Geochemical and mineralogical characteristics of six ore types of Main and Eastern orebodies in the Bayan Obo deposit: A combined PMF and SEM-EDS analysis

Abstract

The Bayan Obo deposit in northern China is the world’s largest source of REE, with substantial Fe and Nb resources. Although different ore types in the deposit have long been recognized, their high mineralogical diversity has hindered systematic evaluation of elemental associations and quantitative characterization of REE geochemistry. In this study, we integrate Positive Matrix Factorization (PMF) of multi-element geochemical datasets with Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) mineral identification to quantify elemental assemblages and mineralogical characteristics, and further analyze the REE patterns across ores representing these known types. Six distinct elemental association groups are identified, dominated respectively by: dolomite-type (MgO-CaO-Mn-Co-Sr-Zn), amphibole-type (SiO₂-MgO-K₂O-Na₂O-Th-Zn-Co-V), aegirine-REE type (Na₂O-SiO₂-TFe₂O₃-FeO-REE-Pb-Ti), iron oxide-type (TFe₂O₃-FeO-Co-Mn-Th), fluorite-REE type (CaO-F-Nb-P-REE), and barite-REE type ores (Ba-S-Ti-Sr-Pb-REE-Nb-Th-U). Significant variability in REE contents and fractionation patterns was observed across ore types. Barite-REE, fluorite-REE, and aegirine-REE type ores exhibit the highest total REE concentrations (9.54 wt%, 7.74 wt%, and 5.94 wt%, respectively), coupled with pronounced LREE enrichment, as indicated by elevated (La/Yb) _CN ratios of 4964, 1598, and 1572. In contrast, dolomite-type and iron oxide-type ores exhibit moderate REE enrichment (2.45 wt% and 2.34 wt%, respectively), while amphibole-type ores contain the lowest REE contents (0.663 wt%), but display significantly higher proportions of Nd and MREE. Negative Eu anomalies (average δEu = 0.621–0.798) observed in nearly all ore types suggest that extensive hydrothermal alteration has significantly influenced REE distribution. These results establish a geochemical framework for identifying elemental association patterns and refining the understanding of REE distribution in the Bayan Obo deposit.