Update on Gemstone Mining in Luc Yen, Vietnam

 

Marble-hosted ruby and spinel deposits represent the foremost source of colored gems in Central and Southeast Asia (Garnier et al., 2008). They are contained in a series of platform carbonates that underwent amphibolite facies metamorphism to form crystalline marbles during the Cenozoic continental collision between the Indian and Eurasian plates about 21–23 million years ago (Garnier et al., 2006). These deposits occur in Afghanistan, Pakistan, Azad Kashmir, Tajikistan, Nepal, Burma (Myanmar), China, and central and northern Vietnam (Hughes, 1997). In northern Vietnam, the deposits are located in the Yen Bai province (figure 1). The city of Yen Bai, the regional administrative capital, is located some 200 km northwest of Hanoi, about halfway to the Chinese border. It was an important local gem trading center during the 1990s (Kane et al., 1991; Kammerling et al., 1994), though most of the dealers have since relocated to Hanoi. Luc Yen, on the other hand, is a mountainous district located in the north of Yen Bai province. Its main city, Yen The, is also known as Luc Yen, the name given to the ruby and spinel mining district described in the gemological literature (e.g., Kane et al., 1991; Pham et al., 2004a).

The aim of this paper is to present the state of gem mining activities in the Luc Yen district within their geological framework, and to provide insight into the local gem trade. The paper will show the evolution of Luc Yen’s gem industry since 1987, with an eye toward the future.

HISTORY OF THE LUC YEN MINING DISTRICT

The Luc Yen district mainly consists of jungle-covered karsts separated by several narrow valleys where paddy fields and traditional villages are located, as shown in figure 2. It is bordered on the west, south, and east by Thac Ba Lake (figure 3). The lake was created in 1970 by the construction of a dam for Vietnam’s first hydroelectric power plant. Thac Ba Lake is 80 km long, with a maximum width of 10 km, and contains more than a thousand islands. Covering the expanse between Tan Huong and the Luc Yen peninsula, it has submerged a potentially gem-rich area.

The initial discovery of high-quality Vietnamese rubies occurred in the Luc Yen district in late 1987, after farmers recovered the stones from placers. The ruby was found in colluvial and alluvial sediments, and the primary source was believed to be marble. Gem exploitation began in earnest in June 1988, with the establishment of VINAGEMCO (Vietnam Gemstones Company), a state-owned company for exploration, mining, processing, and trading. Mining activity began that September, with a joint venture between VINAGEMCO and B.H. Mining Company of Thailand to work the Khoan Thong Valley placer. From November 1989 to March 1990, 244 kg of gem-quality rubies and sapphires were mined from Khoan Thong, most of them sold and cut in Bangkok (Kane et al., 1991; Garnier et al., 2004; Pham et al., 2004a,b,c; Nguyen et al., 2011; Le et al., 2012).

From 1990 to 1994, thousands of independent miners moved to the Luc Yen district. Many new discoveries were made in both primary deposits and placers. These gems were sold in the Luc Yen market. In 1995, the mines went under the management of the Vietnam National Gems and Gold Corporation (VIGEGO). A 2003 merger formed the Vietnam Minerals Corporation (VIMICO), and the Luc Yen mines are no longer controlled by the state. Local farmers, working in small-scale operations, do much of the mining throughout the region.

Mining activity in Khoan Thong and the surrounding mountains continues, though it is only a shadow of its past. Today, two large memorial stones located at the entrance of the valley serve as reminders of bygone ruby mining.

THE LUC YEN MINING DISTRICT

The deposits of Yen Bai and Luc Yen are hosted by metamorphic belts associated with large-scale shear zones. The Red River shear zone (figure 4) comprises the Day Nui Con Voi metamorphic belt and, on its eastern flank, the Lo Gam tectonic zone (Garnier, 2003; Pham et al., 2004a,c). The Luc Yen ruby and spinel deposits occur in the Lo Gam zone, in a thick metasedimentary sequence of Cambrian age, composed of marble and overlying sillimanite-biotite-garnet schist. This zone is also known for fine tour­maline. These units, bounded by left-lateral faults, are intruded by granitic rocks and related pegmatites of Triassic age. The primary and secondary (placer) deposits of ruby and spinel, as well as sapphire, tourmaline, amazonite, pargasite, and humite, are presented within a geological framework.

Ruby and Sapphire Deposits. Luc Yen’s corundum deposits are set in moderate- to high-temperature recrystallized marble units of Upper Proterozoic to Lower Cambrian age, in the eastern side of the Red River shear zone, specifically the Lo Gam tectonic zone (Hoang et al., 1999; Garnier et al., 2002, 2008; Pham et al., 2004a; Nguyen et al., 2011). Two types of deposits are mined there (Giuliani et al., 2003):

• Primary ruby occurs as: (a) disseminated crystals within marble associated with phlogopite, dravite, margarite, pyrite, rutile, spinel, edenite, and graphite (An Phu, Minh Tien, Luc Yen, and Khoan Thong areas); (b) veinlets associated with calcite, dravite, pyrite, margarite, and phlogopite (An Phu); and (c) fissures with graphite, pyrite, phlogopite, and margarite (Bai Da Lan mine and Minh Tien)
• Secondary deposits (i.e., placers) consist of gravel concentration in karst pockets and alluvial fans found all over the Luc Yen region (Kane et al., 1991; Garnier et al., 2004). The gem-bearing valleys are often narrow and correspond to small depressions typically ranging from 2 to 3 km². Corundum crystals are pink, purple to red, and blue; colorless sapphire coexists with ruby, as well as gray to brown and bipyramidal sapphire and trapiche ruby. Gem corundum is associated with spinel (red, pink, and pale blue), multicolored tourmaline, and garnet. These placers furnish a variety of gem-quality material for the  market, which has been open daily since 1987 (figure 5).

Spinel Deposits. Gem spinel, hosted in either marble or placers, was discovered in Luc Yen in 1987, together with ruby and sapphire. Local farmers mine gem-quality spinel from placers along the streams and from alluvial deposits. The specimens exhibit red, brownish red, pink, purple, sea-blue, and sky-blue colors (figure 6). Faceted spinels of up to several hundred carats are known, but most of the faceted gems weigh less than 10 carats. The red, brownish red, and pink spinels are found in An Phu, Minh Tien, Bai Gau, and Khoan Thong. Cong Troi is the source of pink and purple spinels, while dark sea-blue specimens are mined mostly in Co Ngan and Bai Gau. Fine “electric” blue spinel crystals (usually small and/or included and under three grams) come from Khe Khi, Ba Linh Mot, and Khau Ca. Sky-blue material is produced in the Bai Son and Lung Thin areas, as well as near Bai Gau (Senoble, 2010).

In Luc Yen, rough spinel crystals occur in dolomitic marble with calcite, phlogopite, humite, and pargasite. Formation takes place in metasomatic zones resulting from the percolation of fluids in the marble. The crystals, which range in size from a few millimeters to five centimeters, are of octahedral habit and have a red to brownish red color. Spinel crystals in placers are more transparent and are used for gem cutting. Larger crystals in host rock are usually translucent to opaque, and suitable only as collection samples.

Tourmaline Deposits. Tourmaline-bearing pegmatites have been reported in Luc Yen (Nguyen, 1995), but gem tourmalines have not been found in these rocks thus far. They have been recovered instead from alluvial gravels associated with gem corundum; these gravels may be found in weathered crust. The crystals are striated prisms with rounded triangular cross-sections and various terminations. The color of Luc Yen tourmaline ranges from green to brown, black, yellow, and red (figure 7). Multicolored zoned crystals usually contain alternately pink, purple, and yellowish green colors. Color zoning is often observed from the center of the crystals to their periphery, with a combination of pink, purple, and dark green.

Feldspar Deposits. Gem-quality feldspar, found with tourmaline in weathered pegmatites in the Minh Tien area, was first discovered in 1999. Production has fallen in recent years, because the deposit’s location in paddy fields has restricted mining. This feldspar ranges from light to dark green, and from opaque to transparent. Most of the production has only been suitable for cabochon cutting. The stones are typically traded as amazonite, a variety of microcline feldspar, which is found in rare-element pegmatites (Nguyen, 2010). It is associated with smoky quartz, albite (cleavelandite), tourmaline (elbaite), and Li-mica (lepidolite). Facet-grade transparent green feldspar (figure 8), which is very rare here, has been identified as green orthoclase (Laurs et al., 2005).

Pargasite and Humite Occurrences. Pargasite and humite are often associated with spinel in the marbles around An Phu, Phan Thanh, and Khoan Thong. They tend to be interspersed alongside spinel in alternating bands within the marbles. The pargasite appears as dense clusters or wide bands formed of long prismatic crystals, sometimes with a hexagonal cross-section. They can form as groups of crystals from 0.5 to 2 cm and sometimes up to 1.5 × 7 cm. The pargasite is usually dark to light green (Cong Troi) but can be yellowish green when associated with ruby (Khoan Thong). Its transparency is often very poor, and the material is sold in Yen The for “gem paintings” (described in the “Gemstone Trading Activities” section below).

Humite (clinohumite), distributed with spinel in white marble, forms in groups of crystals and usually has a honey to dark yellow color (figure 9). Their size varies from 1 × 1.5 cm to 3 × 5 cm. Like the Luc Yen pargasite, the humite has poor transparency and is typically used for gem paintings¹. Attractive samples  of spinel, pargasite, and humite in white marble, ranging in height from 6 to 10 cm, are typically sold to collectors; sometimes they are carved into ornamental artworks.