Stratiform Zn-Pb-Fe-Mn mineralization in the Alvlangen- Vikern area, Bergslagen, Sweden

Abstract

The stratiform Zn-Pb sulphide mineralization of the Älvlången-Vikern area, central Sweden, is contained in an iron-rich meta-sedimentary unit at the base of a sequence of dolomitic marble and metachert of Mid-Proterozoic age, deposited in a fault-bounded sedimentary basin. This basin probably developed during a phase of extensional tectonics. The mineralized unit is a steeply south-easterly dipping zone of 6 km long, extending from Lake Vikern in the NE to Lake Älvlången in the SW. Sphalerite, galena, magnetite and minor arsenopyrite, pyrite, pyrrhotite, chalcopyrite and marcasite occur in fine-grained, well banded and laminated garnet-biotite-cummingtonite-tourmaline-rich rocks of presumably sedimentary-exhalative origin. Characteristic are the small quantities of scapolite and orthite, and the high Ba and C1 contents of various micas and amphiboles. Apart from Zn, Pb, Fe, Mn, Ba and Cl the ore zone is enriched in As, Sb, Ag, Ba, Th, U, Ti, Cr, Co, Ni and V. A distinct metal (Zn + Pb à Fe + Mn + Ba) and mineral (sulphides + tourmaline-+ oxides + cummingtonite) zonation has been observed along the ore zone from the centre, where Zn and Pb contents are highest, to the margin of the basin. The marbles just below the ore zone are locally brecciated, showing network veins with phlogopite, tourmaline and traces of chalcopyrite and pyrrhotite, and corroded rhyolitic fragments. This brecciation of marbles is associated with extreme calc-silicate alteration of intercalated metatuffites and metacherts along veins and fractures. The breccias and calc-silicate alteration, occurring in narrow zones following the fault-bounded basin, are considered to represent hydrothermal feeder channels. The metapyroclastic rocks below the ore-hosting marbles are albitized, microclinized, sericitized, biotitized, calc-silicate altered, silicified, and tourmalinized. Of these, the first three are regional alteration types, presumably not significantly related to the ore-forming processes, whereas the latter four are localized, ore-related alteration types, occurring mostly along network veins and fractures. Tourmaline is a characteristic, but minor, constituent of all rock types below and within the ore zone. The highest concentrations occur in and below the central part of the basin, where the highest sulphide contents occur. It is absent in rocks overlying the ore zone, suggesting tourmaline to be a mineralogical exploration guide. Circulation of the hydrothermal fluids is thought to have initiated during rifting and driven by a high geothermal gradient beneath the depression. This high geothermal gradient may partially be attributed to a hidden felsic magma plug below the ore zone, as indicated by the rhyolitic fragments in the hydrothermal marble breccias.