Abstract

The Skellefte district in northern Sweden comprises more than 85 pyritic volcanic-hosted massive sulfide deposits which mainly occur within, and at the top of, a felsic-dominated volcanic unit overlain by a sedimentary sequence. The Boliden Au-Cu-As deposit was one of the first discovered in the district, and it has attracted a continuous interest since then due to its significant size and high gold grade (avg 15 ppm). The Boliden ore can be divided into massive ore, with arsenopyrite- and pyrite-dominated lenses, and vein ore which comprises a quartz-chalcopyrite-sulfosalt-dominated assemblage, occurring in brecciated parts of the arsenopyrite bodies, and quartz-tourmaline veins mainly in host rocks below the massive ore. As a rule, the gold is found in deformational structures in vein ore. Most gold is present as an Au-Ag-Hg alloy with variable compositions, from Au (sub 0.17) Ag (sub 0.68) Hg (sub 0.16) to Au (sub 0.93) Ag (sub 0.07) (in atomic proportions).For the last two decades, the approximately 1.88 Ga massive sulfide ores in the Skellefte district have collectively been interpreted as volcanic exhalative formations resembling the Miocene kuroko ores of Japan. However, this view has recently been challenged and a subsurface replacement origin has been proposed for some of the ores in the district.The Boliden ore is not bound to one particular host rock but occurs in feldspar porphyritic dacite, quartz porphyry, and basalt-andesite. Textural observations suggest that these rocks represent intrusions or lavas. Geochemically, they are typical calc-alkaline volcanic rocks, enriched in large ion lithophile elements, depleted in heavy rare earth elements, and with troughs for Th, Nb, Hf, and Ti. The ore zone, in its present setting, is in a more or less vertical position and oblique to lithological contacts. Ore-related hydrothermal and regional metamorphic processes (lower amphibolite facies) have created a complex alteration system around the ore. This forms a symmetric pattern with an inner sericite-rich zone, locally containing abundant andalusite, and an outer chlorite-dominated zone. The nature of the alteration is consistent with leaching of elements and a silica-alumina-rich residue--features which are often found in epithermal environments.Structural observations suggest that three ductile foliation-forming events have affected the rocks near the ore. These include a regional S 1 foliation, formed during isoclinal folding, which was subsequently sheared causing formation of a strong cleavage S s and extensive deformation of the ore itself. A late S 2 cleavage crenulated earlier fabrics.The available data and observations are not consistent with a volcanic exhalative model for the ore and the following scenario is favored. Shallow intrusions of dacite and andesite into unlithified sediments occurred around 1.87 Ga. At this time, the earlier marine environment had been lifted up to a shallow-marine or possibly subaerial position. Shortly thereafter, fluids which generated the massive ore at Boliden were focused along a fault, and arsenopyrite and pyrite lenses were precipitated in more than one host rock discordantly to lithological contacts. Regional deformation with folding and shearing, possibly at around 1.85 Ga, led to brecciation of previously formed ores and stretching of orebodies. In relation to this shearing event, Au was introduced and/or remobilized and concentrated in brecciated portions of the ore zone. Thereafter, ores and host rocks recrystallized during peak metamorphism at around 1.82 Ga, and a second deformation at around 1.80 Ga caused crenulation of early fabrics.The crosscutting nature of the ore with respect to the host rocks, the hydrothermal alteration pattern with strongly leached host rocks, and the ore association with early massive sulfides followed by gold, chalcopyrite, and sulfosalts in brittle structures all indicate that a modern analogue for ore formation may be a high-sulfidation epithermal environment. The epigenetic nature of the Boliden deposit has significant implications for exploration of gold deposits elsewhere in the region.

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