ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA

Abstract

Tanzanite is the blue vanadium-bearing variety of zoisite. The only known source of gem-quality tanzanite, to date, is the Merelani area of northeastern Tanzania, a structurally complex region consisting of granulite facies metacarbonate and metasilicate rocks overprinted by amphibolite to greenschist facies retrogression. Gem-quality green grossular garnets, known as tsavorite, are also found in the area, as well as a variety of other rare minerals and gemstones (such as axinite, dravite, fluorapatite, prehnite and tremolite). The δ18O values of tanzanite, tremolite, tsavorite and kyanite from the Merelani area are all high (average of 19.1, 19.5, 18.5 and 18.7‰, respectively) and are consistent with high-T O-isotope equilibrium, and the minimal involvement of externally derived fluids. The whole-rock δ18O values of graphite-bearing gneisses that host the tanzanite range from 16.8 to 21.6‰, whereas the marbles have somewhat higher values (24.0 and 25.0‰). Graphite-free garnet-bearing gneisses from the Telele Hills, ~33 km south of the mining area, have much lower δ18O values (~7 to 10‰). The high loss on ignition (LOI) values, ~8 weight % and ~10 weight %, of the graphite-bearing gneisses from the mining area do not correlate with CaO or MgO content, which is consistent with the presence of up to 10% graphite with minimal carbonate being present. The average δ13C value of the graphite from the garnet-kyanite gneiss in the mining area is −23.8‰, whereas most of the associated carbonate rocks have δ13C values close to zero. This lack of C-isotope equilibration between the adjacent carbonate and silicate rocks, together with the lack of change in bulk-rock δ18O values at the contact between carbonate and silicate units suggests that the high δ18O value of the graphite-bearing gneiss reflects that of its protolith. The average δD value of the tanzanite and tremolite is −77 and −27‰, respectively, which is consistent with their formation at in equilibrium with fluids having δD values of −36 and −11‰, respectively, assuming a temperature of formation of ~630°C. High δ18O values calculated for the fluids in equilibrium with tanzanite are consistent with a strongly rock-buffered system and hence a ‘metamorphic origin’. Calculated fluid δD values are close to zero, and suggest that the fluid is formation water, ultimately of marine or meteoric origin. The negative δ13C of the graphite and the high δ18O values of the rocks and minerals are consistent with previous suggestions that the protolith of the graphite-bearing gneisses was black shale. The major and trace element compositions of the rocks and minerals are consistent with this hypothesis