GEOPHYSICAL SETTING OF IRON OXIDE-COPPER-COBALT-GOLD-RARE EARTH ELEMENT DEPOSITS OF SOUTHEAST MISSOURI

The consensus among many workers is that the iron-copper-cobalt-gold-rare earth element (REE) deposits of southeast Missouri belong to the iron oxide-copper-gold (IOCG) family of mineral deposits. The deposits are hosted in Mesoproterozoic granite-rhyolite igneous basement rocks of the St. Francois Mountains terrane. The largest of the known concealed deposits is the Pea Ridge iron ore deposit, which contains rare earth element (REE)-rich minerals that occur in both breccia pipes and in apatite within magnetite ore. Key facets of the geologic setting, resource potential, and ages of these deposits remain uncertain owing to their poor exposure. Among the 12 known iron deposits and prospects, the majority are covered by a thick Paleozoic sedimentary sequence that blankets the basement surface. Depth to the top of mineralized deposits, prospects, and occurrences at Pea Ridge, Bourbon, Camel’s Hump, Boss-Bixby, and Kratz Spring vary from 325 to 415 m below the topographic surface. However, the depth to and topography on the basement surface for much of the prospective terrane is poorly constrained and can range from a few hundred meters to more than 700 m in the southeast part of the study area. This year, the U.S. Geological Survey Mineral Resource Program began a three-year project to investigate the geological setting, origin, and geophysical signature of iron-copper-cobalt-gold-REE deposits in southeast Missouri. The geophysical component includes compilation and interpretation of existing magnetic and gravity data to determine overall basement architecture. The quality and resolution of the airborne magnetic and ground gravity data are adequate to establish where large basement features occur and are used to impart refinement on the present basement geologic map. Petrophysical measurements have been made on an inventory of several hundred drill core, surface, and underground mine samples collected by the USGS over the last decades. Data obtained on these samples are analyzed statistically to determine relationships between petrophysical properties and alteration assemblages, ore mineralogy, and chemistry. The petrophysical data will be used as input to a deposit scale 3-D geophysical model of the Pea Ridge Fe-REE deposit.