INSIGHTS ON THE SHEPHERD MOUNTAIN AND PILOT KNOB IRON ORE DEPOSITS USING FIELD OBSERVATIONS, STRATIGRAPHIC RELATIONSHIPS, AND FLUID/MELT INCLUSION DATA

The Shepherd Mountain and Pilot Knob iron ore deposits, found in the St. Francois Mountains terrane are hosted by early Mesoproterozoic (1500-1400 Ma) rhyolites. The Shepherd Mountain deposit consists of northeast trending veins of massive magnetite and specular hematite. The Pilot Knob deposits consist of both the Pilot Knob hematite deposit (PKH) at the surface containing hematite-rich ore beds, and the Pilot Knob magnetite deposit (PKM), a magnetite-rich tabular ore body located in the subsurface in the Pilot Knob valley. Although these deposits have been historically significant in the region in terms of iron ore production, few studies have focused on the deposits. Recently, Tunnel et al. (2022) suggested a genetic link via shared fluid pathways within the Ironton Fault and Pilot Knob Fault between the Shepherd Mountain and Pilot Knob deposits. This study utilized several visits to the field to observe surface outcrops as well as many visits to the McCracken Core Library to observe core from the subsurface, both of which provided samples for microthermometric study. Primary volcanic features were observed at both the Shepherd Mountain and PKH deposits including vugular features and volcaniclastic features in the ore. Stratigraphic observations conducted using core samples and surface outcrops show that the volcanic units and mineralization associated with the PKM deposit continue below Shepherd Mountain as well as ~2 km east of the deposits. Petrological and microthermometric work done in this study indicates a complex fluid history with heterogenous entrapment of multiple fluid inclusion types. The different fluid inclusion types include: (1) liquid-rich, (2) highly variable liquid-vapor ratios, (3) liquid-rich with a daughter mineral, and (4) polycrystalline inclusions with multiple daughter minerals of various compositions. Positive melting temperatures were observed indicating the presence of sulfate/carbonate-salt hydrates suggesting the involvement of sulfate/carbonate-rich melts in the system (Bain et al., 2020, 2021). The observations made in this study indicate that these deposits are magmatic-hydrothermal in nature and are likely genetically linked as proposed by Tunnel et al. (2022). Future work will include high-temperature microthermometry to better understand the composition of the fluids associated with these deposits.