CHARACTERISTICS OF HYDROTHERMAL INOSILICATE VEINS IN THE BAYAN OBO REE DISTRICT, INNER MONGOLIA, CHINA – A POTENTIAL EXPLORATION TOOL

Hydrothermal inosilicate veins 2-50 mm thick cut the H6 feldspathic sandstone and H7 quartzite 300 – 2,000 meters north of the Bayan Obo Main Ore Body. The veins are symmetrically-banded, rhythmic, and consist of alternating layers of 100-1000 µm richterite>aegirine and aegirine>richterite, each with minor interstitial albite and quartz. Richterite needles grew perpendicular to vein boundaries. Aegirine occurs as blocky intergrowths with richterite. Accessory minerals include apatite, monazite, barite, rutile, ilmenite, magnetite, and zircon. Ilmenite commonly rims rutile cores and contains significant manganese and niobium. Niobium also occurs in baotite (Ba₄Ti₄(Ti,Nb,Fe)₄(Si₄O₁₂)O₁₆Cl with X_Nb=5-10%, X_Ti=90-95%). Rare earth elements occur as disseminated 20-50 µm monazite grains. Apatite contains minor strontium and fluorine. Individual richterite crystals in any band have a magnesian core (X_Mg=0.50) overgrown by an Fe-rich rim (X_Mg=0.15) which suggests rhythmic bands formed from refreshing pulses of hydrothermal fluid of similar composition, possibly from the same source. The abundance and thickness of inosilicate veins, and the number of rhythmic bands within individual veins increased with proximity to the Main Ore Body, suggesting mapping of inosilicate vein characteristics is a potentially useful exploration tool in this type of deposit.