COMPOSITION, FRACTIONATION TREND AND ZONING ACCRETION OF THE COLUMBITE-TANTALITE GROUP OF MINERALS IN THE KENTICHA RARE-METAL FIELD (ADOLA,SOUTHERN ETHIOPIA)

Columbite-tantalite ore mineralogy from the Kenticha rare-metal granit es and pegmatites, Adola (Southern Ethiopia), has been studied in deta il and the results can be used as an indicator of the metallogenic evo lution. On the basis of the chemical data, the compositional fields of the columbite-tantalite collected from alaskitic granite, aplite and different pegmatite groups are discriminated using a quadrilateral dia gram of four end-members. The columbite-tantalite group of minerals in different pegmatite groups show compositional variability and this is directly dependent on the crystallization condition of an individual granite pegmatite group. The composition of these minerals progressive ly changes (fractionates) from Nb- and Fe-rich to Ta- and Mn-rich memb ers depending on the stage of differentiation of the granite pegmatite s. Highly fractionated pegmatites are characterized by columbite-tanta lite crystals relatively rich in Ta and Mn. In the early stage of the granite pegmatite formation, there is a higher fractionation of Fe/Mn with an almost constant Nb/Ta ratio, followed by an extreme Nb/Ta frac tionation with a more or less constant Fe/Mn ratio. Electron microprob e analyses carried out on some of the columbite-tantalite crystals sho w a wide compositional variation. Backscattered electron imagery (BEI) revealed oscillatory, patchy, mixed oscillatory and patchy zoning and other replacement textures. These textures are considered to be a res ult of periodical changes of the major element contents due to the act ivity of successive generations of compositionally different ore miner al forming fluids also rich in gaseous materials. The development of o scillatory zoning is consistent with the periodic variation of Ta, Nb, Mn and Fe contents most probably resulting from the repetitive instab ility produced by the crystal growth process, the concentration and di ffusion of the major elements, and the upward motion of gas-saturated magma. The oscillatory zoning texture mostly consists of a relatively homogenous core, zoned mantle and faceted border zone. The patchy zoni ng and other replacement textures of successive columbite-tantalite ge nerations are formed by resorption and replacement by late-coming gase ous-rich chemical fluids. The later generation is relatively rich in T a and Mn and corrosive in its nature. Copyright (C) 1997 Elsevier Scie nce Ltd.