Mobilization of Cu in the continental lower crust: A perspective from Cu isotopes

Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust. Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits. To test this hypothesis and further explore the behavior of Cu in the lower crust, we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains. The collected samples include hornblendites from the Kohistan arc, granulite xenoliths and hornblendites from the Gangdese arc, hornblendites and gabbros from the Laiyuan complex in the North China Craton, and hornblendite xenoliths from the western margin of the Yangtze Craton. These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation, with significantly varied Cu contents (11.2 to 145 ppm) and δ⁶⁵Cu (−1.05‰ to 1.40‰). Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks, and based on this, they can be further divided into three groups: Group Ⅰ includes the Gangdese granulites and Yunnan hornblendites, which perhaps experienced significant metasomatism. This suite of rocks shows enrichment of ⁶⁵Cu (δ⁶⁵Cu = 0.01‰ to 1.40‰), positively correlated with metasomatism (δ⁶⁵Cu vs. Ce/Pb). We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks, so the feature of heavy isotope enrichment is inherited. Group Ⅱ includes Laiyuan hornblendites and gabbros, derived from the same parental magma and emplaced at different depths (hornblendites, 23.3 – 28.1 km; gabbros 8.4 – 11.1 km). The Cu isotopic compositions are strongly fractionated between these two kinds of rocks, with low δ⁶⁵Cu in the hornblendites (0.00‰ to 0.28‰) and highly polarized δ⁶⁵Cu in the gabbros (−1.05‰ to 0.81‰). Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature. Primary sulfides were decomposed by fluids and reprecipitated at shallower depths. Since this process involves multiple redox reactions, the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated. Group Ⅲ includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction. The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition (−0.09‰ to 0.18‰) and Cu content (83.4 to 128 ppm), suggesting insignificant Cu migration and isotope fractionation. In contrast, the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated (−0.36‰ to 1.27‰). Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation. The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust, resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues. Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.