SEDIMENTARY IRON DEPOSITS WITHIN THE ST. FRANCOIS MOUNTAINS MAGMATIC-HYDROTHERMAL IRON DISTRICT, SOUTHEAST MISSOURI

The Middle Proterozoic igneous terrane known as the St. Francois Mountains in SE MO constitutes an iron metallogenic province and hosts eight known major and numerous minor magnetite and hematite deposits. Considerable debate yet exists surrounding the genesis of these deposits and their relationship to the host rocks and each other. Hydrothermal and magmatic intrusive origin has been proposed for individual deposits as well as for the entire district. However, ore textural evidence clearly indicates that some of these deposits formed in sedimentary environments. The Pilot Knob hematite deposit represents a fine-grained, laminated banded iron formation that exhibits abundant evidence of sedimentary depositional features including ripple marks, mud cracks, rain drop imprints, and continuous thin laminae. The Cedar Hill, Russell Mountain, and College Hill deposits all exhibit thinly laminated banded iron formation interbedded with red jasperoid laminae. These deposits also reflect unique characteristics of the depositional settings in which they formed. Russell Mountain and Cedar Hill contain locally abundant oolitic hematite laminae. The Russell Mountain deposit also displays spectacular channel and scour bedding features within the banded iron formation. The College Hill deposit locally exhibits considerable disturbed bedding of the banded iron formation and associated host materials. Some of these host materials include siltstones, which have been brecciated and redeposited as a flat-pebble sedimentary breccia cemented by specular hematite. Disturbed bedding within these deposits is perhaps the result of nearby volcanic or tectonic processes. The Hogan Mountain deposit consists of stunning crustiform hematite coatings and open space fillings within a rhyolitic breccia, suggesting deposition in a shallow vent system associated with hot spring activity. Each of these deposits was apparently deposited in shallow, restricted basinal settings that were fed by iron-rich solutions from nearby hot springs. The Hogan Mountain deposit represents just one of many vents which fed into these shallow sedimentary basins. These deposits probably represent surficial expressions of the same plumbing system that produced the deeper magmatic/hydrothermal iron deposits within the terrane.