The tectonostratigraphic setting of stratabound sulphide deposits in the southern Trondheim region, Central Norwegian Caledonides.

Abstract

The large number of mainly massive, polymetallic, stratabound sulphide depositswithin the southern Trondheim region are spatially confined to the differentnappe units of the central Scandinavian Caledonides. The rocks of theparautochthon and Lower Allochthon comprise metasandstones and arkoses whichare virtually barren of any sulphide mineralizations, except for a fewshowings of \"sandstone lead\" disseminations. The middle and lowermost partsof the upper Allochton are dominated by sediment-hosted chalcopyrite\/pyrrhotiteassemblages within calcareous mixed peltic and psammitic rocks with minor maficmetavolcanic and plutonic intercalations. The structurally higher Gaula andStøren units of the Trondheim Nappe complex of the upper Allochthon containchiefly pyrite-dominated assemblages in mafic and mixed mafic\/felsicvolcanogenic settings and have highly variable Cu, Zn and Pb contents. Thestratabound sulphides of the Trondheim region were deposited during successivestages and within restricted sedimentary and volcanosedimentary environmentsrelated to development of the lapetus Ocean. Deposition started in basinsediments during Late Proterozoic rifting, but was mainly the result of earlysubduction and the development of an active island-arc magmatism in the EarlyOrdovician. The structural and stratigraphical sequences were subsequentlyaffected by Caledonian folding and metamorphism prior to and during eastwardtransport as composite nappe units in Silurian times. The postdepositionaltectonic and metamorphic events have affected the different ores and theirwallrocks to various degree as revealed by apparent tectonic control and theinternal metamorphic fabric of the deposits.