Geology and Origin of the Cu-sulphide Ores in the Tynset-Alvdal region, southern Scandinavian Caledonides.

Abstract

The Trondsvangen-Baugsberget Cu-bearing ore deposits are located in the Tynset-Alvdal region, Hedmark county, central Norway. The earliest documented work in these mines are from the late 17th century, with extensive mining between 1880 and 1911. The ore deposits are located within volcanic and metasedimentary sequences of the Lower Seve Nappes of the Middle Allochthon, Scandinavian Caledonides. The ore-bearing rocks are tectonostratigraphically located above the Tännäs Augen Gneiss, lowermost Middle Allochthon, and beneath the ultramafic pods that form an extension of the Vågåmo Ophiolite, in the lowest parts of the Köli Nappes.

Based on the petrographical and field observations the host rocks are greenschist and mica schist, with the greenschist located on top. However, the stratigraphic polarity was never determined and the entire sequence may be inverted. The geochemical investigation revealed that the greenschist originated as ocean-floor basalts with a tholeiitic signature associated with a mid-ocean ridge, and the mica schist as a greywacke deposited in a continent-margin environment sea. The altered equivalents show both a depletion and enrichment in the mobile elements, indicating hydrothermal alteration.

The greenschist-hosted ores represent a chalcopyrite-sphalerite-bearing pyrite-rich massive ore type, while the mica schist-hosted ores represent a chalcopyrite-sphalerite-galena pyrrhotite-rich irregular and semi-massive ore type. The stratigraphy is proposed to be inverted, with the mica schist formed on top of the greenschist, which yields similarities with the Cyprus VMS type for the greenschist-hosted ores and with the Escanaba and/or subsea-floor replacement VMS types for the mica schist-hosted ores, where the latter may be the most similar. The entire environment show similarities with the present Red Sea in terms of tectonic setting, magmatism and sedimentation.

The host rocks are believed to have formed during the early rifting and opening up of the Iapetus Ocean during the Ediacaran in the late Precambrian. The ore-bearing sequences at Tronsvangen-Baugsberget may thus be the oldest and lowermost Cu-bearing VMS ore deposits within the Scandinavian Caledonides.