Sedimentary facies and variation of stable isotope composition of Upper Cambrian to Lower Ordovician strata in Southern Missouri: Implications for the origin of MVT deposits, and the geochemical and hydrological features of regional ore-forming fluids

Upper Cambrian and Lower Ordovician 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 Precambrian 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 little lateral variation 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 Cambro-Ordovician 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 Pb-Zn mineralization, have a relatively complex, four zone CL pattern; (2) dolomite cements, in the Bonneterre and Davis Formations, which are not related spatially to mineralization commonly display less complex CL patterns; (3) dolomite cements in the post-Davis part of the Cambrian and in the lower Ordovician section (upper aquifer) display a CL stratigraphy which appears to be 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, δ13C values become higher (from -2.5 toward +3.0%). Above the Davis Formation through lower Ordovician strata, the trend reverses and δ13C values become lower (toward -3.0%o). A similar trend exists for δ18O values in the Bonneterre and Davis Formations, as values become higher up section. However, above the Davis Formation, δ18O values for host dolomites display no statistical trends. The trend of upwardly decreasing δ13C values in post-Davis rocks may be the result of a secular trend in ocean carbon during this time. The trend of upwardly increasing δ13C and δ18O 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 12C- and 16O-enriched fluids moved higher in the section. Distribution of sedimentary facies had a profound effect on the hydrological framework of southern Missouri during the period of MVT Pb-Zn mineralization. Linear facies belts that developed on high-relief topography during Bonneterre and Davis time 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 alteration of the host dolomite. The distinct C and O isotopic trend observed in the Bonneterre-Davis Formations versus that observed in post-Davis rocks, coupled with differences in CL microstratigraphies of dolomite cements, indicate that these two parts of the section acted as distinct aquifers, with relatively little fluid communication during the Pb-Zn mineralizing event --Abstract, pages iii-iv