Native copper formation associated with serpentinization in the Cheshmeh-Bid ophiolite massif (Southern Iran)

In the Cheshmeh-Bid district of the Khajeh-Jamali ophiolitic massifs (Southern Iran), mantle peridotites are intruded by abundant pyroxenite dykes. A few of these dykes are remarkable for the occurrence of native copper associated with the development of a metasomatic reaction zone. The dykes are progressively reacted, from their margins towards the center, with an amphibole + antigorite selvage, followed by a centimeter-thick clinopyroxene + antigorite assemblage and, finally, by the native copper-bearing zone consisting of clinopyroxene + chlorite + antigorite. Native Cu occurs along cleavages and partially healed fractures in clinopyroxene, and as massive grains intergrown with antigorite. Copper isotope signatures and thermodynamic calculations show that the main driver for reaction zone formation is Ca-metasomatism. Native copper forms at the expense of chalcocite in the reaction zone. Such a reaction can only occur in reducing conditions, in agreement with the analysis of fluid inclusions composition displaying H and CH. Such fluids presumably originated from the hydration of mantle rocks. The observed reaction zone and native copper mineralization are thus interpreted as the result of Ca-metasomatism during hydrothermal alteration of the oceanic lithosphere. This is consistent with U/Pb dating of titanite, suggesting formation during the Albian when the dykes were exposed on the seafloor in a supra-subduction setting. The source for copper mineralization, as revealed by Cu isotopes, is probably mantle-like.We would like to thank the two anonymous reviewers, and the editor Marco Scambelluri for their constructive and valuable comments and suggestions on our manuscript. Valentina Batanova and Valérie Magnin (ISTerre) are thanked for their help during the electron microprobe measurements. We would like to express our sincere gratitude to local people in Khajeh-Jamali village and workers from the Fars Chromite Company. The work of L. E. Aradi and Cs. Szabó was supported by the ELTE Institutional Excellence Program (TKP2020-IKA-05) managed by the Hungarian Ministry of Human Capacities. The work of G. Grieco was supported by the Italian Ministry of Education (MIUR) through the projects “PRIN2017 - Mineral reactivity, a key to understand large-scale processes” (2017L83S77) and “Dipartimenti di Eccellenza 2018-2022”. C.M.´s research has been funded by project PID2019-111715GB-I00 / AEI / 10.13039/501100011033