Mississippi Valley Lead and Zinc Deposits

(USA), a large ore-bearing region in the Mississippi-Missouri basin. The deposits are concentrated in four areas: (1) Missouri, Kansas, and Oklahoma, containing major zinc reserves; (2) southeastern Missouri, the main source of lead; (3) Kentucky and Tennessee; and (4) Wisconsin, Illinois, and Iowa. Lead ore was first extracted in 1720, and zinc mining began in 1870. The lead and zinc content of the ores ranges from 1 to 3 percent. In 1970 the deposits yielded 195,000 tons of zinc and 386,000 tons of lead.

The deposits lie in the gently sloping Paleozoic rocks of the sedimentary mantle of the North American platform. The sedimentary rocks are gathered in wide, gently sloping folds broken by faults. The ores form extensive sheetlike deposits among carbonaceous rocks of the Cambrian and Carboniferous periods. The deposits occur only in sedimentary rock without the magmatic rock from which the deposits might have been derived. They are confined to certain parts in the stratigraphic cross section of the sedimentary rock. The ore bodies are predominantly sheets, and the ore has a simple mineral composition. The primary minerals are sphalerite and galena, and the secondary minerals are pyrite, marcasite, chalcopyrite, millerite, siegenite, barite, fluorite, quartz, and calcite.

Some geologists believe that the lead and zinc deposits were formed by the settling of sediments on the bottoms of ancient seas. Others suggest that they were formed after sedimentation from hot mineralized hydrothermal solutions circulating through the strata of porous carbonaceous rock.