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

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

Les gisements de corindons gemmes de Madagascar [The gem corundum deposits of Madagascar]

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Gem corundum deposits of Madagascar: a review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800°C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits

New aspects and perspectives on tsavorite deposits

Tsavorite, the vanadian variety of green grossular, is a high value economic gemstone. It is hosted exclusively in the metasedimentary formations from the Neoproterozoic Metamorphic Mozambique Belt. The deposits are mined in Kenya, Tanzania and Madagascar and other occurrences are located in Pakistan and East Antarctica. They are located within metasomatized graphitic rocks such as graphitic gneiss and calc-silicates, intercalated with meta-evaporites. Tsavorite is found as primary deposits either in nodule (type I) or in quartz vein (type II), and in placers (type III). The primary mineralizations (types I and II) are controlled by lithostratigraphy and/or structure. For the African occurrences, the protoliths of the host-rocks were deposited at the beginning of the Neoproterozoicwithin a marine coastal sabkha environment, located at the margin of the Congo–Kalahari cratons in the Mozambique Ocean. During the East African–Antarctican Orogeny, the rocks underwent high amphibolite to granulite facies metamorphism and the formation of tsavorite deposits occurred between 650 and 550 Ma. The nodules of tsavorite were formed during prograde metamorphism, calcium coming from sulphates and carbonates, whereas alumina, silicates, vanadium and chromium probably came from clays and chlorite. The veins were formed during the deformation of the metasedimentary platform units which experienced shearing, leading to the formation of fault-filled veins. Metasomatism developed during retrograde metamorphism. The metasedimentary sequences are characterized by the presence of evaporitic minerals such as gypsum and anhydrite, and scapolite. Evaporites are essential as they provide calcium and permit the mobilization of all the chemical elements for tsavorite formation. The H2S–S8 metamorphic fluids characterized in primary fluid inclusions of tsavorites and the δ11B values of coeval dravite confirm the evaporitic origin of the fluids. The V2O3 and Cr2O3 contents of tsavorite range respectively from 0.05 to 7.5 wt.%, while their δ18O values are in the range of 9.5–21.1‰. The genetic model proposed for tsavorite is metamorphic, based on chemical reactions developed between an initial assemblage composed of gypsum and anhydrite, carbonates and organic matter deposited in a sabkha-like sedimentary basin