Chemical reaction path modeling of ore deposition in mississippi valley-type Pb-Zn deposits of the Ozark region, U.S. Midcontinent

Abstract

The Ozark region of the U.S. midcontinent is host to a number of Mississippi Valley-type districts, including the world-class Viburnum Trend, Old Lead Belt, and Tri-State districts and the smaller Southeast Missouri barite, Northern Arkm•sas, and Central Missouri districts.There is increasing evidence that the Ozark Mississippi Valley-type districts formed locally within a large, interconnected hydrothermal system that also produced broad fringing areas of trace mineralization, extensive subtle hydrothermal alteration, broad thermal anomalies, and regional deposition of hydrothermal dolomite cement. The fluid drive was provided by gravity flow accompanying uplift of foreland thrust belts during the Late Pennsylvanian to Early Permian Ouaehita orogeny.In this study, we use chemical speeiation and reaction path calculations, based on quantitative chemical analyses of fluid inclusions, to constrain likely hydrothermal brine compositions and to determine which precipitation mechanisms are consistent with the hydrothermal mineral assemblages observed regionally and locally within each Mississippi Valley-type distriet in the Ozark region. Deposition of the regional hydrothermal dolomite cement with trace sulfides likely occurred in response to near-isothermal effervescence of COa from basinai brines as they migrated to shallower crustal levels and lower confining pressures. In contrast, our calculations indicate that no one depositional process can reproduce the mineral assemblages and proportions of minerals observed in each Ozark ore district; rather, individual districts require spedfie depositional mechanisms that reflect the local host-rock composition, 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-Ordovieian dolostone aquifers into shallower carbonate sequences dominated by limestones. In the Northern Arkansas district, jasperoid preferentially replaced limestones in the mixed dolostone-limestone sedimentary packages. Modeling results indicate that the ore and alteration assemblages in the Tri-State and Northern Arkansas districts resulted from the flow of initially dolomite-saturated brines into cooler limestones. Adjacent to fluid conduits where water/rock ratios were the highest, the limestone was replaced by dolomite. As the fluids moved outward into cooler limestone, jasperoid and sulfide replaced limestone. Isothermal boiling of the ore fluids may have produced openspace filling of hydrothermal dolomite with minor sulfides in breeeia and fault zones. Local mixing of the regional brine with locally derived sulfur undoubtedly played a role in the development of sulfide-rich ore runs. Sulfide ores of the Central Missouri district are largely open-space filling of sphalerite plus minor galena in dolostone karst features localized along a broad antidine. Hydrothermal solution collapse during ore deposition was a minor process, indicating dolomite was slightly undersaturated during ore dep...

Characterization of gas chemistry and noble-gas isotope ratios of inclusion fluids in magmatic-hydrothermal and magmatic-steam alunite

“…Magmatic-steam fluids are SO 2 -dominant, with SO 2 JH 2 S (average SO 2 / H 2 Sc202), whereas magmatic-hydrothermal fluids have SO 2 VH 2 S (average SO 2 /H 2 Sc0.7). Concentrations of sulfur species of both fluids are low (tens to hundreds of ppm V), but are typical of the results obtained from studies of fluid-inclusion gases in other deposits (Landis and Rye, 1989;Goldfarb et al, 1989;Hofstra et al, 1991;Landis and Hofstra, 1991;Plumlee et al, 1994). Magmatic-steam fluids are enriched in HCl, but contain less HF relative to magmatic-hydrothermal fluids.…”

Stable isotope studies of paleokarst-hosted uranium deposits in China.

“…The regional tectonism af fected the vast region of South China. Therefore, the regional tectonic events are the most likely driving mechanism for large-scale fluid focusing (Plumlee et al, 1994), leading to formation of not only the paleokarst-hosted uranium deposits men tioned here, but also other types such as granite hosted, sandstone-hosted and volcanics-hosted uranium deposits in South China.…”

Legacy Lead from Past Mining Activity and Gasoline Additives: Evidence from Lead Isotopes and Trace Element Geochemical Studies in the White River Basin, Southern Ozark Region, USA

“…Other mineralized strata are the Mississippian St. Joe (mainly limestone) and the Lower Ordovician Powell and Cotter dolomites [21,38]. The host rocks have been altered to hydrothermal dolomite and jasperoid, as in the Tri-State district [39]. Mineralization developed along faults and occurs as open space filling in solution collapse breccias and as replacement zones bordering the faults and breccia zones [21,38].…”