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Abstract

Upper Cambrian and Lower Ordovician sedimentary rocks in southeastern Missouri host the world-class Mississippi Valley-type (MVT) lead-zinc deposits of the region. Sedimentary facies of the lower part of the Upper Cambrian section are dominated by distinct clastic and carbonate facies belts associated with a high relief Cambrian topography. The upper part of the Upper Cambrian (post Davis Formation) and the Lower Ordovician section were deposited under epeiric sea conditions on a low relief topography. These latter rocks are characterized by cyclic sequences of shallow-water platform carbonates, with good lateral continuity of facies. The Davis Formation is composed of interbedded carbonates and shales and forms an effective aquiclude separating the upper and lower parts of the section into two distinct aquifers.

Petrographic and cathodoluminescent studies of epigenetic dolomite cements in these Cambro-Ordovician sedimentary rocks document that: (1) dolomite cements of the Bonneterre Dolomite (lower aquifer) in the Viburnum Trend and the Old Lead Belt, which are related closely to mineralization, have a relatively complex, four zone cathodoluminescence (CL) pattern; (2) dolomite cements, in the Bonneterre and Davis Formations, which are not related spatially to mineralization commonly display less complex CL patterns; and (3) dolomite cements in the post- Davis part of the Cambrian and in the lower Ordovician section (upper aquifer) display a CL stratigraphy unrelated to that observed lower in the section.

Carbon isotope compositions of host dolomite show two types of statistical variation. From the bottom of the Bonneterre Dolomite to the top of the Davis Formation, 8I3C values become higher (from -2.5 toward +3.0%»). Above the Davis to the Lower Ordovician, the trend reverses and 8nC values become lower (toward — 3.0%o). A similar trend exists for 5I80 values in the Bonneterre and Davis, as values become higher up section. However, above the Davis Formation, 81S0 values for host dolomites display no statistical trends.

The pattern of upwardly decreasing 8I3C values in post-Davis rocks may be the result of a secular trend in ocean carbon. The trend of upwardly increasing S'3C and 8I80 values in the Bonneterre Dolomite and Davis Formation is likely the result of interaction with hydrothermal fluids emanating from the underlying Lamotte Sandstone, reflecting increased buffering by host dolomite as l2C- and l60-enriched light fluids moved higher in the section.

Distribution of sedimentary facies had a profound effect on the hydrological framework of southern Missouri during mineralization. Linear facies belts that developed on high-relief topography during deposition of the Bonneterre and Davis strata resulted in focused fluid flow and a greater degree of alteration of host dolomite. Broad, laterally continuous distribution of sedimentary facies in post-Davis rocks resulted in less focused fluid flow and less alteration of the host dolomite. The distinct C and O isotopic trend observed in the Bonneterre-Davis Formations versus that observed in post-Davis Formation rocks, coupled with differences in CL microstratigraphies of dolomite cements, indicate that the these two parts of the section acted as distinct aquifers, with relatively little fluid communication during mineralization.

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