Dépouillements

The physical hydrogeology of ore deposits / S. E. Ingebritsen in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 559-584 Titre : The physical hydrogeology of ore deposits Type de document : texte imprimé Auteurs : S. E. Ingebritsen, Auteur ; M. S. Appold, Auteur Année de publication : 2012 Article en page(s) : pp. 559-584 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Hydrothermal  ore  deposits  THMC  modeling  Geologic  environments Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Hydrothermal ore deposits represent a convergence of fluid flow, thermal energy, and solute flux that is hydrogeologically unusual. From the hydrogeologic perspective, hydrothermal ore deposition represents a complex coupled-flow problem—sufficiently complex that physically rigorous description of the coupled thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes (THMC modeling) continues to challenge our computational ability. Though research into these coupled behaviors has found only a limited subset to be quantitatively tractable, it has yielded valuable insights into the workings of hydrothermal systems in a wide range of geologic environments including sedimentary, metamorphic, and magmatic. Examples of these insights include the quantification of likely driving mechanisms, rates and paths of fluid flow, ore-mineral precipitation mechanisms, longevity of hydrothermal systems, mechanisms by which hydrothermal fluids acquire their temperature and composition, and the controlling influence of permeability and other rock properties on hydrothermal fluid behavior. In this communication we review some of the fundamental theory needed to characterize the physical hydrogeology of hydrothermal systems and discuss how this theory has been applied in studies of Mississippi Valley-type, tabular uranium, porphyry, epithermal, and mid-ocean ridge ore-forming systems. A key limitation in the computational state-of-the-art is the inability to describe fluid flow and transport fully in the many ore systems that show evidence of repeated shear or tensional failure with associated dynamic variations in permeability. However, we discuss global-scale compilations that suggest some numerical constraints on both mean and dynamically enhanced crustal permeability. Principles of physical hydrogeology can be powerful tools for investigating hydrothermal ore formation and are becoming increasingly accessible with ongoing advances in modeling software. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/559.abstract [article] The physical hydrogeology of ore deposits [texte imprimé] / S. E. Ingebritsen, Auteur ; M. S. Appold, Auteur . - 2012 . - pp. 559-584. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 559-584 Mots-clés : Hydrothermal  ore  deposits  THMC  modeling  Geologic  environments Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Hydrothermal ore deposits represent a convergence of fluid flow, thermal energy, and solute flux that is hydrogeologically unusual. From the hydrogeologic perspective, hydrothermal ore deposition represents a complex coupled-flow problem—sufficiently complex that physically rigorous description of the coupled thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes (THMC modeling) continues to challenge our computational ability. Though research into these coupled behaviors has found only a limited subset to be quantitatively tractable, it has yielded valuable insights into the workings of hydrothermal systems in a wide range of geologic environments including sedimentary, metamorphic, and magmatic. Examples of these insights include the quantification of likely driving mechanisms, rates and paths of fluid flow, ore-mineral precipitation mechanisms, longevity of hydrothermal systems, mechanisms by which hydrothermal fluids acquire their temperature and composition, and the controlling influence of permeability and other rock properties on hydrothermal fluid behavior. In this communication we review some of the fundamental theory needed to characterize the physical hydrogeology of hydrothermal systems and discuss how this theory has been applied in studies of Mississippi Valley-type, tabular uranium, porphyry, epithermal, and mid-ocean ridge ore-forming systems. A key limitation in the computational state-of-the-art is the inability to describe fluid flow and transport fully in the many ore systems that show evidence of repeated shear or tensional failure with associated dynamic variations in permeability. However, we discuss global-scale compilations that suggest some numerical constraints on both mean and dynamically enhanced crustal permeability. Principles of physical hydrogeology can be powerful tools for investigating hydrothermal ore formation and are becoming increasingly accessible with ongoing advances in modeling software. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/559.abstract

Targeting iron ore in banded iron formations using ASTER data / Paul Duuring in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 585-597 Titre : Targeting iron ore in banded iron formations using ASTER data : weld range greenstone belt, Yilgarn Craton, western Australia Type de document : texte imprimé Auteurs : Paul Duuring, Auteur ; Steffen G. Hagemann, Auteur ; Yulia Novikova, Auteur Année de publication : 2012 Article en page(s) : pp. 585-597 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : ASTER  image  analysis  Fe  ore  deposits  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image analysis is a proven method for mapping mineral and geochemical zonation associated with a variety of ore types, including orogenic Au, porphyry Cu-(Mo), porphyry-skarn, Pb-Zn-Au, and Mn systems. Only recently has this technique been applied, in a general sense, to mineral alteration mapping and exploration for Fe ore deposits hosted by banded iron formations (BIFs). For this reason, the Archean Weld Range greenstone belt that hosts the Beebyn and Madoonga Fe ore deposits has been chosen as a case study area to test the effectiveness of ASTER imaging techniques for the identification of Fe orebodies. Banded iron formations in the Weld Range district crop out as a series of parallel, 10- to 500-m-wide, <150-m-high, ENE-trending ridges that continue along strike for up to 70 km. Individual ridges of BIFs are surrounded by mafic igneous rocks, whereas felsic volcanic and volcaniclastic rocks and granitoid intrusions are exposed as rounded low-lying mounds in the northern and eastern areas of the district. All outcrop is at least moderately weathered. Thick layers of unconsolidated alluvial and colluvial sediments dominate low-lying areas. The Beebyn and Madoonga high-grade (>55 wt % Fe) iron ore deposits host Archean hypogene magnetite and specular hematite orebodies that are locally replaced by more recently formed, supergene goethite-hematite ore within several hundred meters of the present erosion surface. A common feature of all ore types hosted by BIFs is a high Fe content relative to SiO2. Consequently, all types of Fe ore in the Weld Range district are best identified by the ferric iron to silica index and the opaques to silica index, for the reason that these ASTER image products detect surfaces that are rich in (opaque) Fe oxide minerals and have a low silica abundance. Gabbro, dolerite, and basalt country rocks located within 20 m of high-grade Fe ore zones in BIFs are altered to hypogene Fe-rich chlorite and, more rarely, are altered by Fe-rich talc. These hypogene alteration zones are best detected by the ferrous iron content in MgOH minerals and carbonates and the FeOH group abundance products, which identify hypogene Fe chlorite and Fe talc. This study demonstrates that integrated remote spectral sensing techniques (ASTER, airborne hyperspectral, and radiometric) used in conjunction with geophysical surveys (aeromagnetic and gravity) are useful for district-scale exploration for Fe orebodies hosted by BIFs. The spectral sensing techniques are a rapid, cost-effective, and efficient means for generating and ranking exploration targets that are located in areas with restricted physical access. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/585.short [article] Targeting iron ore in banded iron formations using ASTER data : weld range greenstone belt, Yilgarn Craton, western Australia [texte imprimé] / Paul Duuring, Auteur ; Steffen G. Hagemann, Auteur ; Yulia Novikova, Auteur . - 2012 . - pp. 585-597. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 585-597 Mots-clés : ASTER  image  analysis  Fe  ore  deposits  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image analysis is a proven method for mapping mineral and geochemical zonation associated with a variety of ore types, including orogenic Au, porphyry Cu-(Mo), porphyry-skarn, Pb-Zn-Au, and Mn systems. Only recently has this technique been applied, in a general sense, to mineral alteration mapping and exploration for Fe ore deposits hosted by banded iron formations (BIFs). For this reason, the Archean Weld Range greenstone belt that hosts the Beebyn and Madoonga Fe ore deposits has been chosen as a case study area to test the effectiveness of ASTER imaging techniques for the identification of Fe orebodies. Banded iron formations in the Weld Range district crop out as a series of parallel, 10- to 500-m-wide, <150-m-high, ENE-trending ridges that continue along strike for up to 70 km. Individual ridges of BIFs are surrounded by mafic igneous rocks, whereas felsic volcanic and volcaniclastic rocks and granitoid intrusions are exposed as rounded low-lying mounds in the northern and eastern areas of the district. All outcrop is at least moderately weathered. Thick layers of unconsolidated alluvial and colluvial sediments dominate low-lying areas. The Beebyn and Madoonga high-grade (>55 wt % Fe) iron ore deposits host Archean hypogene magnetite and specular hematite orebodies that are locally replaced by more recently formed, supergene goethite-hematite ore within several hundred meters of the present erosion surface. A common feature of all ore types hosted by BIFs is a high Fe content relative to SiO2. Consequently, all types of Fe ore in the Weld Range district are best identified by the ferric iron to silica index and the opaques to silica index, for the reason that these ASTER image products detect surfaces that are rich in (opaque) Fe oxide minerals and have a low silica abundance. Gabbro, dolerite, and basalt country rocks located within 20 m of high-grade Fe ore zones in BIFs are altered to hypogene Fe-rich chlorite and, more rarely, are altered by Fe-rich talc. These hypogene alteration zones are best detected by the ferrous iron content in MgOH minerals and carbonates and the FeOH group abundance products, which identify hypogene Fe chlorite and Fe talc. This study demonstrates that integrated remote spectral sensing techniques (ASTER, airborne hyperspectral, and radiometric) used in conjunction with geophysical surveys (aeromagnetic and gravity) are useful for district-scale exploration for Fe orebodies hosted by BIFs. The spectral sensing techniques are a rapid, cost-effective, and efficient means for generating and ranking exploration targets that are located in areas with restricted physical access. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/585.short

Geochemical Evolution of the Banded Iron Formation-hosted high-grade iron ore system in the koolyanobbing greenstone belt, western Australia / Thomas Angerer in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 599-644 Titre : Geochemical Evolution of the Banded Iron Formation-hosted high-grade iron ore system in the koolyanobbing greenstone belt, western Australia Type de document : texte imprimé Auteurs : Thomas Angerer, Auteur ; Steffen G. Hagemann, Auteur ; Leonid V. Danyushevsky, Auteur Année de publication : 2012 Article en page(s) : pp. 599-644 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : BIF-hosted  iron  ore  deposits  Geochemical  evolution  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ~200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor–altered, Ca-Si–depleted mafic rocks in the vicinity of the BIF during early regional (syn-D1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D2 to -D4 dissolution of early carbonate. Enrichment of Fe2O3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y–enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D4 faults and caused specularite-dolomite-quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite-martite–rich ore and recent supergene modification, including the further upgrade of Fe ore. DEWEY : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/599.short [article] Geochemical Evolution of the Banded Iron Formation-hosted high-grade iron ore system in the koolyanobbing greenstone belt, western Australia [texte imprimé] / Thomas Angerer, Auteur ; Steffen G. Hagemann, Auteur ; Leonid V. Danyushevsky, Auteur . - 2012 . - pp. 599-644. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 599-644 Mots-clés : BIF-hosted  iron  ore  deposits  Geochemical  evolution  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ~200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor–altered, Ca-Si–depleted mafic rocks in the vicinity of the BIF during early regional (syn-D1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D2 to -D4 dissolution of early carbonate. Enrichment of Fe2O3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y–enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D4 faults and caused specularite-dolomite-quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite-martite–rich ore and recent supergene modification, including the further upgrade of Fe ore. DEWEY : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/599.short

Crustally contaminated komatiite / H. V. Eales in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 645-665 Titre : Crustally contaminated komatiite : primary source of the chromitites and marginal, lower, and critical zone magmas in a staging chamber beneath the bushveld complex Type de document : texte imprimé Auteurs : H. V. Eales, Auteur ; G. Costin, Auteur Année de publication : 2012 Article en page(s) : pp. 645-665 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Lowermost  rocks  Chromium  paradox  Crystal  settling Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In the lowermost rocks of the Rustenburg Layered Suite (Bushveld Complex), levels of total Cr2O3 are up to ten times higher than its putative solubility (0.16% Cr2O3) in the magmas that many investigators have accepted as parental to the dunites, harzburgites, pyroxenites, and norites of the Lower, Lower Critical, and Upper Critical zones. Various existing models fail to explain this “chromium paradox” in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates. Simple mixing calculations and quantitative Adiabat_1ph runs support our model in which a heterogeneous (liquid + crystals) system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30% of crustal melts at depths appropriate to pressures between 10 and 4.5 kbars. We contend that crystal settling and the effects of elutriation during magma ascent had generated a high SiO2 cap with modal olivine, orthopyroxene, and chromite increasing with depth in the staging chamber. The magma from the high SiO2 cap of the staging chamber was first injected into the largely sedimentary Transvaal Supergroup, generating what is now termed the Marginal zone. Slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower Bushveld chamber to form the Lower and Critical zones. Chromite within the slurries introduced in this way could account for their high gross Cr content. Frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber. The lithologically diverse nature of the floors upon which the chromitite layers accumulated (peridotites, pyroxenites, norites, and anorthosites) was therefore more a function of the timing of emplacement of the Cr-rich slurries generated in the deeper reservoir than of specific evolutionary processes within the Bushveld chamber itself. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/645.short [article] Crustally contaminated komatiite : primary source of the chromitites and marginal, lower, and critical zone magmas in a staging chamber beneath the bushveld complex [texte imprimé] / H. V. Eales, Auteur ; G. Costin, Auteur . - 2012 . - pp. 645-665. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 645-665 Mots-clés : Lowermost  rocks  Chromium  paradox  Crystal  settling Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In the lowermost rocks of the Rustenburg Layered Suite (Bushveld Complex), levels of total Cr2O3 are up to ten times higher than its putative solubility (0.16% Cr2O3) in the magmas that many investigators have accepted as parental to the dunites, harzburgites, pyroxenites, and norites of the Lower, Lower Critical, and Upper Critical zones. Various existing models fail to explain this “chromium paradox” in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates. Simple mixing calculations and quantitative Adiabat_1ph runs support our model in which a heterogeneous (liquid + crystals) system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30% of crustal melts at depths appropriate to pressures between 10 and 4.5 kbars. We contend that crystal settling and the effects of elutriation during magma ascent had generated a high SiO2 cap with modal olivine, orthopyroxene, and chromite increasing with depth in the staging chamber. The magma from the high SiO2 cap of the staging chamber was first injected into the largely sedimentary Transvaal Supergroup, generating what is now termed the Marginal zone. Slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower Bushveld chamber to form the Lower and Critical zones. Chromite within the slurries introduced in this way could account for their high gross Cr content. Frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber. The lithologically diverse nature of the floors upon which the chromitite layers accumulated (peridotites, pyroxenites, norites, and anorthosites) was therefore more a function of the timing of emplacement of the Cr-rich slurries generated in the deeper reservoir than of specific evolutionary processes within the Bushveld chamber itself. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/645.short

Insights into the liquid bismuth collector model through analysis of the bi-au stormont skarn prospect, northwest Tasmania / Amy B. D. Cockerton in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 667-682 Titre : Insights into the liquid bismuth collector model through analysis of the bi-au stormont skarn prospect, northwest Tasmania Type de document : texte imprimé Auteurs : Amy B. D. Cockerton, Auteur ; Andrew G. Tomkins, Auteur Année de publication : 2012 Article en page(s) : pp. 667-682 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Liquid  bismuth  collector  model  Hydrothermal  fluid  Stormont  Bi-Au  skarn  prospect  Tasmania Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The liquid bismuth collector model has previously been proposed to explain the common association between gold and bismuth seen in many ore deposits, and involves the scavenging of gold from hydrothermal fluid by liquid bismuth. Here, textural relationships and temperature and chemical conditions of mineralization at the Stormont Bi-Au skarn prospect in northwestern Tasmania are analyzed to investigate whether the liquid bismuth collector model can explain gold accumulation at this prospect. The calc-silicate skarn assemblage containing grandite, hedenbergite, actinolite, magnetite, epidote, quartz, and calcite suggests that initial metasomatism involved temperatures >400° to 500°C. Visible gold is associated with native bismuth, bismuthinite, maldonite, galena, and an unknown Bi-Te-S mineral. Native bismuth is directly associated with grandite, which formed during initial stages of metasomatism. This implies that bismuth would have precipitated from a hydrothermal fluid as a liquid. Heating experiments showed that bismuth alloy inclusions in garnet melt between temperatures of 250° and 288°C. The mineralization setting at Stormont and textural relationships suggest that gold was scavenged from hydrothermal fluids by liquid bismuth. Rhythmically zoned grandite crystals provide evidence for fluctuating hydrothermal fluid conditions, which may have contributed to dissolution and reprecipitation of gold, thus facilitating an ore zone refining process within the prospect. This refining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly under-saturated fluids. Infiltrating gold-undersaturated fluids can dissolve gold not in contact with bismuth, which can then be preferentially redeposited upon encountering a bismuth droplet, enhancing the correlation between the two elements. A long-lived and focused fluid flow system would maximize the efficiency of this ore zone refining process, which is likely to be relevant to gold deposition at a range of deposit types. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/667.short [article] Insights into the liquid bismuth collector model through analysis of the bi-au stormont skarn prospect, northwest Tasmania [texte imprimé] / Amy B. D. Cockerton, Auteur ; Andrew G. Tomkins, Auteur . - 2012 . - pp. 667-682. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 667-682 Mots-clés : Liquid  bismuth  collector  model  Hydrothermal  fluid  Stormont  Bi-Au  skarn  prospect  Tasmania Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The liquid bismuth collector model has previously been proposed to explain the common association between gold and bismuth seen in many ore deposits, and involves the scavenging of gold from hydrothermal fluid by liquid bismuth. Here, textural relationships and temperature and chemical conditions of mineralization at the Stormont Bi-Au skarn prospect in northwestern Tasmania are analyzed to investigate whether the liquid bismuth collector model can explain gold accumulation at this prospect. The calc-silicate skarn assemblage containing grandite, hedenbergite, actinolite, magnetite, epidote, quartz, and calcite suggests that initial metasomatism involved temperatures >400° to 500°C. Visible gold is associated with native bismuth, bismuthinite, maldonite, galena, and an unknown Bi-Te-S mineral. Native bismuth is directly associated with grandite, which formed during initial stages of metasomatism. This implies that bismuth would have precipitated from a hydrothermal fluid as a liquid. Heating experiments showed that bismuth alloy inclusions in garnet melt between temperatures of 250° and 288°C. The mineralization setting at Stormont and textural relationships suggest that gold was scavenged from hydrothermal fluids by liquid bismuth. Rhythmically zoned grandite crystals provide evidence for fluctuating hydrothermal fluid conditions, which may have contributed to dissolution and reprecipitation of gold, thus facilitating an ore zone refining process within the prospect. This refining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly under-saturated fluids. Infiltrating gold-undersaturated fluids can dissolve gold not in contact with bismuth, which can then be preferentially redeposited upon encountering a bismuth droplet, enhancing the correlation between the two elements. A long-lived and focused fluid flow system would maximize the efficiency of this ore zone refining process, which is likely to be relevant to gold deposition at a range of deposit types. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/667.short

Petrology, geochemistry, and stable isotope studies of the chehelkureh cu-zn-pb deposit, Zahedan, Iran / Mohammad Maanijou in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 683-712 Titre : Petrology, geochemistry, and stable isotope studies of the chehelkureh cu-zn-pb deposit, Zahedan, Iran Type de document : texte imprimé Auteurs : Mohammad Maanijou, Auteur ; Iraj Rasa, Auteur ; David R. Lentz, Auteur Année de publication : 2012 Article en page(s) : pp. 683-712 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Metal  deposit  Petrogenetic  model  Geochemistry  Iran Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Chehelkureh base metal deposit in southeast Iran is in an abandoned historic copper district that is fault controlled and hosted in a sequence of slightly Eocene interbedded graywackes, siltstones, and shales. Several stocks and dikes of Oligo-Miocene granodiorite to quartz monzodiorite and granite composition, oriented parallel to the dominant northwest-southeast fault system, intruded into the sedimentary sequence. The sedimentary rocks were metamorphosed to the hornfels facies in some outcrops. The intrusions are spatially and temporally related to mineralized faults. The deposit consists of numerous irregular lenses and veins located along faults that cover an area 1,500 m long by 80 to 280 m wide. There are two periods of primary Cu-Zn-Pb mineralization that crosscut each other. The first stage includes quartz, calcite, dolomite, ankerite, siderite, ilmenite, rutile, molybdenite, pyrrhotite, arsenopyrite, pyrite, and chalcopyrite. The second stage consists of quartz, dolomite, ankerite, siderite, chalcopyrite, sphalerite, pyrite, galena, selenian galena, marcasite, nevskite, and paraguanajuatite. The gangue minerals are dominated by quartz and various carbonates, locally associated with chlorite. Hypogene alteration consists of silicification, carbonatization (ankerite, magnesite, siderite, and dolomite), chloritization, kaolinitization, sulfidation, and, less commonly, sericitization. The intrusive rocks at Chehelkureh are calc-alkaline and have chemical features typical of I-type granitoids. Based on Nb-Y and Ta-Yb discrimination diagrams, the geotectonic environment of Chehelkureh granitoids is an intracontinental volcanic arc (Nb/Y ~ 0.4), which is the same setting as that in the multiphase granitoid batholiths of Zahedan, located southeast of Chehelkureh. Granitoids of the Chehelkureh area have moderate REE (rare-earth element) contents (∑REE = 110–174 ppm; average 153 ppm), moderate light REE/heavy REE ((La/Lu)cn = 7–8), and strong negative Eu anomalies (Eu/Eu* ~ 0.2), but no Eu anomaly is evident in monzodiorites. Spider diagram patterns for samples of igneous rocks show Nb and Ta, Sr, and Ti-V negative anomalies, with high amounts of Cu, U, and Th. Fluid inclusion studies of quartz intergrown with sulfides in quartz veins show three main types of fluid inclusions: type 1 inclusions consist of liquid + vapor + daughter crystal (solid); type 2 inclusions are composed of liquid + minor vapor, with vapor/liquid ratios of 0.1 to 0.4; and type 3 inclusions are vapor rich with liquid/vapor ratios up to 0.05 (liquid phase is minor or absent). Homogenization temperatures (Th) for type 2 inclusions vary between 330° and 480°C, whereas their salinities range from 5 to 15 wt % NaCl equiv. The δ18Owater values of quartz in sulfide-bearing veins are 8.8 to 11‰, with an average of 10‰. Most of the δ18Owater values of quartz veins are in the range of typical magmatic, metamorphic, and connate waters, so the source of the ore fluids cannot be distinguished based on oxygen isotopes alone. The δ18Owater and δDwater values calculated from chlorite range from 5.6 to 10.6‰ and −31 to −23‰, respectively. These data lie in the metamorphic water box, in the formation water box, or about midway between the magmatic water field and typical seawater. For kaolinite at ~200°C, the calculated δ18Owater varies from 4.2 to 10.7‰ and the δDwater values range from −88 to −49‰. The carbon isotope values (Pee Dee Belemnite) of carbonates vary between −5.7 and −0.9‰, whereas the δ18O values (Standard Mean Ocean Water) are between 12.4 and 14.9‰. The coupled oxygen and carbon isotope values shift from magmatic values and can be related to metasedimentary decarbonation reactions and metasomatic exchange of oxygen with magmatic fluid. The range of δ34S values of sulfide minerals is small (2.0–4.2‰, with a mean value of 3‰), consistent with a magmatic origin for sulfur. Isotope thermometry, based on quartz-carbonate pairs, yields an average of 450°C, which is consistent with fluid inclusion temperatures. Isothermal fO2-pH diagrams constructed at 450°C and 1 kbar suggest that the ore formed in the H2S-predominant field with fO2 and pH ranging from 10−29 to 10−24 and 5.0 to 6.2, respectively. The petrogenetic model proposed for the Chehelkureh deposit suggests that during the Oligocene a quartz-monzodiorite stock intruded the Eocene turbidites and provided a magmatic component to the ore-forming system, but also acted as the heat source for hydrothermal convection cells, although metamorphic and connate waters may also have contributed to the ore-forming fluids. In this model, magma is the main source of sulfur, base metals, and carbon, although Eocene turbidites also contributed some metals and carbon. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/683.abstract [article] Petrology, geochemistry, and stable isotope studies of the chehelkureh cu-zn-pb deposit, Zahedan, Iran [texte imprimé] / Mohammad Maanijou, Auteur ; Iraj Rasa, Auteur ; David R. Lentz, Auteur . - 2012 . - pp. 683-712. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 683-712 Mots-clés : Metal  deposit  Petrogenetic  model  Geochemistry  Iran Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Chehelkureh base metal deposit in southeast Iran is in an abandoned historic copper district that is fault controlled and hosted in a sequence of slightly Eocene interbedded graywackes, siltstones, and shales. Several stocks and dikes of Oligo-Miocene granodiorite to quartz monzodiorite and granite composition, oriented parallel to the dominant northwest-southeast fault system, intruded into the sedimentary sequence. The sedimentary rocks were metamorphosed to the hornfels facies in some outcrops. The intrusions are spatially and temporally related to mineralized faults. The deposit consists of numerous irregular lenses and veins located along faults that cover an area 1,500 m long by 80 to 280 m wide. There are two periods of primary Cu-Zn-Pb mineralization that crosscut each other. The first stage includes quartz, calcite, dolomite, ankerite, siderite, ilmenite, rutile, molybdenite, pyrrhotite, arsenopyrite, pyrite, and chalcopyrite. The second stage consists of quartz, dolomite, ankerite, siderite, chalcopyrite, sphalerite, pyrite, galena, selenian galena, marcasite, nevskite, and paraguanajuatite. The gangue minerals are dominated by quartz and various carbonates, locally associated with chlorite. Hypogene alteration consists of silicification, carbonatization (ankerite, magnesite, siderite, and dolomite), chloritization, kaolinitization, sulfidation, and, less commonly, sericitization. The intrusive rocks at Chehelkureh are calc-alkaline and have chemical features typical of I-type granitoids. Based on Nb-Y and Ta-Yb discrimination diagrams, the geotectonic environment of Chehelkureh granitoids is an intracontinental volcanic arc (Nb/Y ~ 0.4), which is the same setting as that in the multiphase granitoid batholiths of Zahedan, located southeast of Chehelkureh. Granitoids of the Chehelkureh area have moderate REE (rare-earth element) contents (∑REE = 110–174 ppm; average 153 ppm), moderate light REE/heavy REE ((La/Lu)cn = 7–8), and strong negative Eu anomalies (Eu/Eu* ~ 0.2), but no Eu anomaly is evident in monzodiorites. Spider diagram patterns for samples of igneous rocks show Nb and Ta, Sr, and Ti-V negative anomalies, with high amounts of Cu, U, and Th. Fluid inclusion studies of quartz intergrown with sulfides in quartz veins show three main types of fluid inclusions: type 1 inclusions consist of liquid + vapor + daughter crystal (solid); type 2 inclusions are composed of liquid + minor vapor, with vapor/liquid ratios of 0.1 to 0.4; and type 3 inclusions are vapor rich with liquid/vapor ratios up to 0.05 (liquid phase is minor or absent). Homogenization temperatures (Th) for type 2 inclusions vary between 330° and 480°C, whereas their salinities range from 5 to 15 wt % NaCl equiv. The δ18Owater values of quartz in sulfide-bearing veins are 8.8 to 11‰, with an average of 10‰. Most of the δ18Owater values of quartz veins are in the range of typical magmatic, metamorphic, and connate waters, so the source of the ore fluids cannot be distinguished based on oxygen isotopes alone. The δ18Owater and δDwater values calculated from chlorite range from 5.6 to 10.6‰ and −31 to −23‰, respectively. These data lie in the metamorphic water box, in the formation water box, or about midway between the magmatic water field and typical seawater. For kaolinite at ~200°C, the calculated δ18Owater varies from 4.2 to 10.7‰ and the δDwater values range from −88 to −49‰. The carbon isotope values (Pee Dee Belemnite) of carbonates vary between −5.7 and −0.9‰, whereas the δ18O values (Standard Mean Ocean Water) are between 12.4 and 14.9‰. The coupled oxygen and carbon isotope values shift from magmatic values and can be related to metasedimentary decarbonation reactions and metasomatic exchange of oxygen with magmatic fluid. The range of δ34S values of sulfide minerals is small (2.0–4.2‰, with a mean value of 3‰), consistent with a magmatic origin for sulfur. Isotope thermometry, based on quartz-carbonate pairs, yields an average of 450°C, which is consistent with fluid inclusion temperatures. Isothermal fO2-pH diagrams constructed at 450°C and 1 kbar suggest that the ore formed in the H2S-predominant field with fO2 and pH ranging from 10−29 to 10−24 and 5.0 to 6.2, respectively. The petrogenetic model proposed for the Chehelkureh deposit suggests that during the Oligocene a quartz-monzodiorite stock intruded the Eocene turbidites and provided a magmatic component to the ore-forming system, but also acted as the heat source for hydrothermal convection cells, although metamorphic and connate waters may also have contributed to the ore-forming fluids. In this model, magma is the main source of sulfur, base metals, and carbon, although Eocene turbidites also contributed some metals and carbon. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/683.abstract

An re-os date for molybdenite-bearing quartz vein mineralization within the kangerlussuaq alkaline complex, east Greenland / D. A. Holwell in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 713-722 Titre : An re-os date for molybdenite-bearing quartz vein mineralization within the kangerlussuaq alkaline complex, east Greenland : implications for the timing of regional metallogenesis Type de document : texte imprimé Auteurs : D. A. Holwell, Auteur ; D. Selby, Auteur ; A. J. Boyce, Auteur Année de publication : 2012 Article en page(s) : pp. 713-722 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Alkaline  complex  Metal  mineralization  Mo  deposits  Greenland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Kangerlussuaq Alkaline Complex, East Greenland, is one of the largest alkaline complexes in the world. It is known to host a number of occurrences of base and precious metal mineralization, including the Flammefjeld porphyry Mo deposits and a suite of epithermal style base and precious metal veins in an area around the Søndre Syenitgletscher. Here we describe and date a previously unknown occurrence of molybdenite-pyrite-scheelite–bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex. The veins appear to be cogenetic with the dikes, which crosscut and brecciate granodiorites of the Cirque 1320 complex. Sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean δ34S of 2.2 ± 0.7‰, consistent with a magmatic S source related to the intrusion of the dikes. Molybdenite from the veins gave an Re-Os age of 52.74 ± 0.26 Ma, some 13 m.y. older than molybdenite at the Flammefjeld porphyry deposit, thus distinguishing this metallogenic episode as a distinct event, temporally unrelated to the known Mo mineralization at Flammefjeld. Significantly, in terms of the timing of regional metallogenesis and magmatism, our date suggests that the age of the Kangerlussuaq Alkaline Complex itself may be slightly older than the 50 Ma currently accepted, based on 40Ar-39Ar dating from biotites in the main syenites. Given the common discrepancy in ore systems of 40Ar-39Ar, which produce slightly younger dates than those by Re-Os and U-Pb, we suggest the actual age of the Kangerlussuaq Alkaline Complex may be closer to 53 to 52 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/713.abstract [article] An re-os date for molybdenite-bearing quartz vein mineralization within the kangerlussuaq alkaline complex, east Greenland : implications for the timing of regional metallogenesis [texte imprimé] / D. A. Holwell, Auteur ; D. Selby, Auteur ; A. J. Boyce, Auteur . - 2012 . - pp. 713-722. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 713-722 Mots-clés : Alkaline  complex  Metal  mineralization  Mo  deposits  Greenland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Kangerlussuaq Alkaline Complex, East Greenland, is one of the largest alkaline complexes in the world. It is known to host a number of occurrences of base and precious metal mineralization, including the Flammefjeld porphyry Mo deposits and a suite of epithermal style base and precious metal veins in an area around the Søndre Syenitgletscher. Here we describe and date a previously unknown occurrence of molybdenite-pyrite-scheelite–bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex. The veins appear to be cogenetic with the dikes, which crosscut and brecciate granodiorites of the Cirque 1320 complex. Sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean δ34S of 2.2 ± 0.7‰, consistent with a magmatic S source related to the intrusion of the dikes. Molybdenite from the veins gave an Re-Os age of 52.74 ± 0.26 Ma, some 13 m.y. older than molybdenite at the Flammefjeld porphyry deposit, thus distinguishing this metallogenic episode as a distinct event, temporally unrelated to the known Mo mineralization at Flammefjeld. Significantly, in terms of the timing of regional metallogenesis and magmatism, our date suggests that the age of the Kangerlussuaq Alkaline Complex itself may be slightly older than the 50 Ma currently accepted, based on 40Ar-39Ar dating from biotites in the main syenites. Given the common discrepancy in ore systems of 40Ar-39Ar, which produce slightly younger dates than those by Re-Os and U-Pb, we suggest the actual age of the Kangerlussuaq Alkaline Complex may be closer to 53 to 52 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/713.abstract

Petrography, chemistry, and near-infrared microthermometry of indum-bearing sphalerite from the toyoha polymetallic deposit, Japan / Toru Shimizu in Economic geology, Vol. 107 N° 4 (Juin/Juillet 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 723-735 Titre : Petrography, chemistry, and near-infrared microthermometry of indum-bearing sphalerite from the toyoha polymetallic deposit, Japan Type de document : texte imprimé Auteurs : Toru Shimizu, Auteur ; Yuichi Morishita, Auteur Année de publication : 2012 Article en page(s) : pp. 723-735 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : In-bearing  sphalerite  Petrography  X-ray  mapping  Electron  microprobe  analyses  Near-infrared  (NIR)  microscopy  Japan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Textural, chemical, and microthermometric characteristics of In-bearing sphalerite from the Toyoha Zn-Pb-Ag-Cu-Sn-In vein-type deposit are investigated using petrography, X-ray mapping, electron microprobe analyses, and near-infrared (NIR) microscopy. The studied ore is rich in zinc and mainly consists of sphalerite. The In-bearing sphalerite formed as euhedral growth bandings after corrosion of In-free sphalerite and replacement of a precursor mineral. The In-bearing sphalerite, which appears black and dark red in thin sections, is zoned with alternating high and low In bands. This zoning is only visible by X-ray mapping. There is a positive correlation between In and Cu, supporting the case for coupled substitution: In3+ + Cu+ <−> 2 Zn2+. The maximum concentrations of In and Cu in sphalerite are 7.03 and 3.65 wt %, respectively. Indium is also commonly correlated with trace amounts of Ag and Sn. Primary fluid inclusions suggest that the maximum formation temperature of In-bearing sphalerite is 305°C. This indicates that In concentration in Zn-rich ores occurred at lower temperatures than in Cu-rich In-bearing ores (350°–400°C) previously reported at Toyoha. Textural and chemical characteristics indicate that fluids enriched in In, Cu, Ag, and Sn formed In-bearing sphalerite after corrosion of In-free sphalerite and replacement of a precursor mineral. Combined with previous geological, mineralogical, and experimental studies, we propose that In was transported by high-temperature (≥300°C) fluids enriched in In, Cu, Ag, and Sn. Indium may have been derived by remobilization of the metals from the preexisting Cu-rich ores at depth and/or injection of metal-rich fluid from a new source. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/723.short [article] Petrography, chemistry, and near-infrared microthermometry of indum-bearing sphalerite from the toyoha polymetallic deposit, Japan [texte imprimé] / Toru Shimizu, Auteur ; Yuichi Morishita, Auteur . - 2012 . - pp. 723-735. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 4 (Juin/Juillet 2012) . - pp. 723-735 Mots-clés : In-bearing  sphalerite  Petrography  X-ray  mapping  Electron  microprobe  analyses  Near-infrared  (NIR)  microscopy  Japan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Textural, chemical, and microthermometric characteristics of In-bearing sphalerite from the Toyoha Zn-Pb-Ag-Cu-Sn-In vein-type deposit are investigated using petrography, X-ray mapping, electron microprobe analyses, and near-infrared (NIR) microscopy. The studied ore is rich in zinc and mainly consists of sphalerite. The In-bearing sphalerite formed as euhedral growth bandings after corrosion of In-free sphalerite and replacement of a precursor mineral. The In-bearing sphalerite, which appears black and dark red in thin sections, is zoned with alternating high and low In bands. This zoning is only visible by X-ray mapping. There is a positive correlation between In and Cu, supporting the case for coupled substitution: In3+ + Cu+ <−> 2 Zn2+. The maximum concentrations of In and Cu in sphalerite are 7.03 and 3.65 wt %, respectively. Indium is also commonly correlated with trace amounts of Ag and Sn. Primary fluid inclusions suggest that the maximum formation temperature of In-bearing sphalerite is 305°C. This indicates that In concentration in Zn-rich ores occurred at lower temperatures than in Cu-rich In-bearing ores (350°–400°C) previously reported at Toyoha. Textural and chemical characteristics indicate that fluids enriched in In, Cu, Ag, and Sn formed In-bearing sphalerite after corrosion of In-free sphalerite and replacement of a precursor mineral. Combined with previous geological, mineralogical, and experimental studies, we propose that In was transported by high-temperature (≥300°C) fluids enriched in In, Cu, Ag, and Sn. Indium may have been derived by remobilization of the metals from the preexisting Cu-rich ores at depth and/or injection of metal-rich fluid from a new source. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/4/723.short

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