Groat L. A., Giuliani Gaston, Marshall D. D., Turner D. (2008). Emerald deposits and occurrences : a review. Ore Geology Reviews, 34 (1-2 Special Issue), p. 87-112. ISSN 0169-1368.
Auteurs
Groat L. A., Giuliani Gaston, Marshall D. D., Turner D.
Source
Ore Geology Reviews, 2008,
34 (1-2 Special Issue), p. 87-112 ISSN 0169-1368
Emerald, the green gem variety of beryl, is the third Most valuable gemstone (after diamond and ruby). Although it is difficult to Obtain accurate statistics, Colombia supplies most (an estimated 60%, worth more than 500,000,000 per year) of the world's emeralds. However there is speculation that the emerald mines in Colombia are becoming depleted. Brazil currently accounts for approximately 10% of world emerald production. Emeralds have also been mined in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar, Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the United States, and Zimbabwe. Because it is difficult to obtain accurate analyses of beryllium, most published analyses of beryl are renormalized on the basis of 18 oxygen and 3 Be atoms per formula unit. The color of emerald is due to trace amounts of chromium and/or vanadium replacing aluminum at the Y site; in most cases the Cr content is much greater than that of V. To achieve charge balance, the Substitution of divalent cations at the Y site is coupled with the substitution of a monovalent cation for a vacancy at a channel site. Beryl is relatively rare because there is very little Be in the upper continental Crust. Unusual geologic and geochemical conditions are required for Be and Cr and/or V to meet. In the classic model, Be-bearing pegmatites interact with Cr-bearing ultramafic or mafic rocks. However in the Colombian deposits there is no evidence of magmatic activity and it has been demonstrated that circulation processes within the host black shales were sufficient to form emerald. In addition, researchers are recognizing that regional metamorphism and tectonometamorphic processes Such as shear zone formation may play a significant role in certain emerald deposits. A number of genetic classification schemes have been proposed for emerald deposits. Most are ambiguous when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit. Studies of individual emerald deposits show that in most cases a combination of mechanisms (magmatic, hydrothermal, and metamorphic) were needed to bring Be into contact with the chromophores. This Suggests the need for a more flexible classification scheme based on mode of formation. Stable isotopes can be used to estimate the contribution of each mechanism in the formation of a particular deposit. Such estimates could perhaps be more precisely defined using trace element data, which should reflect the mode of formation. Emerald may be identified in the field by color, hardness, and form. It will tend to Show LIP in stream sediment samples but because its specific gravity is relatively low, it will not concentrate in the heavy mineral fraction. In Colombia, structural geology, the sodium content of stream sediment samples, and the lithium. sodium, and lead contents of soil samples have all been used to find emerald occurrences. Exploration for gem beryl could result in the discovery of new occurrences of non-gem beryl or other Be minerals that Could become new sources of Be and Be oxide. Future efforts should go towards creating a comprehensive data base of emerald compositions (including trace elements), determination of the role of metamorphism in the formation of some emerald deposits, improved classification schemes, and more effective exploration guidelines.
