Palaeoproterozoic porphyry Cu–Au, intrusion-hosted Au and ultramafic Cu–Ni deposits in the Fennoscandian Shield: Temporal constraints using U–Pb geochronology

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The Skellefte district, northern Sweden, is known for the occurrence of 1.89Ga Palaeoproterozoic volcanogenic massive sulphide (VMS) deposits. The deposits are hosted by the older part of a volcanosedimentary succession, which was intruded at 1.88–1.86Ga by multiple phases of the syn-volcanic, early orogenic Jörn intrusive complex (JIC). The oldest phase of the JIC hosts different styles of mineralisation, among them porphyry Cu–Mo–Au, intrusion-related Au, and mafic-hosted Fe and Cu–Ni deposits. To discriminate between the different intrusive and ore related events, U–Pb ages of zircons have been obtained for nine intrusive phases and from Na–Ca alteration spatially related to mineralisation, while U–Pb ages of baddeleyite (ZrO2) have been used to constrain intrusive ages of three mineralised and barren mafic–ultramafic intrusive rocks.The two main JIC intrusive phases of a granodioritic–tonalitic composition in the southern study area intruded at 1887±3Ma and 1886±3Ma, respectively, and were succeeded by the intrusion of layered mafic–ultramafic intrusive rocks in the northern and southern study area at 1879±1Ma and 1884±2Ma, respectively. Emplacement of porphyry dykes took place at ca. 1877Ma in the southern, western and northern JIC. The dykes are spatially and temporally associated with formation of porphyry style mineralisation, alteration and Au-mineralisation, as inferred from 1879±5Ma zircons in adjacent Na–Ca alteration zones. High SiO2 and Al2O3 contents together with high Sr/Y ratios, mingling structures, mafic xenoliths and hornblende phenocrysts in the porphyry dykes suggest that the magma originated from hydrated partial melts, possibly from the base of the crust at a mature stage of subduction. Local extension resulted in intrusion of mafic–ultramafic rocks around 1.88Ga prior to and after, the porphyry dykes and associated mineralisation, approximately 10Ma after the formation of the spatially related 1.89Ga VMS deposits in the Skellefte district. This 1.88Ga event correlates with other 1.88Ga mafic–ultramafic units widespread around the world, and could possibly be interpreted as a large scale response to supercontinent formation.