Geology, Geochemistry And Genesis Of The Baishan Molybdenum Deposit In Eastern Tianshan, NW China

The Baishan porphyry Mo deposit is located in the Sanchakou-Baishan porphyry copper-molybednum belt, which is the eastern part of Kangguer-Huangshan ductile shear zone in eastern Tianshan, NW China. The ore-bearing strata of the Baishan Mo deposit is the Lower Carboniferous Gandun Formation, and the Late Triassic granite porphyry is its metallogenetic rock body. In this paper, geology, petrography, geochemistry, SIMS zircon U-Pb dating and in situ Hf isotope of the Baishan deposit and associated granite porphyry were systematically studied. So that we can constrain the petrogenesis and formation age of the granite porphyry emplaced in Baishan, and that we can reveal the granitic magma source, metallogenetic geodynamical setting and ore-forming mechanism of the Baishan porphyry Mo deposit.Geochemical data indicate that the deep granitic rocks in Baishan are characterized by high SiO2% Al2O3 and alkali (Na2O+K2O) contents, with low MgO, Cr and Ni contents, at the same time they show pronounced enrichment in Rb?Ba? Th and U, and depletion in Nb?Ta?Zr and Hf. These features of granites in Baishan suggest that they belong to high-K calc-alkaline I-type granite, and show geochemical affinity with adakites. In situ Hf isotopic analyses of zircons that had previously been dated by SIMS yielded positive ??Hf(t) values ranging from +8.3 to+12. The geochemical and isotopic data imply that the primary magmas of the deep granitic rocks in Baishan could have originated by partial melting of a juvenile lower crust.SIMS zircon U-Pb dating data indicate that the magmatic activities in Baishan occurred at 229.7±3.2Ma. Combined with the Re-Os isochron age (225-229Ma) reported by previous researchers, we can conclude that mineralization of the Baishan porphyry Mo deposit occurred during the same period as that of the rock formation. Therefore, we propose that the Baishan porphyry Mo deposit was formed in an intracontinental extension setting in the Late Triassic.After the closing of the ancient Tianshan Ocean in the Late Carboniferous, eastern Tianshan started the post-collision stage. In the intracontinental extension setting in the Late Triassic, thickness of the lower crust itself and the heat from mantle triggered partial melting of the lower crust, resulting in the formation of the adakitic magmas. During ascending of the adakitic magmas to the shallow crustal levels, abundant ore-forming elements and volatile components were released; then the deposit were formed in the wall rock through filling and replacement by the ore-bearing hydrothermal fluid.