Weathering of Copper Deposits and Copper Mobility: Mineralogy, Geochemical Stratigraphy, and Exploration Implications

Abstract

Abstract Weathering of pyrite and copper sulfide-bearing rocks produces a consistent series of iron- and copper-bearing minerals that reflect vertical and lateral geochemical changes as supergene solutions react with rock and experience loss of oxidizing capacity. Reactive host rocks, comprising feldspars, mafic minerals, chlorite, and carbonates, buffer pH values that limit copper mineral destruction, thus restricting the supergene transport of copper. Generally, rocks that have undergone well-developed hypogene or supergene hydrolysis of aluminosilicates promote copper mobility because they do not react substantially with low-pH supergene solutions generated by oxidation of pyrite and associated copper sulfides. Development of geochemical stratigraphy is characterized by physical and geochemical parameters that determine the maturity of a supergene profile, with cyclical leaching and enrichment periods critical for the production of economically significant copper accumulation. Evidence for multicycle enrichment is recorded by hematite after chalcocite, hanging zones of copper oxides that replace chalcocite, and hematitic capping overlying immature goethitic-pyritic capping. Because pyrite is the most refractory sulfide with respect to chalcocite replacement, geochemically strong supergene enrichment is independent of total copper added to protore and instead is indicated qualitatively by the degree to which chalcocite replaces pyrite. Covellite replacement of chalcopyrite indicates weak copper addition to protore and generally represents the base and lateral extent of supergene enrichment; covellite replacement of chalcocite indicates incipient copper removal by copper-impoverished supergene solutions. Exploration for, and delineation of, mature supergene enrichment profiles benefits from the recognition of paleoweathering cycles and consequent development of mature geochemical stratigraphy.