Abstract Complex breccia host rocks, quartz-topaz alumina-rich alteration, and low-sulfide ore (3%) differentiate the Åkulla Au-Te deposit from nearby volcanogenic massive sulfide (VMS) deposits, which are the main deposit type in the Skellefte district. At Åkulla, mineralization comprises native gold and telluride minerals in disseminated, remobilized fracture-associated, quartz vein–associated, and sulfide-associated styles within a steeply dipping lens-shaped domain of strongly altered 1.9 Ga dacitic to andesitic volcanic rocks. The host rocks and mineralization are flattened and elongated subparallel to the main D2 tectonic foliation and lineation and are metamorphosed to upper greenschist facies. The deposit has an almost concentrically zoned hydrothermal alteration pattern with a quartz-topaz–rich core that is centered on a breccia complex. Topaz occurs as discrete altered clasts in the breccias as well as pervasive to patchy alteration. The quartz-topaz-sericite-andalusite core is poor in sulfide and passes outward to sericite-andalusite ± corundum ± quartz ± chlorite associations, then to sericite and finally chlorite. Stringer veins and small semimassive to massive base metal sulfide lenses occur in the alteration zones outside the topaz core, continue ~300 m updip to the Åkulla Östra massive sulfide deposit, and resemble footwall alteration and mineralization associated with nearby VMS deposits. Four main breccia facies are identified based on textural characteristics: monomict jigsaw-fit breccia; poorly sorted, dominantly polymict breccia with rotated clasts; moderately sorted matrix-supported polymict breccia; and well-sorted breccias with graded bedding. These breccias are attributed to quench and hydrothermal brecciation in a sea-floor to subseafloor setting during emplacement of a dacitic-andesitic volcanic complex. Clast size grading within breccia facies with topaz-rich clasts is dominated by fining to the northeast, the same direction as in nearby volcaniclastic sedimentary rocks, indicating that these breccias were emplaced while the strata were flat lying or gently dipping. This constrains the formation of the topaz-rich breccias and Au-Te mineralization to the early synvolcanic evolution of the succession, possibly prior to or during rotation of strata in extensional fault blocks. The topaz- and alumina-rich alteration of the breccias is attributed to acidic, F-rich, probably magmatic-hydrothermal fluids. The Åkulla Au-Te deposit represents unusual, F-rich, subseafloor synvolcanic mineralization within a larger VMS mineral system. The relationship we demonstrate between VMS mineralization, sulfide-poor Au-Te mineralization, complex breccias, and magmatic ore fluids at Åkulla will modify exploration models for VMS mineralization in the Skellefte district and should be considered in other VMS districts.
Nordfeldt et al. (Mon,) studied this question.