Proterozoic FE-Cu-REE deposits in southwestern China and northwestern Vietnam

Abstract

Numerous Fe-Cu-(REE) deposits are hosted in Proterozoic metasedimentary-metavolcanic successions in southwestern China and northwestern Vietnam, which form the Kangdian IOCG metallogenic province. The Yinachang deposit in southwestern China and the Sin Quyen deposit in northwestern Vietnam are typical of the province. The Yinachang deposit contains economic concentrations of Fe and Cu, and is enriched in the LREE. It consists mainly of massive and banded replacement ores that are hosted in Paleoproterozoic strata. The paragenetic sequence includes Na-(Fe) alteration, Fe mineralization and associated carbonate alteration, and Cu mineralization and associated carbonate-fluorite-biotite alteration. Fluid inclusion and C-O-S isotopic compositions suggest that two types of fluids were involved in the ore-forming process, namely a high-temperature, saline deeply derived magmatic-hydrothermal fluid and a shallow level non-magmatic fluid. The involvement of magmatic-hydrothermal fluids is consistent with the temporal association between mineralization and the ~1700 Ma intraplate rift-related mafic magmatism. The Sm-Nd isotopic compositions suggest that the mafic rocks/magmas were important sources for the ore metals. The Yinachang deposit experienced a complex LREE enrichment and remobilization history. The LREE were transported by CO2- and Cl-rich brines, and were deposited mainly as REE-phosphates and REE-fluorocarbonates. After deposition, REE-bearing minerals were variably altered in, at least, two episodes of hydrothermal activity (~1700 Ma, ~840 Ma), during which the REE behaved differently in terms of transportation and deposition. The Sin Quyen deposit contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive replacement ores are hosted in the Neoproterozoic metapelite. The paragenetic sequence includes pre-ore sodic alteration, Fe-REE-(U) mineralization and associated calcic-silicate alteration, and Cu-(Au) mineralization and associated potassic-silicate alteration. Uranium-lead dating of hydrothermal zircon and monazite from the ores suggests that the Sin Quyen deposit formed at ~840 Ma. The mineralization was synchronous with the regional subduction-related (Andean-type) magmatism, indicating that the deposit formed in an active continental margin. The alteration mineralogy and O-S isotopic compositions suggest that magmatic-hydrothermal fluids were dominant in the ore-forming system, and the ore-forming fluids were reducing and rich in Cl but poor in F and CO2. The Nd-Hf isotopic compositions indicate that ore metals were derived mainly from ancient crust-derived granitic magmas/intrusions. The ore metals were probably transported from the igneous rocks by high-temperature magmatic-hydrothermal fluids, and were deposited due to cooling of the fluid and fluid/wallrock interaction. Both the Yinachang and Sin Quyen deposits share many features common to other IOCG-type deposits, but are of different ages and formed in different tectonic environments. Another notable difference between the two deposits is the mineralogy. The Yinachang deposit contains abundant CO2- and F-rich gangue minerals, with LREE mainly hosted in phosphates and fluorocarbonates. In contrast, the Sin Quyen deposit contains abundant Cl-rich silicates, and the LREE are hosted mainly in allanite. The differences in mineralogy may be due partially to the formation of the deposits in different tectonic environments.