NA-CL-BR SYSTEMATICS OF MINERALIZING BRINES IN MISSISSIPPI VALLEY-TYPE DEPOSITS

New and published analyses of fluid-inclusion leachates from Mississip pi Valley-type (MVT) deposits can be divided into two groups on the ba sis of Na/Br and Cl/Br ratios. MVT leachates from the Illinois-Kentuck y 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-dissolut ion brines. MVT leachate from Polaris and octahedral galena in the Vib urnum Trend have Na/Br and Cl/Br ratios that plot below seawater and a long the compositional trend formed by evaporation. Solubility-volume constraints require that brines formed by seawater evaporation had hig h dissolved metal contents. Preliminary correlation of leachate compos itions allows delineation of two brine provinces in the midcontinent U nited States: (1) an early, high-Br province found only in southeaster n Missouri, and (2) a later, low-Br province of probable Permian age t hat extended from the Cincinnati arch to the Tri-State district. These observations, along with Na-Cl-Br data for modern brines from the Ill inois basin, argue against models for single-stage midcontinent MVT br ine how based on recharge from late Paleozoic (Arkoma) foreland basins to the south.