Dépouillements

The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave, Australia) / Zoran Seat in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 106-527 Titre : The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave, Australia) : Pt. 1. U/Pb zircon ages, whole-rock and mineral chemistry, and O-Sr-Nd isotope compositions of the intrusion, with constraints on petrogenesis Type de document : texte imprimé Auteurs : Zoran Seat, Auteur ; M. A. Mary Gee, Auteur ; Benjamin A. Grguric, Auteur Année de publication : 2011 Article en page(s) : pp. 106-527 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Nebo  babel  Sulfid  deposit  Mineral  chemistry  Intrusion  Petrogenesis Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Nebo-Babel intrusion, which hosts a world-class magmatic Ni-Cu-PGE sulfide deposit, is a concentrically zoned, tube-like gabbronorite body. The emplacement of Nebo-Babel at 1068.0 ± 4.3 Ma was part of a major magmatic event, which included the Giles Complex and the Warakurna large igneous province. Field, mineral, chemical, and isotopic data from the Nebo-Babel intrusion indicate that all of the lithostratigraphic units are cogenetic and that the intrusion is the product of three temporally distinct magma pulses. The initial magma pulse, which formed the marginal units of the intrusion, was the most primitive and has assimilated the highest percentage of the country-rock orthogneiss. The subsequent magma pulses, emplaced into the core of the intrusion, were both less primitive and less contaminated. The final and largest magma pulse underwent in situ closed-system crystal fractionation resulting in the observed stratigraphic mineral and whole-rock chemical trends. Emplacement of Nebo-Babel took place at ca. 10- to 12-km depth. Postmagmatic, structural overturning of the intrusion resulted in stratigraphically reversed mineral and chemical trends. All of the Nebo-Babel units can be related by crystal fractionation of parental tholeiitic magma with 8 to 9 wt percent MgO. Mineral {delta}18O values are consistent with typical mantle values and preclude large-scale crustal contamination of the parental magma with anything other than material with similar values. Although even the least contaminated intrusive units have negative Nb anomalies and enriched 87Sr/86Sr initial ratios compared to bulk earth at the time, the crustal assimilation must have been significantly less than 10 percent based on mantle-like {delta}18O values and 143Nd/144Nd ratios. Trace elements indicate that primitive Nebo-Babel melts equilibrated with spinel facies mantle implying shallow melting beneath a lithosphere less than 80 km thick. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/4/527 [article] The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave, Australia) : Pt. 1. U/Pb zircon ages, whole-rock and mineral chemistry, and O-Sr-Nd isotope compositions of the intrusion, with constraints on petrogenesis [texte imprimé] / Zoran Seat, Auteur ; M. A. Mary Gee, Auteur ; Benjamin A. Grguric, Auteur . - 2011 . - pp. 106-527. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 106-527 Mots-clés : Nebo  babel  Sulfid  deposit  Mineral  chemistry  Intrusion  Petrogenesis Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Nebo-Babel intrusion, which hosts a world-class magmatic Ni-Cu-PGE sulfide deposit, is a concentrically zoned, tube-like gabbronorite body. The emplacement of Nebo-Babel at 1068.0 ± 4.3 Ma was part of a major magmatic event, which included the Giles Complex and the Warakurna large igneous province. Field, mineral, chemical, and isotopic data from the Nebo-Babel intrusion indicate that all of the lithostratigraphic units are cogenetic and that the intrusion is the product of three temporally distinct magma pulses. The initial magma pulse, which formed the marginal units of the intrusion, was the most primitive and has assimilated the highest percentage of the country-rock orthogneiss. The subsequent magma pulses, emplaced into the core of the intrusion, were both less primitive and less contaminated. The final and largest magma pulse underwent in situ closed-system crystal fractionation resulting in the observed stratigraphic mineral and whole-rock chemical trends. Emplacement of Nebo-Babel took place at ca. 10- to 12-km depth. Postmagmatic, structural overturning of the intrusion resulted in stratigraphically reversed mineral and chemical trends. All of the Nebo-Babel units can be related by crystal fractionation of parental tholeiitic magma with 8 to 9 wt percent MgO. Mineral {delta}18O values are consistent with typical mantle values and preclude large-scale crustal contamination of the parental magma with anything other than material with similar values. Although even the least contaminated intrusive units have negative Nb anomalies and enriched 87Sr/86Sr initial ratios compared to bulk earth at the time, the crustal assimilation must have been significantly less than 10 percent based on mantle-like {delta}18O values and 143Nd/144Nd ratios. Trace elements indicate that primitive Nebo-Babel melts equilibrated with spinel facies mantle implying shallow melting beneath a lithosphere less than 80 km thick. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/4/527

The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave Block, Australia): / Bélinda Godel in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 557-584 Titre : The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave Block, Australia): : Pt. 2. Constraints on parental magma and processes, with implications for mineral exploration Type de document : texte imprimé Auteurs : Bélinda Godel, Auteur ; Zoran Seat, Auteur ; Wolfgang D. Maier, Auteur Année de publication : 2011 Article en page(s) : pp. 557-584 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Nebo-babel  Parental  magma  Mineral  exploration  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Nebo-Babel Ni-Cu-platinum-group element (PGE) sulfide deposit (West Musgrave, Australia) is hosted in a gabbronorite chonolith emplaced into sulfur-free country-rock orthogneiss at ca. 1068 Ma. Five different types of mafic dikes are found in the area or are spatially associated with the Nebo-Babel intrusion. The mafic dikes are divided into three groups based on petrography, whole-rock major, trace, and PGE chemistry, Sm-Nd isotope ratios, and silicate mineral chemistry: (1) low Ti basalts (NB-1, NB-2, and NB-3); (2) high Ti basalts (NB-4); and (3) alkali basalts (NB-5). Recent age constraints on similar magma suites intruding in the west Musgrave indicate the low and high Ti basalts are coeval with the Nebo-Babel intrusion and hence can potentially be equivalent to its parental magma composition(s). Based on this, we use whole-rock major, trace, and PGE chemistry, Sm-Nd isotope data, silicate mineral chemistry, and geochemical modeling to constrain the petrogenesis of each mafic dike type and its potential relationships with the Nebo-Babel intrusion. Our results indicate that crustal contamination did not play a major role in the generation or evolution of the different magma series. The geochemical variations observed are rather interpreted to reflect different mantle source compositions and different degrees of partial melting. The low Ti basalts exhibit subduction-related geochemical signatures and are interpreted to have been generated by 5 to 10 percent partial melting of a hydrous spinel-bearing mantle. The melting is inferred to have been triggered as a result of mantle plume impingement in an area of the mantle that has previously been metasomatized. The high Ti basalts are interpreted to have formed by mixing of the sublithospheric mantle with deeper asthenospheric mantle melts from the mantle plume. The decompressional melting of the plume head led to the formation of alkali basalts (characterized by typical ocean island basalt (OIB)-like compositions) generated by 4 to 5 percent of partial melting of a garnet-bearing lherzolite. Our results show that the Nebo-Babel intrusion and its associated Ni-Cu-PGE sulfide mineralization formed between the intrusion of the low- and high Ti basalts and may have originated from either mixing between these two magma types, or as a result of continuous change in the melting conditions between these two magmas types. The spatial analysis of the repartition of the different magma suites allowed highlighting zones where the two magma types are spatially associated and where Ni-Cu-PGE sulfide anomalies were discovered, suggesting that magma mixing may be a key factor in the ore genesis at Nebo-Babel. This observation is strengthened by the modeling of the sulfur concentrations at sulfide saturation, which indicates that the assimilation of country-rock orthogneiss by the mixed magma caused a drop in sulfur solubility which in turn led to sulfide saturation in the early stages of the mixing and/or assimilation and potentially to the formation of the Nebo-Babel Ni-Cu-PGE sulfide deposit. Calculated metal concentrations of the parental magma from which the Ni-Cu-PGE mineralization formed are: 165 ppm Ni, 0.15 ppb Ir, 0.30 ppb Ru, 0.22 ppb Rh, 3.40 ppb Pt, 3.30 ppb Pd, and 150 ppm Cu. These metal concentrations are similar to those obtained from the low Ti basalt compositions after they experienced a small amount (<0.001%) of sulfide segregation prior to their final emplacement. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/557.abstract [article] The nebo-babel Ni-Cu-PGE sulfide deposit (West Musgrave Block, Australia): : Pt. 2. Constraints on parental magma and processes, with implications for mineral exploration [texte imprimé] / Bélinda Godel, Auteur ; Zoran Seat, Auteur ; Wolfgang D. Maier, Auteur . - 2011 . - pp. 557-584. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 557-584 Mots-clés : Nebo-babel  Parental  magma  Mineral  exploration  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Nebo-Babel Ni-Cu-platinum-group element (PGE) sulfide deposit (West Musgrave, Australia) is hosted in a gabbronorite chonolith emplaced into sulfur-free country-rock orthogneiss at ca. 1068 Ma. Five different types of mafic dikes are found in the area or are spatially associated with the Nebo-Babel intrusion. The mafic dikes are divided into three groups based on petrography, whole-rock major, trace, and PGE chemistry, Sm-Nd isotope ratios, and silicate mineral chemistry: (1) low Ti basalts (NB-1, NB-2, and NB-3); (2) high Ti basalts (NB-4); and (3) alkali basalts (NB-5). Recent age constraints on similar magma suites intruding in the west Musgrave indicate the low and high Ti basalts are coeval with the Nebo-Babel intrusion and hence can potentially be equivalent to its parental magma composition(s). Based on this, we use whole-rock major, trace, and PGE chemistry, Sm-Nd isotope data, silicate mineral chemistry, and geochemical modeling to constrain the petrogenesis of each mafic dike type and its potential relationships with the Nebo-Babel intrusion. Our results indicate that crustal contamination did not play a major role in the generation or evolution of the different magma series. The geochemical variations observed are rather interpreted to reflect different mantle source compositions and different degrees of partial melting. The low Ti basalts exhibit subduction-related geochemical signatures and are interpreted to have been generated by 5 to 10 percent partial melting of a hydrous spinel-bearing mantle. The melting is inferred to have been triggered as a result of mantle plume impingement in an area of the mantle that has previously been metasomatized. The high Ti basalts are interpreted to have formed by mixing of the sublithospheric mantle with deeper asthenospheric mantle melts from the mantle plume. The decompressional melting of the plume head led to the formation of alkali basalts (characterized by typical ocean island basalt (OIB)-like compositions) generated by 4 to 5 percent of partial melting of a garnet-bearing lherzolite. Our results show that the Nebo-Babel intrusion and its associated Ni-Cu-PGE sulfide mineralization formed between the intrusion of the low- and high Ti basalts and may have originated from either mixing between these two magma types, or as a result of continuous change in the melting conditions between these two magmas types. The spatial analysis of the repartition of the different magma suites allowed highlighting zones where the two magma types are spatially associated and where Ni-Cu-PGE sulfide anomalies were discovered, suggesting that magma mixing may be a key factor in the ore genesis at Nebo-Babel. This observation is strengthened by the modeling of the sulfur concentrations at sulfide saturation, which indicates that the assimilation of country-rock orthogneiss by the mixed magma caused a drop in sulfur solubility which in turn led to sulfide saturation in the early stages of the mixing and/or assimilation and potentially to the formation of the Nebo-Babel Ni-Cu-PGE sulfide deposit. Calculated metal concentrations of the parental magma from which the Ni-Cu-PGE mineralization formed are: 165 ppm Ni, 0.15 ppb Ir, 0.30 ppb Ru, 0.22 ppb Rh, 3.40 ppb Pt, 3.30 ppb Pd, and 150 ppm Cu. These metal concentrations are similar to those obtained from the low Ti basalt compositions after they experienced a small amount (<0.001%) of sulfide segregation prior to their final emplacement. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/557.abstract

Structural controls and evolution of gold-, silver-, and REE-bearing copper-cobalt ore deposits, blackbird district, East-Central Idaho / K. Lund in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 585-618 Titre : Structural controls and evolution of gold-, silver-, and REE-bearing copper-cobalt ore deposits, blackbird district, East-Central Idaho : epigenetic origins Type de document : texte imprimé Auteurs : K. Lund, Auteur ; R. G. Tysdal, Auteur ; K. V. Evans, Auteur Année de publication : 2011 Article en page(s) : pp. 585-618 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Gold  Silver  Blackbird  district  Metasedimentary  rocks  Idaho  [India] Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Cu-Co ± Au (± Ag ± Ni ± REE) ore deposits of the Blackbird district, east-central Idaho, have previously been classified as Besshi-type VMS, sedex, and IOCG deposits within an intact stratigraphic section. New studies indicate that, across the district, mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems. Quartz-rich and biotite-rich veins (and alteration zones) and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district. Both early and late quartz veins contain chalcopyrite and pyrite, whereas intermediate-stage tourmaline-biotite veins host the cobaltite. Barren early and late albite and late carbonate (generally siderite) form veins or are included in the quartz veins. REE minerals, principally monazite, allanite, and xenotime, are associated with both tourmaline-biotite and late quartz veins. The veins are in mineralized intervals along axial planar cleavage, intrafolial foliation, and shears. Mineralized intervals are hosted by a variety of metasedimentary rocks, including three phyllitic units of Mesoproterozoic age and two schistose units. All of these units are S-tectonites in the footwall of a regional thrust fault. Specifically, the district lies within an oblique thrust ramp containing a series of structural horses (three domains) in a duplex system. The deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower-middle greenschist facies in the lower domain (an inverted metamorphic sequence). Early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late-tectonic retrograde recrystallization and metamorphic mineral growth, to the same extent as the country rocks in each domain. Where little subsequent deformation occurred, early veins are discordant to bedding but, where folding was polyphase and fabrics are penetrative, mineralized zones are concordant with metamorphic compositional layering. Late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed. 40Ar/39Ar dating of unoriented muscovite from the selvage of a late quartz vein yields a Late Cretaceous age of about 83 Ma, the time of retrograde metamorphism associated with introduction of late quartz veins. Textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to Late Cretaceous regional thrust faulting. Heat, fluids, and conduits for generation and circulation of fluids were part of the regional crustal thickening. The faulting also juxtaposed metaevaporite layers in the Mesoproterozoic Yellowjacket Formation over Blackbird district host rocks. We conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements (Cl, K, B, Na) from metaevaporites to produce brines, scavenging of metals (Co, Cu, etc) from rocks in the region, and, finally, concentrating metals in the lower-plate ramp structures. Although the ultimate source of the metals remains undetermined, the present Cu-Co ± Au (± Ag ± Ni ± REE) Blackbird ore deposits formed during Late Cretaceous compressional deformation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/585.abstract [article] Structural controls and evolution of gold-, silver-, and REE-bearing copper-cobalt ore deposits, blackbird district, East-Central Idaho : epigenetic origins [texte imprimé] / K. Lund, Auteur ; R. G. Tysdal, Auteur ; K. V. Evans, Auteur . - 2011 . - pp. 585-618. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 585-618 Mots-clés : Gold  Silver  Blackbird  district  Metasedimentary  rocks  Idaho  [India] Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Cu-Co ± Au (± Ag ± Ni ± REE) ore deposits of the Blackbird district, east-central Idaho, have previously been classified as Besshi-type VMS, sedex, and IOCG deposits within an intact stratigraphic section. New studies indicate that, across the district, mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems. Quartz-rich and biotite-rich veins (and alteration zones) and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district. Both early and late quartz veins contain chalcopyrite and pyrite, whereas intermediate-stage tourmaline-biotite veins host the cobaltite. Barren early and late albite and late carbonate (generally siderite) form veins or are included in the quartz veins. REE minerals, principally monazite, allanite, and xenotime, are associated with both tourmaline-biotite and late quartz veins. The veins are in mineralized intervals along axial planar cleavage, intrafolial foliation, and shears. Mineralized intervals are hosted by a variety of metasedimentary rocks, including three phyllitic units of Mesoproterozoic age and two schistose units. All of these units are S-tectonites in the footwall of a regional thrust fault. Specifically, the district lies within an oblique thrust ramp containing a series of structural horses (three domains) in a duplex system. The deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower-middle greenschist facies in the lower domain (an inverted metamorphic sequence). Early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late-tectonic retrograde recrystallization and metamorphic mineral growth, to the same extent as the country rocks in each domain. Where little subsequent deformation occurred, early veins are discordant to bedding but, where folding was polyphase and fabrics are penetrative, mineralized zones are concordant with metamorphic compositional layering. Late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed. 40Ar/39Ar dating of unoriented muscovite from the selvage of a late quartz vein yields a Late Cretaceous age of about 83 Ma, the time of retrograde metamorphism associated with introduction of late quartz veins. Textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to Late Cretaceous regional thrust faulting. Heat, fluids, and conduits for generation and circulation of fluids were part of the regional crustal thickening. The faulting also juxtaposed metaevaporite layers in the Mesoproterozoic Yellowjacket Formation over Blackbird district host rocks. We conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements (Cl, K, B, Na) from metaevaporites to produce brines, scavenging of metals (Co, Cu, etc) from rocks in the region, and, finally, concentrating metals in the lower-plate ramp structures. Although the ultimate source of the metals remains undetermined, the present Cu-Co ± Au (± Ag ± Ni ± REE) Blackbird ore deposits formed during Late Cretaceous compressional deformation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/585.abstract

The geochemistry of carbonate-replacement Pb-Zn-Ag mineralization in the lavrion district, Attica, Greece / Todd A. Bonsall in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 619-651 Titre : The geochemistry of carbonate-replacement Pb-Zn-Ag mineralization in the lavrion district, Attica, Greece : fluid inclusion, stable isotope, and rare earth element studies Type de document : texte imprimé Auteurs : Todd A. Bonsall, Auteur ; Paul G. Spry, Auteur ; Panagiotis Ch. Voudouris, Auteur Année de publication : 2011 Article en page(s) : pp. 619-651 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Carbonate  Mineralization  Laviron  district  Fluid  inclusion  Isotrope  Rare  earth  element  Greece Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7–10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132° to 365°C from CO2-poor, low- to high-salinity fluids (1–20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite (δ13C = −15.6 to −1.5‰ and δ18O = −9.2 to +17.3‰) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high fO2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides (δ34S = −4.9 to +5.3‰, with one outlier of 9.4‰) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from δ34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250°C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log fO2 = −41 to −36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate-replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/619.abstract [article] The geochemistry of carbonate-replacement Pb-Zn-Ag mineralization in the lavrion district, Attica, Greece : fluid inclusion, stable isotope, and rare earth element studies [texte imprimé] / Todd A. Bonsall, Auteur ; Paul G. Spry, Auteur ; Panagiotis Ch. Voudouris, Auteur . - 2011 . - pp. 619-651. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 619-651 Mots-clés : Carbonate  Mineralization  Laviron  district  Fluid  inclusion  Isotrope  Rare  earth  element  Greece Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7–10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132° to 365°C from CO2-poor, low- to high-salinity fluids (1–20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite (δ13C = −15.6 to −1.5‰ and δ18O = −9.2 to +17.3‰) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high fO2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides (δ34S = −4.9 to +5.3‰, with one outlier of 9.4‰) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from δ34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250°C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log fO2 = −41 to −36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate-replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/619.abstract

The origin and evolution of mineralizing fluids in a sediment-hosted orogenic-gold deposit, Ballarat East, Southeastern Australia / A. M. Fairmaid in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 653-666 Titre : The origin and evolution of mineralizing fluids in a sediment-hosted orogenic-gold deposit, Ballarat East, Southeastern Australia Type de document : texte imprimé Auteurs : A. M. Fairmaid, Auteur ; M. A. Kendrick, Auteur ; D. Phillips, Auteur Année de publication : 2011 Article en page(s) : pp. 653-666 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Mineralizing  fluids  Gold  deposits  Sediments  hosted  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (~4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 × 10−3 and 2.9 × 10−3 and I/Cl values between 150 × 10−6 and 500 × 10−6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have 40Ar/36Ar ratios ranging from 322 (close to the atmospheric 40Ar/36Ar ratio of ~296) up to a maximum of 4503. 40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 × 10−4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm 40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3–2.7 ppb). Fluid inclusion 84Kr/36Ar and 130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/653.abstract [article] The origin and evolution of mineralizing fluids in a sediment-hosted orogenic-gold deposit, Ballarat East, Southeastern Australia [texte imprimé] / A. M. Fairmaid, Auteur ; M. A. Kendrick, Auteur ; D. Phillips, Auteur . - 2011 . - pp. 653-666. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 653-666 Mots-clés : Mineralizing  fluids  Gold  deposits  Sediments  hosted  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (~4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 × 10−3 and 2.9 × 10−3 and I/Cl values between 150 × 10−6 and 500 × 10−6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have 40Ar/36Ar ratios ranging from 322 (close to the atmospheric 40Ar/36Ar ratio of ~296) up to a maximum of 4503. 40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 × 10−4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm 40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3–2.7 ppb). Fluid inclusion 84Kr/36Ar and 130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/653.abstract

In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada / T. Ulrich in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 667-686 Titre : In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada Type de document : texte imprimé Auteurs : T. Ulrich, Auteur ; D. G. F. Long, Auteur ; B. S. Kamber, Auteur Année de publication : 2011 Article en page(s) : pp. 667-686 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Sulfur  analysis  Pyrite  Paleoproterozoic  gold  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at ~1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as “invisible” Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive δ34S values (0.97–9.26‰). The δ34S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative Δ33S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/667.abstract [article] In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada [texte imprimé] / T. Ulrich, Auteur ; D. G. F. Long, Auteur ; B. S. Kamber, Auteur . - 2011 . - pp. 667-686. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 667-686 Mots-clés : Sulfur  analysis  Pyrite  Paleoproterozoic  gold  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at ~1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as “invisible” Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive δ34S values (0.97–9.26‰). The δ34S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative Δ33S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/667.abstract

Heavy mineral sands in the eucla basin, Southern Australia / Baohong Hou in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 687-712 Titre : Heavy mineral sands in the eucla basin, Southern Australia : deposition and province-scale prospectivity Type de document : texte imprimé Auteurs : Baohong Hou, Auteur ; John Keeling, Auteur ; Anthony Reid, Auteur Année de publication : 2011 Article en page(s) : pp. 687-712 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Heavy  mineral  sands  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The marine Eucla basin in southern Australia is emerging as a major new heavy minerals province in Australia. Beach placers are associated with a series of partially buried Cenozoic coastal barrier sands formed along an arcuate 2,000-km-long basin margin, the trace of which is up to 320 km inland of the present coastline. The presence of high-grade deposits with dominant zircon over ilmenite and lesser amounts of rutile and leucoxene was established with the discovery of the Jacinth and Ambrosia heavy mineral deposits in late 2004. An additional 10 heavy mineral prospects were subsequently identified and are at various stages of evaluation. The Eucla basin and its adjacent paleovalley system have a large areal extent that contains a complicated succession of marine and nonmarine strata spanning a wide range of depositional environments. Four distinct constructional phases for the development of shorelines can be recognized and correlated with major third-order sea-level events, established by others from the marine depositional record as occurring during the middle Eocene (~42.5 Ma), late middle Eocene (39–36 Ma), late Eocene (36–34 Ma), and Miocene-Pliocene (15–2.6 Ma). Prevailing westerly winds built extensive dune systems by longshore drift. Sediment movement was from west to east. Detrital zircon rocks from the Ooldea and Barton barriers show a distribution of zircon age that is consistent with the Proterozoic Musgrave province to the north of the basin as the dominant primary source area of the heavy minerals, with a contribution from the Albany Fraser orogen to the west. The likelihood is that these heavy minerals have been recycled via sedimentary basins that flank the Musgrave province and include the Neoproterozoic to Cambrian Officer basin and Permian to Mesozoic deposits of the Bight basin. Our current depositional model is summarized as follows: (1) initial rapid transgression and deposition of a shallow marine sand sheet subsequently overlain by shallow marine limestone during middle Eocene; (2) a major Eocene transgression and deposition of a shelf, barrier, and lagoonal shoreface marine complex during the late middle Eocene; ( 3) further transgression and highstand deposition during the late Eocene; (4) renewed transgression of barrier, lagoonal, and possibly flooding deltaic sand blanket in the southeastern coastal plain with neotectonic uplift tilting in the western Eucla margin during Mio-Pliocene time. Each stage of reworking increased the potential for heavy mineral concentration in placer deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/687.abstract [article] Heavy mineral sands in the eucla basin, Southern Australia : deposition and province-scale prospectivity [texte imprimé] / Baohong Hou, Auteur ; John Keeling, Auteur ; Anthony Reid, Auteur . - 2011 . - pp. 687-712. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 687-712 Mots-clés : Heavy  mineral  sands  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The marine Eucla basin in southern Australia is emerging as a major new heavy minerals province in Australia. Beach placers are associated with a series of partially buried Cenozoic coastal barrier sands formed along an arcuate 2,000-km-long basin margin, the trace of which is up to 320 km inland of the present coastline. The presence of high-grade deposits with dominant zircon over ilmenite and lesser amounts of rutile and leucoxene was established with the discovery of the Jacinth and Ambrosia heavy mineral deposits in late 2004. An additional 10 heavy mineral prospects were subsequently identified and are at various stages of evaluation. The Eucla basin and its adjacent paleovalley system have a large areal extent that contains a complicated succession of marine and nonmarine strata spanning a wide range of depositional environments. Four distinct constructional phases for the development of shorelines can be recognized and correlated with major third-order sea-level events, established by others from the marine depositional record as occurring during the middle Eocene (~42.5 Ma), late middle Eocene (39–36 Ma), late Eocene (36–34 Ma), and Miocene-Pliocene (15–2.6 Ma). Prevailing westerly winds built extensive dune systems by longshore drift. Sediment movement was from west to east. Detrital zircon rocks from the Ooldea and Barton barriers show a distribution of zircon age that is consistent with the Proterozoic Musgrave province to the north of the basin as the dominant primary source area of the heavy minerals, with a contribution from the Albany Fraser orogen to the west. The likelihood is that these heavy minerals have been recycled via sedimentary basins that flank the Musgrave province and include the Neoproterozoic to Cambrian Officer basin and Permian to Mesozoic deposits of the Bight basin. Our current depositional model is summarized as follows: (1) initial rapid transgression and deposition of a shallow marine sand sheet subsequently overlain by shallow marine limestone during middle Eocene; (2) a major Eocene transgression and deposition of a shelf, barrier, and lagoonal shoreface marine complex during the late middle Eocene; ( 3) further transgression and highstand deposition during the late Eocene; (4) renewed transgression of barrier, lagoonal, and possibly flooding deltaic sand blanket in the southeastern coastal plain with neotectonic uplift tilting in the western Eucla margin during Mio-Pliocene time. Each stage of reworking increased the potential for heavy mineral concentration in placer deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/687.abstract

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