The Nature and Origin of Hydrothermal REE Mineralization in the Sin Quyen Deposit, Northwestern Vietnam

The Neoproterozoic (similar to 840 Ma) Sin Quyen deposit in northwestern Vietnam is a rare example of an allanite(Ce)-rich iron oxide copper-gold (IOCG)-type deposit. This deposit contains economic concentrations of Cu and Au and subeconomic concentrations of light rare earth elements (LREEs) (La, Ce, Pr, and Nd) in massive or banded replacement ores hosted in metasedimentary rocks. Three main stages of alteration and mineralization have been identified in this deposit, namely Na alteration (stage I), Ca-K alteration and associated Fe-LREE mineralization (stage II), and K alteration and associated Cu-Au mineralization (stage III). The LREEs are hosted mainly in allanite-(Ce) of stage II, and there are also subordinate REE-bearing fluorapatite and rare monazite-(Ce) and chevkinite-(Ce) in stages I and II. Based on oxygen isotope thermometry, the ore-forming fluids in stages I and II had high temperatures, 466 degrees to 588 degrees C. The delta O-18(V-SOW) values of the fluids range from 8.3 to 12.7700 and are similar to those of magmatic zircon (7.3-12.4 parts per thousand) from nearby Neoproterozoic felsic intrusions. Sulfide minerals of stage III, including chalcopyrite, pyrrhotite, and pyrite, have a narrow range of delta S-34(V-CDT) values near zero per mil (-2.5 to 3.5 parts per thousand), indicating a magmatic source for the sulfur. We propose that the ore-forming fluids were dominantly of magmatic-hydrothermal origin, although insignificant nonmagmatic fluids may also be involved in the ore genesis. The involvement of magmatic-hydrothermal fluids in the ore-forming process is consistent with the temporal association between the mineralization and Neoproterozoic subduction-related magmatism in the region. The bulk ores have epsilon(Nd)(t) values ranging from -4.6 to 2.4, similar to those of the Neoproterozoic crust-derived, felsic intrusions in the region. In addition, hydrothermal zircon has epsilon(Hf)(t) values (-4.9 on avg) similar to those of magmatic zircon from the Neoproterozoic crust-derived, felsic intrusions. Thus, the REEs were likely sourced mainly from related felsic magma(s). Gangue minerals in the Sin Quyen deposit are dominated by Cl-bearing silicates, amphibole, and biotite. Although minor F is present in some minerals, F- and CO2-rich minerals are rarely encountered. These observations are consistent with Cl-rich ore-forming fluids that had relatively low concentrations of F and CO2. We propose that the REEs were transported mainly as chloride complexes and deposited mainly as allanite-(Ce) in response to fluid cooling and fluid/wall-rock interaction. The abundance of allanite-(Ce) and scarcity of REE fluorocarbonates and REE phosphates (the principal ore minerals in many REE deposits) in the Sin Quyen deposit are interpreted to reflect the high-temperature, Si-, Al-, and Ca-rich, and F- and CO2-poor nature of the fluids.