IN SITU LA-ICPMS U-PB GEOCHRONOLOGY OF APATITE FROM IOA AND IOCG DEPOSITS, ST. FRANCOIS MOUNTAINS, SOUTHEAST MISSOURI, USA

St. Francois Mountains terrane, southeast Missouri, USA is located within the Mesoproterozoic Granite-Rhyolite province in the U.S. Midcontinent and includes a number of iron oxide-apatite-rare earth element (IOA-REE) and iron oxide-copper-gold (IOCG) deposits. The province formed during two major pulses of igneous activity at ca. 1.48-1.45 Ga and ca. 1.32-1.27 Ga. Precise and accurate determination of the timing of ore-forming processes is crucial for understanding the origin of deposits and placing them in a regional geologic context. The potential presence of multiple generations of dateable ore minerals, processes of dissolution-reprecipitation, and occurrence of micrometer-sized intergrowths and inclusions complicate measurements and interpretations of the geochronological results and emphasizes the need of applying high-spatial resolution dating techniques. We report new U-Pb ages for ore apatites from Pea Ridge, Kratz Spring, and Iron Mountain IOA deposits and Bourbon IOCG deposit obtained by in situ analyses of polished rock slabs using laser ablation inductively coupled plasma (LA-ICPMS) technique.

Apatite from the Pea Ridge IOA deposit, one of the major magnetite deposits hosted in the ~1.47 Ga rhyolites, contains abundant monazite and xenotime inclusions. Two acid leachates of apatite from a magnetite ore sample yield TIMS ²⁰⁷Pb*/²⁰⁶Pb* dates of ~1.47 Ga, slightly younger than, but within error of, the age of host rhyolites. This result suggests a short time interval between felsic volcanism and magnetite ore formation. At least one younger episode of hydrothermal activity at ~1.44 Ga was identified using in situ LA-ICPMS dating of monazite inclusions in apatite. Intermediate ~1.46 to ~1.45 Ga ages determined using LA-ICPMS on inclusion-free zones in apatite and some monazite inclusions may suggest several superimposed metasomatic events occurred between 1.47 and 1.44 Ga.

Preliminary ~1.45 Ga U-Pb ages of apatite from Kratz Spring, Bourbon, and Iron Mountain deposits suggest that the hydrothermal activity responsible for the IOA-IOCG mineralization at all the studied deposits can be related to the older 1.48-1.45 Ga pulse of igneous activity within the St. Francois Mountains terrane.