Ore Genesis and Hydrothermal Evolution of the Donggebi Porphyry Mo Deposit, Xinjiang, Northwest China: Evidence from Isotopes (C, H, O, S, Pb), Fluid Inclusions, and Molybdenite Re-Os Dating

The Donggebi porphyry Mo deposit (441 Mt at 0.115% Mo) is located in the center of the Jueluotage metallogenic belt in Eastern Tianshan, on the southern margin of the Central Asian orogenic belt, Xinjiang, northwest China. Molybdenum mineralization mainly occurs as veins or veinlets in sandstone of the lower Carboniferous Gandun Formation or as dissemination within Triassic granitic porphyry stocks. Four stages (I-IV) of hydrothermal activity have been identified. Molybdenite mainly occurs in veins of stage II (quartz-molybdenite and molybdenite-only veins or veinlets within the altered wall rocks) and III (quartz + molybdenite + fluorite + pyrite +/- chalcopyrite +/- bornite +/- galena +/- sphalerite veinlets/veins with phyllic halos). Re-Os dating results for molybdenite samples from stage II yielded an isochron age of 236.6 +/- 1.9 Ma (2 sigma, MSWD = 0.41, n = 10). Fluid evolution during the four stages of mineralization was constrained by a detailed fluid inclusion study: (1) Fluid inclusions during the formation of the early pre-ore stage quartz-K-feldspar veins (stage I) were trapped under two-phase conditions, as evidenced by the presence of liquid-rich fluid inclusions coexisting with vapor-rich ones, and CO2-bearing and solid-bearing inclusions with an opaque daughter crystal; fluid phase separation occurred at 200- to 550-bar pressure (corresponding to a depth of 0.7-2.0 km, assuming lithostatic pressure conditions) and temperatures of 256 degrees to 460 degrees C. (2) Fluid inclusions in quartz veins from the main ore stages (II and III) were also trapped under two-phase conditions (fluid phase separation); these inclusions mostly homogenize to liquid between 220 degrees and 300 degrees C, with salinities of similar to 2 to 7 wt % NaCl equiv, and record a pressure of entrapment of <50 to 200 bar (corresponding to a depth of 0.5-2.0 km under hydrostatic pressure conditions). (3) Late post-ore stage (IV) fluids are represented by two-phase aqueous inclusions in quartz from quartz-calcite veins, characterized by homogenization temperatures ranging from 106 degrees to 243 degrees C and salinities between 1.6 and 5.7 wt % NaCl equiv. All of these observations combined with the stable and radiogenic isotope results reveal that the formation of the Triassic Donggebi porphyry Mo deposit was dominated by a magmatic-hydrothermal system. Hydrogen, oxygen, and carbon isotope data of quartz and calcite at Donggebi indicate that the hydrothermal fluids had a dominantly magmatic signature and were diluted by meteoric waters. Molybdenite samples from stages II and III record a narrow range of delta S-34 values between 3.3 and 5.0 parts per thousand, whereas pyrite samples from stages I to IV show decreasing delta S-34 values between 2.1 and 5.0 parts per thousand. Molybdenite also has Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 ratios similar to those of granitic intrusions. In a diagram of Pb-207/Pb-204 versus Pb-206/Pb-204, the Pb isotope data from sulfide samples display a positive trend transecting the growth curves of crustal lead. Sulfur and lead isotope data for sulfide minerals, together with their paragenesis, suggest that the ore-forming components (metal and sulfur) were sourced from a larger magmatic reservoir and some upper crustal materials in a postcollision extensional setting.