Dépouillements

Origins of mineral deposits, belt-purcell basin, United States and Canada / Stephen E. Box in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1081-1088 Titre : Origins of mineral deposits, belt-purcell basin, United States and Canada : an introduction Type de document : texte imprimé Auteurs : Stephen E. Box, Auteur ; Arthur A. Bookstrom, Auteur ; Robert G. Anderson, Auteur Année de publication : 2012 Article en page(s) : pp. 1081-1088 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : ore  deposits;  Belt-Purcell  basin;  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The fill of the Mesoproterozoic Belt-Purcell Basin, which straddles the United States-Canada border within the Rocky Mountains of western North America (Fig. 1), consists of marine and nonmarine clastic and carbonate strata 15 to 20 km thick. Three giant metal-producing ore deposits or districts account for the bulk of the known metal endowment within the bounds of the Belt-Purcell Basin: (1) the syndepositional Sullivan Pb-Zn-Ag deposit in southern British Columbia (total production: Pb, 8.4 million tonnes [Mt]; Zn, 7.9 Mt; Ag, 0.0093 Mt; Lydon, 2000), (2) the mesothermal Pb-Zn-Ag veins of the Coeur d’Alene district in northern Idaho (total production: Pb, 7.5 Mt; Zn, 3.0 Mt; Ag, 0.052 Mt; Long, 1998; post-1997 data from USGS Annual Minerals Yearbooks), and (3) the Cretaceous porphyry copper deposit and associated polymetallic veins in the Butte district in Montana (total resource: Cu, 35 Mt; Zn, 4.6 Mt; Ag, 0.044 Mt; Long et al., 1998). The Sullivan Mine closed in 2001 after more than 92 years of production. Mining of 26 major vein deposits in the Coeur d’Alene district began in the 1880s and peaked about 1950. Production in the Coeur d’Alene district continues today from the Galena and Lucky Friday Mines (the latter closed for 2012 to refurbish the mile-deep vertical access shaft). Mining at Butte began in 1875, with copper production peaking in 1917. Mining continues today in the eastern upfaulted portion of the Butte porphyry copper deposit at the Continental Mine. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1081.extract [article] Origins of mineral deposits, belt-purcell basin, United States and Canada : an introduction [texte imprimé] / Stephen E. Box, Auteur ; Arthur A. Bookstrom, Auteur ; Robert G. Anderson, Auteur . - 2012 . - pp. 1081-1088. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1081-1088 Mots-clés : ore  deposits;  Belt-Purcell  basin;  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The fill of the Mesoproterozoic Belt-Purcell Basin, which straddles the United States-Canada border within the Rocky Mountains of western North America (Fig. 1), consists of marine and nonmarine clastic and carbonate strata 15 to 20 km thick. Three giant metal-producing ore deposits or districts account for the bulk of the known metal endowment within the bounds of the Belt-Purcell Basin: (1) the syndepositional Sullivan Pb-Zn-Ag deposit in southern British Columbia (total production: Pb, 8.4 million tonnes [Mt]; Zn, 7.9 Mt; Ag, 0.0093 Mt; Lydon, 2000), (2) the mesothermal Pb-Zn-Ag veins of the Coeur d’Alene district in northern Idaho (total production: Pb, 7.5 Mt; Zn, 3.0 Mt; Ag, 0.052 Mt; Long, 1998; post-1997 data from USGS Annual Minerals Yearbooks), and (3) the Cretaceous porphyry copper deposit and associated polymetallic veins in the Butte district in Montana (total resource: Cu, 35 Mt; Zn, 4.6 Mt; Ag, 0.044 Mt; Long et al., 1998). The Sullivan Mine closed in 2001 after more than 92 years of production. Mining of 26 major vein deposits in the Coeur d’Alene district began in the 1880s and peaked about 1950. Production in the Coeur d’Alene district continues today from the Galena and Lucky Friday Mines (the latter closed for 2012 to refurbish the mile-deep vertical access shaft). Mining at Butte began in 1875, with copper production peaking in 1917. Mining continues today in the eastern upfaulted portion of the Butte porphyry copper deposit at the Continental Mine. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1081.extract

Strata-bound Fe-Co-Cu-Au-Bi-Y-REE deposits of the idaho cobalt belt / John F. Slack in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1089-1113 Titre : Strata-bound Fe-Co-Cu-Au-Bi-Y-REE deposits of the idaho cobalt belt : multistage hydrothermal mineralization in a magmatic-related iron oxide copper-gold system Type de document : texte imprimé Auteurs : John F. Slack, Auteur Année de publication : 2012 Article en page(s) : pp. 1089-1113 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : strata-bound;  REE  deposits Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Mineralogical and geochemical studies of strata-bound Fe-Co-Cu-Au-Bi-Y-rare-earth element (REE) deposits of the Idaho cobalt belt in east-central Idaho provide evidence of multistage epigenetic mineralization by magmatic-hydrothermal processes in an iron oxide copper-gold (IOCG) system. Deposits of the Idaho cobalt belt comprise three types: (1) strata-bound sulfide lenses in the Blackbird district, which are cobaltite and, less commonly, chalcopyrite rich with locally abundant gold, native bismuth, bismuthinite, xenotime, allanite, monazite, and the Be-rich silicate gadolinite-(Y), with sparse uraninite, stannite, and Bi tellurides, in a gangue of quartz, chlorite, biotite, muscovite, garnet, tourmaline, chloritoid, and/or siderite, with locally abundant fluorapatite or magnetite; (2) discordant tourmalinized breccias in the Blackbird district that in places have concentrations of cobaltite, chalcopyrite, gold, and xenotime; and (3) strata-bound magnetite-rich lenses in the Iron Creek area, which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime. Most sulfide-rich deposits in the Blackbird district are enclosed by strata-bound lenses composed mainly of Cl-rich Fe biotite; some deposits have quartz-rich envelopes. Whole-rock analyses of 48 Co- and/or Cu-rich samples show high concentrations of Au (up to 26.8 ppm), Bi (up to 9.16 wt %), Y (up to 0.83 wt %), ∑REEs (up to 2.56 wt %), Ni (up to 6,780 ppm), and Be (up to 1,180 ppm), with locally elevated U (up to 124 ppm) and Sn (up to 133 ppm); Zn and Pb contents are uniformly low (≤821 and ≤61 ppm, respectively). Varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite, allanite, and xenotime; biotite and gold; detrital minerals; chalcopyrite and sparse stannite; quartz; and cobaltite with sparse selenides and tellurides. Significantly, Cu is statistically separate from Co and As, consistent with the general lack of abundant chalcopyrite in cobaltite-rich samples. Paragenetic relations determined by scanning electron microscopy indicate that the earliest Y-REE-Be mineralization preceded deposition of Co, Cu, Au, and Bi. Allanite, xenotime, and gadolinite-(Y) commonly occur as intergrowths with and inclusions in cobaltite; the opposite texture is rare. Monazite, in contrast, forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime, reflecting a later metamorphic paragenesis. Allanite and xenotime also show evidence of late dissolution and reprecipitation, forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral. Textural data suggest extensive deformation of the deposits by folding and shearing, and by pervasive recrystallization, all during Cretaceous metamorphism. Sensitive high resolution ion microprobe U-Pb geochronology by Aleinikoff et al. (2012) supports these paragenetic interpretations, documenting contemporaneous Mesoproterozoic growth of early xenotime and crystallization of megacrystic A-type granite on the northern border of the district. These ages are used together with mineralogical and geochemical data from the present study to support an epigenetic, IOCG model for Fe-Co-Cu-Au-Bi-Y-REE deposits of the Idaho cobalt belt. A sulfide facies variant of IOCG deposits is proposed for the Blackbird district, in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides. This new model includes Mesoproterozoic vein mineralization and related Fe-Cl metasomatism that formed the biotite-rich lenses, a predominantly felsic magmatic source for metals in the deposits, given their local abundance of Y, REEs, and Be, and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore-related tourmaline, respectively. DEWEY : 620.1 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1089.short [article] Strata-bound Fe-Co-Cu-Au-Bi-Y-REE deposits of the idaho cobalt belt : multistage hydrothermal mineralization in a magmatic-related iron oxide copper-gold system [texte imprimé] / John F. Slack, Auteur . - 2012 . - pp. 1089-1113. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1089-1113 Mots-clés : strata-bound;  REE  deposits Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : Mineralogical and geochemical studies of strata-bound Fe-Co-Cu-Au-Bi-Y-rare-earth element (REE) deposits of the Idaho cobalt belt in east-central Idaho provide evidence of multistage epigenetic mineralization by magmatic-hydrothermal processes in an iron oxide copper-gold (IOCG) system. Deposits of the Idaho cobalt belt comprise three types: (1) strata-bound sulfide lenses in the Blackbird district, which are cobaltite and, less commonly, chalcopyrite rich with locally abundant gold, native bismuth, bismuthinite, xenotime, allanite, monazite, and the Be-rich silicate gadolinite-(Y), with sparse uraninite, stannite, and Bi tellurides, in a gangue of quartz, chlorite, biotite, muscovite, garnet, tourmaline, chloritoid, and/or siderite, with locally abundant fluorapatite or magnetite; (2) discordant tourmalinized breccias in the Blackbird district that in places have concentrations of cobaltite, chalcopyrite, gold, and xenotime; and (3) strata-bound magnetite-rich lenses in the Iron Creek area, which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime. Most sulfide-rich deposits in the Blackbird district are enclosed by strata-bound lenses composed mainly of Cl-rich Fe biotite; some deposits have quartz-rich envelopes. Whole-rock analyses of 48 Co- and/or Cu-rich samples show high concentrations of Au (up to 26.8 ppm), Bi (up to 9.16 wt %), Y (up to 0.83 wt %), ∑REEs (up to 2.56 wt %), Ni (up to 6,780 ppm), and Be (up to 1,180 ppm), with locally elevated U (up to 124 ppm) and Sn (up to 133 ppm); Zn and Pb contents are uniformly low (≤821 and ≤61 ppm, respectively). Varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite, allanite, and xenotime; biotite and gold; detrital minerals; chalcopyrite and sparse stannite; quartz; and cobaltite with sparse selenides and tellurides. Significantly, Cu is statistically separate from Co and As, consistent with the general lack of abundant chalcopyrite in cobaltite-rich samples. Paragenetic relations determined by scanning electron microscopy indicate that the earliest Y-REE-Be mineralization preceded deposition of Co, Cu, Au, and Bi. Allanite, xenotime, and gadolinite-(Y) commonly occur as intergrowths with and inclusions in cobaltite; the opposite texture is rare. Monazite, in contrast, forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime, reflecting a later metamorphic paragenesis. Allanite and xenotime also show evidence of late dissolution and reprecipitation, forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral. Textural data suggest extensive deformation of the deposits by folding and shearing, and by pervasive recrystallization, all during Cretaceous metamorphism. Sensitive high resolution ion microprobe U-Pb geochronology by Aleinikoff et al. (2012) supports these paragenetic interpretations, documenting contemporaneous Mesoproterozoic growth of early xenotime and crystallization of megacrystic A-type granite on the northern border of the district. These ages are used together with mineralogical and geochemical data from the present study to support an epigenetic, IOCG model for Fe-Co-Cu-Au-Bi-Y-REE deposits of the Idaho cobalt belt. A sulfide facies variant of IOCG deposits is proposed for the Blackbird district, in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides. This new model includes Mesoproterozoic vein mineralization and related Fe-Cl metasomatism that formed the biotite-rich lenses, a predominantly felsic magmatic source for metals in the deposits, given their local abundance of Y, REEs, and Be, and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore-related tourmaline, respectively. DEWEY : 620.1 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1089.short

Geologic setting, sedimentary architecture, and paragenesis of the mesoproterozoic sediment-hosted sheep creek Cu-Co-Ag deposit, Helena Embayment, Montana / Garth Graham in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1115-1141 Titre : Geologic setting, sedimentary architecture, and paragenesis of the mesoproterozoic sediment-hosted sheep creek Cu-Co-Ag deposit, Helena Embayment, Montana Type de document : texte imprimé Auteurs : Garth Graham, Auteur ; Murray W. Hitzman, Auteur ; Jerry Zieg, Auteur Année de publication : 2012 Article en page(s) : pp. 1115-1141 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sheep  creek  Cu-Co-Ag  deposit;  mineralization;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The northern margin of the Helena Embayment contains extensive syngenetic to diagenetic massive pyrite horizons that extend over 25 km along the Volcano Valley-Buttress fault zone and extend up to 8 km basinward (south) within the Mesoproterozoic Newland Formation. The Sheep Creek Cu-Co deposit occurs within a structural block along a bend in the fault system, where replacement-style chalcopyrite mineralization is spatially associated mostly with the two stratigraphically lowest massive pyrite zones. These mineralized pyritic horizons are intercalated with debris flows derived from synsedimentary movement along the Volcano Valley-Buttress fault zone. Cominco American Inc. delineated a geologic resource of 4.5 Mt at 2.5% Cu and 0.1% Co in the upper sulfide zone and 4 Mt at 4% Cu within the lower sulfide zone. More recently, Tintina Resources Inc. has delineated an inferred resource of 8.48 Mt at 2.96% Cu, 0.12% Co, and 16.4 g/t Ag in the upper sulfide zone. The more intact upper sulfide zone displays significant thickness variations along strike thought to represent formation in at least three separate subbasins. The largest accumulation of mineralized sulfide in the upper zone occurs as an N-S–trending body that thickens southward from the generally E trending Volcano Valley Fault and probably occupies a paleograben controlled by normal faults in the hanging wall of the Volcano Valley Fault. Early microcrystalline to framboidal pyrite was accompanied by abundant and local barite deposition in the upper and lower sulfide zones, respectively. The sulfide bodies underwent intense (lower sulfide zone) to localized (upper sulfide zone) recrystallization and overprinting by coarser-grained pyrite and minor marcasite that is intergrown with and replaces dolomite. Silicification and paragenetically late chalcopyrite, along with minor tennantite in the upper sulfide zone, replaces fine-grained pyrite, barite, and carbonate. The restriction of chalcopyrite to inferred synsedimentary E- and northerly trending faults and absence of definitive zonation with respect to the Laramide Volcano Valley Fault in the lower sulfide zone suggest a diagenetic age related to basin development for the Sheep Creek Cu-Co-Ag deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1115.short [article] Geologic setting, sedimentary architecture, and paragenesis of the mesoproterozoic sediment-hosted sheep creek Cu-Co-Ag deposit, Helena Embayment, Montana [texte imprimé] / Garth Graham, Auteur ; Murray W. Hitzman, Auteur ; Jerry Zieg, Auteur . - 2012 . - pp. 1115-1141. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1115-1141 Mots-clés : Sheep  creek  Cu-Co-Ag  deposit;  mineralization;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The northern margin of the Helena Embayment contains extensive syngenetic to diagenetic massive pyrite horizons that extend over 25 km along the Volcano Valley-Buttress fault zone and extend up to 8 km basinward (south) within the Mesoproterozoic Newland Formation. The Sheep Creek Cu-Co deposit occurs within a structural block along a bend in the fault system, where replacement-style chalcopyrite mineralization is spatially associated mostly with the two stratigraphically lowest massive pyrite zones. These mineralized pyritic horizons are intercalated with debris flows derived from synsedimentary movement along the Volcano Valley-Buttress fault zone. Cominco American Inc. delineated a geologic resource of 4.5 Mt at 2.5% Cu and 0.1% Co in the upper sulfide zone and 4 Mt at 4% Cu within the lower sulfide zone. More recently, Tintina Resources Inc. has delineated an inferred resource of 8.48 Mt at 2.96% Cu, 0.12% Co, and 16.4 g/t Ag in the upper sulfide zone. The more intact upper sulfide zone displays significant thickness variations along strike thought to represent formation in at least three separate subbasins. The largest accumulation of mineralized sulfide in the upper zone occurs as an N-S–trending body that thickens southward from the generally E trending Volcano Valley Fault and probably occupies a paleograben controlled by normal faults in the hanging wall of the Volcano Valley Fault. Early microcrystalline to framboidal pyrite was accompanied by abundant and local barite deposition in the upper and lower sulfide zones, respectively. The sulfide bodies underwent intense (lower sulfide zone) to localized (upper sulfide zone) recrystallization and overprinting by coarser-grained pyrite and minor marcasite that is intergrown with and replaces dolomite. Silicification and paragenetically late chalcopyrite, along with minor tennantite in the upper sulfide zone, replaces fine-grained pyrite, barite, and carbonate. The restriction of chalcopyrite to inferred synsedimentary E- and northerly trending faults and absence of definitive zonation with respect to the Laramide Volcano Valley Fault in the lower sulfide zone suggest a diagenetic age related to basin development for the Sheep Creek Cu-Co-Ag deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1115.short

Constraints on the timing of Co-Cu ± Au mineralization in the blackbird district, Idaho, using SHRIMP U-Pb ages of monazite and xenotime plus zircon ages of related mesoproterozoic orthogneisses and metasedimentary rocks / 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. 1143-1175 Titre : Constraints on the timing of Co-Cu ± Au mineralization in the blackbird district, Idaho, using SHRIMP U-Pb ages of monazite and xenotime plus zircon ages of related mesoproterozoic orthogneisses and metasedimentary rocks Type de document : texte imprimé Auteurs : John N. Aleinikoff, Auteur ; John F. Slack, Auteur ; Karen Lund, Auteur Année de publication : 2012 Article en page(s) : pp. 1143-1175 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Co  reserves;  strata-hosted  Co-Cu  ±  Au  mineralization;  xenotime;  monazite;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206Pb/238U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1143.short [article] Constraints on the timing of Co-Cu ± Au mineralization in the blackbird district, Idaho, using SHRIMP U-Pb ages of monazite and xenotime plus zircon ages of related mesoproterozoic orthogneisses and metasedimentary rocks [texte imprimé] / John N. Aleinikoff, Auteur ; John F. Slack, Auteur ; Karen Lund, Auteur . - 2012 . - pp. 1143-1175. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1143-1175 Mots-clés : Co  reserves;  strata-hosted  Co-Cu  ±  Au  mineralization;  xenotime;  monazite;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206Pb/238U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1143.short

Reconnaissance lead isotope characteristics of the blackbird deposit / K. Panneerselvam in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1177-1188 Titre : Reconnaissance lead isotope characteristics of the blackbird deposit : implications for the age and origin of cobalt-copper mineralization in the Idaho cobalt belt, United States Type de document : texte imprimé Auteurs : K. Panneerselvam, Auteur ; Andrew W. Macfarlane, Auteur ; Vincent J.M. Salters, Auteur Année de publication : 2012 Article en page(s) : pp. 1177-1188 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : cobalt-copper  deposits;  isotope  measurements;  Idaho  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Strata-bound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation (Mesoproterozoic) in a linear belt called the Idaho cobalt belt, near Salmon, Idaho. We report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores. The Pb isotope compositions of cobalt-copper (Co-Cu) ores (206Pb/204Pb = 30.8–40.4; 207Pb/204Pb = 16.8–17.6; 208Pb/204Pb = 49.7–63.9) are very radiogenic, much more radiogenic than any known sedimentary exhalative (sedex) deposits. The data plot well beyond the crustal growth model curves, a feature shared by the Mississippi Valley-type (MVT) deposits which is characteristic of an upper crustal source. It is unlikely that the ore lead could have come from any mafic igneous source of mantle origin (Proterozoic Moyie sill and its equivalent, or the Tertiary Challis Volcanic Group). The Cretaceous felsic igneous rocks of the region (the Idaho batholith and related rocks) also have much lower Pb isotope ratios than the Co-Cu ores. The Pb isotope ratios of Proterozoic crystalline rocks partially overlap Pb isotope ratios of the Apple Creek Formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 Ma. Only the host Apple Creek Formation is known to have appropriate Pb isotope compositions (206Pb/204Pb = 26.8–86.7; 207Pb/204Pb = 16.3–21.1; 208Pb/204Pb = 47.9–64.8) to be the source of Co-Cu ores. Leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of Co-Cu deposits in the Idaho cobalt belt. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1177.short [article] Reconnaissance lead isotope characteristics of the blackbird deposit : implications for the age and origin of cobalt-copper mineralization in the Idaho cobalt belt, United States [texte imprimé] / K. Panneerselvam, Auteur ; Andrew W. Macfarlane, Auteur ; Vincent J.M. Salters, Auteur . - 2012 . - pp. 1177-1188. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1177-1188 Mots-clés : cobalt-copper  deposits;  isotope  measurements;  Idaho  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Strata-bound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation (Mesoproterozoic) in a linear belt called the Idaho cobalt belt, near Salmon, Idaho. We report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores. The Pb isotope compositions of cobalt-copper (Co-Cu) ores (206Pb/204Pb = 30.8–40.4; 207Pb/204Pb = 16.8–17.6; 208Pb/204Pb = 49.7–63.9) are very radiogenic, much more radiogenic than any known sedimentary exhalative (sedex) deposits. The data plot well beyond the crustal growth model curves, a feature shared by the Mississippi Valley-type (MVT) deposits which is characteristic of an upper crustal source. It is unlikely that the ore lead could have come from any mafic igneous source of mantle origin (Proterozoic Moyie sill and its equivalent, or the Tertiary Challis Volcanic Group). The Cretaceous felsic igneous rocks of the region (the Idaho batholith and related rocks) also have much lower Pb isotope ratios than the Co-Cu ores. The Pb isotope ratios of Proterozoic crystalline rocks partially overlap Pb isotope ratios of the Apple Creek Formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 Ma. Only the host Apple Creek Formation is known to have appropriate Pb isotope compositions (206Pb/204Pb = 26.8–86.7; 207Pb/204Pb = 16.3–21.1; 208Pb/204Pb = 47.9–64.8) to be the source of Co-Cu ores. Leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of Co-Cu deposits in the Idaho cobalt belt. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1177.short

Ore genesis constraints on the Idaho cobalt belt from fluid inclusion gas, noble gas isotope, and ion ratio analyses / Gary P. Landis in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1189-1205 Titre : Ore genesis constraints on the Idaho cobalt belt from fluid inclusion gas, noble gas isotope, and ion ratio analyses Type de document : texte imprimé Auteurs : Gary P. Landis, Auteur ; Albert H. Hofstra, Auteur Année de publication : 2012 Article en page(s) : pp. 1189-1205 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Idaho  cobalt  belt;  cobaltite;  Co  pyrite;  chalcopyrite;  gold  deposit;  REEs  elements;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Idaho cobalt belt is a 60-km-long alignment of deposits composed of cobaltite, Co pyrite, chalcopyrite, and gold with anomalous Nb, Y, Be, and rare-earth elements (REEs) in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group. It is the largest cobalt resource in the United States with historic production from the Blackbird Mine. All of the deposits were deformed and metamorphosed to upper greenschist-lower amphibolite grade in the Cretaceous. They occur near a 1377 Ma anorogenic bimodal plutonic complex. The enhanced solubility of Fe, Co, Cu, and Au as chloride complexes together with gangue biotite rich in Fe and Cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved. The isotopes of B in gangue tourmaline are suggestive of a marine source, whereas those of Pb in ore suggest a U ± Th-enriched source. The ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post-ore minerals and metamorphic minerals. Such components can potentially be identified and distinguished by their relative abundances in contrasting samples. Therefore, we obtained samples of Co and Cu sulfides, gangue quartz, biotite, and tourmaline and post-ore quartz veins as well as Cretaceous metamorphic garnet and determined the gas, noble gas isotope, and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography. The most abundant gases present in extracts from each sample type are biased toward the gas-rich population of inclusions trapped during maximum burial and metamorphism. All have CO2/CH4 and N2/Ar ratios of evolved crustal fluids, and many yield a range of H2-CH4-CO2-H2S equilibration temperatures consistent with the metamorphic grade. Cretaceous garnet and post-ore minerals have high RH and RS values suggestive of reduced sulfidic conditions. Most extracts have anomalous 4He produced by decay of U and Th and 38Ar produced by nucleogenic production from 41K. In contrast, some ore and gangue minerals yield significant SO2 and have low RH and RS values of a more oxidized fluid. Three extracts from gangue quartz have high helium R/RA values indicative of a mantle source and neon isotope compositions that require nucleogenic production of 22Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated 40K/40Ar that permit a Precambrian age. Extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios (Na, K, NH4, Cl, Br, F) suggestive of a single fluid. Their Na, Cl, Br ratios suggest this fluid was a mixture of magmatic and basinal brine. Na-K-Ca temperatures (279°–347°C) are similar to homogenization temperatures of hypersaline inclusions. The high K/Na of the brine may be due to albitization of K silicate minerals in country rocks. Influx of K-rich brines is consistent with the K metasomatism necessary to form gangue biotite with high Cl. An extract from a post-ore quartz vein is distinct and has Na, Cl, Br ratios that resemble metamorphic fluids in Cretaceous silver veins of the Coeur d’Alene district in the Belt Basin. The results show that in some samples, for certain components, it is possible to “see through” the Cretaceous metamorphic overprint. Of great import for genetic models, the volatiles trapped in gangue quartz have 3He derived from a mantle source and 22Ne derived from fluorite, both of which may be attributed to nearby ~1377 Ma basalt-rhyolite magmatism. The brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater. The former requires a granitic intrusion that is present in the bimodal intrusive complex, and the latter equatorial paleolatitudes that existed in the Mesoproterozoic. The results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the Lemhi Group. While the inferred fluid sources in the Idaho cobalt belt are similar in many respects to those in iron oxide copper-gold deposits, the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1189.short [article] Ore genesis constraints on the Idaho cobalt belt from fluid inclusion gas, noble gas isotope, and ion ratio analyses [texte imprimé] / Gary P. Landis, Auteur ; Albert H. Hofstra, Auteur . - 2012 . - pp. 1189-1205. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1189-1205 Mots-clés : Idaho  cobalt  belt;  cobaltite;  Co  pyrite;  chalcopyrite;  gold  deposit;  REEs  elements;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Idaho cobalt belt is a 60-km-long alignment of deposits composed of cobaltite, Co pyrite, chalcopyrite, and gold with anomalous Nb, Y, Be, and rare-earth elements (REEs) in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group. It is the largest cobalt resource in the United States with historic production from the Blackbird Mine. All of the deposits were deformed and metamorphosed to upper greenschist-lower amphibolite grade in the Cretaceous. They occur near a 1377 Ma anorogenic bimodal plutonic complex. The enhanced solubility of Fe, Co, Cu, and Au as chloride complexes together with gangue biotite rich in Fe and Cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved. The isotopes of B in gangue tourmaline are suggestive of a marine source, whereas those of Pb in ore suggest a U ± Th-enriched source. The ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post-ore minerals and metamorphic minerals. Such components can potentially be identified and distinguished by their relative abundances in contrasting samples. Therefore, we obtained samples of Co and Cu sulfides, gangue quartz, biotite, and tourmaline and post-ore quartz veins as well as Cretaceous metamorphic garnet and determined the gas, noble gas isotope, and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography. The most abundant gases present in extracts from each sample type are biased toward the gas-rich population of inclusions trapped during maximum burial and metamorphism. All have CO2/CH4 and N2/Ar ratios of evolved crustal fluids, and many yield a range of H2-CH4-CO2-H2S equilibration temperatures consistent with the metamorphic grade. Cretaceous garnet and post-ore minerals have high RH and RS values suggestive of reduced sulfidic conditions. Most extracts have anomalous 4He produced by decay of U and Th and 38Ar produced by nucleogenic production from 41K. In contrast, some ore and gangue minerals yield significant SO2 and have low RH and RS values of a more oxidized fluid. Three extracts from gangue quartz have high helium R/RA values indicative of a mantle source and neon isotope compositions that require nucleogenic production of 22Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated 40K/40Ar that permit a Precambrian age. Extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios (Na, K, NH4, Cl, Br, F) suggestive of a single fluid. Their Na, Cl, Br ratios suggest this fluid was a mixture of magmatic and basinal brine. Na-K-Ca temperatures (279°–347°C) are similar to homogenization temperatures of hypersaline inclusions. The high K/Na of the brine may be due to albitization of K silicate minerals in country rocks. Influx of K-rich brines is consistent with the K metasomatism necessary to form gangue biotite with high Cl. An extract from a post-ore quartz vein is distinct and has Na, Cl, Br ratios that resemble metamorphic fluids in Cretaceous silver veins of the Coeur d’Alene district in the Belt Basin. The results show that in some samples, for certain components, it is possible to “see through” the Cretaceous metamorphic overprint. Of great import for genetic models, the volatiles trapped in gangue quartz have 3He derived from a mantle source and 22Ne derived from fluorite, both of which may be attributed to nearby ~1377 Ma basalt-rhyolite magmatism. The brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater. The former requires a granitic intrusion that is present in the bimodal intrusive complex, and the latter equatorial paleolatitudes that existed in the Mesoproterozoic. The results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the Lemhi Group. While the inferred fluid sources in the Idaho cobalt belt are similar in many respects to those in iron oxide copper-gold deposits, the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1189.short

Sulfur, carbon, hydrogen, and oxygen isotope geochemistry of the Idaho cobalt belt / Craig A. Johnson in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1207-1221 Titre : Sulfur, carbon, hydrogen, and oxygen isotope geochemistry of the Idaho cobalt belt Type de document : texte imprimé Auteurs : Craig A. Johnson, Auteur ; Arthur A. Bookstrom, Auteur ; John F. Slack, Auteur Année de publication : 2012 Article en page(s) : pp. 1207-1221 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cobalt-copper  ±  gold  deposits;  Blackbird  district;  geochemistry;  Idaho  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cobalt-copper ± gold deposits of the Idaho cobalt belt, including the deposits of the Blackbird district, have been analyzed for their sulfur, carbon, hydrogen, and oxygen isotope compositions to improve the understanding of ore formation. Previous genetic hypotheses have ranged widely, linking the ores to the sedimentary or diagenetic history of the host Mesoproterozoic sedimentary rocks, to Mesoproterozoic or Cretaceous magmatism, or to metamorphic shearing. The δ34S values are nearly uniform throughout the Blackbird district, with a mean value for cobaltite (CoAsS, the main cobalt mineral) of 8.0 ± 0.4‰ (n = 19). The data suggest that (1) sulfur was derived at least partly from sedimentary sources, (2) redox reactions involving sulfur were probably unimportant for ore deposition, and (3) the sulfur was probably transported to sites of ore formation as H2S. Hydrogen and oxygen isotope compositions of the ore-forming fluid, which are calculated from analyses of biotite-rich wall rocks and tourmaline, do not uniquely identify the source of the fluid; plausible sources include formation waters, metamorphic waters, and mixtures of magmatic and isotopically heavy meteoric waters. The calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide (VMS) deposits. Carbon and oxygen isotope compositions of siderite, a mineral that is widespread, although sparse, at Blackbird, suggest formation from mixtures of sedimentary organic carbon and magmatic-metamorphic carbon. The isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin. Several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper, so its significance for the ore-forming event is uncertain. From the stable isotope perspective, the mineral deposits of the Idaho cobalt belt contrast with typical VMS and sedimentary exhalative deposits. They show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes, although the isotopic evidence for magmatic components is relatively weak. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1207.short [article] Sulfur, carbon, hydrogen, and oxygen isotope geochemistry of the Idaho cobalt belt [texte imprimé] / Craig A. Johnson, Auteur ; Arthur A. Bookstrom, Auteur ; John F. Slack, Auteur . - 2012 . - pp. 1207-1221. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1207-1221 Mots-clés : Cobalt-copper  ±  gold  deposits;  Blackbird  district;  geochemistry;  Idaho  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cobalt-copper ± gold deposits of the Idaho cobalt belt, including the deposits of the Blackbird district, have been analyzed for their sulfur, carbon, hydrogen, and oxygen isotope compositions to improve the understanding of ore formation. Previous genetic hypotheses have ranged widely, linking the ores to the sedimentary or diagenetic history of the host Mesoproterozoic sedimentary rocks, to Mesoproterozoic or Cretaceous magmatism, or to metamorphic shearing. The δ34S values are nearly uniform throughout the Blackbird district, with a mean value for cobaltite (CoAsS, the main cobalt mineral) of 8.0 ± 0.4‰ (n = 19). The data suggest that (1) sulfur was derived at least partly from sedimentary sources, (2) redox reactions involving sulfur were probably unimportant for ore deposition, and (3) the sulfur was probably transported to sites of ore formation as H2S. Hydrogen and oxygen isotope compositions of the ore-forming fluid, which are calculated from analyses of biotite-rich wall rocks and tourmaline, do not uniquely identify the source of the fluid; plausible sources include formation waters, metamorphic waters, and mixtures of magmatic and isotopically heavy meteoric waters. The calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide (VMS) deposits. Carbon and oxygen isotope compositions of siderite, a mineral that is widespread, although sparse, at Blackbird, suggest formation from mixtures of sedimentary organic carbon and magmatic-metamorphic carbon. The isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin. Several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper, so its significance for the ore-forming event is uncertain. From the stable isotope perspective, the mineral deposits of the Idaho cobalt belt contrast with typical VMS and sedimentary exhalative deposits. They show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes, although the isotopic evidence for magmatic components is relatively weak. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1207.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

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

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

Iron oxide ± Cu ± Au deposits in the iron range, purcell basin, southeastern British Columbia / M. Galicki in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1293-1301 Titre : Iron oxide ± Cu ± Au deposits in the iron range, purcell basin, southeastern British Columbia Type de document : texte imprimé Auteurs : M. Galicki, Auteur ; D. Marshall, Auteur ; R. Staples, Auteur Année de publication : 2012 Article en page(s) : pp. 1293-1301 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : iron  oxide  ±  Cu  ±  Au  deposits;  mineralization;  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Iron Range iron oxide ± Cu ± Au deposits in southeastern British Columbia comprise massive lenses and veins of hematite and martite with lesser magnetite in the Iron Range fault zone, which crosscuts the Proterozoic Aldridge Formation and Moyie sills. The mineralized zones are flanked by albite-quartz-iron oxide breccia within sedimentary rocks and by chlorite-altered iron oxide breccia where they are in contact with Moyie sills. Oxygen isotope analyses indicate 340° to 400°C precipitation temperatures for the albite-quartz-magnetite assemblages in the mineralized zones. Magnetite trace element compositions closely compare with those in iron oxide-(copper-gold) (IOCG) to porphyry-type mineralization worldwide. Paleomagnetic studies show consistent paleopole orientations concordant with Cretaceous poles and support links to a porphyry-type genesis associated with phases of the nearby 80 to 105 Ma magnetite-bearing Bayonne Suite plutonic rocks. The Iron Range iron oxide mineralized zones share many characteristics of major IOCG deposits, with the exception of economic Cu (± Au) concentrations in the exposed rocks; however, recent drilling intersected minor chalcopyrite, pyrite, and gold. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1293.short [article] Iron oxide ± Cu ± Au deposits in the iron range, purcell basin, southeastern British Columbia [texte imprimé] / M. Galicki, Auteur ; D. Marshall, Auteur ; R. Staples, Auteur . - 2012 . - pp. 1293-1301. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1293-1301 Mots-clés : iron  oxide  ±  Cu  ±  Au  deposits;  mineralization;  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Iron Range iron oxide ± Cu ± Au deposits in southeastern British Columbia comprise massive lenses and veins of hematite and martite with lesser magnetite in the Iron Range fault zone, which crosscuts the Proterozoic Aldridge Formation and Moyie sills. The mineralized zones are flanked by albite-quartz-iron oxide breccia within sedimentary rocks and by chlorite-altered iron oxide breccia where they are in contact with Moyie sills. Oxygen isotope analyses indicate 340° to 400°C precipitation temperatures for the albite-quartz-magnetite assemblages in the mineralized zones. Magnetite trace element compositions closely compare with those in iron oxide-(copper-gold) (IOCG) to porphyry-type mineralization worldwide. Paleomagnetic studies show consistent paleopole orientations concordant with Cretaceous poles and support links to a porphyry-type genesis associated with phases of the nearby 80 to 105 Ma magnetite-bearing Bayonne Suite plutonic rocks. The Iron Range iron oxide mineralized zones share many characteristics of major IOCG deposits, with the exception of economic Cu (± Au) concentrations in the exposed rocks; however, recent drilling intersected minor chalcopyrite, pyrite, and gold. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1293.short

Geochemistry and metallogeny of Ag-Pb-Zn veins in the purcell basin, British Columbia / Jean-Philippe Paiement in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1303-1320 Titre : Geochemistry and metallogeny of Ag-Pb-Zn veins in the purcell basin, British Columbia Type de document : texte imprimé Auteurs : Jean-Philippe Paiement, Auteur ; Georges Beaudoin, Auteur ; Suzanne Paradis, Auteur Année de publication : 2012 Article en page(s) : pp. 1303-1320 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Ag-Pb-Zn  veins;  Metallogeny;  Geochemistry;  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mesoproterozoic Purcell Basin in Canada contains three different types of veins: (1) type 1 Pb-Zn-Ag veins are composed of Fe-rich sphalerite, pyrrhotite, galena, freibergite, and pyrite with minor quartz gangue; (2) type 2 Pb-Ag-Cu-Au veins are characterized by galena, pyrite, freibergite, and gold in a quartz gangue; and (3) type 3 Ag-Pb-Zn vein and replacement deposits contain Fe-poor sphalerite, galena, pyrite, and freibergite in a quartz-dolomite gangue that locally replaces host dolostone. The sulfur isotope composition of type 1, 2, and 3 vein and replacement deposits closely reflects their respective stratigraphic positions. Type 1 and 2 veins hosted in lower Purcell sedimentary rocks have light sulfur isotope compositions (−4.8 to +2.5‰) similar to diagenic pyrite disseminated in clastic sedimentary host rocks. Type 3 veins and the replacement deposits hosted in upper Purcell Supergroup rocks have heavy sulfur isotope compositions (8.3–17.2‰) indicative of thermo-chemical reduction of marine sulfate in carbonate host rocks. Quartz from type 1 Pb-Zn-Ag veins has δ18OSMOW values ranging from 7 to 17‰, whereas type 2 and 3 vein and replacement deposits have δ18O values ranging from 9 to 20‰. The range of δ18O values for type 1, 2, and 3 veins is the result of mixing of metamorphic fluids with a δ18O value higher than 11.5‰ and an upper crustal fluid with a δ18O lower than 1.2‰, in equilibrium with the host rocks during Proterozoic (type 1) and Mesozoic-Cenozoic (type 2 and 3) hydrothermal events. Galena and freibergite lead isotope compositions plot in two groups, consistent with previous studies. Type 1 Pb-Zn-Ag veins have a nonradiogenic lead signature (206Pb/204Pb: 16.322–16.435) and are considered Proterozoic in age. Type 2 Pb-Ag-Au-Cu veins and type 3 Ag-Pb-Zn vein and replacement deposits have a radiogenic lead signature (206Pb/204Pb: 17.842–19.347) and are interpreted to be Mesozoic-Cenozoic in age. Hydrothermal sericite from the type 3 Ptarmigan replacement deposit yields a 40Ar/39Ar age of 133.1 ± 0.7 Ma, indicating a Cretaceous age for type 3 deposits. Type 1 veins are interpreted to be the result of hydrothermal fluids generated during the Proterozoic East Kootenay Orogeny, whereas type 2 and 3 veins and replacement deposits are interpreted to be the result of Late Cretaceous-Paleocene tectonism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1303.short [article] Geochemistry and metallogeny of Ag-Pb-Zn veins in the purcell basin, British Columbia [texte imprimé] / Jean-Philippe Paiement, Auteur ; Georges Beaudoin, Auteur ; Suzanne Paradis, Auteur . - 2012 . - pp. 1303-1320. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1303-1320 Mots-clés : Ag-Pb-Zn  veins;  Metallogeny;  Geochemistry;  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mesoproterozoic Purcell Basin in Canada contains three different types of veins: (1) type 1 Pb-Zn-Ag veins are composed of Fe-rich sphalerite, pyrrhotite, galena, freibergite, and pyrite with minor quartz gangue; (2) type 2 Pb-Ag-Cu-Au veins are characterized by galena, pyrite, freibergite, and gold in a quartz gangue; and (3) type 3 Ag-Pb-Zn vein and replacement deposits contain Fe-poor sphalerite, galena, pyrite, and freibergite in a quartz-dolomite gangue that locally replaces host dolostone. The sulfur isotope composition of type 1, 2, and 3 vein and replacement deposits closely reflects their respective stratigraphic positions. Type 1 and 2 veins hosted in lower Purcell sedimentary rocks have light sulfur isotope compositions (−4.8 to +2.5‰) similar to diagenic pyrite disseminated in clastic sedimentary host rocks. Type 3 veins and the replacement deposits hosted in upper Purcell Supergroup rocks have heavy sulfur isotope compositions (8.3–17.2‰) indicative of thermo-chemical reduction of marine sulfate in carbonate host rocks. Quartz from type 1 Pb-Zn-Ag veins has δ18OSMOW values ranging from 7 to 17‰, whereas type 2 and 3 vein and replacement deposits have δ18O values ranging from 9 to 20‰. The range of δ18O values for type 1, 2, and 3 veins is the result of mixing of metamorphic fluids with a δ18O value higher than 11.5‰ and an upper crustal fluid with a δ18O lower than 1.2‰, in equilibrium with the host rocks during Proterozoic (type 1) and Mesozoic-Cenozoic (type 2 and 3) hydrothermal events. Galena and freibergite lead isotope compositions plot in two groups, consistent with previous studies. Type 1 Pb-Zn-Ag veins have a nonradiogenic lead signature (206Pb/204Pb: 16.322–16.435) and are considered Proterozoic in age. Type 2 Pb-Ag-Au-Cu veins and type 3 Ag-Pb-Zn vein and replacement deposits have a radiogenic lead signature (206Pb/204Pb: 17.842–19.347) and are interpreted to be Mesozoic-Cenozoic in age. Hydrothermal sericite from the type 3 Ptarmigan replacement deposit yields a 40Ar/39Ar age of 133.1 ± 0.7 Ma, indicating a Cretaceous age for type 3 deposits. Type 1 veins are interpreted to be the result of hydrothermal fluids generated during the Proterozoic East Kootenay Orogeny, whereas type 2 and 3 veins and replacement deposits are interpreted to be the result of Late Cretaceous-Paleocene tectonism. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1303.short

Age and origin of quartz-carbonate veins associated with the coeur d'alene mining district, Idaho and western Montana / Frank C. Ramos in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1321-1339 Titre : Age and origin of quartz-carbonate veins associated with the coeur d'alene mining district, Idaho and western Montana : insights from isotopes and rare-earth elements Type de document : texte imprimé Auteurs : Frank C. Ramos, Auteur ; Philip E. Rosenberg, Auteur Année de publication : 2012 Article en page(s) : pp. 1321-1339 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sm-Nd,  Pb,  and  Sr  isotope;  rare-earth  element;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sm-Nd, Pb, and Sr isotope and rare-earth element (REE) analyses of quartz-carbonate veins associated with the Coeur d’Alene mining district, Idaho and western Montana, constrain the age and origin of Coeur d’Alene veining. An Sm-Nd isochron age of siderites from ore-bearing veins (1511 ± 45 Ma) and a Pb-Pb isochron age of siderites, ankerites, and calcites from ore-barren veins (1523 ± 41 Ma) are similar to the model age of Coeur d’Alene-type Pb (~1450 Ma) and constrain Mesoproterozoic events in the earliest history of the Belt-Purcell Basin. Xenotime, an accessory mineral found in all veins, is zoned with a core laser ablation age of 1420 ± 90 Ma, confirming vein emplacement early in the diagenetic and/or metamorphic history of the Belt-Purcell Basin, and an overgrowth age of 990 ± 130 Ma results from later, Grenville-age metamorphism. Pb isotope ratios of carbonates from both ore-bearing and ore-barren veins define a single linear array resulting from Mesoproterozoic vein emplacement with an initial Pb isotope ratio similar to that of Coeur d’Alene-type Pb observed in galena, while moderately radiogenic Pb isotope ratios originate from variable amounts of accompanying xenotime. This array is similar to the previously determined “more radiogenic” array of Leach et al. (1998a). Ore-barren veins are characterized by initial 143Nd/144Nd and 87Sr/86Sr ratios of 0.5108 and 0.769, respectively, which may be partially inherited from Belt-Purcell Supergroup and/or Archean rocks. In contrast, initial 143Nd/144Nd and 87Sr/86Sr ratios of 0.5083 and 1.146 of ore-bearing veins require a source with highly unradiogenic Nd and highly radiogenic Sr, most likely Archean crust. Ore minerals may result from interaction of hydrothermal fluids from Archean sources with potential Belt-Purcell Basin sedimentary exhalative (sedex) deposits. REEs are concentrated in carbonate fractions of veins. Siderite and ankerite are light REE depleted and heavy REE enriched, resulting in significant Sm/Nd fractionation from the dual effects of REE complexation in carbonate fluids and mineralogical control, which in the case of siderite and ankerite favors the heavy REEs. Positive Eu anomalies characterize ore-bearing veins, whereas ore-barren veins retain negative Eu anomalies. Normalization of REE patterns of ore-barren veins to those of adjacent Belt-Purcell wall rocks results in a near-zero anomaly, implying that REEs from Belt-Purcell metasediments were locally scavenged by carbonate-rich fluids responsible for ore-barren veins. In contrast, leaching of Archean gneisses followed by probable interaction with sedex deposits produced fluids that formed ore-bearing deposits. Further faulting and deposition from successive fluxes of carbonate-rich solutions derived in part from leaching Belt-Purcell metasediments generated ore-barren siderite-, ankerite-, and, finally, calcite-dominant veins. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1321.short [article] Age and origin of quartz-carbonate veins associated with the coeur d'alene mining district, Idaho and western Montana : insights from isotopes and rare-earth elements [texte imprimé] / Frank C. Ramos, Auteur ; Philip E. Rosenberg, Auteur . - 2012 . - pp. 1321-1339. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - pp. 1321-1339 Mots-clés : Sm-Nd,  Pb,  and  Sr  isotope;  rare-earth  element;  Montana Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Sm-Nd, Pb, and Sr isotope and rare-earth element (REE) analyses of quartz-carbonate veins associated with the Coeur d’Alene mining district, Idaho and western Montana, constrain the age and origin of Coeur d’Alene veining. An Sm-Nd isochron age of siderites from ore-bearing veins (1511 ± 45 Ma) and a Pb-Pb isochron age of siderites, ankerites, and calcites from ore-barren veins (1523 ± 41 Ma) are similar to the model age of Coeur d’Alene-type Pb (~1450 Ma) and constrain Mesoproterozoic events in the earliest history of the Belt-Purcell Basin. Xenotime, an accessory mineral found in all veins, is zoned with a core laser ablation age of 1420 ± 90 Ma, confirming vein emplacement early in the diagenetic and/or metamorphic history of the Belt-Purcell Basin, and an overgrowth age of 990 ± 130 Ma results from later, Grenville-age metamorphism. Pb isotope ratios of carbonates from both ore-bearing and ore-barren veins define a single linear array resulting from Mesoproterozoic vein emplacement with an initial Pb isotope ratio similar to that of Coeur d’Alene-type Pb observed in galena, while moderately radiogenic Pb isotope ratios originate from variable amounts of accompanying xenotime. This array is similar to the previously determined “more radiogenic” array of Leach et al. (1998a). Ore-barren veins are characterized by initial 143Nd/144Nd and 87Sr/86Sr ratios of 0.5108 and 0.769, respectively, which may be partially inherited from Belt-Purcell Supergroup and/or Archean rocks. In contrast, initial 143Nd/144Nd and 87Sr/86Sr ratios of 0.5083 and 1.146 of ore-bearing veins require a source with highly unradiogenic Nd and highly radiogenic Sr, most likely Archean crust. Ore minerals may result from interaction of hydrothermal fluids from Archean sources with potential Belt-Purcell Basin sedimentary exhalative (sedex) deposits. REEs are concentrated in carbonate fractions of veins. Siderite and ankerite are light REE depleted and heavy REE enriched, resulting in significant Sm/Nd fractionation from the dual effects of REE complexation in carbonate fluids and mineralogical control, which in the case of siderite and ankerite favors the heavy REEs. Positive Eu anomalies characterize ore-bearing veins, whereas ore-barren veins retain negative Eu anomalies. Normalization of REE patterns of ore-barren veins to those of adjacent Belt-Purcell wall rocks results in a near-zero anomaly, implying that REEs from Belt-Purcell metasediments were locally scavenged by carbonate-rich fluids responsible for ore-barren veins. In contrast, leaching of Archean gneisses followed by probable interaction with sedex deposits produced fluids that formed ore-bearing deposits. Further faulting and deposition from successive fluxes of carbonate-rich solutions derived in part from leaching Belt-Purcell metasediments generated ore-barren siderite-, ankerite-, and, finally, calcite-dominant veins. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1321.short

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