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Gacun |
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Sichuan, China |
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Zn Pb Cu Ag Au
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Super Porphyry Cu and Au
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The Gacun volcanic hosted massive sulphide zinc-lead-copper-silver-gold deposit is hosted by Triassic submarine calcalkalic volcanics in the northern Yidun Terrane of western Sichuan in southwestern China (#Location: 31° 11' 6"N, 99° 32' 49"E).
The lens shaped, ~550 km long, NNW-SSE aligned Yidun Terrane, is located to the east of the parallel but more extensive Qiangtang Terrane. It is bounded to the west by the Jinshajiang Suture and to the east by the Ganzi-Litang Suture which separates it from the Songpan-Ganze Complex/Terrane. For detail of the setting and detail of the terrane, see the Regional Setting section of the separate Yulong Porphyry Belt record.
The Yidun Terrane contains more than 100 polymetallic VHMS deposits, of which Gacun is the largest. The volcanic arc within the Yidun Terrane was developed on a basement of thick Palaeozoic to upper Triassic platformal carbonates and clastics with subordinate alkaline basalts. Arc-continent collision at the end of the late Triassic resulted in the arc sedimentary-volcanic sequence and intrusion of 200 ±2 Ma collisional granites, followed by compressional deformation, uplift and lower greenschist facies metamorphism in the Mesozoic and Cenozoic.
Gacun lies within the 20 x 8 km Changtai volcano-sedimentary basin which was filled by the bi-modal volcanic dominated Gacun Formation. The basal 50 to 250 m of this unit is composed of conglomeratic sediments, andesite detritus, sandstone, shale and marble interbeds. This is followed by a mafic package (basaltic volcanics and associated dyke swarms) which is in turn overlain by an extensive 100 to 460 m thick succession of interbedded sandstone, siltstone, conglomerate, deeper marine phyllite and chert. At Gacun the basalic volcanics are dominant, with a lower 50 m thick lava flow below 100 m of tuffaceous rocks. The basalt is tholeiitic, massive and porous. The basaltic-tuffaceous unit includes around 30 m of porous tuff and pumice with the 20 to 30% of the tuff being pore space filled by hydrothermal chlorite, epidote, quartz, calcite with pyrite, galena and sphalerite.
Higher in the sequence, felsic volcanics comprise around 65% of the district and are around 1000 m thick, forming a North to NNW trending 14 km long belt. There are two felsic units in fault contact. The lower is a dacite or dacitic rhyolite with massive and autobrecciated lava, tuff breccias and tuffs. These are in turn followed by tuffaceous siltstone, limestone and phyllite. The upper felsic unit is exposed on both flanks of a NNW trending anticline and comprises mainly rhyolitic pyroclastics.
The host volcanic rocks at Gacun belong to the upper felsic package, comprising from the base:
• dacitic volcanics - 70 to 100 m of dacite and dacite-rhyolite tuff breccia and lesser dacite-rhyolite lava and domes;
• dacitic-rhyolitic volcanics - 300 m of dacitic rhyolite, with lesser dacite, rhyolite and sediments, composed of a lower dacite-rhyolitic breccia lava, a middle tuff breccia and an upper rhyolitic tuff and limestone lenses - sulphide ore is mainly in the tuff breccia and tuffaceous rocks; and
• rhyolitic volcanics - >100 m of rhyolitic volcanic rocks overlain by a package of sedimentary and exhalative rocks. It comprises a lower rhyolite tuff breccia with sheeted sulphide orebodies, a middle rhyolitic breccia lava hosting stringer and stockwork mineralisation, and an upper 5 to 10 m of rhyolitic tuff and tuffaceous rocks conformably overlain by a unit that includes chemical sediments. The latter is composed of 3 sulphide bearing barite horizons, an iron oxide-silica band (red quartz-magnetite and grey chert within 30 to 50 m above the sulphide equivalent horizon), carbonate rocks (dolomite, dolomitic limestone and bioclastic limestone which conformably overlie the massive sulphides and barite) and carbonaceous shale.
Three main sulphide zones are recognised, namely:
• Lower stringer zone as a series of 11 tabular sheets dipping 70 to 80° W over a strike length of 1150 m and 700 m downdip in tuff breccias, the largest is up to 19 m thick with 4% Pb+Zn+Cu. These transgressive stringer ore zones are stratabound, being restricted to particular units and contain pyrite, sphalerite, galena, chalcopyrite and minor arsenopyrite, pyrrhotite and ilmenite.
• Middle stringer-stockwork zone occurring as 6 tabular sheets within a 1700 m wide by 1900 m long interval which is about 20 m thick and dips at 80° W. This zone is concordant with the overlying massive sulphides and is above the stringer zone, usually in the uppermost strongly silicified rhyolite. Sulphides include: pyrite, sphalerite, galena and chalcopyrite with subordinate tetrahedrite, bornite, arsenopyrite, bournonite, boulangerite and minor native gold cuproaurite and konigsbergite.
• Upper massive zone comprising 4 (M1 to M4) Zn-Pb-Cu-Ag-Au rich sheets of conformable massive sulphides, dipping at 70 to 80° westward. Two (M1 & M2 are closely associated with dark and white massive barite. M1, the largest is 8.5 m thick and has a strike length of 1400 m. Five zones are recognised in the massive sulphide ores, namely: i). the lowest is a massive sulphide lens with a sulphide-silica-barite cap, ii). massive sulphide ore followed by a dark barite lens iii). massive sulphide ore (usually with barite fragments) with a grey-white thick massive barite layer and thin rhyolite tuff interlayers, iv). clastic black massive sulphide ore and an overlying barite lens and tuffaceous shale, v). dolomite and dolomitic limestone lenses (with no bioclastic component) overlian by fine grained, black massive sulphides with siliceous breccia fragments. The zone is continuous over a strike length of 730 m.
The associated alteration is mainly stratigraphic in porous rocks of the footwall extending along strike in association with the favourable horizon from 500 to 1000 m. There are four alteration zones, namely: i). an uppermost silica-hyalophane-pyrite with stockwork sulphides; ii). quartz-sericite-pyrite conformably below the previous zone; iii). a lower semi-conformable sericite-chlorite zone, and iv). chlorite-epidote patches.
Reserves in 1990 were 4 Mt @ 5.4% Zn, 3.7% Pb, 0.44% Cu, 160 g/t Ag, 0.31 g/t Au (Hou, et al., 2001).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Gacun
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Selected References:
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Hou Zengqian, Khin Zaw, Qu Xiaoming, Ye Qingtong, Yu Jinjie, Xu Mingji, Fu Deming,Yin Xianke 2001 - Origin of the Gacun volcanic-hosted massive Sulfide deposit in Sichuan, China: fluid inclusion and Oxygen Isotope evidence: in Econ. Geol. v96 pp 1491-1512
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Hou, Z.,Khin Zaw, Pan, G., Mo, X., Xu, Q., Hu, Y. and Li, X., 2007 - Sanjiang Tethyan metallogenesis in S.W. China: Tectonic setting, metallogenic epochs and deposit types: in Ore Geology Reviews v.31, pp. 48-87. doi.org/10.1016/j.oregeorev.2004.12.007.
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Sun Haitian 1992 - A general review of volcanogenic massive sulphide deposits in China : in Ore Geology Reviews v7 pp 43-71
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Yu, H.-J., Jiang, J.-W. and Li, W.-C., 2020 - Controls of variable crustal thicknesses on Late Triassic mineralization in the Yidun Arc, Eastern Tibet: in J. of Asian Earth Sciences v.195, 13p. doi.org/10.1016/j.jseaes.2020.104285
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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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