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

摘要

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.