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Stibnite from the Wuling antimony mine Jiangxi Province, China.

Two Chinese antimony mines have produced collector-quality stibnite crystals in recent years. The most recent of these to yield fine specimens is the Wuling antimony mine in Jiangxi Province. Highly lustrous, razor-sharp stibnite crystals and crystal groups exceeding 50 cm in length were recovered there in late 2000.

INTRODUCTION

China is believed to account for 50% of the world's reserves of antimony and 80% of world production, primarily from huge ore deposits in Hunan Province, the largest of which is the Xikuangshan mine in Xinhua County. Other deposits are found in the provinces of Guangdong, Guizhou, Yunnan, Anhui, Zhejiang, Fujian, Gansu, Jilin, Shaanxi, Julin, the Guansgxi Zhuang Autonomous Region, and Jiangxi Province (Anonymous, 1980). Until recently, most specimens of Chinese stibnite in collections have come from Hunan, and a few also from Guangxi and Guizhou (Guo and Zhou, 1996). But in the closing months of 2000, two spectacular pockets of stibnite crystals were encountered at the Wuling antimony mine (also known as the Qingjiang mine), northwestern Jiangxi Province, People's Republic of China.

A significant number of Wuling mine stibnite specimens were recovered, many of which are essentially damage free thanks to the professional collecting techniques used. While the Wuling mine specimens have been collected and transported in a manner that has significantly reduced the numerous nicks and dings which have plagued stibnite specimens from other localities, the collecting of matrix specimens remains as uncommon at the Wuling mine as it has been at other classic stibnite-producing occurrences.

The mine and town name has been spelled "Wuling" or "Wuning," both of which are considered correct. The "Wuling" transliteration is based upon the pronunciation in use at the mine, in the dialect which is prevalent in the surrounding areas of northern Jiangxi, southeast Hubel, and northeast Hunan provinces. The spelling "Wuning" or "Wu-ning' on the other hand, is in accordance with standard rules for transliterating Mandarin Chinese.

HISTORY

The Wuling antimony mine is situated on the southern bank of the Xiushui [= Hsiu Shui] River, near the small hamlet of Qingjiang, 40 km southwest of the city of Wuling (Wuning), in northern Jiangxi Province, China. The deposit, situated along Tuobeishan Hill, was discovered in the 1960's. The Wuling mine was established in the mid-1980's by the Waling County government, which owned the mine until 1998. Since 1998, the mine has been contracted to the Star Antimony Limited Company.

Tuobeishan Hill, in the lower foothills of the Chiu-ling Range, is littered with old shafts and adits. The underground workings currently being exploited, however, are accessed by four vertical shafts which reach a maximum depth of approximately 65 meters. Mining methods are simple, traditional, and dominated by manual labor. Surface activities such as ore sorting, crushing, and concentrating are likewise accomplished with minimal mechanization. A total of about 300 workers are employed at the Wuling mine.

In 2000, antimony production from the Wuling mine was reported to be 1,200 metric tons (tonnes). (By comparison, the Xikuanshan mine produced well over 30,000 tonnes of antimony and antimony concentrate in 2000.) According to sources at the Wuling mine, more than half of the known orebodies and reserves have been mined thus far.

In 1995 several mineral specimen dealers from Hunan Province began monitoring mining activities at the Wuling mine. These dealers had previously been involved in recovering stibnite specimens from the Xikuangshan mine in Hunan Province and selling them on the international specimen market during the early to middle 1990's. The dealers were the first to recognize the Wuling mine's potential for stibnite specimen production; prior to late 2000 almost all stibnite crystals found at the locality had been processed as ore.

In December of 1997, a very large pocket measuring 20 meters in length, 5 to 6 meters in width and 3 meters in height was found. Stibnite crystals from this pocket were large, up to 15 centimeters in diameter and exceeding 1 meter in length. Unfortunately, the pocket was strongly oxidized and the stibnite crystals had been partially altered to stibiconite.

From 1997 to late 2000, there were no significant stibnite pocket discoveries. Then, in late November of 2000, a large pocket was encountered off of Shaft #2. The dimensions of the pocket were approximately 1 x 2 x 4 meters. Individual crystal size ranged from a few millimeters to 4 cm in diameter and 10 to 30 cm in length. Although the miners at Wuling had, by this time, an increased awareness that stibnite crystals had value as mineral specimens, the collecting and transport techniques employed resulted in heavy damage to most of the stibnite specimens. Most of the specimens recovered were single crystals or small clusters of crystals without matrix.

In general, matrix stibnite specimens from the Wuling antimony mine are extremely uncommon. The recovery of stibnite crystals attached to their silicified limestone matrix rock presents a nearly insurmountable challenge to the miners because of the lack of adequate specimen recovery tools and technology and because of time constraints imposed by the pre-eminent goal of maintaining ore production schedules.

Remarkably, one large, spectacular stibnite matrix specimen was preserved from the Shaft #2 pocket. The miners were able to collect this superb matrix specimen because, in this case, the stibnite had crystallized on a fragment of silicified limestone that had become dislodged within the pocket during previous tectonic movement. The specimen, recognized as the best specimen to have been produced from the mine, measures 57 cm wide by 38 cm high and 25 cm deep with the longest crystals exceeding 27 cm in length. It was originally placed on display in the offices of the Star Antimony Limited Company. The specimen was purchased from the mine owners in early 2001, was transported to the United States (occupying a commercial airline seat ... in first-class!), and now resides in a private U.S. collection.

Approximately two weeks later, yet another crystal-lined pocket was discovered, this one off of Shaft #3. At the time of the Shaft #3 pocket discovery, several mineral dealers familiar with the specimen collecting and preservation problems that had been experienced at the Xikuangshan mine were visiting the Wuling mine to examine the specimens from the Shaft #2 pocket. These dealers advised the miners about the importance of exercising great care in the collecting and packing of the stibnite crystals from this new pocket. Consequently, the stibnite specimens from the Shaft #3 discovery were very well preserved.

The Shaft #3 pocket measured 1.5 x 4 x 10 meters. Individual crystals from this pocket have superb luster on the prism faces and are very large, some reaching 5 cm in diameter and 50 cm in length, with the average individual crystal length being over 15 cm. Again, the pocket yielded, primarily, single crystals and clusters of three to five or more crystals. Several hundred specimens from this astounding pocket were featured at the 2001 Denver Gem and Mineral Show. Superb specimens from this discovery were also available at the 2002 Tucson Gem and Mineral Show.

GEOLOGY

The antimony deposits of China are distributed primarily to the south of the Yellow River, in four major ore belts: (1) the South China belt, (2) the Western Yunnan belt, (3) the Qinling belt, and (4) the Jilin-Dahinggan-Tianshan belt. Deposits in these belts have been classified into seven genetic types: (1) carbonate, (2) clastic, (3) epimetamorphic, (4) marine volcanic, (5) continental volcanic, (6) post-magmatic, and (7) exogenic accumulation. The Wuling antimony orebodies in the South China belt are characteristic of type 5: they occur along the margin of a volcanic fault basin in an active platform, associated metallogenetically with continental fissure-type volcanism related to the Late Yanshanian-Himalayan Orogeny. Gases and fluids escaping from the basalt, andesite and lamprophyre layers carried antimony in solution, which eventually became concentrated by various means and deposited as stibnite in open fractures and shear zones (Wu et al., 1990).

The east-west-striking zone of antimony occurrences in Jiangxi province is 100 km long and 5 to 10 km wide, extending from the Baoshan deposit in De-an [= Te-an] County in the east to Wuling (Qingjiang) and on to Xianglushan, Hubei Province, in the west. The country rocks are Sinian [= latest Precambrian] to Silurian carbonates, mainly limestone, dolostone and arenaceous shale. In the Wuling area the ore-bearing strata are Cambrian to Ordovician; the richest orebodies and finest stibnite crystallization occur in Middle Cambrian silicified limestone. Mineralization has been characterized as low to medium temperature, and mesothermal grading into epithermal, accompanied by metasomatic silicification.

Chinese antimony deposits are generally strata-bound and are overlain by an argillaceous "shielding layer" which has acted as a semi-permeable membrane. The result was to greatly increase antimony (and silica) concentration in the ore-forming fluid at this boundary, because [H.sub.2]S, [H.sub.2]O and [CO.sub.3] could readily pass through the shielding layer. With gradually decreasing temperature came a series of reactions involving [Na.sub.2]S, [Na.sub.2][CO.sub.3], [H.sub.2]S and [H.sub.2]O, accompanied by variations in pH, Eh and f [O.sub.2] which resulted in repeated phases of stibnite crystallization and silicification (Wu et al., 1990).

Stibnite orebodies have been grouped into five types: (1) brecciated stibnite, (2) massive stibnite, (3) stockwork stibnite, (4) stibnite disseminated in lamprophyre, and (5) stibnite disseminated in silty shale, the first three types being dominant. Ore grade prior to hand-sorting is usually 15-20% Sb.

The most well-developed crystal pockets in the Wuling deposit have formed along the axis of a syncline (the Tuobeishan syncline), where the spaces available for mineralization, opened by intra-strata gliding, tend to be relatively large. There are five ore-bearing belts within the deposit, with the Tongjia-Jieshangping belt having the most well-developed crystal pockets. Stibnite crystals were first encountered at the deposit in the 1980s, within intra-strata openings and occasionally in small karst holes.

According to the mine geologist, crystal pockets normally occur at a depth of 20 to 50 meters. Above the 40 meter level, most pockets are highly oxidized. Stibnite crystals in these upper-level pockets are dull-lustered or are altered to stibiconite. Only at depths of 40 to 50 meters can good, unoxidized crystals be found. No economic antimony deposits have been encountered at the Wuling mine below 65 meters.

MINERALOGY

Chinese antimony deposits of the Wuling type generally contain stibnite as the main ore mineral (with stibiconite and valentinite in the oxidized zone), plus quartz, barite and calcite. According to Wu et al. (1990), some deposits also contain microscopic gold and copper particles; other sulfides such as arsenical pyrite, arsenopyrite, pyrrhotite, chalcopyrite, bornite, tetrahedrite, and sphalerite; oxides including hematite, magnetite, cassiterite, ilmenite, chromite, goethite and psilomelane; and sometimes also siderite, dolomite, orpiment, apatite, epidote, chlorite, micas, and even topaz, spinel and tourmaline (although the latter six or seven species are probably unrelated in origin to the antimony mineralization). The species listed below are those which have been preserved as specimens from the Wuling mine.

Barite Ba[SO.sub.4]

Barite is encountered infrequently in the Wuling mine. It occurs as colorless to pale yellow, tabular, pseudo-rhombohedral crystals reaching 2 mm in thickness and 1.5 cm on edge. The barite occurs directly on the silicified limestone matrix rock, occasionally associated with stibnite crystals. The miners generally do not attempt to preserve barite crystal specimens, because the barite crystals have frequently suffered significant damage from blasting methods and because the barite crystals found to date have been associated with small and/or unimpressive stibnite crystals.

Calcite Ca[CO.sub.3]

Calcite occurs in massive form as colorless to white vein material and fracture fillings that are occasionally included with stibnite crystals. No significant calcite crystal specimens have been encountered.

Stibiconite [Sb.sup.3+][Sb.sup.5+.sub.2][O.sub.6](OH)

Stibiconite is a common secondary mineral in the oxidized zones of the Wuling mine, where it occurs as partial or complete replacements of stibnite crystals.

Stibnite [Sb.sub.2][S.sub.3]

Stibnite, as mentioned above, is the primary ore of antimony at the Wuling mine. Occasionally, large open pockets containing magnificent, stainless-steel-gray crystals of stibnite have been encountered. The stibnite occurs as brilliantly lustrous, lightly striated prisms that can exceed 5 cm in width and 50 cm in length; the majority of the stibnite crystals from the recent discoveries have ranged from 1 to 3 cm in diameter and 10 to 30 cm in length. Terminations are generally matte-gray and a simple chisel shape, some of which are slightly beveled or rounded; some individual crystals ramify into amazingly complex multiple terminations. Crystals exhibiting multiple terminations, begin as rather ordinary elongated stibnite prisms that are crowned by tens to hundreds of thin individuals in tightly packed parallel growth. In some cases the small crystals diverge into a broom-like habit, especially where the large, main crystal has grown across the pocket and contacted the opposite wall. A small number of doubly terminated "floater" crystals have also been recovered.

By far, the most common stibnite specimens from the Wuling mine are single crystals without matrix. However, the locality also produces stibnite crystal groups in a wide array of aesthetic compositions: some groups are composed of multiple stibnite crystals in parallel growth; some occur as slightly divergent clusters of individuals radiating from a common base; others occur as individuals intersecting, like crossed-swords, higher on their prism faces; still others exhibit interesting secondary crystal formation from the sides of large, single stibnite prisms, at times these crystal "arms" are naturally bent or curved, mimicking the appearance of a saguaro cactus. Jumbled aggregates of prismatic crystals also occur.

CONCLUSIONS

Remaining antimony reserves at the Wuling mine are estimated at nearly 10,000 tonnes, which should enable the mining operation to continue commercial antimony production for several more years. The history and geology of the occurrence suggest that additional undiscovered stibnite-bearing pockets may exist nearby.

Employing modern specimen collecting tools and techniques at this locality (diamond chain saws, splitters, etc.) might facilitate the extraction of some astounding matrix stibnite specimens. Market forces will dictate whether specimen recovery utilizing these relatively sophisticated collecting tools will be attempted in the future. Whatever the future may hold, the mineralogical treasures already preserved from the Wuling antimony mine rank it among the most important specimen occurrences for stibnite.

ACKNOWLEDGMENTS

The authors wish to thank the owner of the Wuling antimony mine, Mr. Wu Hefu, and the mine geologist, Mr. Yang Moxiang, for their kind hospitality and willingness to share information about the history, geology, and mineralogy of the deposit. We would also like to thank Liu Guanghui and Duan Shengdong for their contributions to this article and for their assistance in China. Dr. Anthony R. Kampf and Thomas P. Moore kindly reviewed the manuscript and provided useful suggestions. Finally, Ken Roberts, formerly of Collector's Edge Minerals, Inc., should also be acknowledged for his keen eye in recognizing the significance of the stibnite specimens from the Wuling mine and for his efforts to ensure that these incredible specimens were made available to mineral collectors around the world.

REFERENCES

WU, J., XIAO, Q., and ZRAO, S. (1990) Antimony deposits of China; in: Mineral Deposits of China, Vol. 1. Geological Publishing House, Beijing, China, p. 209-287.

GUO, K., and ZHOU, Z. (eds.) (1996) Mineral Treasures. Geological Publishing House, Beijing, China, 222 p.

ANONYMOUS (1980) Minerals in China. Museum of Geology and Shanghai Scientific and Technical Publishers, Shanghai, China, 165 p.
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Author:Behling, Steven C.; Liu, Guanghua; Wilson, Wendell E.
Publication:The Mineralogical Record
Geographic Code:9CHIN
Date:Mar 1, 2002
Words:2566
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