Arsenic minerals realgar and orpiment from China

--- at China University of Geosciences- Beijing, China

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Figure 1. CUGB / Yifu Geoscience Museum specimen K033.


"Rock of the Month #180, posted for June 2016" ---

Arsenic sulphides realgar and orpiment from China

are beautifully displayed in these two samples from the museum in the Yifu Building, China University of Geosciences - Beijing.

Realgar (AsS) and orpiment (As2S3) are colourful low-temperature arsenic minerals, often easier to recognize than the silvery-grey arsenopyrite (FeAsS2) which is probably the most widespread mineral with an essential arsenic component in its formula. The two arsenic sulphides vary in colour from yellow to orange to red, with the most striking colours seen in well-developed crystals, as opposed to the typical dusty masses and crusts. Both minerals are monoclinic in symmetry. They occur in low-temperature veins, in hot springs, and as sublimates in volcanic fumaroles. They may form as secondary minerals upon alteration of other arsenical mineral species.

REALGAR

Over the last 25 years, the international mineral market has seen an influx of excellent mineral specimens from China. A good example is realgar from the Shimen mine in Hunan province (Liu et al., 2013; White and Moore, 2014; Huizing and Wilson, 2015). The Shimen realgar mine has been in operation, almost continuously, for the past 1,500 years (Wilson, 2007). The Shimen deposit (Zhang et al., 1993) is the largest producer of arsenic in China. It is a pipe- shaped orebody with silica sinter and hydrothermal explosion breccia, an extended hot spring system. No igneous rocks have been found in the district.

ASSOCIATION WITH GOLD

Arsenic, often with other elements such as antimony, mercury and thallium, tellurium, tungsten and barium, is variably enriched in many gold deposits, and may serve as a pathfinder element (indicator element) of use as a proxy for (or vector toward) gold in geochemical exploration for the yellow metal.

Arsenic anomalies occur for 30 to 60 m from Au mineralization in sediment-hosted deposits, such as Banqi, Yata, Getang and Lannigou (He, 1996). However, main-stage ores may show inverse Au-As correlations: late-stage ores are generally As-poor, and As phases such as realgar and orpiment contain little or no gold.

These ores have obvious environmental effects wherever exposed by erosion or mining: they are enriched in elements such as Sb, As, Hg and Tl (Zhang and Zhang, 1996; Wang and Zhang, 2001; Zhang et al., 2003; Su et al., 2009).

An example of the sediment-hosted disseminated gold deposits of southwest Guizhou province is the Lannigou Au- As- Sb deposit, hosted by turbidites deposited along the continental slope during rapid rift-related subsidence. Lannigou was a 1986 discovery around a former arsenic (realgar) working, and is the largest Au deposit in the “gold triangle” region (Bao, 2001). Lannigou is one of several deposits around the Laizishan anticline, in strata affected by numerous faults and at least 2 generations of folds. Mineralization and hydrothermal alteration are observed along the various fault structures. The orebodies at Lannigou are all hosted by sandstone, siltstone and argillite of the Triassic-age Bianyang Formation. Alteration in the mine area is commonly along fractures. Pyrite and other sulphides are one component of each of the 3 stages of silicification. The pyritization is irregular, with higher pyrite content in richer ores. Pyrite occurs in three phases, as grains 10-500 microns in diameter. There is some arsenopyrite. Carbonatization includes early ankerite and late calcite, the latter associated with Hg, Sb and As minerals (such as cinnabar, stibnite, realgar, orpiment). Arsenical pyrite is said to host circa 68% of the total Au in the ores. Geochemical exploration may benefit from the average trace element content of the primary ores: Au (8 ppm), As (0.53%) and Hg (108 ppm).

A CURIOUS FACT

In China, a preparation known as realgar wine (xionghuangjiu) is traditionally drunk at the time of the Dragon Boat festival in high summer. Fifty or 100 grams of powdered realgar may be added to a litre of Chinese cereal wine (yellow wine) and left to steep for several hours. Personally, I would not recommend this nor drink it myself (I probably absorb enough arsenic by handling rock samples!). Because of its use as an insecticide and pesticide, realgar has a role in traditional Chinese medicine. Realgar is not readily adsorbed in the body, but other arsenic compounds may be more soluble. Further discussion of realgar wine is available. Natural arsenic-rich districts are scattered around the world, with examples in China, Borneo, Canada, Switzerland and elsewhere.

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Figure 2. CUGB / Yifu Geoscience Museum specimen 3739. Apparently, a little arsenic in wine can be good for you, a medicinal libation in China (see main text for more details). One wonders if this is one of the origins of today's homeopathic treatments. Certainly, a trace of an inorganic toxin will not prove fatal, hence the various national or world limits on elements such as Pb and Ni, Cd and Hg, As and Tl in water, food and soil.


Acknowledgements Thanks to Dr Patrick Wen for original translations.

References

Bao,Z (2001) Geochemistry of the Sediment-hosted Disseminated Gold Deposits in Southwestern Guizhou Province, China. PhD Thesis, Université du Québec à Chicoutimi, 244pp.

He,M (1996) Physicochemical conditions of differential mineralization of Au and As in gold deposits, southwest Guizhou province, China. Chinese J.Geoc. 15 no.2, 189-192.

Huizing,T and Wilson,WE (2015) Mineral Collections in the American Midwest. Mineral.Record 46 no.4, supplement, 240pp.

Liu,G, Lavinsky,RM, Meieran,ES, Schmitt,HH, Moore,TP and Wilson,WE (2013) Crystalline Treasures: the Mineral Heritage of China. Mineral.Record 44, supplement, 104pp.

Su,W, Heinrich,CA, Pettke,T, Zhang,X, Hu,R and Xia,B (2009) Sediment-hosted gold deposits in Guizhou, China: products of wall-rock sulfidation by deep crustal fluids. Econ.Geol. 104, 73-93.

Wang,X-C and Zhang,Z-R (2001) Geology of sedimentary rock-hosted disseminated gold deposits in northwestern Sichuan, China. Int.Geol.Rev. 43, 69-90.

White,JS and Moore,TP (2014) New standards of excellence in the mineral kingdom. Mineral.Record 45, 217-230.

Wilson,WE (2007) The Shimen mine, Jiepaiyu, Shimen county, Hunan province, China. Mineral.Record 38, 43-53.

Zhang,J, Wang,W, Yang,F and Cai,T (1993) The hot-spring genesis of the Shimen realgar deposit, northwest Hunan. Chinese J.Geoc. 12 no.2, 137-147.

Zhang,X-C, Spiro,B, Halls,C, Stanley,CJ and Yang,K-Y (2003) Sediment-hosted disseminated gold deposits in southwest Guizhou, PRC: their geological setting and origin in relation to mineralogical, fluid inclusion, and stable-isotope characteristics. Int.Geol.Rev. 45, 407-470.

Zhang,Z and Zhang,B (1996) Thallium in low temperature ore deposits, China. Chinese J.Geoc. 15 no.1, 87-96.

Graham Wilson, 26 April 2016, 10 May 2016, 09 June 2016.

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