Mineralogical fingerprints of crustal silica contamination in the Bayan Obo carbonatite

Abstract

Carbonatites are carbonate-rich and silica-undersaturated igneous rocks. The presence of various silicates in carbonatites has sparked discussions about the source of silica. In this study, abundant fluorbritholite-(Ce) and humite group minerals are identified in the No. 1 carbonatite dike at the Bayan Obo REE-Nb-Fe deposit. These silicates are relatively rare and poorly understood in carbonatite systems. Mineral textures, in-situ EPMA and LA-ICP-MS analyses have been combined to constrain the mineral genesis in the carbonatite. Fluorbritholite-(Ce), member of the apatite super-group, occurs as euhedral to subhedral crystals in the dike. They are characterized by remarkably high concentrations of REE2O3 (56.0-63.7 wt.%), SiO2 (19.6-21.2 wt.%) and F (2.47-3.47 wt.%), along with relatively lower P2O5 (0.25-3.69 wt.%) and CaO (10.3-14.2 wt.%) contents compared to common fluorapatite species. Additionally, their high Y (961-3435 ppm) and low Sr/Y (0.59-2.70) values suggest a hydrothermal origin from a fluid rich in SiO2, REE and F. Humite group minerals, mainly chondrodite and humite, display irregular mineral textures. They also exhibit elevated SiO2 (32.5-34.7 wt.%), and F content (3.59-7.32 wt.%), with notably low TiO2 content (0.02-0.08 wt.%), indicating a hydrothermal origin induced by fenitization in the shallow crust. Our results favor a model of crustal silica contamination for the fenitization fluids, enriched in F, LREE and SiO2. More importantly, the fluid-assisted silica contamination from wall rocks within carbonatites is likely to be a critical trigger of REE deposition in the carbonatite ore-forming systems.