Abstract

The back reef facies of the Bonneterre and Davis Formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones. These subfacies were deposited on tidal flats and in shallow lagoons that existed in the St. Francois Mountains of southeast Missouri during late Cambrian time. The back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast Missouri Mississippi Valley-type (MVT) lead-zinc-copper ore deposits. Cryptalgalaminates are replaced by both fine crystalline, planar dolomite and coarse crystalline, nonplanar dolomite. Peloid mudstones are partially replaced, but mainly by coarse crystalline, nonplanar dolomite. Planar dolomite, replacing cryptalgalaminates, shows gradational transitions into coarse crystalline, nonplanar dolomite. The transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction. Planar dolomite is enriched in 18 O relative to stratigraphically equivalent marine limestones. Coarse crystalline, nonplanar dolomite, which replaces both cryptalgalaminates and peloid mudstones, has delta 13 C and delta 18 O values transitional between those of planar dolomite and late dolomite cements associated with MVT mineralization. Fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures (60 to 114 degrees C) by saline, basinal fluids. These data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism. Fine crystalline, planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition. Coarse crystalline, nonplanar dolomite was formed as a result of neomorphism of pro-existing planar dolomite and dolomitization of peloid mudstones, after burial, accompanying MVT mineralization in southeast Missouri.

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