THE PILOT KNOB IRON ORE DEPOSIT IN SOUTHEAST MISSOURI, USA: A HIGH-TO-LOW TEMPERATURE MAGMATIC-HYDROTHERMAL CONTINUUM

Although there are currently no active iron ore mining activities in Missouri, there is a renewed interest in these deposits as they are speculated to contain economically important cobalt and rare earth element (REE) resources. One of the most prominent iron ore deposits in Missouri is Pilot Knob where underground magnetite ore has been mined from 1968 to 1980. However, overlying the deep magnetite ore (PKM) is a second hematite-rich ore body (PKH) consisting of bedded and breccia hematite that outcrops at the surface. Whereas the PKM ore was studied intensively over the past years, the origin of the PKH ore, and its relation to the PKM, remain uncertain.

C1-chondrite-normalized REE patterns for the PKH ore show a strong fractionation between the light REE and heavy REE with a notable depletion in middle REE and a subtle positive Eu anomaly. Primitive mantle-normalized patterns display an overall negative slope from Cs to Lu with negative Nb, Ta, Ti, Y, Al, V, and Sc anomalies and positive in Ba, U, Pb, and La peaks. These results fall within the range of that elemental patterns observed for the PKM deposit.

Hematite iron isotopic results from 7 bedded samples range from δ⁵⁶Fe 0.06 to 0.30‰ while the δ⁵⁶Fe result from one hematite breccia is -0.19‰. The δ⁵⁶Fe value from the hematite breccia falls within the low temperature hydrothermal event range. The iron isotopic values for bedded hematite are similar to published isotopic data for magnetite from the PKM of which has been suggested to be of magmatic and/or magmatic-hydrothermal origin (Childress et al., 2016). Our data are interpreted to reflect that the PKM and PKH are genetically related. δ⁵⁶Fe values for PKH hematite are slightly lower than that of magnetite from the PKM, which is interpreted to reflect oxidation of ascending hydrothermal fluids. The increase in local redox conditions facilitated conversion of Fe²⁺ into Fe³⁺ which can explain the lighter iron isotope composition of hematite. Therefore, we suggest that the PKM and PKH are part of a high-to-low temperature magmatic-hydrothermal continuum.

References: Childress, T.M., Simon, A.C., Day, W.C., Lundstrom, C.C. and Bindeman, I.N., 2016. Iron and oxygen isotope signatures of the Pea Ridge and Pilot Knob magnetite-apatite deposits, southeast Missouri, USA. Economic Geology, 111, pp.2033-2044.