Geology and geochemistry of an early Proterozoic volcanic arc sequence at Kristineberg, Skellefte district, Sweden

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Abstract
In the Kristineberg area, early Proterozoic volcanic rocks underlie a sedimentary sequence, and base-metal massive sulfide deposits are intercalated within the volcanics. The volcanic rocks are bimodal with abundant rhyolites and a subordinate population of mafic rocks ranging in composition from andesite to basalt. The petrography and field relationships suggest that most of the volcanic activity was submarine. The geochemistry of the volcanic rocks is similar to those volcanic rocks formed at subduction-related destructive plate margins, especially to volcanics from thin continental margins or island arcs developed on continental crust. The bimodal character of the volcanic sequence and the large amount of pyroclastic material are consistent with a tensional stress regime during the deposition of the volcanics. The initial extrusion of a large amount of felsic pyroclastic rocks and synvolcanic faulting resulted in development of large volcano-tectonic depressions with subsidence of the volcanic pile. Synvolcanic regional hydrothermal alteration produced the general spilitization of the volcanic rocks. Na-enrichment is characterized by depletion of K, Mg, Rb, Ba, Cr, Zn and Pb, whereas Ti, Y, Sc, Zr and in part REE behaved immobile. Ni and Cu are depleted in the Na-enriched andesite, but only slightly changed in the Na-enriched rhyolite. At the waning stage of the volcanism, massive sulfide deposits were formed by venting hydrothermal solutions on the sea-floor within local volcano-tectonic depressions.