The metasediments associated with stratabound base metal mineralization, Ljusnarsberg District, Central Sweden

Abstract

An extensive suite of exhalites of sub-economic potential occurs in a succession of Lower Proterozoic quartzofeldspathic gneisses in the Ljusnarsberg district, central Sweden. Previously economic varieties include Fe-Mn oxides, silicates, and carbonates, Cu-Pb-Znbearing and W bearing rocks all of which have been mined in the past. The stratigraphic succession of the Ljusnarsberg-Ställdalen area is conformable and changes from a lower metavolcaniclastic pile of >5 km thickness (the Kumlan Group) to a mixed metavolcanic and metasedimentary sequence (the Ställdalen Group). To the east the mixed metavolcanic and metasedimentary succession (the Wigström Group) is thought to be stratigraphically equivalent to the Upper Kumlan Group and Lower Ställdalen Group. The Kumlan Group is dominated by primary rhyodacitic volcaniclastic material partly subaerial. The rapid decline in volcanism is coeval with an increase in the sedimentary component and marks the conformable contact of the upper and lower groups. The dominantly stratabound mineralization is concentrated in two main ore horizons in the upper group: (1) a lower Cu-Pb-Zn sulphide-rich horizon with some magnetite and (2) an upper Fe-Mn enrichted horizon. The ore deposits are spatially associated with the metamorphic equivalents of a variety of clastic and chemical sediments including quartz-feldspar-biotite rocks, with minor calcite, hornblende, garnet, pyroxene, epidote and magnetite; quartz-K feldspar-plagioclase-garnet-amphibole-pyroxene-calcite-epidote- biotite rocks with minor chlorite, apatite and sphene; and pyroxene-epidote- K feldspar rocks. The varied chemistry of both ores and barren chemical sediments suggests the palaeoenvironment was continental with minor basinal development. Composition and pathways of the circulating fluids were restricted causing sharp changes in chemical precipitation. Clastic sediments are not common and reflect a low energy environment starved of sediment. Subsidence seems to have been the overriding tectonic trend.