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The Dugupi-Maanshan antimony deposit: Weishan County, Yunnan Province, China.

Specimens from the Dugupi-Maanshan deposit have been available on the international mineral market since 2004 or earlier, but have been misattributed to various existing and fictitious localities scattered across southwestern China. The mine has produced spectacular aragonite "flos ferri" specimens, large, transparent gypsum crystals with red inclusions, and colorful fluorite-stibiconite combinations.

INTRODUCTION

Localities for valuable Chinese mineral specimens are often a well-kept secret in the Chinese specimen trade. This secrecy is intended to hinder competition and is usually of little importance to Eastern collectors who generally seem more concerned with specimen size and aesthetics than with chemistry and locality. Moreover, small-scale mining in China often operates in an environment where keeping a low profile is advantageous to the mine owners. In other cases the information about the specific locality of origin is simply lost in the process of trading. As a consequence, even in Chinese museums the locality information for domestic specimens is typically quite vague.

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So how was the Dugupi-Maanshan locality "discovered?" That is a long story, stretching over four years, most of it taking place in Kunming, the capital of Yunnan Province (several hundred kilometers from the mine). The story contains all of the inevitable ingredients: time, money, luck and a mine owner who needed to be persuaded. Some would call it "investigative journalism."

LOCATION

The Dugupi-Maanshan deposit is located at the southwestern extension of Dali Prefecture of Yunnan Province. Administratively it belongs to Weishan County. The county capital to the east-southeast, Weishan, which is also the nearest major city, can be reached from the mining area by paved and unpaved roads in several hours.

GEOLOGY

Geologically the Dugupi-Maanshan deposit is situated in a region that hosts ore deposits of considerable economic importance. The regional geological background has therefore been extensively studied. Tectonically the deposit is located in the Mesozoic-Cenozoic Lanping-Simao Basin that developed on the Changdu-Lanping-Simao micro-plate between the Yangtze Plate to the east and the Tibet-Yunnan Plate to the west, separated by the Lancangjiang and Jinshajiang-Ailaoshan faults (Yin et al., 1990 in Xue et al., 2007).

The Lanping-Simao Basin is essentially an intracontinental terrestrial basin. Six horizons of evaporites, dominantly of gypsum and halite, and occasionally also sylvite, have been found in the sequence. The total thickness of the evaporite layers may well exceed 2,000 meters in some places. The basin is filled with siliciclastic rocks, except for the lowest part of the sequence, and the Upper Triassic Sanhedong Formation, which consists mainly of marine limestone. There are several sedimentary gaps in the stratigraphic column of the basin (Qin and Zhu, 1991; Xue et al., 2002b in Xue et al., 2007).

No direct reference is available for the Dugupi-Maanshan deposit itself; however, it can be assumed that it belongs to the same hydrothermal system as the neighboring Bijiashan antimony mine. Chang (2007) describes the Bijiashan deposit as a medium to low-temperature hydrothermal deposit hosted by upper Triassic limestone of the Sanhedong Formation.

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MINING

The Dugupi-Maanshan mining area consists of more than a hundred individual mines scattered over a steep hillside forming the east bank of the Yangbi River. A small tributary of the Yangbi divides the district; the area on the north side is referred to as "Dugupi," whereas the southern section is called "Maanshan." The deposit, discovered in the 1980s, was initially worked exclusively for antimony ore. Specimen mining began much later, not before the late 1990s, in reaction to the development of the Chinese mineral specimen market.

Individual mines in the Dugupi-Maanshan mining area usually consist of a single drift that follows the stibnite vein several hundred to more than a thousand meters into the mountain. Simple mining methods without forced ventilation prevail. Ore beneficiation on site is done manually and by conventional gravity concentration techniques.

Specimen mining makes use of expanding mortar. Large-diameter holes are drilled into the rock matrix around the area of interest and filled with expanding mortar, so that the rocks are gently fractured when the mortar hardens. The fragility of the crystal specimens permits no other means of recovery. Large specimens are then carefully trimmed, wrapped and levered onto wooden crates. These are hoisted up the inclined shaft at an angle of 45 degrees and more. Much of the procedure is reminiscent of the methods used in the construction of the great pyramids of ancient Egypt.

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PARAGENESIS

The initial stage of mineralization in the orebody consisted almost exclusively of stibnite followed by fluorite and calcite. Both stibnite and calcite were subsequently heavily altered. Almost all of the stibnite was oxidized to secondary antimony minerals, and much of the calcite was dissolved. The resulting fluid, rich in calcium, carbonate and sulfate ions, later precipitated in the cavities to form aragonite, secondary calcite and gypsum.

MINERALS

Aragonite [CaCO.sub.3]

The spectacular coralloidal flos ferri specimens are the "cash cows" of specimen mining at Dugupi-Maanshan. In size and aesthetic appearance they compare favorably with flos ferri specimens from any other locality in the world. Because of this high level of quality, large specimens can be quite expensive. The collecting of aragonite specimens is highly labor-intensive and does not permit concomitant ore mining. Some workings in the "aragonite zone" in the lower levels of the deposit are now operated exclusively for aragonite specimens--once discarded as worthless gangue.

The filigreed stalks and branches may reach more than half a meter in length. They are often too fragile to be retrieved as matrix specimens. Other habits have an irregular "etched" appearance or show crystal faces and can be difficult to distinguish from calcite. The aragonite is usually pure white and rarely pale bluish green. Surface coatings, often on one side only, may be yellow, brown or gray.

Aragonite from this locality is typically quite fluorescent in various hues ranging from yellow to shades of green, blue and pure white. Occasionally, different colors appear on the same specimen. The fluorescence almost always differs under different wavelengths of ultraviolet light.

Calcite [CaCO.sub.3]

A common mineral, early-stage calcite is found as pale brown to yellow scalenohedral crystals which are translucent to opaque, commonly with black inclusions and colored surface coatings. Various stages of corrosion and partial replacement by aragonite, gypsum and other minerals is apparent, including possible pseudo-morphs after calcite. Rarely, corroded specimens, inconspicuous in daylight, exhibit a deep green fluorescence under shortwave ultraviolet light.

Late-stage calcite seems to be less common at this locality and can typically be found in the aragonite zone. It forms crusts of white opaque or colorless to yellow transparent scalenohedrons and other crystal habits, usually less than 2 cm long and commonly exhibiting an orange fluorescence.

Gypsum [CaSO.sub.4]*[2H.sub.2]O

Cavities in the upper levels of the deposit are commonly filled with masses of gypsum and are sometimes lined with large transparent crystals ("selenite") that reportedly can reach up to 3 meters in length. Usually they have colorless to pale yellow, very transparent interiors whereas the surfaces may be matte or lustrous.

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The source of these large, transparent gypsum crystals (many showing characteristic red to black inclusion phantoms) had long been a subject of speculation, but may now be considered confirmed. The identity of the red inclusions remains to be determined, though. Some have been described as cinnabar or realgar, but an antimony oxysulfide or metastibnite seems more likely.

As with aragonite specimen mining, some mines at Dugupi-Maanshan specialize in selenite specimen mining. Sections of cavities, some quite large, are meticulously separated from the host rock by use of expanding mortar and hand tools.

Selenite crystals from Dugupi-Maanshan commonly exhibit yellow or orange fluorescent zones, and some show fluorescent phantoms. Occasionally, fluorescent phantoms and inclusion phantoms occur in the same crystal at different depths. Inclusions of gases, liquids and other minerals are relatively common.

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Fluorite [CaF.sub.2]

Fluorite is ubiquitous in the deposit, in all shades of purple and blue, sometimes in alternating purple and blue bands. Freestanding crystal clusters usually consist of tightly intergrown cubes, occasionally modified by dodecahedron faces. The resulting specimens show thick, colorful crusts without conspicuous reflections because of the absence of large crystal faces. The fluorite typically has good transparency, and phantoms in different colors are common. Crystals with smooth faces are rarely larger than 1 cm on edge and fetch high prices when of deep color and good transparency. Disseminated crystals, if present at all, tend to be small and colorless.

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Also noteworthy are pale blue to almost colorless "botryoidal" crusts of fluorite crystals showing abundant twinning and edge modifications. This peculiar habit is similar to that of fluorite from Mogok, Burma.

Some of the lighter colored varieties exhibit a noticeable color change from blue under diffuse sunlight to pale purple under incandescent light.

Most fluorite at this locality is completely overgrown by calcite, aragonite, gypsum or antimony minerals.

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

Practically all freestanding stibnite crystals from this locality have been thoroughly oxidized. Intact stibnite occurs as intergrown crystal cores that have little appeal to the collector. Some or all of the red antimony minerals may in fact be metastibnite, the rare amorphous polymorph of stibnite.

Secondary Antimony Minerals

Stibiconite is a characteristic mineral of this locality and occurs as yellow or greenish, brittle pseudomorphs after stibnite in various states of decay. White, brown, yellow and red secondary antimony minerals commonly occur in the vicinity of the stibiconite. In combination with purple fluorite and silvery stibnite remnants, this assemblage can be quite colorful. Most secondary antimony minerals at Dugupi-Maanshan are found as earthy masses and microcrystal-line crusts. Analyses of secondary antimony minerals to determine the exact species have not yet been performed, but there are strong indications that rare species such as coquandite may be present.

Other Metals

According to local miners, white to yellow zinc oxide and sulfide minerals are found in the southern section of the deposit. Specimens show unidentified greenish yellow crusts associated with calcite.

A secondary uranium mineral has been found on corroded calcite and as inclusions in gypsum. It occurs as thinly disseminated, highly fluorescent, platy yellow crystals to 3 mm.

Trace element analysis (Chang, 2007) of the orebody at the neighboring Bijiashan mine shows a 50:1 ratio of antimony to the next most abundant analyzed elements, namely zinc and arsenic. Gold seems to be the only other heavy element occurring in commercial quantities at this deposit. It is unclear whether the gold occurs in native form.

Fluorescent Minerals

The presence of patchy, bright yellow-green fluorescence, under shortwave ultraviolet light, on surfaces of species that are inherently non-fluorescent is fairly common. The fluorescence may be caused by thin crusts of hyaline opal.

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The principal secondary minerals from this locality (aragonite, calcite and gypsum) show a rainbow of fluorescence colors, usually with medium intensity, frequently with several colors occurring on the same specimen and commonly with different colors at different ultraviolet wavelengths. This type of fluorescence is typically caused by impurities and crystal lattice defects, in contrast to the fluorescence exhibited by inherently fluorescent compounds.

Some fluorite specimens from this locality exhibit zones of fluorescence other than the "usual" blue. This may be caused by surface coatings or inclusions.

ON-SITE COLLECTING

Small specimens of fluorite, calcite and antimony minerals can be found on the mine dumps, but the quality is insufficient to justify either the physical danger involved or the tedious trip to the mine. Good specimens leave the mining area very quickly through the established specimen market channels.

REFERENCES

CHANG, K. Y. (2007) On the genesis of Bijiashan Sb deposit in Weishan, Yunnan. Yunnan Geology, 26 (2), 197-206. (in Chinese with English abstract)

XUE, C. J., ZENG, R., et al. (2007) Geologic, fluid inclusion and isotopic characteristics of the Jinding Zn-Pb deposit, western Yunnan, South China: A review. Ore Geology Reviews, 31, 337-359.

Chris Schroeder

Kunming, Yunnan Province, P.R. China

gold@rare-X.com
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Author:Schroeder, Chris
Publication:The Mineralogical Record
Geographic Code:9CHIN
Date:Jan 1, 2010
Words:2004
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