Trace elements characterization of the hydrothermally deposited tanzanite and green grossular in the Merelani – Lelatema shear zone, northeastern Tanzania

Abstract

Geochemical investigations of trace element contents of different rock samples were determined in the late Proterozoic metamorphic sequences of the Mozambique Belt along the Merelani–Lelatema shear zone, northeastern Tanzania. The area is composed of the following rock types: pelitic and semi-pelitic gneisses, psammitic, hydrothermally altered including carbonate-rich, carbonate-gypsum-rich, and iron-rich rocks, pegmatite dykes and crystalline limestones. Trace element characteristics of the principal rock units in and outside the shear zone have been compiled and studied. With the exception of higher barium contents in pelites, trace element contents in the psammitic and pelitic gneisses show some similarities while in the hydrothermally altered rocks where gemstones (tanzanite and green grossular) are localized reveal that the following trace elements: Ba, Cu, Mo, Ni, Rb, Sr, U, V and Zn have been enriched, while As, Ag, Co, Cr, Cs, Pb, Th and Zr were relatively depleted compared to the other rock types. Calculated CI/ Br ratios are at maximum of 900, which are the same as found in waters of deep drill holes and in salt waters associated with oil deposits. Calculated Co/Ni ratios are low, less than unity in most rocks except the psammitic gneisses and carbonate-rich rocks whose values are approximately 3.4 and 6.5 respectively. The low Co and Ni values in these rocks may be a result of low a_co2+ and a_ni2+ in the hydrothermal fluids and / or lower temperature of crystallization. The high V contents result in an unusual degree of V substitution in a number of silicate and oxide phases. The high levels of V in the metasediments, together with that of Ni and Sr, is reflected in their enhanced concentration in the hydrothermally altered zone indicating that they may have been partly re-distributed by the hydrothermal fluids. Vanadium contents in the hydrothermally altered rocks are strongly enriched especially in the calcium-magnesium-aluminium-rich rocks but low in carbonate-sulphate-rich rocks. It is most likely that at the stage when tanzanite and green grossular were crystallized Al, Ca, Mg, Si, and V were migrating together and were incorporated into the minerals precipitated. Vanadium (V³⁺) substitutes for iron (Fe³⁺) and aluminium (Al³⁺) diadochically in the crystal lattices of silicates mainly zoisite and green grossular.


The data suggest that the gemstone deposits are the result of a sequence of long-continued and overlapping geological processes culminating in intense shearing, considerable mobilization of trace elements and syn-tectonic hydrothermal intrusion.The Proterozoic metasedimentary rocks of the shear zone, as well as those enclosing it, served as the source of trace elements, while diffusion of the trace element constituents and their precipitation in physico-chemically favorable traps, formed in response to folding and shearing, resulted in the gemstone deposition.


Tanz. J. Sci. Vol.29(1) 2003: 45-60