Origin of Tanzanite and Associated Gemstone Mineralization at Merelani, Tanzania

Research Article| June 01, 2014 ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA C. HARRIS; C. HARRIS Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa, e-mail: chris.harris@uct.ac.za Search for other works by this author on: GSW Google Scholar W. HLONGWANE; W. HLONGWANE Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa, e-mail: wandisa.hlongwane@riotinto.com Search for other works by this author on: GSW Google Scholar N. GULE; N. GULE Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa, e-mail: glxnonto@yahoo.com Search for other works by this author on: GSW Google Scholar R. SCHEEPERS R. SCHEEPERS Department of Geology, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South Africa. e-mail: mapsltd1@gmail.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2014) 117 (1): 15–30. https://doi.org/10.2113/gssajg.117.1.15 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. HARRIS, W. HLONGWANE, N. GULE, R. SCHEEPERS; ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA. South African Journal of Geology 2014;; 117 (1): 15–30. doi: https://doi.org/10.2113/gssajg.117.1.15 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search 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 You do not have access to this content, please speak to your institutional administrator if you feel you should have access.