RARE EARTH ELEMENT ORE POTENTIAL OF APATITE MINERALIZATION IN THE BOURBON AND IRON MOUNTAIN DEPOSITS IN THE SOUTHEAST MISSOURI IRON METALLOGENIC PROVINCE

The Southeast Missouri Iron Metallogenic Province consists of 15 known iron-oxide-apatite magmatic-hydrothermal deposits. Six of these deposits were mined for iron between the early 19^th century and 2001. The largest of these deposits, Pea Ridge, also contains significant enrichments of rare earth elements (REE’s) in late-stage hydrothermal breccia pipes, making Pea Ridge the 11^th largest and highest grade known REE deposit in the U.S. An important unanswered question about the Southeast Iron Metallogenic Province is whether similarly REE-enriched breccias occur in any of the other deposits besides Pea Ridge, as this could significantly increase the REE resource in the province. To begin to answer that question, drill hole logs from the Bourbon and Iron Mountain deposits, archived by the Missouri Department of Natural Resources, were reviewed. Like Pea Ridge, Bourbon and Iron Mountain are regarded as two of the deeper-formed deposits in the province. Numerous breccia occurrences are noted in the Bourbon and Iron Mountain logs, though none have descriptions closely resembling the REE-breccias in Pea Ridge. Nonetheless, several drill core intervals from Bourbon and Iron Mountain breccia zones were sampled to see if they were enriched in REE’s and what minerals carried the greatest concentrations of REE’s. Apatite, andradite, actinolite, biotite, and chlorite in thin sections prepared from these samples were analyzed using LA-ICP-MS. The results showed that apatite by far contains the greatest concentrations of REE’s, ranging from 100’s to 1000’s of ppm for the light REE’s and from 10’s to 100’s of ppm for the heavy REE’s. Apatite REE concentrations are very similar in the Bourbon and Iron Mountain deposits. These apatite REE concentrations comprise a much lower grade REE resource at Bourbon and Iron Mountain than at Pea Ridge, as even ore zones that consisted of 100% apatite at Bourbon and Pea Ridge would only contain 0.25 to 0.3 weight percent rare earth oxides (REO), whereas the Pea Ridge breccias contain 12.0 weight percent REO.