Documents disponibles écrits par cet auteur

Separation of molybdenum and copper in porphyry deposits / Jung Hun Seo in Economic geology, Vol. 107 N° 2 (Mars/Avril 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 2 (Mars/Avril 2012) . - pp. 333-356 Titre : Separation of molybdenum and copper in porphyry deposits : the roles of sulfur, redox, and pH in ore mineral deposition at Bingham Canyon Type de document : texte imprimé Auteurs : Jung Hun Seo, Auteur ; Marcel Guillong, Auteur ; Christoph A. Heinrich, Auteur Année de publication : 2012 Article en page(s) : pp. 333-356 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Mo-Au  deposits  Porphyry  deposits  Bingham  Canyon Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene subvolcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90–260 bars and 320°–430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about postentrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 μg/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive factor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large subvolcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of both veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/2/333.short [article] Separation of molybdenum and copper in porphyry deposits : the roles of sulfur, redox, and pH in ore mineral deposition at Bingham Canyon [texte imprimé] / Jung Hun Seo, Auteur ; Marcel Guillong, Auteur ; Christoph A. Heinrich, Auteur . - 2012 . - pp. 333-356. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 2 (Mars/Avril 2012) . - pp. 333-356 Mots-clés : Cu-Mo-Au  deposits  Porphyry  deposits  Bingham  Canyon Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene subvolcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90–260 bars and 320°–430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about postentrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 μg/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive factor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large subvolcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of both veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/2/333.short

Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada / Darren LeFort in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 781-808 Titre : Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada Type de document : texte imprimé Auteurs : Darren LeFort, Auteur ; Jacob Hanley, Auteur ; Marcel Guillong, Auteur Année de publication : 2011 Article en page(s) : pp. 781-808 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Prophyry  deposits  Alkalic  porphyry  Cu-Au  mineralization  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu (<740 ppm; rarely > 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/781.abstract [article] Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada [texte imprimé] / Darren LeFort, Auteur ; Jacob Hanley, Auteur ; Marcel Guillong, Auteur . - 2011 . - pp. 781-808. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 781-808 Mots-clés : Prophyry  deposits  Alkalic  porphyry  Cu-Au  mineralization  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu (<740 ppm; rarely > 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/781.abstract