COMPARISON OF QUARTZ CATHODOLUMINESCENCE (CL) AND FLUID INCLUSION (FI) DATA FROM THE PEA RIDGE AND SHEPHERD MOUNTAIN IRON DEPOSITS IN THE ST. FRANCOIS MOUNTAINS, SE MISSOURI

The St. Francois Mountains igneous terrane of SE Missouri contains multiple styles of Fe mineralization hosted in Mesoproterozoic rhyolitic and intermediate to mafic igneous rocks that are similar to iron oxide apatite (IOA) and iron oxide copper gold deposits (IOCG) globally. To advance understanding of their origin, previous quartz CL and FI microthermometric data from the large Pea Ridge IOA replacement deposit were compared to new quartz CL and FI data from the small Shepherd Mtn IOA vein deposit. LA-ICP-MS analyses of FI are underway.

At Pea Ridge, the magnetite orebody is enveloped by broad amphibole, silicic, and K-spar zones that are cut by REE-bearing breccia pipes. Early quartz that fills voids and fractures in the magnetite and adjacent alteration zones exhibits short-lived blue CL. Later quartz with dark red CL precipitated after episodes of fracturing and brecciation. Early CL-zoned quartz lacks primary FIs, but contains abundant secondary or pseudosecondary FIs along healed fractures with red CL. Liquid-rich FIs have T_h of 80-390°C, P_h 1-150 bars, and bimodal salinities of 1-10 and 10-27 eq. wt. % NaCl. Episodes of decompression are evidenced by a population of hypersaline and vapor-rich FIs that occasionally contain liquid CO₂. In the REE breccia pipes, late quartz cements with dark red CL contain liquid- and vapor-rich FIs with variable T_hand salinities near 0 wt. %.

At Shepherd Mtn., steeply dipping magnetite veins are enveloped by narrow amphibole, epidote, chlorite, and K-spar alteration halos. Voids and fractures are filled with quartz and calcite. Early quartz with short-lived blue CL is crosscut by later quartz with red-CL that contains secondary liquid-rich FIs with T_h of 142-169°C, P_h of 3-7 bars, and salinities of 16-20 wt. %. Rare vapor-rich and hypersaline FIs appear to have formed by accidental trapping of halite during episodes of decompression. CO₂ was not observed or detected by laser Raman. Calcite contains liquid- and vapor-rich FIs with low salinities.

The results suggest that Pea Ridge and Shepherd Mtn. formed from saline fluids at different pressures and depths that unmixed during episodic decompression. Both were cemented by a later stage of dilute fluids that boiled. The veins at Shepherd Mtn. may therefore be the shallow expression of a larger concealed replacement IOA deposit at depth.