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Early Proterozoic Volcanic Suites of the Baltic Shield |
Institute of Materials, University College, Newport Road, Cardiff CF2 1TA, UK
Broken Hill-type mineralization is characterized by abundant exhalites in a sulphur-poor, felsic volcanic environment. It is typically Proterozoic. The Ljusnarsberg area in central Sweden is a microcosm of this mineralization type. One part of the amphibolite-grade, supracrustal succession is characterized by a thick sequence of subaerial volcaniclastics which grade into diverse chemical sediments interpreted as exhalities containing sulphide, iron oxide and manganese mineralization. The environment is interpreted as terrestrial with frequent lakes which are fed by volcanic exhalations. Evaporites may also have been present. The non-marine environment may not only have helped determine the nature of the mineralization in the area but also affected the chemistry of the volcanic host rocks. Thus, the occurrence of potassic and sodic metavolcanics may be related to the effects on hydrothermal solution chemistry and water-rock ratios of non-marine surface conditions, as well as alteration temperature and primary volcanic chemistry.
