Abstract
The refertilized mantle wedge is an important source of ore-forming metals in subduction-related Cu–Au deposits. However, the source and migration of Cu in the mantle wedge are poorly constrained. Here, we present a combined study of the Cu elemental and isotopic compositions (δ65Cu) as well as Fe3+/∑Fe ratios on a well-characterized suite of the Mg-lherzolites, Fe-rich peridotites and pyroxenites from the Bohemian Massif in a Variscan subduction zone. The Mg-lherzolites represent melting residues moderately affected by metasomatism of slab-derived melts/fluids. The Fe-rich peridotites and pyroxenites are, respectively, results of Mg-lherzolite-melt reaction and crystalline products of evolved melts in the lithospheric mantle.The peridotites and pyroxenites display higher Fe3+/∑Fe ratios (0.14–0.56) than the cratonic peridotites and mid-ocean ridge basalts, indicating that the mantle wedge was oxidized by slab-derived components. The Mg-lherzolites have relatively low Cu contents (7.35–39.6 μg/g) and normal mantle-like δ65Cu values (−0.13 to 0.26 ‰), suggesting an insignificant slab-to-mantle wedge Cu transfer. In contrast, the Fe-rich peridotites and pyroxenites have variable but overall high Cu contents (37.0–513 μg/g) and heavier δ65Cu values (up to 0.83 ‰). These signatures are ascribed to secondary sulfide precipitation from the evolved Cu- and 65Cu-enriched melts, which were produced by reaction of Mg-lherzolites with subduction-related oxidative SiO2‐undersaturated basaltic melts. Such melt-peridotite reaction at high oxygen fugacity can cause the partial oxidative decomposition of primary sulfides in peridotites with the preferential release of 65Cu into the evolved melts. Our results thus demonstrate that oxidative melt-rock reaction can result in the Cu migration, redistribution and its local enrichment in the mantle wedge, which may serve as an important source of Cu for subduction-related porphyry copper deposits.
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