The Upper Mississippi Valley zinc-lead district covers 2, 500 square miles of Wisconsin, Illinois, and Iowa. It is one of the oldest mining districts in the United States, as lead mining by settlers began in 1788. Zinc has been mined since 1859, and the present production is more than ten times that of lead.
Recks exposed in the district range from Early Ordovician to Middle Silurian in age, and, except for the St. Peter sandstone and the Maquoketa shale, consist mainly of dolomite. Structural compression has resulted in gentle folds and faults; principal trends are northwestward, northeastward, and eastward.
Galena (lead sulfide) has been mined principally from vertical joints in the upper, noncherty part of the Galena dolomite. On the other hand, sphalerite (zinc sulfide) and a minor amount of smithsonite (zinc carbonate) are found as veins, breccia ore, and disseminations in the lower, cherty part of the Galena dolomite, in strata of the Decorah formation, and in the upper beds of the Platteville formation; these 'lower-run' ore bodies are in inclined reverse faults (pitches) and associated bedding-plane faults (flats) that are localized along synclinal trends. Lead and zinc minerals are found in beds of the Prairie du Chien group where they are exposed along the northern edge of the district.
Since 1942 the U. S. Geological Survey has been studying the geology of the zinc-lead district and has been mapping the structure, stratigraphy, and the occurrences of ore bodies. The program here described was centered in two areas at the margin of the district. Twenty-five holes that totalled 7,466 feet were drilled in 1950-51. In the Tete des Morts area, Iowa, the drilling showed lithology, structure, and evidences of mineralization that are favorable indications of the possible existence of pitch-type lower-run ore bodies; it showed a lateral extension of the potentially productive part of the district. In the Highland area, Wisconsin, lithology and evidences of mineralization found in strata of the Prairie du Chien group indicate that this unit might warrant further investigation as a potential source of ore at a lower stratigraphic position than that now being prospected in the main part of the zinc-lead district, farther south. Drilling in beds of the Prairie du Chien showed a vertical extension of the potentially productive part of the district.