Using Gravity and Magnetic Data for Insights into the Mesoproterozoic St. Francois Terrane, Southeast Missouri: Implications for Iron Oxide Deposits

Gravity and magnetic data in conjunction with available drill hole data were used to decipher the nature of the volcanic lithologies and their relationship to iron oxide ore deposits within the northwest portion of the Mesoproterozoic St. Francois Terrane in southeast Missouri. Analyses of the data included creating a deep-seated gravity anomaly map by removing the gravitational effect of the Paleozoic sedimentary units, residual gravity and magnetic anomaly maps, and two and one-half dimensional gravity and magnetic models. The residual gravity and magnetic anomaly maps highlighted a series of short-wavelength maxima and minima anomalies that could be interpreted to be granitic within collapsed caldera systems surrounded later by ring intrusions. The gravity and magnetic models illustrate that the sources of the anomalies are situated mainly within the upper 5km of the crust and help to constrain the location of suspected calderas. The iron oxide deposits, some of which contain significant quantities of REE minerals, are rarely associated with both residual gravity and magnetic maxima but may be on the edge of these maxima and/or occur over gravity minima. The REE-rich Pea Ridge deposit is located over a gravity minimum where modeling shows the main ore body extending down to 1.5km with a low-density region extending to 5km below the ore body. The cause of this low-density body is unknown but may be associated with the formation of the ore deposit. Potential iron oxide deposits may occur to the south and southwest of Pea Ridge based on our analysis while to the northeast of the Pea Ridge deposit an anomaly pattern similar to that over Pea Ridge was found.