Gravity and Magnetic Investigations into Possible Economic Mineral Deposits within the Northwest St. Francois Terrane, Southeastern Missouri

The exposed Precambrian St. Francois Mountains in southeast Missouri are a well-studied terrane of rhyolites, granites, and basaltic dikes, but much of the buried basement lithology west of the exposed region is still poorly delineated. The western St. Francois is host to large hydrothermal Pb-Zn and Fe-oxide ore deposits, some of which were located with previous geophysical investigations. The economic ore deposits specifically in the Pea Ridge Mine in Washington County contain known economic minerals and rare earth element deposits. In order to further investigate the Precambrian basement lithologies and the possible locations of additional economic ore deposits, a gravity survey was conducted during the summers of 2013 and 2014, collecting over 700 new gravity stations. The new gravity data were merged with previous gravity data showing anomalies that with further processing (e.g., wavelength filtering and derivative analysis) and 2.5-D computer modeling some of the basement lithologies were better defined and previously unknown gravity maximums were identified. The 2.5-D models allowed the development of answers to questions raised by the complete Bouguer gravity anomaly map which also gives insight into the history of the region during the Precambrian after the collapse of the central pluton. The Pea Ridge Mine and many of the other Fe mines are located over gravity minimums, and low density bodies in the Precambrian below the denser ore bodies compensated for the low gravity anomalies. Additionally, the low density bodies may give a greater understanding into the origins of the hydrothermal ore deposits themselves