My OperaGEMSTONES IN VIETNAM
Tuesday, December 2, 2008 2:20:23 AM
By: Pham Van Long PhD
91 Dinh Tien Hoang Hanoi, Vietnam
* Note : This article was published in The Australian Gemmologist in Volume 22, Number 4, October–December 2004.
Abstract:
Vietnamese research in geology has indicated a high potential for gemstones in Vietnam and has found in this last decade numerous gem deposits, especially ruby and sapphire. In northern Vietnam, gem corundums are found in the Luc Yen, Yen Bai and Quy Chau areas, in primary deposits hosted by metamorphic rocks and in placers. In secondary deposits, ruby and sapphire are associated with gem spinel and garnet. In southern Vietnam, sapphires are related to alkaline basalts, with blue sapphires being economic. Sapphires are recovered with gem zircons and peridots in placers. Aquamarine, beryl, topaz, quartz crystals (amethyst, citrine, morion), tektite, fluorite, opal, chalcedony, jadeite, nephrite and amazonite are the other gemstones exploited in Vietnam. Ruby, sapphires and pearls provide important commercial exchanges in the gemstones markets of Vietnam and other foreign countries.
Introduction
In the late 1980s, reports emerge from Vietnam of a major discovery of high quality rubies in the northern part of the country, from deposits in the Luc Yen and Yen Bai areas. Ruby occurred in colluvial and alluvial sediments, and the primary source of ruby was suspected to be marble and pegmatite. Placer deposits of ruby were recovered by farmers during routine agricultural activities. Following the discovery of ruby in Luc Yen in 1987, others occurrences of gemstones were found in Thuong Xuan (aquamarine and topaz), Co Phuong (jadeite, nephrite), Thach Khoan (beryl, quartz), Quy Chau (ruby) and in the Dak Lak and Binh Thuan provinces (sapphire). After these discoveries, gemstone exploitation really started in 1988 with the establishment of Vinagemco by the government—a state-owned company for the investigation, mining, processing, and trading of gem materials in Vietnam (Fig. 1). The purpose of this paper is to realize an overview of the gemstones occurrences in Vietnam with a special dedication to ruby and sapphire.
Figure 1. Map of Vietnam showing the location of gemstone occurrences.
Distribution of gemstones
Ruby and sapphire deposits
In northern Vietnam
Yen Bai mining district
The primary corundum occurrences of Yen Bai (Fig. 2) occur within the high-grade metamorphic gneisses of the Day Nui Con Voi range (Pham Van 1996, 2003), which extends to the southeast from the Ailao Shan in Yunnan (China). This range is bounded by lateral strike-slip faults forming the major Cenozoic geological discontinuity in East Asia known as the Ailao Shan-Red River shear zone (Phan Trong et al. 1998, 1999, Leloup et al. 2001). The Day Nui Con Voi range is composed of high-grade metamorphic rocks with sillimanite-biotite-garnet gneisses, mica-schists with local alternation of marbles and amphibolites.
Figure 2. Geological map of Yen Bai area with locations of ruby and sapphire deposits.
Corundum occurs as:
grey to blue sapphires in garnet-sillimanite mica schists and gneisses which contain leucosome and leucocratic granitoidic dykes (Truc Lau gneisses and Khe Nhan meta-pegmatite);
in amphibolites converted by the effect of metasomatism in biotite schists with some layers containing centimetre-sized grey to dark sapphires (north of Tan Huong mine, km 15 occurrence;
as rubies in large marble boudins intercalated with gneiss, mica schist and amphibolite (Tan Huong drill cores). These marbles represent previous limestones intergrown with mudstones, which were sheared and metamorphosed during the tectonic activity along the Red River shear zone.
Figure 3. Exploitation of ruby in Tan Huong mine.
The gem deposits are placers exploited along the shear zone as in Tan Huong and Truc Lau (Fig. 2). In Tan Huong, ruby was found and exploited by local farmers in 1994. In 1996, the deposit was managed and exploited by the Vietnam National Gem and Gold Corporation (Fig. 3). From 1994 to 1996, hundreds kilograms of rubies and star rubies were exploited illegally and sold to foreign dealers. The area is composed of metamorphic gneiss, micaceous-quartz schist, marble and amphibolite of the Nui Voi complex that have been intruded by granites, syenites and pegmatite dykes. Ruby and spinel have been found in the magmatic rocks in minor grains as well as in marbles (Nguyen Kinh Quoc et al. 1995). In placers, ruby grains are eroded but the crystals present a prismatic shape, are from 1.0 to 19 mm long, and range in colour from red to reddish and purplish to red. The main associated gem mineral are red and octahedral spinels and blue trapiche-like sapphires. In April 1997, two ruby crystals of 2.58 kg (Fig. 4) and 1.96 kg (star ruby) respectively, of very high quality, were found and declared State treasure.
Figure 4. The ruby called Star of Vietnam weighing 2.58 kg that was found in the Tan Huong mine.
The Truc Lau paleoplacer consists of 10 m thick of sediments overlying the bedrock. The rubies and blue sapphires are contained in a gravel layer of 5 m thickness that is overlain by a 3.5 m of quaternary sediments and 1-1.5 m of soil. In 2002, up to two boulders (1-2 kg) per month made of pink sapphire and star ruby were recovered from this paleoplacer.
In the Tan Dong placer (Fig. 2), assemblages of blue sapphire-margarite-plagioclase are the remainder of metasomatised pegmatites.
Luc Yen mining district
The ruby and sapphire deposits of Luc Yen (Fig, 2) are set in moderate to high temperature recrystallized marble units of Upper Proterozoic-Lower Cambrian age in the eastern side of the Red River shear zone of the Lo Gam tectonic zone (Hoang Quang et al. 1999, Garnier, 2003, Pham Van 2003).
Primary ruby occurs as:
disseminated crystals within marbles with phlogopite, dravite, margarite, pyrite, rutile, spinel, edenite and graphite (Bai Da Lan, An Phu, Minh Tien, Nuoc Ngap, Luc Yen and Khoan Thong mines);
veinlets associated with calcite, dravite, pyrite, margarite and phlogopite (An Phu mine);
fissures with graphite, pyrite, phlogopite and margarite (Bai Da Lan mine); Minh Tien region (Fig. 5).
Figure 5. Flakes of phlogopite (brown) associated with a 1.5 cm long ruby crystal from Minh Tien area.
Secondary deposits consist of gravel concentration in karst pockets and in alluvial fans in the Luc Yen valleys (Kane et al.. 1991). The gem-bearing valleys are often narrow, small depressions ranging from 0.5 to 0.7 km2 in area, but most common 2-3 km2. The corundums are pink, purple to red (Fig. 6), and blue and colourless sapphires coexist with rubies as well as with grey to brown and bi-pyramidal sapphires and trapiche rubies. Associated gem minerals include red, pink and pale blue spinel, gem quality multi-colour tourmaline and garnet. The great variety and high quality of the gem material recovered in the placers make the gemstone market in the center of Luc Yen town opened daily for dealers since 1987 (Fig. 7).
Figure 6. Gem-quality rubies and pink sapphires from Luc Yen deposits.
Ba Be sapphire occurrence
This is located in the Bac Kan province, 320 km north of Hanoi (Fig. 2). It is located at the proximity of the Nui Chua granite and the Hoang Tri gabbro-monzonite. The colourless to pale blue sapphire is found in a pegmatite, composed of quartz, K-feldspar and muscovite, which intrudes schist and marble. All sapphires are opaque and cannot be used for gem cutting.
Figure 7. The Luc Yen gemstones market.
Quy Chau mining district
This area, located 200 km south of the Red River shear zone, is formed by the Bu Khang dome (Fig. 1). It consists in a broad antiform of Paleozoic and Mesozoic sedimentary and meta-sedimentary rocks overlaying a core of micaschists, granitoids, paragneisses and orthogneisses (Nguyen Kinh Quoc et al. 1995; Jolivet et al. 1999). The northeastern part of the dome is limited by the major extensional Cenozoic shear zone of Quy Chau. This is where the corundum deposits are located.
Figure 8. Rough ruby from Quy Chau deposit.
Rubies and sapphires have been mined since 1987 from the placer deposits of Doi Ty, Doi San, Mo Coi and Quy Hop (Pham Van, 2003). The corundum occurs principally in the Quy Chau area as:
very rare and uneconomic rubies disseminated in marbles associated with pyrite and graphite;
in placers which form the economic deposit. In the Doi San and Doi Ty area, granitic intrusions resulted in the injection of pegmatites and the formation of calcium-magnesium-rich skarns in the surrounding marbles, amphibolites, gneiss and micaschists. Rubies were neither observed in the skarn nor in the pegmatite. The genetic origin of this ruby has remained unclear since oxygen isotopes and fluid inclusions studies on these rubies show that they their isotopic signature was metamorphic and similar to that found for typical ruby-hosted marble deposits in northern Vietnam (Garnier, 2003; Giuliani et al., 2003a, b). The gem material consists of ruby, with smaller amount of blue to violet and orange sapphire. Ruby tends to be slightly purplish red in hue position; their crystals are usually in hexagonal prism and barrel shape (Fig. 8).
In Southern Vietnam
The sapphire deposits of southern Vietnam consist of placers formed by the erosion of alkali-basalt flows (Smith et al. 1995). The sapphires present usually prismatic and pyramidal shapes. The size of the crystals is up to 2-7 mm long, but in Dak Nong, Ngoc Yeu and Da Ban areas they sometimes reach 30-40 mm in diameter. Their colour is usually dark blue, sometimes green blue, sky blue, and rarely honey-yellow (except in the Tien Co area in Binh Thuan province).
Several mining district were exploited from 1985 to present. These include:
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91 Dinh Tien Hoang Hanoi, Vietnam
* Note : This article was published in The Australian Gemmologist in Volume 22, Number 4, October–December 2004.
Abstract:
Vietnamese research in geology has indicated a high potential for gemstones in Vietnam and has found in this last decade numerous gem deposits, especially ruby and sapphire. In northern Vietnam, gem corundums are found in the Luc Yen, Yen Bai and Quy Chau areas, in primary deposits hosted by metamorphic rocks and in placers. In secondary deposits, ruby and sapphire are associated with gem spinel and garnet. In southern Vietnam, sapphires are related to alkaline basalts, with blue sapphires being economic. Sapphires are recovered with gem zircons and peridots in placers. Aquamarine, beryl, topaz, quartz crystals (amethyst, citrine, morion), tektite, fluorite, opal, chalcedony, jadeite, nephrite and amazonite are the other gemstones exploited in Vietnam. Ruby, sapphires and pearls provide important commercial exchanges in the gemstones markets of Vietnam and other foreign countries.
Introduction
In the late 1980s, reports emerge from Vietnam of a major discovery of high quality rubies in the northern part of the country, from deposits in the Luc Yen and Yen Bai areas. Ruby occurred in colluvial and alluvial sediments, and the primary source of ruby was suspected to be marble and pegmatite. Placer deposits of ruby were recovered by farmers during routine agricultural activities. Following the discovery of ruby in Luc Yen in 1987, others occurrences of gemstones were found in Thuong Xuan (aquamarine and topaz), Co Phuong (jadeite, nephrite), Thach Khoan (beryl, quartz), Quy Chau (ruby) and in the Dak Lak and Binh Thuan provinces (sapphire). After these discoveries, gemstone exploitation really started in 1988 with the establishment of Vinagemco by the government—a state-owned company for the investigation, mining, processing, and trading of gem materials in Vietnam (Fig. 1). The purpose of this paper is to realize an overview of the gemstones occurrences in Vietnam with a special dedication to ruby and sapphire.
Figure 1. Map of Vietnam showing the location of gemstone occurrences.
Distribution of gemstones
Ruby and sapphire deposits
In northern Vietnam
Yen Bai mining district
The primary corundum occurrences of Yen Bai (Fig. 2) occur within the high-grade metamorphic gneisses of the Day Nui Con Voi range (Pham Van 1996, 2003), which extends to the southeast from the Ailao Shan in Yunnan (China). This range is bounded by lateral strike-slip faults forming the major Cenozoic geological discontinuity in East Asia known as the Ailao Shan-Red River shear zone (Phan Trong et al. 1998, 1999, Leloup et al. 2001). The Day Nui Con Voi range is composed of high-grade metamorphic rocks with sillimanite-biotite-garnet gneisses, mica-schists with local alternation of marbles and amphibolites.
Figure 2. Geological map of Yen Bai area with locations of ruby and sapphire deposits.
Corundum occurs as:
grey to blue sapphires in garnet-sillimanite mica schists and gneisses which contain leucosome and leucocratic granitoidic dykes (Truc Lau gneisses and Khe Nhan meta-pegmatite);
in amphibolites converted by the effect of metasomatism in biotite schists with some layers containing centimetre-sized grey to dark sapphires (north of Tan Huong mine, km 15 occurrence;
as rubies in large marble boudins intercalated with gneiss, mica schist and amphibolite (Tan Huong drill cores). These marbles represent previous limestones intergrown with mudstones, which were sheared and metamorphosed during the tectonic activity along the Red River shear zone.
Figure 3. Exploitation of ruby in Tan Huong mine.
The gem deposits are placers exploited along the shear zone as in Tan Huong and Truc Lau (Fig. 2). In Tan Huong, ruby was found and exploited by local farmers in 1994. In 1996, the deposit was managed and exploited by the Vietnam National Gem and Gold Corporation (Fig. 3). From 1994 to 1996, hundreds kilograms of rubies and star rubies were exploited illegally and sold to foreign dealers. The area is composed of metamorphic gneiss, micaceous-quartz schist, marble and amphibolite of the Nui Voi complex that have been intruded by granites, syenites and pegmatite dykes. Ruby and spinel have been found in the magmatic rocks in minor grains as well as in marbles (Nguyen Kinh Quoc et al. 1995). In placers, ruby grains are eroded but the crystals present a prismatic shape, are from 1.0 to 19 mm long, and range in colour from red to reddish and purplish to red. The main associated gem mineral are red and octahedral spinels and blue trapiche-like sapphires. In April 1997, two ruby crystals of 2.58 kg (Fig. 4) and 1.96 kg (star ruby) respectively, of very high quality, were found and declared State treasure.
Figure 4. The ruby called Star of Vietnam weighing 2.58 kg that was found in the Tan Huong mine.
The Truc Lau paleoplacer consists of 10 m thick of sediments overlying the bedrock. The rubies and blue sapphires are contained in a gravel layer of 5 m thickness that is overlain by a 3.5 m of quaternary sediments and 1-1.5 m of soil. In 2002, up to two boulders (1-2 kg) per month made of pink sapphire and star ruby were recovered from this paleoplacer.
In the Tan Dong placer (Fig. 2), assemblages of blue sapphire-margarite-plagioclase are the remainder of metasomatised pegmatites.
Luc Yen mining district
The ruby and sapphire deposits of Luc Yen (Fig, 2) are set in moderate to high temperature recrystallized marble units of Upper Proterozoic-Lower Cambrian age in the eastern side of the Red River shear zone of the Lo Gam tectonic zone (Hoang Quang et al. 1999, Garnier, 2003, Pham Van 2003).
Primary ruby occurs as:
disseminated crystals within marbles with phlogopite, dravite, margarite, pyrite, rutile, spinel, edenite and graphite (Bai Da Lan, An Phu, Minh Tien, Nuoc Ngap, Luc Yen and Khoan Thong mines);
veinlets associated with calcite, dravite, pyrite, margarite and phlogopite (An Phu mine);
fissures with graphite, pyrite, phlogopite and margarite (Bai Da Lan mine); Minh Tien region (Fig. 5).
Figure 5. Flakes of phlogopite (brown) associated with a 1.5 cm long ruby crystal from Minh Tien area.
Secondary deposits consist of gravel concentration in karst pockets and in alluvial fans in the Luc Yen valleys (Kane et al.. 1991). The gem-bearing valleys are often narrow, small depressions ranging from 0.5 to 0.7 km2 in area, but most common 2-3 km2. The corundums are pink, purple to red (Fig. 6), and blue and colourless sapphires coexist with rubies as well as with grey to brown and bi-pyramidal sapphires and trapiche rubies. Associated gem minerals include red, pink and pale blue spinel, gem quality multi-colour tourmaline and garnet. The great variety and high quality of the gem material recovered in the placers make the gemstone market in the center of Luc Yen town opened daily for dealers since 1987 (Fig. 7).
Figure 6. Gem-quality rubies and pink sapphires from Luc Yen deposits.
Ba Be sapphire occurrence
This is located in the Bac Kan province, 320 km north of Hanoi (Fig. 2). It is located at the proximity of the Nui Chua granite and the Hoang Tri gabbro-monzonite. The colourless to pale blue sapphire is found in a pegmatite, composed of quartz, K-feldspar and muscovite, which intrudes schist and marble. All sapphires are opaque and cannot be used for gem cutting.
Figure 7. The Luc Yen gemstones market.
Quy Chau mining district
This area, located 200 km south of the Red River shear zone, is formed by the Bu Khang dome (Fig. 1). It consists in a broad antiform of Paleozoic and Mesozoic sedimentary and meta-sedimentary rocks overlaying a core of micaschists, granitoids, paragneisses and orthogneisses (Nguyen Kinh Quoc et al. 1995; Jolivet et al. 1999). The northeastern part of the dome is limited by the major extensional Cenozoic shear zone of Quy Chau. This is where the corundum deposits are located.
Figure 8. Rough ruby from Quy Chau deposit.
Rubies and sapphires have been mined since 1987 from the placer deposits of Doi Ty, Doi San, Mo Coi and Quy Hop (Pham Van, 2003). The corundum occurs principally in the Quy Chau area as:
very rare and uneconomic rubies disseminated in marbles associated with pyrite and graphite;
in placers which form the economic deposit. In the Doi San and Doi Ty area, granitic intrusions resulted in the injection of pegmatites and the formation of calcium-magnesium-rich skarns in the surrounding marbles, amphibolites, gneiss and micaschists. Rubies were neither observed in the skarn nor in the pegmatite. The genetic origin of this ruby has remained unclear since oxygen isotopes and fluid inclusions studies on these rubies show that they their isotopic signature was metamorphic and similar to that found for typical ruby-hosted marble deposits in northern Vietnam (Garnier, 2003; Giuliani et al., 2003a, b). The gem material consists of ruby, with smaller amount of blue to violet and orange sapphire. Ruby tends to be slightly purplish red in hue position; their crystals are usually in hexagonal prism and barrel shape (Fig. 8).
In Southern Vietnam
The sapphire deposits of southern Vietnam consist of placers formed by the erosion of alkali-basalt flows (Smith et al. 1995). The sapphires present usually prismatic and pyramidal shapes. The size of the crystals is up to 2-7 mm long, but in Dak Nong, Ngoc Yeu and Da Ban areas they sometimes reach 30-40 mm in diameter. Their colour is usually dark blue, sometimes green blue, sky blue, and rarely honey-yellow (except in the Tien Co area in Binh Thuan province).
Several mining district were exploited from 1985 to present. These include:
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