Lateral variations of carbonate rock adjacent to ore deposits in the Upper Mississippi Valley zinc-lead district, Wisconsin, Illinois, Iowa

The lateral variation and nature of carbonate host rock in the Upper Mississippi Valley zinc-lead district were petrographically studied within the McGregor and Quimbys Mill Members of the Platteville Formation of the Middle Ordovician System. These members are dolomitic limestone in the Upper Mississippi Valley district of southwestern Wisconsin, northwestern Illinois and northeastern Iowa. One hundred and twenty-seven specimens were collected or were available from pull drifts in the Shullsburg mine. The specimens were collected at regular intervals of 50 or lOO feet in seven pull drifts between eight ore bodies. Thin sections prepared from the carbonate specimens were studied, described and classified according to the Folk and Pettijohn classifications. The McGregor and Quimbys Mill carbonates consist of the following lithologic varieties: biosparite, dolomitic limestone, calcitic dolomite and dolomite. The host rock carbonate distal from ore bodies consists of an intimate mixture of very fine-grained calcite and dolomite in roughly equal proportions. Allochems may be moderately abundant locally; clay minerals constitute a minor constituent. Toward ore bodies, the composition of the carbonate rock gradually becomes more dolomitic. Closer yet to the ore zones the rock consists entirely of the coarser dolomite rhombs. The gradual change in carbonate composition toward the margin of ore bodies begins at an average distance of about 200 feet from the ore zones. The writer believes this change in composition to be due to the solutions which deposited the oers --Abstract, page ii-iii