The Älgträsk Au±Cu deposit, northern Sweden: a Palaeo-proterozoic porphyry-related hydrothermal system? Therese Bejgarn, Pär Weihed Department of Civil, Mining and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden Curt Broman Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden Ulf Söderlund Department of Earth and Ecosystem Sciences, Lund University, SE-223 62 Lund, Sweden Ross Large CODES, University of Tasmania, Hobart TAS 7001, Australia Hans Årebäck, Juhani Nylander Boliden Mineral AB, SE-936 81 Boliden, Sweden Abstract. The Älgträsk Au±Cu deposit is situated in the northern part of the Skellefte mining district, northern Sweden. The deposit is hosted by the 1.89 Ga phase of the early orogenic, synvolcanic, Jörn Granitoid Complex, which intruded a continental margin arc or island arc volcanosedimentary succession. Subsequent intrusion of quartz-feldspar porphyritic dykes at 1.88 Ga, and related hydrothermal activity, formed the Tallberg and Granberg porphyry Cu±Mo±Au deposits. The Älgträsk deposit is spatially related to the Tallberg porphyry deposit and characterized by steeply dipping zones of intense phyllic-silicic alteration enveloped by pervasive distal propylitic alteration, with dissemination and veins of pyrite, locally enriched in chalcopyrite, sphalerite, arsenopyrite, Te-minerals and Au. Low salinity fluid inclusions with variable CO2/H2O suggest formation temperatures of 150-200°C. In contrast, fluid inclusion data and trace element zonation in pyrite indicate that mineralization in the Tallberg deposit formed from two hydrothermal fluids at higher temperatures interpreted from fluid inclusion data and the presence of different stages of mineralization defined by trace element zonation in pyrite. The Älgträsk deposit is interpreted to have formed shortly after the formation of the porphyry mineralization at Tallberg, representing a higher level, later phase of the hydrothermal system, or alternatively a similar porphyry system. Keywords. Skellefte district, porphyry related mineralization, gold, copper, Palaeoproterozoic 1 Introduction The Älgträsk and Tallberg deposits are situated in the northern part of the Skellefte mining district, northern Sweden (Fig.1). The Skellefte mining district comprises numerous VMS deposits, e.g. the Boliden, Renström, Petiknäs, Kristineberg and Maurliden deposits (Allen et al. 1996; Bergman Weihed et al. 1996; Årebäck et al.

2005; Barrett et al. 2005; Montelius et al. 2007). The VMS deposits are hosted by the older parts of a volcanosedimentary succession interpreted as a remnant of an early Proterozoic island arc or continental margin arc succession (Weihed et al. 1992; Allen et al. 1996). The Jörn granitoid Complex (JGC) is situated directly north of the volcanosedimentary succession. The JGC hosts several different types of mineralization, such as porphyry Cu±Mo±Au, Au±Cu, ultramafic-mafic hosted Cu±Ni±Au, and Fe±PGE (Bejgarn et al. 2009, Bejgarn et al. 2011). All mineral deposits are situated in the outer, older parts of the JGC, but little has been known regarding the timing of mineralization. In this paper we present a summary of geological, geochronological (U-Pb SIMS on zircon and U-Pb TIMS on baddeleyite (ZrO2), LA-ICP-MS and fluid inclusion results from the Älgträsk and Tallberg deposits that provide evidence for a better understanding of the temporal relationships between intrusive events and mineralization in the JGC. 1.1 Regional geology The VMS bearing Skellefte district is situated in the northern part of the Fennoscandian shield (Fig. 1). The VMS deposits formed during the early Proterozoic, in what has been interpreted as the remnant of an ancient volcanic arc, situated behind a northward dipping subduction zone (Lundberg 1980; Weihed et al. 1992; Allen et al. 1996). The Palaeoproterozoic JGC intruded the volcano-sedimentary succession at c. 1.89-1.87 Ma and has been interpreted as an early orogenic-synvolcanic intrusive complex. Similarities in composition and age led many authors to suggest that the volcanic rocks of the Skellefte Group in the Skellefte district are comagmatic with the JGC (Lundberg 1980, Claesson 1985, Billström and Weihed 1996, Wilson et al. 1987). The JGC is a calc-alkaline, I-type intrusion with four distinct magmatic phases. The oldest phase, the “GI”, is the least fractionated and most heterogeneous, with a

composition ranging from gabbro to granodiorite (Wilson et al. 1987).

The JGC intrusive phases have previously been dated by the U-Pb zircon method at e.g., 1886 ± 3 Ma (GI), 1874 ± 6 Ma (GII) and 1863 ± 5 Ma (GIII) (González-Roldán 2010). The intrusion of the later phases into the GI unit likely caused metamorphism, hydrothermal alteration and deformation of the GI (Wilson et al. 1987; González-Roldán 2010). Two major fold periods have been proposed for the

Skellefte district; tight to isoclinal upright folds with variably plunging fold axes formed during EW shortening at 1.87-1.82 Ga, and a set of later open folds with steep north to north-east striking axial surfaces and fold axes that are coaxial with earlier folds (Bergman Weihed 2001). Generally, W-NW striking shear zones are correlated with the former event and N-NE trending shear zones correlated with the later event (Bergman Weihed 2001). Large scale NE-SW striking fault zones are interpreted to crosscut the JGC, based on interpretation of regional geophysical maps and could possibly be correlated with these events. 1.2 Deposit geology The Tallberg porphyry Cu±Mo±Au (Fig. 1a) and Älgträsk Au±Cu deposits (Fig.1b) are hosted by the earliest phase (GI) of the JGC. Mineralization in Tallberg (mineral resource, 45 Mt @ 0.27% Cu; Weihed et al. 1992) occurs as disseminated pyrite, chalcopyrite, molybdenite, pyrrhotite, magnetite and quartz vein stockworks with similar sulphide mineral assemblage associated with propylitic and phyllic alteration and 1886+16/-9 Ma quartz-feldspar porphyritic (QFP) dykes (Weihed 1991; Weihed and Schöberg 1992).

The Älgträsk Au deposit (indicated mineral resource, 2.9 Mt @ 2.6 g/t Au), situated approximately 3 km east of the Tallberg deposit, is mainly hosted by coarse grained JGC granodiorite. It is characterised by several steeply dipping, NE-SW striking zones of varying width with disseminations and quartz veins with pyrite locally enriched in chalcopyrite, sphalerite, arsenopyrite, accessory Te-minerals and Au. The mineralized zones are structurally controlled and display intense proximal phyllic-silicic alteration and pervasive distal propylitic alteration of the host rock (Bejgarn et al. 2011). QFP dykes in the Älgträsk area have similar composition and age as in Tallberg, but predate the Älgträsk mineralization. In the southern Tallberg area, similar style Au-mineralization is commonly closely associated with the QFP dykes, but quartz vein-stockwork style mineralization has not yet been observed in the Älgträsk area. The mineralization in Tallberg and Älgträsk is crosscut by the Älgliden dyke - a steeply dipping, NE striking ultramafic-mafic intrusion (Fig. 1). The dyke is approximately 100 m wide and 3 km long, with disseminated magnetite, pyrrhotite, chalcopyrite and pentlandite.

A deposit with similar characteristics as seen in

Tallberg is present at Granberg in the northern part of the JGC (Fig. 1). This deposit is spatially related to a layered gabbro at Näsberg (Bejgarn et al. 2009). 2 Results The GI tonalite in the Tallberg area and the granodiorite in the Älgträsk areas yield preliminary U-Pb zircon ages of 1886 ± 3 Ma and 1887 ± 3 Ma, respectively. Rims of complex zircon in quartz-feldspar porphyritic dykes from Granberg Älgträsk and Tallberg yield preliminary U-Pb ages between 1876-1880 Ma whereas the zircon cores yield slightly older ages between 1884-1900 Ma. Mafic intrusions adjacent to porphyry style mineralisation yield precise baddeleyite U-Pb TIMS ages of 1884 ± 2 at Torrspiggen (ca. 1 km east of Älgträsk), 1876 ± 2 Ma at Älgliden (crosscutting porphyry style mineralization) and 1881 ± 2 Ma at Näsberg (Fig. 1).

Preliminary results from microscopic and LA-ICP-MS studies of sulphide minerals show that refractory gold in arsenopyrite is common, while Au commonly occurs as micro-inclusions, in grain boundaries and as fracture fillings in pyrite together with Te-minerals.

Figure 1. Simplified geological map of the Jörn Granitoid Complex and central part of the Skellefte district with major VMS deposits indicated. Discussed intrusion hosted mineralization at a) Tallberg b) Älgträsk c) Älgliden d) Näsberg and e) Granberg (Modified from Wilson et al. 1987 and Antal et al. 2003). Inset: Outline of the Fennoscandian shield with the Skellefte district indicated.

Textural and element proportions observed in different pyrite growth stages indicate at least two different generations of pyrite in the Tallberg and the Älgträsk deposits. The Tallberg porphyry Cu-deposit and adjacent Au bearing zones formed from magmatic hydrothermal water mixed with seawater at higher temperatures and possibly from an unmixed fluid giving two unmixed salinity groups (Weihed 1992). Fluid inclusions from the Älgträsk deposit have variable CO2/H2O proportions, low salinity and P-T results indicating a trapping of fluids at 150-200°C at a depth of ca. 2 km. 3 Conclusions U-Pb dates of zircon and baddeleyite indicate a 10 Ma age difference between VMS and porphyry ores in the Skellefte district, in spite of their relatively close spatial relationship. This could correspond to a shift in tectonic regime from extension to compression during the Svecokarelian orogeny. The age of the porphyry dykes, their geochemical signatures and radial structural expression surrounding the GII intrusion in the southern part of the JGC suggest that the GII granitoid may have contributed magma, heat and opening of pathways for porphyry dykes and mineralizing fluids. Furthermore, the spatial correlation of coeval mafic-ultramafic intrusions, evidence for magma mingling/mixing and porphyry style mineralization in both the southern and northern JGC, suggests that the formation of porphyry style mineralization may have been triggered by the upwelling of mafic magma, as has been proposed in younger orogens (c.f. Hattori and Keith, 2001).

The classification of the Älgträsk Au deposit is not straightforward, but the fluid forming the Älgträsk deposit is interpreted to represent a near-surface, later phase of the hydrothermal system forming the Tallberg deposit, or alternatively a similar, deeper-seated porphyry system. The mafic intrusion at Älgliden crosscuts Tallberg mineralization and alteration, hence marking the end of the formation of porphyry style and related mineralization at 1876 ± 2 Ma. Acknowledgements Luleå University of Technology, Boliden Mineral AB, ÅF Forskningsstiftelse, Nordea Bank AB and SEG Student Foundation are greatly acknowledged for making this study possible. Therese Bejgarn acknowledges Wallenbergsstiftelsen and SGA for travel support to attend this meeting. References Allen RL, Weihed P, Svenson SA (1996) Setting of Zn-Cu-Au-Ag

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