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Abstract.:  The Proterozoic Pahtohavare Cu-Au deposit is located in the greenstone belt near Kiruna, northern Sweden. The greenstone consists of mafic volcanic rocks with pillow lavas, mafic sills and albitized rocks, including tuffites, black schists and mafic sills, together with carbonates and mineralized zones. Mineralization occurs as impregnations, epigenetic quartz-rich breccias and fracture fillings with pyrite, chalcopyrite, pyrrhotite and gold in a complex tectonic environment. Fluid inclusions indicate an early formation of quartz and pyrite at temperatures initially near 500 �C and a pressure of 2–2.4 kbar from a supersaturated aqueous solution of magmatic origin. In addition to halite cubes, daughter minerals of sylvite, calcite, hematite, graphite and two unknown phases are found. The main stage of chalcopyrite and gold deposition is characterized by aqueous fluids of variable salinity (up to 30 eq. wt.% NaCl including CaCl2), at temperatures below 350 �C and pressures between 1 and 2 kbar. A minor CO2 phase with some N2 accompanies this stage. Gold was transported as a chloride complex which destabilized due to an increase in pH (as a consequence of the CO2 loss) as well as cooling and dilution of the solution. The ore deposition occurred as a result of mixing with a low salinity aqueous solution during tectonic fracturing with pressure fluctuations and CO2 unmixing. Late oxidation of ores was caused by low to moderately saline (3 to 13 eq. wt.% NaCl) low temperature aqueous solutions.

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 d18O signature of around +8‰ and dD 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.