Genesis of an early Proterozoic zinc deposit in high-grade metamorphic terrane, Saxberget, central Sweden

The Saxberget strata-bound Zn-Pb-Cu sulfide deposit of early Proterozoic age (ca. 1.9 Ga) is located in central Sweden. It is hosted by a supracrustal sequence of quartz-feldspar and quartz-mica gneisses, with minor intercalations of amphibolite, marble, and calc-silicate gneiss (skarn). The area has undergone several metamorphic events subsequent to syndepositional hydrothermal alteration: regional high-temperature and moderate- to low-pressure metamorphism in the upper amphibolite facies up to granite melting; medium-grade, low-pressure metamorphism associated with granite intrusions; and retrograde metamorphism in the greenshist facies probably related to a burial metamorphic event. The metamorphism was essentially isochemical and bulk chemical changes to the host rocks were premetamorphic and probably prelithification.Geochemical and petrographic evidence indicates a volcanic protholith for the gneisses and amphibolite. They are the metamorphic equivalents of mainly felsic pyroclastic rocks, whereas amphibolites are metamorphosed mafic volcanic rocks of basaltic composition. The quartz-mica gneisses represent the metamorphic equivalents of hydrothermally altered felsic volcanic rocks. The intercalation of metavolcanoclastic and metasedimentary rocks (marble, skarn, and reworked volcanics) indicates a shallow-water submarine environment for this ore-bearing sequence. Geochemically the present composition of the metavolcanic rocks displays calc-alkaline affinity, and their bimodal character suggests a tensional environment of deposition.The Zn-Pb-Cu sulfide mineralization at Saxberget is mainly bounded to a calc-silicate horizon which laterally grades into dolomitic marble. The orebodies trend in a general east-west direction and dip 35 degrees to 45 degrees S, with a horizontal extension on the surface of about 500 m and more than 1,500 m in the pitch direction (southeast); the thickness can reach up to 40 m. From the footwall to the hanging-wall side, the ores include stratiform massive ball ore, semi-massive to disseminated strata-bound Zn-Pb ores in calc-silicate gneiss ("skarn ore"), and disseminated Cu-rich ore in quartz-mica gneiss. Large-scale zoning is observed in the mineralization, the massive Zn-Pb-rich ball ore dominates in the western part of the mine and disseminated skarn ore and Cu-rich ore are more common in the eastern part of the mine. The present footwall rocks are banded felsic metavolcanics which are normally less altered and deformed than the hanging-wall rocks. The contact between the footwall rocks and the ores is partly defined by the ball ore, which appears to represent a tectonically deformed sulfide body along a significant thrust. Upward, the ball ore grades into or is in contact with the skarn ore, which shows intense small-scale folding. The hanging wall of the ore-bearing horizon consists of strongly altered metavolcanic and metasedimentary rocks. Geochemically, the hanging-wall rocks are depleted in Na and Ca and are enriched in Mg and Fe, displaying a geochemical zoning relative to the orebodies. This geochemical zoning is mineralogically expressed in the hanging-wall rocks by orthoamphibole-bearing and spinel-bearing assemblages close to the ore and followed outward by the garnet-bearing quartz-mica gneisses which in turn are succeeded by the cordierite-bearing quartz-mica gneisses.The geochemistry and interrelationships of the various petrologies strongly suggest that the stratigraphic sequence at Saxberget is inverted and that the ore represents a proximal exhalite located on a subsurface alteration pipe. The deposit is classified with the Broken Hill type of middle Proterozoic stratiform sulfides, which is represented in the Phanerozoic by the familiar sedimentary exhalative group of stratiform zinc-lead-copper deposits in sedimentary rocks