Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin

• Published in: Mineralium deposita Vol. 42; no. 3; pp. 251 - 270
• Main Authors: Giuliani, Gaston, Fallick, Anthony, Rakotondrazafy, Michel, Ohnenstetter, Daniel, Andriamamonjy, Alfred, Ralantoarison, Théogène, Rakotosamizanany, Saholy, Razanatseheno, Marie, Offant, Yohann, Garnier, Virginie, Dunaigre, Christian, Schwarz, Dietmar, Mercier, Alain, Ratrimo, Voahangy, Ralison, Bruno
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.02.2007
• Subjects: Geology; Isotopes; ISW, Indian Ocean, Madagascar; ISW, Sri Lanka; ISW, India; ISW, Tanzania
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-006-0105-3

The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (^sup 18^O/^sup 16^O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ^sup 18^O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6[per thousand]. Metamorphic rubies are defined by two groups of δ^sup 18^O values in the range of 1.7 to 2.9[per thousand] (cordieritite) and 3.8 to 6.1[per thousand] (amphibolite). "Magmatic" rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ^sup 18^O values ranging between 1.3 and 4.7[per thousand]. Sapphires are classified into two main groups with δ^sup 18^O in the range of 4.7 to 9.0[per thousand] (pyroxenite and feldspathic gneiss) and 10.7 to 15.6[per thousand] (skarn in marble from Andranondambo). The δ^sup 18^O values for gem corundum from secondary deposits have a wide spread between -0.3 and 16.5[per thousand]. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ^sup 18^O values between 3.5 and 6.9[per thousand]. Ruby from the placers of Vatomandry and Andilamena has δ^sup 18^O values of 5.9[per thousand], and between 0.5 and 4.0[per thousand], respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ^sup 18^O values between -0.3 and 16.5[per thousand], and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have a large variety of colored sapphires and rubies with a large variation in δ^sup 18^O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire, syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire.[PUBLICATION ABSTRACT]