ND-PB ISOTOPE GEOCHEMISTRY AND PB-PB AGE ESTIMATES OF THE MESOPROTEROZOIC PEA RIDGE IRON-OXIDE-APATITE-RARE EARTH ELEMENT DEPOSIT, SOUTHEAST MISSOURI, USA

Nd and Pb isotope data were obtained on the Pea Ridge Fe-oxide-apatite-rare earth element (REE) deposit and host rocks in order to characterize their petrologic setting, better understand the regional geologic framework, and advance exploration techniques. REE-rich breccias and iron oxide-apatite (IOA) ores at Pea Ridge are hosted in a volcanic sequence dominated by felsic tuffs with minor mafic-intermediate composition rocks. Radiogenic ²⁰⁷Pb/²⁰⁶Pb age estimates for host rocks and ores are broadly coeval: ~1.50 Ga for rhyolitic host rocks, ~1.48 Ga for REE-rich breccias, ~1.43 Ga for Fe-ores; these estimates are similar to zircon U-Pb ages determined for the host rocks and mineralizing events (ca. 1.46 Ga, Aleinikoff et al., E. Geol., in press). Nd isotope data yield depleted mantle model ages (T_DM) of ~1.51 to 1.59 Ga for rhyolites, 1.51 to 1.6 Ga for REE-rich breccias, and 1.51 to 1.6 Ga for Fe-ores. Immobile trace elements (e.g., Nb/Y, Zr/Ti) indicate that the felsic host rocks are subalkaline dacite and rhyolite having geochemical attributes typical of rocks produced by subduction. The felsic host rocks are depleted in HFSE (e.g., Nb, Ta, Ti, P) and Sr, and lack positive Pb anomalies in mantle-normalized plots. Nd isotope data show: (1) felsic hosts have e_Nd values from 3.44 to 4.25; (2) REE-rich breccias have similar e_Nd values from 3.04 to 4.11; (3) Fe-ores display e_Nd values from 3.04 to 4.21. Our results suggest that host rocks, REE-rich breccias, and Fe-ores share a common origin from a similar source. No evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Pb isotope results for the felsic hosts overlap those of galena in the REE-breccia ore. Whole rock Pb isotopes of REE-rich breccia, Fe-ores and host rocks have a wide range in values and are more radiogenic than galena. The Nd and Pb isotope data indicate that the Pea Ridge deposit was part of a system that produced a uniform isotopic composition. The Pea Ridge deposit represents an excellent example of an igneous-dominated hydrothermal system and defines a distinctive Nd-Pb isotopic target that involved coeval igneous magmatism and REE mineralization. The Nd-Pb isotopic attributes can be used in exploration for additional REE deposits in the Mesoproterozoic rocks of the eastern Granite-Rhyolite Province.