The Vargbäcken orogenic gold deposit, Skellefte district, northern Sweden: mineralization style, alteration and setting of gold

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The Vargbäcken orogenic gold deposit is located in the Palaeoproterozoic part of the Fennoscandian Shield within the western part of the Skellefte mining district, northern Sweden. The Skellefte district is a product of a ca. 1.90 Ga Palaeoproterozoic volcanic arc with numerous massive sulphide deposits and orogenic gold deposits of which Vargbäcken is one with an indicated resource of ca. 1.2 Mt with 1.44 ppm Au. Mineralized veins exhibit several structural styles, ranging from breccias to vein stockworks predominantly within competent host rocks close to the metasediment-diorite contact, and laminated quartz veins in brittle-ductile shear zones. The quartz veins are of two distinctive generations, the milky and opaque fine to medium grained quartz veins with a high amount of visible, coarse grained, gold and the smoky dark translucent coarse grained quartz veins with disseminated sulphides and low amount of visible gold. The ore zone is situated on the metagreywackes-diorite contact, and the silicified diorite close to the contact. The best mineralized areas include zones where changes in strike and dip of the contact occur, and structural deformation zones along the contact, or within the diorite. Only sphalerite and rarely pyrrhotite is visibly correlated with the gold which is found in small cracks or interstices at grain boundaries between quartz grains. Gold is also seen as free grains in the silicate matrix of the host diorite. The vein related alteration is considered to be spatially restricted and most quartz veinlets have thin or no alteration envelopes. Alteration mineral assemblage suggests a greenschist metamorphic facies environment, indicating mesozonal conditions during mineralization. ...