Hydrothermal Mineralization of the Mississippi Valley Type at the Martin-Marietta Quarry, Linn County, Iowa

A hydrothermal sulfide mineral deposit is exposed at the Martin-Marietta Quarry (MMQ) near Cedar Rapids, Iowa. Mineralization occurs along solution-enlarged vertical joints in host rocks of the Silurian Scotch Grove and Gower formations. The hydrothermal minerals in general order of deposition are: marcasite, pyrite, sphalerite, calcite. Wall rock alteration is not extensive, and consists of solution enlargement of joints and dissemination of microscopic marcasite in the host rock. Fluid inclusion homogenization temperatures for sphalerite and calcite range from 69° to 99°C. The physical mineralogical similarities between the MMQ deposit and main-district Upper Mississippi Valley hydrothermal deposits evidence cogenesis. The origin of the MMQ deposit is considered in light of the East Central Iowa Basin model of Ludvigson, et al (1983a)