Steady and transient flow models for regional brine migration have been constructed for quantifying the role of groundwater in the genesis of carbonate-hosted lead-zinc ore deposits in the U.S. Midcontinent region. Earlier theoretical studies suggested that ores of the Mississippi Valley type formed as deep groundwater was driven out of basins and onto platform margins by elevated topography (Garven and Freeze, 1984a, b). Several basins surround the major ore districts of the Midcontinent region, but it was the tectonic uplift after the Alleghanian orogeny of Late Pennsylvanian time that created the topography necessary for driving brines out of the basins and onto the adjacent domes where the ore deposits formed. A typical paleohydrologic reconstruction extending across the Arkoma basin and onto the Ozark dome shows that Cambrian-Ordovician strata acted as regional aquifers in focusing metal-bearing brines at Darcy flow rates of 1.0 to 5.0 m/yr in topography-driven flow systems. Numerical simulations of basin compaction and thrust-induced flow suggest a minor role for sediment compaction and the ''squeegee'' effect in ore formation, especially for the huge ore districts far-removed from the orogenic belts. Ore mineralization associated with topography-driven flow occurred in less than a few million years at temperatures between 80-degrees and 130-degrees-C in broad discharge areas in southeast Missouri, although much warmer thermal transients may have lasted for about 100,000 yrs. Geothermal gradients in discharge areas were strongly elevated by regional flow associated with foreland uplift, yet lateral temperatures gradients are predicted to have been very small in the platform aquifers. Other hydrogeologic simulations predicted similar broad discharge areas in southern Wisconsin and southern Illinois with transient temperatures of ore formation between 150-degrees and 220-degrees-C because of brine movement through the deep Illinois Basin and Reelfoot Rift, respectively. Alleghanian uplift of the Appalachians evolved such that paleo-relief probably reached a maximum first in the northeast and then migrated south, culminating with subaerial exposure of the Ouachita fold belt and Arkoma platform. Based on this tectonic interpretation, regional fluid migration pathways are likely to have varied considerably throughout the Late Paleozoic. For example, ores along the Old Lead Belt in Missouri may reflect a discharge pathway for brines driven to the west out of the Appalachian foredeep and Illinois sag as illustrated in two of the simulation models. A similar scenario probably applied for the ore districts in Tennessee. Later uplift in the southern Appalachians drove brines northwesterly out of the Black Warrior Basin and into southeast Missouri, perhaps adding another chemical signature to ore formation in the Old Lead Belt. Mineralization in the Upper Mississippi Valley District is most likely to have originated through the migration of brines out of the Appalachian foredeep and across part of the Illinois sag as a direct result of uplift of the Appalachian Mountains rather than later by uplift of the Pascola Arch in southern Illinois (Bethke, 1986). Uplift of the Ouachita Mountains and foreland platform resulted in the massive migration of brines to the north and in part to the northeast. Ores in the Tri-State District, Northern Arkansas, Viburnum Trend, and Central Missouri record this hydrologic system of which there is little dispute. Deep brines also would have moved easily along the axis of the Reelfoot Rift under a gravity-drive to form the fluorite deposits in southern Illinois. Ore genesis waned in the earliest Mesozoic as erosion dissipated the topography-driven flow systems. Emergence of the Rocky Mountains in the Tertiary resulted in easterly brine migration across the Denver and Forest City basins, but this flow system was too weak in Missouri to play a role in ore formation.
