A NEW LOOK AT IRON OXIDE-APATITE AND IRON OXIDE-COPPER-GOLD-RARE EARTH ELEMENT DEPOSITS IN THE MESOPROTEROZOIC ST. FRANCOIS MOUNTAINS TERRANE OF SOUTHEAST MISSOURI

Large iron oxide-apatite (IOA) and iron oxide-copper-gold (IOCG) deposits in southeastern Missouri are hosted within ~1.47 Ga volcanic rocks of the St. Francois Mountains terrane. A current USGS project is studying these deposits using newly acquired data from U-Pb, Re-Os, and Ar-Ar geochronology, geophysics, whole rock and mineral chemistry, stable and radiogenic isotopes, and fluid inclusions. The deposits occur in the Eastern Granite-Rhyolite Province, near the western margin of a regional crustal boundary between eastern and western Proterozoic basements having T_DM ages of <1.55 Ga and >1.55 Ga, respectively. Structural control by Proterozoic basement is suggested by broad spatial association of the deposits with northwest-trending gravity and magnetic anomalies. Some deposits are buried ca. 300-500 m beneath Cambrian sedimentary rocks. The IOA group includes the 136-Mt Pea Ridge, 30-Mt Iron Mountain, and 20-Mt lower Pilot Knob deposits, all of which are massive to brecciated bodies chiefly in rhyolite or trachyte, with magnetite being the predominant iron oxide mineral accompanied by minor apatite, albite, calcite, and sparse pyrite and chalcopyrite. Altered wall rocks, best developed at Pea Ridge, include zones rich in hematite, amphibole, K-feldspar, and/or quartz. Margins of the magnetite-apatite orebody at Pea Ridge also contain four unmined REE-rich breccia pipes that together constitute 600,000 t @ 12% REO having relatively high HREE/LREE ratios, abundant barite, and locally high Au (to 10.8 oz/t), Mo (to 1500 ppm), Sn (to 4090 ppm), Th (to 1.59 wt %), and U (to 610 ppm); REE reside in monazite, lesser xenotime and allanite, and sparse bastnäsite and synchysite. Stratiform hematite deposits, hosted in rhyolitic volcanic rocks, include upper Pilot Knob and Cedar Hill. The largest known IOCG deposit is Boss-Bixby (40 Mt @ 0.8% Cu with minor Co, Au, and REE) that occurs mainly in trachyte; the Bourbon Fe-Cu-REE-Th-U deposit may belong to the IOCG group.

Geological settings and nature of the IOA and IOCG deposits in southeast Missouri are similar to those in the Stuart Shelf region of South Australia and the Norbotten-Kolari region of northern Sweden and adjacent Finland. Significant potential exists for undiscovered REE-rich IOCG deposits in southeast Missouri, hidden under Cambrian sedimentary rocks.