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Gemstones Formed Directly from Molten Rock: the Ruby Deposits of Chanthaburi-Trat, Thailand

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Gemstones Formed Directly from Molten Rock: the Ruby Deposits of Chanthaburi-Trat, Thailand 5 Gemstones Formed Directly from Molten Rock: The Ruby Deposits of Chanthaburi-Trat, Thailand Molten lava erupting from fissures or volcanoes on temperature rhyolitic lavas. Moonstone is found in the earth's surface may be a source for gemstones. all of these environments, although rarely in sizes These hot lavas may contain gems in one of three and qualities suitable for use as a gemstone. ways: (1) Gemstones such as moonstone may be an Sri Lanka is by far the most important source essential constituent of the lava and crystallize as of moonstone. In Sri Lanka, the moonstone occurs such as the lava cools; (2) in the final stages of in feldspar-rich dikes near Ambalangoda in the cooling, when much of the magma has solidified, Southern Province and in the Kandy district in the minerals such as topaz may crystallize in litho­ Central Province. Characteristically reddish brown physal (gas) cavities formed in the cooling lava; and moonstone occurs in the Coimbatore district of (3) the lava may simply act as the vehicle that trans­ Madras, India (Webster, 1975). ports materials that crystallized at great depth to Moonstone is also found in numerous locali­ the surface. Such is the case with many alkali­ ties in the United States. It is generally believed that olivine basalts, which may bring peridot, zircon, the finest are found in the interior of large, chalky sapphire, or ruby up from 100-km depths (Fig. 5-11. white crystals associated in granitic dikes on the Moonstone is a variety of the potassium-rich west slope of the Black Range in Grant County, feldspar, orthoclase, and its high-temperature New Mexico (Sinkankas, 1959). polymorph, sanidine. It is distinguished by its dis­ Topaz is typically found in gem-bearing grani­ play of a silver-blue sheen known as adulerescence. tic pegmatites, in hydrothermal veins, and in litho­ Orthoclase is a common constituent of granitic physal cavities in rhyolite. Topaz as a late-stage pegmatites, and sanidine is common in the higher- lithophysal cavity filling is best known from the 71 P. C. Keller, Gemstones and Their Origins © Van Nostrand Reinhold 1990 72 Gemstones Formed Directly from Molten Rock topaz-bearing rhyolites in the southern end of the Thomas Mountains in Juab County, Utah (Sinkan­ kas, 1959). A similar topaz occurrence is found in San Luis PotosI, Mexico. Unfortunately, these topaz crystals as well as the similar ones from Utah tend to fade to colorlessness when exposed to light for pro­ longed periods of time. Rare, red beryl has recently been commercially produced from similar lithophysal cavities in rhyolite in the Wah Wah mountains of southwest­ ern Utah. These unique beryls are believed to have formed from a gas or vapor phase given off by a rhyolitic magma as it was cooling (Shigley and Foord, 1984), and the intense red color of these unique beryls seems to be caused by trace amounts of manganese. From a gemologist's point of view, the mate­ rials brought up as passengers in the voluminous alkali basalts found around the world are the most interesting. These basalts, sometimes known as flood basalts because they appear to be so wide­ spread, may bring with them nodules containing zircon, ruby, and/or sapphire or volcanic "bombs" of olivine (peridot) that are thought to represent broken fragments of the Earth's upper mantle. One of the best examples of an upper mantle material, olivine (peridot), being transported tens of kilometers to the earth's surface is an alkali ba­ salt found on Peridot Mesa near San Carlos, Ari­ zona. Here, elliptical to angular bombs of olivine (peridot) up to 20 cm in diameter make up 25 to 40 percent of the rock volume (Broomfield and Shride, 1956; Holloway and Cross, 1978; Vuich and Moore, 1977). The roughly 3-square-km Mesa itself is capped by the basalt, which is 10 to 100 feet thick and was believed to have been erupted from a vol­ canic core that occupies the southwest corner of Peridot Mesa. The olivine bombs are mostly gran­ ular, but contain irregular fragments of peridot up to 2 cm in diameter. These fragments are mined by the local Apache Indians and currently represent a very high percentage of the peridot supplied to the world's gem market (Koivula, 1981). Mineralog­ ically similar nodules are commonly characteristic of alkali basalts, but included fragments of gem peridot are considered quite rare. Recently, a sim­ ilar occurrence with tremendous gem potential was reported from the Tertiary age Hanluoba basalt in the Xuanhua area of Hebei Province, China (Keller and Wang, 1986). Other than peridot, a principal constituent of Figure 5-1. Diagram showing processes leading to the the earth's mantle, relatively few gemstones are magmatic occurrence of rubies. Courtesy of the Natural thought to be formed at great depth and trans­ History Museum of Los Angeles County. ported to the surface as xenocrystic passengers in The Ruby Deposits of Chanthaburi-Trat, Thailand 73 alkali basalts. However, ruby and/or sapphiEe and of widespread lava flows that quickly weathered to zircon occur in such an environment and typically an iron-rich (lateritic) soil (Thompson, 1983). are found together as a suite, although they are The classic occurrence of gems coming from also found individually. At Yoga Gulch, Montana, depth is found in the alkali basalt fields extending where fine cornflower blue sapphires occur in a across Indochina. In Indochina, the occurrence of gabbroic dike 2 to 7 m wide (Clabaugh, 1952), no gems varies regularly from east to west in very sim­ zircons or rubies are present, although some of the ilar alkali basalt flows across the peninsula. In pink Yoga sapphires approach ruby in color. More Vietnam, the eastern extension of the flows, only typical, however, are the sapphire and zircon facil­ zircon has been recovered, the best examples being ities on Hainan Island and at Mingsi in Fujian Prov­ found in the Kha district (Webster, 1975). Zircon ince, China (Keller and Keller, 1986), which have occurrences continue westward into eastern and not yet been fully developed. The sapphires and north-central Kampuchea. The occurrence of zircons are found eluvially in what was a deeply gem-quality zircon has been described from the weathe~d alkali basalt. Very similar deposits have Ratanakiri Quaternary basaltic maslif in northern recently been discovered in the Mercaderes-Rio Kampuchea by Lacombe (1970). In western Kam­ Mayo area in the state of Cauca, Colo~bia (Keller puchea, however, zircon diminishes in impor­ et al., 1985). tance, and sapphire becomes dominant along with The geologic situation is almost identical to the ruby at the famous Pailin deposits (Berrange and gem fields at Anakie, Queensland, and in the New Jobbins, 1976). These deposits are almost continu­ England district of New South Wales, Australia ous westward into Thailand, where the Chantha­ (Coldham, 1985). The New England district centers buri-Trat area produces huge quantities of gem on the towns of Inverell and Glen Innes. Due to the ruby from deeply weathered alkali basalta, and zir­ production of Anakie and New England, Australia con and sapphire diminish in importance. Further is currently the supplier of between 50 and 70 per­ west, ruby gives way to sapphire in the Kanchana­ cent of the world's sapphires. Both areas produce buri area west of Bangkok. Today, the Chanthaburi­ abundant yellow and dark blue-green sapphires, Trat gem field is of greatest importance. along with pale yellow to brown zircons. The gem­ stones are all recovered from alluvial and eluvial The Chanthaburi-Trat Gem Field, Thailand deposits derived from deeply weathered alkali ba­ salts (MacNevin, 1971). The gemstones are associ­ Thailand has been a world supplier of gem ruby ated with black spinel (pleonaste) and are thought and sapphire since the latter part of the nineteenth originally to have been constituents of xenocrysts century, although the deposits were not thought to brought up from great depths. The alkali basalt be as important as those of neighboring Burma. In acted, as in many other areas of the world, as an 1963, however, the Burmese deposits were nation­ elevator for the gemstones that moved them up to alized, and supplies of fine gems from these mines the surface where they spilled out as constituents declined rapidly. The fact that so little is produced in Burma today has catapulted the Thai deposits into importance. An estimated 70 percent of the world's high-quality gem rubies now come from Thailand. Of these, 85 to 90 percent come from the Chanthaburi-Trat district alone (Aranyakanon and Vichit, 1979). Although the overall quality of these rubies is not as high as those found in Burma, some stones are exceptional (Fig. 5-2). The gem deposits of the Chanthaburi-Trat (for­ merly Krat) area are entirely alluvial, having eroded out of deeply weathered basalt flows (Fig. 5-3). The region can be divided into two mining districts based on the type of corundum produced. The first lies to the west, near the town of Chanthaburi and in the Chanthaburi province. It includes the fa­ mous Khao Ploi Waen and Bang Kha Cha mining Figure 5-2. Two fine Thai rubies weighing 3.41 and 3.04 areas, known for their production of blue, blue­ carats. Photo courtesy ofJack S. D. Abraham, Precious green, and yellow sapphires, as well as black star Gem Resources, Inc., New York. sapphires. The second district, approximately 45 74 Gemstones Formed Directly from Molten Rock Figure 5-3. Geologic sketchmap of the Chanthaburi-Trat area's major ruby and sapphire mines. Modifiedfrom Vichit and associates (1978) and Javanaphet (1969). Courtesy of the Gemological Institute ofAmerica.
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