Spatial distribution and chemical characteristics of bastnäsite and monazite provide insights into the Bayan Obo deposit, the world's largest rare earth element mineralization
Jin Wang , Lin Li , M. Santosh , Guo-Ying Yan , Jun-Feng Shen , Mao-Wen Yuan , Masroor Alam , Zhao-Jing Wang , Sheng-Rong Li
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引用次数: 0
Abstract
Bastnäsite and monazite are the main carriers of thorium (Th) and rare earth elements (REEs) in Bayan Obo, the world's largest REE deposit. Understanding the spatial distribution of their chemical compositions is crucial for deciphering mineralization processes and improving resource utilization. Meanwhile, the precise mechanisms by which Th enters the lattice of these minerals remain poorly understood. Petrographic studies reveal that bastnäsite and monazite predominantly occur as fine-grained (<50 μm) aggregates, classified into three distinct paragenetic types: associated only with dolomite (Bsn/Mnz-I), associated with fluorite (Bsn/Mnz-II), and associated with alkali silicate minerals (Bsn/Mnz-III). Th4+ enters the lattice of all types of bastnäsite through Th4++(Ca, Ba)2+ → 2ΣREE3+. In contrast, monazite exhibits distinct Th4+ incorporation mechanisms depending on its type. Specifically, Th4+ is incorporated in Mnz-I exclusively through cheralite substitution (Th4+ + Ca2+ → 2REE3+), while in Mnz-II and Mnz-III, it involves cheralite (Th4++Ca2+ → 2REE3+) and huttonite (Th4++Si4+ → REE3++P5+) substitutions. Our data also show that the chemical composition of bastnäsite and monazite within the Main Orebody is highly heterogeneous, exhibiting a tendency towards enrichment in La2O3 and Pr2O3 and depletion in ThO2 and Nd2O3 from the deep eastern part to the shallow western part of the Main Orebody. Based on the metallogenic temperature, mineralized materials, and REE migration characteristics, it is inferred that the ore-forming hydrothermal fluids originate from the eastern depths of the Main Orebody and subsequently migrate upwards towards the western domain.
期刊介绍:
Lithos publishes original research papers on the petrology, geochemistry and petrogenesis of igneous and metamorphic rocks. Papers on mineralogy/mineral physics related to petrology and petrogenetic problems are also welcomed.