Equilibrium Cu isotope fractionation in copper minerals: a first-principles study

Abstract

Copper is of importance in geosciences, biosciences, and industry. Its isotopes can be used to study planetary processes, trace Cu biogeochemical cycling, identify the type of Cu-bearing ore minerals, investigate the formation of ore deposits, and explore concealed ore-bodies. However, Cu isotope fractionation parameters of the majority of copper minerals remain poorly known, impeding the application of Cu isotopes as a geochemical tracer. In this study, the first-principles methods are used to compute Cu isotope fractionation parameters of chalcocite (Cu2S), covellite (CuS), villamaninite (CuS2), chalcopyrite (CuFeS2), cubanite (CuFe2S3), bornite (Cu5FeS4), cuprite (Cu2O), malachite (Cu2(CO3)(OH)2), azurite (Cu3(CO3)2(OH)2), antlerite (Cu3(SO4)(OH)4), brochantite (Cu4(SO4)(OH)6), liebauite (Ca3Cu5Si9O26), Cu-bearing enstatite (Mg31/16Cu1/16Si2O6), and native copper (Cu). The reduced partition function ratio of 65Cu/63Cu (103 ln β65−63) decreases in the order of Cu silicates > Cu carbonates > Cu sulfates > Cu oxide > native Cu > Cu sulfides. The103 ln β65−63values of these minerals display a weak correlation with oxidation state and Cu coordination number, while display a strong correlation with the average Cu-S bond lengths in Cu sulfides and the average Cu-O bond lengths in Cu oxysalt minerals with the same Cu coordination number. For Cu2S and Cu5FeS4 polymorphs, the extent of Cu isotope fractionation arising from polymorphic transformations is different. Our calculated results are useful for understanding the enrichment of Cu isotopes in nature. Cu isotope fractionation between sulfides and silicates may be a potential geothermometer.