Cu and S isotopic signatures of the Erdenetiin Ovoo porphyry Cu-Mo deposit, northern Mongolia: Implications for their origin and mineral exploration

Abstract

The copper (δ65Cu) and sulfur (δ34S) isotope compositions of ore minerals from the Erdenetiin Ovoo porphyry Cu-Mo deposit in northern Mongolia were measured. The δ65Cu values of Cu (I) sulfide minerals ranged from 0.14‰ to 2.69‰, suggesting that Cu predominantly originated from magmatic sources, whereas Cu (II) minerals such as chrysocolla, malachite and azurite presented much larger variations of δ65Cu values from −1.01‰ to 10.0‰. The small difference between the primary and secondary Cu sulfide minerals indicates an insignificant influence of Cu isotope fractionation processes during their formation, which may be explained by large mass transport and/or the involvement of biogenic activities. The δ65Cu values of primary chalcopyrite suggest source heterogeneity and/or the occurrence of isotope fractionation under a high-temperature environment. The positive ΔCu (II) mineral − Cu (I) mineral values imply little transport of Cu in the deposit, with a rough mass balance and fast redox reaction.The δ34S values of the primary sulfide minerals (pyrite, molybdenite and chalcopyrite) clustered near 0‰, indicating that the sulfur is mainly derived from a homogeneous magmatic source. By contrast, the δ34S values of secondary Cu sulfide minerals ranged from −3.2‰ to −0.3‰, with an average of −1.6‰. The lower δ34S values are likely influenced by either S isotope fractionation processes or input of sulfur with different S isotope compositions during their formation.The measured δ65Cu and δ34S values of these ore minerals suggest a large mass transportation of Cu to an adjacent location, indicating little possibility of a hidden Cu occurrence in the Erdenetiin Ovoo deposit area.