GENESIS AND EVOLUTION OF KENTICHA RARE-ELEMENT GRANITE-PEGMATITE, SOUTHERN ETHIOPIA

Abstract

A series of fracture-controlled barren to rare-metal bearing pegmatites were intruded into the Neoproterozoic Adola Belt (530 Ma). World-class Nb-Ta deposit and significant reserve of Li and Be metals occur in association with these pegmatites. The Adola belt constitutes the southern Ethiopian crystalline rocks which occupy a transition position between the high-grade metamorphic Pan-African Mozambique belt and the juvenile Neoproterozoic crust of the Arabian-Nubian shield of the East African Orogen. 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 metals, gemstones and industrial minerals. In this study, a detail investigation of mineralogy, mineral chemistry, whole-rock geochemistry and microthermometric studies of fluid inclusions have been carried out to rigorously constrain the genesis and condition of crystallization for the Kenticha rare-metal granite-pegmatite. EPMA of minerals - apatite, beryl, gahnite, garnet, columbite group of minerals, feldspars, mica and spodumene; XRF and ICP-MS based whole-rock geochemical analysis of pegmatites and associated granites; and microthermometric and Raman spectroscopic studies of fluid inclusions have been carried out to achieve these objectives. Mineralogical and geochemical study indicates strongly peraluminous character for the Kenticha rare-metal granite-pegmatite and mildly peraluminous for the nearby two mica granite (Kilta Shanbali pluton). The strong peraluminous character is marked by the presence of several indicator minerals including garnet, gahnite, muscovite, tourmaline and aluminosilicates; high alumina saturation index (ASI >1.1) and % of corundum normative (C > 1). Beryl, garnet, and gahnite compositions indicate crystallization of the rare-metal pegmatite from a highly evolved residual melt, enriched with a fluxing component (Li, F, P, B) via fractional crystallization processes. Furthermore, the Zn/Fe and (Fe+Mg)/Al ratios in gahnite, the Mn/(Mn+Fe) ratio in columbite group of mineral and MnO/(FeOt+MgO) ratios in garnet suggest characteristic internal evolution. Compositions of apatite and mica indicate a peraluminous S-type magma source for the two-mica granite and associated pegmatites; however, the latter was crystallized under more of reduced condition. i From textural and compositional data of columbite-group minerals, it could be interpreted that enrichment of the Ta-Nb and other rare-elements were controlled largely by magmatic processes that were overprinted by later hydrothermal processes. Whole-rock geochemical data indicate a clear genetic relationship between the Kenticha pegmatites and the two-mica granite and characteristic regional fractionation. Mineralogical and geochemical studies show that occurrence of the Kenticha rare metal pegmatite was resulted from extended fractionation of S-type magma that developed possibly in the course of crustal melting and nappe stacking in the syn collisional setting. Microthermometric studies carried out on fluid inclusions in quartz and spodumene indicate that the earliest fluid was a single supercritical aqueous carbonic fluid of low salinity which was cooled along isochors from pressure of around 3.5-4.2 kbar. This fluid was separated into two immiscible fluids at a solvus temperature of around 350°C and a pressure of 1.4 kbar. Equilibrium temperature of the two-feldspar geothermometer and the trapping temperature determined based on microthermometric data are similar. Accordingly, a final crystallization temperature of around 400°C is constructed for the rare-metal granite-pegmatite. Such a low P-T condition of crystallization is attributed to the presence of volatile and fluxing components. These components have considerably depressed the solidus and liquidus temperatures, delayed crystal nucleation and enhanced the enrichment of rare-metals in the late pegmatitic fluid.