Dépouillements

P-T-X Conditions of Fluids in the Sunrise dam gold deposit, western australia, and implications for the interplay between deformation and fluids / T. Baker in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 873-894 Titre : P-T-X Conditions of Fluids in the Sunrise dam gold deposit, western australia, and implications for the interplay between deformation and fluids Type de document : texte imprimé Auteurs : T. Baker, Auteur ; M. Bertelli, Auteur ; T. Blenkinsop, Auteur Année de publication : 2011 Article en page(s) : pp. 873-894 Note générale : Economic Geology Langues : Anglais (eng) Mots-clés : Gold  deposit  Fluids  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow- to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures < 300°C and pressures of ~1 kbar. CO2 inclusions crosscut the early CO2-H2O inclusions and are the dominant inclusion type in Group II, III, and IV structures. They likely represent a major late influx of separately sourced fluid (magmatic or mantle?) during D4 and were trapped at lower pressure conditions than the earlier CO2-H2O fluids. The combined structural and fluid history for the deposit suggests that the early CO2-H2O fluid ponded beneath moderately dipping shear zones during late D2, early D3, and that fluid pressures increased to near- or supralithostatic conditions (>1–3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/873.abstract [article] P-T-X Conditions of Fluids in the Sunrise dam gold deposit, western australia, and implications for the interplay between deformation and fluids [texte imprimé] / T. Baker, Auteur ; M. Bertelli, Auteur ; T. Blenkinsop, Auteur . - 2011 . - pp. 873-894. Economic Geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 873-894 Mots-clés : Gold  deposit  Fluids  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow- to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures < 300°C and pressures of ~1 kbar. CO2 inclusions crosscut the early CO2-H2O inclusions and are the dominant inclusion type in Group II, III, and IV structures. They likely represent a major late influx of separately sourced fluid (magmatic or mantle?) during D4 and were trapped at lower pressure conditions than the earlier CO2-H2O fluids. The combined structural and fluid history for the deposit suggests that the early CO2-H2O fluid ponded beneath moderately dipping shear zones during late D2, early D3, and that fluid pressures increased to near- or supralithostatic conditions (>1–3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/873.abstract

Crustal-scale fluid pathways and source rocks in the victorian gold province, australia / C. E. Willman in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 895-915 Titre : Crustal-scale fluid pathways and source rocks in the victorian gold province, australia : insights from deep seismic reflection profiles Type de document : texte imprimé Auteurs : C. E. Willman, Auteur ; R. J. Korsch, Auteur ; D. H. Moore, Auteur Année de publication : 2011 Article en page(s) : pp. 895-915 Note générale : Economic Geology Langues : Anglais (eng) Mots-clés : Crustal-Scale  Fluid  Pathways  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/895.abstract [article] Crustal-scale fluid pathways and source rocks in the victorian gold province, australia : insights from deep seismic reflection profiles [texte imprimé] / C. E. Willman, Auteur ; R. J. Korsch, Auteur ; D. H. Moore, Auteur . - 2011 . - pp. 895-915. Economic Geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 895-915 Mots-clés : Crustal-Scale  Fluid  Pathways  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/895.abstract

The BIF-hosted high-grade iron ore deposits in the archean koolyanobbing greenstone belt, western Australia / Thomas Angerer in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 917-945 Titre : The BIF-hosted high-grade iron ore deposits in the archean koolyanobbing greenstone belt, western Australia : structural control on synorogenic- and weathering-related magnetite-, hematite-, and goethite-rich iron ore Type de document : texte imprimé Auteurs : Thomas Angerer, Auteur ; Steffen G. Hagemann, Auteur Année de publication : 2011 Article en page(s) : pp. 917-945 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Australia  Archean  Iron  Ore Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Koolyanobbing banded iron formation (BIF)-hosted iron ore deposits (total premining resources ~150 million metric tons (Mt), indicated reserves ~32 Mt) are located in the Mesoarchean lower succession BIF of the Koolyanobbing greenstone belt, Younami terrane, Yilgarn craton in Western Australia. In the Koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the Southern Cross domain includes: D1 structures (mainly small-scale F1a and F1b folds, formed in a north-south to northwest-southeast compressional regime), a ductile to brittle deformation sequence, D2 to D4 (generated during east-west compression) and, a late-stage brittle segmentation of BIF and reactivation of faults, attributed to D5. The formation of the seven known medium- (45–58 wt % Fe) to high-grade (58–68 wt % Fe) magnetite-, martite-, specularite-, and goethite-bearing orebodies can be subdivided into four Archean stages and one weathering-related upgrade from the Permian and/or Mesozoic to recent times. The Archean ore-forming stages comprise: (1) early Fe-Mg ± Ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz-magnetite BIF protolith; (2) sequential syn-D2a (coaxial) to syn-D4 (transpressional) tight folding-driven removal of carbonate, quartz and minor ferroan talc by solution and mechanical transfer, producing residual enrichment of medium- to high-grade magnetite ore; (3) magnetite mineralization in syn-D2b and syn-D4 breccias and fractures, forming medium-grade ore zones, or overprint magnetite in BIF and first-stage magnetite ore; and (4) mineralization of hydrothermal specularite and locally associated ferroan dolomite-quartz alteration, and local oxidation of magnetite in and near brittle D4 faults, fractures, and reactivated F1 and F2a fold cores. Modern weathering-related leaching of carbonate (and minor quartz), pseudomorphic goethite replacement of existing iron oxides and gangue, and coeval or subsequent to oxidation in the vadose zone formed goethite-martite ore with local relics of specularite or magnetite and/or kenomagnetite. The intensity and localization of this supergene modification is, in most deposits at Koolyanobbing, controlled by existing hypogene magnetite, specularite-rich medium- to high-grade ore zones and/or carbonate-altered BIF at depth. The existence of high-grade ore below the weathering horizons suggests the possibility of further concealed magnetite- and/or specularite-rich orebodies within the deposits and region. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/917.abstract [article] The BIF-hosted high-grade iron ore deposits in the archean koolyanobbing greenstone belt, western Australia : structural control on synorogenic- and weathering-related magnetite-, hematite-, and goethite-rich iron ore [texte imprimé] / Thomas Angerer, Auteur ; Steffen G. Hagemann, Auteur . - 2011 . - pp. 917-945. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 917-945 Mots-clés : Australia  Archean  Iron  Ore Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Koolyanobbing banded iron formation (BIF)-hosted iron ore deposits (total premining resources ~150 million metric tons (Mt), indicated reserves ~32 Mt) are located in the Mesoarchean lower succession BIF of the Koolyanobbing greenstone belt, Younami terrane, Yilgarn craton in Western Australia. In the Koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the Southern Cross domain includes: D1 structures (mainly small-scale F1a and F1b folds, formed in a north-south to northwest-southeast compressional regime), a ductile to brittle deformation sequence, D2 to D4 (generated during east-west compression) and, a late-stage brittle segmentation of BIF and reactivation of faults, attributed to D5. The formation of the seven known medium- (45–58 wt % Fe) to high-grade (58–68 wt % Fe) magnetite-, martite-, specularite-, and goethite-bearing orebodies can be subdivided into four Archean stages and one weathering-related upgrade from the Permian and/or Mesozoic to recent times. The Archean ore-forming stages comprise: (1) early Fe-Mg ± Ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz-magnetite BIF protolith; (2) sequential syn-D2a (coaxial) to syn-D4 (transpressional) tight folding-driven removal of carbonate, quartz and minor ferroan talc by solution and mechanical transfer, producing residual enrichment of medium- to high-grade magnetite ore; (3) magnetite mineralization in syn-D2b and syn-D4 breccias and fractures, forming medium-grade ore zones, or overprint magnetite in BIF and first-stage magnetite ore; and (4) mineralization of hydrothermal specularite and locally associated ferroan dolomite-quartz alteration, and local oxidation of magnetite in and near brittle D4 faults, fractures, and reactivated F1 and F2a fold cores. Modern weathering-related leaching of carbonate (and minor quartz), pseudomorphic goethite replacement of existing iron oxides and gangue, and coeval or subsequent to oxidation in the vadose zone formed goethite-martite ore with local relics of specularite or magnetite and/or kenomagnetite. The intensity and localization of this supergene modification is, in most deposits at Koolyanobbing, controlled by existing hypogene magnetite, specularite-rich medium- to high-grade ore zones and/or carbonate-altered BIF at depth. The existence of high-grade ore below the weathering horizons suggests the possibility of further concealed magnetite- and/or specularite-rich orebodies within the deposits and region. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/917.abstract

Baogutu porphyry cu-mo-au deposit, west junggar, northwest china / Ping Shen in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 947-970 Titre : Baogutu porphyry cu-mo-au deposit, west junggar, northwest china : petrology, alteration, and mineralization Type de document : texte imprimé Auteurs : Ping Shen, Auteur ; Yuanchao Shen, Auteur ; Hongdi Pan, Auteur Année de publication : 2011 Article en page(s) : pp. 947-970 Note générale : Economic Geology Langues : Anglais (eng) Mots-clés : Mineralization  Alteration  Petrology  Northwest  China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Baogutu is the first porphyry Cu-Mo-Au deposit discovered in West Junggar, Xinjiang, China. It is part of the Central Asian orogenic belt. Baogutu is associated with a Carboniferous intrusive complex that was emplaced into lower Carboniferous volcano-sedimentary sequences of the Baogutu and Xibeikulasi Groups. The intrusive complex is made up of main stage equigranular to porphyritic diorites and quartz diorites, and minor late stage diorite porphyries. Intrusive activity occurred at about 325 Ma based on U-Pb (SHRIMP) analyses of zircons. The main stage diorites host the bulk of the Cu-Mo-Au mineralization at Baogutu. They have been overprinted by three alteration assemblages, including an early potassic (biotite) assemblage that occurs in the center of the deposit. A propylitic assemblage surrounds the potassic zone concentrically. Both of these alteration assemblages have been overprinted locally by phyllic alteration (quartz-sericite-pyrite), which is associated with the highest Cu-Mo grades. Mineralized and hydrothermally cemented breccias have disrupted the main stage diorites. The late stage diorites have undergone moderate potassic alteration and contain weak Cu mineralization. Matrix-rich breccias with very weak mineralization have cut the intrusive complex and disrupted the ore-body. Re-Os dating of molybdenite indicates that mineralization occurred at about 310 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/947.abstract [article] Baogutu porphyry cu-mo-au deposit, west junggar, northwest china : petrology, alteration, and mineralization [texte imprimé] / Ping Shen, Auteur ; Yuanchao Shen, Auteur ; Hongdi Pan, Auteur . - 2011 . - pp. 947-970. Economic Geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 947-970 Mots-clés : Mineralization  Alteration  Petrology  Northwest  China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Baogutu is the first porphyry Cu-Mo-Au deposit discovered in West Junggar, Xinjiang, China. It is part of the Central Asian orogenic belt. Baogutu is associated with a Carboniferous intrusive complex that was emplaced into lower Carboniferous volcano-sedimentary sequences of the Baogutu and Xibeikulasi Groups. The intrusive complex is made up of main stage equigranular to porphyritic diorites and quartz diorites, and minor late stage diorite porphyries. Intrusive activity occurred at about 325 Ma based on U-Pb (SHRIMP) analyses of zircons. The main stage diorites host the bulk of the Cu-Mo-Au mineralization at Baogutu. They have been overprinted by three alteration assemblages, including an early potassic (biotite) assemblage that occurs in the center of the deposit. A propylitic assemblage surrounds the potassic zone concentrically. Both of these alteration assemblages have been overprinted locally by phyllic alteration (quartz-sericite-pyrite), which is associated with the highest Cu-Mo grades. Mineralized and hydrothermally cemented breccias have disrupted the main stage diorites. The late stage diorites have undergone moderate potassic alteration and contain weak Cu mineralization. Matrix-rich breccias with very weak mineralization have cut the intrusive complex and disrupted the ore-body. Re-Os dating of molybdenite indicates that mineralization occurred at about 310 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/947.abstract

Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada / de Almeida, Carolina Michelin in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 971-1004 Titre : Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada : implications for sources of ore-forming elements, ore genesis, and mineral exploration Type de document : texte imprimé Auteurs : de Almeida, Carolina Michelin, Auteur ; Gema Ribeiro Olivo, Auteur ; Annick Chouinard, Auteur Année de publication : 2011 Article en page(s) : pp. 971-1004 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  ore  Host  rocks  Mineral  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada. Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II. The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite. this study reveals that Al2O3 and TiO2 are the most immobile compounds, and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks. Among the ore-related trace elements, Tl best correlates with Au grade (R2 = 0.69) and shows some relationship with the calculated amount of pyrite (R2 = 0.49), indicating that Tl would be the best element to vector toward zones of high-grade Carlin-type Au mineralization. Gold grades do not correlate with the amount of pyrite, degree of alteration, or organic C. Our results integrated with available thermodynamic data for AU, ore-related elements, and SiO2 lead us to suggest that the formation of Ore I occurred more proximal to the major mineralizing conduits as the hot, more acid, SiO2- and trace element-rich auriferous fluids interacted with Fe-bearing impure carbonate host rocks, intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the Betze-Post and Rodeo deposits. As the fluids moved laterally throughout the favorable host rocks, the pH increased, leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica, favoring the formation of more distal Ore II in the central-northern part of the Screamer deposit. Significantly, the gold concentrations in whole rock and in pyrite are, in some way, very similar in both ore types, being slightly higher in Ore II, suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite, even at lower concentrations of other trace elements. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/971.abstract [article] Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada : implications for sources of ore-forming elements, ore genesis, and mineral exploration [texte imprimé] / de Almeida, Carolina Michelin, Auteur ; Gema Ribeiro Olivo, Auteur ; Annick Chouinard, Auteur . - 2011 . - pp. 971-1004. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 971-1004 Mots-clés : Gold  ore  Host  rocks  Mineral  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada. Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II. The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite. this study reveals that Al2O3 and TiO2 are the most immobile compounds, and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks. Among the ore-related trace elements, Tl best correlates with Au grade (R2 = 0.69) and shows some relationship with the calculated amount of pyrite (R2 = 0.49), indicating that Tl would be the best element to vector toward zones of high-grade Carlin-type Au mineralization. Gold grades do not correlate with the amount of pyrite, degree of alteration, or organic C. Our results integrated with available thermodynamic data for AU, ore-related elements, and SiO2 lead us to suggest that the formation of Ore I occurred more proximal to the major mineralizing conduits as the hot, more acid, SiO2- and trace element-rich auriferous fluids interacted with Fe-bearing impure carbonate host rocks, intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the Betze-Post and Rodeo deposits. As the fluids moved laterally throughout the favorable host rocks, the pH increased, leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica, favoring the formation of more distal Ore II in the central-northern part of the Screamer deposit. Significantly, the gold concentrations in whole rock and in pyrite are, in some way, very similar in both ore types, being slightly higher in Ore II, suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite, even at lower concentrations of other trace elements. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/971.abstract

Microbial sphalerite formation in carbonate-hosted Zn-Pb ores, Bleiberg, Austria / H. Kucha in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1005-1023 Titre : Microbial sphalerite formation in carbonate-hosted Zn-Pb ores, Bleiberg, Austria : micro- to nanotextural and sulfur isotope evidence Type de document : texte imprimé Auteurs : H. Kucha, Auteur ; E. Schroll, Auteur ; J. G. Raith, Auteur Année de publication : 2011 Article en page(s) : pp. 1005-1023 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Zn-Pb  ores  Sulfur  isotope  Austria Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Microglobular sphalerite (90–180 μm) is a major form of sphalerite in the abandoned, Triassic carbonate-hosted Zn-Pb deposit at Bleiberg, Austria. It is common in ores from the Crest horizon and the first Cardita horizon, which contain about 0.5- to 3-mm-thick and up to 5-cm-long, wavy, discontinuous, sphalerite-rich bands composed of agglomerations of microglobular sphalerite. Based on comparisons with modern environments, these textures are interpreted as peloids and probably represent fossil microbial mats. These are inter-layered with carbonate- and marcasite-rich, as well as finely banded, sphalerite layers (±fibrous galena and relics of oxysulfides), resulting in the characteristic rhythmically banded macrotexture of these Zn-dominated ores. Microbial nanotextures, made visible by field emission scanning electron microscopy (FESEM) after etching, include sphalerite nanospheres (10–90 nm) and bacterial filaments. The sphalerite nanospheres are identical to those previously reported for mixed calcite-sphalerite peloids from Bleiberg and are very similar to nanotextures observed in recent bacterial biofilms made by Desulfobacteriaceae. The observed sphalerite nanospheres are interpreted as in situ metabolic products of sulfate-reducing bacteria. Micro- and nanotextures suggest that the larger, μm-sized sphalerite globules formed by agglomeration of sphalerite nanospheres, as well as by replacement of peloids representing former bacterial colonies; the latter are now composed of Zn-calcite cores surounded by serrated sphalerite rims. Most samples studied evidence recrystallization of sphalerite that is broadly coeval with formation of fluorite. Involvement of sulfate-reducing bacteria in the formation of this early sphalerite mineralization is supported by sulfur isotope data. Sphalerite is generally characterized by isotopic light sulfur with δ34S per mil values ranging from −30.5 to −20.3 per mil. The most negative sulfur isotope values are recorded from sphalerite (−30.5‰, −30.2‰) and galena with unusual fibrous to dendritic morphology (−31.9‰; −31.8‰); both are associated with oxysulfides. Microglobular sphalerite with well-preserved sphalerite globules has a δ34S value of −28.8 to −28.2 per mil; colloform sphalerite (schalenblende) ranges from −29.0 to −25.0 per mil. Microglobular sphalerite associated with fine-grained fluorite and euhedral sphalerite formed through recrystallization processes show a trend to less negative values; i.e., −26.3 to −22.2 per mil and −22.9 to −20.3 per mil, respectively. We suggest that the combined biogenic nano- to macrotextures and sulfur isotope data are evidence that microbes played a key role in formation of Alpine carbonate-hosted Zn-Pb deposits. Results of this study contradict the MVT model suggested for Alpine Pb-Zn deposits, which calls for formation of Pb-Zn mineralization during late diagenetic burial in the Late Triassic-Early Jurassic. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1005.abstract [article] Microbial sphalerite formation in carbonate-hosted Zn-Pb ores, Bleiberg, Austria : micro- to nanotextural and sulfur isotope evidence [texte imprimé] / H. Kucha, Auteur ; E. Schroll, Auteur ; J. G. Raith, Auteur . - 2011 . - pp. 1005-1023. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1005-1023 Mots-clés : Zn-Pb  ores  Sulfur  isotope  Austria Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Microglobular sphalerite (90–180 μm) is a major form of sphalerite in the abandoned, Triassic carbonate-hosted Zn-Pb deposit at Bleiberg, Austria. It is common in ores from the Crest horizon and the first Cardita horizon, which contain about 0.5- to 3-mm-thick and up to 5-cm-long, wavy, discontinuous, sphalerite-rich bands composed of agglomerations of microglobular sphalerite. Based on comparisons with modern environments, these textures are interpreted as peloids and probably represent fossil microbial mats. These are inter-layered with carbonate- and marcasite-rich, as well as finely banded, sphalerite layers (±fibrous galena and relics of oxysulfides), resulting in the characteristic rhythmically banded macrotexture of these Zn-dominated ores. Microbial nanotextures, made visible by field emission scanning electron microscopy (FESEM) after etching, include sphalerite nanospheres (10–90 nm) and bacterial filaments. The sphalerite nanospheres are identical to those previously reported for mixed calcite-sphalerite peloids from Bleiberg and are very similar to nanotextures observed in recent bacterial biofilms made by Desulfobacteriaceae. The observed sphalerite nanospheres are interpreted as in situ metabolic products of sulfate-reducing bacteria. Micro- and nanotextures suggest that the larger, μm-sized sphalerite globules formed by agglomeration of sphalerite nanospheres, as well as by replacement of peloids representing former bacterial colonies; the latter are now composed of Zn-calcite cores surounded by serrated sphalerite rims. Most samples studied evidence recrystallization of sphalerite that is broadly coeval with formation of fluorite. Involvement of sulfate-reducing bacteria in the formation of this early sphalerite mineralization is supported by sulfur isotope data. Sphalerite is generally characterized by isotopic light sulfur with δ34S per mil values ranging from −30.5 to −20.3 per mil. The most negative sulfur isotope values are recorded from sphalerite (−30.5‰, −30.2‰) and galena with unusual fibrous to dendritic morphology (−31.9‰; −31.8‰); both are associated with oxysulfides. Microglobular sphalerite with well-preserved sphalerite globules has a δ34S value of −28.8 to −28.2 per mil; colloform sphalerite (schalenblende) ranges from −29.0 to −25.0 per mil. Microglobular sphalerite associated with fine-grained fluorite and euhedral sphalerite formed through recrystallization processes show a trend to less negative values; i.e., −26.3 to −22.2 per mil and −22.9 to −20.3 per mil, respectively. We suggest that the combined biogenic nano- to macrotextures and sulfur isotope data are evidence that microbes played a key role in formation of Alpine carbonate-hosted Zn-Pb deposits. Results of this study contradict the MVT model suggested for Alpine Pb-Zn deposits, which calls for formation of Pb-Zn mineralization during late diagenetic burial in the Late Triassic-Early Jurassic. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1005.abstract

The mineralogy and crystallography of pyrrhotite from selected nickel and PGE ore deposits / Megan Becker in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1025-1037 Titre : The mineralogy and crystallography of pyrrhotite from selected nickel and PGE ore deposits Type de document : texte imprimé Auteurs : Megan Becker, Auteur ; Johan De Villiers, Auteur ; Dee Bradshaw, Auteur Année de publication : 2011 Article en page(s) : pp. 1025-1037 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Pyrrhotite  Mineral  chemistry  Cristallography Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The nonstoichiometric sulfide pyrrhotite (Fe(1−x)S) common to many ore deposits occurs in a variety of crystallographic forms and compositions and occasionally is also intergrown with stoichiometric troilite (FeS). In this study, the mineralogy of pyrrhotite derived from several different nickel and PGE ore deposits in South Africa, Botswana, and Canada was examined in detail in terms of pyrrhotite association, crystallography, and mineral chemistry. Pyrrhotite samples were subdivided into two-phase 6C Fe11S12 pyrrhotite intergrown with 2C FeS troilite, two-phase 4C Fe7S8 pyrrhotite intergrown with 5C Fe9S10 pyrrhotite, single-phase 5C Fe9S10 pyrrhotite and single-phase 4C Fe7S8 pyrrhotite. None of the pyrrhotite samples analyzed were classified as two-phase 4C pyrrhotite intergrown with pyrite due to the scarcity of pyrite in these samples. Average solid solution Ni contents of NC pyrrhotite (0.75 ± 0.10 wt % Ni) in this study were found to be greater than in 4C pyrrhotite (0.43 ± 0.10 wt % Ni), but only when the pyrrhotite occurred as two-phase 4C pyrrhotite intergrown with NC pyrrhotite. For single-phase pyrrhotite occurrences in this study, 4C pyrrhotite was more Ni rich (up to 2 wt % Ni) than NC pyrrhotite (0.75 ± 0.19 wt % Ni). The average atomic metal/S ratios obtained for 4C Fe7S8 pyrrhotite was 0.869 ± 0.013 (n = 699), for 5C Fe9S10 pyrrhotite was 0.895 ± 0.013 (n = 316) and for 6C Fe11S12 pyrrhotite was 0.918 ± 0.017 (n = 101). The histogram comparing metal/S ratios of all the pyrrhotite samples analyzed showed a continuum of metal/S ratios, although with frequency maxima corresponding to the ideal compositions of 4C, 5C, and 6C pyrrhotite. The presence of the continuum however, was interpreted to be representative of nonstoichiometry in the pyrrhotite structure. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1025.abstract [article] The mineralogy and crystallography of pyrrhotite from selected nickel and PGE ore deposits [texte imprimé] / Megan Becker, Auteur ; Johan De Villiers, Auteur ; Dee Bradshaw, Auteur . - 2011 . - pp. 1025-1037. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1025-1037 Mots-clés : Pyrrhotite  Mineral  chemistry  Cristallography Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The nonstoichiometric sulfide pyrrhotite (Fe(1−x)S) common to many ore deposits occurs in a variety of crystallographic forms and compositions and occasionally is also intergrown with stoichiometric troilite (FeS). In this study, the mineralogy of pyrrhotite derived from several different nickel and PGE ore deposits in South Africa, Botswana, and Canada was examined in detail in terms of pyrrhotite association, crystallography, and mineral chemistry. Pyrrhotite samples were subdivided into two-phase 6C Fe11S12 pyrrhotite intergrown with 2C FeS troilite, two-phase 4C Fe7S8 pyrrhotite intergrown with 5C Fe9S10 pyrrhotite, single-phase 5C Fe9S10 pyrrhotite and single-phase 4C Fe7S8 pyrrhotite. None of the pyrrhotite samples analyzed were classified as two-phase 4C pyrrhotite intergrown with pyrite due to the scarcity of pyrite in these samples. Average solid solution Ni contents of NC pyrrhotite (0.75 ± 0.10 wt % Ni) in this study were found to be greater than in 4C pyrrhotite (0.43 ± 0.10 wt % Ni), but only when the pyrrhotite occurred as two-phase 4C pyrrhotite intergrown with NC pyrrhotite. For single-phase pyrrhotite occurrences in this study, 4C pyrrhotite was more Ni rich (up to 2 wt % Ni) than NC pyrrhotite (0.75 ± 0.19 wt % Ni). The average atomic metal/S ratios obtained for 4C Fe7S8 pyrrhotite was 0.869 ± 0.013 (n = 699), for 5C Fe9S10 pyrrhotite was 0.895 ± 0.013 (n = 316) and for 6C Fe11S12 pyrrhotite was 0.918 ± 0.017 (n = 101). The histogram comparing metal/S ratios of all the pyrrhotite samples analyzed showed a continuum of metal/S ratios, although with frequency maxima corresponding to the ideal compositions of 4C, 5C, and 6C pyrrhotite. The presence of the continuum however, was interpreted to be representative of nonstoichiometry in the pyrrhotite structure. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1025.abstract

Mining, society and a sustainable world / Jeremy P. Richards in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - p. 1039 Titre : Mining, society and a sustainable world Type de document : texte imprimé Auteurs : Jeremy P. Richards, Auteur Année de publication : 2011 Article en page(s) : p. 1039 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Mining  society Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1039.extract [article] Mining, society and a sustainable world [texte imprimé] / Jeremy P. Richards, Auteur . - 2011 . - p. 1039. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - p. 1039 Mots-clés : Mining  society Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1039.extract

Metallogeny and petrogenesis of lamprophyres in the mid-european variscides / Thomas Seifert in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1040-1041 Titre : Metallogeny and petrogenesis of lamprophyres in the mid-european variscides : post-collisional magmatism and its relationship to late-variscan ore forming processes in the Erzgebirge (Bohemian massif) Type de document : texte imprimé Auteurs : Thomas Seifert, Auteur Année de publication : 2011 Article en page(s) : pp. 1040-1041 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Lamprophyres  Post-collisional  magmatism  Bohemia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1040.extract [article] Metallogeny and petrogenesis of lamprophyres in the mid-european variscides : post-collisional magmatism and its relationship to late-variscan ore forming processes in the Erzgebirge (Bohemian massif) [texte imprimé] / Thomas Seifert, Auteur . - 2011 . - pp. 1040-1041. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 1040-1041 Mots-clés : Lamprophyres  Post-collisional  magmatism  Bohemia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1040.extract

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