THE CEDAR HILL DEPOSIT, IRON COUNTY, MO--A PROTEROZOIC OOLITIC SEDIMENTARY BANDED IRON FORMATION

At the Cedar Hill iron deposit north of Pilot Knob, MO, hematite ores are contained within rhyolite volcanic breccias of the Proterozoic St. Francois Mountains terrane. These hematite ores show excellent thin bedding and lamination. The ores are alternating laminations of fine grained hematite and oolitic hematite, both with interstitial quartz, i.e., the ores are siliceous taconite banded iron formations. The oolitic nature of some of the laminations at Cedar Hill leads us to the conclusion that the bedded hematite ores are of undoubted sedimentary origin and not of hydrothermal replacement origin. In addition to the interbedded fine grained and oolitic ores, some of the iron is present as coarse, crosscutting specular hematite, filling fractures and breccia interstices, and is apparently of late hydrothermal origin. Associated with this late specular hematite is abundant red jasperoid, perhaps the result of hot spring activity. The bedded ores are, in places, considerably disturbed as a result of volcanic or tectonic processes. Minor magnetite is present and some has been partly converted to hematite as martite. Minor pyrite is also present.

The laminated fine grained hematite ores at Cedar Hill are very similar to the ores at the nearby Pilot Knob Hematite Deposit. Several previous authors have suggested that the bedded ores at the Pilot Knob Hematite Deposit are of hydrothermal origin, having replaced fine grained tuffs. No oolites have been found at Pilot Knob Hematite, but the similarity of the Pilot Knob ores to the fine grained ores at Cedar Hill which are of undoubted sedimentary origin, strongly supports our past assertions that the Pilot Knob Hematite Deposit is of sedimentary origin and not of hydrothermal replacement origin.