Distribution and mineralogical features of thorite in the Bayan Obo deposit: Implications for hydrothermal metasomatic Th re-enrichment

Abstract

Thorite, as a principally Th-bearing independent mineral, is an important indicator for Th mineralization. However, its genesis and significance for Th enrichment processes are still unclear. Herein, we address this issue based on systematic mineralogical observation and major element analyses of thorites with related monazite and bastnäsite from different type ores in the Bayan Obo deposit. The thorites are mainly hosted in biotite-type and riebeckite-type ores with minor distribution in aegirine-type and banded-type ores, but are scarce in dolomite-type ores. The thorites in each type ores principally contain Th, O and Si with variable LREE and Fe concentrations. They commonly occur as vein (or veinlet) or aggregation crosscutting, cavity-infilling or around REE, Fe and Nb minerals, and some of these minerals exist as relict or inclusion within the thorites. In addition, late-stage huanghoite and pyrite are intergrown with the thorites or appear as inclusions within the thorites. These petrographic characteristics suggest that thorite formation was posterior to REE, Fe and Nb mineralization and was related to Paleozoic hydrothermal mineralization events. Barite and alkali alteration minerals (e.g., biotite and riebeckite) show a closely spatial relationship with the thorites, which indicates that hydrothermal fluids for thorite formation were enriched in alkalis (K+ and Na+) and sulfate (SO42-). Major element analyses of monazite and bastnäsite exhibit variable ThO2 concentration (unaltered region: 7.13 ∼ 14.34 wt% for monazite, 0.64 ∼ 1.33 wt% for bastnäsite; altered region: 2.50 ∼ 5.41 wt% for monazite, 0.17 ∼ 0.53 wt% for bastnäsite), which indicates that later-formed monazite and bastnäsite underwent Th leaching by chemical-coupled substitutions including: (1) 2REE3+ = Th4+ + Ca2+; and (2) P5+ + REE3+ = Si4+ + Th4+. In combination with relevant published data, our study suggests that later-stage Th-rich monazite and bastnäsite in the Bayan Obo deposit experienced mineral dissolution and Th leaching by metasomatism of hydrothermal fluids, which induced Th re-enrichment in form of thorite precipitation. On the other hand, as intrinsically high Th content of the thorites, this study also evaluates grain size and Th abundance of the thorites within different type ores, and proposes that biotite-type and riebeckite-type are targeted type ores for thorite extraction and Th collection.