COMPARISON OF BANDED ORES IN THE MAIN/EAST, WEST, AND SOUTH OREBODIES OF THE GIANT BAYAN OBO REE DISTRICT, INNER MONGOLIA, CHINA

The Bayan Obo REE District in Inner Mongolia, China, consists of three mineralized areas distributed over a 13 x 1.5 km region: the Main/East, West, and South Orebodies. This is the first analysis of regional spatial trends in mineralization in the world's largest known REE deposit and the first study of the South Bayan Obo Orebodies.

The Middle Protereozoic Bayan Obo Group H8 dolomitic marble hosts massive, disseminated, and banded ore types in all three areas. Banded ore is characterized by mm to cm scale layers that vary both mineralogically and texturally. We surveyed and sampled each ore body and distinguished five broad varieties of banded ore: 1) equant, medium-grained quartz-fluorite with bands of fluorite containing disseminated monazite, highly variable magnetite, and trace pyroxene; 2) fine-grained aegerine augite bands with variable fluorite, magnetite, and trace apatite; 3) weakly banded magnetite alternating with blue amphibole-phlogopite-dolomite bands; 4) fine-grained carbonate-fluorite-limonite with trace amounts of apatite, magnetite, and pyrite; 5) evenly disseminated magnetite in weakly to moderately banded coarse-grained dolomite-fluorite-magnetite with fine-grained phlogopite and diopside. Samples from the Main/East Orebodies are typified by varieties 1, 2, and 3, with much variation in the content of REE minerals, whereas varieties 4 and 5 typify the West and South Orebodies, respectively.

Limonite is much more common in the distant West Orebodies and sulfides are sparse in the Main/East and South Orebodies, suggesting the West Orebodies were initially richer in pyrite. If the South Orebodies are continuous with the Main/East Orebodies beneath the H9 syncline, the dominantly disseminated nature of the South Orebodies, the paucity of fluorite and aegerine-augite, the lower REE grades, and greater calcite content may indicate a more distal position of the South Orebodies within the ore-forming system.

The presence of rhythmic pyroxene/fluorite-carbonate bands that wrap around breccia fragments indicates some banding formed due to metasomatic replacement. Boudinage in some banded ores and granoblastic textures indicate post-banding deformation and a possible metamorphic origin or modification for some banding.