MINERALOGY, TEXTURES AND ENVIRONMENTS OF DEPOSITION OF IRON DEPOSITS, ST. FRANCOIS IGNEOUS TERRANE, SOUTHEAST MISSOURI

The St. Francois Mountains igneous terrane of southeast MO hosts several major iron deposits which exhibit features indicating magmatic/hydrothermal origin. However, some of the minor iron deposits in the terrane have features which strongly indicate a sedimentary origin. The Cedar Hill and Russell Mountain deposits exhibit fine-grained, thinly laminated banded iron formation (BIF) hematite interbedded with oolitic hematite laminae and abundant red jasperoid. The oolitic nature of some of the laminations at Cedar Hill and Russell Mountain leads us to the conclusion that these bedded hematite ores are of undoubted sedimentary origin.

The Pilot Knob Hematite deposit (PKH) represents a fine-grained, laminated banded iron formation that exhibits abundant evidence of sedimentary depositional features including ripple marks, mud cracks, and continuous thin laminae. Though the ores at PKH lack oolites, the close similarity of the laminated BIF ores at PKH to the Cedar Hill and Russell Mountain ores leads us to the conclusion that PKH is also of sedimentary origin.

The College Hill deposit is a laminated BIF with considerable disturbed bedding. Also found in association are siltstones, some of which have been disturbed and redeposited as a flat-pebble sedimentary breccia cemented by specular hematite. Bedding disturbance at College Hill is probably the result of volcanic or tectonic processes.

The Hogan Mountain deposit consists of spectacular crustiform hematite coatings and open space fillings within a rhyolitic breccia, suggesting deposition in a vent system associated with hot spring activity.

The Shepherd Mountain and Shut-Ins deposits are composed of magnetite and hematite deposited in steep veins. They apparently represent a somewhat deeper look into the plumbing system than the surface deposits described above.

The Cedar Hill, Russell Mountain, and Pilot Knob Hematite BIF’s apparently were deposited in shallow, restricted basinal settings (caldera lakes?) that were fed by iron-rich waters from nearby hot springs. The Hogan Mountain deposit appears to represent one of the hot spring vents, while the Shepherd Mountain and Shut-Ins deposits apparently represent conduits from the developing magmatic/hydrothermal system below, feeding iron-rich waters to the hot springs and to the sedimentary basins.