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.