Residual barite deposits of the Southeast Missouri barite district were derived from the weathering of lower Paleozoic dolostones, chiefly the Upper Cambrian Potosi and Eminence Dolomites. Open-space, Mississippi Valley-type mineralization in the bedrock was controlled by the intersection of permeable lithologies with late Paleozoic near-vertical faults or linear zones of intense jointing. Linear trends of residual deposits are the surface expression of these bed-rock controls. A consistent mineralogical and barite textural zonation is disposed about the central structures. Sulfide mineralization is concentrated about the central structures, whereas barite is more widespread. Barite textures vary systematically from coarse bladed near the central structures to fine plumose more distally. Such zoning indicates outward ore fluid flow from the central faults and joint zones.Fluid inclusions in sphalerite indicate that warm Na-Ca-Cl brines (69 degrees -105 degrees C, 20-25.5 equiv wt % NaCl) were present during early mineralization. Salinity data for fluid inclusions in barite (0-14 equiv wt % NaCl) may be representative of the barite mineralizing fluid, but contamination by later fluids is possible. Sulfur isotope studies show that the paragenetically early sulfides are isotopically lighter than later barite. Individual minerals involved in mainstage mineralization have the following ranges of delta 34 S values: pyrite-marcasite, 8.4 to 34.7 per mil; galena, 3.2 to 11.8 per mil; sphalerite, 7.2 to 15.8 per mil; barite, 20.5 to 32.2 per mil. Barite delta 34 S values show enrichment in 34 S away from the central fault. This spatial distribution is due to the incorporation of sulfate from two reservoirs. Isotopically light sulfate, developed most strongly about the central fault, originated from the oxidation of H 2 S in the ore fluid emanating from the central fault. The heavy sulfate that prevailed distally was indigenous to the host dolostone formation water. The delta 18 O values of main-stage barite vary from 17.0 to 19.6 per mil and the range is attributed to mixing of two fluids of different oxygen isotope composition and to variable isotopic fractionation between sulfate and water due to changing temperature. The light sulfur and oxygen isotope composition of supergene barite (delta 34 S = 15.8ppm, delta 18 O = 9.0ppm) is due to incorporation of isotopically light sulfate produced locally during the oxidation of sulfide minerals in the soil environment.Barite precipitation was caused by fluid mixing, in which a Ba-Pb-Zn-H 2 S-bearing brine discharged from faults and joints into lower Paleozoic dolostones and mixed with an oxidizing, sulfate-bearing, dilute fluid. Increased supersaturation of barite with progressive mixing caused the systematic spatial variation of barite textures from coarse bladed near the central structures to fine plumose more distally. Similar barite-sulfide mineralization in the Central Missouri barite district probably formed in the same manner. Barite mineralization across the north flank of the Ozark dome postdates local Appalachian-Ouachita-Marathon orogenic deformation and suggests the existence of dilute sulfate-bearing water, probably shallow meteoric ground water, in the Paleozoic section during barite mineralization. Uplift of the Ouachitas in late Paleozoic time apparently initiated Mississippi Valley-type mineralization of the Ozark region.

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