Abstract

Paleozoic strata of the Midwest host (1) Mississippi Valley-type (MVT) ore deposits and fossilfuel resources, which are localized in ore deposits, coal seams, and hydrocarbon fields; (2) widespread country-rock occurrences of -60-200°C fluid inclusions containing brines and hydrocarbons; and (3) extremely metal-rich black shales. The metalliferous shales occur in Mississippian strata but are especially prevalent in overlying Pennsylvanian cyclothems throughout the Midwest.

The paucity of petrographic information and absolute dates for MVT ores has hampered the development of a coherent hypothesis relating these phenomena. The discovery of additional occurrences of hydrothermal minerals and new dates have set the stage for refinement of a genetic model that begins with introduction of metals from migrating basinal brines to shales and surrounding rocks during sedimentation and early diagenesis. Substantial augmentation of metal values in permeable beds, including thin-jointed black shales, took place at a later time. Consequent plugging of numerous minor fluid passageways in carbonate and shale beds may have been a major factor in confining the subsequent development of major ore and hydrocarbon deposits within discrete locations.