The 87 Sr/ 86 Sr ratio measurements in Mississippi Valley-type gangue minerals, and host marine carbonates ranging in age from upper Cambrian to Mississippian, show that in northwestern Ohio, where the mineralization is sparse, the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced. In contrast, most gangue minerals from districts of major mineralization in Illinois, Kentucky, Virginia, and Tennessee contain strontium with higher 87 Sr/ 86 Sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession. No evidence has been found for a magmatie component in the ore fluid strontium.Data from Elmwood, Tennessee, show that the early ore fluids, like those of northwestern Ohio, were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontium.These findings are consistent with the two-fluid mixing model for the Mississippi Valley mineralization, where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal-bearing and low sulfide oil field brines, with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession.

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