Facies analysis of a 1.9 Ga, continental margin, back-arc, felsic caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden

Abstract

The Bergslagen mining district in south-central Sweden has been a major metal producer for more than 1,000 years. In geologic terms, Bergslagen is the intensely mineralized part of a large, Early Proterozoic (mainly 1.90-1.87 Ga), felsic magmatic region of mainly medium to high metamorphic grade in the Baltic Shield. The district contains a diverse range of ore deposit types, including banded iron-formation, magnetitecalc-silicate skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and strata-bound Zn-Pb-Ag-(Cu-Au) sulfide ores, and W skarn. Most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns.This study is a field-oriented, regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration, deformation, and metamorphism. The region is interpreted as an extensional, probably back-arc, active continental margin magmatic region. It evolved through stages of intense magmatism, thermal doming, and crustal extension, followed by waning extension, waning volcanism, thermal subsidence, reversal from extension to compressional deformation, regional metamorphism, and structural inversion. The volcanic successions comprise interfingering proximal (near vent), medial (volcano flanks), and distal (volcano margin) facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite-rhyolite complexes. Nonwelded to poorly welded pyroclastic flow and fallout units, and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies. Subvolcanic porphyritic intrusions and cryptodomes are also abundant. The pyroclastic debris was shed from mainly shallow-water and subaerial proximal areas to shallow- and moderately deep-water (above and below storm wave base) distal areas.Most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash-siltstone, limestone, and vitric crystal sandstone and breccia. These mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base, but locally above the storm wave base. Water depths are interpreted to have been mainly 10 to 500 m, with local deeper areas. Although the ore deposits occur in medial to distal facies associations, many occur above, or laterally adjacent to, subsided volcanic vent complexes. This suggests a spatial, temporal, and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers. Except for the apatite-bearing iron deposits, it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes. This is possible, especially for the base metal sulfide deposits, which have intense localized footwall alteration in felsic volcanic rocks. However, an important role of the volcanic centers may have been to provide strong positive pertubations (+ or - magmatic hydrothermal input) to the regional convective geothermal system.The main Zn-Pb-Ag-(Cu-Au) ore deposits span a range between two end-member types: stratiform ash-siltstone-hosted sea-floor deposits that have similarities to some stratiform sediment-hosted Zn-Pb-Ag deposits, especially the Broken Hill-type deposits; and strata-bound volcanic-associated limestone-skarn subsea-floor replacement deposits that more closely resemble felsic volcanic-associated massive sulfide deposits. Facies models are presented for both. The banded iron-formations are interpreted as chemical suspension deposits, rhythmically interbedded with rhyolitic ash-siltstone. The skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements. Apatite-bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite-rhyolite volcanic center.