Age and genesis of the W-Bi-Cu-F (Au) Nui Phao deposit, Northeast Vietnam: Constrains from U-Pb and Ar-Ar geochronology, fluid inclusions study, S-O isotope systematic and scheelite geochemistry

Abstract

Northeast Vietnam comprises several tectonic units, which host number of metallic deposits of different types from orogenic gold to intrusion hosted PGE deposits. The Lo Gam domain being part of the NE Vietnam, hosts the Lang Vai gold district, a several Pb–Zn deposits, and the recently discovered super-large W-Bi-Cu-F (Au) Nui Phao deposit. The Nui Phao deposit is 87.9 Mt at 0.19% WO3, 7.95% CaF2, 0.18% Cu, 0.19 g/t Au, and 0.09% Bi, while the measured resource is 25.1 Mt at 0.25% WO3, 8.4% CaF2, 0.23% Cu, 0.26 g/t Au, and 0.108% Bi. The Nui Phao deposit is a very interesting object for research due to location of the orebodies between of two granite massifs of the Cretaceous (Da Lien) and Triassic (Nui Phao) age. Consequently, the temporal and genetic relationships of mineralization with a certain magmatic events are unclear. Here we present mineralogical, compositional, and geochronological data of the Nui Phao deposit in conjunction with physical-chemical researches to understand the ore genesis and its genetic link to a certain magmatic events. According to our observations, at least three different mineral associations are distinguished at the Nui Phao deposit, which include greisens, skarns, and quartz-sulfide ore. Skarn and sulfide ore mineral assemblages generally contain abundant scheelite associated with pyrrhotite, chalcopyrite, and native bismuth. The greisens consist of fluorite, muscovite, and rare molybdenite. The results of fluid inclusions study indicate that at the Nui Phao deposit the initial ore-forming fluid was represented by the reduced (CO2-CH4)–H2O–NaCl(CaCl2) system characterized by high temperatures and moderate salinities. The fluid boiling is the dominant mechanism for ore deposition at the Nui Phao deposit. Relatively homogeneous isotopic compositions of oxygen (δ18OH2O = +6.73 to +9.24‰) and sulfur (δ34S = +1.6 to +3.5‰) that closely correspond to magmatic source, suggest that the ore-forming fluid was generated from cooling granite pluton without contribution meteoric water. The systematic studies of scheelite composition show that ΣREE contents decrease from early to late scheelite generations, which suggest gradual REE depletion of the ore-forming fluid without significant fractionation between LREE, MREE, and HREE. The REEs of scheelite are mainly controlled by the substitution mechanisms Ca2++W6+ ↔ REE3+ + Nb5+ and 3Ca2+ = 2REE3+ + □(Ca). By combining REE features, Y/Ho ratios and other characteristics of the Nui Phao deposit, we propose a single magmatic source for hydrothermal ore-firming fluids without fluid mixing; a significantly contribution of material during fluid/rock interaction is also unlikely. The combination of mineralogical and geochemical features as well as complex studies of the formation conditions allowed us to conclude that gold at the Nui Phao deposit occur as impurities in the native bismuth. The ages of the Da Lien and Nui Phao granite massifs and the Nui Phao deposit have been estimated to be ca. 84 Ma (Ar-Ar), 244 Ma (U-Pb) and ca. 85 Ma (Ar-Ar), respectively, indicating a genetic relationship between the mineralization and the Cretaceous magmatic event.