The Chanthaburi-Trat Gem Field, Thailand
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THE CHANTHABURI-TRAT GEM FIELD, THAILAND By Peter C. Keller With th e recent drastic decline in hailand has been a world supplier of gem ruby and production from the classic ruby m ine s of T sapphire since the latter part of the 19th century, Burma, Thailand has become the wo rld's although the deposits were not thought to be as impor major source of gem ruby. The majority tant as those of neighboring Burma. In 1963, however, the of the ruby produ ction, as well as som e Burmese deposits were nationalized, and supplies of fine significant sapphire produ ction, comes gems from these mines declined rapidly. The fact that so from the provin ces of Chan tliabuti and little is produced in Burma today has catapulted the Thai Ttat in southeastern Thailand. Thi s article reviews the geology of th ese deposits into importance. An estimated 70% of the world's alluvial deposits and describes the wide high-quality gem rubies come from Thailand. Of these, range of methods used to mine th e 85% to 90% come from the Chanthaburi-Trat district corundum. Gemologically, the Thai alone (Aranyakanon and Vichit, 1979). While the overall rubies are distinguished by their violet quality of these rubies is not as high as those found in overtone, inhibited fluor escence, and Burma, exceptional stones are seen (figure 1). characteristic inclusion s. Also discussed The gem deposits of the Chanthaburi-Trat (formerly is the heat treatment of corundum to Krat] area are entirely alluvial, having eroded out of deeply enhance color, which has become a major weathered basalt flows . The region can, however, be di industry in Clianthaburi.The newest vided into two mining districts based on the type of co product, "golden" sapphire, is examined, rundum produced. The first lies to th e west, near the including the apparent stability of the color produced and clues to the det ection town of Chanthaburi (figure 2) and in the Chanthaburi of the heat treatment. Province. It includes th e famous Khao ploi Waen and Bang Kha Cha mining areas, known for their production of blue, blue-green, and yellow sapphires as well as black ABOUT THE AUTHOR star sapphires. The second district, approximately 45 krn Dr. Keller is director of educa tion at the to the east of Chanthaburi, in Trat Province, is the cur Gemological Institute of America,Santa Monica, California. rently very active Bo RailBo Waen mining area known for its significant production of ruby. In fact , Bo Rai/Bo Waen, Acknowledgments: The author wishes to thank the many people who con tributed to together with the Pailin area 27 km to the northeast in this article in a variety of ways. John Koivula Kampuchea (formerly Cambodia), comprises the most and Chuck Fryer helped ident ify many of the important ruby-producing area in the world today. Oc inclusions. The trip into the mining region would have been very difficufl if not for the casionally, green sapphires and color-change sapphires generous office of Henry Ho, of the AIGS in occur in this district as well. Bangkok, and the two knowledgable guides, In 1980, an estimated 39.4 million carats of ruby and Narong Saeovi .and Dick Hughes. Sp ecial thanks also go to Dr. George Rossman of the sapphire were mined in the Chanthaburi-Trat area by an California Institute of Techn ology for estimated work force of 20,000 miners (Stamm, 1981). It sharing his insights into the heal is difficult to translate this production figure into official Ireatment of corundum . This study was made possible by a grant from Ihe Western export figures because the latter do not include tourist Foundati on of Vertebrate Zoology. purchases that do not have to be declared at customs or ))1983 Gemolo gical Institute of America smuggled material. In addition, the tremendous amount 186 Chanthaburi-Trat, Thailand GEM S & GEMOLOGY Winter 1982 1 Figure 1. Two fine Thai rubies, 3.41 and 3.04 ct. Photo courtesy of Jack Abraham, Precious Gem Resources, Inc., New York, NY. of corundum from elsewhere, particularly Sri visitors to the area, most notably Moreau (19761, Lanka and Australia, that is brought to Thailand Chang (1969), and Pavitt (1973, 1976). for heat treatment and thus sold in the gem mar- None of these papers, however, has put all the kets of Bangkok undoubtedly has a major effect- aspects of geology, mining methods, production, on any 'export statistics. The actual production and gemological characteristics together into a and export figures are probably much greater than single, comprehensive report. The purpose of this those estimated by the Department of Customs article is to fill this void, as well as to present an and the Department of Business Economics. It is update on current activities in the Chanthaburi- very significant, however, that these figures have Trat area, including the heat treatment of corun- more than doubled since 1973 (Stamm, 1981). dum, based on observations made by the author Considering the importance of the Thai de- on a visit to the area in June 1982. posits, surprisingly little has been written about them. Bauer (1904)provides one of the best early LOCATION AND ACCESS descriptions and includes a detailed map of the The Chanthaburi-Trat mining district is located deposits. Most of the relatively recent work done in southeast Thailand, approximately 330 lzm east- by the Geological Survey of Thailand has been southeast of the capital city of Bangkok. The dis- published in Thai journals which are, for the most trict extends from approximately 1020001 to part, difficult to obtain in the United States. An 103¡00west longitude and 12O00' north latitude excellent general study of the gem deposits of to 13O00' north latitude. The district is contig- Thailand was done by Aranyakanon and Vichit uous with the very productive Pailin ruby and (1979).This report included details on the Chan- sapphire gem field, 27 km northeast of Bo Rai in thaburi-Trat area. Charaljavanaphet (1951)did one Kampuchea (Cambodia). of the earliest geological reports on the Bo Rai Access to Chanthaburi is quite easy now com- area. One of the most important geological stud- pared to the late 19th century, when, according ies of the corundum-related basalts in Thailand to Bauer (1904), a 20-hour journey by steamer was by Vichit et al. (1978). Berrangk and Jobbins from Bangkok was required. The new highway (1976) did a superbly detailed study of the gem has shortened the trip from Bangkok to Chantha- deposits at Pailin, Cambodia, in which they in- buri to three hours. Bo Rai is about an additional cluded many references to the deposits in the 45 lzm from Chanthaburi eastward via highway Chanthaburi-Trat area. A summary of this work no. 3157. Currently, there are more than 20 active was recently published by Jobbins and Berrangk mining operations in the district, ranging from (1981). There have also been several articles by hand operations to highly mechanized ones. Chanthaburi-Trat, Thailand GEMS & GEMOLOGY Winter 1982 187 HISTORY with tropical climates, the corundum deposits of Gem mining in the Chanthaburi-Trat area was the Chanthaburi-Trat district occur exclusively first reported at Khao Ploi Waen, also lznown as in alluvial, eluvial, or residual lateritic soil de- 'Pagoda Hill" or the "Hill of Gems," in 1850. posits derived from underlying basalt flows. The Khao Ploi Waen is about 8 lzm southwest of gems have long been thought to have been de- Chanthaburi, near the village of Bang Kha Cha. rived from these basalts, although reports of in- In 1850, the Shans and Burmese were mining sap- situ occurrences are rare (Vichit et al., 1978). phires here (Pavitt, 1973). According to Bauer While the precise age of the basalts has not yet (1904))a missionary report dated 1859 stated that been determined, they are thought to be relatively it was possible to collect a handful of rubies from young; estimates range from Tertiary to Pleisto- the "Hill of Gems" in half an hour. Interestingly, cene (Leon Silver, personal communication, 1982). the pagoda that Bauer cites still stands. According to Jobbins and Berrange (1981), the Bauer (1904) described the state of the ruby closely related Pailin basalts have been radi- and sapphire deposits in the Trat Province (then ometrically determined to be 1.4 to 2.14 million lznown as Krat) in the 1870s and 1880s. He di- years old. This places the basalts in Upper Pli- vided the district into two groups, roughly 50 lzm ocene to Lower Pleistocene age. They have been apart: Bo Nawang, an area of about 40 square lzi- informally designated the Chanthaburi-Trat ba- lometers; and Bo Channa, 50 lzm to the northeast salts by Vichit et al. (1978).These flows uncon- and a little over one square kilometer in extent. formably overlie the Devonian-age Kanchanaburi Bauer noted that the mines at Bo Nawang were formation, which consists of phyllites and quartz- small, typically pits one meter deep, and sunk in ites, and the JurassicITriassic Phu Kradung vol- coarse yellow-brown sand overlying a bed of clay. canic siltstones, sandstones, and conglomerates The rubies occurred at the base of the sand unit (Javanaphet, 1969). These units are undoubtedly in a layer 15-25 cm thick. These mines have been comparable to the Devonian-age 0 Smoet for- worlzed since about 1875. mation and Triassic-age Tadeth Group that Ber- In 1895, an English company, The Sapphires range and Jobbins (1976)discuss in their compre- and Rubies of Siam, Ltd., obtained the rights to hensive study of the Pailin gem field. Other mine in what was then Siam.