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Keketale |
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Xinjiang, China |
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Zn Pb Ag
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Super Porphyry Cu and Au
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The Keketale volcanogenic massive sulphide (VHMS) Pb-Zn deposit is located in the Southern Altai Metallogenic Belt of northwestern Xinjiang, China. It is located ~110 km SW and ~250 km SE of the borders with Mongolian and Kazakhstan respectively, and ~110 km SE of Altai City.
The Keketale deposit, which is the largest Pb-Zn deposit in the Chinese Altai, was discovered in 1986 and has been exploited since 1999. The Chinese Altai, which extends over a NW-SE strike length of ~450 km, extends into the neighbouring countries of Kazakhstan, Russia and Mongolia, and lies within the southern margin of the Central Asian Orogenic Belt. It is composed of Cambrian to Carboniferous volcanic and sedimentary rocks, intruded by voluminous masses of Ordovician to Triassic-Jurassic granitoids, and is separated from the Junggar Block to the south by the major dextral NW-SE trending Erqis Shear Zone. The Southern Altai Metallogenic Belt, which lies within the Chinese Altai, comprises a string of significant Zn-Pb, Au and Cu-Ni deposits. This metallogenic belt is developed within a polyphase fold system, bounded by a series of faults, and comprises four distinct volcanosedimentary basins, which are from NW to SE, the Ashele, Chonhuer, Kelang and Maizi basins. Each represents a large synclinal structure hosting numerous VHMS deposits or small occurrences.
The Keketale and Mengku deposits are 20 km apart on the northern limb of the Maizi basin, whereas Akeharen and Daqiao on the southern limb of the same structure/basin are 14 and 16 km to the NW of Keketale. The northern limb of the syncline is overturned, with an axial plane that dips NE and strikes NW–SE. The Keketale deposit is the largest and most important economic producer of Pb and Zn in the Southern Altai Metallogenic Belt. It comprises 12 orebodies that outcrop over an area of nearly 0.7 km2. All of the orebodies at Keketale strike NW and dip SE concordant with the host strata.
The exposed rocks in the Keketale district belong to the upper section of the Lower Devonian Kangbutiebao Formation, deposited within a continental arc to back-arc tectonic setting (Wan et al., 2010). All of the orebodies at Keketale are hosted by meta-calcareous sandstones, and are intercalated with dacitic to rhyolitic volcanic and pyroclastic rocks. The structural footwall to the SE of the deposit is mainly composed of fragmental meta-tuff, whilst the structural hanging wall to the NW is mainly meta-rhyolite, agglomeratic meta-tuff and meta-tuff. The orebodies have considerably variable sizes, ranging from >700 m in strike length x 80 m in thickness and up to 550 m down dip at the No. 7 orebody, to <100 m in length x 10 m thick x 200 m down dip at the smallest. The No. 7 orebody comprises one main and several smaller lenses. All are stratabound within the meta-calcareous sandstone of the upper Kangbutiebao Formation. The main lens is slightly lenticular, plunges in the down the dip direction, and is more deformed than other orebodies, possibly due to its proximity to the hinge of the overturned syncline. The main lens has been oxidized at surface, with an oxide zone that extends from the surface to a depth of ~20 m. The primary sulphides of the main lens are dominated by massive (>60 vol.%) sulphides, accompanied by dense to finely disseminated and minor banded and laminated sulphide mineralisation that is commonly found on the margin of the ore lens. The disseminated mineralisation is well developed on the structural uppermost sections of the lens, extending into the overlying calcareous meta-sandstone. In the immediate structural hanging wall of the main lens there are several vein type ores. The smaller lenses in the footwall of the main lens are mainly composed of massive ore with high metal grades up to 8.5% combined Pb+Zn.
The principal sulphide minerals, in decreasing order of abundance, are pyrite, sphalerite, galena and pyrrhotite, with minor chalcopyrite. Gangue minerals include quartz, calcite, biotite and plagioclase. Pyrrhotite and chalcopyrite, which are located in the uppermost part of the main len of orebody No. 7, have a xenomorphic texture, and are always associated with vein-type pyrite. Pyrite occurs as idiomorphic grains and hypidiomorphic granular aggregates throughout the sulphide lenses. Sphalerite and galena are the principal economic minerals. Sphalerite is mostly found as xenomorphic aggregates, sometimes as matrix to pyrite grains, and mainly in the upper and middle parts of the ore lens. Galena is mainly present as idiomorphic grains and sometimes as the interstitial phase amongst many other sulphides (e.g., pyrite, sphalerite). It is located in the middle and lower sections of the main lens. The Pb/Zn ratio increases with depth. The overall zonation ranges from (Fe+Cu) in the structurally uppermost parts of No. 7 orebody → (Fe+Zn+Pb with Zn>Pb) in the middle → (Fe+Pb+Zn with Pb>Zn) at the structural base of the orebody. Chalcopyrite is very minor and has no economic significance at Keketale.
Strong hydrothermal alteration is evident in the immediate vicinity of the ore lenses at Keketale (Qin et al., 1998), and includes zones characterised by chlorite, sericite, carbonates, pyrite and skarn minerals. An intense chlorite zone, which is characterised by chlorite ±pyrite ±quartz ±carbonate, is found immediately structurally above the ore lenses, both in calcareous sandstone and in rhyolitic tuff. This chlorite zone also includes pyrite and skarn minerals, which form a narrow stratabound sheet immediately below the ore lenses. The skarn minerals are the product of alteration of marbles and the calcareous meta-sedimentary rocks. A broad sericite zone characterised by sericite ±chlorite plusmn;carbonate envelopes the envelops the chlorite zone, and mainly affects the meta-rhyolite, and to a minor extent sedimentary host rocks.
Structural footwall rhyolite and hanging-wall dacite have been dated at 410.5 ±1.3 Ma and 394.8 ±1.9 Ma, respectively (Yang et al., 2018). Rb-Sr dating of sphalerite yield an age of 398.2 ±3.3 Ma (Yang et al. (2018). Muscovite from selvages to sulphide-bearing veins that cut across the massive sulphide mineralisation and host rocks along faults and fractures has been dated at 259.3 ±2.6 Ma (Ar-Ar; Zheng et al., 2013) suggesting a Middle Permian tectonic-thermal event contributed additional resources to the deposit.
The deposit has a resource of 1.94 Mt and 0.89 Mt of contained Zn and of Pb metal respectively @ grades of 3.16% Zn, 1.51% Pb, as well as 650 t of contained Ag @ grades of up to 40 g/t Ag. This would equate to ~60 Mt of Pb-Zn ore. The No.7 and 9 orebodies account for almost 70% of the total resource.
The most recent source geological information used to prepare this decription was dated: 2022.
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.
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Selected References:
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Sun, C., Yang, X., Zhang, H., Ji, W., Chen, B., Dong, Z., Faisal, M and Xi, D., 2022 - Tracing the formation and modification of the Keketale VMS-type Pb-Zn deposit, Altai Mountains: Insights from ore deposit geology, geochronology, and magnetite geochemistry: in Ore Geology Reviews v.144, 22p. doi.org/10.1016/j.oregeorev.2022.104852.
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Wan, B., Zhang, L. and Xiao, W., 2010 - Geological and geochemical characteristics and ore genesis of the Keketale VMS Pb-Zn deposit, Southern Altai Metallogenic Belt, NW China: in Ore Geology Reviews v.37, pp. 114-126
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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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