The Aitik Cu-Au-Ag deposit in northern Sweden: a product of high salinity fluids

摘要

The Aitik Cu-Au-Ag depoit is located in northern Sweden and is hosted by strongly deformed 1.9 Ga Svecofennian volcano-sedimentary rocks. The main copper mineralization, which occurs as disseminations and veinlets, is hosted by garnet-biotite schist. Subeconomic mineralization in the footwall to the ore is hosted by feldspar-biotite-amphibole gneiss and porphyritic quartz monzodiorite. The deposit has been affected by post-mineralization metamorphic and igneous activity. Fluid inclusions in six samples of copper-mineralized quartz veins record the presence of three different fluid populations. The main ore was deposited from an aqueous, highly saline (31-37 eq. wt% NaCl + CaCl_2) fluid. This fluid was trapped in inclusions intimately associated with the main chalcopyrite mineralization. Later bornite deposition took place from a less saline (18-27 eq. wt% NaCl + CaCl_2), aqueous fluid. A third fluid composed of almost pure CO_2, interacted with the copper-rich system during a post-ore event. The Aitik Cu-Au-Ag deposit shares some features with both porphyry-type and Fe-oxide-Cu-Au deposits. A high calcium content of the ore fluids, similar to other Cu-Au deposits in northern Scandinavia, suggests a contribution to the salinity of the mainly magmatic-hydrothermal fluids from evaporitic rocks in stratigraphically lower units.