INSIGHTS INTO CONCEALED IRON OXIDE-RARE EARTH ELEMENT DEPOSITS FROM NEW AIRBORNE GEOPHYSICAL DATA, SOUTHEAST MISSOURI

The iron-copper-gold-rare earth element (IOCG-REE) deposits in southeast Missouri are not well defined owing to limited outcrops of the host Precambrian igneous rocks of the St. Francois Mountains terrane. There is widespread sedimentary cover, as much as 450 m thick, of Cambrian carbonate and sandstone rocks. As a result, the geometry, age, and petrology of buried plutons and volcanic rocks—and potentially undiscovered metal deposits—are unknown except where sparse drill hole data exist. Late in 2013, the U.S. Geological Survey Mineral Resource Program contracted a high-resolution magnetic and gravity gradiometry survey as one component of a large research study on the setting and origin of IOCG-REE deposits of southeast Missouri. The geophysical survey covers 576 sq. km centered on the Pea Ridge mine, located about halfway between the towns of Sullivan and Potosi, Missouri. The data were collected from a helicopter flying 80 to 100 m above ground along 400-m-spaced, north-south flight lines. The survey was designed to specifically target the important Pea Ridge deposit, the top of which is located at a depth of approximately 300 m from the topographic surface. The survey also covers other undeveloped and concealed iron oxide deposits including Kratz Spring and Bourbon. This will be the second airborne geophysical survey flown over the area by the USGS. Magnetic data were first collected from a fixed-wing aircraft in the late 1940s in cooperation with the Missouri Geologic Survey. The older survey was the basis for the discovery of the Pea Ridge deposit, which was actively mined for iron from 1963 until its closing in 2001. Preliminary results from the new airborne survey will be presented and used to begin a re-interpretation of the Precambrian rocks, and incorporate current concepts of IOCG-REE deposits and their geologic setting using modern geophysical data.