Geology, lithogeochemistry and paleotectonic setting of the host sequence to the Kangasjärvi Zn-Cu deposit, central Finland: Implications for volcanogenic massive sulphide exploration in the Vihanti-Pyhäsalmi district

Abstract

The Kangasjarvi Zn-Cu deposit is a highly deformed and metamorphosed Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located in the Vihanti-Pyhasalmi base metal mining district of central Finland. The host sequence to the deposit, referred to as the Inner Volcanic Sequence (IVS), is comprised of a bimodal suite of metavolcanic rocks and a regionally extensive tonalite-trondhjemite gneiss (sub-volcanic intrusions?). A separate and perhaps younger sequence of mafi c volcanic rocks, with irregular intervals of undifferentiated intermediate to felsic schists and metalimestones, referred to as the Outer Volcanic Sequence (OVS), are separated from the IVS sequence by intervals of metagreywacke and U-P-bearing graphitic schists. A stratigraphic scheme for rocks within the IVS is proposed based on outcrop observations, locally preserved volcanic textures, aspects of seafl oor-related hydrothermal alteration and lithogeochemistry. In this scheme, rare andesites form the lowermost volcanic stratigraphy and are overlain by typical island-arc basalts that were erupted in a subaqueous setting. Tonalite-trondhjemite subvolcanic intrusions were locally emplaced within andesites and coeval rhyolites were extruded on the basaltic substrate. The extrusion of rhyolites, including high-silica rhyolites, was coeval with regional-scale, pre-metamorphic seafloor hydrothermal alteration and local sulphide mineralization. Extensively altered rhyolites envelope massive sulphides and are underlain by altered basalts. The latter rocks are now characterized by a variety of low-variance metamorphic mineral assemblages (e.g. orthoamphibole-cordierite rocks) and define a domain of intense pre-metamorphic chlorite ± sericite alteration in the stratigraphic footwall of the deposit. The altered nature of these rocks is attributed to reaction with seawater-related hydrothermal fluids within a zone of upflow at or near the seafloor. The fundamental controls on convective hydrothermal circulation and subsequent alteration and massive sulphide mineralization at Kangasjarvi, and possibly elsewhere in the district, share many characteristics with other well-described, ancient VMS deposits (e.g. massive sulphide deposits in the Flin Flon Belt, Manitoba, Canada). These characteristics include: 1) an association with bimodal volcanism developed in extensional settings; 2) a close spatial association with regionally extensive felsic subvolcanic intrusions; and 3) petrogenesis of ore-associated volcanic rocks (e.g. high-silica rhyolites, felsic subvolcanic intrusions) indicative of substantial heat transfer from the mantle to the upper crust and the development of anomalous thermal corridors. These features translate into geochemically distinctive rock types that, when combined with aspects of stratigraphy and pre-metamorphic alteration, may be used to develop regional exploration strategies in the Vihanti-Pyhasalmi district.