GENESIS OF STRATABOUND ORE-DEPOSITS IN THE MIDCONTINENT BASINS OF NORTH-AMERICA .1. THE ROLE OF REGIONAL GROUNDWATER-FLOW

Steady and transient flow models for regional brine migration have bee n constructed for quantifying the role of groundwater in the genesis o f carbonate-hosted lead-zinc ore deposits in the U.S. Midcontinent reg ion. Earlier theoretical studies suggested that ores of the Mississipp i 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 Midcontin ent region, but it was the tectonic uplift after the Alleghanian oroge ny of Late Pennsylvanian time that created the topography necessary fo r driving brines out of the basins and onto the adjacent domes where t he ore deposits formed. A typical paleohydrologic reconstruction exten ding across the Arkoma basin and onto the Ozark dome shows that Cambri an-Ordovician strata acted as regional aquifers in focusing metal-bear ing 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-in duced flow suggest a minor role for sediment compaction and the ''sque egee'' effect in ore formation, especially for the huge ore districts far-removed from the orogenic belts. Ore mineralization associated wit h topography-driven flow occurred in less than a few million years at temperatures between 80-degrees and 130-degrees-C in broad discharge a reas in southeast Missouri, although much warmer thermal transients ma y have lasted for about 100,000 yrs. Geothermal gradients in discharge areas were strongly elevated by regional flow associated with forelan d uplift, yet lateral temperatures gradients are predicted to have bee n very small in the platform aquifers. Other hydrogeologic simulations predicted similar broad discharge areas in southern Wisconsin and sou thern Illinois with transient temperatures of ore formation between 15 0-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 ma ximum first in the northeast and then migrated south, culminating with subaerial exposure of the Ouachita fold belt and Arkoma platform. Bas ed 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 di scharge pathway for brines driven to the west out of the Appalachian f oredeep and Illinois sag as illustrated in two of the simulation model s. A similar scenario probably applied for the ore districts in Tennes see. Later uplift in the southern Appalachians drove brines northweste rly out of the Black Warrior Basin and into southeast Missouri, perhap s adding another chemical signature to ore formation in the Old Lead B elt. Mineralization in the Upper Mississippi Valley District is most l ikely to have originated through the migration of brines out of the Ap palachian foredeep and across part of the Illinois sag as a direct res ult of uplift of the Appalachian Mountains rather than later by uplift of the Pascola Arch in southern Illinois (Bethke, 1986). Uplift of th e Ouachita Mountains and foreland platform resulted in the massive mig ration of brines to the north and in part to the northeast. Ores in th e Tri-State District, Northern Arkansas, Viburnum Trend, and Central M issouri record this hydrologic system of which there is little dispute . Deep brines also would have moved easily along the axis of the Reelf oot Rift under a gravity-drive to form the fluorite deposits in southe rn Illinois. Ore genesis waned in the earliest Mesozoic as erosion dis sipated the topography-driven flow systems. Emergence of the Rocky Mou ntains in the Tertiary resulted in easterly brine migration across the Denver and Forest City basins, but this flow system was too weak in M issouri to play a role in ore formation.