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Tonglushan |
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Hubei, China |
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Au Cu
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Super Porphyry Cu and Au
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The Tonglushan skarn Au-Cu deposit is located ~3 km southwest of the Daye city, in Hubei Province, ~200 km WNW of Nanchang (#Location: 30° 4' 57"N, 114° 56' 3"E).
The deposit has a long mining history dating back to the Shang Dynasty between 1600 and 1046 BC. Ancient workings adjacent to the current modern operation are estimated to have operated until the Han Dynasty, early in the first century BC. They are estimated to have produced ~80 000 to 120 000 tonnes of copper by manual digging and processing, based on the volume of slag on site. The copper-rich veins contained 12 to 20% Cu, 20% Fe in deposits of cuprite and malachite.
This deposit lies within the Edong Mining District of the Middle to Lower Yangtze River Valley Metallogenic Belt on the northern margin of the Yangtze craton. For more detail on the regional setting see the Yangtze River Belt record.
The Yangtze Valley Gold Province lies within the Yangtze River Trough which was developed on the eastern margin of the Yangtze Craton. The trough is composed of Mesozoic fault-bounded volcanic basins, with mainly 135 to 127 Ma felsic to intermediate volcanic rocks of the Yangtze Igneous Group emplaced along the basin margins and as zones of uplift within the basin. This tectonism is interpreted to be the result of back-arc, post-collisional extension that occurred subsequent to amalgamation of the North and South China cratons, and during subduction of oceanic plates beneath eastern China.
The stratigraphy at Tonglushan includes the Lower Triassic Daye Formation limestone and dolostone. Triassic NWW- to EW-trending folds and faults were developed, which were overprinted/crosscut by Jurassic and Cretaceous NNE trending folds and faults. The carbonates have been metamorphosed to (dolomitic)-marble and/or altered to skarn, which occur as concealed and discontinuous NNE-trending blocks (Liu et al., 2005; Wei et al., 2007; Zhang et al., 2018, 2019). The major mineralisation-related intrusive rocks are the quartz monzodiorite and its porphyritic phase. The quartz monzodiorite, which is spatially and genetically related to the Cu-Au-Fe skarn mineralisation, is widespread at Tonglushan, and intruded the Triassic carbonate rocks at 141.0 ±0.8 Ma, and is composed of 50 to 60 vol.% plagioclase, ~15 vol.% hornblende, 15 to 20 vol.% K feldspar, 10 to 15 vol.% quartz and minor biotite, with accessory magnetite, titanite, apatite, zircon and monazite (Zhang et al., 2018). It contains 62.51 to 66.95 wt.% SiO2, relatively high in total alkalis (K2O + Na2O = 5.67 to 9.63 wt.%) with [Al2O3/ (CaO + Na2O + K22O)] of 0.73 to 0.90 (Zhao et al., 2010; Zhang et al., 2018). It is rich in light rare earth elements (LREE) and large ion lithophile elements (LILE), but relatively depleted in Nb, Ta and Ti contents, and show I-type and magnetite-series affinity (Zhao et al., 2010; Zhang et al., 2018). In addition, Sr-Nd-Pb-Hf isotope geochemistry suggest that the quartz monzodiorite was mainly derived from enriched lithospheric mantle and had experienced lower crustal contamination and fractional crystallization (Li et al., 2009; Zhao et al., 2010; Xie et al., 2011).
These intrusives belong to a Late Jurassic to Early Cretaceous 157 to 137 Ma suite that included Tonglushan porphyritic quartz-monzodiorite, emplaced into Triassic dolomitic limestones within the southern limb of an anticlinorium. Mineralisation is concentrated at the intersections of NW, NNE and ENE striking faults with the fold to form three ore zones. The major ore zone dips subvertically and covers an area of about 2100 x 300 to 350 m.
In summary, mineralisation occurs as massive or disseminated sulphides associated with retrograde alteration mainly within skarn, with minor mineralisation formed along the margins of the intrusion and within the surrounding marbles. The main ore phases comprise chalcopyrite, bornite, pyrite, magnetite, hematite, chalcocite, molybdenite, sphalerite, gold and tennantite. The principal gangue assemblages include diopside, garnet, calcite, dolomite, quartz, phlogopite, epidote, white mica, chlorite, serpentine, tremolite, actinolite and plagioclase. Gold is associated with sulphide minerals and quartz, commonly occurring as inclusions in pyrite and chalcopyrite.
Thirteen Cu-Au-Fe skarn-type and local Cu-Au breccia bodies have been delineated at Tonglushan (Liu et al., 2005; Zhao et al., 2012; Zhang et al., 2018), of which the No. 1, 3, 4 and 13 orebodies are more important. These lenticular/stratabound bodies are mainly distributed along NNE-trending faults and the intrusive contact between the quartz monzodiorite porphyry and the dolomitic-marble lithologies.
Prograde and retrograde skarn alteration, and Cu-Au-Fe mineralization mainly occurred at the external intrusive contact. In detail, five alteration/mineralisation stages have been recognised, namely:
• Pre-ore skarn-potassic alteration, mainly as abundant anhydrous minerals in the endo- and exoskarn. Three generations of garnet are recognised, of which the first two (Grt1 and Grt2) are pre-mineral. Grt1 is commonly dark brown to dark green, while Grt2 is light brown/green and usually accompanies Grt1 with distinctive oscillatory zoning. Distal to the intrusive contact, Grt1 coexists with diopside/wollastonite in the exoskarn. Meanwhile, potassic/garnet/diopside alterations were also developed in the quartz monzodiorite porphyry. The younger Grt3 accompanies mineralisation as detailed below;
• Fe mineralisation and retrograde alteration, occurs in marble, as epidote-quartz-specularite and epidote-actinolite-apatite assemblages, where it is seen to replace Grt1. In dolomitic marble, phlogopite-magnetite (-serpentine) replaces diopside. This alteration is accompanied by extensive magnetite and hematite mineralisation. The hematite bodies are mainly found in the mid- and shallow-sections of the dolomitic-marbles, while the magnetite bodies are closely related to the skarn and retrograde alteration. In thin section saponite-epidote is seen to replace Grt1, and the phlogopite-serpentine-saponite assemblage is cut by late calcite veins. In this stage, epidote, actinolite, magnetite and biotite alteration also occur in the quartz monzodiorite porphyry.
• Cu-Au quartz-sulphide mineralisation, which mainly includes chalcopyrite, bornite, chalcocite and pyrite, with local molybdenite, sphalerite, digenite and native gold. The main gangue minerals include quartz, calcite and garnet Grt3. Magnetite-hematite-phlogopite replaces Grt1, which is in turn replaced by chalcopyrite. The chalcocite-bornite-digenite assemblage is also observed in the marble. Disseminated chalcopyrite-sphalerite is seen to replace the magnetite, whilst the latter is also cut by Grt3-calcite veins with residual diopside. Chlorite is one of the most significant alteration minerals at this stage. Apart from the chlorite-bearing quartz-sulphides veins (with K feldspar alteration halos) cutting the quartz monzodiorite in the internal intrusive contact, chlorite replaced epidote and pyrite-chlorite veins cutting the magnetite-hematite mineralisation.
• Carbonate alteration is characterised by extensively developed calcite ±ankerite ±pyrite veins crosscutting the skarn, Fe-oxides and quartz-sulphides ores, and locally the quartz monzodiorite porphyry. Lower crystallinity phyllosilicates, mainly kaolinite, illite and montmorillonite are also evident.
• Supergene alteration, manifested as abundant supergene malachite and azurite, occur in the shallow parts of the No. 1, 3 and 8 orebodies (Zhao and Lin, 1990; Shu et al., 1992)
Tonglushan is reported to contain 69 t of Au and 1.1 Mt Cu at grades averaging 1.2 g/t Au, >1% Cu.
The most recent source geological information used to prepare this decription was dated: 2002.
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.
Tonglushan
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Selected References:
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Zhang, S., Chu, G., Cheng, J., Zhang, Y., Tian, J., Li, J., Sun, S. and Wei, K., 2020 - Short wavelength infrared (SWIR) spectroscopy of phyllosilicate minerals from the Tonglushan Cu-Au-Fe deposit, Eastern China: New exploration indicators for concealed skarn orebodies: in Ore Geology Reviews v.122, 20p. doi.org/10.1016/j.oregeorev.2020.103516
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Zhao, H., Xie, G., Wei, K. and Ke, Y., 2011 - Mineral compositions and fluid evolution of the Tonglushan skarn Cu-Fe deposit, SE Hubei, east-central China: in International Geology Review v.54, pp. 737-764.
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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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