Winter 2008

Featured Gem News International: Visit to Andesine Mines in Tibet and Inner Mongolia

Gem-quality plagioclase feldspar (labradorite) has been recovered for years from the U.S. state of Oregon (e.g., A. M. Hofmeister and G. R. Rossman, "Exsolution of metallic copper from Lake County labradorite," Geology, Vol. 13, 1985, pp. 644–647; C. L. Johnston et al., "Sunstone labradorite from the Ponderosa mine, Oregon," Winter 1991 Gems & Gemology, pp. 220–233). In 2002, red andesine-labradorite appeared in the gem market that was reportedly sourced from an unspecified locality in the Democratic Republic of the Congo (Spring 2002 GNI, pp. 94–95), but some believe that this material actually came from China. In late 2005, a red andesine called "Tibetan sunstone" was supplied by Do Win Development Co. Ltd. of Tianjin, China, reportedly from Nyima (actually Nyemo) in central Tibet (Winter 2005 GNI, pp. 356–357). Then, at the February 2007 Tucson gem shows, King Star Jewellery Co. (Hong Kong) and M. P. Gem Corp. (Kofu, Japan) introduced a similar red andesine from Tibet called Lazasine. A large supply of red andesine allegedly from China was offered for sale as an official gemstone of the 2008 Summer Olympic Games in Beijing. Despite claims to the contrary, there has been widespread suspicion that the red Chinese andesines are diffusion treated. In fact, recent studies have proved the viability of diffusion-treating such material (e.g., G. Roskin, "JCK web exclusive: The andesine report," posted November 12, 2008, www.jckonline.com/article/CA6613857.html).

In October-November 2008, this contributor visited two andesine deposits in the Chinese autonomous regions of Tibet and Inner Mongolia. The investigation was made possible by the cooperation of mine owners Li Tong of Tibet and Wang Gou Ping of Inner Mongolia, as well as trip organizers Wong Ming (King Star Jewellery Co.) and Christina Iu (M. P. Gem Corp.), who are partners in the Tibetan andesine mine. Also participating in the expedition were Masaki Furuya (Japan Germany Gemmological Laboratory, Kofu, Japan), David Chiang (BBJ Bangkok Ltd., Bangkok), and Marco Cheung (Litto Gems Co. Ltd., Hong Kong).

The Tibetan andesine mine we visited is located 70 km south of the regionís second largest city, XigazÍ (or Shigatse), in southern Tibet. This area is well south of the Nyima/Nyemo area (Lhasa region), and our guides were not aware of an andesine mine in that part of Tibet. We drove seven hours from the capital city of Lhasa to the mine, which lies at an elevation of more than 4,000 m. The site is divided into north and south areas with a total coverage spanning 3–4 km east-west and 5–7 km north-south. During our visit, fewer than 10 miners were digging pits in the south area, near a piedmont riverbed (located at the base of a mountain). Organized mining began there in January 2006 under the supervision of Li Tong. The work is done by hand, from April to November. According to the miners, red andesine was originally found in this area in the 1970s, and beads of this material first appeared in Lhasa's largest bazaar (Bakuo Street) in 2003.

The surface layer at the site consists of humic soil that is 0.5–3 m thick. The andesine is mined from an underlying layer consisting of greenish gray or dark gray sand/gravel in the south area, and yellowish red or greenish gray soil in the north area. The andesine-bearing layers are apparently derived from Tertiary volcano-sedimentary deposits (Qin Zang Gao Yuan [Tibet Highland] area geologic map, Chengdu Institute of Multipurpose Utilization of Mineral Resources, China Geological Survey, Chengdu, Sichuan Province, 2005). In the south mining area, a few tunnels penetrate several meters horizontally into the andesine-bearing horizons. In addition, a shaft was sunk several meters deep in the north area, but mining there was discontinued after the devastating Chengdu earthquake in May 2008. The andesine is concentrated in patches consisting of several to more than a dozen pieces (100–200 g total) mixed with sand/gravel or soil. These accumulations appear to have been concentrated across a wide area by water from seasonal snowmelt.

The andesine from Inner Mongolia is mined from an alluvial deposit of sand/gravel in the Guyang area, north of Baotou city. The mine is situated in the Yinshan tectonic belt of Mesozoic-Cenozoic age (Inner Mongolia Guyang-Xiaoyutai area geologic map, Inner Mongolia Autonomous Region Geological Survey, Hohhot, 1982). Andesine has been recovered from a region measuring 20 km east-west and 5 km north-south. Humic topsoil overlies Tertiary (Pliocene) and Cretaceous sand/gravel; some areas also show layers of tuff or basaltic rock. The andesine is restricted to a light gray layer (locally iron stained) that is 1–3 m thick and lies several meters beneath the surface—down to more than 10 m—within the sand/gravel. Organized mining has taken place near Shuiquan and Haibouzi villages, producing up to 100 tonnes annually. The andesine seen by this contributor commonly had high transparency and was somewhat rounded, except for broken pieces that showed well-developed cleavage surfaces. The stones were typically 0.3–5.5 cm in diameter, with 70–80% in the 1–2 cm range. Most of the andesine was pale yellow. Colorless or deep yellow stones were uncommon, while other colors have not been reported from this area.

This field investigation confirmed that the XigazÍ region of Tibet does indeed produce natural red andesine, while the Guyang area of Inner Mongolia is a source of pale yellow andesine that may be used as the starting material for diffusion treatment. Additional images from this expedition can be found in the G&G Data Depository.

Ahmadjan Abduriyim
Gemmological Association of All Japan ñ Zenhokyo
Tokyo, Japan