Hydrothermal alteration of pyrochlore from carbonatite: An experimental study

Niobium is a critical metal used in various superconducting materials and alloys. It is predominantly mined from ores rich in pyrochlore-group minerals with the general formula A2B2O6Y (B ≥ 50% Nb). Fluorcalciopyrochlore, (Ca,Na)2Nb2O6F, is considered as primary magmatic pyrochlore. However, metasomatism by hydrothermal fluids, amongst other processes, can lead to a wide range of chemical compositions (Mitchell 2015). Different stages of pyrochlore alteration were assigned to specific p-T-ranges using associated minerals in the host rock ores. Enrichment in Ba for example is documented in low-temperature hydrothermal environments (Lumpkin & Ewing 1995).

However, it is difficult to unravel the role of temperature, pressure, fluid composition or time in alteration processes of primary fluorcalciopyrochlore in natural carbonatite systems. Therefore, alteration experiments using an autoclave are necessary to identify and quantify alteration mechanisms. To our knowledge, such experiments have only been conducted for microlite and betafite (Geisler et al. 2005), but not for pyrochlore sensu stricto.

Hence, we studied the interaction of natural fluorcalciopyrochlore with Cl-rich, acidic hydrothermal fluids of varying composition in teflon-lined hydrothermal reactor vessels at 200 °C. Here we will present preliminary results of our experiments and discuss consequences of hydrothermal alteration of pyrochlores, with application to Nb-rich carbonatite ores.

R. H. Mitchell, Ore Geology Reviews, 64, 2015; G. R. Lumpkin & R. C. Ewing, American Mineralogist, 80, 1995; T. Geisler et al., American Mineralogist, 90, 2005