Feneyrol J. auteur aut IRD Giuliani Gaston auteur aut IRD Ohnenstetter D. auteur aut IRD Fallick A. E. auteur aut IRD Martelat J. E. auteur aut IRD Monie P. auteur aut IRD Dubessy J. auteur aut IRD Rollion-Bard C. auteur aut IRD Le Goff E. auteur aut IRD Malisa E. auteur aut IRD Rakotondrazafy A. F. M. auteur aut IRD Pardieu V. auteur aut IRD Kahn T. auteur aut IRD Ichangi D. auteur aut IRD Venance E. auteur aut IRD Voarintsoa N. R. auteur aut IRD Ranatsenho M. M. auteur aut IRD Simonet C. auteur aut IRD Omito E. auteur aut IRD Nyamai C. auteur aut IRD Saul M. auteur aut IRD text journalArticle eng text/pdf born digital access 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 fades 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-S-8 metamorphic fluids characterized in primary fluid inclusions of tsavorites and the delta B-11 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 delta O-18 values are in the range of 9.5-21.1 parts per thousand. 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. specialized Neoproterozoic Metamorphic Mozambique Belt Tsavorite Graphitic gneiss Evaporites Metasomatism Genetic model 064 53 1-25 2013 0169-1368 https://www.documentation.ird.fr/hor/PAR00010568 10.1016/j.oregeorev.2013.01.016 0169-1368 [F B010060328] https://www.documentation.ird.fr/hor/PAR00010568 https://www.documentation.ird.fr/intranet/publi/2013/06/010060328.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2013-07-04 2017-08-23 PAR00010568 fre