The geochemistry of early Proterozoic metavolcanic rocks hosting massive sulphide deposits in the Skellefte district, northern Sweden

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Abstract
The Skellefte district, an early Proterozoic massive sulphide ore province in Northern Sweden, contains a well-preserved volcano-sedimentary rock association, referred to as the Skellefte Group, metamorphosed predominantly under low-grade conditions. The volcanic rocks in the lower part of the succession host the massive sulphide deposits and comprise a bimodal sequence, composed of a calc-alkaline group of felsic rocks and a tholeiitic group of mafic rocks displaying a basaltic to andesitic composition. Derivation of the felsic rocks by fractional crystallization from the mafic rocks is not supported by the geochemical data. Application of tectonic environment discriminant diagrams involving the elements Ti, Zr, Y and Cr suggests that these rocks were deposited in a volcanic arc setting. Stratigraphically above the volcanic rocks hosting the massive sulphide deposits, there follows a sequence of metamorphosed greywackes and pelitic sediments with intercalation of primitive basaltic to ultrabasic volcanic and high-level intrusive rocks. The volcanic rocks show geochemical similarities to both Archaean basaltic komatiites and Phanerozoic boninites. The primitive nature of these rocks present difficulties in assessing their palaeotectonic setting. A volcanic arc or transition to an inter- or back-arc rift situation is suggested.