In situ elemental and isotopic study of diorite intrusions: implication for Jurassic arc magmatism and porphyry Cu-Au mineralisation in southern Tibet

The Gangdese porphyry Cu-Au belt (South Tibet, China) in the eastern Tethyan metallogenic belt hosts many deposits, which formed in subduction and collisional settings. Several porphyry Cu-Au deposits have been discovered in the Xietongmen region, including a large porphyry Cu-Au deposit at Xiongcun. However, the formation mechanism for the Xietongmen Cu-Au deposits and their igneous host rocks remain enigmatic. This study reports whole-rock major- and trace-element compositions, Sr-Nd isotopes, in situ zircon Hf- and plagioclase Sr-isotope compositions, and major- and trace-element compositions of amphiboles to constrain the magmatic emplacement and evolution related to the porphyry Cu-Au mineralisation in the Xietongmen region. New zircon U-Pb ages show that the Xietongmen diorites formed at 185Ma, consistent with the age of Cu-Au mineralisation. The Xietongmen diorites are characterised by enrichment in LILEs and LREE, and depletion in HFSEs. These features suggest that the diorites were produced by arc magmatism in an oceanic-slab subduction environment. In situ isotopic compositions exhibit radiogenic ε_Hf(t) (+9.8 to +14.4) and low initial ⁸⁷Sr/⁸⁶Sr values (0.7039-0.7043); whole-rock results show positive ε_Nd(t) (+5.26 to +5.31) and unradiogenic initial ⁸⁷Sr/⁸⁶Sr values (0.7042-0.7043). These isotopic results are similar to coeval mantle-derived hornblende gabbros and the Sangri volcanic rocks in the Gangdese belt. Amphibole clots in the Xietongmen diorites have higher MgO and Ni contents, with AlT>1.0, Mg#>0.5, Ni>143ppm, and REE patterns different from those of magmatic and idiomorphic amphiboles in the host diorites, suggesting that the clots are xenocrysts. All the petrological and geochemical evidence suggests that the Xietongmen mineralised diorites were derived from a Jurassic hydrous mantle wedge induced by slab dehydration. These magmas contain higher contents of water and other volatiles and have higher oxidation states. These conditions favored the transfer of metals (such as Cu, Au) to the upper crust, and then formed the Jurassic porphyry Cu-Au deposits.