Abstract

Mississippi Valley-type deposits of the Upper Mississippi Valley mineral district probably formed during a period of regional ground-water flow across the Illinois basin initiated by uplift of the Pascola arch in post-Early Permian and pre-Late Cretaceous time. Numerical modeling of this inferred paleohydrologic regime shows that district temperatures attained by this process depend on flow rates through the basin, heat flow along flow paths, and presence of structures to cause convergence and upwelling of fluids. Predicted flow rates and timing of mineralization agree with previous estimates. Modeling results also offer explanations of banding in district mineralization and district silicification patterns. Modeling of ground-water flow due to sediment compaction during basin subsidence, however, shows that this process was not responsible for mineralization. Fluids displaced from the deep basin by compaction-driven flow moved too slowly to avoid conductive cooling to the surface before reaching the district. Episodic dewatering events are unlikely to have occurred, because the basin did not develop significant overpressures during subsidence. Results of the compaction-driven flow modeling probably also apply to the Michigan and Forest City basins. Study results suggest that exploration strategies for Mississippi Valley-type deposits should account for tectonic histories of basin margins distant from targets.

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