Origin and tectonic environment of the Longbohe Fe-Cu-(LREE) deposit in the Ailao Shan–Red River shear zone, Southwest China: a Neoproterozoic subduction-related IOCG deposit?

• Published in: Mineralium deposita 2024, Vol.59 (1), p.23-46
• Main Authors: Liu, Lei, Chen, Wei Terry, Li, Xiao-Chun, Zhao, Xin-Fu
• Format: Article
• Language: English
• Subjects: Belts; Chalcopyrite; Copper; Earth and Environmental Science; Earth Sciences; Fluids; Fluvial deposits; Geology; Intrusion; Iron; Lava; Low temperature; Mafic magma; Magma; Magnetite; Mineral Resources; Mineralization; Mineralogy; Precambrian; Pyrite; Pyrrhotite; Radiometric dating; Shear zone; Subduction; Subduction (geology); Tectonics; Zircon
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-023-01198-4

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 ºC, and have δ 18 O fluid values of 4.3 to 11.7 ‰, indicating dominantly magmatic fluids. Furthermore, they have hydrothermal zircon with ε 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 ºC) but exhibit magmatic-like δ 34 S values (1.2 to 3.7 ‰) similarly suggestive of a major magmatic source. However, they have distinctly lower ε Hf(t) and ε 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 ~ 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.