Mineralogy and weathering of REE minerals in the Liuchen granite, Guangxi, southern China: Implications for HREE enrichment in the granite regolith

Regolith-hosted ion-adsorption REE deposits (IARDs) in southern China were derived predominantly from the weathering of granitoids. However, the role of primary REE minerals in the formation of a REE-rich regolith during weathering of the granites has not been fully understood. To investigate the formation process of IARDs, fresh granite samples and regolith samples from different horizons (i.e., fully-, highly-, and semi-weathered horizons) in the Liuchen (LC) super-large IARD of Guangxi Province, southern China were collected and studied the compositions of the REE minerals. Allanite-(Ce), monazite, apatite, and zircon are major primary LREE-bearing minerals in the LC granite, and xenotime-(Y) is the major host of HREEs, including Gd, Dy, Er, Yb and Y (up to 38.68 wt%). Altered anhedral xenotimes are observed recrystallized as a result of the breakdown of apatite. These xenotimes have elevated contents of HREEs compared with their precursors. The sizes of these altered xenotimes are significantly smaller (mostly 0.5–5 μm) compared to the primary xenotimes (>50 μm). The increased surface-to-volume ratio for smaller particle sizes of these altered xenotimes make them more susceptible to weathering during supergene alteration. Except for zircon, the volume of primary REE minerals is reduced in zones of intense weathering (fully- and highly-weathered horizons) in the granite regolith, but relics of xenotime which covered by biofilms on their surfaces were observed in the incipient zones of weathering (e.g., in the semi-weathered horizon). Dissolution that begins at their surfaces from nano-scale was observed in these mineral relics. Microbial solubilization of this previously thought insoluble REE phosphate is likely to control the mobilization REE content, especially Y, in the LC granite regolith. Secondary cerianite-(Ce) and LREE- and HREE-bearing phosphates, mainly rhabdophane and florencite, were identified in the regolith, and this is evidence that HREE minerals such as xenotime-(Y) that were undergoing bio-weathering were an important source of materials for the formation of REE-bearing secondary minerals. This may provide new insights into the formation of so-called "ion-adsorption" REE deposits.