Stable-isotope study of volcanogenic- and sedimentary-manganese deposits

As part of the program for the comprehensive study of volcanogenic-and sedimentary-manganese deposits, we have conducted stable-isotope analyses of manganiferous samples to (1) determine the H-and O-isotope systematics of manganese minerals and (2) improve our understanding of the origin and evolution of manganese deposits in order to better assess and evaluate their resource potential.Our methodology, analytical results, and preliminary interpretations are given here. 18The iV o values of manganese-oxide minerals from stratiform deposits in Nevada and Arizona range from about +1 to -9°/oo, the *D values of romanechite and nsutite from Arizona range from -87 to -109°/oo, and are only slightly lower than those of most hydroxyt-bearing minerals of similar occurrence.The estimated oxygen-isotope fractionation factors between manganese oxide minerals and water are ~1.001 at 2°C, and ~0.995 at 120°C; the estimated hydrogen-isotope fractionation factor is about 0.900 at 120°C.The deposits from both Arizona (Artillery Mountains) and Nevada (Three Kids, Virgin River) seem to have formed under similar conditions; however, Nevada deposits have a slightly higher thermal history than the Arizona deposits.The average formation temperature of the deposits is about 120°C.The Ladd and Buckeye deposits of the Franciscan Complex in California formed at lower temperatures (<100°C) in the presence of methane in a marine environment.The carbonate was derived from the oxidation of methane.The Molango deposits (Sierra Madre Oriental, Mexico) probably formed on or close to the sea floor in organic-rich carbonate sediment within an anoxic basin.During early diagenesis rhodochrosite, kutnahorite, Mn-calcite, and Mn-chlorite formed, probably from recrystallization of the sediment and breakdown of the earlier-formed carbonate.Carbon-isotope values range from -15.5 to +1.1, suggesting that both biogenic carbonate and organic matter provided carbon for the manganese carbonates.