STUDYING THE GENESIS OF THE SHEPHERD MOUNTAIN MAGNETITE VEIN-TYPE DEPOSIT USING FLUID AND MELT INCLUSIONS

The Shepherd Mountain vein-type iron ore deposit in the St. Francois Mountains terrane, SE Missouri, USA, consists of two northeast trending veins of massive magnetite and specular hematite hosted by early Mesoproterozoic (1500-1440 Ma) rhyolites. The mine was active in the early 19^th century and ceased production sometime between 1848-1863. In spite the historical importance of this deposit, there are only few studies that have focused on the Shepherd Mountain deposit, partly because the veins are fully mined out and studies are restricted to tailings piles. Recently, Tunnel et al. (2022) suggested that there were five ore-related fluid pulses with earlier stages containing higher temperature fluids and later stages containing lower temperature fluids. These authors also suggest that there could be a genetic link to the nearby (i.e., <5 km) Pilot Knob Iron Oxide Apatite (IOA) deposit. In this study, we use fluid and melt inclusions from samples from the tailing piles (ore and host rock) to constrain the pressure, temperature and compositional evolution of the fluids/melts involved in the formation of the Shepherd Mountain magnetite deposit, and to further investigate the possible link between the Shepherd Mountain and Pilot Knob IOA deposits. Our preliminary petrographic and microthermometric work reveals a complex fluid history with heterogenous entrapment of multiple fluid inclusion types that suggests potential fluid immiscibility at different stages of the deposition: (1) liquid-rich, (2) liquid and vapor-rich with daughter minerals, and (3) liquid and vapor rich coexisting with inclusions with multiple large crystals that look like recrystallized melt inclusions. Preliminary microthermometry data from fluid inclusions (type 1 and 2) hosted in quartz veins of the later stages of the mineralization, shows T_h ranges from ~130 - 280 ^oC and T_m -19 to - 8 ^oC. Future work will include continued petrography of fluid and melt inclusions, continued microthermometry of both fluid and melt inclusions, and analyzing the chemistry of the fluids with LA-ICP-MS from inclusions representing different mineral stages.