A FLUID APPROACH TO UNDERSTANDING THE IOA MINERALIZATIONS IN SE MISSOURI

The Iron-Oxide Apatite (IOA) deposits are major sources of Fe, and rare earth elements (REEs). However, the exact genesis of these deposits is still a matter of intense debate in the scientific community. Some studies suggest that the IOA deposits formed through a combination of magmatic and hydrothermal processes, while others point to a purely hydrothermal origin. Despite the increasing relevance of these deposits in the literature and the ever-increasing demand for REEs for the energy transition, the IOA deposits of SE Missouri remain relatively understudied. These deposits are hosted in middle Mesoproterozoic (1.5 – 1.4 Ga) felsic to intermediate rocks within the St. Francois Mountain Complex in SE Missouri. This study combines field and rock core observations with in situ fluid-melt inclusion analysis to understand the thermal and chemical evolution of the fluids responsible for the formation of the Pea Ridge, Shepherd Mt., and Pilot Knob IOA deposits. Fluid inclusion (FI) petrography shows that most of the FIs are hosted in quartz, calcite, barite, and actinolite showing a large diversity of FI types: (1) liquid-rich, (2) coexisting liquid and vapor-rich, (3) liquid and vapor-rich FI with daughter minerals coexisting with (4) polymineralic inclusions that we interpret as melt inclusions. These polymineralic inclusions show evidence of gypsum, calcite, hematite, and a hydrous silicate phase, suggesting that they are crystallized melt inclusions, very similar to the ones identified recently on IOA deposits as sulfate and carbonate melts. The microthermometry of FI types 1 to 3 from Pea Ridge and Shepherd Mt. show wide distribution of T_H in different FI assemblages (155 – 183ºC, 200– 220.2º C, and 317.3 – 346º C) and a wide distribution of salinities (10 to 40 wt.% NaCl). A subset of the FIAs show anomalous positive melting temperatures from 11 to 25ºC, but no CO₂, potentially indicating other fluid compositions with CO₃^- and/or SO₄^- complexes. Further work needs to be done to fully understand these deposits' complex fluid-melt evolution, including more microthermometric data, Raman analyses, and LA-ICPMS.