Les gisements de saphirs et rubis associés aux basaltes alcalins de Madagascar : caractéristiques géologiques et minéralogiques 1ère partie : caractéristiques géologiques des gisements

Rakotosamizanany, S., Giuliani, G., Ohnenstetter, D., Rakotondrazafy, A.F.M. and Fallick, A.E. (2009) Les gisements de saphirs et rubis associés aux basaltes alcalins de Madagascar : caractéristiques géologiques et minéralogiques 1ère partie : caractéristiques géologiques des gisements. Revue de l’Association Française de Gemmologie, 169, pp. 13-21.

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Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy from the Antsiranana Province in the northern part of the country (sapphire-bearing palaeoplacer and placer) Soamiakatra - Mandrosohasina from the Antananarivo Province in the central part (primary ruby deposit, and ruby and/or sapphire-bearing placer and palaeoplacer) and Vatomandry from the Toamasina Province in the eastern part of the island (ruby and sapphire-bearing paleoplacer). Soamiakatra is the only known primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, which were entrained and brought up to the upper crust by the Ankaratra volcanic event. Petrographic studies have demonstrated the existence of two different conditions of ruby formation at the boundary of the eclogite domain (HT ~ 1100°C, HP ~ 20Kb) and granulite facies (HT ~ 1100°C, BP less than 15Kb). In contrast, the sapphires originated from alkaline mafic magmatic chambers at the lower continental crust-mantle boundary. They occur within syenite and anorthoclasite xenoliths in the basalts or are associated with them. These alkali basalts transported corundum and their host-rocks to the surface; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene-Quaternary. The mineralogical study of ruby and sapphire permitted us to establish the relationships that exist between their chemical composition, their colour, and the nature of their solid inclusions. Iron is the main chromophorous element of sapphire and the Fe2+/Fe3+ ratio in its structure, in addition to the quantity of titanium and chromium available in the fluid, determine the colour of the sapphire and the hue of the polychrome sapphires. Chromium is the second most important chromophorous element for pink, violet blue and some blue green to light blue sapphires. For ruby, chromium is the chromophorous element but iron and vanadium interfere to produce red brown to purplish-blue crystals. The deep coloured high quality rubies are characterized by a Fe2O3/Cr2O3 less than 1. The chemical diagrams used for the genetic classification of corundum indicate a metamorphic origin for all rubies from the central and eastern regions of Madagascar: rubies are hosted either by mafic and/or ultramafic rocks or by metasomatites formed by fluid-mafic rock alteration ("plumasite" or biotitite). Two processes have been identified to explain the origin of sapphires from the different regions: (i) a dominant magmatic process linked to syenites (anorthoclasite), (ii) a metamorphic process (probably of skarn type). The oxygen isotopic compositions of the corundum corroborate these two origins. Solid inclusions such as pyrochlore, samarksite, uraninite and anorthoclase in the sapphires confirm their syenitic origin. The presence of pentlandite, talc, sillimanite, titanite and phlogopite in the rubies testify to a metamorphic paragenesis.

Item Type:Articles
Keywords:basalt; xenolith; ruby; sapphire; deposit; syenite; pyroxenite; meta-gabbro magmatic; metamorphic; paragenesis; solid inclusion; trace elements; geology of gemstones
Glasgow Author(s) Enlighten ID:Fallick, Professor Anthony
Authors: Rakotosamizanany, S., Giuliani, G., Ohnenstetter, D., Rakotondrazafy, A.F.M., and Fallick, A.E.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Revue de l’Association Française de Gemmologie

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