CL MICROSTRATIGRAPHY AND STABLE ISOTOPE COMPOSITIONS OF DOLOMITE CEMENT IN THE ZINC-RICH WEST FORK MVT MINE, SOUTHEAST MISSOURI

Dolomite cement with a distinctive 4-zone cathodoluminescent (CL) microstratigraphy is associated with MVT ore sulfide mineralization in the upper Bonneterre Dolomite (Cambrian) of the Viburnum Trend district. Regional extent of the CL stratigraphy indicates that a large-scale fluid flow system was likely responsible for the district’s world-class Pb-Zn deposits. The West Fork mine is unusually zinc rich compared to other more lead-rich mines (Pb/Zn = 5:1) of the district. Previous studies of West Fork mine (Mavrogenes and Hagni, 1992) documented a distinct lateral zonation of ore metals/minerals that follows a general sequence, east to west, from iron- to zinc- to lead-rich. Microscopic and isotopic study of dolomite cement from West Fork and the adjacent Fletcher mine was undertaken to determine if cement CL stratigraphy in this anomalously Zn-rich area shows similar geochemical and temporal variations as cements associated with more typical Pb-rich ores of the district. In the West Fork-Fletcher area, dolomite cement CL stratigraphies are variations on the regional 4-zone pattern, however there are notable differences in CL that appear to be associated spatially with the distinct lateral metal zoning in these mines. The survival and textural appearance of specific CL cement zones appear to depend on the dominant type of associated metal sulfide. Zinc-rich areas are associated with a thicker zone 2 and resorbed textures, whereas lead-rich areas are associated with a thinner zone 2 and thicker zone 3b. Dissolution of dolomite cement and host rock associated with iron sulfide deposition that postdates early sphalerite precipitation and a later, lead-rich mineralizing event each appear to have contributed to the development of resorbed textures and localized partial or complete removal of CL zones 2 and 3a. These observations indicate that dolomite CL cement stratigraphy may have been the product of multiple, ore metal-specific mineralizing fluids whose influence varied spatially and temporally. Stable isotope compositions of dolomite and associated ore sulfide show distinct zoning across the orebodies and also may indicate that multiple, chemically distinct fluids (localized, fault-related and regional, basin-derived fluids) were integral to formation of the unusual zinc-rich ores of West Fork mine.