Metallogenic Mechanism of the Shamai Tungsten Deposit in Dong Ujimqin Banner of Inner Mongolia:Evidence from Fluid Inclusions and Stable Isotopes
The Shamai tungsten deposit is located in the easternmost section of the Central Asian giant suture zone,with widespread development of biotitization. The tungsten ore body is mainly controlled by NW-trending fault structures and is produced in the form of wolframite quartz veins in the Yanshanian Shamai rock body (mainly composed of biotite diorite) and the Devonian surrounding rock strata (mainly composed of brecciated sandstone and sandstone). The mineralization stages from early to late are as follows:quartz topaz stage (Ⅰ),greisen-wolframite stage (Ⅱ),and late silicification stage (Ⅲ). Fluid inclusions and C-H-O-S isotopes of the Shamai deposit show that CO2-H2O inclusions and aqueous gas-liquid two-phase fluid inclusions are commonly developed. From the early to late stages of mineralization,the CO2 rich inclusions in the ore deposit gradually decrease,and the CO2 content gradually decreases. In the early stages of mineralization,the coexistence of melt inclusions and fluid inclusions was observed,belonging to low salinity and CO2 rich magma derived fluids. From early to late ore-forming process of the ore deposit,the homogenization temperatures of fluid inclusions are 279～341 ℃,196～286 ℃,and 103～201 ℃,respectively,and the fluid salinity is (1.6～9.1)％ NaCleq,(3.0～11.1)％ NaCleq,and (0.9～4.7)％ NaCleq,respectively. The results of Laser Raman spectroscopy analysis show that the gas phase in the fluid is mainly CO2,with some containing H2O and CH4. The ore-forming fluid evolved from early medium-high temperature and medium salinity fluids to late low temperature and low salinity fluids,belonging to the NaCl-H2O-CO2±CH4 hydrothermal system. Research on stable isotopes of the deposit indicates that the ore-forming fluid of the Shamai tungsten deposit mainly comes from magmatic water,mixed with atmospheric water in the late stage of mineralization. The source of sulfur in the deposit is related to the Early Cretaceous magmatic hydrothermal system or deep mantle derived magma,and some surrounding rock-derived sulfur. Comprehensive analysis suggests that the Shamai tungsten deposit is a post magmatic hydrothermal tungsten deposit. The Yanshanian intrusive body carried ore-forming materials from deep and underwent processes such as magma differentiation and flow melt interaction,forming a magmatic hydrothermal ore-forming system within the region. The immiscibility of fluids is the main reason for the decomposition and ultimate precipitation of tungsten containing complexes.
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