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Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States / Patrick Nadoll in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - p p. 1275-1292 Titre : Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States Type de document : texte imprimé Auteurs : Patrick Nadoll, Auteur ; Jeffrey L. Mauk, Auteur ; Timothy S. Hayes, Auteur Année de publication : 2012 Article en page(s) : p p. 1275-1292 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : magnetite;  mineral  deposits;  geochemistry;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magnetite (Fe3O4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe3O4, but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1275.short [article] Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States [texte imprimé] / Patrick Nadoll, Auteur ; Jeffrey L. Mauk, Auteur ; Timothy S. Hayes, Auteur . - 2012 . - p p. 1275-1292. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - p p. 1275-1292 Mots-clés : magnetite;  mineral  deposits;  geochemistry;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magnetite (Fe3O4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe3O4, but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1275.short

SHRIMP U-Pb ages of xenotime and monazite from the spar lake red bed-associated Cu-Ag deposit, western Montana / John N. Aleinikoff in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1251-1274 Titre : SHRIMP U-Pb ages of xenotime and monazite from the spar lake red bed-associated Cu-Ag deposit, western Montana : implications for ore genesis Type de document : texte imprimé Auteurs : John N. Aleinikoff, Auteur ; Timothy S. Hayes, Auteur ; Karl V. Evans, Auteur Année de publication : 2012 Article en page(s) : pp. 1251-1274 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : SHRIMP  U-Pb  ages;  Xenotime  occurs;  Cu-Ag  deposit;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1–30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornite-calcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207Pb/206Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207Pb/206Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1251.short [article] SHRIMP U-Pb ages of xenotime and monazite from the spar lake red bed-associated Cu-Ag deposit, western Montana : implications for ore genesis [texte imprimé] / John N. Aleinikoff, Auteur ; Timothy S. Hayes, Auteur ; Karl V. Evans, Auteur . - 2012 . - pp. 1251-1274. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1251-1274 Mots-clés : SHRIMP  U-Pb  ages;  Xenotime  occurs;  Cu-Ag  deposit;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1–30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornite-calcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207Pb/206Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207Pb/206Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1251.short

The spar lake strata-bound Cu-Ag deposit formed across a mixing zone between trapped natural gas and metals-bearing brine / Timothy S. Hayes in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1223-1249 Titre : The spar lake strata-bound Cu-Ag deposit formed across a mixing zone between trapped natural gas and metals-bearing brine Type de document : texte imprimé Auteurs : Timothy S. Hayes, Auteur ; Gary P. Landis, Auteur ; Joseph F. Whelan, Auteur Année de publication : 2012 Article en page(s) : pp. 1223-1249 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : strata-bound  Cu  deposit;  ore  formation;  fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Ore formation at the Spar Lake red bed-associated strata-bound Cu deposit took place across a mixing and reaction zone between a hot oxidized metals-transporting brine and a reservoir of “sour” (H2S-bearing) natural gas trapped in the host sandstones. Fluid inclusion volatile analyses have very high CH4 concentrations (≥1 mol % in most samples), and a sample from the fringe of the deposit has between 18 and 36 mol % CH4. The ratio of CH4/CO2 in fluid inclusions appears to vary regularly across the deposit, with the lowest CH4/CO2 ratios from high-grade chalcocite-bearing ore, and the highest from the chalcopyrite-bearing fringe. The helium R/Ra isotope ratios (0.23–0.98) and concentrations define a mixture between crustal and atmospheric helium. The volatiles in fluid inclusions (CH4, CO2, H2S, SO2, H2, H2O, and other organic gases) and values of fO2 and temperature calculated from the volatiles data all show gradations across the deposit that are completely consistent with such a mixing and reaction zone. Other volatiles from the fluid inclusions (HCl, HF, 3He, 4He, N2, Ar) characterize the brine and give evidence for only shallow crustal fluids with no magmatic influences. The brine entered the gas reservoir from below and along the axis of the deposit and migrated out along bedding to the southwest, northeast, and northwest. Metals-transporting brines may have been fed into the host sandstones from the East Fault, but that remains uncertain. Abundant ore-stage Fe and Mn calcite cements from the reduced fringe have δ13C values as low as −18.4‰, and many values less than −10‰, which indicate that significant carbonate was generated by oxidation of organic carbon from the natural gas. The zone of calcite cements with very low δ13C values approximately envelopes chalcocite-bearing ore. Sulfur isotope data of Cu, Pb, and Fe sulfides and barite indicate derivation of roughly half of the orebody sulfide directly from sour gas H2S. That sour gas H2S had developed in steps known from other sedimentary basins, starting with (1) bacterial sulfate reduction (BSR) of seawater sulfate having δ34S of about 20‰ and sequestering of the sulfide in organic matter in source rocks stratigraphically below the deposit host rocks, followed by (2) maturation of the sulfide-bearing organic matter into liquid petroleum with relatively homogeneous sulfide having δ34S of 5 ± 5‰, then by (3) thermal cracking of the oil to CH4 and H2S with relatively homogeneous sulfide having δ34S closely distributed, about 6‰. The CH4 and H2S migrated and were trapped in sandstones of the upper member of the Revett Formation, where they were later met by the 200°C metals-transporting brine. There was additional contribution of sulfide to ore from later thermochemical sulfate reduction (TSR) operating on sulfate δ34S of 20 to 29‰ in both formation waters and metals-transporting solutions. A large range of δ34S in sulfides resulted as the 6‰ sour gas sulfide was supplemented with varying proportions of 20 to 29‰ sulfide from TSR. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1223.short [article] The spar lake strata-bound Cu-Ag deposit formed across a mixing zone between trapped natural gas and metals-bearing brine [texte imprimé] / Timothy S. Hayes, Auteur ; Gary P. Landis, Auteur ; Joseph F. Whelan, Auteur . - 2012 . - pp. 1223-1249. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1223-1249 Mots-clés : strata-bound  Cu  deposit;  ore  formation;  fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Ore formation at the Spar Lake red bed-associated strata-bound Cu deposit took place across a mixing and reaction zone between a hot oxidized metals-transporting brine and a reservoir of “sour” (H2S-bearing) natural gas trapped in the host sandstones. Fluid inclusion volatile analyses have very high CH4 concentrations (≥1 mol % in most samples), and a sample from the fringe of the deposit has between 18 and 36 mol % CH4. The ratio of CH4/CO2 in fluid inclusions appears to vary regularly across the deposit, with the lowest CH4/CO2 ratios from high-grade chalcocite-bearing ore, and the highest from the chalcopyrite-bearing fringe. The helium R/Ra isotope ratios (0.23–0.98) and concentrations define a mixture between crustal and atmospheric helium. The volatiles in fluid inclusions (CH4, CO2, H2S, SO2, H2, H2O, and other organic gases) and values of fO2 and temperature calculated from the volatiles data all show gradations across the deposit that are completely consistent with such a mixing and reaction zone. Other volatiles from the fluid inclusions (HCl, HF, 3He, 4He, N2, Ar) characterize the brine and give evidence for only shallow crustal fluids with no magmatic influences. The brine entered the gas reservoir from below and along the axis of the deposit and migrated out along bedding to the southwest, northeast, and northwest. Metals-transporting brines may have been fed into the host sandstones from the East Fault, but that remains uncertain. Abundant ore-stage Fe and Mn calcite cements from the reduced fringe have δ13C values as low as −18.4‰, and many values less than −10‰, which indicate that significant carbonate was generated by oxidation of organic carbon from the natural gas. The zone of calcite cements with very low δ13C values approximately envelopes chalcocite-bearing ore. Sulfur isotope data of Cu, Pb, and Fe sulfides and barite indicate derivation of roughly half of the orebody sulfide directly from sour gas H2S. That sour gas H2S had developed in steps known from other sedimentary basins, starting with (1) bacterial sulfate reduction (BSR) of seawater sulfate having δ34S of about 20‰ and sequestering of the sulfide in organic matter in source rocks stratigraphically below the deposit host rocks, followed by (2) maturation of the sulfide-bearing organic matter into liquid petroleum with relatively homogeneous sulfide having δ34S of 5 ± 5‰, then by (3) thermal cracking of the oil to CH4 and H2S with relatively homogeneous sulfide having δ34S closely distributed, about 6‰. The CH4 and H2S migrated and were trapped in sandstones of the upper member of the Revett Formation, where they were later met by the 200°C metals-transporting brine. There was additional contribution of sulfide to ore from later thermochemical sulfate reduction (TSR) operating on sulfate δ34S of 20 to 29‰ in both formation waters and metals-transporting solutions. A large range of δ34S in sulfides resulted as the 6‰ sour gas sulfide was supplemented with varying proportions of 20 to 29‰ sulfide from TSR. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1223.short