THERMAL RECONSTRUCTION OF PEA RIDGE IOA-REE DEPOSITS, SE MISSOURI, USING FLUID & MELT INCLUSIONS

The genesis of Iron Oxide Apatite (IOA) or Kiruna-type deposits has been intensely debated over the last decade. Some proposed genetic models favor the hydrothermal process or magma immiscibility between silicate, Fe-rich melts, and potentially more exotic melts. The debate becomes imminent because these types of deposits also emplace REE that are critical for the ongoing green energy transition. The Pea Ridge IOA-REE deposit is hosted in middle Mesoproterozoic (~1.4 Ga) felsic rock in the St. Francois Mountain Complex in SE Missouri. The Pea Ridge deposit consists of five major mineralization zones: 1) magnetite breccia and pseudobreccia, 2) amphibole-quartz zone, 3) silicified zones, 4) hematite zone, and 5) REE-bearing breccia pipe. Despite Pea Ridge IOA being one of the most studied IOA-REE deposits, there is a gap in understanding this deposit's fluid and thermal history.

We aim to reconstruct the thermal history by studying the fluid and melt inclusions of the major mineralization zones and to study the evolution of fluid-melt compositions through time to understand the REE mobilization and enrichment. We conducted careful petrographic examinations of the samples from the mineralization zones. Petrographic work reveals a large diversity of fluid inclusion types 1) liquid rich, 2) coexisting liquid- and vapor-rich, 3) co-existing liquid and vapor-rich inclusion with daughter minerals, and 4) polymineralic inclusion that we interpret as melt inclusions. From microthermometry analysis, we collected a wide distribution in homogenization temperatures for FI types 1, 2, and 3: 62 – 473.6 ºC, 121 – 489 ºC, and 149 – 462 ºC while type 4 exhibits homogenization temperature > 500 ºC. The last ice melting temperatures indicate NaCl eq. salinities of 15 – 25%wt. Some inclusions show anomalous last ice melting temperatures suggesting an unknown fluid composition and the presence of clathrates in the inclusions. BSE images of the minerals denotes that REEs are distributed on apatites and barites, which raises the question of whether sulfate melts have the potential to transport these REEs. LA-ICP-MS analysis showed that there are substantial REE concentrations in these minerals too. Future work will include melt inclusion microthermometry and composition analysis to understand the nature of exotic melts which plays an essential role in the Pea Ridge IOA-REE deposit.