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

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

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

The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension / Gillian Gruen in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 69-90 Titre : The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension Type de document : texte imprimé Auteurs : Gillian Gruen, Auteur ; Christoph A. Heinrich, Auteur ; Kim Schroeder, Auteur Année de publication : 2011 Article en page(s) : pp. 69-90 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Mo-Au  deposit  Vein  geometry  Porphyry  ore  shell Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/69.abstract [article] The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension [texte imprimé] / Gillian Gruen, Auteur ; Christoph A. Heinrich, Auteur ; Kim Schroeder, Auteur . - 2011 . - pp. 69-90. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 69-90 Mots-clés : Cu-Mo-Au  deposit  Vein  geometry  Porphyry  ore  shell Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/69.abstract