New aspects and perspectives on tsavorite deposits

• Published in: Ore Geology Reviews
• Main Authors: Feneyrol, J., Giuliani, G., Ohnenstetter, D., Fallick, A. E., Martelat, J. E., Monie, Patrick, Dubessy, J., Rollion-Bard, C., Le Goff, Elisabeth, Malisa, E., Rakotondrazafy, A. F. M., Pardieu, V., Kahn, T., Ichangi, D., Venance, E., Voarintsoa, N. R., Ranatsenho, M. M., Simonet, C., Omito, E., Nyamai, C., Saul, M.
• Other Authors: Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Scottish Universities Environmental Research Centre (SUERC), University of Glasgow-University of Edinburgh (Edin.), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Géosciences Montpellier, Université des Antilles et de la Guyane (UAG)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS), Bureau de Recherches Géologiques et Minières (BRGM), Université de Dar es Salaam (UDSM), Faculté des Sciences, Université d'Antananarivo, GIA, Bangkok, GIA, Department of Earth and Environmental Sciences Islamabad, Bahria University, University of Nairobi (UoN), Akili Mineral Services Ltd Company Nairobi, Akili Mineral Services, Department of Mines and Geological Wundanyi, Taita Taveta University (TTU), Swala Gem Traders, Arusha, Swala Gem Traders
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: Neoproterozoic Metamorphic Mozambique; Belt; Tsavorite; Graphitic gneiss; Evaporites; Metasomatism; Genetic model; [SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography; Antarc*; Antarctica; East Antarctica
• Online Access: https://hal.science/hal-00853279; https://doi.org/10.1016/j.oregeorev.2013.01.016

International audience 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 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 ...