Reconstructing the architecture of highly deformed and metamorphosed Zn-Cu massive sulphide deposits in the Vihanti-Pyhäsalmi district, central Finland

Abstract

Highly deformed and metamorphosed Zn-Cu deposits in the Vihanti-Pyhäsalmi district of central Finland preserve the geochemical signature of pre-metamorphic seafloor alteration and are thus considered as volcanogenic massive sulphide (VMS) deposits. The distribution and chemistry of alteration associated with several deposits (Kangasjärvi, Ruostesuo) constrains an overall stratigraphic-hydrothermal framework. Massive sulphide deposits are hosted by chlorite-sericite altered rhyolites (biotite-garnet-orthoamphibole assemblages) associated with ca. 1.93-1.91 Ga bimodal volcanic sequences. On the mine-scale, alteration of these rocks is contiguous with chloritic stockwork zones (orthoamphibole-cordierite assemblages; depleted δ18O values) developed in adjacent mafic volcanic rocks which define hydrothermal upflow zones within the stratigraphic footwall. Local epidotization and metal leaching of basal basalts and andesites suggests that these rocks were located within high-temperature (>400°C) hydrothermal reaction zones, likely in the immediate hanging wall of felsic subvolcanic intrusions. On a regional scale, low-temperature, semi-conformable alteration of felsic volcanic rocks (alkali exchange; enriched δ18O values) reflects broad areas of hydrothermal recharge and indicate the upper levels of stratigraphy. The above stratigraphic model can be used to guide lithogeochemistry-based exploration strategies in the district and also provides a framework to gauge and test the significance of components within the stratigraphy (e.g., felsic intrusions, high-silica rhyolites) and their fundamental relationship with mineralization.