Documents disponibles écrits par cet auteur

The chemistry of manganese ores through time / J. Barry Maynard in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 535-552 Titre : The chemistry of manganese ores through time : a signal of increasing diversity of earth-surface environments Type de document : texte imprimé Auteurs : J. Barry Maynard, Auteur Année de publication : 2011 Article en page(s) : pp. 535-552 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Manganese  ores  Earth-surface  environments Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Almost all economic manganese ores are ultimately or directly derived from hydrothermal vents associated with intermediate volcanic rocks. This source is in contrast to deep-sea nodules, which likely have a larger component of sediment-derived manganese and whose volcanic sources are more mafic. Manganese deposits can be divided into sedimentary rock-hosted, volcanic rock-hosted and karst-hosted, in order of predominance. Two genetic types of sedimentary rock-hosted deposits can also be identified: those with Mn derived via upwelling from oxygen-minimum zones and those formed on the margins of euxinic basins. Most of the large tonnage deposits appear to form by the euxinic mechanism. Manganese ores, like those of Fe, show a strong concentration of deposits in the Paleoproterozoic and a lesser occurrence in the Neoproterozoic, but, unlike Fe, there is an additional strong peak in the Oligocene. Therefore, Mn is not controlled entirely by the level of oxygen in the Earth’s atmosphere. At each peak of Mn deposition, the associated ore deposits are concentrated in a few districts, suggesting a more local than global control on manganese metallogenesis. Age trends can, however, be discerned in some chemical properties of manganese deposits. Overall, there is a trend to progressive increases in chemical diversity from the Archean to the Recent, with a particularly steep increase in the Neoproterozoic-Early Cambrian, corresponding in time to the radiation of metazoans. Also beginning in the Cambrian is the development of upwelling-linked deposits. There is another sharp increase in chemical diversity at the Jurassic-Cretaceous boundary, which includes increased SiO2/Al2O3 ratios and corresponds to the radiation of diatoms. There is a conspicuous gap in sedimentary rock-hosted Mn deposits between 1800 and 800 Ma that may correspond to a monotonous, low-oxygen ocean, but one without sulfidic deep water. Alternatively, Mn may have been precipitated entirely in the deep ocean, beneath a sulfidic oxygen minimum layer. The positive Eu anomalies, which in iron formations are equated to vent-sourced metals, are not seen in most Mn deposits, although they are found in Mn-rich iron formations. By contrast, Fe deposits interbedded with major Mn ores lack the usual Eu signal. Therefore, mechanisms of transport between hydrothermal vents and the sites of deposition differed for Fe and for Mn deposits in the Archean-Paleoproterozoic. The dominant pattern in the time trend of Mn deposition is increasing chemical diversity, which reflects an increasing compartmentalization of the Earth’s depositional environments. This compartmentalization was a response to, but also provided a spur to, the diversification of life forms. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/535.abstract [article] The chemistry of manganese ores through time : a signal of increasing diversity of earth-surface environments [texte imprimé] / J. Barry Maynard, Auteur . - 2011 . - pp. 535-552. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 535-552 Mots-clés : Manganese  ores  Earth-surface  environments Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Almost all economic manganese ores are ultimately or directly derived from hydrothermal vents associated with intermediate volcanic rocks. This source is in contrast to deep-sea nodules, which likely have a larger component of sediment-derived manganese and whose volcanic sources are more mafic. Manganese deposits can be divided into sedimentary rock-hosted, volcanic rock-hosted and karst-hosted, in order of predominance. Two genetic types of sedimentary rock-hosted deposits can also be identified: those with Mn derived via upwelling from oxygen-minimum zones and those formed on the margins of euxinic basins. Most of the large tonnage deposits appear to form by the euxinic mechanism. Manganese ores, like those of Fe, show a strong concentration of deposits in the Paleoproterozoic and a lesser occurrence in the Neoproterozoic, but, unlike Fe, there is an additional strong peak in the Oligocene. Therefore, Mn is not controlled entirely by the level of oxygen in the Earth’s atmosphere. At each peak of Mn deposition, the associated ore deposits are concentrated in a few districts, suggesting a more local than global control on manganese metallogenesis. Age trends can, however, be discerned in some chemical properties of manganese deposits. Overall, there is a trend to progressive increases in chemical diversity from the Archean to the Recent, with a particularly steep increase in the Neoproterozoic-Early Cambrian, corresponding in time to the radiation of metazoans. Also beginning in the Cambrian is the development of upwelling-linked deposits. There is another sharp increase in chemical diversity at the Jurassic-Cretaceous boundary, which includes increased SiO2/Al2O3 ratios and corresponds to the radiation of diatoms. There is a conspicuous gap in sedimentary rock-hosted Mn deposits between 1800 and 800 Ma that may correspond to a monotonous, low-oxygen ocean, but one without sulfidic deep water. Alternatively, Mn may have been precipitated entirely in the deep ocean, beneath a sulfidic oxygen minimum layer. The positive Eu anomalies, which in iron formations are equated to vent-sourced metals, are not seen in most Mn deposits, although they are found in Mn-rich iron formations. By contrast, Fe deposits interbedded with major Mn ores lack the usual Eu signal. Therefore, mechanisms of transport between hydrothermal vents and the sites of deposition differed for Fe and for Mn deposits in the Archean-Paleoproterozoic. The dominant pattern in the time trend of Mn deposition is increasing chemical diversity, which reflects an increasing compartmentalization of the Earth’s depositional environments. This compartmentalization was a response to, but also provided a spur to, the diversification of life forms. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/535.abstract