Silver in the copper minerals of the Narodinsky uranium-copper occurrence (Subpolar Urals)

Abstract

Introduction: In the Kozhim region of the Subpolar Urals, copper minerals are widespread in rocks of various genesis, composition, and age. In all manifestations of copper mineralization, the host rocks are characterized by an increased content of silver. This study aimed to establish the typomorphic and geochemical features of silver-bearing sulphides and copper oxides. Methods: The morphological features and chemical composition of copper and silver minerals from the Narodinsky uranium-copper ore occurrence in the zone of contact between basement igneous rocks and metaterrigenous Lower Ordovician deposits were studied by EPMA (electron probe microanalysis). Results and discussion: The main silver-concentrating mineral is silver-bearing covellite, previously unknown in the Subpolar Urals. Silver also forms microinclusions of acanthite in malachite and langite. We suggested that in this case, a physicochemical model for the formation of supergene silver minerals with the participation of alkaline "granodiorite" or "basalt" waters formed by the interaction of atmospheric waters with the corresponding rocks could occur. The staging of mineral formation is well illustrated by an example of a sample in which the original chalcopyrite with an insignificant content of silver was transformed into covellite and malachite as a result of oxidation in the hypergenesis zone. In close to the surface and fractured areas of the grains, transformations led to the release of silver and the appearance of acanthite and native silver. Conclusions: The forms of occurrence of silver in copper minerals of the Narodinsky uranium-copper ore occurrence in the zone of contact between magmatic rocks of the basement and metaterrigenous Lower Ordovician deposits have been established. Silver-bearing covelline was discovered for the first time in the Subpolar Urals. It has been suggested that silver-bearing covelline and silver sulphides were formed as a result of its release when copper sulphides were replaced by malachite and langite as a result of hypergene oxidation.