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Axi |
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Xinjiang, China |
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Au Ag
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Super Porphyry Cu and Au
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The Axi low sulphidation epithermal gold deposit is located on a plateau south of Mount Keguerqirt in the western Tianshan, about 40 km northeast of Yining, ~500 km WNW of Urumqi and less than 100 km west of the Kazakhstan border (#Location: 44° 14' 4"N, 81° 36' 37"E).
The Axi deposit lies on the northern margin of the east tapering, wedge shaped Yili Block, which separates the Junggar microcontinent to the north from the Tarim Block to the south. It is structurally-hosted within a tilted succession of auto-brecciated andesitic lava flows and fragmental andesitic tuffs and brecclas of the Lower Carboniferous Dahalajunshan Formation. Rare syn-volcanic felsic dykes are found within the region, although none are found in the vicinity of the deposit. The host rocks are unconformably overlain by conglomerate and coarse clastic sedimentary rocks of the late Early to Middle Carboniferous Aqialehe Group, the basal conglomerate of which contains gold-beating clasts derived from erosion of the Axi deposit.
The Axi deposit, which comprises of a northern and a southern orebody, lies within a steep (65 to 80°), east-dipping, curvilinear fault that exhibits evidence of east-side-down motion. The northern of the two orebodies is 480 m long, from 8 to 20 m wide, and has been drill tested to a depth of at least 450 m. The southern orebody is 750 m to the south and 110 m lower than the northern orebody, and is around 4 m wide, although it is encompassed by a much wider zone of alteration and brecciation.
The auriferous quartz of the northern orebody is dominantly chalcedonic and cryptocrystalline, druzy, sinter-like, and locally banded with a variety of colors that likely reflect differences in trace element chemistry and oxidation. Some of the northern orebody is jasperoidal with minor brecciation.
The southern orebody is more diffuse, with extensive sificification and stockworking, rather than being a discreet feature. Brecclation is much more extensive. Gangue mineralogy is dominated by various forms of silica with sericite, calcite, siderite, and adularia. Various supergene iron ochres and oxides are common, as well as scoradite.
Four types have been recognised, namely: i). quartz veins; ii). brecciated quartz ore; iii). quartz-altered and cemented brecciated andesite ore; and iv). altered country-rock ore.
The total sulphide mineral content is typically around 1%, with fine-grained pyrite being the most abundant sulphide, although arsenopyrite is also a common minor constituent of the ores. In the southern pit, pyrite is coarser-grained and arsenopyrite is more common. Much of the sulphide mineralisation is paragenetically late and fills fractures. Some of the fresh broken ore is very dark,with fracture surfaces coated in fine-grained marcasite. Rare, fine grained tetrahedrite, chalcopyrite, galena, marcasite, tetrahedrite, pyrargyrite, calaverite and sphalerite have been reported. Gold occurs as very fine-grained native gold and electrum, but also occurs as a refractory phase within some sulphide minerals.
The northern orebody averages 5.6 g/t Au, with higher grades (~30 g/t Au) in the central part of the ore zone and local concentrations of as much as 150 g/t Au. The southern orebody averages 4 g/t Au. Au:Ag ratios are typically 1:2.
The dominant alteration in close proximity to the ore is strong silicification, with peripheral phyllic (sericite-pyrite) alteration passing outwards into widespread chlorite and carbonate alteration. Adularia has been variably reported. Silicification of incorporated wallrock fragments with disseminated pyrite, is locally intensive, particularly in the footwall and in brecciated zones in the vein. These altered wallrock zones locally grade 2 to 5 g/t Au.
The Axi deposit has a total resource of approximately 50 t Au, although some estimates as high as 70 t Au have been published. Curent mining is at a head grade which averages about 5.6 g/t Au.
An and Zhu (2018) quote the deposit as containing 12 Mt @ 5 g/t Au.
The most recent source geological information used to prepare this decription was dated: 2003.
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.
Axi
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Selected References:
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An, F. and Zhu, Y., 2018 - Geology and geochemistry of the Early Permian Axi low-sulfidation epithermal gold deposit in North Tianshan (NW China): in Ore Geology Reviews v.100, pp. 12-30.
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Hart C J R, Yitian Wang, Goldfarb R, Begg G, Jingwen Mao and Lianhui Dong, 2003 - Axi and associated epithermal gold deposits in the western Tianshan, Xinjiang, P.R. China: in Mao, Goldfarb, Seltmann, Wang, Xiao and Hart (Eds.), 2003 Tectonic Evolution and Metallogenesis of the Chinese Altay and Tianshan, Proceedings Volume of the International Symposium of the IGCI - 473 Project in Urumqi and Guidebook of the Field Excursion in Xinjiang, China: August 9-21, 2003. IAGOD Guidebook Series 10: CERCAMS/NHM London, pp 209-226
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Pirajno, F., Seltmann, R. and Yang, Y., 2011 - A review of mineral systems and associated tectonic settings of northern Xinjiang, NW China: in Geoscience Frontiers v.2, pp. 157-185.
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Yang, F., Mao, J., Bierlein, F.P., Pirajno, F., Zhao, C., Ye, H. and Liu, F., 2009 - A review of the geological characteristics and geodynamic mechanisms of Late Paleozoic epithermal gold deposits in North Xinjiang, China: in Ore Geology Reviews v.35, pp. 217-234.
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Zheng, J., Shen, P. and Li, C., 2020 - Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: Constraints from U-Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes: in Ore Geology Reviews v.117, 14p. doi.org/10.1016/j.oregeorev.2019.103290
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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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