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

The genesis of Iron Oxide-Apatite (IOA) deposits has been the focus of an intense debate in the literature in the last decade. Some genetic models favor the hydrothermal process as the main process, or magma immiscibility between silicate and Fe-rich melts and potentially other more exotic melts (sulfate and carbonate melts). Adding to the importance of the debate is that these types of deposits are associated with the emplacement of REE-rich intervals, critical for the present green energy transition. The Pea Ridge IOA-REE deposit is hosted in middle Mesoproterozoic (1.5 – 1.4 Ga) felsic rocks within the St. Francois Mountain Complex in SE Missouri. The Pea Ridge deposit has been associated with a Kiruna-type deposit and consists of 5 major mineralization zones; (1) massive magnetite breccias and pseudobreccias, (2) amphibole-quartz zone, (3) silicified zone, (4) hematite zone, and the (5) REE-bearing breccia pipe. Although the Pea Ridge deposit has been one of the most studied IOA-REE deposits in the USA, there is a gap in understanding the fluid and thermal history of the deposit. We aim to study this fluid-melt history by studying in detail the fluid and melt inclusions of the major mineralized zones and see how the fluid-melt compositions evolved through time to understand the REE mobilization and enrichment better. We conducted detailed petrographic examinations and microthermometry analyses of samples from all the mineralized zones. Petrographic work reveals that the REE-bearing breccia pipe has apatite, barite, quartz, and alkali-feldspar accompanied by a large diversity of fluid inclusion types: (1) liquid-rich, (2) coexisting liquid and vapor-rich, (3) coexisting liquid and vapor-rich FI with daughter minerals, and (4) polymineralic inclusions that we interpret as melt inclusions. From microthermometry analyses, we obtained a wide distribution in homogenization temperatures for types 1, 2, and 3: 155.7 – 183.2ºC, 199.0 – 220.2º C, 317.3 – 346º C. Last ice melting temperatures between -23.3 to -14.8º C indicating NaCl eq. salinities of 15-25%, as well as a group of inclusions that melt at positive temperatures suggesting the presence of clathrates in the inclusions. Future work will include melt inclusions microthermometry and LA-ICP-MS to characterize the chemistry of the fluids and melts of the deposit.