GEOLOGY, RE-OS AND U-PB GEOCHRONOLOGY AND SULFUR ISOTOPE OF THE DONGGEBI PORPHYRY MO DEPOSIT, XINJIANG, NW CHINA, CENTRAL ASIAN OROGENIC BELT

The Donggebi porphyry Mo deposit in the eastern section of the Eastern Tianshan Orogenic Belt in the Central Asian Orogenic Belt contains Mo metal reserves of 0.5 Mt. The deposit is hosted in Early Carboniferous metasedimentary rocks, namely; metasandstone, meta-sandy mudstone . Multiple hydrothermal activities have resulted in propylitic, phyllic, and argillic alteration in this deposit. Four stages (I-IV) of hydrothermal activity are identified. Stage I is represented by a mineral assemblage of K-feldspar, quartz and wolframite. Stage II consists of quartz + magnetite+pyrite ± chalcopyrite veinlets/veins with phyllic halos. Stage III consists of quartz + molybdenite + pyrite ± galena ± sphalerite ± chalcopyrite veins that are commonly related to phyllic alteration in the altered rocks. Stage IV has an assemblage of calcite+gypsum. Molybdenite mainly occurs in Stages III. Re-Os dating results for molybdenite samples from these two stages yielded an isochron age of 234.2 ± 1.6 Ma (2σ, MSWD = 0.25, n = 8). Porphyritic granites have a SIMS U-Pb zircon age of ∼236 Ma and it was probably related to the Triassic felsic magmatism in this area. Values of δ³⁴S of sulfides range from 1.5‰ to 3.8‰, with an average value of 2.81 ± 2.24‰ (n = 22), reflecting a deep sulfur source. Most molybdenite samples have high δ³⁴S values (≥3.36‰) relative to other sulfide minerals (i.e., pyrite and chalcopyrite) of Stages I to III (δ³⁴S = 1.5–3.8‰, n = 18). Based on the geological history and spatial-temporal distribution of the granitoids, it is proposed that the Mo deposits in the eastern part of the East Tianshan Orogenic Belt formed in a post-collision extensional setting in the Early Mesozoic.