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Northwest China, covering northern Xinjiang, northern Gansu and westernmost Inner Mongolia, mainly includes Junggar Basin and its surrounding mountains such as Chinese Altay, Junggar, Chinese Tianshan and Beishan. It lies at the junction of Siberia, Tarim and Kazakhstan plates, and is a key sector of the Central Asian Orogenic Belt (CAOB), characterized by multistage Phanerozoic continental growth. Herein at least nine Mo-only or Mo-dominated, fourteen Cu-Mo, two W-Mo and one Be-Mo deposits have been discovered. These 27 deposits occur in Altay, West Jungar, West Tianshan, East Tianshan and Beishan areas, and have been formed during accretionary or collisional orogenies. The majority of the deposits are porphyry type, followed by the skarn and quartz vein types. The orebodies occur mainly as veins, lens, pods in the positions from inner intrusions through contact zones to the hostrocks distal to causative intrusions. The host-rocks are variable in lithologies, including granites, porphyries, volcanic breccias and tuffs, and sedimentary rocks. Outward from orebodies to hostrocks, the wallrock alteration is zoning from potassic (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alterations, with skarn specifically occurring in skarn-type systems. Hydrothermal mineralization generally includes four stages, from early to late, represented by (1) potassic feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-polymetallic sulfide stockworks, and (4) quartz ± carbonate ± fluorite veins or veinlets. The ore-forming fluids were initially magmatic in origin and shew high-temperature and high-salinity, containing daughter mineral- and/or CO 2 -bearing fluid inclusions; and eventually evolved to low-temperature, low-pressure, low-salinity and CO 2 -poor meteoric water. The porphyry Mo deposits can be further subdivided into two subtypes, i.e., Dabie- and Endako-types. The Endako-type Mo deposits, e.g., Suyunhe and Hongyuan, together with all the Cu-Mo systems, were formed in the Palaeozoic subduction-related magmatic arcs. The Dabie-type porphyry Mo deposits, represented by giant Donggebi and Baishan, together with the Mo-only, Mo-dominated and W-Mo or Be-Mo deposits were formed in syn- to post-collisional tectonic setting, with isotope ages ranging 260–213.2 Ma, with the Kumutage skarn-type Mo system being an exception aged 319 Ma. The Dabie-type porphyry Mo deposits are characterized by the CO 2 -bearing fluid inclusions that cannot be observed in the Endako-type porphyry Mo systems. The Re contents in molybdenites from porphyry and porphyry-skarn Cu-Mo systems are mainly > 100 ppm, suggesting a source significantly contributed by the mantle; whereas the Re contents in molybdenites from the Mo-only or W-Mo deposits are mainly < 100 ppm, indicating a genetic relation to the crust-sourced granitic magmatism. Therefore, the types of porphyry Mo deposits and their contrasting geological and geochemical characteristics are a powerful indicator of the tectonic settings; and the available data from the Mo deposits in NW China indicate a Late Carboniferous-Permian transformation from subduction-related accretionary orogeny to continental collision orogeny. [ABSTRACT FROM AUTHOR]

The Tianmugou molybdenum (Mo) deposit is a newly discovered porphyry deposit in the Dabie Shan of eastern China. The Mo-mineralization is spatially and genetically related to the Tianmushan granite batholith, and mainly occurs as thin molybdenite-bearing pegmatite dykes and numerous hydrothermal veins, with the development of potassic alteration, silicification, sericitization, propylitization, and fluoritization. The Tianmushan batholith is composed of outermost fine-grained K-feldspar granite (Phase 1), medium-grained K-feldspar granite (Phase 2), coarse-grained biotite monzogranite (Phase 3), porphyritic granite (Phase 4), and innermost fine-grained K-feldspar granite (Phase 5). Zircon crystals from the biotite monzogranite yield a weighted average 206 Pb/ 238 U age of 123.0 ± 0.8 Ma, but those from the porphyritic granite do not yield a good age because of their high contents of Th, U and Pb. Seven molybdenite samples from the ores yield individual Re–Os isotopic ages of 120.5 ± 1.7 Ma to 122.5 ± 1.9 Ma, which are slightly younger than the U–Pb age for the biotite monzogranite. These ages constrain the ore-forming magmatic-hydrothermal activity at the Tianmugou Mo deposit that took place during the post-collisional tectonism subsequent to the collision between the Yangtze and North China continents, consistent with the 142–111-Ma regional Mo mineralization event in Dabie Shan. The biotite monzogranite and porphyritic granite have high contents of SiO 2 , K 2 O and Al 2 O 3 , and low contents of TiO 2 , MgO and CaO, showing a metaluminous to peraluminous high-K calc-alkaline affinity, with significant depletion in Eu, Ba, Sr, P and Ti, and enrichment in Rb, Th, U, Nb, Ta, Hf and Y. The rocks have high initial 87 Sr/ 86 Sr ratios of 0.73546–0.89204, and negative εNd( t ) values of − 11.6 to − 11.0, with T DM2 (Nd) of 1.81–1.86 Ga. Their ( 206 Pb/ 204 Pb)t, ( 207 Pb/ 204 Pb)t and ( 208 Pb/ 204 Pb)t are 17.337–17.554, 15.450–15.465 and 37.992–38.147, respectively. The Sr-Nd-Pb isotopic signatures indicate that the causative granitic intrusions for the Tianmugou deposit mainly originated from partial melting of the Qinling Group during Early Cretaceous post-collisional tectonic extension. The Tianmugou Mo deposit belongs to a collisional- or Dabie-type porphyry Mo system. [ABSTRACT FROM AUTHOR]