“Olympic Dam-Type” Deposits and Geology of Middle Proterozoic Rocks in the St. Francois Mountains Terrane, Missouri

Abstract The midcontinent of the United States hosts some of the world's largest, economically most valuable mineral districts and metallogenic provinces. The Precambrian iron deposits of the Lake Superior region include both Algoma- and Lake Superior-type Fe and Fe-Mn ores. Native cu and Cu-Ni sulfides occur in the mafic igneous rocks of the Keweenawan rift, and in the Late Precambrian Nonesuch Shale. The largest Au producer in the Western Hemisphere, the Homestake Mine in the Black Hills, yields its gold from cummingtonite schist, a metamorphosed iron-carbonate formation. (The most famous ores of this region, however, are not Precambrian; they are the Mississippi-Valley type Pb, Zn, fluorite, and barite deposits in the Paleozoic platform carbonates.) The Middle Proterozoic Fe-cu-REE metallogenic province of the st. Francois terrane, although not as large as the aforementioned districts, is a unique and interesting metal province and has the potential for new discoveries. The ores may be variants of the Kiruna- and Olympic Dam-type class of deposits. The St. Francois Mountains constitute the exposed part of an extensive anorogenic terrane of granite ring complexes and associated rhyolites that underlie most of southeastern Missouri. This igneous terrane is characterized by the predominance of silicic over mafic rocks and by alkalic-intermediate rocks (trachytes), and is not metamorphosed. Its distinctive ore deposits include volcanic-hosted magnetite-hematite-apatite (e.g., Pea Ridge, Pilot Knob); hypo-xenothermal vein deposits of W, Ag, and Pb (Silver Mine district); and vein and replacement deposits of Mn. The Precambrian terrane of the St. Francois Mountains has been deeply eroded and dissected, resulting in a rugged topography and the unroofing of granite. Upper Cambrian marine sedimentary rocks are in nonconformable contact with the underlying igneous rocks. Near the crest of the Ozark dome, the dominant structural feature, the Precambrian outcrops of the ;St. Francois Mountains represent a structural and topographic high. The granite ring complexes correspond to the deeply eroded roots of a formerly more extensive volcanic terrane comprising several calderas, cauldron subsidence structures, ring intrusions, and resurgent cauldrons with central plutons (Kisvarsanyi, 1981). The outcrops of the Precambrian rocks, the network of major roads in the St. Francois Mountains, and the scheduled field trip stops are shown in Fig. 1. The Precambrian rock units compiled by Pratt et al. (1979) and Kisvarsanyi (1981) are shown in Table 1. The volcanic superstructure of the st. Francois terrane has been largely removed by pre-Paleozoic erosion, but as much