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

摘要

Highly deformed and metamorphosed Zn-Cu deposits in the Vihanti-Pyhaesalmi 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 (Kangasjaervi, 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 δ~(18)O values) developed in adjacent mafic volcanic rocks which define hydrothermal up-flow zones within the stratigraphic footwall. Local epidotization and metal leaching of basal basalts and ande-sites 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 δ~(18)O 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.