Petrogenesis of H9 litho-unit and deposit model for Bayan Obo giant Fe-Nb-REE deposit, Inner Mongolia, China

The Bayan Obo giant Fe-Nb-REE deposit is recognized as the world’s largest rare earth elements (REE) deposit. The Precambrian outcrop in the area includes Archean gneiss and amphibolite, and Mesoproterozoic Bayan Obo group which has been divided into H1-H9 litho-units. The H8 is now widely accepted as ore-hosting carbonatite, the other litho-units (H1-H7 and H9 litho-units) have been referred to as meta-sedimentary and meta-volcanic rocks. Both intrusive and extrusive origins have been proposed for the ore-hosting carbonatite (H8), the petrogenesis of the other units in Bayan Obo group is also controversial. The different genetic understandings to the ore-hosting carbonatites and their wall rocks have led to diverse deposit models. Through integrated geological profile investigation, combined with micrographic observations, SEM/EDS, TIMA analyses, zircon U-Pb and monazite Th-Pb geochronology, this study investigates the influences of deformations and alterations to the lithologies and the petrogenesis of the wall rock of carbonatite. A schematic model for the ore deposit genesis is proposed, showing the two overprinting stages of REE-Nb mineralization, and the alteration zonation. Our findings reveal that the H9 litho-unit, together with H1-H7 litho-units, may constitute a Paleoproterozoic felsic intrusive complex (zircon U-Pb age ~1.95 Ga). This complex is intruded by variable scale carbonatite-biotitite and biotite syenite dikes (zircon U-Pb ages ~1.3 Ga), resulting in pervasive aegirine-augite, biotite, albite, and graphite alteration. The H9 litho-unit, the direct wall rock of carbonatite, displays intensive biotite, albite and graphite alteration in an alteration halo to the carbonatite. Both the ore-hosting carbonatite and its wall rocks exhibit varying degrees of ductile shearing. The intensely altered H9 litho-unit shows stronger ductile shear deformation than the other litho-units leading to their foliated texture. Post-shearing hydrothermal alterations, characterized by arfvedsonite-pyrite mineralization, occur as veins or disseminated along NE-trending faults/fractures and in shear cleavages. Post-shearing hydrothermal veins crosscut both the ore-hosting carbonatite and shear fabrics. In addition to arfvedsonite and pyrite, veins include bastnaesite, monazite, columbite, aeschynite, Sr-Ba-rich carbonates (e.g., Sr-bearing calcite, strontianite, norsethite, barytocalcite), and sulfate minerals (e.g., barite, celestine), indicating REE-Nb-Sr-Ba-SO₄^2--rich hydrothermal fluids. Vein monazite Th-Pb ages (~417 Ma) reveal Paleozoic REE-Nb mineralization. Our results indicate that besides the Mesoproterozoic carbonatite-related REE-Fe-Nb mineralization, Early Paleozoic hydrothermal overprinting significantly contributed to the formation of this giant deposit. Key exploration targets, particularly for high-grade REE-Nb ores, occur at intersections between NE-trending faults and H8/H9 litho-units. In addition to fenitization, biotite and graphite alteration may be significant types of carbonatite-related alteration and may serve as critical indicators for carbonatite-related REE-Nb mineralization.