Abstract

Two distinct varieties of Mississippi Valley-type ore occur in the northern Arkansas zinc-lead district: (1) open-space fillings in solution collapse breccia, and (2) strata-bound replacement deposits generally conformable with their carbonate host rocks. The relationship between these two types of ore is unusually well exposed in the Monte Cristo mine, as is the relationship between these ores and their host rocks. Primary pore spaces in the micritic and stromatolitic limestones and dolostones (Ordovician Everton Formation) were largely occluded by cementation during diagenesis. Dissolution of limestone, sometime after the Early Mississippian period, locally produced solution collapse breccias composed chiefly of dolostone fragments in a quartz sand and dolomite matrix. After lithification of this matrix, further fracturing opened channels later used by the ore fluid.Mineralization (deposition of sphalerite and pink sparry dolomite) was preceded and/or accompanied by silicification of limestone in the bedded orebodies and the outer margin of the breccias. Fluid inclusion and stable isotope (C and O) data suggest a mixing of two brines during mineralization, both of probable deep burial origin. The mixtures were hot (90 degrees - 150 degrees C) and highly saline (19-25 equiv. wt % NaCl) and were displaced by cool, dilute, presumably meteoric waters during deposition of late gangue quartz and calcite. It is noteworthy that, in the Monte Cristo mine, the bedded and breccia mineralizations are physically and mineralogically continuous and yield indistinguishable fluid inclusion, stable isotope, and rare earth element signatures. Hence, the bedded ores, which possess many characteristics of Mississippi Valley-type deposits attributed to a syngenetic or diagenetic origin elsewhere in the world, formed simultaneously with the discordant breccia ore and are, at least in northern Arkansas, epigenetic in origin.

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