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Porphyry copper deposits (PCDs) represent the most important type of Cu reservoir on Earth, but the mechanism of Cu enrichment in PCDs is debated due to the lack of direct constraints. This issue may be resolved by the study of copper isotopes (δ65Cu), which are strongly fractionated during formation and/or precipitation of sulfides. Here we report high-precision Cu-isotope data on a large set of porphyries, mafic magmatic enclaves (MMEs), and sulfide ores from PCDs in southern Tibet. For comparison, barren intrusions from southern Tibet were also analyzed. The fertile porphyries and MMEs from PCDs have high Cu contents and elevated δ65Cu values compared with the barren intrusions and global average felsic rocks. These features are inconsistent with the known supergene processes after sulfide formation (e.g., leaching and weathering). Because sulfides formed from secondary Cu-rich fluids are enriched in heavy Cu isotopes, the elevated Cu contents and Cu-isotope ratios indicate that the Cu source for PCDs was a refertilized lithosphere enriched in sulfides. This suggests that initial Cu enrichment in magma sources could be a key step in the formation of giant PCDs in continental collision zones. [ABSTRACT FROM AUTHOR]

Miocene postcollisional porphyry Cu deposits in southern Tibet are genetically associated with dacitic-rhyolitic intrusions with unusually high Sr/Y ratios (>40), which have been attributed to dehydration melting of garnet amphibolite in a thickened lower crust. To test this hypothesis and examine the hydration state of copper ore-forming high Sr/Y magmas, we utilize a geohygrometer for granitoid rocks, entailing zircon-saturation thermometry and H2O-dependent phase equilibria. The results show that these Tibetan high Sr/Y magmas had dissolved H2O contents >10 wt%, which considerably exceeds the water supply by dehydration melting of basaltic amphibolites (maximum of 6.7 ± 1.4 wt%). Our results indicate that high Sr/Y dacitic-rhyolitic magmas cannot be produced by dehydration melting of basaltic amphibolites. While H2O-added melting of basaltic amphibolites can produce high Sr/Y dacitic-rhyolitic melts, it does not yield high enough Mg# (>50) to match the Tibetan ore-forming porphyries. We propose an alternative model for the genesis of copper ore-forming high Sr/Y magmas in Tibet, and suggest that the high Sr/Y dacitic-rhyolitic porphyries in southern Tibet are residually H2O-enriched, high-pressure differentiation products of hydrous mafic partial melts of Tibetan mantle. This hypothesis is based on the previous investigation of Miocene mafic microgranular enclaves (mantle-derived melts), which define a fractionation trend with, and have Sr-Nd-Hf isotopic compositions similar to, the host Tibetan ore-forming porphyries. [ABSTRACT FROM AUTHOR]