Fluid chemistry of the hypozonal Fäboliden orogenic gold deposit, northern Sweden

• Southwest of the well-known Skellefte District in northern Sweden a new ore province is presently being explored, the so called Gold Line. Today the largest known gold deposit in the Gold Line is the Fäboliden orogenic gold deposit.The gold mineralization is commonly hosted in quartz veins, which parallel the steep main foliation, within a shear zone in the metagreywacke host rocks. The fine-grained (2-40 μm) gold is closely associated with arsenopyrite in the quartz veins.Two main groups of fluid inclusions are present in the Fäboliden quartz veins. 1) Primary inclusions with a CO2-CH4 or a H2S (±CH4) composition (the latter recognized for the first time in a Swedish ore deposit). 2) Secondary fluid inclusions composed of pure CH4 and low-salinity aqueous fluids. The primary fluid inclusions are associated with arsenopyrite (+gold) and the CO2-CH4 fluid was also involved in precipitation of graphite. The graphite-forming reactions should generate a H2O phase as well. However, the presence of a H2O phase was not detected in any of the primary fluid inclusions and is suggested to have been consumed by wall rock reactions, generating hydrated alteration minerals such as Ca-amphibole, biotite, and minor tourmaline. Fluid inclusion data indicate arsenopyrite and graphite deposition at a pressure condition of ~4 kbars. Graphite is useful as an indicator of the metamorphic grade because the graphitization process is irreversible with no effects on the graphite structure during retrogression (Beyssac et al., 2002). Graphite in the mineralized quartz veins at Fäboliden indicates maximum temperatures of 520-560°C for the hydrothermal alteration system.Pyrrhotite was deposited after a subsequent pressure decrease and a later input of pure CH4 and low-salinity aqueous fluids, as suggested by the secondary fluid inclusions. These later fluids were trapped at a substantially lower pressure of ~0.3 kbars and a temperature of ~400°C.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

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Ore Geology

Malmgeologi

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