COMPARISON OF CARBONATE VEIN/DIKE WITH EAST ORE BODY CARBONATE, BAYAN OBO REE MINING DISTRICT, INNER MONGOLIA, CHINA

We compared the mineralogy and geochemistry of samples of two 20-70 cm wide discordant, planar, NE-striking bodies of carbonate rock cutting H7 quartzite southeast of the Bayan Obo East Ore Body with carbonate rock samples from the East Ore Body to determine if they are carbonatite dikes or hydrothermal veins. The discordant bodies consist of 1-2 mm calcite intergrown with richterite amphibole. The discordant bodies widths vary along strike, locally pinching to 20 cm. Richterite concentrations are highest (50%) and needles randomly oriented within the 5-cm-wide, fine-grained (<3 mm) margins of the discordant bodies, and sparser (10-15%), coarser (3-10 mm), and slightly aligned near the center.

Mineralogically, the discordant bodies consist of strontian calcite (0.7-2.1 mole% Sr and 1.0-5.0 mole% Mg) with subordinate ferroan richterite (Fe:Mg ratio 0.2-0.3) and trace amounts of aegirine, barite, apatite, scheelite, wollastonite. REE in the discordant bodies occur mainly in apatite and lesser carbocernaite and synchesite. REE rich apatites occur as overgrowth pure hydroxyapatites or as discrete grains. La and Ce in REE phases are subequal, and Nd is less than 15 mole% of REE. Fluorine-bearing phases are absent in the discordant body samples. If the discordant body is a sövite carbonatite dike, then it shares mineralogical similarities with the Afrikanda carbonatite (Kola Peninsula).

In contrast, carbonate samples from the northern contact of the East Ore Body consisted of Fe-Mn-bearing dolomite with lesser amounts of aegirine, barite, apatite, fluorite, and pyrite. REE occurred as bastnäsite, synchysite, and parisite, and only rarely in REE-apatite. East Ore contact dolomite lacked Sr. La:Nd:Ce ratios in REE phases are similar between ore and discordant body samples. East Ore Body carbonate is distinct in mineralogy and F-content from the discordant carbonate bodies.

Calcite and dolomite in both samples are geochemically similar to published compositions of the Bayan Obo Group H8 Unit. We find no unequivocal evidence that distinguishes between a magmatic sövite dike vs. hydrothermal vein origin of the discordant carbonate bodies in this study.