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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Our preliminary investigations suggest that numerous stratiform manganese deposits in the western United States were
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formed by hydrothermal-volcanogenic processes. These deposits can be grouped into five categories on the basis of the manganese mineral assemblage, lithologic association, and tectonic setting: (1) deposits in chert-graywacke sequences (e.g., in melange of the Franciscan complex of California--lenses and beds of manganese carbonate and manganiferous opal associated with bedded chert within thick sequences of graywacke and conglomerate; (2) deposits in chert-greenstone (ophiolite) sequences (e.g., the late Paleozoic Havallah Formation, Nevada)--occurrences of Mn-jasper, Mn-oxides, and the Mn-silicates braunite, bementite, and rhodonite in lenses within bedded chert, at the interface between basalt and chert, and within basalt; (3) deposits in metachert-metavolcanic sequences (e.g., lat Paleozoic to Jurassic strata of the Sierra Nevada and Klamath Mountains)--deposits that are similar in occurrence and general lithology to those in the Franciscan and Havallah sequences but that have undergone a higher grade of metamorphism, so that the manganese mineral assemblage includes rhodochrosite, rhodonite, spessartine, piemontite, and Mn-rich pyroxene and amphibole; (4) deposits in pelagic-limestone/oceanic-basalt sequences (e.g., the Eocene Crescent Formation on the Olympic Peninsula, Washington)--marine carbonate and spilitized volcanic rocks hosting a diverse manganese mineralogy dominated by silicate (e.g., bementite) and oxide (e.g., hausmannite) phases; and (5) deposits in Miocene and Pliocene sequences of conglomerate-sandstone-tuff-gypsum of the Colorado River-Lake Mea area, Nevada and Arizona--high-tonnage low-grade stratiform and largely strata bound deposits consisting mainly of amorphous manganese oxide cement in clastic sedimentary rocks.
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