THE ST. FRANCOIS MOUNTAINS IN SE MISSOURI: A NATURAL LABORATORY TO STUDY IOA AND REES MINERALIZATIONS

The Iron-Oxide Apatite (IOA) deposits represent significant sources of both Fe and rare earth elements (REEs). However, the precise origin of these deposits remains a subject of intense debate within the scientific community. While some studies propose that the IOA deposits resulted from a combination of magmatic and hydrothermal processes, others argue for a purely hydrothermal genesis. Despite their growing importance in scientific literature and the rising demand for REEs due to the energy transition, the IOA deposits in SE Missouri have received relatively little attention in research. These deposits, comprising 30 minor and 6 major IOA occurrences, are hosted within mid-Mesoproterozoic (1.5 – 1.4 Ga) felsic to intermediate rocks within the St. Francois Mountain Complex. To gain insights into the nature of this mineralization, we conducted a comprehensive study, employing field and rock core observations along with in situ fluid-melt inclusion analysis. This approach allowed us to explore the thermal and chemical evolution of the fluids responsible for forming the Pea Ridge, Shepherd Mt., and Pilot Knob IOA deposits. By combining these analytical methods, we aimed to shed light on the complex processes that led to the formation of these deposits and contribute to a deeper understanding of their genesis. 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 are gypsum, calcite, hematite, and a hydrous silicate phase, suggesting that they are crystallized melt inclusions, very similar to the ones identified recently on other IOA deposits as sulfate and carbonate melts. Preliminary microthermometric data shows that the fluids and melts are quite complex. For example, FI types 1 to 3 from Pea Ridge and Shepherd Mt. show wide distribution of TH 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.