40AR/39AR GEOCHRONOLOGY OF HYDROTHERMAL MINERALS FROM IOA AND IOCG DEPOSITS IN THE SOUTHEAST MISSOURI IRON PROVINCE, USA – RELATION TO 1.4 AND 1.3 GA MAGMATISM

The southeast Missouri iron province contains several significant magnetite and hematite iron oxide apatite (IOA)-REE deposits, a breccia pipe REE deposit and a magnetite iron oxide copper gold (IOCG)-Co deposit. Previous U-Pb geochronologic studies of igneous zircon and hydrothermal IOA and REE mineralization were mostly conducted using in-situ SHRIMP and LA-ICP-MS U-Pb spot analyses of zircon (1.48-1.44 Ga and 1.34-1.27 Ga), apatite (1.47-1.38 Ga), monazite (1.46-1.38 Ga), xenotime (1.45 Ga), and rutile (1.46-1.42 Ga).TIMS analyses of apatite (1.47-1.46 Ga), a whole rock Pb-Pb isochron (1.46 Ga) and Re-Os analyses of molybdenite (1.44 Ga) yield similar ages. Previous ⁴⁰Ar/³⁹Ar analyses of muscovite and biotite from the Pea Ridge IOA deposit showed that K-bearing alteration minerals can yield meaningful ages (1.47 Ga). Therefore, K-bearing alteration and vein minerals from several other IOA deposits and the IOCG deposit were analyzed at the USGS Denver Argon Geochronology Laboratory and the results compared to previous U-Pb ages.

Abundant actinolite and phlogopite and minor biotite and muscovite from IOA deposits (Pea Ridge, Bourbon, Lower Pilot Knob, and Kratz Spring) yield ⁴⁰Ar/³⁹Ar plateau ages of 1.51-1.46 Ga that are the same or older than previous U-Pb dates on apatite, monazite, and xenotime. Barren amphibole veins from the Boss IOCG-Co deposit are also dated at ca. 1.46 Ga.

Abundant muscovite and biotite alteration at the Boss IOCG-Co deposit yield ⁴⁰Ar/³⁹Ar plateau ages of 1.36-1.31 Ga. A small patch of muscovite alteration associated with fluorite at the Pea Ridge IOA deposit records a ca. 1.28 Ga hydrothermal overprint.

Together, previous and new ⁴⁰Ar/³⁹Ar analyses of K-bearing minerals record two periods of hydrothermal activity that correspond to each pulse of bimodal magmatism in the iron province. IOA-REE deposits and the REE breccia pipe are associated with the older 1.4 Ga pulse and the IOCG-Co deposit is associated with the younger 1.3 Ga pulse.