Arusha, Tanzania
Arusha, Tanzania

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Feneyrol J.,University of Lorraine | Giuliani G.,University of Lorraine | Ohnenstetter D.,University of Lorraine | Le Goff E.,Bureau de Recherches Géologiques et Minières | And 5 more authors.
Comptes Rendus - Geoscience | Year: 2010

The first study of the Lemshuku 'tsavorite' mining district is presented. From bottom to top, the lithostratigraphic column corresponds to a metasedimentary sequence composed of quartzite, fine-grained graphitic gneiss, kyanite-graphite gneiss, biotite-almandine gneiss, metasomatized graphitic gneiss and dolomitic marble. 'Tsavorite' occurs in quartz veins and rarely as nodular concretions. Two factors control mineralization: (1) lithostratigraphy, with 'tsavorite' in association with pyrite and graphite confined to quartz veins within the metasomatized graphitic gneiss; and (2) structure, with the mineralized veins characteristically controlled by tight isoclinal folds associated with shearing. © 2010 Académie des sciences.

Giuliani G.,British Petroleum | Fallick A.E.,Isotope Geosciences Unit | Feneyrol J.,British Petroleum | Ohnenstetter D.,British Petroleum | And 2 more authors.
Mineralium Deposita | Year: 2011

The combination of oxygen isotope composition with V-Cr-Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ18O values between 9. 5‰ and 11. 0‰, 11. 6‰ and 14. 5‰, and 15. 5‰ and 21. 1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ18O value of tsavorite is a good tracer of the environment of its formation; the δ18O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide. © 2011 Springer-Verlag.

Feneyrol J.,University of Lorraine | Giuliani G.,University of Lorraine | Giuliani G.,University Paul Sabatier | Ohnenstetter D.,University of Lorraine | And 18 more authors.
Ore Geology Reviews | Year: 2013

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 Neoproterozoic within 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 550Ma. 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 7.5wt.%, 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. © 2013 Elsevier B.V.

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