Origin of Precambrian copper sulfides of the Outokumpu District, Finland

Abstract

The Outokumpu region is an integral part of the Precambrian formations of the Karelidic orogeny. The copper deposits discovered in the western part of Karelian mica schist area are associated with a lithologic complex that regularly consists of black schists, serpentinites, and siliceous quartz-rich rocks. This rock association makes up the coherent stratigraphic sequence, known as the Outokumpu zone, a prerequisite for the occurrence of Outokumpu-type ores.The lithological and structural setting, together with the high nickel content of the serpentinite-siliceous rock association, indicates a genetic relation between its components. The location of this rock association in a specific stratigraphic position corroborates the concept that they were emplaced concurrently with sedimentation.The copper deposits show clear stratigraphic affiliation with the enclosing metasedimentary rocks of the Outokumpu sequence. The stratified structure and systematic relationship between sulfur isotope ratios and stratigraphy in the Outokumpu ore suggest that the sulfides were deposited during sedimentation of the siliceous host rock. The conformable alteration of carbonaceous intercalations with the siliceous host and country rock of the Outokumpu ores indicates a common depositional environment. Several lithological and geochemical features of the carbonaceous schists in the Outokumpu district indicate that they were deposited under stagnant marine conditions.The simple mineralogy and high cobalt content of the ore suggest volcanic origin. Sulfur isotopic evidence and the compatibility between the radiometric ages of the Outokumpu galenas, metavolcanics resulting from Precambrian volcanic activity, and black schists support the notion that the Outokumpu stratiform copper ores were formed under conditions characterized by submarine volcanism and a restricted environment of sedimentation.The Outokumpu-type sulfide deposits seem to be the Precambrian equivalent of more recent massive pyrite deposits of sedimentary-volcanogenic origin that differ in location and geologic ages. The ores underwent regional metamorphism which modified the structure and composition of the ores and their surroundings.