Abstract

New and published analyses of fluid-inclusion leachates from Mississippi Valley–type (MVT) deposits can be divided into two groups on the basis of Na/Br and Cl/Br ratios. MVT leachates from the Illinois-Kentucky and Cincinnati arch districts and from cubic galena in the Viburnum Trend have Na/Br and Cl/Br ratios that extend to values significantly above that of seawater, which are characteristic of evaporite-dissolution brines. MVT leachates from Polaris and octahedral galena in the Viburnum Trend have Na/Br and Cl/Br ratios that plot below seawater and along the compositional trend formed by evaporation. Solubility-volume constraints require that brines formed by seawater evaporation had high dissolved metal contents. Preliminary correlation of leachate compositions allows delineation of two brine provinces in the midcontinent United States: (1) an early, high-Br province found only in southeastern Missouri, and (2) a later, low-Br province of probable Permian age that extended from the Cincinnati arch to the Tri-State district. These observations, along with Na-Cl-Br data for modern brines from the Illinois basin, argue against models for single-stage midcontinent MVT brine flow based on recharge from late Paleozoic (Arkoma) foreland basins to the south.

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