Critical rare metal deposits are strategic resources as these metals are significant for high-tech industries. Among the critical rare metals, stannum (Sn) in nature is mostly found in Sn-oxide mineral, cassiterite (SnO2), and closely associated with granite or pegmatite. Carbonatite and alkaline rocks are more likely to contain huge amounts of critical rare metals, especially REEs and niobium (Nb). We reported abundant cassiterite (SnO2) and evaluated potential Sn mineralization in the Bayan Obo Fe-Nb-REE deposit in northern China, the largest REE deposit worldwide. In this paper, evidence for the Sn enrichment in a carbonatite-hosted REE deposit is given for the first time.

REE-Fe ores are dominantly mined in the Bayan Obo deposit. Disseminated, banded and massive ores contain tens to hundreds ppm Sn and vein-type ores are notably rich in Sn (up to 1500 ppm). Through in situ micro-zonation mineralogical analyses, two occurrences of cassiterite and several Sn-rich minerals are identified in these REE-Fe ores. Abundant early-stage nanoscale cassiterite inclusions are present within magnetite grains in banded and massive REE-Fe ores, and ubiquitous late-stage granular cassiterite, Sn-rich rutile, titanite, and bafertisite are present in vein-type REE-Fe ores. Multiple U-Th-Pb dating of monazite and columbite-Mn in association with cassiterite yields peak ages of 425 Ma and 419±18 Ma, respectively, revealing coeval Sn and Nb mineralization. We concluded that Sn was derived from carbonatitic magmas, and the dense distribution of cassiterite inclusions in magnetite marked the pre-enrichment of Sn in the Bayan Obo deposit. Subsequent Early Paleozoic hydrothermal events led to the reactivation and further Sn mineralization. Similar to Nb, Sn was mineralized in the Bayan Obo deposit probably to form economically important resources. Our study highlights the potential of Sn mineralization associated with carbonatite-hosted REE deposits.

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