Discovery of the REE minerals in the Wulong–Nanchuan bauxite deposits, Chongqing, China: Insights on conditions of formation and processes

Abstract

The karstic bauxite deposits in the south Chongqing–north Guizhou Province bauxite belt are enriched in rare earth elements (REEs). REEs in this belt have been considered to be adsorbed on the surface of clay or diaspore minerals in the ion state. However, extracting tests have demonstrated that the amount of REE adsorption in bauxite ore is very low. In order to comprehensively understand the occurrence of REE in bauxite ores, the chemical and mineralogical compositions of Wulong–Nanchuan karst bauxites and their textural components have been studied. The bulk samples are composed of diaspore, boehmite, kaolinite, illite, hematite, anatase, and pyrite. The ∑REE concentrations vary widely, from 17 to 3610ppm REE, in the bauxite ores and most of the ores show an obvious positive Ce anomaly.Many kinds of REE minerals were identified in these bauxite ores including synchysite, bastnaesite, cerianite, and churchite. These minerals seldom occur in the same sample (with the exception of synchysite and bastnaesite). Synchysite occurs rarely as homogeneously single crystals, but typically contains a microfine phase of bastnaesite. Cerianite occurs mainly as framboidal clusters among chamosite or as fracture or micro-crack fillings. The existence of REE mineral phases is responsible for the high REE content and positive Ce anomaly in the ores. The compositions of the REE minerals show that the Eu anomaly can change subtly during the weathering. The REE compositions of the bauxite ores suggest that the REE mainly derives from the weathered remnants of the underlying Hanjiadian Formation shales and probably, a minor fraction derives from the Huanglong carbonates.The formation processes of these REE minerals can be explained assuming: (1) Ce oxidation and cerianite precipitation in the uppermost part of the deposits; (2) the mobile phosphates combined with the Y-enriched sediments and the HREE-enriched solutions under an acidic environment allow the precipitation of churchite-(Y); and (3) remobilization of cerium as fluoride complex, as a consequence of more acidic conditions in the uppermost part of the deposit, and precipitation of Ce3+ as fluorocarbonate mineral toward the carbonate bedrock barrier, at alkaline pH.