STUDY OF THE SHEPHERD MOUNTAIN AND PILOT KNOB IRON ORE DEPOSITS USING STRATIGRAPHIC RELATIONSHIPS, FIELD OBSERVATIONS, AND FLUID INCLUSION DATA

The Shepherd Mountain iron ore deposit in southeast Missouri consists of two veins of massive magnetite and hematite in rhyolites from the early Mesoproterozoic period (1500-1440 Ma). The mine was active in the early 19th century but stopped production between 1848-1863. There have been only a couple of studies on the deposit due to the depletion of the veins and limited access to waste rock piles. Recent findings by Tunnel et al. (2022) suggest a possible genetic connection to the nearby underground Pilot Knob Iron Oxide Apatite deposit. In our study we explore the link between the Shepherd Mt. deposit and Pilot Knob using stratigraphic relationships with core samples from both deposits, field Observations, and comparing the thermal and chemical evolution of the deposits using fluid and melt inclusions. To better understand the stratigraphic relationships between the ore deposits, we mapped the volcanic units outcropping in Shepherd Mountain and compared them to core samples obtained from the McCracken Core Library in Rolla, MO. We compared two cores from Shepherd Mountain and several from the lower Pilot Knob Magnetite deposit. The core from Shepherd Mountain showed that the mineralization continues at depth finding the same alteration mineralogy (amphibole-magnetite-quartz-calcite) of the underground Pilot Knob deposit at similar depth than Pilot Knob also hosted by the same andesitic sequence and intruded by the Shepherd Mountain gabbro unit. Our petrological and microthermometric analyses conducted thus far suggest a complex fluid history with multiple fluid inclusions, potentially indicating fluid immiscibility at different stages of deposition. Our initial microthermometry data from fluid inclusions in quartz veins of later mineralization stages shows temperatures ranging from 130-280°C and -19 to -8°C, with a group of fluid inclusions that have final ice crystal melting temperatures between 11 and 25ºC suggesting complex fluid compositions. Future work will focus on more microthermometry work and LA-ICPMS analyses to better understand the fluid evolution and find the potential link between Shepherd Mountain and Pilot Knob ores.