The underground mining operation at the Linwood Mine in Scott County, Iowa exposes a large section of the Cedar Rapids Member of the Otis Formation. The eastern portion of the mine was mapped to determine the spatial and temporal distribution of cavities, mineralogy, and the sequence of mineral formation of the deposit. The majority of cavities occurs in the lowest part of the Cedar Rapids Member and is controlled by bedding plane fractures. Minerals from the mine consist of calcite in four habits, marcasite, pyrite, microcrystalline iron sulfide, barite, sphalerite, and chalcopyrite. The sequence of mineralization begins with iron sulfides, followed by calcite and barite. Mineralization within the deposit occurred in both vadose and phreatic conditions as evidenced in large part by the presence of sulfides, euhedral calcite, barite, and boxwork. During phreatic stages, a reducing environment would have formed allowing the precipitation of marcasite, pyrite, sphalerite, and chalcopyrite. The presence of pyritic stalactites is a mystery because it appears to require mineral formation in both phreatic conditions, because of the presence of pyrite, and vadose conditions, because of the stalactitic form. Some characteristics of the deposit suggest a genetic relationship with the Upper Mississippi Valley Zinc-Lead District, while other characteristics suggest a local origin.
Jacob Spinsby, ’03 Le Claire, IA
Sponsor: Paul Garvin