High Spatial Resolution U-Pb Geochronology and Pb Isotope Geochemistry of Magnetite-Apatite Ore from the Pea Ridge Iron Oxide-Apatite Deposit, St. Francois Mountains, Southeast Missouri, USA

• DOI: 10.2113/econgeo.111.8.1915
• Publication Date: 2016-11-17T10:46:56Z
• Authors: L. A. Neymark; C. S. Holm-Denoma; A. J. Pietruszka; J. N. Aleinikoff; C. M. Fanning; R. M. Pillers; R. J. Moscati
• Keywords: Geochronology; Metasomatism; Ore genesis

The Pea Ridge iron oxide-apatite (IOA) deposit is one of the major rhyolite-hosted magnetite deposits St. Francois Mountains terrane, which located within Mesoproterozoic (1.5–1.3 Ga) Granite-Rhyolite province in U.S. Midcontinent. Precise and accurate determination timing duration oreforming processes this crucial for understanding its origin placing it a deposit-scale regional geologic context. Apatite monazite, well-established U-Pb mineral geochronometers, are abundant orebody. However, potential presence multiple generations dateable minerals, dissolution-reprecipitation, occurrence micrometer-sized intergrowths inclusions complicate measurements interpretations geochronological results. Here, we employ combination several techniques, including ID-TIMS high spatial resolution geochronology apatite monazite using LA-SC-ICPMS SHRIMP, Pb isotope geochemistry pyrite to obtain first direct age constraints on formation alteration history IOA deposit. oldest TIMS 207 Pb*/ 206 Pb* dates 1471 ± 1 1468 Ma, slightly younger than (but error of) ~1474 ~1473 Ma zircon ages host rhyolites. Dating provides evidence at least metasomatic event ~1.44 Ga, possibly superimposed events between 1.47 1.44 Ga. Lead isotop analyses show extremely radiogenic Pb/ 204 ratios up ~80 unsupported by situ U decay. This excess may have been derived from spatially associated as recrystallized tens million years after formation. low initial ratio ~16.5 ~15.4 individual grains indicate closed system behavior consistent with derivation mantle-like source.