2 research outputs found
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