Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc, Skellefte District, Sweden

The Skellefte mining district occurs in an Early Proterozoic, mainly 1.90-1.87 Ga (Svecofennian) magmatic province of low to medium metamorphic grade in the Baltic Shield in northern Sweden. The district contains over 85 pyritic Zn-Cu-Au-Ag massive sulfide deposits and a few vein Au deposits and subeconomic porphyry Cu-Au-Mo deposits. The massive sulfide deposits mainly occur within, and especially along the top of, a regional felsic-dominant volcanic unit attributed to a stage of intense, extensional, continental margin arc volcanism. From facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow-water areas, surrounded by deeper seas. All the major massive sulfide ores occur in below-wave base facies associations; however, some ores occur close to stratigraphic intervals of above-wave base facies associations, and the summits of some volcanoes that host massive sulfides emerged above sea level. Intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential uplift and subsidence resulting in a complex horst and graben paleogeography. Uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels. A few massive sulfide ores formed within the basal strata of this second stage. The horst and graben system was filled by prograding fluvial-deltaic sediments and mainly mafic lavas, and during this stage the Skellefte district was a transitional area between renewed arc volcanism of more continental character to the north, and subsidence and basinal mudstone-turbidite sedimentation to the south. This whole volcanotectonic cycle occurred within 10 to 15 m.y. We define 26 main volcanic, sedimentary, and intrusive facies in the Skellefte district. The most abundant facies are (1) normal-graded pumiceous breccias, which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris, (2) porphyritic intrusions, and (3) mudstone and sandstone turbidites. Facies associations define seven main volcano types, which range from basaltic shields to andesite cones and rhyolite calderas. Despite this diversity of volcano types, most massive sulfide ores are associated with one volcano type: subaqueous rhyolite cryptodome-tuff volcanoes. These rhyolite volcanoes are 2 to 10 km in diameter, 250 to 1,200 m thick at the center, and are characterized by a small to moderate volume rhyolitic pyroclastic unit, intruded by rhyolite cryptodomes, sills, and dikes. Massive sulfide ores occur near the top of the proximal (near vent) facies association. The remarkable coincidence in space and time between the ores and this volcano type indicates an intimate, genetic relationship between the ores and the magmatic evolution of the volcanoes.Many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies. Some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits. We suggest that massive sulfides in the Skellefte district span a range in ore deposit style from deep-water sea-floor ores, to subsea-floor replacements, to shallow-water and possibly subaerial synvolcanic replacements. Facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes.