UNDERSTANDING MISSOURI’S RARE EARTH ELEMENT AND COBALT POTENTIAL

The St Francis igneous province of Southeast Missouri contains a series of iron ore deposits hosted by 1.47 Ga granites and rhyolites. The origin of these deposits remains debated; however, several deposits show characteristics commonly associated with Kiruna-type iron ore deposits and/or iron oxide copper gold (IOCG) deposits. Several of these deposits, most notably at Pilot Knob, Iron Mountain, and Pea Ridge, where mined for iron from the late 19th century until the early 2000s. The last mine (Pea Ridge) closed in 2001; however, there is renewed exploration interest in Missouri’s iron ore deposits due to their notably high content of rare earth elements (REE) and cobalt (Co). REE are predominantly hosted by the phosphate minerals apatite, monazite, and xenotime that occur as disseminations in magnetite ore bodies and/or are locally concentrated in breccia pipes. Several iron ore deposits in Missouri are notably Co-rich based on high bulk-rock Co concentrations observed in drill core assays. However, cobalt’s mode of occurrence remains to be constrained in these deposits.

Here we present the preliminary results of a study that aims to better understand the origin of Missouri’s iron ore deposits, particularly focusing on the REE and Co potential of the Bourbon, Camel’s Hump and Kratz Spring iron ore deposits. These deposits do not outcrop at the surface, but have been discovered by the Doe Run Mining Company during exploration programs. Preliminary observations show that the deposits occur along igneous discontinuities in lens-shaped magnetite ore bodies, sometimes cross cut by phosphate breccia pipes. A mineral systems approach will be applied to better understand (i) the origin of these deposits (magmatic vs. hydrothermal), (ii) the physical and chemical mechanisms that controlled their formation/emplacement, and (iii) the reasons for the REE and/or Co enrichment in these deposits. The results will be used to reflect on the role of the geodynamic evolution of the St Francis igneous province in the formation of these deposits in order to develop conceptual exploration tools for similar deposits elsewhere.