REVIEW OF THE IRON OXIDE DEPOSITS OF MISSOURI—MAGMATIC END MEMBERS OF THE IRON OXIDE-CU-AU-U-REE DEPOSIT FAMILY

The 1.47 Ga igneous terrane of the St. Francois Mountains of southeastern Missouri hosts several iron-oxide+/-apatite+/-Cu-Au-REE deposits. The terrane is made up of high-silica rhyolitic pyroclastic flows and associated sediments, cogenetic granite massifs, and high-iron trachytic to andesitic flows and ring dikes. Highly evolved late-stage granite intruded some source calderas as did late basaltic dikes. The iron-oxide deposits are temporally equivalent to their host volcanic rocks. Trace element modeling and isotopic studies suggest that the deposits are genetically related to the host igneous terrane. Partial melting of the lower crust, coupled with crystal fractionation, can account for formation of the early volcanic rocks and related plutons. The ore-forming sources ranged from iron-rich “magmas” to magmatic-hydrothermal fluids that were probably derived from immiscible liquids related to the iron-rich trachytes.

Textural and mineralogical evidence supports a conceptual framework in which the iron-oxide deposits were emplaced at various crustal depths. The deeper hypothermal deposits (Pea Ridge, Kratz Springs, lower Pilot Knob, Iron Mountain) are dominantly cored by magnetite-apatite ores with outer zones of hematite and amphibole skarns. Textural evidence supports both a magmatic and hydrothermal replacement origin for various parts of the deposits. Later magmatic-hydrothermal REE-Au-barite-rich breccia pipes cut the Pea Ridge deposit. Such pipes may have been present in other hypothermal deposits that are now currently inaccessible. Midlevel deposits are represented by magnetite vein deposits (Shepard Mountain) and mesothermal magnetite- and hematite-volcanic breccia deposits (Boss-Bixby, Bourbon, Camels Hump). The Boss-Bixby deposit has the greatest known base-metal content at ~ 40 mt of 0.8 percent copper. The shallower epithermal deposits (Upper Pilot Knob) are hematite-rich exhalative deposits with some associated hematite-volcanic breccias, locally contain anomalous gold abundances, and are intercalated with intracaldera sediments that were deposited during the waning stages of regional volcanism.