Composition, fractionation trend and zoning accretion of the columbite-tantalite group of minerals in the Kenticha rare-metal field (Adola, southern Ethiopia)

Abstract

Columbite-tantalite ore mineralogy from the Kenticha rare-metal granites and pegmatites, Adola (Southern Ethiopia), has been studied in detail and the results can be used as an indicator of the metallogenic evolution. 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 diagram 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 progressively changes (fractionates) from Nb- and Fe-rich to Ta- and Mn-rich members depending on the stage of differentiation of the granite pegmatites. Highly fractionated pegmatites are characterized by columbite-tantalite 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 fractionation with a more or less constant Fe/Mn ratio. Electron microprobe analyses carried out on some of the columbite-tantalite crystals show 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 result of periodical changes of the major element contents due to the activity of successive generations of compositionally different ore mineral forming fluids also rich in gaseous materials. The development of oscillatory zoning is consistent with the periodic variation of Ta, Nb, Mn and Fe contents most probably resulting from the repetitive instability produced by the crystal growth process, the concentration and diffusion 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 zoning and other replacement textures of successive columbite-tantalite generations are formed by resorption and replacement by late-coming gaseous-rich chemical fluids. The later generation is relatively rich in Ta and Mn and corrosive in its nature.