Sedimentary manganese and iron mineralization in the St. Francois mountains magmatic-hydrothermal iron district; Southeast Missouri.

Though carbonate-clastic sequences within the middle-Proterozoic St. Francois igneous terrane are quite rare, manganese and iron mineralization at Cuthbertson Mountain, Missouri, is found within a sedimentary section of limestone and siltstone that is intercalated within rhyolitic tufts. The limestones are laminated and contain stromatolites, first reported by Stinchcomb (1976). The siltstones are arkosic, have very angular grains, and apparently represent immature volcanic sediments. Manganese minerals predominate over iron (hematite) and include pyrolusite, psilomelane/cryptomelane, hausmanite, and braunite. The manganese mineralization occurs mainly as nodules within the sedimentary section, and both Mn and Fe minerals exhibit colloform textures. Within this terrane we have described a number of banded iron formation deposits that appear to have formed in shallow, restricted basins, which were fed by iron-rich hot springs. The Cuthbertson Mountain deposit appears to have been formed in a similar setting, but one where hot springs were delivering manganese to the basinal waters.

* Dudley, M.A. and J.L. Nold. Department of Earth Science, Central Missouri State University.