Manganese in the United States, exclusive of Alaska and Hawaii

The manganese deposits in the United States (exclusive of Alaska and Hawaii) are shown on the accompanying map. The deposits have been divided into several genetic types distinguished on the map by symbols. The principal distinction is between syngenetic deposits, in which the manganese was deposited contemporaneously with the enclosing rocks, and epigenetic, in which manganese was introduced into sedimentary rocks after their deposition or into igneous rocks after solidification. Because hydrothermal, metamorphic, and particularly supergene processes modify and in places concentrate manganese minerals after deposition of the element, classification of many deposits is subject to uncertainties. A third category, deposits of unknown or uncertain origin, is therefore included also. Subdivisions of these principal categories are based on the geologic history subsequent to original accumulation of the syngenetic deposits, and on gross mineralogic nature of the epigenetic ones.