FLUID AND THERMAL RECONSTRUCTION OF THE PEA RIDGE IOA-REE DEPOSIT, SE MISSOURI, USING FLUID AND MELT INCLUSIONS

The Pea Ridge IOA-REE deposit, in SE Missouri, is one of several major IOA and IOCGs deposits hosted in early Mesoproterozoic (1500-1440 Ma) felsic rocks in the St. Francois Mountains Terrane. The Kiruna-type Pea Ridge deposit is by far the most widely studied deposit in the region and consists of at least 5 major alteration zones composed mainly of amphibole, magnetite, hematite, silica, and an REE-rich breccia pipe respectively. The interest in the IOA deposits has increased over the last decade owing to an intense debate in the literature about the genesis of these deposits (Bain et al., 2020 and reference within) and the increasing demand for REE metals for the green energy transition. Despite all the attention to deposits like Pea Ridge, there is an incomplete understanding of the fluid and thermal history of the deposit. In this study, we aim to better constrain the genesis of the deposit, its fluid-melt history, and how these fluids and melts associate with the enrichment of REEs by conducting careful characterization of fluid pulses using fluid and melt inclusions by conducting detailed fluid/melt inclusion petrography, microthermometry, and LA-ICPMS analyses. Preliminary observations on samples from the REE breccia pipe show a large diversity of primary and secondary fluid inclusions (FI) types: from (1) liquid-rich FI, (2) liquid and vapor-rich FI, and (3) liquid and vapor-rich FI coexisting with inclusions with multiple larger crystals (calcite, hematite, halite/sylvite) that look like recrystallized melt inclusions similar to those observed by Bain et al. (2020). Homogenization temperatures of type (1) and (2) FI assemblages show a wide distribution in temperatures 155.7 – 183.2ºC, 199.0 – 220.2º C, 317.3 – 346º C, and last ice melting temperatures ranging from -23.3 to -14.8º C. Future work will focus on petrography, microthermometry, and LA-ICP-MS analyzes of the fluid and melt inclusions of the different mineralization zones to understand the role of different episodes of fluid-rock interaction in relation to the formation of the main ore body and the REE enrichment.