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Ajouter le résultat dans votre panierHigh Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits / Jeremy P. Richards in Economic geology, Vol. 106 N° 7 (Novembre 2011)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1075-1081
Titre : High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits : just add water Type de document : texte imprimé Auteurs : Jeremy P. Richards, Auteur Année de publication : 2011 Article en page(s) : pp. 1075-1081 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Arc magmas Au deposits Mineral exploration Index. décimale : 001.424 Résumé : Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere.
While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H2O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.DEWEY : 001.424 ISSN : 0160-5682 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1075.abstract [article] High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits : just add water [texte imprimé] / Jeremy P. Richards, Auteur . - 2011 . - pp. 1075-1081.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1075-1081
Mots-clés : Arc magmas Au deposits Mineral exploration Index. décimale : 001.424 Résumé : Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere.
While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H2O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.DEWEY : 001.424 ISSN : 0160-5682 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1075.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1083-1110
Titre : The Santa Rita nickel sulfide deposits in the Fazenda mirabela intrusion, Bahia, Brazil : Geology, sulfide geochemistry, and genesis Type de document : texte imprimé Auteurs : Stephen J. Barnes, Auteur ; Grant A. Osborne, Auteur ; Doug Cook, Auteur Année de publication : 2011 Article en page(s) : pp. 1083-1110 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Santa rita ore zone Sulfide tenors Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Santa Rita ore zone within the Fazenda Mirabela intrusion is an unusual example of high-tonnage magmatic Ni-Cu-PGE sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion. The intrusion as a whole is about 4 × 2.5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat-shaped morphology. The ore zone comprises a 50- to 200-m-thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites. This orthopyroxene-dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half.
Sulfide tenor variations and silicate host-rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores. Consistent patterns of sulfide tenor—gradually decreasing Ni from over 20 to about 10 wt percent, near-constant Cu and gradually decreasing PGEs with cyclically varying Pt/Pd ratios—are found within the Santa Rita ore zone across the entire intrusion and display relatively little lateral variability. The position of the sulfide mineralization envelope within the harzburgite-orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion. Maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene-bearing cumulates, asymmetrically disposed toward the southern margin of the intrusion.
Nickel tenors of the sulfides range from 15 to 25 wt percent, exceptionally high for rocks with relatively low values of MgO/(MgO + FeO) and are closer to the typical values observed in komatiite-hosted deposits, although Cu tenors (typically 5–8 wt %) are much higher than komatiite values. Sulfide PGE tenors range from less than 100 ppb to greater than 10 ppm of both Pt and Pd, with maximum values in S-poor rocks at the base of the sulfide-bearing interval. PGEs become significantly more depleted toward the top of the Santa Rita zone. We attribute these features, along with the localization of the sulfide-bearing interval at the contact between ultramafic and mafic cumulates, to mixing of an initially S-undersaturated, moderately Ni enriched resident magma with a relatively low Ni, PGE-depleted, significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1083.abstract [article] The Santa Rita nickel sulfide deposits in the Fazenda mirabela intrusion, Bahia, Brazil : Geology, sulfide geochemistry, and genesis [texte imprimé] / Stephen J. Barnes, Auteur ; Grant A. Osborne, Auteur ; Doug Cook, Auteur . - 2011 . - pp. 1083-1110.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1083-1110
Mots-clés : Santa rita ore zone Sulfide tenors Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Santa Rita ore zone within the Fazenda Mirabela intrusion is an unusual example of high-tonnage magmatic Ni-Cu-PGE sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion. The intrusion as a whole is about 4 × 2.5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat-shaped morphology. The ore zone comprises a 50- to 200-m-thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites. This orthopyroxene-dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half.
Sulfide tenor variations and silicate host-rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores. Consistent patterns of sulfide tenor—gradually decreasing Ni from over 20 to about 10 wt percent, near-constant Cu and gradually decreasing PGEs with cyclically varying Pt/Pd ratios—are found within the Santa Rita ore zone across the entire intrusion and display relatively little lateral variability. The position of the sulfide mineralization envelope within the harzburgite-orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion. Maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene-bearing cumulates, asymmetrically disposed toward the southern margin of the intrusion.
Nickel tenors of the sulfides range from 15 to 25 wt percent, exceptionally high for rocks with relatively low values of MgO/(MgO + FeO) and are closer to the typical values observed in komatiite-hosted deposits, although Cu tenors (typically 5–8 wt %) are much higher than komatiite values. Sulfide PGE tenors range from less than 100 ppb to greater than 10 ppm of both Pt and Pd, with maximum values in S-poor rocks at the base of the sulfide-bearing interval. PGEs become significantly more depleted toward the top of the Santa Rita zone. We attribute these features, along with the localization of the sulfide-bearing interval at the contact between ultramafic and mafic cumulates, to mixing of an initially S-undersaturated, moderately Ni enriched resident magma with a relatively low Ni, PGE-depleted, significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1083.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1111-1128
Titre : The Lagoa Salgada orebody, Iberian pyritr belt, Portugal Type de document : texte imprimé Auteurs : de Oliveira, D. P. S., Auteur ; J. X. Matos, Auteur ; C. J. P. Rosa, Auteur Année de publication : 2011 Article en page(s) : pp. 1111-1128 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Orebody Iberian pyritr belt Sulfide deposits Mineral chemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Iberian Pyrite Belt is one of the most outstanding European ore provinces and hosts one of the largest concentrations of massive sulfide deposits today, totaling 1,850 million metric tons (Mt) in more than 90 deposits. Lagoa Salgada is a small orebody (estimated to have at least 4 Mt) and, as yet, an unexploited orebody found within this ore province. It is located 80 km northwest of Neves Corvo and occurs ~130 m beneath sediments of the Sado Tertiary basin, limiting interpretation to drill hole data. Lagoa Salgada is folded, faulted, and interpreted to occur mostly on the subvertical-overturned and intensely faulted limb of a southwest-verging anticline. It is represented by a central stockwork zone and a massive sulfide lens zone in the northwestern part of the orebody.
Mineralization is mainly composed of pyrite with minor sphalerite, tetrahedrite-tennantite, arsenopyrite, chalcopyrite, galena, stannite, and supergene minerals. The orebody is hosted by a volcanic succession of rhyodacitic composition. These small orebodies and some of the other abandoned mines within the Iberian Pyrite Belt may represent interesting and feasible mining projects as a result of the added value generated by the presence of trace metals, such as In, serving a significant future demand for the high-tech industry. Lagoa Salgada is one such case.
Indium is a significant trace metal in the ores of Lagoa Salgada as indicated by whole-rock analyses. This element is preferentially contained by sphalerite. Electron probe microanalyses (EPMA) of In contents within sphalerite show a large variability, ranging from below detection limits to an obtained maximum of 0.8 percent In. Discrete inclusions of In-bearing minerals have not been observed, thus favoring the idea that In occurs either dispersed or in nanodomains within the host mineral.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1111.abstract [article] The Lagoa Salgada orebody, Iberian pyritr belt, Portugal [texte imprimé] / de Oliveira, D. P. S., Auteur ; J. X. Matos, Auteur ; C. J. P. Rosa, Auteur . - 2011 . - pp. 1111-1128.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1111-1128
Mots-clés : Orebody Iberian pyritr belt Sulfide deposits Mineral chemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Iberian Pyrite Belt is one of the most outstanding European ore provinces and hosts one of the largest concentrations of massive sulfide deposits today, totaling 1,850 million metric tons (Mt) in more than 90 deposits. Lagoa Salgada is a small orebody (estimated to have at least 4 Mt) and, as yet, an unexploited orebody found within this ore province. It is located 80 km northwest of Neves Corvo and occurs ~130 m beneath sediments of the Sado Tertiary basin, limiting interpretation to drill hole data. Lagoa Salgada is folded, faulted, and interpreted to occur mostly on the subvertical-overturned and intensely faulted limb of a southwest-verging anticline. It is represented by a central stockwork zone and a massive sulfide lens zone in the northwestern part of the orebody.
Mineralization is mainly composed of pyrite with minor sphalerite, tetrahedrite-tennantite, arsenopyrite, chalcopyrite, galena, stannite, and supergene minerals. The orebody is hosted by a volcanic succession of rhyodacitic composition. These small orebodies and some of the other abandoned mines within the Iberian Pyrite Belt may represent interesting and feasible mining projects as a result of the added value generated by the presence of trace metals, such as In, serving a significant future demand for the high-tech industry. Lagoa Salgada is one such case.
Indium is a significant trace metal in the ores of Lagoa Salgada as indicated by whole-rock analyses. This element is preferentially contained by sphalerite. Electron probe microanalyses (EPMA) of In contents within sphalerite show a large variability, ranging from below detection limits to an obtained maximum of 0.8 percent In. Discrete inclusions of In-bearing minerals have not been observed, thus favoring the idea that In occurs either dispersed or in nanodomains within the host mineral.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1111.abstract
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
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1155-1171
Titre : Mineral chemistry and in situ dating of Allanite, and geochemistry of its host rocks in the Bagjata uranium mine, singhbhum shear zone, India : implications for the chemical evolution of REE mineralization and mobilization Type de document : texte imprimé Auteurs : Dipak C. Pal, Auteur ; Trisrota Chaudhuri, Auteur ; Chris McFarlane, Auteur Année de publication : 2011 Article en page(s) : pp. 1155-1171 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Host rocks Mineral chemistry Uranium mine Mineralization India Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The present study combines textures, compositions, and in situ dating of allanite and whole-rock geochemistry from the Bagjata uranium deposit in the Singhbhum shear zone, the most important uranium-producing belt in India, to investigate the chemical evolution of rare earth element (REE) mineralization. Allanite, volumetrically the most important sink of light rare earth elements (LREEs) in this deposit, occurs as disseminated grains, pockets, veins, and stringers in biotite schist and chlorite schist. Metasomatic addition of LREEs in the rock was associated with Ca-K-Fe ± B metasomatism, as evident from the vein mineralogy. The allanite-bearing metasomatized rocks are locally extremely enriched in REEs, reaching up to ~4.8 wt percent ∑REEs. The textures and compositions of allanite demonstrate that early REE-rich allanite was replaced by later, relatively less REE bearing allanite-epidote, and this alteration followed a substitution scheme of REE+3 + (Fe+2, Mg+2) → Ca+2 + (Al, Fe+3). The presence of halite-bearing fluid inclusions in associated tourmaline, high Cl content of biotite, and other circumstantial evidence indicate that a high-salinity, Cl-rich fluid was likely responsible for initial LREE metasomatism. A later hydrothermal event altered preexisting allanite and removed REEs. The more oxidized nature of altered allanite and the presence of unidentified secondary REE minerals in association with sulfide might indicate the involvement of oxidized, sulfate-rich fluid in the alteration of allanite and the removal of REEs. Alternatively, fluoride complexing of REEs and selective removal of HREEs during this alteration are possible.
In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of allanite and monazite indicates that LREE metasomatism took place at ~1.88 Ga. The next hydrothermal event, which altered existing allanite, removed some REEs, and perhaps was associated with U (+HREE) addition, is dated at ~1.66 Ga. The youngest age determined from the thin rim of allanite is ~1.02 Ga. These three different ages, determined from allanite, overlap with the known thermotectonic events in and around the Singhbhum shear zone.
Comparison between the calculated ∑REE content of allanite-rich rock prior to allanite alteration and whole-rock ∑REE composition of the same rock suggests that the addition of a large quantity of REEs was followed by significant REE removal. Strong variations in the modal abundances of the constituent minerals and in whole-rock compositions, including REEs, indicate open-system behavior. The whole-rock REE pattern suggests that decoupling between LREEs and HREEs might be possible during superimposed alteration. Therefore, petrogenetic modeling using an REE-based discrimination diagram and REE patterns should be done with caution for rocks that underwent multiple events of hydrothermal fluid flux.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1155.abstract [article] Mineral chemistry and in situ dating of Allanite, and geochemistry of its host rocks in the Bagjata uranium mine, singhbhum shear zone, India : implications for the chemical evolution of REE mineralization and mobilization [texte imprimé] / Dipak C. Pal, Auteur ; Trisrota Chaudhuri, Auteur ; Chris McFarlane, Auteur . - 2011 . - pp. 1155-1171.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1155-1171
Mots-clés : Host rocks Mineral chemistry Uranium mine Mineralization India Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The present study combines textures, compositions, and in situ dating of allanite and whole-rock geochemistry from the Bagjata uranium deposit in the Singhbhum shear zone, the most important uranium-producing belt in India, to investigate the chemical evolution of rare earth element (REE) mineralization. Allanite, volumetrically the most important sink of light rare earth elements (LREEs) in this deposit, occurs as disseminated grains, pockets, veins, and stringers in biotite schist and chlorite schist. Metasomatic addition of LREEs in the rock was associated with Ca-K-Fe ± B metasomatism, as evident from the vein mineralogy. The allanite-bearing metasomatized rocks are locally extremely enriched in REEs, reaching up to ~4.8 wt percent ∑REEs. The textures and compositions of allanite demonstrate that early REE-rich allanite was replaced by later, relatively less REE bearing allanite-epidote, and this alteration followed a substitution scheme of REE+3 + (Fe+2, Mg+2) → Ca+2 + (Al, Fe+3). The presence of halite-bearing fluid inclusions in associated tourmaline, high Cl content of biotite, and other circumstantial evidence indicate that a high-salinity, Cl-rich fluid was likely responsible for initial LREE metasomatism. A later hydrothermal event altered preexisting allanite and removed REEs. The more oxidized nature of altered allanite and the presence of unidentified secondary REE minerals in association with sulfide might indicate the involvement of oxidized, sulfate-rich fluid in the alteration of allanite and the removal of REEs. Alternatively, fluoride complexing of REEs and selective removal of HREEs during this alteration are possible.
In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of allanite and monazite indicates that LREE metasomatism took place at ~1.88 Ga. The next hydrothermal event, which altered existing allanite, removed some REEs, and perhaps was associated with U (+HREE) addition, is dated at ~1.66 Ga. The youngest age determined from the thin rim of allanite is ~1.02 Ga. These three different ages, determined from allanite, overlap with the known thermotectonic events in and around the Singhbhum shear zone.
Comparison between the calculated ∑REE content of allanite-rich rock prior to allanite alteration and whole-rock ∑REE composition of the same rock suggests that the addition of a large quantity of REEs was followed by significant REE removal. Strong variations in the modal abundances of the constituent minerals and in whole-rock compositions, including REEs, indicate open-system behavior. The whole-rock REE pattern suggests that decoupling between LREEs and HREEs might be possible during superimposed alteration. Therefore, petrogenetic modeling using an REE-based discrimination diagram and REE patterns should be done with caution for rocks that underwent multiple events of hydrothermal fluid flux.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1155.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1173-1196
Titre : Metalliferous manganese oxide mineralization associated with the boléo Cu-Co-Zn district, Mexico Type de document : texte imprimé Auteurs : Andrew G. Conly, Auteur ; Steven D. Scott, Auteur ; Hervé Bellon, Auteur Année de publication : 2011 Article en page(s) : pp. 1173-1196 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Co-Zn district Mineralization Mexico Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boléo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanèchite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %).
Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO2-Fe2O3t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boléo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boléo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boléo Formation strata at elevations above the present-day water table.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1173.abstract [article] Metalliferous manganese oxide mineralization associated with the boléo Cu-Co-Zn district, Mexico [texte imprimé] / Andrew G. Conly, Auteur ; Steven D. Scott, Auteur ; Hervé Bellon, Auteur . - 2011 . - pp. 1173-1196.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1173-1196
Mots-clés : Cu-Co-Zn district Mineralization Mexico Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boléo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanèchite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %).
Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO2-Fe2O3t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boléo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boléo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boléo Formation strata at elevations above the present-day water table.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1173.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1197-1223
Titre : Structural and stratigraphic controls on carbonate-osted base metal mineralization in the mesoproterozoic borden basin (Nanisivik district), Nunavut Type de document : texte imprimé Auteurs : Elizabeth C. Turner, Auteur Année de publication : 2011 Article en page(s) : pp. 1197-1223 Note générale : Economic geologfy Langues : Anglais (eng) Mots-clés : Carbonate Metal mineralization Nunavut Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Field mapping of carbonate-hosted base metal showings in the Milne Inlet graben of the Mesoproterozoic Borden basin, Nunavut, has identified the main geologic settings for mineralization in the district that hosts the past-producing Nanisivik deposit. All known showings are associated with faults, fractures, or dikes; these include major, graben-bounding structures with significant displacement, extensive synsedimentary and reactivated intragraben structures, and comparatively minor structures with negligible displacement. These brittle structural features, together with stratigraphic factors and primary lithofacies, control the distribution of base metal showings, and define four main settings for sulfide concentrations. The most volumetrically impressive mineralization (1) for example, Nanisivik orebody, is spatially constrained by erosional highs on an unconformity surface that separates host dolostone and overlying shale; (2) and (3) widespread but volumetrically limited fault- and fracture-controlled showings are spatially associated with intragraben fracture and dike systems—some of these showings (2) are stratigraphically limited to dolostone immediately above shale, whereas others (3) appear to be stratigraphically random and show a close spatial association with structural features; and (4) lithofacies-controlled mineralization that displays replacement textures is present in the immediate vicinity of graben-bounding faults. The most distinctive features of the district are the predominance of structural controls on the spatial distribution of sulfides, the nonplatformal, nonpassive-margin origin of the main dolostone host, the fundamental spatial control imposed by unconformity shape at the most important showings, and association of some highly prospective showings with long-lived crustal-scale faults. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1197.abstract [article] Structural and stratigraphic controls on carbonate-osted base metal mineralization in the mesoproterozoic borden basin (Nanisivik district), Nunavut [texte imprimé] / Elizabeth C. Turner, Auteur . - 2011 . - pp. 1197-1223.
Economic geologfy
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1197-1223
Mots-clés : Carbonate Metal mineralization Nunavut Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Field mapping of carbonate-hosted base metal showings in the Milne Inlet graben of the Mesoproterozoic Borden basin, Nunavut, has identified the main geologic settings for mineralization in the district that hosts the past-producing Nanisivik deposit. All known showings are associated with faults, fractures, or dikes; these include major, graben-bounding structures with significant displacement, extensive synsedimentary and reactivated intragraben structures, and comparatively minor structures with negligible displacement. These brittle structural features, together with stratigraphic factors and primary lithofacies, control the distribution of base metal showings, and define four main settings for sulfide concentrations. The most volumetrically impressive mineralization (1) for example, Nanisivik orebody, is spatially constrained by erosional highs on an unconformity surface that separates host dolostone and overlying shale; (2) and (3) widespread but volumetrically limited fault- and fracture-controlled showings are spatially associated with intragraben fracture and dike systems—some of these showings (2) are stratigraphically limited to dolostone immediately above shale, whereas others (3) appear to be stratigraphically random and show a close spatial association with structural features; and (4) lithofacies-controlled mineralization that displays replacement textures is present in the immediate vicinity of graben-bounding faults. The most distinctive features of the district are the predominance of structural controls on the spatial distribution of sulfides, the nonplatformal, nonpassive-margin origin of the main dolostone host, the fundamental spatial control imposed by unconformity shape at the most important showings, and association of some highly prospective showings with long-lived crustal-scale faults. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1197.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1225-1239
Titre : The evaluation of brine prospects and the requirement for modifications to filing standards Type de document : texte imprimé Auteurs : John Houston, Auteur ; Andrew Butcher, Auteur ; Peter Ehren, Auteur Année de publication : 2011 Article en page(s) : pp. 1225-1239 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Brine prospects Lithium exploration Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The recent increase in demand for lithium has led to the development of new brine prospects, particularly in the Central Andes. The brines are hosted in closed basin aquifers of two types: mature, halite dominant, and immature, clastic dominant. The estimate of elemental resources in these salars depends on a detailed knowledge of aquifer geometry, porosity, and brine grade. The geometry of the aquifers can be evaluated by classical geophysical and drilling techniques, but because the resource is a fluid, with the attendant problems of in-aquifer mixing and reorganization, existing codes for filing resource and reserve estimates need modification. Total porosity is relatively straightforward to measure, but effective porosity and specific yield, which are required to estimate the resource, are more difficult. Recovery factors are low compared with most metalliferous and industrial mineral deposits due to reliance on pumping of the brine from wells for extraction. These and related issues lead us to believe that modifications to the existing standards for reporting mineral resources and reserves are required for these prospects. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1225.abstract [article] The evaluation of brine prospects and the requirement for modifications to filing standards [texte imprimé] / John Houston, Auteur ; Andrew Butcher, Auteur ; Peter Ehren, Auteur . - 2011 . - pp. 1225-1239.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1225-1239
Mots-clés : Brine prospects Lithium exploration Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The recent increase in demand for lithium has led to the development of new brine prospects, particularly in the Central Andes. The brines are hosted in closed basin aquifers of two types: mature, halite dominant, and immature, clastic dominant. The estimate of elemental resources in these salars depends on a detailed knowledge of aquifer geometry, porosity, and brine grade. The geometry of the aquifers can be evaluated by classical geophysical and drilling techniques, but because the resource is a fluid, with the attendant problems of in-aquifer mixing and reorganization, existing codes for filing resource and reserve estimates need modification. Total porosity is relatively straightforward to measure, but effective porosity and specific yield, which are required to estimate the resource, are more difficult. Recovery factors are low compared with most metalliferous and industrial mineral deposits due to reliance on pumping of the brine from wells for extraction. These and related issues lead us to believe that modifications to the existing standards for reporting mineral resources and reserves are required for these prospects. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1225.abstract
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1241-1249
Titre : Electron backscatter diffraction-based identification and quantification of diamonds from the rif gneisses (Spain and Morocco) : economic implication Type de document : texte imprimé Auteurs : María Dolores Ruiz Cruz, Auteur ; de Galdeano, Carlos Sanz, Auteur ; Carmen Garrido, Auteur Année de publication : 2011 Article en page(s) : pp. 1241-1249 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Electron Backscatter Diamond Spain Morocco Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Microdiamonds from granulite facies gneisses of the Northern Rif (Spain and Morocco) have been studied by electron backscatter diffraction (EBSD). Diamond identification using other techniques, such as Raman spectroscopy, is hindered by microdiamond size and by the presence of graphite bands. In contrast, EBSD enabled the following: (1) the rapid identification of diamonds and their distribution, (2) the estimate of the diamond concentration, and (3) the analysis of the crystallographic relationships between diamonds and the surrounding phases. EBSD study has revealed that (1) diamonds are concentrated mainly in large mesoperthite and quartz + coesite grains, (2) the diamond concentration in the rocks is notably higher than that estimated on the basis of diamond concentration in garnet, and (3) clear orientation relationships exist among diamonds, coesite, and quartz. These data would not have been revealed using classical methods, since most diamonds can be recognized by optical microscopy only when EBSD maps have previously revealed their location. The high diamond grades determined in this preliminary study suggest that these deposits could be of economic interest. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1241.abstract [article] Electron backscatter diffraction-based identification and quantification of diamonds from the rif gneisses (Spain and Morocco) : economic implication [texte imprimé] / María Dolores Ruiz Cruz, Auteur ; de Galdeano, Carlos Sanz, Auteur ; Carmen Garrido, Auteur . - 2011 . - pp. 1241-1249.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1241-1249
Mots-clés : Electron Backscatter Diamond Spain Morocco Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Microdiamonds from granulite facies gneisses of the Northern Rif (Spain and Morocco) have been studied by electron backscatter diffraction (EBSD). Diamond identification using other techniques, such as Raman spectroscopy, is hindered by microdiamond size and by the presence of graphite bands. In contrast, EBSD enabled the following: (1) the rapid identification of diamonds and their distribution, (2) the estimate of the diamond concentration, and (3) the analysis of the crystallographic relationships between diamonds and the surrounding phases. EBSD study has revealed that (1) diamonds are concentrated mainly in large mesoperthite and quartz + coesite grains, (2) the diamond concentration in the rocks is notably higher than that estimated on the basis of diamond concentration in garnet, and (3) clear orientation relationships exist among diamonds, coesite, and quartz. These data would not have been revealed using classical methods, since most diamonds can be recognized by optical microscopy only when EBSD maps have previously revealed their location. The high diamond grades determined in this preliminary study suggest that these deposits could be of economic interest. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1241.abstract
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