The role of evaporites in the formation of gems during metamorphism of carbonate platforms: a review

Giuliani, G., Dubessy, J., Ohnenstetter, D., Banks, D., Branquet, Y., Feneyrol, J., Fallick, A. E. and Martelat, J.-E. (2018) The role of evaporites in the formation of gems during metamorphism of carbonate platforms: a review. Mineralium Deposita, 53(1), pp. 1-20. (doi: 10.1007/s00126-017-0738-4)

Abstract

The mineral and fluid inclusions trapped by gemstones during the metamorphism of carbonate platform successions are precious markers for the understanding of gem genesis. The nature and chemical composition of inclusions highlight the major contribution of evaporites through dissolution or fusion, depending on the temperature of formation from greenschist to granulite facies. The fluids are highly saline NaCl-brines circulating either in an open system in the greenschist facies (Colombian and Afghan emeralds) and with huge fluid-rock metasomatic interactions, or sulphurous fluids (ruby, garnet tsavorite, zoisite tanzanite and lapis-lazuli) or molten salts formed in a closed system with a low fluid mobility (ruby in marble) in the conditions of the amphibolite to granulite facies. These chloride-fluoride-sulphate ± carbonate-rich fluids scavenged the metals essential for gem formation. At high temperature, the anions SO4 2−, NO3 −, BO3 − and F− are powerful fluxes which lower the temperature of chloride- and fluoride-rich ionic liquids. They provided transport over a very short distance of aluminium and/or silica and transition metals which are necessary for gem growth. In summary, the genetic models proposed for these high-value and ornamental gems underline the importance of the metamorphism of evaporites formed on continental carbonate shelves and emphasise the chemical power accompanying metamorphism at moderate to high temperatures of evaporite-rich and organic matter-rich protoliths to form gem minerals.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fallick, Professor Anthony
• Authors: Giuliani, G., Dubessy, J., Ohnenstetter, D., Banks, D., Branquet, Y., Feneyrol, J., Fallick, A. E., and Martelat, J.-E.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Mineralium Deposita
• Publisher: Springer
• ISSN: 0026-4598
• ISSN (Online): 1432-1866
• Published Online: 24 May 2017
• Copyright Holders: Copyright © 2017 Springer-Verlag Berlin Heidelberg
• First Published: First published in Mineralium Deposita 53(1):1-20
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher