Petrography, geochemistry and genesis of the Skiftesmyr Cu-Zn VMS deposit, Grong, Norway

Abstract

The Skiftesmyr Cu-Zn VMS-deposit is located in the Grong municipality of Northern Trøndelag, Norway. The mineralization has been known since at least 1903, when mention of small workings in the area were first published, and has later been the subject of several exploration projects by different companies, of which MetPro AS is the latest. The Skiftesmyr deposit is a part of the Gjersvik Nappe, which is a part of the Köli Nappe Complex, which in turn is a part of the Upper Allochthon of the Scandinavian Caledonides, and is likely of Mid- Ordovician age. Petrographic and geochemical studies were carried out on samples from trenches cutting across the mineralized horizons. Geochemical analyses performed on the igneous rocks suggest that the deposit formed in a back-arc environment in relation to a relatively mature (compared to e.g. the Skorovas and Gjersvik deposits) island arc. In terms of lithological associations Skiftesmyr can be classified as bimodal-mafic and/or maficdominated, depending on the interpretation of the available geochemical data, and has several traits in common with both the Noranda deposits, Quebec, and the deposits of the Troodos Massif, Cyprus. The dominant ore minerals are chalcopyrite and sphalerite, with only minor amounts of Cu-enriched sulphides associated with supergene enrichment processes present.