R. K. McLimans, H. L. Barnes, and H. Ohmoto
Sphalerite stratigraphy of the upper Mississippi Valley zinc-lead district, southwest Wisconsin
Economic Geology and the Bulletin of the Society of Economic Geologists (May 1980), 75(3):351-361

Abstract:
Sphalerites in the ores of the upper Mississippi Valley zinc-lead district exhibit color banding. This banding is uniform across the district and a sphalerite stratigraphy can be described. The stratigraphy can be divided into three stages: A (early), B (middle), and C (late). It is possible to correlate individual bands over a few hundred meters in an orebody, and certain bands may be correlated many kilometers across the ore district.The color of the sphalerite bands is related to the FeS content. Light colors correspond to a low FeS content and dark colors to a high FeS content. The variability of the FeS content within any single band is small, suggesting the existence of uniform chemical conditions during deposition of any band across the district.Ore textures and the uniformity of the sphalerite stratigraphy across the district during any ore deposition interval are more compatible with ore formation from a fluid carrying both metals and sulfur than from mixing of metal-rich and sulfur-rich fluids of different origin.