Stable isotope (C, O, H), major- and trace element studies on hydrothermal alteration and related ore mineralization in the volcano-sedimentary belt of Bergslagen, Sweden

Abstract

The 1.90 - 1.86 Ga volcano-sedimentary belt of West Bergslagen, central Sweden, is situated in the Svecofennian domain, which forms part of the Baltic Shield. The West Bergslagen belt comprizes more than 10 km of felsic volcanics and over 2 km of volcaniclastic sediments. Carbonate, chert and iron-oxide lenses are intercalated in cthe volcanics. The supracrustal rocks were intruded by diabase dykes and sills and by granitic plutons. The supracrustal rocks have been tilted into a sub-vertical position. Foliation follows approximately the bedding planes, or is developed in schistose shear or fault zones. Lineations are generally steeply dipping and positioned in foliation planes. The rocks of the West Bergslagen belt are metamorphosed to greenschist or lower amphibolite mineral parageneses. Hydrothermal alterations and related ore mineralizations occur in the West Bergslagen belt, and are studied in this thesis. The aims of the study were to understand the working of the hydrothermal systems operative in the Proterozoic crust of West Bergslagen, and include topics such as: hydrothermal fluid genesis, interaction between rock minerals and fluids, mobility of elements in the systems and estimates of mass in/decrease during the processes, and relation of hydrothermal fluids with the different ore forming processes. An important aim was to determine the amount of involvement of sea water in the hydrothermal processes, and to evaluate the Early Proterozoic chemical and isotopic composition of sea water compared with present sea water.