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
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 δ18OH2Ovalue >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.