Addition of H2O at the Baishiquan and Tianyu magmatic Ni-Cu sulfide deposits, southern Central Asian Orogenic Belt, China: evidence from isotopic geochemistry of olivine and zircon

Magmatic Ni-Cu sulfide deposits in orogenic belts have distinct petrological and geochemical features compared with world-class intraplate magmatic deposits. One such feature is the presence of abundant primary hydrous minerals such as phlogopite and amphibole, indicating a H2O-rich parental magma. However, the origin and nature of the fluids have not been established. In the southern Central Asian Orogenic Belt, the Permian Baishiquan and Tianyu mafic-ultramafic intrusions host magmatic Ni-Cu sulfide deposits. Both intrusions are composed of lherzolite, olivine websterite, gabbronorite, gabbro, and diorite. The main ore-host rocks are lherzolite and websterite, with disseminated, net-textured, and massive ores comprising the dominant primary ore types. Low zircon epsilon(Hf) (- 6.9 to 7.9 parts per thousand) and varying delta O-18 values (5.2 to 6.6 parts per thousand), as well as both arc-type and MOR-type zircon Nb/Yb (0.0013-0.011) and U/Yb (0.33-10.22), identify an altered lower oceanic crust component in the mantle source. A combination of altered lower oceanic crust and upper crust can account for the zircon Hf-O isotopic characteristics. A mixing model further reveals that the Baishiquan and Tianyu magmatic sulfide deposits originated from a depleted mantle source that experienced two stages of subduction metasomatism with a 30% addition of altered oceanic crust in the first stage and 20% in the second. In the second stage, > 10% oceanic sediment with high delta O-18 must also have been added to result in olivine delta O-18 values higher than those in mantle. This staged evolution is reflected in both the olivine and zircon signatures with olivine from disseminated and net-textured ores at the Tianyu deposit having high delta O-18 (4.6 to 6.3 parts per thousand) with a similar range to zircon and high delta Li-7 isotopic compositions (12.03 to 19.46 parts per thousand). Contamination in the source by oceanic sediment with high delta O-18 values can account for the olivine delta O-18 values (4.8 to 5.7 parts per thousand at Baishiquan) being higher than those in the mantle, while the addition of brine (5% at the Baishiquan deposit) can explain the olivine delta Li-7 values (20.96 to 31.01 parts per thousand). The results suggest that H2O content, Li content, and delta Li-7 and delta O-18 isotopic signatures in olivine and zircon are good indicators of mantle metasomatism and olivine fluid processes in magmatic Ni-Cu sulfide deposits.

Automated summary

Magmatic Ni-Cu sulfide deposits in orogenic belts exhibit unique petrological and geochemical features, with the presence of primary hydrous minerals indicating a water-rich magma source. The origin of fluids in these deposits has not been definitively established. Geochemical analysis of zircon and olivine in the deposits suggests a mantle source influenced by subduction metasomatism involving altered oceanic crust components. The isotopic signatures in olivine and zircon further reveal a staged evolution of the deposits, with contributions from altered oceanic crust and sediment during different subduction events.