Lead isotopes from the Upper Mississippi Valley district: A regional perspective
Bulletin
2094-B
By: Timothy M. Millen, Robert E. Zartman, and Allen Van Heyl
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Abstract
New lead isotopic data on galena from within and peripheral to the Upper Mississippi Valley lead-zinc district make it possible, by extending coverage to outlying locations, to trace the pathway traversed by the mineralizing fluids beyond the boundary of the main district. All but one of the samples exhibit elevated ratios of the radiogenic isotopes typical of the Upper Mississippi Valley ore deposits; 206PbP04Pb ranges from 19.38 to 24.46, 207PbP04Pb ranges from 15.73 to 16.24, and 208PbP04Pb ranges from 39.24 to 43.69. Galena from the Pints quarry near Waterloo, Iowa, has distinctly lower values of these ratios and may not be related paragenetically to the other samples. Otherwise, the lowest ratios are for samples in the southern part of the region in north-central Illinois, and the highest ratios are for samples to the northeast of the main district in the vicinity of Madison, Wisconsin. Thus, an isotopic pattern rather similar to that observed originally by Heyl and others (1966) prevails regionally, although the predominant fluid flow is now believed to have emanated from the Illinois Basin rather than from the Forest City Basin. Metal-bearing brines being driven northward out of the Illinois Basin probably played the key role in mineralization of the Upper Mississippi Valley district.
Both the new and the previously reported lead ratios for the Upper Mississippi Valley district are plotted on 207PbP04Pb and Pb208/pb204Pb versus 206PbP04Pb diagrams, which permit their comparison and the calculation of refined slopes for the expanded data set. A two-stage model age for the time of mineralization can be determined from the 207PbP04Pb_Pb206/Pb204 slope, provided that the source age of the lead is known. With our limited know ledge of this source age, the time of mineralization cannot be tightly constrained but is permissive of a Permian or younger lateral secretion event, as suggested by other geochronological results.
Suggested Citation
Millen, T.M., Zartman, R.E., and Heyl, A.V., 1995, Lead isotopes from the Upper Mississippi Valley district: A regional perspective: U.S. Geological Survey Bulletin 2094, iii, 13 p., https://doi.org/10.3133/b2094B.
Study Area
| Publication type | Report |
|---|---|
| Publication Subtype | USGS Numbered Series |
| Title | Lead isotopes from the Upper Mississippi Valley district: A regional perspective |
| Series title | Bulletin |
| Series number | 2094 |
| Chapter | B |
| DOI | 10.3133/b2094B |
| Year Published | 1995 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Contributing office(s) | U.S. Geological Survey |
| Description | iii, 13 p. |
| Larger Work Type | Report |
| Larger Work Subtype | USGS Numbered Series |
| Larger Work Title | Evolution of sedimentary basins - Illinois Basin |
| Country | United States |
| State | Illinois, Iowa, Wisconsin |