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Jinshan |
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Jiangxi, China |
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Au
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Super Porphyry Cu and Au
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The Jinshan gold deposit located in the northeast of Jiangxi Province, South China, 25 km ENE of Dexing, and 200 km ENE of Nanchang.
Gold mining at Jinshan is recorded as early as the thirteenth century during the Song Dynasty. Mylonite hosted massive gold ore bodies with grades locally as high as 18 g/t Au were discovered at the mine in the early 1980s. During the ensuing 10 years, several other smaller deposits were discovered near Jinshan, including Huaqiao and Bashiyuan, followed by Hamashi, Naikeng and Dongjia to the west of Jinshan in the 1990s.
The Jinshan gold deposit lies within the Jiangnan Orogenic Belt which incorporates multi-stage Neoproterozoic orogenic events (Guo et al., 1980; Zhou and Zhu 1993; Charvet et al., 1996) caused by Neoproterozoic orogenesis along the boundary between the Huaiyu terrane of the Cathaysia Block to the SE, and the Jiuling terrane, part of the Yangtze craton in the NW at (Shu et al., 1995).
The Jiuling terrane is predominantly composed of Mesoproterozoic to Neoproterozoic continent-derived flysch turbidites and littoral sedimentary rocks, with mafic and ultramafic intrusions dated at 1515±241 Ma (Shu 1991) in flysch towards its southern margin.
The Huaiyu terrane is essentially composed of metavolcanic rocks, mainly Mesoproterozoic basalt and dolerite which have an oceanic or back-arc basin affinity, with lesser Neoproterozoic basalt, andesite, and rhyolite, and turbiditic flysch. The Mesoproterozoic volcanic sequence includes a 1000 to 930 Ma ophiolitic mélange (Shu 1991). These rocks are generally metamorphosed to greenschist facies except for a strip of blueschist facies ophiolitic mélange. The age of the Huaiyu terrane ranges from 1401 Ma (Rb-Sr on phyllite) to between 800 and 700 Ma (Charvet et al., 1996).
Age data and geological relationships suggest orogenesis along the boundary between the Huaiyu (Cathaysia Block) and Jiuling (Yangtze craton) terranes peaked in Neoproterozoic time, with intense ductile thrust shearing and deformation, mostly within the Huaiyu terrane during pre-Sinian times. Charvet et al. (1996) suggested a multi-stage Neoproterozoic orogenesis, including early southeastward obduction and nappe emplacement at around 950±40 Ma, with subsequent intrusions of S-type granitoids at 930±20 Ma, and an intracontinental shortening along ductile shear zones. The orogenesis also produced regional shortening and folding in the Shuangqiaoshan Group sedimentary rocks of the Jiuling terrane. The regional-scale Sizhoumiao synclinorium, formed parallel to the regional structures in the Group, accompanied by regional greenschist-facies metamorphism with newly formed chlorite and muscovite in the Shuangqiaoshan Group rocks.
The suture between the Huaiyu and Jiuling terranes is a NE-trending, steeply NW dipping, regional deep crustal fault system expressed as the 10 to 20 km wide Jiangxi Deep Fault. This structural zone is characterised by a 4 to 5 km wide ophiolite and mélange sequence, and is the dominant linear tectonic feature along the Jiangnan Orogenic Belt, extending continuously for at least 200 km from Dongxiang in Jiangxi Province northeastward to Xixian in Anhui Province. The geochronology of the Jinshan ductile shear zone, which is associated with this structure, yielded a Rb-Sr whole rock age of 717±6 Ma (Zhang 1994), and Rb-Sr isochron ages of 714.5±60.5 to 732.1±61.9 Ma (Wei 1996) on the ultramylonite and mylonite produced in the ductile shear zones.
Jinshan and other gold deposits in the region, are closely associated with either the first order Northeast Jiangxi Deep Fault, and several second-order shear zone splays, such as the NE-striking Bashiyuan-Tongchang and Jiangguang-Fujiawu ductile shear zones, and the third-order Jinshan, Xijiang, and Jinshankou shear zones, from the northeast to the southwest of the area respectively. The third-order shear zones up to 650 m wide, and dip shallowly to the NW, north and NE at 5 to 30°. These structures directly host gold deposits such as Jinshan, Huaqiao and Bashiyuan in the Neoproterozoic Shuangqiaoshan Group slate, tuffaceous slate, phyllite, greywacke and andesitic basalt dated at 880 Ma (zircon LA-ICP-MS, Wang et al., 2008). The kinematics of the third-order ductile shears indicate dominant dip-slip motion with the SSE hanging wall thrust over the NNW footwall (Wei 1995).
The ore bodies at Jinshan are confined to the Jinshan ductile shear zone, which comprises sub-zones of mylonitised slate, protomylonite, mylonite, ultramylonite and phyllitic mylonite resulting from to intense strain partitioning. The ultramylonite, mylonite and phyllitic mylonite high strain zone coincides with the strongest hydrothermal alteration and gold mineralisation.
Most of the gold in the Jinshan deposit is within the Jinshan shear, and most ore bodies occur as either disseminated pyrite-bearing mylonite (with 75% of the gold reserve @ 3 to 10 g/t Au) or laminated quartz veins (with grades of 5 and 20 g/t Au). The disseminated mineralisation surrounds the laminated quartz veins. High-grade ores are characterised by a greater abundance of base metal sulphides, with coarse, visible gold grains commonly observable in hand specimens and polished sections of laminated quartz veins. Native gold grains occur as irregular sheets, dendrites and fine veinlets, with high-grade pockets carrying grades of >1600 g/t Au (Wei 1995). Microscopically, native gold may occur as fine veins cutting fragmented pyrite crystals, as crack-filling veinlets within pyrite grains, or isolated in quartz grains. Intergrowth between native gold and pyrite has also been observed in quartz.
Ore bodies are lensoid and dip at 10 to 35° NE to NNE. They are 1.2 to 6.0 m (averaging 3.5 m) thick and mostly parallel to shear planes, being predominantly concentrated at shear zone bends, dilational jogs and contacts between rocks of different competencies. Orebody-host rock boundaries are gradational and may only be delineated by chemical analysis. Gold mineralisation is closely associated with extensive hydrothermal alteration that includes silica, pyritic, sericite and chlorite. Three alteration zones have been defined from the centre of the shear zone outward: i). a quartz-albite-ankerite-pyrite zone that occurs in high-strain cores of the shear zone, over widths of several to ~50 m, containing high-grade gold ore; ii). a quartz-sericite-ankerite zone next to the high-strain zones, and iii). a chlorite-calcite-sericite zone distal to the high-strain zones.
Proven pre-mining gold reserves at the Jinshan deposit were estimated at 200 tonnes of gold from ore with grades ranging from 4 to 20 g/t Au (Li et al., 2010)
The most recent source geological information used to prepare this decription was dated: 2010.
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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Li X, Wang C, Hua R and Wei X, 2010 - Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China: in Mineralium Deposita v.45 pp. 583-597
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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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