The Longbohe Fe-Cu-(LREE) deposit in the Longbohe-Sin Quyen (LSQ) Fe-Cu belt in the border zone of South China and northern Vietnam has a resource of > 1 million tonnes Cu, and was once speculated to be a potentially important subduction-related deposit of Precambrian age, but with poor constraints. Our study reveals that the major Fe-, Cu-, and LREE mineralization comprises three stages and are comparable to typical IOCG deposits worldwide. Stage I is characterized by pervasive Na alteration, followed by Fe mineralization stage (Stage II) consisting of Ti-poor magnetite associated with Ca-Na alteration forming amphibole and albite. Stage III, the Cu-LREE mineralization stage, is composed mainly of chalcopyrite and REE-rich allanite with minor pyrrhotite and pyrite. Stages I and II formed at broadly comparable temperatures of 515 to 670 degrees C, and have delta O-18 fluid values of 4.3 to 11.7 parts per thousand, indicating dominantly magmatic fluids. Furthermore, they have hydrothermal zircon with epsilon(Hf(t)) values (7.5 to 12.7) similar to coeval mafic intrusions in the region, likely indicating a mafic magma-source for the fluids. Stage III fluids have lower temperatures (332 to 402 degrees C) but exhibit magmatic-like delta S-34 values (1.2 to 3.7 parts per thousand) similarly suggestive of a major magmatic source. However, they have distinctly lower epsilon(Hf(t)) and epsilon(Nd(t)) values comparable to those of coeval regional felsic intrusions, suggesting that an additional component of REE-rich external fluid from felsic magmas was involved during the Cu-REE mineralization. Hydrothermal zircons from Stages II and III have similar U-Pb ages of similar to 850 Ma, providing tight constraints on the timing of Fe-Cu mineralization. The new U-Pb ages indicate that the Longbohe deposit is coeval with the Sin Quyen IOCG deposit in northern Vietnam, and with the regional subduction-related magmatism, indicating that the Longbohe-Sin Quyen belt is of Neoproterozoic age and formed in a convergent setting.

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