Stratabound copper sulfides in a banded iron-formation and in basaltic tuffs in the early Precambrian Isua supracrustal belt, West Greenland

Abstract

In the pre-3,760-m.y.-old Isua supracrustal belt, West Greenland, copper sulfides with minor amounts of pyrrhotite are found in sulfide, silicate, and carbonate facies iron-formation. In oxide facies iron-formation pyrite is the sole sulfide, occurring in small quantities. In metamorphosed basaltic tuffs thin layers of chalcopyrite and pyrrhotite have been found. Metamorphism to lower amphibolite facies resulted in formation of high-temperature chalcopyrite-cubanite crystals. Subsequent cooling resulted in unmixing and separation of the chalcopyrite and cubanite as lamellae, which in some localities occur together with spindles of pyrrhotite. Lead isotopic ratios in galenas show that the sulfides are coeval with the supracrustals. The sulfides in the iron-formation and in the tuffaceous amphibolites are considered to be of submarine exhalative origin.