Ore Genesis of the Baishitouwa Quartz-Wolframite Vein-Type Deposit in the Southern Great Xing'an Range W Belt, NE China: Constraints from Wolframite In-Situ Geochronology and Geochemistry Analyses

The Baishitouwa deposit is a medium-scale quartz-wolframite vein-type deposit in the southern Great Xing'an Range tungsten (W) belt. The W mineralization occurs mainly as veins and dissemination within the mica schist of the Mesoproterozoic Baiyunebo Group. The formation of the deposit can be divided into four stages. The wolframite yielded a lower intercept Pb-206/U-238 age of 221.0 +/- 3.4 Ma (1 sigma, MSWD = 2.0), which records a late Triassic W mineralization event in the Baishitouwa deposit. In combination with previous geochronological data, we suggest that NE China may have an enormous potential for Triassic W mineralization and more attention should be given to the Triassic ore prospecting in the region. This work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary ore-forming fluid. Trace-element compositions suggest that wolframite (I) was controlled by the substitution mechanism of 4(A)(Fe, Mn)(2+) + 8(B)W(6+) + (B) <-> 3(A)M(3+) + N-A(4+) + 7(B)(Nb, Ta)(5+) + 2(B)N(4+), whereas wolframite (II) was controlled by the substitution mechanism of (A)(Fe, Mn)(2+) + (A) + 2(B)W(6+) <-> 2(A)M(3+) + 2(B)N(4+). Wolframite (I) contains higher concentrations of Nb, Ta, Sc, and heavy rare earth elements (HREEs), and lower Mn/(Mn + Fe) ratios than wolframite (II). Both wolframite (I) and (II) have similar trace elements and left-dipped REEN patterns, and analogical Nb/Ta ratios. They have similar Y/Ho ratios to Mesozoic highly fractionated W-mineralized granitoids in NE China. These data indicate that the W mineralization at Baishitouwa is genetically related to an underlying highly fractionated granite, and the compositional variation of fluids is likely driven by crystallization of wolframite during the processes of fluid evolution. A change of the ore-forming fluids from an oxidized to a relatively reduced state during the evolution occurred from stage 1 to 2.