Geological features and genesis of the Bjorkdal gold deposit in northern Sweden

The Bjorkdal gold deposit is located in the eastern part of the Early Proterozoic Skellefte district in northern Sweden. The ore zone is hosted by a granitoid intrusion. Three types of fluids are represented by fluid inclusions observed in quartz, scheelite and calcite. δ18O values of quartz samples range between +8.3‰ and + 14.3‰. The two whole-rock samples yielded oxygen isotope values of + 9.0‰ and 10.4‰. Sm-Nd isotope data were collected，and the obtained isochron age is rather well constrained at 1915 ±32Ma (MSWD=0.25). The resulting ε(Nd)(t=1.88Ga) values are between +1.1 and +1.7. Three main stages of mineralization are inferred from investigation of fluid inclusions in mineralized quartz veins. The first stage that produced quartz-veins with minor scheelite and sulfides were mainly barren with respect to gold; the main stage of mineralization is considered to have occurred through mixing of the magmatic fluid with surface-derived fluids such as seawater, and this mixing was contemporaneous with the pressure decrease and pH increase. The third stage, which caused the gold-calcite-telluride mineralization and remobilization of the primary ore, occurred at 130-220℃.