STABLE ISOTOPE GEOCHEMISTRY OF THE PEA RIDGE IRON OXIDE-RARE EARTH ELEMENT DEPOSIT, ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI: TEMPERATURE OF ORE FORMATION, SOURCE OF HYDROTHERMAL FLUID, AND SOURCE OF SULFUR

The Pea Ridge deposit is one of several iron oxide bodies (magnetite±apatite, hematite) that have been discovered in the Mesoproterozoic granite-rhyolite terrane of the St. Francois Mts. The Pea Ridge mine produced iron from 1964 until 2001. The remaining mineralization and existing mine tailings are of interest as possible new sources of rare earth elements (REE), which reside mainly in monazite and xenotime, and high-grade iron ore.

Oxygen isotope equilibration temperatures for magnetite-quartz in ore samples average 481°C (range: 269–688, n=11), consistent with the 460 to >530°C values previously reported for homogenization of fluid inclusions in quartz. Temperatures for amphibole-quartz (n=2) and apatite-magnetite (n=2) are similar, 419–725°C. These values are below the probable solidus temperatures for the ore assemblages and thus imply a hydrothermal rather than a melt origin for the ores.

The calculated temperatures and measured δ¹⁸O values (quartz is 13–20 per mil) indicate that the ore fluid had a δ¹⁸O_H2Ovalue >10 per mil. This is at least 2 per mil higher than expected for magmatic fluids evolved from the Pea Ridge igneous system, which appears from Nd isotopic and whole rock chemistry data to have had a depleted-mantle source. Magmatic fluids may have been important in the formation of the iron ore, but our isotopic data suggest the presence of another fluid, or a fluid component, that was isotopically-heavy and externally-derived.

REE-rich breccia pipes that crosscut the margins of the magnetite+apatite deposit contain barite, pyrite, and hematite. The sulfur isotopic composition of barite varies inversely with that of pyrite suggesting that the minerals precipitated from a cooling fluid (450 to 200°C) with an SO₄/H₂S ratio of 1.5 and a δ³⁴S value for total sulfur of ~9 per mil. The δ³⁴S value of the system during the earlier magnetite+apatite ore stage could have been no lower than 4–5 per mil based on analyses of associated pyrite. These values are higher than expected for a mantle-derived magma and thus suggest an external sulfur source.

The Pea Ridge deposit has traditionally been attributed to magmatic-hydrothermal processes, but the isotopic data presented here provide intriguing evidence for the involvement of nonmagmatic fluid and nonmagmatic sulfur in ore genesis.