CHEMICAL-REACTION PATH MODELING OF ORE DEPOSITION IN MISSISSIPPI VALLEY-TYPE PB-ZN DEPOSITS OF THE OZARK REGION UNITED-STATES MIDCONTINENT

The Ozark region of the U.S. midcontinent is host to a number of Missi ssippi Valley-type districts, including the world-class Viburnum Trend , Old Lead Belt, and Tri-State districts and the smaller Southeast Mis souri barite, Northern Arkansas, and Central Missouri districts. There is increasing evidence that the Ozark Mississippi Valley-type distric ts formed locally within a large, interconnected hydrothermal system t hat also produced broad fringing areas of trace mineralization, extens ive subtle hydrothermal alteration, broad thermal anomalies, and regio nal deposition of hydrothermal dolomite cement. The fluid drive was pr ovided by gravity flow accompanying uplift of foreland thrust belts du ring the Late Pennsylvanian to Early Permian Ouachita orogeny. In this study, we use chemical speciation and reaction path calculations, bas ed on quantitative chemical analyses of fluid inclusions, to constrain likely hydrothermal brine compositions and to determine which precipi tation mechanisms are consistent with the hydrothermal mineral assembl ages observed regionally and locally within each Mississippi Valley-ty pe district in the Ozark region. Deposition of the regional hydrotherm al dolomite cement with trace sulfides likely occurred in response to near-isothermal effervescence of CO2 from basinal brines as they migra ted to shallower crustal levels and lower confining pressures. In cont rast, our calculations indicate that no one depositional process can r eproduce the mineral assemblages and proportions of minerals observed in each Ozark ore district; rather, individual districts require speci fic depositional mechanisms that reflect the local host-rock compositi on, structural setting, and hydrology. Both the Northern Arkansas and Tri-State districts are localized by normal faults that likely allowed brines to rise from deeper Cambrian-Ordovician dolostone aquifers int o shallower carbonate sequences dominated by limestones. In the Northe rn Arkansas district, jasperoid preferentially replaced limestones in the mixed dolostone-limestone sedimentary packages. Modeling results i ndicate that the ore and alteration assemblages in the Tri-State and N orthern Arkansas districts resulted from the flow of initially dolomit e-saturated brines into cooler limestones. Adjacent to fluid conduits where water/rock ratios were the highest, the limestone was replaced b y dolomite. As the fluids moved outward into cooler limestone, jaspero id and sulfide replaced limestone. Isothermal boiling of the ore fluid s may have produced open-space filling of hydrothermal dolomite with m inor sulfides in breccia and fault zones. Local mixing of the regional brine with locally derived sulfur undoubtedly played a role in the de velopment of sulfide-rich ore runs. Sulfide ores of the Central Missou ri district are largely open-space filling of sphalerite plus minor ga lena in dolostone karst features localized along a broad anticline. Hy drothermal solution collapse during ore deposition was a minor process , indicating dolomite was slightly undersaturated during ore depositio n. No silicification and only minor hydrothermal dolomite is present i n the ore deposits. The reaction path that best explains the features of the Central Missouri sulfide deposits is the near-isothermal mixing of two dolomite-saturated fluids with different H2S and metal content s. Paleokarst features may have allowed the regional brine to rise str atigraphically and mix with locally derived, H2S-rich fluids.