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Xuebaoding: Pingwu County, Sichuan Province, China.

Beautiful specimens showing large, lustrous, deep orange crystals of scheelite associated with brilliant black twinned cassiterite crystals and tabular, gemmy crystals of aquamarine beryl on beds of muscovite crystals are characteristic of Mount Xuebaoding, Sichuan Province, China. These specimens have already become familiar to mineral collectors in the West, and supplies of them should continue to be available, thanks to active specimen mining and marketing by the Chinese.

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INTRODUCTION: THE SETTING

"Pingwu," "Songpan," "Mount Xuebaoding," "Xue Bao Diang" and occasionally "Huya" are the locality designations generally cited for specimens of orange scheelite associated with cassiterite and beryl from Sichuan Province in central China. Xuebaoding (pronounced "Shway-bow-ding") Mountain, known to the local Tibetan population as Shardungri, is the source of the scheelite; it lies in the autonomous Tibetan Aba Prefecture, on the eastern border of Songpan County. Part of the mountain lies in Songpan County and part lies in Pingwu County. At 5,588 meters, the mountain boasts a year-round snowcap and is one of the highest peaks in Sichuan Province (the highest is the 7,556-meter Gongga).

The town of Songpan, lying at an elevation of 3,000 meters in the valley of the Minjiang River, is an important center of tourism thanks to its picturesque appearance and its many Tibetan inhabitants. During the Ming Dynasty (1368-1644 A.D.), the Tibetanstyle old quarter of the town was encircled by a 6,200-meter-long city wall made of brick, and several impressive gates are still preserved. From Songpan, trekking tours may be undertaken--those done on horseback are especially popular. The town lies 320 km from Chengdu, the capital of Sichuan Province, and it may be reached from there by public bus in about 10 hours. The area around Songpan, called Munigou, is a special attraction for tourists, and is now under development by the government of Songpan County, as are the adjoining Jiuzahaigou and Hualong areas. These parts of Aba Prefecture offer a characteristic local charm because they display the customs and folkways of the cultures of Tibet and of the Qiang peoples. 50 km northeast of Songpan one traverses a mountain pass at 4,000 meters and comes into the impressive Hualong National Park, at 3,300-3,600 meters above sea level. In the valley of Hualong ("yellow dragon"), which is regarded as holy, there are wonderful terraced formations of travertine enclosing countless little lakes. From the head of the valley, lying at 3,700 meters elevation, one can enjoy on clear days a tremendous view of the peak of Xuebaoding Mountain.

The town of Pingwu lies on the Fujiang River about 100 km southeast of Xuebaoding Mountain and on the border between the Longmenshan and Minshan Mountains (Nakamura, 2003). A 500-year-old Buddhist temple there was saved from destruction during the Cultural Revolution, and is now one of the town's few points of interest for visitors. The nature preserves near the towns of Wanglang and Huya presently hold more than 250 living panda bears.

MINE WORKINGS

To reach the scheelite specimen workings--locally called a mine--the visitor travels from Pingwu (elevation 800 meters) up a narrow, partly canyon-like valley, along an unpaved, often very muddy road, towards Huya. This village (elevation 1,500 meters) is ringed by steep mountains, and lies on the edge of Huya Xuebaoding National Park, one of the protected panda bear habitats. From there the collecting area is reached by a laborious climb on foot along a steep path, through wonderfully varying vegetation. Depending on elevation, one can admire orchids, different species of azaleas, bamboo plants with mushrooms growing among them, and, after 3,500 meters, high-alpine flora. Since the mountain people travel this route entirely on horseback, the path is heavily trodden and very muddy, but not dangerous. The last part of the climb leads over a ridge at an elevation of about 4,000 meters, and then to the mining settlement called Zhibaisha, at 3,900 meters.

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During the snow-free season (May through October), about 100 people live at the workings and dig for scheelite ore and specimens, more or less in large groups. A greenhouse-like structure serves as accommodation: constructed of bamboo and torn plastic sheeting, partly collapsed and thoroughly stained, it is in the middle of a deep marsh, and gives an impression of total dilapidation. All food supplies, fuel, and mining equipment must be transported on horseback from Huya; the trip takes a day, even for local people accustomed to the high mountains.

To the northeast of Zhibaisha are steeply ascending rock faces to elevations from 4,100 to 4,300 meters, with clearly delineated quartz veins and traces of earlier mining activity. Directly below the rock faces, groups of people are busy sorting the scree and passing it through very simple washing flumes to concentrate the grains of scheelite. For the concentrate won through all their hard work the miners receive 12 RMB per kilogram. A year's earnings for this kind of labor comes to only 2,000 to 3,000 RMB (one U.S. dollar = about 8.2 RMB). Of the 100 people living in Zhibaisha, about 70 work at winning ore by washing the rocky detritus and 30 work, in groups of 4 to 8, at digging specimens for collectors.

This locality's specimens of intensely orange scheelite crystals associated with large cassiterite crystals and with tabular beryl have, of course, become famous during the past ten years. Before that, the scheelite and cassiterite found here were treated merely as ore, unfortunately. But the farmers of the region received very scant compensation for their hard work as miners, and as they came to realize that they could earn many times more money by collecting minimally damaged mineral specimens, they learned quickly to accommodate the requirements of collectors. Whereas previously nearly all of the scheelite crystals had been found by washing the loose detritus, with only very limited collecting from the country rock, some of the workers now turned their attention to the tunnel workings. Recognizing that other minerals which occurred here were of interest also, and therefore were valuable, they came to pay increasing attention to minerals less commonly seen in the workings, e.g. fluorite and apatite, and to rare species such as kesterite.

The major collecting site is marked by quartz veins cropping out on steep hill slopes. Some of the veins which were fully exposed on the rock faces were worked at first by simple methods, but presently the deposit must be worked through underground tunnels. The timber line in the Minshan Mountains is at about 3,300 meters elevation; at the collecting site, between 4,000 and 4,500 meters high, there is only moss and lichen. Despite the high elevation the locality is free of snow between May and roughly the end of October. In August, at 4,000 meters, the daytime temperature is around 20[degrees]C and the nighttime temperature somewhat over 10[degrees]C. Because of the monsoons the summer months are very rainy; dry, frost-free weather arrives in September and October.

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Having visited this locality personally, I can offer some further details. Mining activities take place in an area of about 4 square kilometers, at an elevation between 4,000 and 4,500 meters, on the south slope of the Xuebaoding massif. The nearest mountain peak with its own name is the 5,440-meter-high Xiaoxuebaoding (the "Xiao" prefix means "little"). According to the accounts of local people, there are other collecting sites in the mountains west of Huya, in the direction of Songpan; however, no detailed information about them is available. For the specimens offered on the worldwide mineral market, no special mine names are given, and as a rule it is not known from which particular working a given specimen has come. It seems most advisable simply to give "Xuebaoding" as the locality. It should be remembered that the occurrence lies within a nature-conservation area subject to strict rules. A conversation in May 2004 with the chief executive of Pingwu County and with the director of the conservation area established that the mining activities are being watched with critical concern, and that a forced termination of them is not out of the question.

One mine which lies at an elevation of 4,480 meters can be reached from Zhibaisha by an acclimatized person in two hours. There, two quartz veins only 10 meters apart are worked in an underground operation. The longer, horizontal tunnel is 2 to 3 meters high and about 200 meters long. The mine's relatively young owner, Zheng Bo, works here along with six other people, including a young woman. The roughly 50-year-old Huang Kai Hua, who runs another mine in the immediate area, is a pioneer of ore mining at Xuebaoding, where he began about 30 years ago to gather ore, particularly scheelite. He became aware of the value of mineral specimens in the Western world when China opened up after 1990. Individual tunnel workings here have no special names. The area is simply called Shuijingchang, which means, approximately, "crystal mine." No electrical generators are available, so the work underground must proceed by candlelight.

Inspecting a tunnel in which blasting has just taken place, one can see abundant quartz as opaque crystals which reach 15 cm long and 25 cm in diameter, but which are not attractive. Surprisingly, there are also green fluorite crystals to 20 cm, including very sharp cubes to more than 5 cm on edge. In the vicinity of the quartz veins neither scheelite, cassiterite or beryl appear, but in fissures separated from the quartz, mica has formed as a wall-lining, and scheelite, beryl and cassiterite crystals are embedded in it. These minerals are not observed in direct association with quartz in the mine.

Some specimens fresh from the mountain are completely clean, but there are also specimens coated with films of iron oxides from weathering. The specimens receive a crude trimming on-site to reduce their weight. Cleaning with concentrated hydrochloric acid and finer trimming with a diamond saw will be handled by the dealers in Huya. Preparing specimens exclusively with a hammer or with a pincer-vise is possible, but difficult, since the grain of the rock runs diagonally to the open faces of pockets; in any case, the technique is unknown here. A majority of collectors today accept the practice of using saws to shape specimens. Although the miners and the dealers in Huya know the importance of keeping crystals damage-free, the prevailing methods of mining and of packing specimens sometimes make this impossible. At the mine site, many specimens, not individually wrapped, are taken out of a sack to be shown and offered the visitor, and it is no wonder that every one shows damage.

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In a good year a mine like this, with the whole group of miners working together, can show a clear profit of up to 30,000 RMB, which comes to 4,000 RMB, or $500, per person. Specimens are sold regularly to the dealers who live in Huya, who in their turn dispose of them to other dealers, especially those going to Changsha. If visitors to Huya want to acquire good specimens, they need to be lucky enough to find just-mined ones, for all good finds are sold very quickly. In fact, news of them is, as a rule, called in immediately via cell phone to the dealers in Changsha, who grab up the specimens right away to forestall competition.

GEOLOGY

The Xuebaoding beryl-scheelite vein deposits in Pingwu County belong structurally to the sub-Pankouwan Dome, which is north of Longmenshan, west of the Huyaguan fault, and east of the Xuebaoding Dome of the western Yangtze paraplatform. Frequent tectonic movements and magmatic activities in this region in the course of geologic history have given rise to deposits of tungsten, tin, copper, lead, zinc and gold. Up to now, only a very few scientific studies have been available, but because of the economic significance of the area, further research is to be expected.

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The regional stratigraphic formations belong to a Triassic system, including the lower Bocigou, middle Zhagashan, and upper Zhowou Series, a sequence of metamorphosed rocks with intercalated carbonate rocks whose total thickness is between 1,500 and 2,000 meters. These are the host rocks of the tungsten, tin and beryllium minerals. The dominant igneous rocks are alkali granites, occurring as scattered intrusions in the area of the Xuebaoding massif. The intrusive rock is an adamellite-monzodiorite with muscovite; it is composed of plagioclase (to 64.33%); K-feldspar, chiefly microcline (12%); muscovite (20%); quartz (2%); tourmaline (2 to 3%); and trace minerals including magnetite, ilmenite, zircon and apatite. On the basis of its chemical composition this alkali granite can be considered aluminum-supersaturated. Studies have established that igneous rocks of the Yanshanian period (170 million years ago)--the matrix material of the scheelite crystals--contain a high concentration of W[O.sub.3] (0.03-0.3%). The main orecontrolling structure is the Xuebaoding anticlinorium, on the southeastern side of the Xuebaoding dome.

In the mining area, the orebodies occur in muscovite-rich quartz veins and arizonite (a rock type composed of 80% quartz and 18% alkali feldspar with accessory mica and apatite). The ore-bearing veins are contiguous to the alkali granite intrusions. Two types of veins can be distinguished; muscovite-quartz and muscovite-feldspar-quartz. In addition to scheelite, cassiterite and beryl, other minerals, including tourmaline, fluorite and calcite have been observed in various veins. Malachite and pyrite may be found on the mine dumps.

A Chinese scientific study (Cao Zhimin et al., 2002) on fluid and gas inclusions in beryl and scheelite crystals is pertinent to the Xuebaoding vein occurrence, as it sheds light on the conditions of formation of the minerals. Mineralization took place at temperatures between 147[degrees] and 343[degrees] C, and chiefly between 200[degrees] and 310[degrees] C. The hydrothermal fluids, emanating from a pluton, were relatively low in NaCl (3-6% by weight), but high in volatile C[O.sub.2]. On the basis of a verified pressure of 15.2-27.4 MPa, the depth of mineralization can be determined to have been 160-280 meters--thus the process can be described as near-surface and mesothermal. On the basis of a study of the [.sup.40]Ar/[.sup.39]Ar isotopes, ore formation can be dated to 187 million years ago, during the Yanshanian Epoch.

MINERALS

Apatite Group [Ca.sub.5](P[O.sub.4])[.sub.3](F,OH,Cl)

Bright pink, thick-tabular to equant crystals to 4 cm of an apatite-group species, probably fluorapatite, occur at Xuebaoding. The pink apatite crystals have been found there more often than pink beryl crystals, which they resemble, though neither are common. Forms include a first-order and second-order prism, two first-order bipyramids, one second order bipyramid and the basal pinacoid. As with beryl, the crystals occur on coarse druses of muscovite crystals lining vein fractures in the rock, sometimes in association with cassiterite. The pink apatite crystals are easily distinguishable from pink beryl in that they show very little transparency and are less lustrous; furthermore the apatite, but not the morganite, shows good fluorescence.

Beryl [Be.sub.3][Al.sub.2][Si.sub.6][O.sub.18]

Beryl occurs at Xuebaoding in three color varieties: colorless (variety goshenite), pale blue (variety aquamarine) and, very rarely, pink (variety morganite). The crystal habit is always thick-tabular, regardless of color, and in some cases the crystals have re-entrant angles around the rim due to parallel growth. And, whereas tabular beryl from other localities always contains elevated levels of alkali metals, particularly cesium, beryl from Xuebaoding contains relatively little Cs, only 0.23 weight % (White and Richards, 1999). The highly lustrous crystals reach several cm in diameter, and up to 20 cm in exceptional cases. A few heavily corroded crystals have been recovered; while the prism faces of some of these are intact, others show marked dissolution of the prism faces. Regrettably, some artificially irradiated crystals have been offered; these display an unnaturally intense blue color along the edges and at the corners.

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Cassiterite Sn[O.sub.2]

Cassiterite is one of the most common minerals at the locality, occurring in high-quality crystals and twins to more than 7 cm. Most of the black, wedge-shaped crystals are highly lustrous, and are typically intergrown as V-twins. Single, short-prismatic crystals displaying the forms {100}, {110}, {101} and {111} occur rarely; in some cases these are twinned, like rutile, on the (101) plane. The cassiterite crystals and twins occur on the coarse drusy muscovite layers which line fissures.

Euclase BeAlSi[O.sub.4](OH)

There was a surprise in store for the observant visitor to the Munich Show in October 2003. Among the enormous and often bewildering stocks of the countless Chinese dealers there was a specimen from Xuebaoding with colorless, transparent crystals to 2 cm; the species in question was not clearly determinable upon visual inspection. Only after examination at the University of Dresden were the crystals identified as euclase. Inevitably this specimen, and perhaps others, disappeared into private hands before it could be determined whether the specimen(s) show other species which also had not yet been seen, or are very rare, at Xuebaoding.

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Feldspar Minerals

The locality has excited the interest of feldspar collectors by producing perfectly formed albite crystals, some almost entirely transparent, to several cm across. Orthoclase has also been observed in very well-developed, opaque white crystals and Manebach twins on muscovite matrix with beryl.

Fluorite Ca[F.sub.2]

Fluorite is comparatively rare at the locality, and specimens are therefore highly coveted. The cubic crystals are colorless to pale blue and can be quite large; the largest found so far measures about 15 cm along an edge. A frosty surface appearance apparently caused by partial resorption is characteristic.

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Kesterite [Cu.sub.2](Zn,Fe)Sn[S.sub.4] and Mushistonite CuSn(OH)[.sub.6]

Two very interesting rare species occurring at Xuebaoding are kesterite and mushistonite. In 2001 and 2002 a few black crystals measuring about 1 cm and showing a greenish coating were recovered and were thought to be the species "pandaite," but soon it turned out that these in fact are extraordinarily large crystals of kesterite, a copper and zinc sulfide related to stannite. The best discovery took place early in 2003, when a number of very well-developed crystals, probably the finest known for the species, were found in a cleft. Individual kesterite crystals from this find reach more than 3 cm and carry a thin partial coating of white to pale yellow to pale green mushistonite, a copper-tin hydroxide. The sharp tetragonal disphenoidal crystals are usually intergrown, perhaps with some twinning, and are difficult to orient. They occur on muscovite, on beryl and on cassiterite crystals, as well as next to (but not on) scheelite.

Muscovite K[Al.sub.2][square]Al[Si.sub.3][O.sub.10](OH)[.sub.2]

Specimens from Xuebaoding characteristically show larger muscovite crystals perched on the typical cavity-lining coatings of muscovite.

Quartz Si[O.sub.2]

The most attractive specimens from Xuebaoding are those in which scheelite, beryl and cassiterite appear on the same specimen, especially if well-developed crystals of quartz are also present. Quartz occurs in crystals to more than 50 cm long, and in beautiful crystal clusters, but specimens in which quartz is associated with the other major Xuebaoding species are unfortunately rare.

Scheelite CaW[O.sub.4]

Although scheelite--a common ore species in China--is rarely of collector quality at other Chinese localities, it is one of the extraordinary highlights of Xuebaoding. The tetragonal dipyramidal crystals are of typical pseudo-octahedral habit and reach 10 cm across. The color ranges from pale beige to a beautiful deep orange and a stunning red-orange. The more richly colored crystals, especially those that are in large part transparent and have a mirrorbright luster, are the most highly valued and rank among the world's finest scheelite crystals.

Because of the brittleness of the crystals, their points are very frequently damaged during mining. Resourceful dealers quickly learned to create new crystal "faces" by filing and polishing, thereby concealing the problem areas. A practiced eye can easily recognize that the new faces do not match the ones possible for the crystal class of scheelite, and a close examination with a handmagnifier clearly reveals the signs of filing and polishing. Sometimes, too, crystals which have fallen off the original matrix are seen to be glued back on.

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Topaz [Al.sub.2]Si[O.sub.4](F,OH)[.sub.2]

While investigating Internet offerings of minerals from Xuebaoding, I came across a specimen with crystals, associated with scheelite, identified as topaz. This did not seem believable, since the form resembled that of fluorite. During a visit to a dealer in Huya in the course of a trip in early 2004, a specimen was found with very similar-looking crystals: colorless and wholly transparent, the crystals reach 4 cm and are integrown in a cluster 10 cm across. Inspecting the cleavage and crystal angles and testing for hardness quickly revealed that the mineral is indeed topaz, not fluorite, and that the specimen seen on the Internet had after all been correctly labeled. Surely more surprises like this from Xuebaoding await us in the future.

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Other Minerals

Calcite crystals, white and corroded, have been seen on some scheelite specimens. Dolomite and phenakite have also been reported but have not been confirmed.

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BIBLIOGRAPHY

CAO ZHIMIN, ZHENG JIANBIN, LI YOUGUO, XU SHIJIN, and WANG RUCHENG (2002) Geological features of the volatile-rich ore fluid and its tracing and dating in the Xuebaoding beryl-scheelite vein deposit, China. Earth Science in China, 25, (8), 719-729.

NAKAMURA, T. (2003) East of the Himalayas--to the Alps of Tibet. The Japanese Alpine Club, Vol. 4, May 2003, Special Submission.

OTTENS, B. (2004) Xuebaoding. ExtraLapis, no. 26/27, 68-87.

WHITE, J. S. and RICHARDS, R.P. (1999) Chinese beryl crystals mimic twinning. Rocks & Minerals, 74, 318-320.

YANG ZUNYI, CHENG YUQI, and WANG HONGZHEN (1986) The Geology of China. Oxford: Clarendon Press. 303 pages.

Berthold Ottens

Klingenbrunn Bahnhof 24

D-94518 Spiegelau

Germany
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Author:Ottens, Berthold
Publication:The Mineralogical Record
Geographic Code:9CHIN
Date:Jan 1, 2005
Words:3757
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