Fuwan, Changkeng, Fuwang

Guangdong, China

Main commodities: Ag Au
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The Changkeng Au and Fuwan (Fuwang) Ag deposits are located 45 km SW of Guangzhou, Guangdong Province in the southern portion of the People's Republic of China, and 2 km NW of the town of Fuwan.

Both Changkeng Gold and Fuwan Silver Deposits are on the northwestern margin of a triangular Upper Palaeozoic fault basin, bounded by the NE trending Shizhou fault in the NW, the east-west Dashi fault to the south and the NW trending Xijiang fault to the northeast. Known precious and base metal occurrences and deposits are largely along the margins of the 550 sq. km basin.

The basin contains two major sedimentary sequences, separated by a low angle fault zone, possibly activated along an unconformity surface, and intruded by Mesozoic granite on the southeast corner of the basin area only. The stratigraphic succession is as follows:

i). Lower Carboniferous Limestone Sequence, comprising,
   Lower Unit - neritic grey and dark-grey, thickly-bedded, bioclastic limestone;
   Middle Unit - terrestrial grey-whitish and reddish quartz sandstone intercalated with grey calcareous siltstone, mudstone, carbonaceous shale and coal;
   Upper Unit - Grey and dark-grey, medium to thickly bedded, argillaceous limestone and mudstone, with light-grey brecciated bioclastic limestone intercalated with yellowish silicified limestone and silty mudstone. Some gold and most of the silver mineralisation occurs in the brecciated bioclastic limestone.
ii). Upper Triassic Clastics, comprising variegated sandstone, sandy conglomerate and conglomerate, dark-grey mudstone, carbonaceous mudstone and siltstone.

There are no outcrops of intrusive rocks in the immediate vicinity of Changkeng Gold and Fuwan Silver Deposits.

The principal structural feature that hosts both deposits is an upright, open syncline which occurs on the NW margin of the basin and has a NE trending axis. It is cut by a low angle fault zone that follows the contact between the Lower Carboniferous and Triassic units. This fault zone varies from several to several tens of metres in width and is occupied by lenticular brecciated and silicified country rocks, brecciated limestone, and silicified sandy conglomerate. The brecciation and mineralisation occurs in both the upper parts of the Lower Carboniferous carbonate sequence and the lower part of the Upper Triassic clastic rocks. The fault zone is believed to have acted as both a feeder conduit and a host structure for the gold and silver mineralisation. A set of second-order faults were developed in the limestone of the footwall, parallel to the main fault. Silver mineralisation is also found within these second-order faults. Other mineralised occurrences are found in the southwest and northeast extensions of this fault zone.

Gold and silver mineralisation are closely associated spatially but occur at different positions in the low-angle fault zone. Most of the gold is found in the Triassic clastic rocks while the majority of the silver mineralisation occurs in the brecciated, siliceous fault zone which separates the two units. There is also a lesser volume of silver mineralisation associated with fractures parallel to the main fault and lying within the bioclastic limestone of the Lower Carboniferous sequence.

Both the Changkeng Gold and Fuwan Silver Deposits are considered to be sediment hosted, epithermal deposits.

The Fuwan Silver Deposit is characterised by vein and veinlet style mineralisation within zones of silicification.

Two styles of mineralisation are recognised at Changkeng and Fuwan: i). Siliceous material - grey to dark grey in colour, composed mainly of secondary quartz, illite, argillaceous and carbonaceous material, and pyrite, with well developed fractures and mariolitic cavities; ii). Calcareous-siliceous material in silicified limestone - light to dark grey in colour, composed of secondary quartz, residual limestone, calcite, and pyrite, occuring in second-order faults in the footwall limestone of the contact zone.

The dominant sulphide minerals within the ore deposits are sphalerite and galena with lesser pyrite and rare arsenopyrite, chalcopyrite and bornite. Major silver ore minerals include freibergite, paragyrite, silver-antimony, brongriarite, and argentite; jamesonite as a secondary mineral and eugenite, a Sb-Cu-Ag sulphide mineral, a 'black silver' mineral, a silver sulphur mineral, and native silver as minor minerals. The Ag ore at Fuwan is only accompanied by low, <0.2 ppm Au.

Both the Changkeng Au and Fuwan Ag deposits are confined to a fault zone separating the Lower Carboniferous limestone sequence from the Upper Triassic Clastic sequence, as described above. Two of the zones of gold mineralisation at Changkeng, Zone 1 and Zone 2, trend NE and dip at 30 to 50° SE in their upper sections, shallowing to 15 to 30° in the lower portion of the fault. These two zones merge both down dip, andalong strike to the NE. The Au veins occur as lenticular bodies within the brecciated Triassic clastics in the upper section of the synform and tend to pinch out toward the hinge of the syncline where they are replaced by silver mineralisation at the Fuwan Ag deposit.

The bulk of the Ag mineralisation lies within the brecciated and silicified fault zone in Zone 1 (completely within the fault plane) and in Zone 2 (where it is only partially within the fault plane).

Zone 1 is continuous over a strike length of 2.7 km, varying from 1.5 to 23.6 m in intersected width, averaging 5.5 m. Silver grades vary from 1 to 3285 g/t with a length weighted average grade of 133 g/t.

Zone 2 has a total strike length of 1.96 km with an average intersected width of 3.4 m, varying from 0.5 to 17.7 m and grades ranging from 1 to 2200 g/t Ag, with a length weighted average grade of 157 g/t.

Zones 3, 4, 5, 6, 7 and 8 are entirely within the footwall limestone sequence and lie along second-order faults in the limestone, parallel to the major fault. They have strike lengths of between 250 m and 1.1 km, and average thickinesses of from 1.5 to 4 m, with grades of 70 to 325 g/t Ag.

The total indicated resources at Fuwan (Minco, 2007): 4.48 Mt @ 203 g/t Ag, 0.20 g/t Au, 0.18% Pb, 0.53% Zn - containing 908 t Ag,
The total inferred resources at Fuwan and Changkeng (Minco, 2007): 17.95 Mt @ 172 g/t Ag 0.32 g/t Au 0.22% Pb, 0.61% Zn
        - containing 3080 t Ag.

Total geological resource (White & Kaihui Yang, 2007) at Fuwan: 6000 t of contained Ag at a grade of 268 g/t Ag,
Total geological resource (White & Kaihui Yang, 2007) at Changkeng: 32 t of contained Au at a grade of 7 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2007.    
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.

  References & Additional Information
   Selected References:
Mao, J., Cheng, Y., Chen, M. and Pirajno, F.,  2013 - Major types and time-space distribution of Mesozoic ore deposits in South China and their geodynamic settings: in    Mineralium Deposita   v.48, pp. 267-294.

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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