The carbonate-hosted Buick mine Pb-Zn-Cu deposit in the Viburnum Trend, southeast Missouri, is immediately overlain by the shale of the Davis Formation. The mineralogy of clay and clay-sized minerals and selected geochemical aspects of this shale were examined along a traverse normal to the Viburnum Trend. The shale consists of illitc, illite-smectite, mono-clinic K feldspar, and quartz; minor minerals include chlorite, dolomite, calcite, pyrite, and gypsum. Aluminous dioctahedral illite was found to be the dominant clay mineral in the <2-mu m fraction of the shale. Alterations to shale of the Davis Formation within the Viburnum Trend include lowering of the illitc crystallinity index, changes in the amount of K in whole-rock shale samples, and the presence of gypsum in the <2-mu m fraction. The crystallinity index of illite in the Davis shale is lowest over the ore zone of the Buick mine and within the Viburnum Trend relative to samples collected greater than 6 km from the ore zone. Decrease in the illitc crystallinity index may have been penecontemporaneous with the early stages of mineralization, the initial enrichment of Davis shale with metals, and the mobilization of K in the Davis and subjacent formations.