Petrology and lithogeochemistry of the mineralized tanzanite-grossular bearings rocks in the Merelani-Lelatema area, northeastern Tanzania

The Merelani-Lelatema area, which is occupied by late Proterozoic metasedimentary rocks, lies along the Lelatema fault zone. The rocks are mainly pelitic and semi-pelitic gneisses, psammitic gneisses, hydrothermally altered rocks, crystalline dolostones, pegmatites and quartz veins. The occurrence of gem-quality tanzanite and grossular has been found to occur mainly in boudinaged pegmatitic veins and hydrothermal fracture fillings. These fracture fillings occur in brecciated and hydrothermally altered graphite-bearing gneiss in a mineral association containing glass-clear quartz, diopside, zoisite, graphite and calcite. The psammitic gneisses are more siliceous and enriched in feldspars. They are grouped into graphite-kyanite gneiss with thin interlayers of quartz-feldspar fels. The pelitic and semi-pelitic gneisses are aluminium-rich and thus have been grouped into garnet-biotite-sillimanite and/or kyanite graphite gneisses. The hydrothermally altered rocks are strongly deformed and contain varying amounts of clay minerals, which characteristically show distinct banded layers due to alterations of minerals. The rocks have been folded and the fold structures have undergone three deformation episodes (D1, D2 and D3) D1 and D2 were accompanied by high-grade metamorphism whereas D3 was associated with retrograde metamorphism and the formation of retrograde shear zones. Evidence from metamorphosed graded bedding confirms the existence of the antiform and also shows that the macroscopic F2 folds are superimposed on a previously inverted stratigraphic succession. This inversion is the consequence of D1 deformation. The polyphase nature of these rocks with which pegmatites and quartz veins occur is shown by rotation of boudins, presence of overturned tight isoclinal folds resembling F1, presence of F2 folds and refolded folds shown by quartz veins. Features like boudins shown by quartz veins and pegmatites at the limbs and along the axial planes of F1 folds with S1 development in the host rocks and disruption of limbs of tight folds resembling F1, shown by these veins also point to significant amount of flattening during F1 folding phase and longitudinal stretching of the late stage of their development. Slippage in the later stages of deformation phase (D1) is also distinct in displaced veins. Lithogeochemical studies reveal that the gneissses originated from miogeosynclinal sediments composed of sand; organic matter rich in V, U and fine Al-rich materials. These underwent advanced weathering, deposited in a lake and during metamorphism and after the Pan-African tectonothermal event, the silicates were altered, trace elements released and migrated with hydrothermal solutions rich in Ca, Mg, CO2 and SO3. Finally these elements together with V, U, Sr, Zn and heavy REE elements were deposited in the hydrothermal zone. Emplacement of gemstones (tanzanite and green grossular) in the area is structurally controlled along the Lelatema fault system. Tanz. J. Sci. Vol.29(2) 2003: 55-7