Sulfides of the Copper‒Sulfur System as Indicators of the Conditions of Formation and Transformation of Cupriferous Sandstone and Shale Deposits

This paper examines cupriferous sandstones and shales hosted in sediments of varying ages (from P2 to PR2) and altered to different degrees (from early catagenesis to the epidote–amphibolite stage). The formation of these rocks is associated with the removal of Cu and other metals by formation waters from the red-colored ore-hosting rocks and their precipitation as sulfides on hydrosulfuric biogeochemical barriers. After their formation, cupriferous sandstones and shales can undergo metamorphic and supergene alterations, which are reflected in the crystallochemical properties of sulfide minerals, the composition of their paragenetic assemblages, the mineral zoning of mineralization, the morphology of ore bodies, and other features. This paper focuses on sulfides of the Cu—S system formed under various conditions. Their composition, structure, physicochemical and thermobaric stability, and occurrence in natural ores are discussed. The composition of copper sulfide ores in cupriferous sandstones, shales, and hypergenesis zones is elucidated. In particular, the paper explains why only nonstoichiometric Cu‒S sulfides (djurleite, geerite, spionkopite, yarrowite) are present in unaltered cupriferous sandstone ores and hypergenesis zones, whereas stoichiometric copper sulfides (chalcocite, digenite, and tetragonal phase) occur in metamorphosed ores. This fact is explained from a physical and chemical perspective. It has been established that each stage of rock formation and transformation is characterized by specific typomorphic minerals of the Cu‒S system and their parageneses that are stable under the conditions of a given stage. With increasing metamorphic grade, the mineral composition of ores and mineral zoning of the deposits change consistently toward simplification due to the gradual disappearance of low-temperature compounds and associations. Indicator minerals of the Cu‒S system, reflecting the thermobaric and chemical conditions of ore formation and transformation, have been identified.