4 research outputs found
New P-T-X conditions for the formation of gem tsavorite garnet in the Voi area (southwestern Kenya)
International audienceTsavorite nodules-bearing deposits from southwestern Kenya are located in the Kurase Group, a metasedimentary unit of the Neoproterozoic Metamorphic Mozambique Belt. This unit is composed of graphitic paragneisses intercalated with metacarbonates and metaevaporites, surrounded by migmatites. The rocks underwent high grade metamorphism at 615-600 Ma. The main goal of this work is to link tsavorite formation to the metamorphic evolution of the Kurase Group. The new thermobarometric data indicate widespread granulite facies conditions at 800 +/- 50 degrees C and 10 +/- 1 kbar, with no significant difference between the tsavorite-bearing metasediments and the surrounding migmatitic gneisses. Pseudosection calculation for a tsavorite-bearing metasediment indicates that tsavorite grew close to peak-T conditions at around 800 degrees C. The tsavoritebearing formations have not melted extensively despite the high-grade metamorphism, in contrast with the surrounding migmatites. The lack of partial melting is probably due to an enrichment in vanadium, chromium and titanium in the protoliths that have increased the stability field of micas toward high-T. We suggest that the primary source of V and Cr was the evaporite-bearing mudstones. Crystallisation of high grade V and Cr rich tsavorite occurred in a closed system with little or no strain, in the presence of molten salts and H2S-S-8 fluids
New typology and origin of tsavorite based on trace-element chemistry
New electron-microprobe analyses of 'tsavorites' from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V > Cr > Mn (in atoms per formula unit); type 2, vanadian grossular with V > Mn > Cr; type 3, Mn-bearing vanadian grossular with Mn > V > Cr; type 4, Mn-bearing chromian grossular with Mn > Cr > V; and type 5, Cr- and Mn-bearing grossular with Cr > Mn > V. These types are also characterized by different absorption spectra in the ultraviolet-visible-near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn2+, whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr3+ at 697 and 701 nm. The different absorption spectra also indicate Fe2+-Ti4+ charge transfer. We measured OH equivalent to 0.08 to 0.38 wt% eq. H2O within the structure. Concentrations of vanadium, chromium and manganese are good chemical "fingerprints" for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar
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