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Shiyingtan, Xitan |
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Xinjiang, China |
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Au
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Super Porphyry Cu and Au
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The Shiyingtan or Xitan epithermal gold deposit is located in the Kanggurtag Gold Belt of the East Tianshan Mountains, ~175 km SE of Urumqi in northern Xinjiang, China (#Location: 42° 5' 25"N, 90° 12' 26"E).
See the Kanggur and Matoutan record for the setting of the Kanggurtag Gold Belt.
The orefield hosting the Shiyingtan deposit extends over a strike length of ~1.8 km and width of ~1.2 km, located in a Permian continental facies volcanic basin, which overlies the Yamansu-Jueluotag Palaeozoic arc-back-arc system in Eastern Tianshan. This basin comprises continental facies volcanic rocks of the Early Permian Arbashayi Formation, consisting of purple andesitic to dacitic lavas, breccias and tuffs, dated at 281±10 and 285±7 Ma (Rb-Sr isochron; Li et al., 1998). Basement consists of Early Carboniferous basalt, andesite and dacite of the Xiaorequanzi Formation.
The deposit is located on the NW margin of an ~20 km diameter caldera structure and is infilled with felsic volcanic and associated pyroclastic rocks. The gold mineralisation is hosted in an undeformed and unmetamorphosed breccia pipe-like structure, suggesting that it was emplaced during a post-tectonic volcanic episode and possibly during a subsequent rifting event. The fragmentals within this structure is pervasively silicified and cur by multidirectional veins. The breccia comprises lithic clasts now pervasively replaced by alteration phases, which include chalcedonic quartz, quartz and pyrophyllite.
The central zone of silicification has associated pyrophyllite and chlorite alteration and is surrounded by an envelope of pyrite-sericite. This is, in turn, overprinted by calcite, dolomite, chlorite and ankerite. Ore minerals include free gold, which is usually interstitial to grain boundaries, pyrite, various sulphosalts of Ag and As, some selenides (Ag2Se) and chlorides (Ag2Cl). Veins are composed of coarse-grained calcite + quartz, with zones of bladed calcite crystals, interpreted to be the result of to precipitation from CO2-rich boiling solutions, with pyrophyllite ± quartz at the vein margins. Late veinlets of gypsum are also recognised.
The Shiyingtan orefield contains 18 orebodies, occurring as veins, lenses and tear-shaped zones. Four of these are economically important and designated as L1, L2-1, L2-4 and L3. Orebody L1 is 340 m long and 0.89 to 8.92 m thick (averaging 3.8 m), and is hosted in a fracture zone dipping at angles ranging from 34 to 51°N. The average grade of orebody L1 is 12.5 g/t Au.
The ore assemblages include electrum-pyrite-quartz, electrum-native gold-quartz-sericite and electrum-native gold-quartz-calcite. Hydrothermal alteration proximal to mineralisation is characterised by silica, sericite, carbonate, limonite and laumontite with occasional adularia, whilst distal alteration includes kaolinite, propylitisation and carbonates. Electrum, native gold and calaverite, tend to occur as very small grains, flakes or dendritic forms. Fluid inclusion suggest homogenisation temperatures in quartz are 109 to 191°C, with an average of 150°C (Feng et al., 2000; Wang et al., 2005)
The δDwater and δ18Owater values are from -81 to -95‰ and -2.5 to -10.4‰, respectively, indicating the fluid-system is dominated by meteoric water. The δ34OS value of pyrite in ore ranges from 0.6 to 1.3‰, averaging 1.0‰, implying that it was sourced from igneous rocks or mantle material. This data suggests, the Shiyingtan deposit was formed at low-temperature, shallow depth, by a meteoric water-dominated fluid-system, which continuously leached ore-forming elements from the volcanicsubvolcanic rocks and precipitated Au into favourable loci, such as fault zones (Pirajno et al., 2011).
Isotopic ages of the host rocks and related intrusions are between 293±1 and 255±21 Ma (Early Permian); whilst ages for metallogenesis are between 288±7 and 237±12 Ma (Early Permian to Early Triassic; Li et al., 1998). This shows that the metallogenesis just postdated magmatic activity and both magmatism and metallogenesis took place in a postcollision tectonic setting. The deposit has been classified as a low-sulphidation epithermal system and the breccia pipe might be the remnant of a hydrothermal explosion crate.
Resources have been estimated at about 6.4 t of contained gold, with grades ranging from 5 to 10 g/t Au. Pirajno et al. (1997) reported that the deposit contained about 5.5Mt @ 7 g/t Au, at a cutoff value of 1 g/t Au.
The most recent source geological information used to prepare this decription was dated: 2011.
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.
Shiyingtan, Xitan
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Selected References:
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Han, C., Xiao, W., Zhao, G., Su, B., Sakyi, P.A., Ao, S., Zhang, J. and Zhang, Z., 2014 - Late Paleozoic Metallogenesis and Evolution of the East Tianshan Orogenic Belt (NW China, Central Asia Orogenic Belt): in Geol. of Ore Deposits (Pleiades Publishing) v.56, pp. 493-512.
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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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Wang, Y.T., Mao, J.W., Chen, W., Yang, J.M., Wang Z.L. and Yang F.Q., 2005 - Strike-slip fault controls on mineralization in the Kanggurtag gold belt in the Eastern Tianshan, Xinjiang, NW China: in Mineral Deposit Research: Meeting the Global Challenge, Springer, Berlin, Heidelberg, pp. 1347-1349.
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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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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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