The Åkulla Au-Te deposit is located in the Skellefte District (SD), Northern Sweden, a world-class mineral district with abundant volcanic massive sulfide (VMS) deposits. The deposit is one of 4 active mines in SD, 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. 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 strata. The hydrothermal alteration is strongly zoned with a quartz-topaz±sericite±andalusite core, surrounded by sericite-quartz±chlorite±andalusite±sulfide association with minor occurrences of corundum in sericite-chlorite rock, followed outwards by sericite±andalusite±sulfide and chlorite±sulfide associations. The outer sulfide-bearing alteration zones continue to surface to the small Åkulla Östra semi-massive sulfide body. The mineralization and host rocks have been metamorphosed to upper greenschist facies, are flattered and elongated sub-parallel to the D2 foliation and lineation, indicating their formation pre-deformation. The Åkulla Au-Te deposit differs from the VMS deposits in SD by being sulfide poor (<3%), native Au and Te enriched.

The four main breccia facies hosting 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 several elements (major, chalcophile and some HFS elements) but enriched in Si, Ag, Bi, Sb and Mo. The corundum occurrences correlate with Si depletion and Al, Ti, Zr, Nb enrichment. Above and surrounding the central core, a zone of Fe, S, Cu, Zn, Ni and Pb enrichment occurs (Paper III). This pattern reflects changes 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 with 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, abundance of topaz, zonation of alteration minerals and chemical 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 should have implications for exploration models in the SD and other VMS districts.