Paleozoic strata of the Midwest host (1) Mississippi Valley-type (MVT) ore deposits and fossil fuel resources which are localized in discrete mining districts and hydrocarbon fields, (2) widespread country-rock occurrences of ~60-1500C fluid inclusions containing brines and hydrocarbons, and (3) extremely metal-rich black shales which occur in Mississippian strata but which are especially prevalent in overlying Pennsylvanian cyclothems throughout the Midwest.
In the past, the lack of key petrographic information and the
paucity of absolute dates for some of the most important deposits,
the MVT ores, has hampered the development of a coherent hypothesis
relating these phenomena. The discovery of additional occurrences
of hydrothermal minerals and the availability of key radiometric
dates has led to the development of a genetic model which begins
with the early introduction of certain metal values, such as Zn
and Ba, 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, likely took place at a later time. Consequent plugging
of numerous minor 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.