Epigenetic lead, zinc, and copper sulfides have been found in old mines, prospects, and other occurrences over a wide area on all sides of the Upper Mississippi Valley district of Illinois, Iowa, and Wisconsin. These sulfides have the same crystal habits, trace elements, lead isotope patterns, and sequence of formation as those within the main mining area. They are interpreted here as guides to possible extension of the district. The known host rocks for ore deposits include rocks from Late Cambrian to Late Silurian age, but the most favorable strata are the Middle Ordovician dolomites and limestones. An area of major potential is defined here, peripheral to the southern edge of the district in Illinois and Iowa. The largest known deposits in the main district are close to its southern margin, and little prospecting has been done south of these deposits, even in the most favorable strata, because of a cover of glacial materials and less favorable Silurian rocks. Other favorable ground surrounds the district, especially where cover by drift has discouraged exploration.

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