The mineral chemistry of gahnite, garnet and columbite-group minerals (CGM): Implications for genesis and evolution of the Kenticha Rare-element granite-pegmatite, Adola, Ethiopia

Abstract

Fracture-controlled barren to rare-element pegmatites were emplaced into the Adola belt of the Southern Ethiopian Precambrian crystalline basement during Neoproterozoic (530 Ma). The Adola belt occupies an important transitional position between the juvenile Neoproterozoic crust of the Arabian Nubian Shield and high-grade metamorphic Pan-African Mozambique belt. The Adola belt is made up of narrow N–S trending synclinal greenstone belts and the granite-gneiss tectonostratigraphic terranes. These greenstone belts are renowned for the occurrence of abundant mineral resources including rare-metals, precious metal, gemstones and industrial minerals. The Kenticha rare-element granite-pegmatite is the largest repository of rare-elements (Ta, Nb, Li, Be), known to date in the Horn of Africa. Ubiquitous accessory minerals occur in association with this pegmatite. The presence of garnet, gahnite, muscovite, tourmaline and aluminosilicates indicates the granite-pegmatite is typical of the peraluminous nature. The present study involves the mineral chemistry of gahnite, garnet and columbite-tantalite phases using electron probe micro-analyzer to constrain the genesis of rare-element granite-pegmatite and evolution of the pegmatite-forming melt. Accordingly, crystal chemistry of garnet, gahnite and tantalite-columbite are very much consistent. The Zn/Fe and (Fe+Mg)/Al ratios in gahnite range from 6.99 to 14.14 and 0.075–0.065, respectively. Garnet compositions vary only in narrow range within the pegmatite zones and also in the individual grains. The calculated structural formula is Sps53-64Alm33-46Prp0-3Grs0-2Adr0-2, showing typical of the spessartine-almandine variety. The characteristic normal zoning and high mol (Zn+Mn)/Al and low mol (Fe+Mg)/Al in gahnite, and the high MnO/(FeOt+MgO) ratios in garnet unequivocally indicate the pegmatite is crystallized from a highly evolved residual melt, enriched with a fluxing component (Li, F, P, B), via fractional crystallization processes. The textural characteristics of garnet grains, nature of zoning, high spessartine content and lack of reaction rim indicate that the granite-pegmatite was crystallized at low-pressure from a Li–H2O–F rich melt. The Fe/Mn ratio both in garnet and columbite-tantalite decreases from the wall to the core zone of the pegmatite. The positive correlation between SiO2 and MnO and the high MnO/(FeOt+MgO) ratios in garnet suggest crystallization of the granite-pegmatite from highly evolved MnO-rich melt. A systematic fractionation trend is also recognized from the quadrilateral compositional plot of tantalite-columbite minerals. It shows ferrocolumbite - manganocolumbite - manganotantalite evolutional trend from lower to the upper unit. The Mn/(Mn+Fe) ratio in columbite group of mineral progressively increases from the lower to the upper unit of the pegmatite. In general, from textural and compositional data, it could be interpreted that the columbite-tantalite mineral and other rare-elements were developed from magmatic source, however, later on, overprinted by hydrothermal replacements. The compositions of all the accessory phases consistently suggest the rare-element granite-pegmatite is crystallized from a magma rich in SiO2 and Li–F–H2O fluids derived at low pressure and shows a progressive evolutional trend from wall to the core zone of the pegmatite.