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ABSTRACT: The Illinois Basin as a Flow Path for Ore Fluids

Martin B. Goldhaber, E. Mosier, S. Church, H. Whitney, G. Gacetta, J. Eidel, K. Hackley

Three major Mississippi Valley-type Pb-Zn±F districts may be the result of fluid migration through the Illinois basin. To establish aquifers and flow vectors possibly associated with mineralizing fluids, we measured trace element and F abundances on acid insoluble residues in samples from 63 drill holes in the southern part of the basin and S and Pb isotopes for a subset of these samples.

Anomalously high amounts of F associated with barite and sphalerite are common in Ordovician and Mississippian strata of the basin, as well as in an area to the southwest of the basin in Ste. Genevieve and Perry counties, Missouri. Fluorine anomalies also extend north of the Illinois-Kentucky fluorspar district into Galatin County, Illinois. Previous studies report elevated Zn (>200 ppm) and Pb (>100 ppm) contents at several stratigraphic intervals, with elevated Pb contents predominant in Cambrian rocks and Zn relatively more abundant upsection.

A prominent Pb enrichment in the deepest part of the basin resides largely in FeS2. Similar Pb isotope data for these Pb-rich pyrites and for galena from the overlying fluorspar district suggests possible vertical transport of ore-forming fluids. This Pb is isotopically distinct from and was not involved in the formation of the southeast Missouri Pb belts. Sulfur isotope data suggest that isotopically heavy H2S (^dgr34S > 10^pmil), characteristic of thermochemical sulfate reduction has sulfidized parts of the Mt. Simon formation and lighter H2S, having small positive or negative ^dgr34S values, has sulfidized the overlying carbonate strata. We have not recognized a signature coincident with the upper Mississippi Valley ores at he north end of the basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990