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Dolomite cement crystals from the Viburnum Trend of the Southeast Missouri Lead District exhibit four regionally correlative cathodoluminescent zones. These zoned cements record major mineralizing events that altered rocks throughout the southeast Missouri region, covering an area greater than 30,000 km 2 and affecting Upper Cambrian to at least Lower Ordovician rocks. Major dissolution surfaces in the dolomite cement appear to correspond to an early period of karst development and later periods of solution brecciation, associated with Mississippi Valley-type sulfide mineralization.Combined cathodoluminescence and ore microscopic studies have determined the positions of the dolomite zones with respect to the known sulfide mineral paragenetic sequence. The earliest generation of dolomite cement (zone I) precipitated before the main generation of disseminated sphalerite. Zone 2 dolomite formed after emplacement of disseminated sphalerite but before cube-octahedral galena. Zone 3 dolomite precipitated during the interval between the deposition of cube-octahedral galena and prior to cubic galena. The last zone of dolomite cement (zone 4) formed after the deposition of the early vug-lining crystals of cubic pyrite and chalcopyrite, but prior to the deposition of cubic galena, marcasite, later chalcopyrite, and calcite.

Abstract. Previous studies of galena and sphalerite fromPaleozoic MVT deposits in the Viburnum Trend, southeast Missouri documented large variations in 634S values throughout the ore-forming event. The present study of Cu-Fe-sulfides reveals a similar 634S variation that reflects two end-member sulfur reservoirs whose relative importance varied both temporally and spatially. More 34S-enriched sulfides (634S approaching 25%0) indicate introduction of sulfur from basinal sedimentary sources, whereas more 32S-enriched sulfides (63~S < 5%0 ) may reflect fluids moving through underlying granitic basement. Two areas containing Precambrian, igneous-hosted FeCu mineralization in southeast Missouri (West and Central Domes of Boss-Bixby) were investigated to elucidate their relationship to Cu-rich MVT orebodies hosted nearby within the overlying Cambrian Bonneterre Dolomite. Mineralization at Boss-Bixby is composed of an early phase of iron oxide deposition followed by Cu-Fe-sulfides. The Central Dome is faulted and its mineralization is more fracture-controlled than the typically podiform ores of the West Dome. The 634S values of West Dome sulfides are 0.9 to 6.5%0 and pyrite-chalcopyrite indicate a temperature of 525~ 50~These data indicate an igneous source of sulfur during Precambrian ore deposition. In contrast, 634S values of Central Dome sulfides are 9.4 to 20.0%0 and pyrite-chalcopyrite indicate temperatures of 275 ~ + 50 ~ Similar 634S values are obtained for chalcopyrite from the overlying MVT deposits. We speculate that deeply circulating, basin-derived MVT fluids mobilized sulfur and copper from the underlying igneous basement and redeposited them in overlying Curich MVT orebodies, as well as overprinting earlier Precambrian sulfides of the Central Dome with a later, Paleozoic MVT sulfur isotope signature. Many models for MVT fluid circulation in the Midcontinent region of North America assume that igneous basement rocks are an impermeable boundary, but in southeast Missouri, evidence exists for structurally controlled MVT fluid movement > 600m vertically through underlying Precambrian igneous rocks. Such basement involvement has been suggested for other carbonate-hosted base-metal districts (e.g. Irish base metal deposits) and should be considered an integral part of the ore-forming process in southeast Missouri. Geological settingSoutheastern Missouri is part of the stable cratonic interior region of North America.

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