The Älgträsk intrusion-related Au (Cu, Zn) deposit, Skellefte district, northern Sweden : Relationships between ore forming events in the Palaeoproterozoic Jörn intrusive complex

Sammanfattning: Palaeoproterozoic intrusion hosted and intrusion related deposits such as the Älgträsk Au±Cu±Zn, Åkerberg Au, Älgliden Cu–Ni±Au, and the porphyry style Järvträsk Cu±Mo±Zn, Granberg Cu±Mo and Tallberg Cu–Au±Mo deposits are situated in the northern part of the Skellefte District, northern Sweden. These deposits are all hosted within the oldest unit of the Jörn intrusive complex (JIC). This first synvolcanic, early orogenic phase of the JIC ranges from gabbro to granodiorite in composition and shows a alc–alkaline, I–type affinity. It intruded a continental margin or island arc volcanosedimentary succession at 1.89 Ga and is spatially, temporally and possibly genetically linked to the oldest volcanic unit in the area, which hosts over 80 volcanogenic massive sulphide deposits of varying sizes. At 1.88 Ga, the JIC was intruded by quartz–feldspar porphyritic (QFP) dykes and related hydrothermal activity formed porphyry style Cu±Mo±Au deposits. A spatial relationship with coeval mafic–ultramafic intrusions, evidence for magma mingling/mixing and porphyry style mineralisation in both the southern and northern part of the JIC suggests that the formation of porphyry style mineralisation may have been triggered by the upwelling of mafic magma and partial melting at the base of the crust.While the porphyry style deposits are dominated by both sulphides in quartz stockworks and disseminated sulphides, the Älgträsk Au deposit is structurally controlled and characterised by NE–SW striking, steeply dipping zones of intense phyllic–silicic alteration enveloped by pervasive distal propylitic alteration. The mineralisation occurs both as dissemination and in veins, dominated by pyrite and locally enriched in chalcopyrite, sphalerite, arsenopyrite, Te–minerals and gold. The mineralisation overprints QFP dykes that are associated with porphyry style mineralisation in other parts of the JIC. Whole rock lithogeochemical data from Älgträsk suggest that gold is positively correlated with SiO2, K2O and Ag, Te, Bi, Fe, Cu, Zn, Mo±As, coupled with feldspar destructive alteration and Sr depletion. Early Na–Ca alteration in the north–eastern part of the Älgträsk deposit, however, shows a depletion in base and precious metals, coupled with an enrichment in Na2O, CaO and Sr, related to the formation of secondary feldspar associated with this alteration. At Älgträsk, several stages of mineralisation occur, that likely formed during one continuous hydrothermal event. Gold is associated with both early and late stages of pyrite precipitation, although one sample from the Tallberg deposit records a transition from base metal– to precious element–rich hydrothermal activity, where gold mainly is introduced early during the later stage of hydrothermal activity. Fluid inclusions associated with gold deposition at Älgträsk are aqueous and characterised by low salinity, variable CO2/H2O, homogenisation temperatures of 150–200°C and pressures corresponding to precipitation at 3–4 km depth in hydrostatic conditions. In contrast, fluid inclusion data and trace element zonation in pyrite indicate that mineralisation in the Tallberg deposit formed from two hydrothermal fluids, one similar to the fluids associated with gold from the Älgträsk deposit. However, fluid inclusions from the Tallberg deposit generally record higher homogenisation temperatures. The results indicate that the Älgträsk deposit represent a telescoping, later phase of the hydrothermal system forming the Tallberg deposit, or alternatively Tallberg represents a similar, deeper–seated porphyry system. The 3 km long, 100 m wide, NE–SW striking mafic dyke at Älgliden, dated at 1876±1 Ma, crosscuts porphyry style mineralisation at Tallberg and, hence is younger than the formation of porphyry style mineralisation. Together with U–Pb geochronology of zircon and baddeleyite from QFP dykes and the JIC units, this indicates a c. 10 Myr age difference between the spatially related VMS and intrusion related deposits in the Skellefte district. This also tentatively corresponds to the tectonic evolution of the volcanic arc where the arc during formation of the VMS deposits was under extension, while basin inversion and exhumation of the arc during later compression or transpression is coeval with the formation of intrusive hosted Au–Cu and porphyry Cu–Au deposits.