Origin of massive sulfide ores in the Skellefte district, as indicated by fluid inclusions
Sammanfattning: The Proterozoic SkeUefte massive sulfide ore district is confined to a 200 km long belt of a submarine metavolcanic and metasedimentary rock sequence in northern Sweden. It is located between an area of terrestrial metavolcanics in the north and gneisses, migmatites and granites in the south. Intrusions of granitoid complexes occur in several places, some of them are cogenetic with the volcanics and some post-date the emplacement of the supacrustals. The regional metamorphism has reached greenschist facies conditions. The district has characteristics typical of a subduction-related volcanic arc formation, which is reflected by the geology, the chemistry of rocks and the appearance of massive sulfide deposits. In the district approximately 100 sulfide mineralizations have been discovered, about 9 are currently mined with a total annual output of roughly 2 million tons of base metal ore. These deposit exhibit many ore features which are common in the Kuroko-type massive sulfide deposits.Microthermometric measurements of aqueous two-phase fluid inclusions in sphalerite, quartz and calcite from the massive sulfide ores indicate that they were deposited from Ca-Na-Cl solutions with a salinity of 1-6 eq. wt. % NaCl under nonboiling conditions at seawater depths between 1 and 2.5 km. The inclusions reveal trapping temperatures of 160-370 °C, similar to the temperatures obtained for the sulfide forming hydrothermal fluids which emerge from the sea floor vents at the East Pacific Rise.The massive ores are underlain by chalcopyrite-rich stockwork mineralizations. The aqueous inclusions hosted by them display temperatures and salinities equal to those in the massive ores, but were C02- and CH4-bearing as well. The characteristics of these inclusions suggest a heterogeneous system with gas-bubbles in a liquid phase. The lack of gases in the inclusions from the massive parts may indicate that the escaping gas have reacted with bottom water and formed the carbonate horizons which are associated with the massive ores.The ores have been affected in various degrees by subsequent metamorphic processes involving fluids of different compositions. Fluid inclusions in healed microfractures and quartz veins indicate that the metamorphic fluids during the peak of the regional metamorphic alteration were dominated by C02-rich compositions. The pressures deduced from the molar volumes of these inclusions can be used to construct a metamorphic isograd which follows the contour of the southern late-orogenic granite, creating a northern and a southern subarea with pressures below and above 3 kbar respectively.Retrograde conditions are recorded by N2-bearing hydrocarbon-rich fluids trapped in microfractures under low metamorphic pressures. No regional gradient was indicated by the N2/CH4 ratio and therefore implying that the amounts were controlled by local mineral reactions. However, the distribution trend of higher molecular weight hydrocarbons in these inclusions suggest a steeper cooling path in the western part of the district.At the final stage of the metamorphism, aqueous solutions with temperatures below 150 “C percolated through the rocks, these were highly saline brines in the western district and solutions with low to medium salinities in the eastern part.
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