THE ASSOCIATION OF COLUMBITE, TANTALITE AND TAPIOLITE IN THE SUZHOU GRANITE, CHINA

We present the results of an investigation of the chemical composition of Nb-Ta oxide minerals from the Suzhou granitic complex, China. This complex is composed of three map-units, granite II being the most imp ortant. In the upper part of granite II, three well-evolved facies are distinguished; they show signs of disseminated mineralization in rare metals (Nb, Ta, Zr, Hf, REE, Th). From the top down, these are (1) th e albite-dominant facies, (2) the topaz-bearing facies, and (3) the bi otite-bearing facies. Optical microscopy studies and electron-micropro be analyses have shown that ferrocolumbite occurs in the biotite-beari ng facies, and ferrocolumbite to manganocolumbite is found in the topa z-bearing facies. Ferrotantalite and tapiolite are concentrated in the albite-dominant facies, these two latter minerals being identified fo r the first time in the Suzhou granite. The tapiolite has an unusual c omposition, beyond the known field of stability of tapiolite, suggesti ve of disequilibrium crystallization. The chemical evolution of the Nb -Ta minerals from the biotite-bearing granite to the albite-dominant f acies reveals an important fractionation of Nb and Ta, accompanied by a more moderate evolution of the Fe:Mn ratio. Tapiolite in the albite- dominant granite indicates an increase in the activity of Fe at the en d stage of differentiation of the residual magma. By comparison with t he Beauvoir granite, in France, it is likely that this pattern of Fe a nd Mn fractionation is due to the reduced activity of fluorine.