Formation of columbite and microlite after alteration of Nb- and Ta-bearing biotite from the Lizaizhai pegmatite (Guangning ore district, Guangdong, South China): Identification of a new potential Nb–Ta mineralization type

Abstract

The Lizaizhai pegmatite is located in Guangning, Guangdong Province, South China. Unlike common rare-element (Li–Sn–Nb–Ta) pegmatites in Guangning, which contain muscovite as the dominant mica, the Lizaizhai pegmatite represents a different type of rare-element mineralization with biotite as the dominant mica. Our detailed study shows that the Lizaizhai pegmatite can be divided into a border zone and a core zone. Biotite in the border zone contains lower contents of Ta (mean 121 ppm) and higher contents of Ti (mean TiO2 = 0.31 wt%) and Nb (mean 409 ppm) compared with biotite in the core zone, which contains slightly more Ta (mean 381 ppm) than Nb (mean 375 ppm) and Ti (mean TiO2 < 0.1 wt%). The primary Nb–Ta oxide minerals consist of Ta–Nb-rich rutile, columbite-group minerals (CGMs), tapiolite, and microlite, which are found as inclusions in quartz and feldspar. Ta–Nb-rich rutile and tapiolite are present only in the core zone of pegmatite. Biotite in the pegmatite dike has been intensely altered by acidic and F-rich hydrothermal fluids, mostly by chloritization. With H+ and F− being consumed continuously with alteration, Nb and Ta complex anions became unstable. By combining with other ions (e.g. Ca, Fe and Ti), alteration-stage Nb–Ta-bearing minerals (titanite, ilmenite, and rutile) and Nb–Ta oxide minerals (CGMs and microlite) were able to be precipitated in chlorite and alteration-formed muscovite. The border and core zones can be distinguished by distinct assemblages of alteration products: Alteration-stage minerals such as titanite, ilmenite, rutile, and Nb–Ta oxides are present in the border zone, whereas only Nb–Ta oxide minerals are found in the core zone. Considering its distinct petrology, mineralogy, and texture, we propose that the Lizaizhai pegmatite represents a new potential type of Nb–Ta mineralization from biotite alteration.