Late Triassic granitic magmatism and tungsten mineralization in NE China: Geochronological and geochemical constraints from the Tantoushan quartz-wolframite vein-type deposit

Abstract

NE China, located at the eastern Central Asian Orogenic Belt, experienced extensive magmatism during the Mesozoic and hosts multistage granitic plutons and accompanying W mineralization. However, due to the limited number of studies on Triassic W deposits and spatially related granitoids, the petrogenesis of these granitoids and their relation to W mineralization remain enigmatic. The Tantoushan quartz-wolframite vein-type deposit is located on the southern margin of NE China. Tungsten mineralization occurs mainly in the veins and veinlets within monzogranites. A lower intercept 206Pb/238U age of 234.3 ± 6.2 Ma (1σ, MSWD = 0.41) was obtained for wolframite, which is identical within uncertainties to the zircon weighted mean 206Pb/238U age of 233.1 ± 1.8 Ma (1σ, MSWD = 0.41) from the W-bearing monzogranites. The monzogranites have the petrological, mineralogical, and geochemical characteristics of highly fractionated I-type granitoids. The rocks are enriched in Rb, Th, U, K, and Pb, and depleted in Ba, Sr, P, and Ti. They have higher W concentrations and Rb/Sr ratios, and lower Nb/Ta, Zr/Hf, and K/Rb ratios than the contemporary W-barren granitoids in NE China. These geochemical characteristics and negative zircon εHf(t) values (−17.7 to −8.6), as well as old two-stage model ages (TDM2 = 1807–2378 Ma), suggest that the monzogranites were derived as a product of the partial melting of the Paleoproterozoic lower crust and subsequently underwent extreme fractional crystallization. Geochronological and geochemical evidence collectively suggest that the W mineralization in the Tantoushan deposit is genetically related to the W-bearing monzogranites, and extreme fractional crystallization was essential for W enrichment in the granitic magma. In contrast, Triassic W-barren granitoids did not induce W mineralization, probably because of their low fractionated signatures. We preliminarily demonstrate that an isovalent substitution mechanism of 4A(Fe, Mn)2+ + 8BW6+ + B□ ↔ 3AM3+ + AN4+ + 7B(Nb, Ta)5+ + 2BN4+ played a critical role in the formation of hydrothermal wolframite in the Tantoushan deposit, and the trace elements compositions of wolframite were controlled by both the crystallochemical parameters and composition of the initial hydrothermal fluids. In the context of the regional geology, we propose that the Tantoushan monzogranites and corresponding W mineralization were formed in a post-collision extensional setting controlled by the closure of the Paleo-Asian Ocean during the Late Triassic. In combination with previous studies, we suggest that NE China may have enormous potential for Triassic W mineralization and the Triassic highly fractionated granitoids distributed on both sides of the Solonker-Xar Moron-Changchun Fault represent potential targets for future exploration of additional W resources.