Temporal and genetical constraints of the Cu-Co Vena-Dampetorp deposit, Bergslagen, Sweden

Sammanfattning: The Bergslagen Province is known for containing major Zn-Pb-Ag and Zn-Pb-Cu-Ag sulphide mineralizations with associated iron-oxide skarns and a lot of research have focused on understanding them. However, Cu-Co deposits like the Vena-Dampetorp deposit have received less attention. The Vena-Dampetorp deposit is a disseminated and vein Cu-Co mineralization hosted primarily in a 1.89 Ga rhyolitic to dacitic volcanoclastic sequence with thin interbedded calcareous horizons and associated mafic and granodioritic intrusions. Two stages of deformation and regional high-temperature and low-pressure metamorphism affected this shallow subaqueous back-arc assemblage. The aim of this study is to constrain the spatial, temporal and genetical relationships of ore minerals and the effects of metamorphism on the host rocks and their assemblages. Building on existing work, it aims to explore the origin and processes responsible for the concentration of Co and genesis of Cu-Co deposits in the Bergslagen area. Textural relationships and geochemical data of drill cores show that the mineralization is zoned from pyrite-pyrrhotite zones to chalcopyrite-sphalerite richer zones, with localized Cu-Co areas. At the deposit scale, Co rich zones occur in the SE while Pb-Sb enriched zones concentrate in the NW. The U-Pb dating of metamorphic titanite with LA-ICP-MS gave a mean concordant 207Pb/206Pb date of 1882±16 Ma for a skarn sample. This age is interpreted as the onset of regional metamorphism. In contrast, the lower intercept age of 1882±16 Ma obtained for titanite in one metavolcanic sample could either represent the onset of metamorphism or a cooling age. Sulphide thermobarometric estimates, EDS-SEM mapping, and the presence of sulphosalt-rich associations together with deformational features in the ore minerals, indicate that the Fe-oxide mineralization at the Vena-Dampetorp deposit occurred as a Bastnäs-type deposit due to the interaction of magmatic fluid with carbonate horizons. In comparison, the sulphide Cu-Co-rich mineralization was deposited in a sub-seafloor hydrothermal exhalative system, where the higher Cu-Co concentrations are related to high temperature stringers near the feeder zone. Later mechanical, hydrothermal and melt-assisted re-mobilization during metamorphism concentrated certain metals like Co in secondary veins and assemblages.