FLUID INCLUSIONS, STABLE ISOTOPES AND GOLD DEPOSITION AT BJORKDAL, 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 hos ted by a granitoid which intrudes a 1.9 Ga old supracrustal sequence a nd consists of a network of quartz veins between two shear zones. The ore mineralogy, alteration assemblages, ore fluid characteristics and general setting of the Bjorkdal deposit reveal many similarities with mesothermal Archean systems. Three types of fluids are represented by fluid inclusions observed in quartz, scheelite and calcite. The first type consists of a CO2-rich fluid which is syngenetic with the formati on of the quartz veins. These inclusions occur in quartz and scheelite . Isotopic equilibrium temperatures derived from quartz-scheelite pair s reflect depositional temperatures around 375-degrees-C. Molar volume s of the carbonic fluid inclusions, ranging down to 55 cm3/mole, indic ate a maximum lithostatic trapping pressure of 1.8 kbar. These fluids were generated at depth in conjunction with early orogenic magma-formi ng processes. The gold was introduced to the vein system by the carbon ic fluid but the gold was deposited after reactions between this fluid and the wall-rock, producing a slightly alkaline, more CH4-rich aqueo us type 2 fluid. Fluid inclusions of this chemically modified fluid in dicate that the precipitation of the gold, together with pyrrhotite, p yrite and chalcopyrite, occurred under heterogenous conditions at a te mperature of 220-degrees-C and a hydrostatic pressure of 0.5 kbar. The gold deposition occurred from fluids with a delta O-18 signature of a round + 8 parts per thousand and deltaD values close to zero per mil. Any metamorphic influence on the stable isotopic signatures is regarde d as minimal. The isotope data suggest rather that a surface-derived f luid component had access to the vein system during this process. At a post-vein forming stage (metamorphic stage ?) a secondary episode of gold mobilization occurred as suggested by the aqueous type 3 inclusio ns trapped in cross-cutting microfractures in quartz and randomly in c alcite, and with homogenization temperatures between 145-220-degrees-C and a salinity up to 11 eq. wt. % NaCl. The Skellefte district is a m ajor ore province, which forms a 200 by 50 km area in northern Sweden (Fig. 1), comprising numerous stratabound massive sulfide ore deposits . During the last decade epigenetic gold deposits have received increa sing interest from a prospecting point of view. The Bjorkdal deposit i s one of several epigenetic gold discoveries made recently in the Skel lefte district. In 1985 a geochemical survey, designed on a grid-patte rn basis, revealed a gold anomaly about 12 km north-east of the Bolide n community and three years later the Bjorkdal gold mine was in operat ion. The annual production is about 960 000 metric tons of ore (1992) and the total reserves are estimated at a minimum of 7 Mton of ore wit h a gold grade of 2.9 ppm. This paper reports on the geological featur es of the Bjorkdal deposit and discusses the genesis of the deposit on basis of fluid inclusions and distribution of oxygen and hydrogen iso topes.