Abstract

Increased understanding of carbonate rocks, solution chemistry, lead and sulfur isotopes, and fluid inclusions has led to near consensus that the southeast Missouri ores occur where brines moving up from the Lamotte Sandstone encountered sulfide produced locally in the Bonneterre Formation.The early belief that the ores were deposited by descending or artesian ground water was disproved by the discovery of brines in fluid inclusions. The more recently held concept that the ore fluids originated in an unknown magma is incompatible with the lead isotope data. There were never adherents of syngenesis among those familiar with the deposits because abundantly exposed geometry clearly indicates otherwise.The following postulates an early diagenetic model, specific to the southeast Missouri district, based largely on temporal and spatial relationships and chemical considerations:(1) Metal-rich algal limestones were deposited in the back reef zone.(2) An "evaporation pan" in the back reef zone produced magnesian brines which percolated downward and seaward, in late Bonneterre time, dolomitizing the back reef and reef facies (seepage reflux).(3) Metals were released from limestones by dolomitization, and the dolomitizing fluid became a metal-bearing brine which followed permeable strata to the edge of the bank facies and followed vertical permeability (i.e., breccias) to the top of the sediment pile.(4) Sulfide (in sour gas?) moved updip in the Lamotte sand under poorly sorted Bonneterre to the edge of the bank facies where it could permeate the overlying carbonate sands or, alternatively, was produced locally by sulfate reduction.(5) Metals were precipitated from brines as sulfides.

First Page Preview

First page PDF preview
You do not currently have access to this article.