Dépouillements

Secular variation in economic geology / Richard J. Goldfarb in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 459-465 Titre : Secular variation in economic geology Type de document : texte imprimé Auteurs : Richard J. Goldfarb, Auteur ; Dwight Bradley, Auteur ; David L. Leach, Auteur Année de publication : 2011 Article en page(s) : pp. 459-465 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Economic  geology  Secular  variation  Ores  Mineral  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/459.extract [article] Secular variation in economic geology [texte imprimé] / Richard J. Goldfarb, Auteur ; Dwight Bradley, Auteur ; David L. Leach, Auteur . - 2011 . - pp. 459-465. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 459-465 Mots-clés : Economic  geology  Secular  variation  Ores  Mineral  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/459.extract

Iron formation / Andrey Bekker in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 467-508 Titre : Iron formation : the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes Type de document : texte imprimé Auteurs : Andrey Bekker, Auteur ; John F. Slack, Auteur ; Noah Planavsky, Auteur Année de publication : 2011 Article en page(s) : pp. 467-508 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Iron  Oxidation  mechanism  Geochemistry  Isotopes  Hydrothermal  processes Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/467.abstract [article] Iron formation : the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes [texte imprimé] / Andrey Bekker, Auteur ; John F. Slack, Auteur ; Noah Planavsky, Auteur . - 2011 . - pp. 467-508. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 467-508 Mots-clés : Iron  Oxidation  mechanism  Geochemistry  Isotopes  Hydrothermal  processes Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/467.abstract

Connections between sulfur cycle evolution, sulfur isotopes, sediments, and base metal sulfide deposits / James Farquhar in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 509-533 Titre : Connections between sulfur cycle evolution, sulfur isotopes, sediments, and base metal sulfide deposits Type de document : texte imprimé Auteurs : James Farquhar, Auteur ; Nanping Wu, Auteur ; Donald E. Canfield, Auteur Année de publication : 2011 Article en page(s) : pp. 509-533 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sulfure  Isotopes  Metal  sulfide  Ore  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Significant links exist between the sulfur cycle, sulfur geochemistry of sedimentary systems, and ore deposits over the course of Earth history. A picture emerges of an Archean and Paleoproterozoic stage of the sulfur cycle that has much lower levels of sulfate (<200 μM), carries a signal of mass-independent sulfur, and preserves evidence for temporal and spatial heterogeneity that reflects lower amounts of sulfur cycling than today. A second stage of ocean chemistry in the Paleoproterozoic, with higher atmospheric oxygen and oceanic sulfate at low millimolar levels, follows this stage. The isotopic record in sedimentary rocks and in sulfide-bearing ore deposits suggests abundant pyrite burial and implies a missing 34S-depleted pool that may have been lost via deep ocean deposition and possibly subduction. Proterozoic ocean chemistry appears to be quite complex. The surface waters of the Proterozoic oceans are believed to have been oxygenated, but geologic evidence from ore deposits and sedimentary rocks supports coexistence of significant sulfidic and nonsulfidic, anoxic, intermediate water and deep-water pools in the Mesoproterozoic. This stage in ocean chemistry ends with the second major global oxidation event in the latest Neoproterozoic (~600 Ma). This event started the transition to more oxygenated intermediate and deep waters, and higher but variable oceanic sulfate concentrations. The event set the scene for the formation in the Phanerozoic of the first significant MVT deposits and possibly is reflected in changes in other sedimentary rock-hosted base metal sulfide deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/509.abstract [article] Connections between sulfur cycle evolution, sulfur isotopes, sediments, and base metal sulfide deposits [texte imprimé] / James Farquhar, Auteur ; Nanping Wu, Auteur ; Donald E. Canfield, Auteur . - 2011 . - pp. 509-533. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 509-533 Mots-clés : Sulfure  Isotopes  Metal  sulfide  Ore  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Significant links exist between the sulfur cycle, sulfur geochemistry of sedimentary systems, and ore deposits over the course of Earth history. A picture emerges of an Archean and Paleoproterozoic stage of the sulfur cycle that has much lower levels of sulfate (<200 μM), carries a signal of mass-independent sulfur, and preserves evidence for temporal and spatial heterogeneity that reflects lower amounts of sulfur cycling than today. A second stage of ocean chemistry in the Paleoproterozoic, with higher atmospheric oxygen and oceanic sulfate at low millimolar levels, follows this stage. The isotopic record in sedimentary rocks and in sulfide-bearing ore deposits suggests abundant pyrite burial and implies a missing 34S-depleted pool that may have been lost via deep ocean deposition and possibly subduction. Proterozoic ocean chemistry appears to be quite complex. The surface waters of the Proterozoic oceans are believed to have been oxygenated, but geologic evidence from ore deposits and sedimentary rocks supports coexistence of significant sulfidic and nonsulfidic, anoxic, intermediate water and deep-water pools in the Mesoproterozoic. This stage in ocean chemistry ends with the second major global oxidation event in the latest Neoproterozoic (~600 Ma). This event started the transition to more oxygenated intermediate and deep waters, and higher but variable oceanic sulfate concentrations. The event set the scene for the formation in the Phanerozoic of the first significant MVT deposits and possibly is reflected in changes in other sedimentary rock-hosted base metal sulfide deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/509.abstract

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

Evolution of uranium fractionation processes through time / Michel Cuney in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 553-569 Titre : Evolution of uranium fractionation processes through time : driving the secular variation of uranium deposit types Type de document : texte imprimé Auteurs : Michel Cuney, Auteur Année de publication : 2011 Article en page(s) : pp. 553-569 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Uranium  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Uranium deposit types have evolved considerably from the Archean to the Present. The major global drivers were (1) change of geotectonic conditions during the Late Archean, (2) strong increase of atmospheric oxygen from 2.4 to 2.2 Ga, and (3) development of land plants during the Silurian. Other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the Cretaceous. Earth’s uranium fractionation mechanisms evolved through four major periods. The first, from 4.55 and 3.2 Ga, corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite-tonalite-granodiorite (TTG) rocks attained uranium concentrations of at most a few parts per million. Moreover, the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent, no uranium deposit could be expected to have formed during this period. The second period, from about 3.1 to 2.2 Ga, is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in U, Th, and K. Late in this period peraluminous granite was selectively enriched in U and to a lesser extent K. These were the first granite and pegmatite magmas able to crystallize high-temperature uraninite. The erosion of these granitic suites liberated thorium-rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals (e.g., zircon, monazite, Fe-Ti oxides) within huge continental basins (e.g., Witwatersrand, South Africa, and Bind River, Canada). The lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on Earth, but only during this period. The third period, from 2.2 to 0.45 Ga, records increased oxygen to nearly the present atmospheric level. Tetravalent uranium from uraninite was oxidized to hexavalent uranium, forming highly soluble uranyl ions in water. Uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation, especially during the Late Paleoproterozoic. A series of uranium deposits formed through redox processes; the first of these developed at a formational redox boundary at about 2.0 Ga in the Oklo area of Gabon. All known economically significant uranium deposits related to Na metasomatism are about 1.8 Ga in age. The high-grade, large tonnage unconformity-related deposits also formed essentially during the Late Paleoproterozoic to early Mesoproterozoic. The last period (0.45 Ga-Present) coincided with the colonization of continents by plants. The detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period: basal, roll front, tabular, and tectonolithologic types. However, the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone-hosted uranium deposits to be found within permeable sandstone older than the Silurian. Large uranium deposits related to high-level hydrothermal fluid circulation and those related to evapotranspiration (calcretes) are only known during this last period of time, probably because of their formation in near-surface environments with low preservation potential. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/553.abstract [article] Evolution of uranium fractionation processes through time : driving the secular variation of uranium deposit types [texte imprimé] / Michel Cuney, Auteur . - 2011 . - pp. 553-569. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 553-569 Mots-clés : Uranium  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Uranium deposit types have evolved considerably from the Archean to the Present. The major global drivers were (1) change of geotectonic conditions during the Late Archean, (2) strong increase of atmospheric oxygen from 2.4 to 2.2 Ga, and (3) development of land plants during the Silurian. Other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the Cretaceous. Earth’s uranium fractionation mechanisms evolved through four major periods. The first, from 4.55 and 3.2 Ga, corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite-tonalite-granodiorite (TTG) rocks attained uranium concentrations of at most a few parts per million. Moreover, the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent, no uranium deposit could be expected to have formed during this period. The second period, from about 3.1 to 2.2 Ga, is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in U, Th, and K. Late in this period peraluminous granite was selectively enriched in U and to a lesser extent K. These were the first granite and pegmatite magmas able to crystallize high-temperature uraninite. The erosion of these granitic suites liberated thorium-rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals (e.g., zircon, monazite, Fe-Ti oxides) within huge continental basins (e.g., Witwatersrand, South Africa, and Bind River, Canada). The lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on Earth, but only during this period. The third period, from 2.2 to 0.45 Ga, records increased oxygen to nearly the present atmospheric level. Tetravalent uranium from uraninite was oxidized to hexavalent uranium, forming highly soluble uranyl ions in water. Uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation, especially during the Late Paleoproterozoic. A series of uranium deposits formed through redox processes; the first of these developed at a formational redox boundary at about 2.0 Ga in the Oklo area of Gabon. All known economically significant uranium deposits related to Na metasomatism are about 1.8 Ga in age. The high-grade, large tonnage unconformity-related deposits also formed essentially during the Late Paleoproterozoic to early Mesoproterozoic. The last period (0.45 Ga-Present) coincided with the colonization of continents by plants. The detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period: basal, roll front, tabular, and tectonolithologic types. However, the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone-hosted uranium deposits to be found within permeable sandstone older than the Silurian. Large uranium deposits related to high-level hydrothermal fluid circulation and those related to evapotranspiration (calcretes) are only known during this last period of time, probably because of their formation in near-surface environments with low preservation potential. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/553.abstract

The geology and metallogeny of volcanic-hosted massive sulfide deposits / David L. Huston in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 571-591 Titre : The geology and metallogeny of volcanic-hosted massive sulfide deposits : variations through geologic time and with tectonic setting Type de document : texte imprimé Auteurs : David L. Huston, Auteur ; Sally Pehrsson, Auteur ; Bruce M. Eglington, Auteur Année de publication : 2011 Article en page(s) : pp. 571-591 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Volcanic  hosted  massive  Sulfide  deposits  Geodynamic  processes  Metallogeny  Earth  evolution Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Analysis of metallogenic data, including grade and tonnage, host-rock succession, ore and alteration mineralogy, and lead and sulfur isotope data, indicates significant secular changes in the character of volcanic-hosted massive sulfide (VHMS) deposits, which appear to be related to changes in tectonic processes, tectonic cycles, and changes in the composition of the hydrosphere and atmosphere. The distribution of these deposits, whether measured in number of deposits, tons of ore, or tons of metal, is episodic, with major peaks at 2740 to 2690, 1910 to 1840, 510 to 460, and 390 to 355 Ma. These peaks correspond to the assembly of major continental land masses, including Kenorland, Nuna, Gondwana, and Pangea, respectively. Periods when fewer VHMS deposits formed correspond to periods of supercontinent and/or supercraton stability. The VHMS deposits do not form during supercontinent and/or supercraton breakup; rather, these intervals are associated with deposition of clastic-dominated sediment-hosted zinc-lead deposits. The main exception to these generalizations is the amalgamation of Rodinia, which was not accompanied by significant VHMS formation. Rodinian amalgamation may have been dominated by advancing accretionary orogenesis, whereby the overriding plate did not go into extension. In this case, slab rollback and the associated extension to form back-arc basins would not have been common, a setting typically conducive to the formation and preservation of VHMS deposits. Large ranges in source 238U/204Pb (μ) that characterized VHMS deposits in the Archean and Proterozoic indicate early (Hadean to Paleoarchean) differentiation of the Earth. A progressive decrease in μ variability may indicate homogenization with time of these differentiated sources. Secular increases in the amount of lead and decreases in 100Zn/(Zn+Pb) relate to an increase in felsic rock-dominated successions as hosts to deposits and to an apparent absolute increase in the abundance of lead in the crust with time. The increase in the abundance of barite and other sulfate minerals in VHMS deposits, from virtually absent in the Mesoarchean and Neoarchean to relatively common in the Phanerozoic, relates to the progressive oxidation of the atmosphere and hydrosphere. The total sulfur content of the oceans also increased, resulting in the enhanced importance of seawater sulfur in VHMS ore fluids with time. In Archean to Paleoproterozoic deposits, the bulk of the sulfur was derived by leaching rocks underlying the deposits, with little contribution from seawater, resulting in uniform, near-zero per mil values of δ34Ssulfide. In contrast, the more variable δ34Ssulfide values of younger deposits reflect the increasing importance of seawater sulfur in the hydrothermal systems. Unlike Mesoarchean and Neoarchean deposits, Paleoarchean deposits contain abundant barite. This sulfate is inferred to have been derived from photolytic decomposition of atmospheric SO2 and does not reflect overall oxidized oceans. Archean and Proterozoic seawater was significantly more saline than that in the Phanerozoic, particularly upper Phanerozoic seawater. The VHMS ore fluids reflect this, being on average more saline in Archean and Proterozoic deposits. This variability introduces uncertainty into genetic models advocating brine pools or magmatic-hydrothermal contributions based on high-salinity ore fluids. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/571.abstract [article] The geology and metallogeny of volcanic-hosted massive sulfide deposits : variations through geologic time and with tectonic setting [texte imprimé] / David L. Huston, Auteur ; Sally Pehrsson, Auteur ; Bruce M. Eglington, Auteur . - 2011 . - pp. 571-591. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 571-591 Mots-clés : Volcanic  hosted  massive  Sulfide  deposits  Geodynamic  processes  Metallogeny  Earth  evolution Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Analysis of metallogenic data, including grade and tonnage, host-rock succession, ore and alteration mineralogy, and lead and sulfur isotope data, indicates significant secular changes in the character of volcanic-hosted massive sulfide (VHMS) deposits, which appear to be related to changes in tectonic processes, tectonic cycles, and changes in the composition of the hydrosphere and atmosphere. The distribution of these deposits, whether measured in number of deposits, tons of ore, or tons of metal, is episodic, with major peaks at 2740 to 2690, 1910 to 1840, 510 to 460, and 390 to 355 Ma. These peaks correspond to the assembly of major continental land masses, including Kenorland, Nuna, Gondwana, and Pangea, respectively. Periods when fewer VHMS deposits formed correspond to periods of supercontinent and/or supercraton stability. The VHMS deposits do not form during supercontinent and/or supercraton breakup; rather, these intervals are associated with deposition of clastic-dominated sediment-hosted zinc-lead deposits. The main exception to these generalizations is the amalgamation of Rodinia, which was not accompanied by significant VHMS formation. Rodinian amalgamation may have been dominated by advancing accretionary orogenesis, whereby the overriding plate did not go into extension. In this case, slab rollback and the associated extension to form back-arc basins would not have been common, a setting typically conducive to the formation and preservation of VHMS deposits. Large ranges in source 238U/204Pb (μ) that characterized VHMS deposits in the Archean and Proterozoic indicate early (Hadean to Paleoarchean) differentiation of the Earth. A progressive decrease in μ variability may indicate homogenization with time of these differentiated sources. Secular increases in the amount of lead and decreases in 100Zn/(Zn+Pb) relate to an increase in felsic rock-dominated successions as hosts to deposits and to an apparent absolute increase in the abundance of lead in the crust with time. The increase in the abundance of barite and other sulfate minerals in VHMS deposits, from virtually absent in the Mesoarchean and Neoarchean to relatively common in the Phanerozoic, relates to the progressive oxidation of the atmosphere and hydrosphere. The total sulfur content of the oceans also increased, resulting in the enhanced importance of seawater sulfur in VHMS ore fluids with time. In Archean to Paleoproterozoic deposits, the bulk of the sulfur was derived by leaching rocks underlying the deposits, with little contribution from seawater, resulting in uniform, near-zero per mil values of δ34Ssulfide. In contrast, the more variable δ34Ssulfide values of younger deposits reflect the increasing importance of seawater sulfur in the hydrothermal systems. Unlike Mesoarchean and Neoarchean deposits, Paleoarchean deposits contain abundant barite. This sulfate is inferred to have been derived from photolytic decomposition of atmospheric SO2 and does not reflect overall oxidized oceans. Archean and Proterozoic seawater was significantly more saline than that in the Phanerozoic, particularly upper Phanerozoic seawater. The VHMS ore fluids reflect this, being on average more saline in Archean and Proterozoic deposits. This variability introduces uncertainty into genetic models advocating brine pools or magmatic-hydrothermal contributions based on high-salinity ore fluids. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/571.abstract

Sediment-hosted lead-zinc deposits in earth history / David L. Leach in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 593-625 Titre : Sediment-hosted lead-zinc deposits in earth history Type de document : texte imprimé Auteurs : David L. Leach, Auteur ; Dwight C. Bradley, Auteur ; David Huston, Auteur Année de publication : 2011 Article en page(s) : pp. 593-625 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sediments  Lead-zinc  deposits  Geochemistry  Tectonic Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sediment-hosted Pb-Zn deposits can be divided into two major subtypes. The first subtype is clastic-dominated lead-zinc (CD Pb-Zn) ores, which are hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequence. This subtype includes deposits that have been traditionally referred to as sedimentary exhalative (SEDEX) deposits. The CD Pb-Zn deposits occur in passive margins, back-arcs and continental rifts, and sag basins, which are tectonic settings that, in some cases, are transitional into one another. The second subtype of sediment-hosted Pb-Zn deposits is the Mississippi Valley-type (MVT Pb-Zn) that occurs in platform carbonate sequences, typically in passive-margin tectonic settings. Considering that the redox state of sulfur is one of the major controls on the extraction, transport, and deposition of Pb and Zn at shallow crustal sites, sediment-hosted Pb-Zn ores can be considered a special rock type that recorded the oxygenation of Earth’s hydrosphere. The emergence of CD and MVT deposits in the rock record between 2.02 Ga, the age of the earliest known deposit of these ores, and 1.85 to 1.58 Ga, a major period of CD Pb-Zn mineralization in Australia and India, corresponds to a time after the Great Oxygenation Event that occurred at ca 2.4 to 1.8 Ga. Contributing to the abundance of CD deposits at ca 1.85 to 1.58 Ga was the following: (1) enhanced oxidation of sulfides in the crust that provided sulfate to the hydrosphere and Pb and Zn to sediments; (2) development of major redox and compositional gradients in the oceans; (3) first formation of significant sulfate-bearing evaporites; (4) formation of red beds and oxidized aquifers, possibly containing easily extractable Pb and Zn; (5) evolution of sulfate-reducing bacteria; and (6) formation of large and long-lived basins on stable cratons. Although MVT and CD deposits appeared for the first time in Earth history at 2.02 Ga, only CD deposits were important repositories for Pb and Zn in sediments between the Great Oxygenation Event, until after the second oxidation of the atmosphere in the late Neoproterozic. Increased oxygenation of the oceans following the second oxidation event led to an abundance of evaporites, resulting oxidized brines, and a dramatic increase in the volume of coarse-grained and permeable carbonates of the Paleozoic carbonate platforms, which host many of the great MVT deposits. The MVT deposits reached their maximum abundance during the final assembly of Pangea from Devonian into the Carboniferous. This was also a time for important CD mineral deposit formation along passive margins in evaporative belts of Pangea. Following the breakup of Pangea, a new era of MVT ores began with the onset of the assembly of the Neosupercontinent. A significant limitation on interpreting the secular distribution of the deposits is that there is no way to quantitatively evaluate the removal of deposits from the rock record through tectonic recycling. Considering that most of the sedimentary rock record has been recycled, most sediment-hosted Pb-Zn deposits probably have also been destroyed by subduction and erosion, or modified by metamorphism and tectonism, so that they are no longer recognizable. Thus, the uneven secular distribution of sediment-hosted Pb-Zn deposits reflects the genesis of these deposits, linked to Earth’s evolving tectonic and geochemical systems, as well as an unknown amount of recycling of the sedimentary rock record. DEWEY : 553 ISSN : 0391-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/593.abstract [article] Sediment-hosted lead-zinc deposits in earth history [texte imprimé] / David L. Leach, Auteur ; Dwight C. Bradley, Auteur ; David Huston, Auteur . - 2011 . - pp. 593-625. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 593-625 Mots-clés : Sediments  Lead-zinc  deposits  Geochemistry  Tectonic Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sediment-hosted Pb-Zn deposits can be divided into two major subtypes. The first subtype is clastic-dominated lead-zinc (CD Pb-Zn) ores, which are hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequence. This subtype includes deposits that have been traditionally referred to as sedimentary exhalative (SEDEX) deposits. The CD Pb-Zn deposits occur in passive margins, back-arcs and continental rifts, and sag basins, which are tectonic settings that, in some cases, are transitional into one another. The second subtype of sediment-hosted Pb-Zn deposits is the Mississippi Valley-type (MVT Pb-Zn) that occurs in platform carbonate sequences, typically in passive-margin tectonic settings. Considering that the redox state of sulfur is one of the major controls on the extraction, transport, and deposition of Pb and Zn at shallow crustal sites, sediment-hosted Pb-Zn ores can be considered a special rock type that recorded the oxygenation of Earth’s hydrosphere. The emergence of CD and MVT deposits in the rock record between 2.02 Ga, the age of the earliest known deposit of these ores, and 1.85 to 1.58 Ga, a major period of CD Pb-Zn mineralization in Australia and India, corresponds to a time after the Great Oxygenation Event that occurred at ca 2.4 to 1.8 Ga. Contributing to the abundance of CD deposits at ca 1.85 to 1.58 Ga was the following: (1) enhanced oxidation of sulfides in the crust that provided sulfate to the hydrosphere and Pb and Zn to sediments; (2) development of major redox and compositional gradients in the oceans; (3) first formation of significant sulfate-bearing evaporites; (4) formation of red beds and oxidized aquifers, possibly containing easily extractable Pb and Zn; (5) evolution of sulfate-reducing bacteria; and (6) formation of large and long-lived basins on stable cratons. Although MVT and CD deposits appeared for the first time in Earth history at 2.02 Ga, only CD deposits were important repositories for Pb and Zn in sediments between the Great Oxygenation Event, until after the second oxidation of the atmosphere in the late Neoproterozic. Increased oxygenation of the oceans following the second oxidation event led to an abundance of evaporites, resulting oxidized brines, and a dramatic increase in the volume of coarse-grained and permeable carbonates of the Paleozoic carbonate platforms, which host many of the great MVT deposits. The MVT deposits reached their maximum abundance during the final assembly of Pangea from Devonian into the Carboniferous. This was also a time for important CD mineral deposit formation along passive margins in evaporative belts of Pangea. Following the breakup of Pangea, a new era of MVT ores began with the onset of the assembly of the Neosupercontinent. A significant limitation on interpreting the secular distribution of the deposits is that there is no way to quantitatively evaluate the removal of deposits from the rock record through tectonic recycling. Considering that most of the sedimentary rock record has been recycled, most sediment-hosted Pb-Zn deposits probably have also been destroyed by subduction and erosion, or modified by metamorphism and tectonism, so that they are no longer recognizable. Thus, the uneven secular distribution of sediment-hosted Pb-Zn deposits reflects the genesis of these deposits, linked to Earth’s evolving tectonic and geochemical systems, as well as an unknown amount of recycling of the sedimentary rock record. DEWEY : 553 ISSN : 0391-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/593.abstract

Formation of sedimentary rock-hosted stratiform copper deposits through earth history / Murray W. Hitzman in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 627-639 Titre : Formation of sedimentary rock-hosted stratiform copper deposits through earth history Type de document : texte imprimé Auteurs : Murray W. Hitzman, Auteur ; David Selley, Auteur ; Stuart Bull, Auteur Année de publication : 2011 Article en page(s) : pp. 627-639 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sedimentary  rock  Copper  deposits  Mineralization Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/627.abstract [article] Formation of sedimentary rock-hosted stratiform copper deposits through earth history [texte imprimé] / Murray W. Hitzman, Auteur ; David Selley, Auteur ; Stuart Bull, Auteur . - 2011 . - pp. 627-639. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 627-639 Mots-clés : Sedimentary  rock  Copper  deposits  Mineralization Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/627.abstract

Iron oxide copper-gold (IOCG) deposits through earth history / David I. Groves in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 641-654 Titre : Iron oxide copper-gold (IOCG) deposits through earth history : implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits Type de document : texte imprimé Auteurs : David I. Groves, Auteur ; Frank P. Bierlein, Auteur ; Lawrence D. Meinert, Auteur Année de publication : 2011 Article en page(s) : pp. 641-654 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Iron  opxide  Gold  deposits  Copper  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/641.abstract [article] Iron oxide copper-gold (IOCG) deposits through earth history : implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits [texte imprimé] / David I. Groves, Auteur ; Frank P. Bierlein, Auteur ; Lawrence D. Meinert, Auteur . - 2011 . - pp. 641-654. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 641-654 Mots-clés : Iron  opxide  Gold  deposits  Copper  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/641.abstract

Lateritization and bauxitization events / Gregory J. Retallack in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 655-667 Titre : Lateritization and bauxitization events Type de document : texte imprimé Auteurs : Gregory J. Retallack, Auteur Année de publication : 2011 Article en page(s) : pp. 655-667 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Laterites  Bauxites Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Laterites and bauxites are produced in tropical soils by weathering, which enriches iron (of laterites) and alumina (of bauxites)—as well as trace elements such as nickel, gold, phosphorus, and niobium—to ore grade. Laterites and bauxites can be redeposited into sedimentary sequences, and remain as ores if not transported far and diluted with other materials. The age of redeposited laterites and bauxites, and of bauxitic and lateritic paleosols, can be established from the geologic age of overlying rocks, an approach especially effective in paleosols within sequences of isotopically datable volcanic rocks. Lateritic profiles can also be dated by paleomagnetic inclination in special cases in which land masses such as in Australia and India drifted long distances northward during Cenozoic time. In addition, cryptomelane and other K-Mn oxides can be dated by K-Ar and 40Ar-39Ar techniques to obtain multiple ages from different crystals in a single relict paleosol. Compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k.y. duration at 2, 12, 16, 35, 48, 55, 65 and 100 Ma. Such events can also be inferred at times older than 100 Ma from paleolatitudinal distribution of laterites and bauxites, but these are poorly sampled. Laterite and bauxite peaks were coeval with times of global high warmth and precipitation, elevated atmospheric carbon dioxide, oceanic anoxia, exceptional fossil preservation, and mass extinction. These CO2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact, flood basalt volcanism, and methane outbursts. Concentration of bauxite and laterite resources, in particular stratigraphic horizons formed during greenhouse crises, suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/655.abstract [article] Lateritization and bauxitization events [texte imprimé] / Gregory J. Retallack, Auteur . - 2011 . - pp. 655-667. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 655-667 Mots-clés : Laterites  Bauxites Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Laterites and bauxites are produced in tropical soils by weathering, which enriches iron (of laterites) and alumina (of bauxites)—as well as trace elements such as nickel, gold, phosphorus, and niobium—to ore grade. Laterites and bauxites can be redeposited into sedimentary sequences, and remain as ores if not transported far and diluted with other materials. The age of redeposited laterites and bauxites, and of bauxitic and lateritic paleosols, can be established from the geologic age of overlying rocks, an approach especially effective in paleosols within sequences of isotopically datable volcanic rocks. Lateritic profiles can also be dated by paleomagnetic inclination in special cases in which land masses such as in Australia and India drifted long distances northward during Cenozoic time. In addition, cryptomelane and other K-Mn oxides can be dated by K-Ar and 40Ar-39Ar techniques to obtain multiple ages from different crystals in a single relict paleosol. Compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k.y. duration at 2, 12, 16, 35, 48, 55, 65 and 100 Ma. Such events can also be inferred at times older than 100 Ma from paleolatitudinal distribution of laterites and bauxites, but these are poorly sampled. Laterite and bauxite peaks were coeval with times of global high warmth and precipitation, elevated atmospheric carbon dioxide, oceanic anoxia, exceptional fossil preservation, and mass extinction. These CO2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact, flood basalt volcanism, and methane outbursts. Concentration of bauxite and laterite resources, in particular stratigraphic horizons formed during greenhouse crises, suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/655.abstract

Secular variation of magmatic sulfide deposits and their source magmas / A. J. Naldrett in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 669-688 Titre : Secular variation of magmatic sulfide deposits and their source magmas Type de document : texte imprimé Auteurs : A. J. Naldrett, Auteur Année de publication : 2011 Article en page(s) : pp. 669-688 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Magmatic  sulfide  deposits  Magmas  sources  Komatiites Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood basalt-related class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic. Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this group), but as yet no examples have been found with economic sulfide bodies. Seventy-five percent of known PGE resources occur in three intrusions—the Bushveld, Great Dyke, and Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO2, MgO, and Cr and low Al2O3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma (T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma (possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age. All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga) komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) or island-arc (Perseverance-Mount Keith, Western Australia) environments, possibly because it was these environments that offered the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in the Archean was an additional control on the age distribution of these deposits. It is postulated that the restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise to U-type magmas. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/669.abstract [article] Secular variation of magmatic sulfide deposits and their source magmas [texte imprimé] / A. J. Naldrett, Auteur . - 2011 . - pp. 669-688. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 669-688 Mots-clés : Magmatic  sulfide  deposits  Magmas  sources  Komatiites Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood basalt-related class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic. Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this group), but as yet no examples have been found with economic sulfide bodies. Seventy-five percent of known PGE resources occur in three intrusions—the Bushveld, Great Dyke, and Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO2, MgO, and Cr and low Al2O3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma (T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma (possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age. All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga) komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) or island-arc (Perseverance-Mount Keith, Western Australia) environments, possibly because it was these environments that offered the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in the Archean was an additional control on the age distribution of these deposits. It is postulated that the restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise to U-type magmas. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/669.abstract

Diamonds through time / J. J. Gurney in Economic geology, Vol. 105 N° 3 (Mai 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 689-712 Titre : Diamonds through time Type de document : texte imprimé Auteurs : J. J. Gurney, Auteur ; H. H. Helmstaedt, Auteur ; S. H. Richardson, Auteur Année de publication : 2011 Article en page(s) : pp. 689-712 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Diamonds  Isotopic  dating Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Diamonds form in the upper mantle during episodic events and have been transported to the Earth’s surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon, but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world’s major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids. Disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds, G10 garnets, and diamond inclusion-type chromites, well used in diamond exploration. Within the mantle domains of diamond stability, there have been repeated episodes of further diamond crystallization and/or growth. These are associated with old, often Proterozoic, subduction-related melt generation, metasomatic fluid migration, and reaction with preexisting mantle eclogite, websterite, and peridotite. Using improved methods of isotope analysis, diamond formation ages can be correlated with specific major processes such as craton accretion, craton edge subduction, and magmatic mantle refertilization. Fibrous cuboid diamond and fibrous coats on SCD are rough-surfaced diamonds with abundant fluid inclusions. They have low mantle residence time, forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain SCD. The symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond-friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption, continuing a process of volatile enrichment in the lithospheric mantle already identified in the Archean harzburgite diamond event. Mantle-derived SCD commonly shows evidence of resorption, illustrating that diamond-unfriendly processes, including lamproite and kimberlite generation, are also active and may have a substantial negative effect in extreme cases on SCD crystal size. Exposure of SCD to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion-free diamonds with our current state of knowledge. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/689.abstract [article] Diamonds through time [texte imprimé] / J. J. Gurney, Auteur ; H. H. Helmstaedt, Auteur ; S. H. Richardson, Auteur . - 2011 . - pp. 689-712. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 3 (Mai 2010) . - pp. 689-712 Mots-clés : Diamonds  Isotopic  dating Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Diamonds form in the upper mantle during episodic events and have been transported to the Earth’s surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon, but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world’s major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids. Disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds, G10 garnets, and diamond inclusion-type chromites, well used in diamond exploration. Within the mantle domains of diamond stability, there have been repeated episodes of further diamond crystallization and/or growth. These are associated with old, often Proterozoic, subduction-related melt generation, metasomatic fluid migration, and reaction with preexisting mantle eclogite, websterite, and peridotite. Using improved methods of isotope analysis, diamond formation ages can be correlated with specific major processes such as craton accretion, craton edge subduction, and magmatic mantle refertilization. Fibrous cuboid diamond and fibrous coats on SCD are rough-surfaced diamonds with abundant fluid inclusions. They have low mantle residence time, forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain SCD. The symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond-friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption, continuing a process of volatile enrichment in the lithospheric mantle already identified in the Archean harzburgite diamond event. Mantle-derived SCD commonly shows evidence of resorption, illustrating that diamond-unfriendly processes, including lamproite and kimberlite generation, are also active and may have a substantial negative effect in extreme cases on SCD crystal size. Exposure of SCD to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion-free diamonds with our current state of knowledge. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/3/689.abstract

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