VARIATIONS IN CL MICROSTRATIGRAPHY AND ISOTOPE COMPOSITIONS OF DOLOMITE CEMENTS IN THE WORLD-CLASS SOUTHEAST MISSOURI MVT PB-ZN-CU DISTRICT: EARLY FAULT-RELATED SYSTEMS VERSUS REGIONAL FLUID FLOW

Epigenetic dolomite cements with a distinctive cathodoluminescent (CL) microstratigraphy are associated with main stage Pb-Zn sulfide mineralization primarily in the upper Bonneterre Dolomite of the Viburnum Trend subdistrict. The regional extent of the dolomite CL stratigraphy points to a large-scale, basin-derived fluid flow system being responsible for the district’s Pb-Zn deposits. Recently discovered Zn-Cu-Co-Ni-rich ores above the Lamotte Sandstone-Bonneterre Dolomite contact reflect early, fault/fracture-control of ore fluids and are associated with two generations of dolomite cement whose CL patterns are distinct from and pre-date the regional dolomite cement. CL and stable isotope studies of these and other dolomite cements from subdistricts throughout southeast Missouri were undertaken to determine whether fault-related ores and their dolomite cements represent a single fluid that evolved chemically along different flow paths or instead reflect multiple, perhaps more localized fluid sources unrelated to regional fluid flow. Disparities in CL patterns of dolomite cements among ore subdistricts indicate mineralizing fluids utilized multiple, local- to regional-scale fault/fracture systems. Variability in δ¹⁸O and δ¹³C values of these dolomite cements likely reflects fluid-rock reactions along specific flow paths. In deep ores of the Viburnum Trend, decreasing δ¹⁸O values (from ~ -3 toward -8‰ VPDB) with increasing distance above the Lamotte-Bonneterre contact reflect progressive reaction of deeper, fault-related fluids with host dolomite formed in an isotopically distinct overlying flow system. In some subdistricts, dolomite cements with distinct CL patterns have similar δ¹⁸O values (~ -8‰), regardless of stratigraphic position or host carbonate facies. They may reflect a common fluid-dominated, regional flow system in which stratigraphically separated fluids experienced more local control of their chemistry and CL. The regional dolomite CL stratigraphy within the upper Bonneterre Dolomite likely resulted from a large-scale fluid flow system. However, CL patterns and stable isotope compositions of dolomite cements elsewhere in southeast Missouri, especially in stratigraphically lower rocks, require the presence of multiple, early fault-related ore-fluid systems.