New styles of intrusive related copper-gold deposits in northern Sweden
Sammanfattning: The Älgträsk Au-deposit is situated in the Jörn Granitoid Complex (JGC) immediately north of the Skellefte VMS district, northern Sweden. The Skellefte VMS district comprise a complex volcanosedimentary sequence, which has been interpreted as the remnant of a Palaeoproterozoic island arc or continental margin arc situated behind a northward dipping subduction zone. The volcanosedimentary succession was intruded by the early orogenic-synvolcanic JGC, constituting I-type, calc-alkaline granitoids at c. 1.89-1.87 Ma. The outer zone of the JGC represents the oldest part and is the most compositionally heterogeneous, ranging from gabbro to granodiorite, while younger units are of a more felsic character ranging from granodiorite to granite in composition. The Älgträsk intrusive hosted Au-deposit is situated in the southern part of the oldest JGC intrusion, mainly hosted within a coarse grained, often quartz porphyritic, granodiorite. Mineralisation occurs as veins and disseminations of mainly pyrite with locally abundant arsenopyrite, chalcopyrite, sphalerite and minor electrum within a proximal silicic to phyllic hydrothermal alteration and distal propylitic hydrothermal alteration in close vicinity to deformation zones. Geochemical data suggest that hydrothermal alteration associated with mineralisation in the granodiorite caused addition of Si, Fe and K together with a significant increase in Au, Te, Cu, Zn, As and loss of Ca and Na. Gold alloys and telluride minerals such as petzite, tellurobismuthinite, krennerite, volynskite and hessite occur intergrown within fractures, along grain boundaries or as inclusions in pyrite and/or arsenopyrite and less common by together with quartz and phengite. Besides propylitic, phyllic and silicic alteration associated with mineralisation, sodic-calcic and quartz destructive alteration characterize the deposit, although with an unclear relationship to mineralisation. The tonalite hosted Tallberg porphyry Cu-deposit, situated only 3 km west of Älgträsk, is associated with quartz brecciation and quartz feldspar porphyritic dykes. These dykes are suggested to be genetically related to similar porphyry dykes overprinted by mineralisation in Älgträsk and the tonalitic host rock in Tallberg. However, the granodiorite hosting the Älgträsk Au-deposit does not appear to be genetically related to the tonalite in Tallberg or the porphyry dykes in neither Älgträsk nor Tallberg. Both styles of mineralisation are cut by mafic dykes, tentatively correlated with 1.87 Ga magmatism, hence representing the last magmatic event in the area. The mineralising fluids at Älgträsk are suggested to have been channelled by structurally weak zones in the upper crust where deposition of sulphide minerals was followed by precious metals. Structural interpretations suggest that the Au-deposit in Älgträsk is hosted by D2 and D3 structures and thus is younger than the porphyry Cu-deposit in Tallberg. Together with the style of mineralisation and associated alteration at the Älgträsk deposit, it resembles either intrusion related Au-systems or shallow level epithermal systems often developed above younger Phanerozoic porphyry Cu-deposits. Alternatively, ductile deformation took place during subsequent tectonic events, likely causing remobilization of sulphide minerals and precious metals, e.g. from a deeper lying porphyry style mineralisation, forming an orogenic style mineralisation.
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