Paper No. 3
Presentation Time: 9:00 AM-6:30 PM


MASON, Kelsey1, ROCK, Marlena1 and FRIEHAUF, Kurt2, (1)Dept. Physical Sciences, Kutztown University, Dept. Physical Sciences, Kutztown University, Kutztown, PA 19530, (2)Dept. Physical Sciences, Kutztown University, Kutztown, PA 19530,

This study examines district-scale REE geochemical trends in the metamorphosed carbonate-hosted Bayan Obo REE district in Inner Mongolia (China) by comparing the mineralogy of samples from the East (EBO), West (WBO), and South (SBO) Bayan Obo ore bodies. Uncoated polished thin sections of samples with similar carbonate gangue material were examined using backscatter electron imaging (BSE) with an environmental scanning electron microscope (ESEM) in low vacuum, gas-phase mode. Automated feature detection based on high Z BSE intensities enabled quantitative analysis of a total of 6,126 REE mineral grains using energy dispersive spectrometry (EDS).

The SBO specimens were rich in REE-Fe-Nb oxides (fergusonite – 84%, aeschynite – 16%), and contained only trace monazite, bastnäsite and barite. The REE minerals made up approximately 0.01% of the samples. The carbonate and fluorite gangue occurred in subequal proportions, along with pyrite and disseminated clinopyroxene. The REE-Fe-Nb oxides occurred more commonly in fluorite-rich bands than carbonate bands. Pyrite occurred both as segregated bands and intergrown with the REE-Fe-Nb oxides. SBO samples had high Nb:Nd mole ratios (2.42, 2.13), lower Ce:Nd mole ratios (0.57, 0.81) and trace La.

Approximately 0.10% of the WBO samples consisted of REE minerals, dominated by bastnäsite (68%) and monazite (32%), but containing trace REE-Fe-Nb oxides. REE minerals were surrounded by quartz and carbonate in subequal amounts, along with pyrite, clinopyroxene, and barite. WBO samples had high Ce:Nd mole ratios (5.50, 13.49), lower La:Ce mole ratios (0.27, 0.09), and trace Nb.

Approximately 0.15% of the EBO samples consisted of REE minerals, containing abundant monazite (74%), bastnäsite (26%) and trace REE-Fe-Nb oxides. The REE minerals occurred in quartz with subordinate clinopyroxene, barite and trace pyrite and carbonate. The REE minerals correlated with fluorite-rich bands. These samples had a high Ce:Nd mole ratio (5.50), a lower La:Ce mole ratio (0.51), and trace Nb.

Samples from the distal WBO deposit (5 km away) have a similar geochemical signature with EBO samples, suggesting no lateral metal zonation in the northern limb of the syncline. SBO, although more proximal to the Main and East Ore Bodies (1 km away) is coarser grained and geochemically distinct.