Clay-mineral alteration in a carbonaceous shale bed seems to be related to hydrothermal zinc-lead depositions in the Upper Mississippi Valley district in southwestern Wisconsin, northwestern Illinois, and northeastern Iowa. A 3-in. thick, dark-brown, carbonaceous shale at the base of the Quimbys Mill Member of the Platteville Formation can be traced accurately in the Thompson-Temperly mine near New Diggings, Wis., through the ore bodies into alteration aureoles around the ore bodies and out into areas barren of mineralization or apparent alteration.

X-ray diffraction studies indicate a progressive alteration of the clay-mineral and, in part, nonclay-mineral components of the shale from unaltered rock into ore-zone rock. Illite of md polymorph in unaltered rock is altered to 1 m and 2 m illite in the altered rock, and 2 m illite polymorph increases markedly within the ore bodies. Accompanying this alteration, calcite decreases and dolomite and microcline increase in amount toward the ore zone; quartz remains unchanged. The conclusion that the alteration was effected by low-temperature and low-pressure environment over a geologically long period of ore deposition is supported by fluid-inclusion studies of the associated ore minerals, which indicate a maximum temperature of 120°C to 130°C, and the known stability of synthesized 1 m and 2 m mica polymorphs at 200°C and 15,000 psi water pressure.