Fengshan ore field - Jilongshan, Fengsandong, Caojiashan, Zhulintang

Hubei, China

Main commodities: Au Cu
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The Jilongshan and Fengsandong porphyry/skarn gold-copper, and the carbonate hosted Caojiashan and Zhulintang gold deposits constitute the Fengshan ore field, located within the Yangtze Valley Gold Province of eastern China, in Hubei Province, ~200 km NW of Nanchang.

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 principal country rocks in the Fengshan ore field comprise the Late Permian Dalong and Early Triassic Daye Formations. The Dalong Formation, mainly to the south, is composed of shale intercalated with limestone. It is concordantly overlain by the Daye Formation, which is widespread in the Fengshan district and consists of 900 m of limestone, dolomitic limestone and dolomite. Dolomitic limestone and limestone are the principal hosts to the Fengsandong and Jilongshan deposits, which are similar to other gold-bearing skarn deposits in the Middle-Lower Yangtze River metallogenic belt (Zhao et al., 1999).

Two subparallel, elongated clusters of WNW- to NW-striking Cretaceous (Yanshanian; 149.9 ±1.3 and 145.9 ±0.7 Ma; U-Pb; Xie et al., 2019) granodiorite porphyry stocks and dykes ~ 2 km apart were intruded into the Early Triassic carbonate rocks. The Fengsandong and Jilongshan Cu-Au skarn deposits are associated with these granodiorite porphyry stocks that are exposed over an area of about 1.3 and 1.2 km2, respectively. The Fengsandong granodiorite porphyry stock has been dated at 146.0 ±2.0 Ma (SHRIMP zircon U-Pb method; Xie et al., 2011). It is characterised by a porphyritic texture with phenocrysts of ~5% quartz, ~20% plagioclase, ~10% hornblende and ~5% biotite in a matrix of very fine grained ~30% plagioclase, ~10% K feldspar and ~10% quartz with accessory magnetite, apatite, titanite and zircon.

The intrusives at Fengsandong and Jilongshan have I-type characteristics, with moderate SiO
2 (63.0 to 66.3 wt.%) and MgO (1.8 to 2.1 wt.%; Xie et al., 2008, 2011). Xie et al. (2019) interpret these rocks to have been derived by partial melting of enriched lithospheric mantle together with the involvement of variable amounts of lower-crustal components (5 to 35 wt.%) during the evolution of the magma (Xie et al., 2011). The economic mineralisation at Fengsandong and Jilongshan is found in two spatially separated zones along the contacts between the stocks and carbonate rocks and comprise irregularly shaped massive sulphide to disseminated sulphide orebodies.

The Jilongshan deposit is 1490 m long and persists to a depth of >340 m along the north contact between the granodiorite porphyry stock and carbonate rocks (Wang et al., 2014). The granodiorite porphyry stock at Jilongshan is surrounded by a marble front. Metal zoning at Jilongshan grades from Cu-Au stockwork veinlets within the stock, to Cu-Au or Au-Cu skarn at the intrusive contact, to carbonate-replacement Pb-Zn ore in marble, and peripheral Au-As ore toward the marble front.

The Fengsandong deposit is >800 m long at the surface and 30 to 40 m wide, extending to depths of >400 m (Wang et al., 2014). Carbonate-replacement Pb-Zn mineralisation occurs in the southeastern part of the deposit area, although its relationship with the main orebody has not been determined due to lack of outcrop.

Wall-rock alteration is widespread and intense in the Cu-Au skarn altered zones, comprising skarn, carbonate and minor potassic alteration. The latter occurs as K feldspar-quartz-sulphide veinlets superimposed on the granodiorite porphyry. Exoskarn alteration is characterised by diopside (Di
80–99:Hd1–19) and andradite (Ad87–100:Gr0–13; Zhao et al., 1999). These prograde assemblages are overprinted by retrograde hydrous minerals including epidote, chlorite and phlogopite as well as disseminated sulphides. Carbonate alteration generally overprints the skarn assemblages and comprises calcite and sulphides. Metallic minerals are principally pyrite, chalcopyrite and bornite, with subordinate molybdenite, tetrahedrite, tennantite, sphalerite and galena. Molybdenite dated at 149.1 ± 2.1 to 145.4 ± 2.9 Ma (Re-Os in Mo; Xie et al., 2019) is present as fine-grained (0.05 to 0.2 mm) flakes in the mineralised skarn and as quartz-sulphide veins in the granodiorite porphyry stock, and typically coexists with chalcopyrite. Some realgar and orpiment are found in the fault-controlled Au-As ore. Native gold infills pyrite fractures in stockwork veinlets, and native gold, electrum, hessite (Ag2Te), tetradymite (Bi2Te2S), aikinite (PbCuBiS3), and wittichenite (Cu3BiS3) coexist with chalcopyrite, pyrite and bornite in the Cu-Au skarn ores, whilst coloradoite (HgTe) coexists with calcite and realgar in the Au-As ores toward the marble front.

Fengsandong is reported to contain 54.6 Mt @ 1.0% Cu and 46.8 Mt @ 0.3 ppm Au.

Jilongshan is reported to contain 13.0 Mt @ 1.3% Cu and 6.3 Mt @ 3.5 ppm Au.

The Caojiashan (between the Fengsandong and Jilongshan porphyry stocks) and Zhulintang (ESE of the Fengsandong stock) are carbonate-hosted Au deposits. Both are closely associated with porphyry dykes and are each located <1 km distal to the stocks of the two main porphyry/skarn deposits of Fengshan ore field.

Together Caojiashan and Zhulintang have a total total indicated + inferred resource of 12 Mt @ 5.0 g/t Au (Liu et al., 2004; Shu, 2004).

Local Pb-Zn-Ag-Au ores are also present at Caojiashan. Early Triassic limestone of the Daye Formation is the principal host for these two Au deposits. The granodiorite porphyry dykes with which they are spatially associated are of similar compositions and ages to the larger stocks associated with the Fengsandong and Jilongshan deposits. Mineralisation occurs as veins that cut the limestone and granodiorite porphyry dykes, with WNW (280°) to ENE (70°) strikes and 50 to 70°S dips. Pb-Zn-Ag-Au mineralisation locally occurs as veins trending east-west and dipping at ~50°S, with an average grade of 6.2 wt.% Pb + Zn, 76.9 g/t Ag and 0.8 g/t Au.

Two vein-type Au orebodies have been mined by open pit at Zhulintang. The No. 1 orebody is hosted by limestone and is 120 m long x 5.5 m wide, whilst the No. 2 orebody occurs in limestone and granodiorite porphyry dykes and is 160 m long x 26 m wide. Both trend WNW (290°), dip 45°SW, and contain 1.9 to 13.9 g/t Au. These deposits are characterised by trace (<2%) sulphides, mainly of realgar and orpiment, with finely disseminated pyrite, sphalerite and tetrahedrite. They comprise:
• Irregular calcite-sulphide veinlets in fractures in limestone, the economically the most important and widespread. Carbonatisation occurs as veinlet and disseminated calcite-sulphide assemblages. Sericite accompanies disseminated pyrite and electrum in limestone. Realgar and/or orpiment coexist with calcite in limestone, and locally realgar and orpiment aggregates coexist with calcite. Fine-grained (40 to 100 µm) euhedral pyrite and sphalerite coexist with calcite in veinlets. Gold occurs in electrum, calaverite (AuTe
2), and petzite (Ag3AuTe2) coexisting with altaite (PbTe), pyrite, sphalerite, and calcite.
• Breccias with limestone clasts and a matrix of calcite-sulphide or fine-grained sulphide aggregates that infill fractures in limestone. Carbonatisation is represented by calcite-fluorite-sulphide assemblages. Sericite accompanied by minor silicification occurs with disseminated pyrite, sphalerite, coloradoite, tetrahedrite and altaite. Rare lorandite (TlAsS
2) crystals up to 22 × 28 mm are found at Zhulintang (Xie et al., 2017), intergrown with calcite, fluorite and realgar. Gold occurs as petzite, coexisting with altaite, pyrite, sphalerite and tetrahedrite
• Argillic-altered granodiorite porphyry dykes with veinlet and disseminated pyrite, locally containing elevated Au of up to 1.1 g/t. Argillic alteration comprises trace illite and kaolinite.

The most recent source geological information used to prepare this summary was dated: 2019.     Record last updated: 14/6/2019
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:
Xie, G., Mao, J., Richards, J.P., Han, Y. and Fu, B.,  2019 - Distal Au Deposits Associated with Cu-Au Skarn Mineralization in the Fengshan Area, Eastern China: in    Econ. Geol.   v.114, pp. 127-142.

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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