POSSIBLE SYNGENETIC SEA-FLOOR EXHALATIVE REE ORES OF BAYAN OBO WEST ORE BODY, INNER MONGOLIA, CHINA

Mineralogical and textural evidence from the Bayan Obo West Ore Bodies suggest these peripheral deposits may have formed in a syngenetic sea-floor exhalative environment. Unlike the Bayan Obo Main and East Ore bodies that are silicate-rich, West Ore bodies are carbonate-rich with subordinate Fe-oxides >> sulfides. We discerned four types of ore: dolomite, siderite, magnetite-sulfide, and goethitic.

Dolomite-hosted ore contains disseminated monazite with fluorite, and accessory magnetite, barite, phlogopite, sphalerite, pyrochlore, and trace galena, huanghoite, and parisite. Early Mn-rich-dolomite grains (Dol₆₄Ank₀₅Kut₃₁) are rimmed by later-stage lower-Mn dolomite (Dol₇₈Ank₀₆Kut₁₅).

Siderite ore contains disseminations of richterite and F-phlogopite. Monazite, bastnaesite, synchysite, and parasite contribute REE to ore in late-stage dolomite veins with barite and magnetite. Most siderite is Mn-rich (Sid₇₁Mag₂₁Rho₈). A distinct stage of Fe-Mn-rich magnesite (Sid₃₈Mag₄₃Rho₁₉) overgrows siderite. Fe-Mn-rich dolomite (Dol₅₇Ank₂₅Kut₁₇) cuts siderite and decreases in Fe and Mn over time to Dol₈₅Ank₁₃Kut₀₃. Early siderite may have formed in a reduced synsedimentary or diagenetic environment. Later-stage hydrothermal fluids created Mg-Mn-rich siderite reaction rims on clasts and introduced REE minerals. The decrease in Fe and Mn contents in late-stage carbonates may either be due to the increasingly oxidized nature of fluids or decreasing salinity over time.

Magnetite ore consists of 70% magnetite with interstitial phlogopite, Fe-rhodochrosite, Mg-Mn-siderite, and traces of later-stage richterite, barite, and late pyrite. Monazite and bastnaesite occur within magnetite grains, but not in infilling carbonate.

Goethitic weathered ore consists of magnetite, rhodochrosite, phlogopite, calcite, fluorite, dolomite, and traces of parisite, monazite, and apatite, depending on the type of ore that was oxidized.

These ores may represent different facies of a sea-floor hot spring deposit in an active carbonate depositional environment and related to a carbonatite magma at depth. Siderite deposited by direct deposition as carbonate sediment or by diagenetic interaction of early hydrothermal fluids with unlithified carbonate sediment, then REE deposited from post-siderite fluids.