HYDROTHERMAL ALTERATION OF THE BUTLER HILL GRANITE, ST. FRANCOIS MOUNTAINS, SE MISSOURI

The St. Francois Mountains in southeastern Missouri reveal a roughly 900 km² 1.5 Ga igneous complex that shows signs of post-emplacement alteration. This study focuses on a hydrothermal event affecting the Butler Hill granite near Fredericktown, MO, manifested by green boulder fragments within the largely red granite quarry. Although our sample boulder was not in place when taken, we know its approximate original location relative to a quarried-out hydrothermal vein, based on information from the quarry geologist. Our sample was divided into six lateral zones by visual inspection, each showing different stages of mineralization. Zone 1, the least altered, shows slightly altered feldspar and plagioclase, biotite/chlorite, and quartz. Zone 2 shows progressive alteration of feldspar and plagioclase, typically unaltered microcline, and quartz. Zone 3 has substantially altered plagioclase, slightly altered microcline, biotite in solution pathways, and epidote in place of chlorite. Zone 4 shows pervasive alteration throughout, large quartz crystals, and epidote in interstitial areas. Zone 5 has smaller subhedral quartz crystals, plagioclase appears broken up, and epidote/opaque minerals are located in alteration routes. Zone 6 shows disseminated alteration throughout, excluding small anhedral quartz and (~3-4mm) vugs of calcite. The sample shows opaque minerals disappearing as the zones grade from slightly to significantly altered. X-ray fluorescence spectrometry (XRF) data shows trending oxide patterns from Zone 1 to Zone 6 indicative of hydrothermal alteration: decreasing SiO₂ (75.6-61.0%) and increasing Al₂O₃ (13.7-16.2%), CaO (0.9-3.6%), and MgO (0.3-2.7%). These trends suggest that a calcium rich fluid was responsible for producing the pervasive epidotization and vugs. Continued petrographic and SEM analyses will give further insight into the composition and source of the hydrothermal fluids that affected the original granite composition.