Mineralogy and the question of zoning, northwestern Illinois zinc-lead district

A detailed study of the mineralogy and paragenesis of the Zn-Pb ores of northwestern Illinois reveals that conditions of ore deposition were the same in the stratigraphically higher "crevice" deposits as they were in the deeper "flat-and-pitch" deposits. Except for an increase of galena relative to sphalerite in the crevice deposits, the minerals, their habits, and their relation to one another are the same in the 2 classes of deposits. In addition, spectrographic analyses show no differences in trace element content between samples of the same mineral from the different stratigraphic horizons. It is proposed that the 2 classes of deposits do not represent zones of different conditions of deposition. Two possible causes of the increase of galena in the stratigraphically higher deposits are discussed.

This record provided courtesy of AGI/GeoRef.