Abstract

Characteristic minerals are psilomelane, hollandite, cryptomelane, and coronadite, more rarely ramsdellite and pyrolusite. Host rocks are Mn-deficient; 80 percent of examples are middle to late Tertiary layered volcanics. Though deposits are shallow, mostly mined to only 100-200 feet (maximum 500 feet), a hypogene origin is indicated by their persistent association with barite and fluorite, a peripheral position in the zonal pattern of some metal-mining districts, alteration of plagioclase to K-spar, and abundance of W, Pb, Cu, Mo, Ti, As, Sb. They represent the subzone of Mn-bearing epithermal vein deposits lying nearest the surface, succeeded in depth by four other subzones: barite, fluorite, gold-silver, and base metals.

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