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Donggebi |
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Xinjiang, China |
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Mo
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Super Porphyry Cu and Au
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The Donggebi porphyry-style Mo deposit is located in the eastern section of the Eastern Tianshan Orogenic Belt, part of the Central Asia Orogenic Belt, ∼110 km south of Hami City (Kumul) and ~650 km SE of Urumqi in Xinjiang, NW China (#Location: 41° 55' 00", 93° 20' 15"E).
The Donggebi deposit, which was discovered in 2008, is ~200 km ESE of the Tuwu-Yandong porphyry Cu-Au cluster. It is the largest and economically most important Mo deposit in the East Tianshan belt, with a total metal reserve of 0.5 Mt of Mo (Huang et al., 2011). It was drilled out during 2009–2010, followed by construction of the now-operating mine which commenced production in 2011. Proved ore reserves in 2017 were 441 Mt @ 0.115% Mo for ~0.5 Mt of contained Mo (Sun et al., 2017).
The deposit is located near the southern margin of the Central Asia Orogenic Belt, to the north of the junction between the east-west elongated Tarim and North China cratons that define the southern margin the belt.
The country rocks are mostly of metasedimentary origin, belonging to the Lower Carboniferous Gandun Formation, comprising meta-sandstone, meta-sandy mudstone, meta-argillaceous sandstone, meta-mudstone, meta andesite, tuff and hornfels. They generally strike WNW and dip at 50 to 75°ENE, and are metamorphosed by deeper-seated intrusive rocks (Ma et al., 2012).
The Gandun Formation sequence is intruded by some granite porphyry dykes which cut the mineralised area, and by unmineralised biotite granite in the NW of the mining area. The intrusive complex at Donggebi mainly comprises:
• Porphyritic granite, the coarser grained varieties of which are composed of 2 to 9% orthoclase, 6 to 51% plagioclase, 20 to 35% quartz and 1 to 3% biotite, with minor muscovite and sericite. Plagioclase crystals are commonly replaced by granular saussurite (sodic-calcic plagioclase feldspars) and fine-grained clay minerals.
• Granite porphyry which contains 30% orthoclase, 40% plagioclase, 25% quartz and 5% biotite, with minor amounts of apatite, muscovite and chlorite. The orthoclase crystals range from 0.5 to 1 mm in size. Alteration minerals include saussurite, sericite and chlorite.
Zircon LA-ICP-MS U-Pb dating yielded concordant ages of 234.6±2.7 and 231.8±2.4 Ma for the porphyritic granite and the fine-grained granite, respectively (Sun et al., 2017). A zircon SHRIMP U-Pb age of 227.6±1.3 Ma has also been obtained from the porphyritic granite (Huang et al., 2011). These ages suggest the granitic rocks were intruded after the Mo mineralisation, which has been dated at 234.6±1.3 and 234.0±2.0 Ma by Molybdenite Re-Os isochron determinations; Han et al., 2014 and Sun et al., 2017 respectively).
Five mineralised bodies of ore have been identified, distributed along the contact zone between the porphyritic granite and metasedimentary rocks of the Gandun Formation, predominantly external to, but also in part within the intrusions. Individual bodies vary from 280 to 850 m in length and 10 to 65 m in thickness. They fall within stratabound-fracture zones that are predominantly within volcaniclastic rocks of the Gandun Formation above the contact with the concealed porphyritic granite. These zones trend NE and dip at 30 to 60°S. The mineralised bodies that have been delineated extend to >320 m below the surface. Whilst most mineralisation is external to the intrusives, molybdenite also occurs within moderate to strongly silicified granite porphyry stocks.
The mineralisation is characterised by Mo-bearing veinlet-disseminated and brecciated structures, mainly containing assemblages of volatile-rich K feldspar-quartz-oxide, K feldspar-quartz, polymetallic sulphides and calcite-quartz. The principal metallic minerals are molybdenite and pyrite with minor chalcopyrite, galena, magnetite, scheelite and wolframite. The gangue assemblage includes orthoclase, plagioclase and quartz, with lesser calcite, muscovite and chlorite. The molybdenite ranges from 0.03 to 3 mm in size.
Several phases of hydrothermal alteration are recognised, based on chemical and mineralogical analyses. The highest Mo content is found in zones with complex hydrothermal overprinting. Potassic alteration (K feldspar + secondary biotite) affected the entire mineralised area, and involved microcline growing in the matrix, around and between primary biotite and plagioclase. Minor disseminated pyrite and magnetite, and veinlets of magnetite±pyrite occur within the potassic alteration zones.
The potassic assemblages are overprinted by pale-coloured to white, irregular, phyllic (quartz-sericite/muscovite) alteration zones, where sericite/muscovite, together with fine-grained quartz have replaced feldspar. Euhedral pyrite is common in this zone. Minor chalcopyrite occurs with disseminated molybdenite.
Silicification with fine-grained quartz is associated with stockwork quartz veins and veinlets with chalcopyrite, pyrite, molybdenite and magnetite. Biotite related to a late, weakly developed quartz and biotite veining episode only hosts minor Mo mineralisation.
The mineral assemblages and crosscutting relationships of the ore veins, suggest five mineralising phases (Han et al., 2014):
• Stage I - characterised by fluorite, K feldspar and quartz veins, also accompanied by wolframite;
• Stage II - producing an assemblage of quartz, magnetite, scheelite and a little molybdenite, accompanied by potassic and phyllic alteration;
• Stage III - the main mineralisation stage, which consists of molybdenite, chalcopyrite and pyrite, with minor galena and sphalerite;
• Stage IV - a late hydrothermal stage that is characterised by the formation of calcite and gypsum as well as molybdenite;
• Stage V - a post-hydrothermal oxidation stage marked by a supergene assemblage of limonite and malachite.
This summary is principally drawn from Han et al., 2014 and Sun et al., 2017, as cited below.
The most recent source geological information used to prepare this decription was dated: 2017.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Donggebi
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Selected References:
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Gao, J., Qin, K., Zhou, M.-F. and Zaw, K., 2018 - Large-scale porphyry-type mineralization in the Central Asian Metallogenic Domain: Geodynamic background, magmatism, fluid activity and metallogenesis: in J. of Asian Earth Sciences Online, https://doi.org/10.1016/j.jseaes.2018.08.023.
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Han, C., Xiao, W., Zhao, G., Sun, M., Qu, W. and Du, A., 2014 - Re-Os Geochronology on Molybdenites from the Donggebi Mo Deposit in the Eastern Tianshan of the Central Asia Orogenic Belt and its Geological Significance: in Resource Geology v.64, pp. 136-148.
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Sun, H., Li, H., Danisik, M., Xia, Q., Jiang, C., Wu, P., Yang, H., Fan, Q. and Zhua, D., 2017 - U-Pb and Re-Os geochronology and geochemistry of the Donggebi Mo deposit, Eastern Tianshan, NW China: Insights into mineralization and tectonic setting: in Ore Geology Reviews v.86, pp. 584-599.
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Wang, Y.-H., Zhang, F.-F., Liu, J.-J., Xue, C.-J., Li, B.-C. and Xian, X.-C., 2018 - 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: in Econ. Geol. v.113, pp. 463-488
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Wu, Y.-S., Zhou, K.-F., Li, N. and Chen, Y.-J., 2017 - Zircon U-Pb dating and Sr-Nd-Pb-Hf isotopes of the ore-associated porphyry at the giant Donggebi Mo deposit, Eastern Tianshan, NW China: in Ore Geology Reviews v.81, pp. 794-807.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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