FATE AND TRANSPORT OF REE IN CARBONATITE-RELATED FLUIDS FROM THE BEAR LODGE ALKALINE COMPLEX, WY: EVIDENCE FROM FLUID INCLUSION STUDIES

Characterization of carbonatite-related fluids is useful in understanding processes that may affect REE ore grade enrichments. Examination of fluid inclusions from the Bear Lodge alkaline complex using laser Raman spectroscopy, scanning electron microscopy, microthermometry, and decrepitate mound analysis show that multiphase inclusions contain H₂O-CO₂-SO₄^2- fluids elevated in Na, Sr, K, with lesser Ca, Ba, REE, and minor Cl. Homogenization temperatures (Th) of fluid inclusions span from near-magmatic conditions down to ~100 ⁰C, with fluids evolving towards lower CO₂ concentrations with decreasing temperature.

Liquid-vapor-solid (LVS) inclusions have complex and variable compositions, with multiple daughter minerals, dissolved sulfate, and ±CO₂ in the vapor phase. LVS inclusions are present as primary fluid inclusion assemblages (FIAs) in zoned fluorite and smoky quartz, as pseudosecondary and secondary FIAs in late clean fluorite, or as secondary FIAs in calcite and less commonly in early clouded fluorite. Daughter minerals most commonly include nahcolite, barite, celestine, (calcio)burbankite, Na-K sulfates, and rarely calcite. Most FIAs hosted in smoky quartz and zoned fluorite lack dissolved CO₂. Vapor homogenization spans a wide range from 92 to 382 ⁰C, with Th of the final solid phase calcioburbankite up to 556 ⁰C.

Secondary FIAs of CO₂-rich “double bubble” inclusions are rare in both early clean and late clouded fluorite. Th of these FIAs are difficult to measure due to the high proportion of vapor, ranging from 272 to 350 ⁰C. Compositions calculated in the H₂O-CO₂-NaCl system suggest that these inclusions contain 1-2 wt% NaCl equivalent and 45 mol% CO₂, with pressures at Th between 600 and 850 bars.

Liquid-vapor (LV) inclusions with trace CO₂ occur as primary and secondary FIAs in coarse to euhedral calcite and in fine-grained recrystallized calcite, as pseudosecondary and secondary FIAs in early clouded fluorite, or as secondary FIAs in fine-grained calcite. Early clouded fluorite and fine-grained calcite also hosts vapor inclusions consisting of ≥80 vol% vapor. Dissolved SO₄^2- is variably present in the liquid phase. Th of LV inclusions ranges widely from 117 ⁰C to >300 ⁰C in coarse-grained and euhedral calcite, from 330 - 432 ⁰C in fine-grained calcite, and 260 - 391 ⁰C in early clouded fluorite.