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Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada / de Almeida, Carolina Michelin in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 971-1004 Titre : Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada : implications for sources of ore-forming elements, ore genesis, and mineral exploration Type de document : texte imprimé Auteurs : de Almeida, Carolina Michelin, Auteur ; Gema Ribeiro Olivo, Auteur ; Annick Chouinard, Auteur Année de publication : 2011 Article en page(s) : pp. 971-1004 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  ore  Host  rocks  Mineral  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada. Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II. The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite. this study reveals that Al2O3 and TiO2 are the most immobile compounds, and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks. Among the ore-related trace elements, Tl best correlates with Au grade (R2 = 0.69) and shows some relationship with the calculated amount of pyrite (R2 = 0.49), indicating that Tl would be the best element to vector toward zones of high-grade Carlin-type Au mineralization. Gold grades do not correlate with the amount of pyrite, degree of alteration, or organic C. Our results integrated with available thermodynamic data for AU, ore-related elements, and SiO2 lead us to suggest that the formation of Ore I occurred more proximal to the major mineralizing conduits as the hot, more acid, SiO2- and trace element-rich auriferous fluids interacted with Fe-bearing impure carbonate host rocks, intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the Betze-Post and Rodeo deposits. As the fluids moved laterally throughout the favorable host rocks, the pH increased, leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica, favoring the formation of more distal Ore II in the central-northern part of the Screamer deposit. Significantly, the gold concentrations in whole rock and in pyrite are, in some way, very similar in both ore types, being slightly higher in Ore II, suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite, even at lower concentrations of other trace elements. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/971.abstract [article] Mineral paragenesis, alteration, and geochemistry of the two types of gold ore and the host rocks from the carlin-type deposits in the southern part of the goldstrike property, northern Nevada : implications for sources of ore-forming elements, ore genesis, and mineral exploration [texte imprimé] / de Almeida, Carolina Michelin, Auteur ; Gema Ribeiro Olivo, Auteur ; Annick Chouinard, Auteur . - 2011 . - pp. 971-1004. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - pp. 971-1004 Mots-clés : Gold  ore  Host  rocks  Mineral  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada. Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II. The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite. this study reveals that Al2O3 and TiO2 are the most immobile compounds, and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks. Among the ore-related trace elements, Tl best correlates with Au grade (R2 = 0.69) and shows some relationship with the calculated amount of pyrite (R2 = 0.49), indicating that Tl would be the best element to vector toward zones of high-grade Carlin-type Au mineralization. Gold grades do not correlate with the amount of pyrite, degree of alteration, or organic C. Our results integrated with available thermodynamic data for AU, ore-related elements, and SiO2 lead us to suggest that the formation of Ore I occurred more proximal to the major mineralizing conduits as the hot, more acid, SiO2- and trace element-rich auriferous fluids interacted with Fe-bearing impure carbonate host rocks, intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the Betze-Post and Rodeo deposits. As the fluids moved laterally throughout the favorable host rocks, the pH increased, leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica, favoring the formation of more distal Ore II in the central-northern part of the Screamer deposit. Significantly, the gold concentrations in whole rock and in pyrite are, in some way, very similar in both ore types, being slightly higher in Ore II, suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite, even at lower concentrations of other trace elements. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/971.abstract

PGE-Rich Ni-Cu sulfide mineralization in the flin flon greenstone belt, Manitoba, Canada / Natalie Eva Bursztyn in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1469-1490 Titre : PGE-Rich Ni-Cu sulfide mineralization in the flin flon greenstone belt, Manitoba, Canada : implications for hydrothermal remobilization of platinum group elements in basic-ultrabasic sequences Type de document : texte imprimé Auteurs : Natalie Eva Bursztyn, Auteur ; Gema Ribeiro Olivo, Auteur Année de publication : 2011 Article en page(s) : pp. 1469-1490 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Platinium  group  element  Greenstone  belt  Sulfide  mineralization  Hydrothermal  remobilization  Basic-ultrabasic  sequences  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Flin Flon greenstone belt is best known for its world-class volcanogenic massive sulfide deposits. However, the recent discovery of the McBratney platinum group element (PGE)-Au occurrence and other PGE occurrences has drawn attention to the potential for Ni-Cu-PGE (Au) deposits in this belt. The McBratney occurrence contains some of the highest PGE and Au grades found in this type of deposit in North America (up to 207 g/t Pd, 34 g/t Pt, 2.6 g/t Rh, and 75 g/t Au). It is hosted in chlorite-actinolite and chlorite schists of the Bear Lake magmatic unit in the Bear Lake block of the Flin Flon greenstone belt. The metamorphosed host rocks are komatiitic and tholeiitic in composition. The mineralization is hydrothermal and postdates the regional, retrograde metamorphic event. It occurs as sulfide-rich veins and surrounding disseminated zones controlled by faults, as well as isolated disseminated zones within chlorite-actinolite and chlorite schists. Platinum group minerals (PGM) and gold are spatially and temporally associated with pyrrhotite, chalcopyrite, carbonate, second generation chlorite, chamosite and, locally, stilpnomelane. In the disseminated zones, these phases replace the metamorphic actinolite-chlorite assemblage in both types of host rocks. Locally the veins are layered with pyrrhotite-rich zones and chalcopyrite-rich zones. The PGM include, in order of abundance, Te-rich sudburyite, borovskite, sperrylite, sudburyite, an unknown Pd-Te-Sb mineral, temagamite, and merenskyite, which occur mainly as inclusions in the sulfides. Minor amounts of PGE also occur within chlorite, carbonate, and stilpnomelane, and Au-Ag alloy is observed locally included in Fe sulfides in association with sphalerite. The mantle-normalized metal distribution shows that the McBratney mineralization is enriched in Bi, As, Au, Pd, Cu, Pt, Rh, Os, Ru, Zn, Re, and Ag; the mineralization is both enriched and depleted in Ir and Cr, and it is depleted in Ni in all analyzed samples when compared with typical komatiitic magmatic ore. The mineral assemblage suggests that the hydrothermal fluids were reduced (pyrrhotite-pyrite stable), neutral to alkaline, and CO2 bearing (carbonate-chlorite stable). Chlorite geothemometry indicates that the hydrothermal assemblage formed at temperatures ranging from 250° to 350°C. Under these conditions, PGE may have been transported mainly as bisulfide complexes and their precipitation likely occurred due to reactions of the hydrothermal fluids with the Fe-rich host rocks, which led to the formation of Fe sulfides, reduction of the activity of the bisulfide, and formation of PGM. Palladium precipitated mainly as tellurides and antimonides; however, Pt formed an arsenide. The abundant As, Te, and Sb may have been crucial in forming high-grade PGE ore. Sulfur isotope data indicate that magmatic rocks or fluids were likely the sources of sulfur, which may include the metamorphosed basic-ultrabasic sequence, hidden magmatic Ni-Cu sulfide mineralization, or fluids derived directly from postmetamorphic magmatic rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1469.abstract [article] PGE-Rich Ni-Cu sulfide mineralization in the flin flon greenstone belt, Manitoba, Canada : implications for hydrothermal remobilization of platinum group elements in basic-ultrabasic sequences [texte imprimé] / Natalie Eva Bursztyn, Auteur ; Gema Ribeiro Olivo, Auteur . - 2011 . - pp. 1469-1490. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1469-1490 Mots-clés : Platinium  group  element  Greenstone  belt  Sulfide  mineralization  Hydrothermal  remobilization  Basic-ultrabasic  sequences  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Flin Flon greenstone belt is best known for its world-class volcanogenic massive sulfide deposits. However, the recent discovery of the McBratney platinum group element (PGE)-Au occurrence and other PGE occurrences has drawn attention to the potential for Ni-Cu-PGE (Au) deposits in this belt. The McBratney occurrence contains some of the highest PGE and Au grades found in this type of deposit in North America (up to 207 g/t Pd, 34 g/t Pt, 2.6 g/t Rh, and 75 g/t Au). It is hosted in chlorite-actinolite and chlorite schists of the Bear Lake magmatic unit in the Bear Lake block of the Flin Flon greenstone belt. The metamorphosed host rocks are komatiitic and tholeiitic in composition. The mineralization is hydrothermal and postdates the regional, retrograde metamorphic event. It occurs as sulfide-rich veins and surrounding disseminated zones controlled by faults, as well as isolated disseminated zones within chlorite-actinolite and chlorite schists. Platinum group minerals (PGM) and gold are spatially and temporally associated with pyrrhotite, chalcopyrite, carbonate, second generation chlorite, chamosite and, locally, stilpnomelane. In the disseminated zones, these phases replace the metamorphic actinolite-chlorite assemblage in both types of host rocks. Locally the veins are layered with pyrrhotite-rich zones and chalcopyrite-rich zones. The PGM include, in order of abundance, Te-rich sudburyite, borovskite, sperrylite, sudburyite, an unknown Pd-Te-Sb mineral, temagamite, and merenskyite, which occur mainly as inclusions in the sulfides. Minor amounts of PGE also occur within chlorite, carbonate, and stilpnomelane, and Au-Ag alloy is observed locally included in Fe sulfides in association with sphalerite. The mantle-normalized metal distribution shows that the McBratney mineralization is enriched in Bi, As, Au, Pd, Cu, Pt, Rh, Os, Ru, Zn, Re, and Ag; the mineralization is both enriched and depleted in Ir and Cr, and it is depleted in Ni in all analyzed samples when compared with typical komatiitic magmatic ore. The mineral assemblage suggests that the hydrothermal fluids were reduced (pyrrhotite-pyrite stable), neutral to alkaline, and CO2 bearing (carbonate-chlorite stable). Chlorite geothemometry indicates that the hydrothermal assemblage formed at temperatures ranging from 250° to 350°C. Under these conditions, PGE may have been transported mainly as bisulfide complexes and their precipitation likely occurred due to reactions of the hydrothermal fluids with the Fe-rich host rocks, which led to the formation of Fe sulfides, reduction of the activity of the bisulfide, and formation of PGM. Palladium precipitated mainly as tellurides and antimonides; however, Pt formed an arsenide. The abundant As, Te, and Sb may have been crucial in forming high-grade PGE ore. Sulfur isotope data indicate that magmatic rocks or fluids were likely the sources of sulfur, which may include the metamorphosed basic-ultrabasic sequence, hidden magmatic Ni-Cu sulfide mineralization, or fluids derived directly from postmetamorphic magmatic rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1469.abstract

The Barry gold deposits, Abitib subprovince, Canada / Kathryn E. Kitney in Economic geology, Vol. 106 N° 7 (Novembre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1129-1154 Titre : The Barry gold deposits, Abitib subprovince, Canada : a greenstone belt-hosted gold deposit coeval with late archean deformation and magmatism Type de document : texte imprimé Auteurs : Kathryn E. Kitney, Auteur ; Gema Ribeiro Olivo, Auteur ; Donald W. Davis, Auteur Année de publication : 2011 Article en page(s) : pp. 1129-1154 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  deposit  Greenstone  Mamatism  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Barry gold deposit is an example of an Archean greenstone-hosted lode gold deposit located in the Urban-Barry greenstone belt in the Abitibi subprovince of Québec, Canada. The host mafic volcanic rocks are part of the 2717 Ma Macho Formation of the Northern Volcanic zone. They are cut by a series of weakly foliated preore diorite, pre- and postore quartz-feldspar porphyry (QFP), and quartz monzonite dikes and plugs, providing a unique opportunity to evaluate the age of the gold mineralization. Auriferous zones are spatially associated with northeast-trending ductile shear zones with moderate south-easterly dip. Gold mineralization occurs within straight N64° E/64° SE and folded N20° E/60° SE, on average, albite-carbonate-quartz veins and within the surrounding carbonate-quartz-pyrite, and locally within biotite-carbonate alteration zones of the host mafic volcanic rocks. The auriferous veins comprise 5 to 15 vol percent of the mafic volcanic rocks, are 1 to 5 cm wide, and are locally boudinaged. Although the volcanic units strike N55–60° E and dip 40° SE, the ore envelope (>2 g/t Au) is constrained from surface to a depth of 30 m in an antiformal shape. Gold generally occurs as microinclusions in pyrite but is also present as free gold in albite-carbonate-quartz veins, synmineralization altered host rocks, and locally within quartz veins cutting early QFP dikes. The deposit has an indicated resource of 309,500 oz Au (7,701,000 t at 1.25 g/t Au) and an inferred gold resource of 471,950 oz Au (10,411,000 t at 1.41 g/t Au). U-Pb dating of single zircon grains from the premineralization diorite and postmineralization QFP yielded indistinguishable ages, averaging 2697 ± 0.6 Ma, which is interpreted as the age of gold mineralization at the Barry deposit. This age and the field relationships reveal that lode gold mineralization was coeval with regional deformation and magmatism, similar to that documented at the Kiena, Norlartic, and Siscoe (Main zone) deposits of the Val d’Or district. These deposits represent the earlier orogenic gold event in the Abitibi greenstone belt, which preceded the postmagmatic, quartz-tourmaline auriferous veins such as those present at Sigma. Barry, Kiena, Norlartic, and Siscoe share many similarities with other orogenic gold deposits worldwide that are spatially associated with felsic and intermediate intrusions. The genesis for these deposits is not well constrained, as it is not obvious to distinguish whether the auriferous fluids were derived from the coeval magmas or whether both magmas and fluids were derived from a thermal event from the deep crust, or possibly the mantle. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1129.abstract [article] The Barry gold deposits, Abitib subprovince, Canada : a greenstone belt-hosted gold deposit coeval with late archean deformation and magmatism [texte imprimé] / Kathryn E. Kitney, Auteur ; Gema Ribeiro Olivo, Auteur ; Donald W. Davis, Auteur . - 2011 . - pp. 1129-1154. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1129-1154 Mots-clés : Gold  deposit  Greenstone  Mamatism  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Barry gold deposit is an example of an Archean greenstone-hosted lode gold deposit located in the Urban-Barry greenstone belt in the Abitibi subprovince of Québec, Canada. The host mafic volcanic rocks are part of the 2717 Ma Macho Formation of the Northern Volcanic zone. They are cut by a series of weakly foliated preore diorite, pre- and postore quartz-feldspar porphyry (QFP), and quartz monzonite dikes and plugs, providing a unique opportunity to evaluate the age of the gold mineralization. Auriferous zones are spatially associated with northeast-trending ductile shear zones with moderate south-easterly dip. Gold mineralization occurs within straight N64° E/64° SE and folded N20° E/60° SE, on average, albite-carbonate-quartz veins and within the surrounding carbonate-quartz-pyrite, and locally within biotite-carbonate alteration zones of the host mafic volcanic rocks. The auriferous veins comprise 5 to 15 vol percent of the mafic volcanic rocks, are 1 to 5 cm wide, and are locally boudinaged. Although the volcanic units strike N55–60° E and dip 40° SE, the ore envelope (>2 g/t Au) is constrained from surface to a depth of 30 m in an antiformal shape. Gold generally occurs as microinclusions in pyrite but is also present as free gold in albite-carbonate-quartz veins, synmineralization altered host rocks, and locally within quartz veins cutting early QFP dikes. The deposit has an indicated resource of 309,500 oz Au (7,701,000 t at 1.25 g/t Au) and an inferred gold resource of 471,950 oz Au (10,411,000 t at 1.41 g/t Au). U-Pb dating of single zircon grains from the premineralization diorite and postmineralization QFP yielded indistinguishable ages, averaging 2697 ± 0.6 Ma, which is interpreted as the age of gold mineralization at the Barry deposit. This age and the field relationships reveal that lode gold mineralization was coeval with regional deformation and magmatism, similar to that documented at the Kiena, Norlartic, and Siscoe (Main zone) deposits of the Val d’Or district. These deposits represent the earlier orogenic gold event in the Abitibi greenstone belt, which preceded the postmagmatic, quartz-tourmaline auriferous veins such as those present at Sigma. Barry, Kiena, Norlartic, and Siscoe share many similarities with other orogenic gold deposits worldwide that are spatially associated with felsic and intermediate intrusions. The genesis for these deposits is not well constrained, as it is not obvious to distinguish whether the auriferous fluids were derived from the coeval magmas or whether both magmas and fluids were derived from a thermal event from the deep crust, or possibly the mantle. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1129.abstract