral assemblage in the proximal alteration zone is diopside, calcic amphibole, biotite, and minor andalusite and tourmaline. Primary fluid inclusions in the Faboliden quartz veins show a CO2-CH4 or a H2S (CH,) composition (the latter recognized for the first time in a Swedish ore deposit). The primary fluid inclusions are associated with arsenopyrite-lollingite (+gold) and the CO2-CH4 fluid was also involved in precipitation of graphite. A prevalence of carbonic over aqueous fluid inclusions is characteristic for a number of hypozonal high-temperature orogenic gold deposits. The Faboliden deposit, thus, shows fluid compositions similar to other hypozonal orogenic gold deposits. The proposed main mechanism for precipitation of gold from the fluids, is a mixing between H2S-rich and H2O?-CO2 +/- CH4 fluids. Fluid inclusion data indicate arsenopyrite-lollingite and graphite deposition at a pressure condition of about 4 kbar. Graphite thermometry indicates maximum temperatures of 520-560 degrees C for the hydrothermal alteration at Faboliden, suggesting that at least the late stages of the mineralizing event took place shortly after peak-metamorphism in the area, i.e. at c. 1.80 Ga.