ANALYZING THE PALEOZOIC BASEMENT STRUCTURE AND LITHOLOGIES OF THE NORTHWEST ST. FRANCOIS TERRANE, MISSOURI USING GRAVITY DATA TO INVESTIGATE POSSIBLE MINERAL DEPOSITS OF ECONOMIC INTEREST

The St. Francois Terrane of southeast Missouri consists of Paleozoic sediments atop a Proterozoic igneous crystalline basement. The St. Francois Terrane contains numerous economic ore deposits including some world class rare earth element deposits. In order to map the Proterozoic lithologies and structures, and how these relate to the Rare Earth Element deposits, a regional gravity survey was undertaken collecting detailed gravity data and merged with the sparse existing data. The analysis will include wavelength filtering, derivative analysis and three-dimensional modeling. One procedure to highlight the Proterozoic structures and lithologies, we will apply gravity stripping where the gravitational response of the sediments will be calculated and removed from the complete Bouguer gravity field to emphasize the effect of the overburden sediments on the gravity field. This technique removes the effect of basin fill deposits above the crystalline basement, improving the Bouguer gravity and allowing a more constrained and accurate modeling and interpretation of gravity anomalies within the Proterozoic basement. This technique requires knowledge of the density of the sediments covering the basement and the thickness of the layers between the surface and basement. A “deep gravity” analysis of this area may lead to the discovery of previously unknown rare earth element and other economic mineral deposits within the terrane. This effort is part of a current USGS project investigating the possibility of unknown economic mineral deposits in Southeast Missouri.