Yaogangxian Mine, Yaogangxian W-Sn ore field, Yizhang Co., Chenzhou, Hunan, Chinai

Regional Level Types
Yaogangxian Mine	Mine
Yaogangxian W-Sn ore field	Ore Field
Yizhang Co.	County
Chenzhou	Prefecture-level City
Hunan	Province
China	Country

Tungsten-tin deposit in the contact aureole of the Mesozoic Yaogangxian composite pluton (consisting of coarse-grained biotite granite, fine-grained porphyritic granite, and quartz porphyry), which intruded into Cambrian-Devonian sediments (mainly sandstones) and Jurassic limestones. The mining field covers an area of about 4 x 2.5 km and includes two large deposits of a distinct type and mineralization style:

(1) The Yaogangxian quartz vein-type tungsten-tin deposit with minor greisen-style mineralization, which has been mined since 1914. The veins are hosted in the biotite granite phase of the Yaogangxian pluton and in its western and northern contact zone. They trend NW to NNW and are grouped in three vein swarms (ore blocks), from west to east: Yangmeiling, Luchangping, and Hamashi. The orebodies contain minor copper, silver, lead, zinc and bismuth minerals. They are strongly zoned both vertically and horizontally. Another group of veins with economic tungsten grades was recently discovered at depth southwest of the Yangmeiling ore block (Yan et al., 2010).

(2) The Heshangtan skarn-type tungsten-tin deposit, which was discovered in 1947, explored during the 1950s and has been mined since the early 1960s. The orebodies are hosted in Devonian sandstone and skarnized slate in the eastern contact zone of the Yaogangxian pluton. They contain significant amounts of associated silver ores.

The mineral list includes all species reported from both deposit types, since there is often no clear distinction made in papers on the Yaogangxian mine, especially in western publications. Apart from the obvious differences between quartz vein and skarn mineralization, there also are some notable differences in the ore mineral assemblages:

(1) Except for bismuthinite, which has been reported from the skarn orebodies (Xu, 1957) and as a microscopic constituent of greisen inclusions in the host granite (Zhou et al., 2013), bismuthiferous minerals have only been reported from the vein-type orebodies.

(2) Most of the silver minerals and most of the sulfosalts have only been reported from the skarn orebodies.

(3) All wolframites from the vein-type orebodies have a significant excess of manganese over iron and are thus hübnerites (Chen, 1981).

Notes on the mineral list:

(1) All fibrous sulphosalts are notoriously traded by Chinese dealers as "bismuthinite". Analyses have shown, however, that they are mostly stibnite, or, more rarely, boulangerite, jamesonite, berthierite, kobellite or cosalite (Jensen, 2009; Ottens, 2011). According to Jensen (2009), independent bismuth minerals listed in earlier reports were never confirmed in a very large number of analyses. Saul Krotki (personal communication, October 10, 2009) had a "bismuthinite" from this locality analyzed by semiquantitative microprobe by CannonMicroprobe and the composition was very bismuthian and ambiguously stibnite/bismuthinite.

These findings do not necessarily discredit all earlier reports on bismuthiferous minerals. It has to be kept in mind that all parts of the deposit were mined long before the first specimens appeared on the western market in the early 1990s. Mineralization zoning is a common phenomenon in intrusion-related deposits, and thus a different mineral assemblage may have been found in parts that are now mined out. They indicate, however, that bismuth minerals are now rare at this locality, if they can still be found at all. Microscopic bismuthinite has been recently found in greisen inclusions in the host granite (Zhou et al., 2013).

(2) Reports on the chemical composition of wolframite are not conclusive. Data presented by Chen (1981) show a significant excess of manganese over iron (Fe:Mn ranging from 1:4 to 3:4) in all analyzed samples of wolframite from the vein-type orebodies. Ottens and Cook (2005), however, state that wolframite is predominantly ferberite and hübnerite was only found in one single pocket, suggesting that their samples came mainly (all ?) from the skarn-type orebodies. Ottens (2011) states that the Fe:Mn ratio is variable, ranging from 2:1 to 1:2 even in black crystals, but which deposit the specimens came from is not known. Consequently, wolframites from the vein-type orebodies are almost certainly hübnerite, while specimens from the skarns or an unknown location within the mining field need to be analyzed to identify the species.

(3) Beryl is listed from the mine, but decent specimens are not known. In particular, the "goshenites" sold by some dealers do not come from the Yaogangxian mine, but actually from Pingwu, Sichuan Province.

(4) Molybdenite is one of the main ore minerals but does not occur in decent specimens. Molybdenites in trade attributed to this locality are really from different localities, many of them probably from the Piaotang mine in Dayu.

(5) Freibergite has been reported by Chinese authors (Zheng, 1989; Zhou et al., 2002), but an analysis to confirm its identity is lacking. In fact, in many reports by Chinese authors, the name "freibergite" refers to silver-rich tetrahedrite and not to true freibergite in which silver is dominant over copper. Analyses of some of the "freibergite" specimens in trade showed them to be argentian tetrahedrite.

(6) Alleged bertrandite specimens from here were really from the Wutong Mine (Wudong Mine), Liubao, in Guangxi Province (Disputed by Berthold Ottens!). Another possible occurrence is the Jiepailing Mine, which works a beryllium-rich deposit in the southernmost part of the Yaogangxian ore field.

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References