Dépouillements

Sulfosalt melts / John Mavrogenes in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 257-262 Titre : Sulfosalt melts : evidence of high-temperature vapor transport of metals in the formation of high-sulfidation lode gold deposits Type de document : texte imprimé Auteurs : John Mavrogenes, Auteur ; Richard W. Henley, Auteur ; Agnes G. Reyes, Auteur Année de publication : 2011 Article en page(s) : pp. 257-262 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sulfosalt  Metals  Vapor  transport  Sulfidation  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In enargite-gold (high-sulfidation) vein deposits, magmatic fluid is considered responsible for the transport of metals and sulfur into the depositional regime. New data from Field-Emission SEM analyses of sulfosalt mineral assemblages (primarily enargite and tennantite) from El Indio, Chile, and Summitville, Colorado, provide direct evidence of high-temperature deposition, including the following: (1) the preservation of delicate euhedral quartz assemblages in sulfosalts, (2) a range of discrete Sb-rich sulfosalt, quartz, feldspar, and flourapatite vug-filling minerals, and (3) symplectic sulfosalt-chalcopyrite textures that are arguably quenched melts. Together, these features indicate formation from the vapor phase at high temperatures. Furthermore, euhedral quartz crystals from El Indio contain high-temperature, vapor-rich fluid inclusions. Combined, these observations are interpreted as suggestive of deposition in response to vapor-phase decompression within fracture arrays that may be considered the analogues of the feeder fractures beneath large low-grade silver-gold deposits such as Yanacocha, Peru. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/257.abstract [article] Sulfosalt melts : evidence of high-temperature vapor transport of metals in the formation of high-sulfidation lode gold deposits [texte imprimé] / John Mavrogenes, Auteur ; Richard W. Henley, Auteur ; Agnes G. Reyes, Auteur . - 2011 . - pp. 257-262. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 257-262 Mots-clés : Sulfosalt  Metals  Vapor  transport  Sulfidation  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In enargite-gold (high-sulfidation) vein deposits, magmatic fluid is considered responsible for the transport of metals and sulfur into the depositional regime. New data from Field-Emission SEM analyses of sulfosalt mineral assemblages (primarily enargite and tennantite) from El Indio, Chile, and Summitville, Colorado, provide direct evidence of high-temperature deposition, including the following: (1) the preservation of delicate euhedral quartz assemblages in sulfosalts, (2) a range of discrete Sb-rich sulfosalt, quartz, feldspar, and flourapatite vug-filling minerals, and (3) symplectic sulfosalt-chalcopyrite textures that are arguably quenched melts. Together, these features indicate formation from the vapor phase at high temperatures. Furthermore, euhedral quartz crystals from El Indio contain high-temperature, vapor-rich fluid inclusions. Combined, these observations are interpreted as suggestive of deposition in response to vapor-phase decompression within fracture arrays that may be considered the analogues of the feeder fractures beneath large low-grade silver-gold deposits such as Yanacocha, Peru. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/257.abstract

Skarn alteration and mineralization at Coroccohuayco, Tintaya district, Peru / Kierran C. Maher in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 263-283 Titre : Skarn alteration and mineralization at Coroccohuayco, Tintaya district, Peru Type de document : texte imprimé Auteurs : Kierran C. Maher, Auteur Année de publication : 2011 Article en page(s) : pp. 263-283 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Ingeous  rocks  Mineralization  Geochemistry  Mineral  chemistry  Fluid  inclusion  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Coroccohuayco is a porphyry-related copper skarn deposit in the Andahuaylas-Yauri batholith of southeast Peru with little evidence for the presence of a significantly mineralized hydrothermal system at surface. The deposit occupies the hinge zone of a synform, hosted by the Cretaceous Ferrobamba Formation. Hydrothermal alteration of the carbonate protolith produced skarn between the underlying hornfels-altered Mara Formation and an overlying preskarn sill-like diorite intrusion, with the main zone of mineralization generally below 200 m from the surface. The spatial distribution of skarn was dominantly controlled by permeability variations in the carbonate protolith. The deposit consists of prograde garnet-dominant calc-silicate alteration with mineralization of chalcopyrite-bornite ± chalcocite disseminated as grains in calc-silicates. Precious metal mineralization occurs as gold and silver minerals mostly associated with bornite-chalcocite mineralization disseminated within more distal granular garnet alteration. Fluids were derived from the crystallization of several porphyry phases that intruded into an upper crustal level, possibly as a cupola above an underlying batholith. Fluid inclusion data from garnet and pyroxene indicate that the prograde skarn alteration formed between 400° to >600°C from highly saline fluids. Data from quartz indicate that copper mineralization formed between 250° to 400°C, with magnetite deposition over the upper part of this range. Retrograde alteration is generally magnetite (+carbonate-silica) replacement of calc-silicates and is locally associated with copper mineralization. Hydrous retrograde alteration, commonly observed in other copper skarn systems, is of lesser importance at Coroccohuayco. This is likely related to the geochemical effects of the overlying diorite intrusion which could have impeded the influx of oxidized meteoric waters into the system or buffered the composition of late-stage fluids, as well as the oxidized Fe-rich character of the prograde skarn. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/263.abstract [article] Skarn alteration and mineralization at Coroccohuayco, Tintaya district, Peru [texte imprimé] / Kierran C. Maher, Auteur . - 2011 . - pp. 263-283. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 263-283 Mots-clés : Ingeous  rocks  Mineralization  Geochemistry  Mineral  chemistry  Fluid  inclusion  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Coroccohuayco is a porphyry-related copper skarn deposit in the Andahuaylas-Yauri batholith of southeast Peru with little evidence for the presence of a significantly mineralized hydrothermal system at surface. The deposit occupies the hinge zone of a synform, hosted by the Cretaceous Ferrobamba Formation. Hydrothermal alteration of the carbonate protolith produced skarn between the underlying hornfels-altered Mara Formation and an overlying preskarn sill-like diorite intrusion, with the main zone of mineralization generally below 200 m from the surface. The spatial distribution of skarn was dominantly controlled by permeability variations in the carbonate protolith. The deposit consists of prograde garnet-dominant calc-silicate alteration with mineralization of chalcopyrite-bornite ± chalcocite disseminated as grains in calc-silicates. Precious metal mineralization occurs as gold and silver minerals mostly associated with bornite-chalcocite mineralization disseminated within more distal granular garnet alteration. Fluids were derived from the crystallization of several porphyry phases that intruded into an upper crustal level, possibly as a cupola above an underlying batholith. Fluid inclusion data from garnet and pyroxene indicate that the prograde skarn alteration formed between 400° to >600°C from highly saline fluids. Data from quartz indicate that copper mineralization formed between 250° to 400°C, with magnetite deposition over the upper part of this range. Retrograde alteration is generally magnetite (+carbonate-silica) replacement of calc-silicates and is locally associated with copper mineralization. Hydrous retrograde alteration, commonly observed in other copper skarn systems, is of lesser importance at Coroccohuayco. This is likely related to the geochemical effects of the overlying diorite intrusion which could have impeded the influx of oxidized meteoric waters into the system or buffered the composition of late-stage fluids, as well as the oxidized Fe-rich character of the prograde skarn. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/263.abstract

Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: / Katharina Wulff in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 285-302 Titre : Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: : constraints from stable (O, H, C, S) isotopes Type de document : texte imprimé Auteurs : Katharina Wulff, Auteur ; Annika Dziggel, Auteur ; Jochen Kolb, Auteur Année de publication : 2011 Article en page(s) : pp. 285-302 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sediment  hosted  Mineralization  Isotopes  Gold  deposits  Namibia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ18O values of 14 to 15 per mil (V-SMOW). The total variations in δ18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ18O and δ13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid (δ18O = 12–14‰ δD = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/285.abstract [article] Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: : constraints from stable (O, H, C, S) isotopes [texte imprimé] / Katharina Wulff, Auteur ; Annika Dziggel, Auteur ; Jochen Kolb, Auteur . - 2011 . - pp. 285-302. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 285-302 Mots-clés : Sediment  hosted  Mineralization  Isotopes  Gold  deposits  Namibia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ18O values of 14 to 15 per mil (V-SMOW). The total variations in δ18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ18O and δ13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid (δ18O = 12–14‰ δD = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/285.abstract

Contrasting patterns of alteration at the wheeler river area, Athabasca basin, Saskatchewan, Canada / Jonathan Cloutier in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 303-324 Titre : Contrasting patterns of alteration at the wheeler river area, Athabasca basin, Saskatchewan, Canada : insights into the apparently uranium-barren zone K alteration system Type de document : texte imprimé Auteurs : Jonathan Cloutier, Auteur ; Kurt Kyser, Auteur ; Gema R. Olivo, Auteur Année de publication : 2011 Article en page(s) : pp. 303-324 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Uranium  Alteration  phases  Isotopy  Weak  acid  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high by ~250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid- and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/303.abstract [article] Contrasting patterns of alteration at the wheeler river area, Athabasca basin, Saskatchewan, Canada : insights into the apparently uranium-barren zone K alteration system [texte imprimé] / Jonathan Cloutier, Auteur ; Kurt Kyser, Auteur ; Gema R. Olivo, Auteur . - 2011 . - pp. 303-324. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 303-324 Mots-clés : Uranium  Alteration  phases  Isotopy  Weak  acid  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high by ~250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid- and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/303.abstract

A group of papers on fluid inclusion research applied to ore deposits / Robert Bodnar in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 325-327 Titre : A group of papers on fluid inclusion research applied to ore deposits : an introduction Type de document : texte imprimé Auteurs : Robert Bodnar, Auteur ; Jean Cline, Auteur Année de publication : 2011 Article en page(s) : pp. 325-327 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Fluid  inclusion  Ore  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/325.extract [article] A group of papers on fluid inclusion research applied to ore deposits : an introduction [texte imprimé] / Robert Bodnar, Auteur ; Jean Cline, Auteur . - 2011 . - pp. 325-327. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 325-327 Mots-clés : Fluid  inclusion  Ore  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/325.extract

Investigation of the H2O-NaCl-LiCl system / Michel Dubois in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 329-338 Titre : Investigation of the H2O-NaCl-LiCl system : a synthetic fluid inclusion study and thermodynamic modeling from −50° to +100°C and up to 12 mol/kg Type de document : texte imprimé Auteurs : Michel Dubois, Auteur ; Christophe Monnin, Auteur ; Teddy Castelain, Auteur Année de publication : 2011 Article en page(s) : pp. 329-338 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : H2O  NaCI  LiCI  Thermodynamic  modeling  Fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Lithium can reach high concentrations in some geological environments, such as rare element pegmatites, continental brines, and hydrothermal systems. The H2O-NaCl-LiCl system was investigated as a model for fluid inclusion studies in lithium-rich fluids. We first compiled experimental data (freezing point depression measurements and halite solubility data). Synthetic fluid inclusions were then prepared in quartz crystals, in order to obtain new solubility data and to observe phase transitions in a system characterized by the presence of numerous hydrates. The combination of Raman microspectrometry and microthermometry leads to identify undoubtedly the phase transition succession. Raman revealed partial crystallization and metastable phase transitions. A new spectrum for LiCl pentahydrate, the stable phase melting at the eutectic, is presented. The Pitzer model was used to calculate phase solubility using published expressions of activity coefficients for the H2O-NaCl and H2O-LiCl systems. We finally propose a representation of the H2O-NaCl-LiCl phase diagram for the interpretation of phase transitions in Li-rich chloride-dominated fluid inclusions in the range −50° to +100°C. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/329.abstract [article] Investigation of the H2O-NaCl-LiCl system : a synthetic fluid inclusion study and thermodynamic modeling from −50° to +100°C and up to 12 mol/kg [texte imprimé] / Michel Dubois, Auteur ; Christophe Monnin, Auteur ; Teddy Castelain, Auteur . - 2011 . - pp. 329-338. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 329-338 Mots-clés : H2O  NaCI  LiCI  Thermodynamic  modeling  Fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Lithium can reach high concentrations in some geological environments, such as rare element pegmatites, continental brines, and hydrothermal systems. The H2O-NaCl-LiCl system was investigated as a model for fluid inclusion studies in lithium-rich fluids. We first compiled experimental data (freezing point depression measurements and halite solubility data). Synthetic fluid inclusions were then prepared in quartz crystals, in order to obtain new solubility data and to observe phase transitions in a system characterized by the presence of numerous hydrates. The combination of Raman microspectrometry and microthermometry leads to identify undoubtedly the phase transition succession. Raman revealed partial crystallization and metastable phase transitions. A new spectrum for LiCl pentahydrate, the stable phase melting at the eutectic, is presented. The Pitzer model was used to calculate phase solubility using published expressions of activity coefficients for the H2O-NaCl and H2O-LiCl systems. We finally propose a representation of the H2O-NaCl-LiCl phase diagram for the interpretation of phase transitions in Li-rich chloride-dominated fluid inclusions in the range −50° to +100°C. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/329.abstract

Conditions for early cretaceous emerald formation at Dyakou, China / G. Xue in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 339-349 Titre : Conditions for early cretaceous emerald formation at Dyakou, China : fluid inclusion, Ar-Ar, and stable isotope studies Type de document : texte imprimé Auteurs : G. Xue, Auteur ; D. Marshall, Auteur ; S. Zhang, Auteur Année de publication : 2011 Article en page(s) : pp. 339-349 Note générale : Econopmic geology Langues : Anglais (eng) Mots-clés : Emerald  formation  Fluid  inclusion  Isotope  China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Dyakou emerald occurrence is located in Malipo County in the province of Yunnan, southern China. The occurrence lies in the northern part of the Laojunshan-Song Chay metamorphic core complex, which is exposed in an area of approximately 2,000 km2 and extends across the border between China and Vietnam. Emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed Proterozoic biotite-muscovite granofels and schist. Hydrogen and oxygen isotope results from the emerald channel waters and emerald, respectively, are consistent with an igneous fluid source. The δ18O fractionation between emerald and quartz yields vein temperatures of 365° to 420°C. Fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10.5 mass percent NaCl equiv. Fluid inclusion isochores intersected with δ18O data yield pressures changing along the geothermal gradient from 1,500 to 3,300 bars. Ar-Ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 ± 1 Ma. These constraints combined with field observations indicate that the Dyakou emerald deposit is consistent with the igneous-related model for emerald formation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/339.abstract [article] Conditions for early cretaceous emerald formation at Dyakou, China : fluid inclusion, Ar-Ar, and stable isotope studies [texte imprimé] / G. Xue, Auteur ; D. Marshall, Auteur ; S. Zhang, Auteur . - 2011 . - pp. 339-349. Econopmic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 339-349 Mots-clés : Emerald  formation  Fluid  inclusion  Isotope  China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Dyakou emerald occurrence is located in Malipo County in the province of Yunnan, southern China. The occurrence lies in the northern part of the Laojunshan-Song Chay metamorphic core complex, which is exposed in an area of approximately 2,000 km2 and extends across the border between China and Vietnam. Emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed Proterozoic biotite-muscovite granofels and schist. Hydrogen and oxygen isotope results from the emerald channel waters and emerald, respectively, are consistent with an igneous fluid source. The δ18O fractionation between emerald and quartz yields vein temperatures of 365° to 420°C. Fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10.5 mass percent NaCl equiv. Fluid inclusion isochores intersected with δ18O data yield pressures changing along the geothermal gradient from 1,500 to 3,300 bars. Ar-Ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 ± 1 Ma. These constraints combined with field observations indicate that the Dyakou emerald deposit is consistent with the igneous-related model for emerald formation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/339.abstract

Direct analysis of ore-precipitating fluids / Kalin Kouzmanov in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 351-373 Titre : Direct analysis of ore-precipitating fluids : combined IR microscopy and LA-ICP-MS study of fluid inclusions in opaque ore minerals Type de document : texte imprimé Auteurs : Kalin Kouzmanov, Auteur ; Thomas Pettke, Auteur ; Christoph A. Heinrich, Auteur Année de publication : 2011 Article en page(s) : pp. 351-373 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Au  deposit  Fluid  inclusion  Ore  minerals Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in combination with near-infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore-precipitating fluids. We applied the two techniques to well-constrained fluid inclusion assemblages hosted by pyrite, enargite, and quartz to trace the source and evolution of the fluids in high-sulfidation epithermal veins overprinting a porphyry copper deposit at Rosia Poieni, Romania. Despite some analytical limitations caused by the sulfide host minerals, the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different. Systematic changes in major and trace element ratios between liquid-vapor, vapor-rich, and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition. Regarding their Cs/(Na + K) ratios, most of epithermal quartz-hosted fluid inclusion assemblages form a well-defined cluster, which coincides with the narrow range of the porphyry-stage fluids trapped in early quartz of the porphyry stockwork veins. Their Cu/(Na + K) ratios are 10 to 100 times lower compared to the pyrite-hosted inclusions and correspond to the lowest Cu/(Na + K) ratios recorded for the porphyry-stage fluids. By contrast, pyrite-hosted, vapor-rich fluid inclusions have the highest Cu/(Na + K) similar to the highest Cu/(Na + K) ratios measured in the porphyry-stage fluid inclusions. The results led to the conclusion that the gangue and ore minerals in the high-sulfidation epithermal veins at Rosia Poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystallizing magma at greater depth. These results are consistent with detailed textural observations, but petrography alone could not have led to this unambiguous conclusion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/351.abstract [article] Direct analysis of ore-precipitating fluids : combined IR microscopy and LA-ICP-MS study of fluid inclusions in opaque ore minerals [texte imprimé] / Kalin Kouzmanov, Auteur ; Thomas Pettke, Auteur ; Christoph A. Heinrich, Auteur . - 2011 . - pp. 351-373. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 351-373 Mots-clés : Cu-Au  deposit  Fluid  inclusion  Ore  minerals Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in combination with near-infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore-precipitating fluids. We applied the two techniques to well-constrained fluid inclusion assemblages hosted by pyrite, enargite, and quartz to trace the source and evolution of the fluids in high-sulfidation epithermal veins overprinting a porphyry copper deposit at Rosia Poieni, Romania. Despite some analytical limitations caused by the sulfide host minerals, the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different. Systematic changes in major and trace element ratios between liquid-vapor, vapor-rich, and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition. Regarding their Cs/(Na + K) ratios, most of epithermal quartz-hosted fluid inclusion assemblages form a well-defined cluster, which coincides with the narrow range of the porphyry-stage fluids trapped in early quartz of the porphyry stockwork veins. Their Cu/(Na + K) ratios are 10 to 100 times lower compared to the pyrite-hosted inclusions and correspond to the lowest Cu/(Na + K) ratios recorded for the porphyry-stage fluids. By contrast, pyrite-hosted, vapor-rich fluid inclusions have the highest Cu/(Na + K) similar to the highest Cu/(Na + K) ratios measured in the porphyry-stage fluid inclusions. The results led to the conclusion that the gangue and ore minerals in the high-sulfidation epithermal veins at Rosia Poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystallizing magma at greater depth. These results are consistent with detailed textural observations, but petrography alone could not have led to this unambiguous conclusion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/351.abstract

Geochemical and isotopic properties of fluids from gold-bearing and barren quartz veins of the sovetskoye gold deposit (Siberia, Russia) / A. Tomilenko in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 375-394 Titre : Geochemical and isotopic properties of fluids from gold-bearing and barren quartz veins of the sovetskoye gold deposit (Siberia, Russia) Type de document : texte imprimé Auteurs : A. Tomilenko, Auteur ; N. Gibsher, Auteur ; Y. Dublyansky, Auteur Année de publication : 2011 Article en page(s) : pp. 375-394 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  deposit  Fluid  inclusion  Barren  quartz  Gold-bearing  Russia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper reports on the comparative study of fluids trapped in inclusions in gold-bearing and barren quartz veins at the Sovetskoye gold deposit, Yeniseisky ridge, Siberia, Russia. The host rocks are greenschist facies metasedimentary rocks of Upper Proterozoic age. Within the deposit, some veins contain 10 to 25 ppm of Au (gold-bearing veins); others contain less than 1 ppm of Au (barren veins). Fluid inclusions trapped in barren veins are predominantly low-salinity-H2O (4.0–8.0 mass % NaCl equiv.) with variable contents of dissolved CO2 (<12.5 mol %), CH4 (<1.3 mol %), and N2 (<1.7 mol %). Homogenization temperatures vary from 200° to 410°C, and the calculated pressures of entrapment are <1.5 kbars. Inclusion fluids trapped in the host schists are predominantly aqueous (97.6–99.3 mol % H2O), with CO2 contents below 2.4 mol percent. The fluids trapped in ore-bearing quartz inclusions are different. Inclusions show higher homogenization temperatures (up to 630°C) and salinity (up to 20–25 mass % NaCl equiv). Calculated pressures range from 0.7 to 2.0 kbars. Bulk gas chromatographic data of fluids extracted from native gold have contents of CO2 that range from 29.3 to 62.0 mol percent, N2 from 2.7 to 13.2 mol percent, and CH4 from 0.0 to 2.8 mol percent. In gold-bearing quartz the contents of CO2 range from 18.0 to 41.0 mol percent, N2 from 0.1 to 0.2 mol percent, and CH4 from 0.3 to 3.0 mol percent. Raman spectroscopic data of fluid from gold-bearing quartz revealed CO2 contents of individual inclusions as high as 98.2 mol percent, N2 up to 66.4 mol percent, and CH4 up to 88.7 mol percent. Fluids of the country-rock schists which host gold-bearing quartz veins are also enriched in CO2 (up to 49.5 mol %). The host rocks for both barren and gold-bearing quartz veins show consistent REE patterns characterized by a negative Eu anomaly. Fluids from barren quartz veins show similar REE patterns although with a less conspicuous negative Eu anomaly, whereas fluids from gold-bearing quartz show a marked positive Eu anomaly. Similar isotopic values of sulfur in pyrite, pyrrhotite, and arsenopyrite disseminated in country rocks (δ34S = 14.7–21.6‰), and in barren (δ34S = 12.9–19.9‰) and gold-bearing quartz veins (δ34S = 13.3–17.9‰) suggest that sulfur in sulfides was derived from the same source, most likely being the adjacent country rocks. CO2 trapped in fluid inclusions of barren and gold-bearing quartz has distinct isotopic compositions (δ13C = −8.1 to −10.2 and −4.9 to +5.2 ‰, respectively), and the δ13C values are positively correlated with Au contents in veins. Helium isotope composition of both types of quartz veins (3He/4He = 0.04–0.05 Ra for gold-bearing veins and 3He/4He = 0.02 Ra for barren veins) indicates a crustal provenance of mineral-forming fluids. The 40Ar/39Ar dating of sericite demonstrates that the barren veins (890-850 Ma) are significantly older than the gold-bearing veins (830-730 Ma). On the basis of their ages, barren veins are interpreted as synmetamorphic. The age of the gold-bearing veins corresponds to the age of the local granitoids (850-720 Ma), suggesting that the origin of the Sovetskoye deposit is related to the tectonic activity syntetctonic magmatism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/375.abstract [article] Geochemical and isotopic properties of fluids from gold-bearing and barren quartz veins of the sovetskoye gold deposit (Siberia, Russia) [texte imprimé] / A. Tomilenko, Auteur ; N. Gibsher, Auteur ; Y. Dublyansky, Auteur . - 2011 . - pp. 375-394. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 375-394 Mots-clés : Gold  deposit  Fluid  inclusion  Barren  quartz  Gold-bearing  Russia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper reports on the comparative study of fluids trapped in inclusions in gold-bearing and barren quartz veins at the Sovetskoye gold deposit, Yeniseisky ridge, Siberia, Russia. The host rocks are greenschist facies metasedimentary rocks of Upper Proterozoic age. Within the deposit, some veins contain 10 to 25 ppm of Au (gold-bearing veins); others contain less than 1 ppm of Au (barren veins). Fluid inclusions trapped in barren veins are predominantly low-salinity-H2O (4.0–8.0 mass % NaCl equiv.) with variable contents of dissolved CO2 (<12.5 mol %), CH4 (<1.3 mol %), and N2 (<1.7 mol %). Homogenization temperatures vary from 200° to 410°C, and the calculated pressures of entrapment are <1.5 kbars. Inclusion fluids trapped in the host schists are predominantly aqueous (97.6–99.3 mol % H2O), with CO2 contents below 2.4 mol percent. The fluids trapped in ore-bearing quartz inclusions are different. Inclusions show higher homogenization temperatures (up to 630°C) and salinity (up to 20–25 mass % NaCl equiv). Calculated pressures range from 0.7 to 2.0 kbars. Bulk gas chromatographic data of fluids extracted from native gold have contents of CO2 that range from 29.3 to 62.0 mol percent, N2 from 2.7 to 13.2 mol percent, and CH4 from 0.0 to 2.8 mol percent. In gold-bearing quartz the contents of CO2 range from 18.0 to 41.0 mol percent, N2 from 0.1 to 0.2 mol percent, and CH4 from 0.3 to 3.0 mol percent. Raman spectroscopic data of fluid from gold-bearing quartz revealed CO2 contents of individual inclusions as high as 98.2 mol percent, N2 up to 66.4 mol percent, and CH4 up to 88.7 mol percent. Fluids of the country-rock schists which host gold-bearing quartz veins are also enriched in CO2 (up to 49.5 mol %). The host rocks for both barren and gold-bearing quartz veins show consistent REE patterns characterized by a negative Eu anomaly. Fluids from barren quartz veins show similar REE patterns although with a less conspicuous negative Eu anomaly, whereas fluids from gold-bearing quartz show a marked positive Eu anomaly. Similar isotopic values of sulfur in pyrite, pyrrhotite, and arsenopyrite disseminated in country rocks (δ34S = 14.7–21.6‰), and in barren (δ34S = 12.9–19.9‰) and gold-bearing quartz veins (δ34S = 13.3–17.9‰) suggest that sulfur in sulfides was derived from the same source, most likely being the adjacent country rocks. CO2 trapped in fluid inclusions of barren and gold-bearing quartz has distinct isotopic compositions (δ13C = −8.1 to −10.2 and −4.9 to +5.2 ‰, respectively), and the δ13C values are positively correlated with Au contents in veins. Helium isotope composition of both types of quartz veins (3He/4He = 0.04–0.05 Ra for gold-bearing veins and 3He/4He = 0.02 Ra for barren veins) indicates a crustal provenance of mineral-forming fluids. The 40Ar/39Ar dating of sericite demonstrates that the barren veins (890-850 Ma) are significantly older than the gold-bearing veins (830-730 Ma). On the basis of their ages, barren veins are interpreted as synmetamorphic. The age of the gold-bearing veins corresponds to the age of the local granitoids (850-720 Ma), suggesting that the origin of the Sovetskoye deposit is related to the tectonic activity syntetctonic magmatism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/375.abstract

Fluid inclusion constraints on the genesis of gold in the darasun district (eastern Transbaikalia), Russia / Vsevolod Yu Prokofiev in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 395-416 Titre : Fluid inclusion constraints on the genesis of gold in the darasun district (eastern Transbaikalia), Russia Type de document : texte imprimé Auteurs : Vsevolod Yu Prokofiev, Auteur ; Paolo S. Garofalo, Auteur ; Nikolay S. Bortnikov, Auteur Année de publication : 2011 Article en page(s) : pp. 395-416 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  district  Ore  minerals  Fluid  inclusion  Microthermometry  Russia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Darasun, Teremkyn, and Talatui are neighboring deposits in the Darasun gold district of Transbaikalia (Russia) that are hosted by Middle-Late Jurassic subduction-related porphyry intrusions and dikes. Darasun consists of vertically extensive, steeply dipping Au mineralized veins and zones, spatially related to a K-rich granodiorite-porphyry intrusion. Within the deposit are also pipelike bodies of tourmaline sulfide hydrothermal breccias. Teremkyn consists of gently and steeply dipping veins and zones, always associated with felsic dikes comagmatic with the Darasun intrusion. Talatui consists of variably shaped mineralized zones systematically associated with dikes of different compositions and with the host Middle-Late Jurassic granite porphyry. A large proportion of both veins and mineralized zones in these deposits consists of sulfide and oxide minerals, with pyrite and arsenopyrite mostly present at Darasun and Teremkyn and pyrite, magnetite, and hematite present at Talaui. The three deposits host also a large number of sulfosalt, bismuth, oxide, and telluride minerals. Gold of variable purity occurs as free nuggets or hosted by opaque phases. The gangue mineralogy is similar in the three deposits and includes commonly quartz, tourmaline, and calcite. Microthermometry and microchemical analyses of fluid inclusions with petrographic and compositional data on ore minerals constrain the stages of ore deposition in the district. At room temperature, the entrapped fluids are of four compositional types: (1) multiphase, with a vapor bubble, liquid, and one or more solids; (2) liquid-vapor aqueous; (3a) vapor-rich with a small proportion of liquid and rare solids, and also (3b) vapor-rich with a small proportion of aqueous liquid and carbonic liquid. Types (1) and (3) or (2) and (3) coexist within the same assemblages and are present mostly in paragenetically early quartz, whereas the type (2) fluid within later quartz, sphalerite, and calcite shows consistent phase proportions. At Darasun and Teremkyn, type (2) fluid is the main ore fluid, but all fluid types are associated with gold deposition at Talatui. Microthermometric measurements from all fluid types show a range of phase transitions that vary systematically at the deposit scale. At Darasun and Teremkyn, coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid states, respectively, within the same 290° to 460°C range. The corresponding bulk salinities vary between 0.7 and 44.8 wt percent NaCl equiv. In contrast, at Talatui coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid phases and show a distribution of Th(total) and bulk salinities between about 300° and 610°C and 0.9 and 48 wt % NaCl equiv, respectively. Mass spectrometric and chromatographic data from bulk samples show that the main-stage ore fluid in the three deposits varies compositionally according to location within the district. In detail, Cl, Na, K, Cs, Br, Sr, Rb, Mn, and Tl are relatively enriched in the Talatui fluid, whereas CO2, CH4, HCO, Cu, Pb, Sb, Fe, Hg, and REE are enriched at Darasun and Teremkyn or show no systematic variations. Au concentrations vary within the 0.01-to 3-g/t range at Darasun and Teremkyn, whereas at Talatui values do not exceed 0.4 g/t. The salinities and homogenization temperatures of the main-stage ore fluids types (1), (2), and (3) are typical of the intrusion-related class of gold deposits. This suggests a dominant magmatic component of the ore fluid during the first stages of mineral precipitation and a magmatic derivation of gold itself in the district. Phase separation was active during the early stages of ore deposition and took place between about 600° to 400°C at Talatui and 450° to 300°C at Teremkyn and Darasun at pressures of 10 to 160 MPa (i.e., 1- to 2.5-km depth). This explains the systematic distribution of ore minerals and fluid chemistry within and around the orebodies. Gold precipitation took place within the three deposits during both early-stage phase separation (Talatui and Darasun) and main-stage ore formation. Microchemical data of fluid inclusions show that the ore fluid was characterized by Au/Cu ratios between 15 and 35 (units: g/t vs. wt %). This range is five orders of magnitude higher than that determined for porphyry Cu deposits (~1·10−4, Ulrich et al., 1999) and compares well with the range of Au/Cu ratios measured in the most fractionated melt hosting the Timbarra intrusion-related deposit of Australia (20–200, Mustard et al., 2006). Hence, similar to porphyry Cu systems, the Au/Cu ratios of the ore fluid at Darasun could have been controlled by the fundamental Au/Cu ratio of the magmatic source rock. The petrographic evidence for an association between gold and galena and the evidence for consistently high concentrations of Pb, Zn, and Fe in the fluid inclusions suggest that a fraction of the fluid that deposited the orebodies did not always reach saturation in gold and other ore minerals during the early magmatic-hydrothermal stage. We speculate that a fraction of the ore fluid was transported through the orebodies and dispersed at lower temperatures into the outer fringes of the deposits, where they formed the documented galena-sphalerite halo around the Darasun stock. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/395.abstract [article] Fluid inclusion constraints on the genesis of gold in the darasun district (eastern Transbaikalia), Russia [texte imprimé] / Vsevolod Yu Prokofiev, Auteur ; Paolo S. Garofalo, Auteur ; Nikolay S. Bortnikov, Auteur . - 2011 . - pp. 395-416. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 395-416 Mots-clés : Gold  district  Ore  minerals  Fluid  inclusion  Microthermometry  Russia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Darasun, Teremkyn, and Talatui are neighboring deposits in the Darasun gold district of Transbaikalia (Russia) that are hosted by Middle-Late Jurassic subduction-related porphyry intrusions and dikes. Darasun consists of vertically extensive, steeply dipping Au mineralized veins and zones, spatially related to a K-rich granodiorite-porphyry intrusion. Within the deposit are also pipelike bodies of tourmaline sulfide hydrothermal breccias. Teremkyn consists of gently and steeply dipping veins and zones, always associated with felsic dikes comagmatic with the Darasun intrusion. Talatui consists of variably shaped mineralized zones systematically associated with dikes of different compositions and with the host Middle-Late Jurassic granite porphyry. A large proportion of both veins and mineralized zones in these deposits consists of sulfide and oxide minerals, with pyrite and arsenopyrite mostly present at Darasun and Teremkyn and pyrite, magnetite, and hematite present at Talaui. The three deposits host also a large number of sulfosalt, bismuth, oxide, and telluride minerals. Gold of variable purity occurs as free nuggets or hosted by opaque phases. The gangue mineralogy is similar in the three deposits and includes commonly quartz, tourmaline, and calcite. Microthermometry and microchemical analyses of fluid inclusions with petrographic and compositional data on ore minerals constrain the stages of ore deposition in the district. At room temperature, the entrapped fluids are of four compositional types: (1) multiphase, with a vapor bubble, liquid, and one or more solids; (2) liquid-vapor aqueous; (3a) vapor-rich with a small proportion of liquid and rare solids, and also (3b) vapor-rich with a small proportion of aqueous liquid and carbonic liquid. Types (1) and (3) or (2) and (3) coexist within the same assemblages and are present mostly in paragenetically early quartz, whereas the type (2) fluid within later quartz, sphalerite, and calcite shows consistent phase proportions. At Darasun and Teremkyn, type (2) fluid is the main ore fluid, but all fluid types are associated with gold deposition at Talatui. Microthermometric measurements from all fluid types show a range of phase transitions that vary systematically at the deposit scale. At Darasun and Teremkyn, coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid states, respectively, within the same 290° to 460°C range. The corresponding bulk salinities vary between 0.7 and 44.8 wt percent NaCl equiv. In contrast, at Talatui coexisting types (1) and (3) and types (2) and (3) homogenize into the vapor and liquid phases and show a distribution of Th(total) and bulk salinities between about 300° and 610°C and 0.9 and 48 wt % NaCl equiv, respectively. Mass spectrometric and chromatographic data from bulk samples show that the main-stage ore fluid in the three deposits varies compositionally according to location within the district. In detail, Cl, Na, K, Cs, Br, Sr, Rb, Mn, and Tl are relatively enriched in the Talatui fluid, whereas CO2, CH4, HCO, Cu, Pb, Sb, Fe, Hg, and REE are enriched at Darasun and Teremkyn or show no systematic variations. Au concentrations vary within the 0.01-to 3-g/t range at Darasun and Teremkyn, whereas at Talatui values do not exceed 0.4 g/t. The salinities and homogenization temperatures of the main-stage ore fluids types (1), (2), and (3) are typical of the intrusion-related class of gold deposits. This suggests a dominant magmatic component of the ore fluid during the first stages of mineral precipitation and a magmatic derivation of gold itself in the district. Phase separation was active during the early stages of ore deposition and took place between about 600° to 400°C at Talatui and 450° to 300°C at Teremkyn and Darasun at pressures of 10 to 160 MPa (i.e., 1- to 2.5-km depth). This explains the systematic distribution of ore minerals and fluid chemistry within and around the orebodies. Gold precipitation took place within the three deposits during both early-stage phase separation (Talatui and Darasun) and main-stage ore formation. Microchemical data of fluid inclusions show that the ore fluid was characterized by Au/Cu ratios between 15 and 35 (units: g/t vs. wt %). This range is five orders of magnitude higher than that determined for porphyry Cu deposits (~1·10−4, Ulrich et al., 1999) and compares well with the range of Au/Cu ratios measured in the most fractionated melt hosting the Timbarra intrusion-related deposit of Australia (20–200, Mustard et al., 2006). Hence, similar to porphyry Cu systems, the Au/Cu ratios of the ore fluid at Darasun could have been controlled by the fundamental Au/Cu ratio of the magmatic source rock. The petrographic evidence for an association between gold and galena and the evidence for consistently high concentrations of Pb, Zn, and Fe in the fluid inclusions suggest that a fraction of the fluid that deposited the orebodies did not always reach saturation in gold and other ore minerals during the early magmatic-hydrothermal stage. We speculate that a fraction of the ore fluid was transported through the orebodies and dispersed at lower temperatures into the outer fringes of the deposits, where they formed the documented galena-sphalerite halo around the Darasun stock. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/395.abstract

A review of fluid inclusion constraints on mineralization in the Irish ore field and implications for the genesis of sediment-hosted Zn-Pb deposits / J. J. Wilkinson in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 417-442 Titre : A review of fluid inclusion constraints on mineralization in the Irish ore field and implications for the genesis of sediment-hosted Zn-Pb deposits Type de document : texte imprimé Auteurs : J. J. Wilkinson, Auteur Année de publication : 2011 Article en page(s) : pp. 417-442 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Fluid  inclusion  Irish  ore  Zn-Pb  deposits  Sediment  hosted Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Many fluid inclusion studies have been carried out in the Irish Midlands basin ore field (Lower Carboniferous) since the earliest work by Ed Roedder in the late 1960s. Results show that, in the ore deposits, the total range in fluid salinity is 4 to 28 wt percent NaCl equiv but with the majority falling in the moderate-salinity range between 8 and 19 wt percent. This variability is interpreted in terms of mixing between moderate-salinity ore fluids and low-temperature brines during ore formation. The most northerly ore deposits of Navan and Abbey-town are distinct in containing fluids of both lower and higher salinity than is typical of the Waulsortian-hosted deposits farther south (Tynagh, Silvermines, Lisheen, and Galmoy). Subeconomic prospects tend to display a narrower range in salinity, mostly at the lower end of the range observed in the ore deposits. In some prospects, and on the margins of some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for mineralization. Homogenization temperatures, a reasonable proxy for true trapping temperatures in the ore field, range from 70° to 280°C but with the majority falling between 130° and 240°C. There is no evidence for systematic stretching or leakage of inclusions related to the postentrapment heating implied by elevated thermal maturity indicators. The highest temperatures are observed in the Waulsortian-hosted systems, with peak temperatures of ~280°C supported by local, high-grade Cu and Ni mineralization. In the Navan and Abbeytown deposits, lower temperature fluids appear to have been more prevalent. The subeconomic prospects formed over essentially the same temperature range as the ore deposits (90°–270°C), with the exception of the morphologically and texturally distinct Mississippi Valley-type (MVT) systems in the region (e.g.,, Kinnitty, Harberton Bridge) that formed at lower temperatures (50°–100°C). Similar hydrothermal fluids to those recorded in both deposits and prospects are widely observed in dolomite (and sometimes calcite) cements within Courceyan-Arundian-age rocks, indicating that hydrothermal fluid activity occurred over an extremely large area (>30,000 km2) and probably over an extended time period. There is a broad regional division in fluid properties, suggesting that the northwestern and southeastern provinces, separated by the trace of the Iapetus suture zone, may represent partly decoupled, large-scale flow regimes. Up to three, low-temperature brine types are also recorded by cements in the host-rock sequence, indicating that a complex range of evaporation and fluid-rock interaction processes were ongoing in the shallow basin succession during the period of hydrothermal activity. Halogen data show that fluids involved in mineralization were originally seawater-derived brines, produced by evaporation to varying degrees. Relatively high temperature, basement-interacted hydrothermal fluids were derived from partially evaporated seawater (molar Cl/Br = 559–825). Their compositions can be explained by dolomitization in the Carboniferous succession prior to circulation to depth; alkali exchange, reduction, and metal-leaching from the lower Paleozoic basement; and mixing with low-temperature brines that locally penetrated the upper parts of the basement rock package. Fertile ore fluids appear to be characterized by higher δ18O (+7 to +9‰), lower δD (−25 to −45‰) and much higher metal contents than otherwise similar fluids sampled in basement-hosted feeder veins distal to deposits. This may reflect highly efficient metal scavenging in deeper and/or higher temperature reaction zones that underlie the principal deposits. In the ore deposits, these fluids mixed with Br-enriched bittern brines (Cl/Br ~290) produced by evaporation of Carboniferous sea-water past halite saturation. It is inferred that bittern brine generation occurred in the shallow marine shelf regions in the footwalls to the synsedimentary fault systems that controlled the localization of mineralization. These brines then migrated into hanging-wall depressions where they ponded within permeable sediments and became enriched in H2S via bacteriogenic sulfate reduction. The coincidence of structurally controlled, high-temperature reaction zones, brine-producing footwalls, and hanging-wall traps, with bacterial blooms above upwelling plumes of hydrothermal fluids, can be interpreted as a self-organizing system that locally converged on ore-forming conditions. Understanding the first-order structural control of the ore systems will therefore be critical for predicting new deposits. The Irish ore field presents arguably the best database available on the thermal and chemical characteristics of hydrothermal fluids involved in sediment-hosted ore genesis. The system shares much of the variety and complexity observed in other intracratonic basin-hosted Zn-Pb(-Ba) ore districts. This includes the coexistence of contrasting styles of mineralization that are typically observed in the more distal and platform-marginal parts of the basinal environment. The thermal and chemical fluid heterogeneity observed is typical of modern intracratonic basin systems and should be expected in large paleohydrothermal systems where recharge of surface-derived fluids is involved. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/417.abstract [article] A review of fluid inclusion constraints on mineralization in the Irish ore field and implications for the genesis of sediment-hosted Zn-Pb deposits [texte imprimé] / J. J. Wilkinson, Auteur . - 2011 . - pp. 417-442. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 417-442 Mots-clés : Fluid  inclusion  Irish  ore  Zn-Pb  deposits  Sediment  hosted Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Many fluid inclusion studies have been carried out in the Irish Midlands basin ore field (Lower Carboniferous) since the earliest work by Ed Roedder in the late 1960s. Results show that, in the ore deposits, the total range in fluid salinity is 4 to 28 wt percent NaCl equiv but with the majority falling in the moderate-salinity range between 8 and 19 wt percent. This variability is interpreted in terms of mixing between moderate-salinity ore fluids and low-temperature brines during ore formation. The most northerly ore deposits of Navan and Abbey-town are distinct in containing fluids of both lower and higher salinity than is typical of the Waulsortian-hosted deposits farther south (Tynagh, Silvermines, Lisheen, and Galmoy). Subeconomic prospects tend to display a narrower range in salinity, mostly at the lower end of the range observed in the ore deposits. In some prospects, and on the margins of some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for mineralization. Homogenization temperatures, a reasonable proxy for true trapping temperatures in the ore field, range from 70° to 280°C but with the majority falling between 130° and 240°C. There is no evidence for systematic stretching or leakage of inclusions related to the postentrapment heating implied by elevated thermal maturity indicators. The highest temperatures are observed in the Waulsortian-hosted systems, with peak temperatures of ~280°C supported by local, high-grade Cu and Ni mineralization. In the Navan and Abbeytown deposits, lower temperature fluids appear to have been more prevalent. The subeconomic prospects formed over essentially the same temperature range as the ore deposits (90°–270°C), with the exception of the morphologically and texturally distinct Mississippi Valley-type (MVT) systems in the region (e.g.,, Kinnitty, Harberton Bridge) that formed at lower temperatures (50°–100°C). Similar hydrothermal fluids to those recorded in both deposits and prospects are widely observed in dolomite (and sometimes calcite) cements within Courceyan-Arundian-age rocks, indicating that hydrothermal fluid activity occurred over an extremely large area (>30,000 km2) and probably over an extended time period. There is a broad regional division in fluid properties, suggesting that the northwestern and southeastern provinces, separated by the trace of the Iapetus suture zone, may represent partly decoupled, large-scale flow regimes. Up to three, low-temperature brine types are also recorded by cements in the host-rock sequence, indicating that a complex range of evaporation and fluid-rock interaction processes were ongoing in the shallow basin succession during the period of hydrothermal activity. Halogen data show that fluids involved in mineralization were originally seawater-derived brines, produced by evaporation to varying degrees. Relatively high temperature, basement-interacted hydrothermal fluids were derived from partially evaporated seawater (molar Cl/Br = 559–825). Their compositions can be explained by dolomitization in the Carboniferous succession prior to circulation to depth; alkali exchange, reduction, and metal-leaching from the lower Paleozoic basement; and mixing with low-temperature brines that locally penetrated the upper parts of the basement rock package. Fertile ore fluids appear to be characterized by higher δ18O (+7 to +9‰), lower δD (−25 to −45‰) and much higher metal contents than otherwise similar fluids sampled in basement-hosted feeder veins distal to deposits. This may reflect highly efficient metal scavenging in deeper and/or higher temperature reaction zones that underlie the principal deposits. In the ore deposits, these fluids mixed with Br-enriched bittern brines (Cl/Br ~290) produced by evaporation of Carboniferous sea-water past halite saturation. It is inferred that bittern brine generation occurred in the shallow marine shelf regions in the footwalls to the synsedimentary fault systems that controlled the localization of mineralization. These brines then migrated into hanging-wall depressions where they ponded within permeable sediments and became enriched in H2S via bacteriogenic sulfate reduction. The coincidence of structurally controlled, high-temperature reaction zones, brine-producing footwalls, and hanging-wall traps, with bacterial blooms above upwelling plumes of hydrothermal fluids, can be interpreted as a self-organizing system that locally converged on ore-forming conditions. Understanding the first-order structural control of the ore systems will therefore be critical for predicting new deposits. The Irish ore field presents arguably the best database available on the thermal and chemical characteristics of hydrothermal fluids involved in sediment-hosted ore genesis. The system shares much of the variety and complexity observed in other intracratonic basin-hosted Zn-Pb(-Ba) ore districts. This includes the coexistence of contrasting styles of mineralization that are typically observed in the more distal and platform-marginal parts of the basinal environment. The thermal and chemical fluid heterogeneity observed is typical of modern intracratonic basin systems and should be expected in large paleohydrothermal systems where recharge of surface-derived fluids is involved. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/417.abstract

Helium isotope composition of fluid inclusions hosted in massive sulfides from modern submarine hydrothermal systems / Volker Lüders in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 443-449 Titre : Helium isotope composition of fluid inclusions hosted in massive sulfides from modern submarine hydrothermal systems Type de document : texte imprimé Auteurs : Volker Lüders, Auteur ; Samuel Niedermann, Auteur Année de publication : 2011 Article en page(s) : pp. 443-449 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Helium  Fluid  inclusion  Sulfides Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Noble gas compositions of fluid inclusions hosted in sulfides from two modern submarine hydrothermal sites of contrasting tectonic settings have been analyzed. The JADE Field (central Okinawa Trough, Japan) is located in an active intracontinental back-arc basin whereas the SO 134 field is related to the central spreading ridge of the North Fiji Basin, which represents an active, highly mature intra-ocean back-arc system. Previous conventional studies of fluid inclusions have revealed compelling evidence for subcritical phase separation of the hydrothermal fluids at both sites. Elemental abundances of Ne, Ar, Kr, and Xe in fluid inclusions in all studied samples of chalcopyrite and sphalerite from both hydrothermal sites indicate that the heavier noble gases are derived from ambient seawater. In contrast, helium concentrations are 1 to 2 orders of magnitude higher than expected for equilibrium solution in seawater, indicating that helium is predominantly derived from a magmatic source beneath the hydrothermal systems. The 3He/4He ratios in sphalerite-hosted fluid inclusions from the JADE Field and the SO 134 field are similar to those reported from related vent fluids and show that fluid inclusions reliably record the helium isotope ratio of the original hydrothermal fluid. However, in the North Fiji Basin, earlier “black smoker” activity and the formation of Fe-rich “Kies-type”sulfides seems to have been related to a more MORB-like volcanism, whereas the younger recent “white smoker” hydrothermal activity in the North Fiji Basin can be attributed to a hot spot magmatism in the study area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/443.abstract [article] Helium isotope composition of fluid inclusions hosted in massive sulfides from modern submarine hydrothermal systems [texte imprimé] / Volker Lüders, Auteur ; Samuel Niedermann, Auteur . - 2011 . - pp. 443-449. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 443-449 Mots-clés : Helium  Fluid  inclusion  Sulfides Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Noble gas compositions of fluid inclusions hosted in sulfides from two modern submarine hydrothermal sites of contrasting tectonic settings have been analyzed. The JADE Field (central Okinawa Trough, Japan) is located in an active intracontinental back-arc basin whereas the SO 134 field is related to the central spreading ridge of the North Fiji Basin, which represents an active, highly mature intra-ocean back-arc system. Previous conventional studies of fluid inclusions have revealed compelling evidence for subcritical phase separation of the hydrothermal fluids at both sites. Elemental abundances of Ne, Ar, Kr, and Xe in fluid inclusions in all studied samples of chalcopyrite and sphalerite from both hydrothermal sites indicate that the heavier noble gases are derived from ambient seawater. In contrast, helium concentrations are 1 to 2 orders of magnitude higher than expected for equilibrium solution in seawater, indicating that helium is predominantly derived from a magmatic source beneath the hydrothermal systems. The 3He/4He ratios in sphalerite-hosted fluid inclusions from the JADE Field and the SO 134 field are similar to those reported from related vent fluids and show that fluid inclusions reliably record the helium isotope ratio of the original hydrothermal fluid. However, in the North Fiji Basin, earlier “black smoker” activity and the formation of Fe-rich “Kies-type”sulfides seems to have been related to a more MORB-like volcanism, whereas the younger recent “white smoker” hydrothermal activity in the North Fiji Basin can be attributed to a hot spot magmatism in the study area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/443.abstract

Exemplaires

Bibliothèque Ecole Nationale Polytechnique d'Alger

Accueil

Se connecter

Météo

Adresse