A discovery of exceptional calcite crystals, concentrically zoned with marcasite and pyrite inclusions, was recently made in the Chicago metropolitan area at the Conco mine, North Aurora. Crystals are found in massive solution cavities that were exposed by underground mining for limestone and dolomite. In 2010, Lafarge North America, owner of the Conco mine, accepted a proposal from Saga Minerals, Inc. to mine the solution cavities for mineral specimens. Crystals to 25 cm and specimens weighing many hundreds of pounds have been recovered. These specimens were first shown to the mineral collecting community at the 2011 Arizona Mineral and Fossil Show at the Hotel Tucson City Center in Tucson, Arizona.
Mineralization at the Conco mine is helping researchers map deep hydrothermal fluid migration across the midcontinent and better understand regional diagenetic processes. Mississippi Valley–type mineralization in northeastern Illinois (the southwest Michigan Basin) and structural dip suggest that the Michigan Basin was a source of mineralizing fluids. Without further study, however, the Illinois Basin cannot yet be ruled out as a potential source.
Jared T. Freiburg, a geologist at the Illinois State Geological Survey, is the founder of Saga Minerals, Inc. (www.sagaminerals.com).
Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University.