Evolution of pyrochloregroup mineral composition in carbonatites of the Belaya Zima massif (Eastern Sayan)
Khromova E. A., Doroshkevich A. G., Sharygin V. V., Izbrodin L. A.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 1. P. 84-102
Pyrochlore-group minerals are the main concentrators of niobium in carbonatite of the Belaya Zima alkaline massif. There were identified among them the following species, by chemical composition: fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore. Their principal characteristics: variations in composition, morphology, zoning, are given in the paper. In general, chemical evolution of these minerals, from early calcite to the late ankerite carbonatites, is directed towards gradual increase of UO2, TiO2, REE and Y concentrations. It is suggested that all types of carbonatites initially contained fluorcalciopyrochlore. However, in calcite-dolomite and ankerite carbonatites it was partially or completely hydrated due to metasomatic/hydrothermal processes at late stages of the massif forma tion. This hydration led to appearance of kenopyrochlore and hydropyrochlore due to loss of Ca, Na and F, and inflow of Ba, H2O, K, Si, Fe, and probably U and REE. At latest stages of the massif formation, this hydrated pyrochlore was replaced by Fe-bearing columbite.
Key words: pyrochlore group, fluorcalciopyrochlore, kenopyrochlore, hydropyrochlore, carbonatites, Belaya Zima massif.