The Åkulla volcanic-hosted Au-Te deposit, Skellefte District, Sweden : characterization of host rocks, alteration and mineralization

Abstract

The Åkulla Au-Te deposit is located in the Skellefte District (SD), Northern Sweden, a worldclass mineral district with abundant volcanic massive sulphide (VMS) deposits. The Åkulla Au-Te deposit is one of 4 active mines in SD. The deposit has produced and currently contains 11.8 Mt of ore grading 4.1 g/t Au, 7.7 g/t Ag, 150.0 g/t Te, 89.7 g/t Bi and 1.0 wt% S since production started in 2012. The Åkulla Au-Te deposit is hosted by an upward-pointing, irregular lens shaped body of strongly altered coherent dacites, andesites and breccias, oriented sub-parallel to the steeply dipping volcanic strata. The hydrothermal alteration is strongly zoned with a quartztopaz±sericite±andalusite core, surrounded by sericite-quartz ± chlorite ± andalusite ± base metal sulfide association with minor occurrences of corundum in sericite-chlorite rock, followed outwards by sericite ± andalusite ± base metal sulfides and chlorite ±base metal sulfides associations. The outer sulfide-bearing alteration zones continue to surface to the small Åkulla Östra semi-massive VMS deposit, which was mined as an open pit. The mineralization and host rocks have been metamorphosed to upper greenschist facies and are flattered and elongated subparallel to the D2 foliation and lineation, indicating their formation pre-deformation. The Åkulla Au-Te deposit differs from other VMS deposits in SD by being sulphide poor (<3%), and native gold and telluride enriched. The four main breccia facies that host the deposit formed through synvolcanic quench brecciation and subsequent synvolcanic hydrothermal brecciation. The occurrence and variations of quartz and topaz alteration within coherent units, breccia clasts and matrices indicate that the rocks were altered before and during brecciation. The near vertical orientation of the graded beds in mass flow deposits is a product of block rotation during synvolcanic extension and subsequent deformation (Paper I). Alteration mineral- and elemental-zoning show a strong correlation. Au and Te enrichments are found in almost every alteration zone. However, the quartz and topaz dominant core is strongly depleted in major, chalcophile and some HFS elements, but enriched in Si, Ag, Bi, Sb and Mo. Corundum occurrences correlate with Si depletion and Al, Ti, Zr, Nb enrichment. Above and irregularly surrounding the central core, a zone of Fe, S, Cu, Zn, Ni and Pb enrichment occurs (Paper III). This distinct pattern reflects a systematic change in the bulk composition of the host rocks due to interaction with a hot and acidic fluid in the core, to progressively lower temperatures and reduced acidity fluids towards the margins. Hydrothermal alteration conditions based on an interpreted synvolcanic formation of topaz and corundum (Paper II) in addition to the elemental zoning suggest a core of relatively low pH (2-4) and temperatures ≥400°C, decreasing to around 375°C in the corundum bearing zone to an outer, near neutral pH and lower temperature (<250°C) zone (Paper II-III). The Åkulla Au-Te deposit is interpreted as a synvolcanic magmatic-hydrothermal deposit that formed at or near the seafloor. The relationship between brecciation and alteration, the abundance of F-rich topaz, the zonation of alteration minerals and elements suggest that an acidic, high temperature, F-rich fluid most likely of magmatic-hydrothermal origin was responsible for the brecciation, alteration and mineralization. The Åkulla Au-Te deposit represents a magmatic end-member style of VMS deposit in the SD. The distinct hydrothermal alteration conditions and mineralization styles at the Åkulla Au-Te deposit have implications for exploration models in the SD and other VMS districts.