Fluid inclusions,stable isotopes and gold deposition at Björkdal,northern Sweden |
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| Authors: | C Broman K Billström K Gustavsson A E Fallick |
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| Institution: | (1) Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden;(2) Swedish Museum of Natural History, P.O. Box 50 007, S-104 05 Stockholm, Sweden;(3) Terra Mining AB, Björkdalsgruvan, S-934 94 Kåge, Sweden;(4) Scottish Universities Research and Reactor Centre, G75 OQU East Kilbride, Glasgow, Scotland, UK |
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| Abstract: | The Björkdal 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 which intrudes a 1.9 Ga old supracrustal sequence and consists of a network of quartz veins between two shear zones. The ore mineralogy, alteration assemblages, ore fluid characteristics and general setting of the Björkdal 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 formation of the quartz veins. These inclusions occur in quartz and scheelite. Isotopic equilibrium temperatures derived from quartz-scheelite pairs reflect depositional temperatures around 375 °C. Molar volumes of the carbonic fluid inclusions, ranging down to 55 cm3mole, indicate a maximum lithostatic trapping pressure of 1.8 kbar. These fluids were generated at depth in conjunction with early orogenic magma-forming processes. The gold was introduced to the vein system by the carbonic fluid but the gold was deposited after reactions between this fluid and the wall-rock, producing a slightly alkaline, more CH4-rich aqueous type 2 fluid. Fluid inclusions of this chemically modified fluid indicate that the precipitation of the gold, together with pyrrhotite, pyrite and chalcopyrite, occurred under heterogenous conditions at a temperature of 220 °C and a hydrostatic pressure of 0.5 kbar. The gold deposition occurred from fluids with a δ 18O signature of around +8‰ and δD values close to zero per mil. Any metamorphic influence on the stable isotopic signatures is regarded as minimal. The isotope data suggest rather that a surface-derived fluid 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 inclusions trapped in cross-cutting microfractures in quartz and randomly in calcite, and with homogenization temperatures between 145–220 °C and a salinity up to 11eq. wt.% NaCl. The Skellefte district is a major 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 increasing interest from a prospecting point of view. The Björkdal deposit is one of several epigenetic gold discoveries made recently in the Skellefte district. In 1985 a geochemical survey, designed on a grid-pattern basis, revealed a gold anomaly about 12 km north-east of the Boliden community and three years later the Björkdal gold mine was in operation. 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 with a gold grade of 2.9 ppm. This paper reports on the geological features of the Björkdal deposit and discusses the genesis of the deposit on basis of fluid inclusions and distribution of oxygen and hydrogen isotopes. |
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