Preliminary Fluid Inclusion Constraints on Fluid Evolution in the Bayan Obo Fe-REE-Nb Deposit, Inner Mongolia, China
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Abstract:
The Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China,
is the world’s largest REE resource and is also exploited as a major Fe
oxide deposit. The deposit consists of replacement bodies hosted in dolomite
marble and of magnetite, hematite, and REE minerals associated with apatite,
aegirine, amphibole, fluorite, calcite, and barite. Fluid inclusions from
monazite-(Ce), bastnasite-(Ce), apatite, aegirine, fluorite, and barite
were studied from eight samples to provide new constraints on ore genesis. During early disseminated monazite mineralization the
fluids are best represented by the system H 2 O-CO 2 -NaCl,
with salinities ranging from 1 to 5 wt percent NaCl equiv and X (CO 2 ) values from 0.3 to 0.45, at temperatures of >280° to 330°C and
pressures of >0.7 kbar. Fluids in apatite and bastnasite from the
main-stage banded ores were initially aqueous, with salinities ranging from
6 to 10 wt percent NaCl equiv. They developed significant CO 2 contents (X (CO 2 ) = 0.1–0.3) due to carbonate dissolution at
temperatures of >400° to ~300°C and pressures of >0.9 to 1.4 kbars.
Evidence for phase separation from coexisting brine and CO 2 -rich
fluid inclusions in vein fluorite indicates pressures of 0.8 to 1.0 kbar
during vein formation. These pressures can be used to infer temperatures of
430° to 460°C for aegirine alteration and of 340° to 240°C for apatite,
bastnasite, and fluorite vein fill. Fluids responsible for aegirine
alteration were dominantly aqueous, with salinities from 5 to 15 wt percent
NaCl equiv, whereas those responsible for apatite and bastnasite
mineralization had salinities of 1 to 6 wt percent NaCl equiv and X (CO 2 ) values from 0.25 to 0.75. Inclusions in late-stage fluorite and barite have
homogenization temperatures from 240° to 150° and 200° to 130°C,
respectively. The general salinity trend in these inclusions suggests
dilution from around 15 to 5 wt percent NaCl equiv down to very low
salinities (<1 wt % NaCl equiv) during barite precipitation.
Halite-bearing inclusions may indicate episodes of boiling or
aqueous-carbonic fluid immiscibility. These fluids are similar to those in some carbonatitic,
alkaline igneous, and other igneous-related REE- and Fe-rich magmatic
hydrothermal systems. However, they are different from fluids in
igneous-related Fe oxide-Cu-Au-U-REE deposits, which have previously been
compared to those at Bayan Obo.Keywords:
Inner mongolia