Dépouillements

Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru / Lawrence S. Winter in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 713-742 Titre : Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru : position and timing of giant VMS deposits Type de document : texte imprimé Auteurs : Lawrence S. Winter, Auteur ; Richard M. Tosdal, Auteur ; James K. Mortensen, Auteur Année de publication : 2011 Article en page(s) : pp. 713-742 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Volcanic  stratigraphy  VMS  deposits  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/713.abstract [article] Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru : position and timing of giant VMS deposits [texte imprimé] / Lawrence S. Winter, Auteur ; Richard M. Tosdal, Auteur ; James K. Mortensen, Auteur . - 2011 . - pp. 713-742. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 713-742 Mots-clés : Volcanic  stratigraphy  VMS  deposits  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/713.abstract

Structural setting and synplutonic fault kinematics of a cordilleran Cu-Au-Mo porphyry mineralization system, Bingham mining district, UUtahUtah / Armelle Kloppenburg in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 743-761 Titre : Structural setting and synplutonic fault kinematics of a cordilleran Cu-Au-Mo porphyry mineralization system, Bingham mining district, UUtahUtah Type de document : texte imprimé Auteurs : Armelle Kloppenburg, Auteur ; John Grocott, Auteur ; David Hutchinson, Auteur Année de publication : 2011 Article en page(s) : pp. 743-761 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cordilleran  Cu-Au-Mo  porphyry  Fault  kinematics  Bingham  mining  district  Utah Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cu-Au-Mo porphyry-type and Cu skarn-type mineralization in the Bingham mining district of Utah are temporally and spatially related to a suite of monzonite and quartz monzonite porphyry intrusions comprising the Bingham complex. Structural control on mineralization at the Bingham Canyon mine has been described at the scale of individual large-scale folds and fracture sets but no integrated pattern for the synmineralization deformation structures has emerged from earlier research. In the work reported here we have investigated hitherto largely unrecognized components of deformation (translations, rotations, and strains) associated with emplacement of many cubic kilometers of granitic rocks in the intrusions of the Bingham district. We have used both the local three-dimensional static mine model and the regional context in digital kinematic two- and three-dimensional forward and reverse modeling to identify a geometrically valid and admissible fault framework for the Bingham district and a kinematic solution for this framework through time. The results indicate that porphyry intrusions and associated mineralization were emplaced during reactivation of a basement-dictated linked system comprising two sets of northwest- and (north)-northeast-trending strike-slip faults. Both sets operated as transfer faults during extensional collapse of the Sevier orogen in the Eocene. Each fault set is characterized by overstep geometries with relay ramps breached by faults (re)activated in extension. Strike-slip was accompanied by progressive dilation of the extensional faults in fault oversteps to permit emplacement of composite stocklike (low aspect ratio) intrusions. The kinematic fault framework identified provides a new structural context for porphyry- and skarn-type mineralization at the Bingham Canyon mine, with potential for linking mineralizing fluid flow to three-dimensional structure and the development of that structure through time. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/743.abstract [article] Structural setting and synplutonic fault kinematics of a cordilleran Cu-Au-Mo porphyry mineralization system, Bingham mining district, UUtahUtah [texte imprimé] / Armelle Kloppenburg, Auteur ; John Grocott, Auteur ; David Hutchinson, Auteur . - 2011 . - pp. 743-761. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 743-761 Mots-clés : Cordilleran  Cu-Au-Mo  porphyry  Fault  kinematics  Bingham  mining  district  Utah Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cu-Au-Mo porphyry-type and Cu skarn-type mineralization in the Bingham mining district of Utah are temporally and spatially related to a suite of monzonite and quartz monzonite porphyry intrusions comprising the Bingham complex. Structural control on mineralization at the Bingham Canyon mine has been described at the scale of individual large-scale folds and fracture sets but no integrated pattern for the synmineralization deformation structures has emerged from earlier research. In the work reported here we have investigated hitherto largely unrecognized components of deformation (translations, rotations, and strains) associated with emplacement of many cubic kilometers of granitic rocks in the intrusions of the Bingham district. We have used both the local three-dimensional static mine model and the regional context in digital kinematic two- and three-dimensional forward and reverse modeling to identify a geometrically valid and admissible fault framework for the Bingham district and a kinematic solution for this framework through time. The results indicate that porphyry intrusions and associated mineralization were emplaced during reactivation of a basement-dictated linked system comprising two sets of northwest- and (north)-northeast-trending strike-slip faults. Both sets operated as transfer faults during extensional collapse of the Sevier orogen in the Eocene. Each fault set is characterized by overstep geometries with relay ramps breached by faults (re)activated in extension. Strike-slip was accompanied by progressive dilation of the extensional faults in fault oversteps to permit emplacement of composite stocklike (low aspect ratio) intrusions. The kinematic fault framework identified provides a new structural context for porphyry- and skarn-type mineralization at the Bingham Canyon mine, with potential for linking mineralizing fluid flow to three-dimensional structure and the development of that structure through time. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/743.abstract

REE-Y, Ti, and P remobilization in magmatic rocks by hydrothermal alteration during Cu-Au deposit formation / van Dongen, Michiel in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 763-776 Titre : REE-Y, Ti, and P remobilization in magmatic rocks by hydrothermal alteration during Cu-Au deposit formation Type de document : texte imprimé Auteurs : van Dongen, Michiel, Auteur ; Roberto F. Weinberg, Auteur ; Andrew G. Tomkins, Auteur Année de publication : 2011 Article en page(s) : pp. 763-776 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Magmatic  rocks  Hydrothermal  fluids  Ok  tedi  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Chondrite-normalized rare earth element (REE) plots of whole-rock geochemical analyses are commonly used to describe the history of magmatic systems. In the particular case of porphyry copper deposits, which are produced by ore-forming magmatic-hydrothermal systems, whole-rock REE concentrations commonly show an unusual depletion in the final phase of magmatism associated with the main ore-forming stage. Interpretation of REE signatures requires establishing whether they result from magmatic or hydrothermal fractionation processes, or a combination of both. We investigated whole-rock trace-element and REE patterns of samples from the giant Ok Tedi porphyry copper-gold deposit, Papua New Guinea, and found that REEs were most depleted in zones of greatest hydrothermal alteration. REE-rich accessory phases zircon, apatite, and titanite have lower modal abundances and become smaller and more anhedral in these intensely altered zones as compared to less altered rocks. This mineralogical change is accompanied by depletions in P, Ti, Y, and Zr, and deviations of Zr/Hf and Y/Ho from chondritic trace-element ratios—chemical signatures indicative of hydrothermal fractionation. We conclude that destruction of REE-rich accessory minerals, particularly apatite, has led to hydrothermal remobilization and depletion of REEs. This implies that intrusive rocks that experienced high-salinity, oxidized fluid-rock interaction at temperatures >400°C may have had their original magmatic REE signature modified. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/763.abstract [article] REE-Y, Ti, and P remobilization in magmatic rocks by hydrothermal alteration during Cu-Au deposit formation [texte imprimé] / van Dongen, Michiel, Auteur ; Roberto F. Weinberg, Auteur ; Andrew G. Tomkins, Auteur . - 2011 . - pp. 763-776. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 763-776 Mots-clés : Magmatic  rocks  Hydrothermal  fluids  Ok  tedi  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Chondrite-normalized rare earth element (REE) plots of whole-rock geochemical analyses are commonly used to describe the history of magmatic systems. In the particular case of porphyry copper deposits, which are produced by ore-forming magmatic-hydrothermal systems, whole-rock REE concentrations commonly show an unusual depletion in the final phase of magmatism associated with the main ore-forming stage. Interpretation of REE signatures requires establishing whether they result from magmatic or hydrothermal fractionation processes, or a combination of both. We investigated whole-rock trace-element and REE patterns of samples from the giant Ok Tedi porphyry copper-gold deposit, Papua New Guinea, and found that REEs were most depleted in zones of greatest hydrothermal alteration. REE-rich accessory phases zircon, apatite, and titanite have lower modal abundances and become smaller and more anhedral in these intensely altered zones as compared to less altered rocks. This mineralogical change is accompanied by depletions in P, Ti, Y, and Zr, and deviations of Zr/Hf and Y/Ho from chondritic trace-element ratios—chemical signatures indicative of hydrothermal fractionation. We conclude that destruction of REE-rich accessory minerals, particularly apatite, has led to hydrothermal remobilization and depletion of REEs. This implies that intrusive rocks that experienced high-salinity, oxidized fluid-rock interaction at temperatures >400°C may have had their original magmatic REE signature modified. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/763.abstract

Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand / James K. Mortensen in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 777-793 Titre : Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand : constraints from lead isotope and 40Ar/39Ar dating studies Type de document : texte imprimé Auteurs : James K. Mortensen, Auteur ; David Craw, Auteur ; Douglas J. MacKenzie, Auteur Année de publication : 2011 Article en page(s) : pp. 777-793 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Orogenic  gold  Lead  isotope Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/777.abstract [article] Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand : constraints from lead isotope and 40Ar/39Ar dating studies [texte imprimé] / James K. Mortensen, Auteur ; David Craw, Auteur ; Douglas J. MacKenzie, Auteur . - 2011 . - pp. 777-793. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 777-793 Mots-clés : Orogenic  gold  Lead  isotope Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/777.abstract

Platinum group element geochemistry of mineralized and nonmineralized komatiites and basalts / Marco L. Fiorentini in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 795-823 Titre : Platinum group element geochemistry of mineralized and nonmineralized komatiites and basalts Type de document : texte imprimé Auteurs : Marco L. Fiorentini, Auteur ; Stephen J. Barnes, Auteur ; C. Michael Lesher, Auteur Année de publication : 2011 Article en page(s) : pp. 795-823 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Platinium  Nonmineralized  komatiites  Geochemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/795.abstract [article] Platinum group element geochemistry of mineralized and nonmineralized komatiites and basalts [texte imprimé] / Marco L. Fiorentini, Auteur ; Stephen J. Barnes, Auteur ; C. Michael Lesher, Auteur . - 2011 . - pp. 795-823. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 795-823 Mots-clés : Platinium  Nonmineralized  komatiites  Geochemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/795.abstract

The shoshonite-absarokite-picrite Karashoho pipe, Uzbekistan / A. V. Golovko in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 825-840 Titre : The shoshonite-absarokite-picrite Karashoho pipe, Uzbekistan : an unusual diamond deposit in an atypical tectonic environment Type de document : texte imprimé Auteurs : A. V. Golovko, Auteur ; F. V. Kaminsky, Auteur Année de publication : 2011 Article en page(s) : pp. 825-840 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Diamond  deposits  Atypical  tectonic  Pipe  rocks  Uzbekistan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The diamondiferous Karashoho pipe was discovered in the Bukantau Mountains, Uzbekistan, in the 1990s and evaluated from 2002 to 2007. It is an elongate, twinned, subvertical-dipping body with an approximate surface expression of 530 × 65–155 m (~45,600 m2). Its northern part is composed of eruptive breccias and shoshonite, whereas the southern part is composed of micaceous shoshonite-absarokite-picritic rocks and shonkinite. The pipe is located within the southern Tian-Shan Hercynian fold system enclosed between the Karakum-Tajik Massif in the south and the middle Tian-Shan system to the north. The Rb-Sr isochron age of the pipe is 353 ± 13 Ma. By petrographic, mineralogical, and chemical compositions the Karashoho rocks belong to a potassic shoshonite-absarokite-picrite rock series, similar to Permian shoshonitic series rocks from the Tian-Shan volcanic belt in north Xinjang, China. However, they differ significantly from common shoshonites and absarokites because of the high magnesian index of rock-forming minerals in the pipe and the presence of some high-pressure mineral phases, such as chromian spinel (including high-chromian varieties of the diamond association) and manganoilmenite. This indicates that the rocks originated under high-pressure conditions. Bulk sampling of the Karashoho rocks yielded a diamond grade in the range of 0.1 to 5 cpht (carats per hundred tons), with the weighted-mean for different rock varieties ranging from 0.87 to 2.55 cpht. The highest grades are in eruptive breccias of the first generation and shoshonitic-absarokitic rocks. The Karashoho pipe is the first example of diamond potential in a shoshonite-absarokite-picrite rock series; and the first primary diamond deposit located within a Phanerozoic fold system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/825.abstract [article] The shoshonite-absarokite-picrite Karashoho pipe, Uzbekistan : an unusual diamond deposit in an atypical tectonic environment [texte imprimé] / A. V. Golovko, Auteur ; F. V. Kaminsky, Auteur . - 2011 . - pp. 825-840. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 825-840 Mots-clés : Diamond  deposits  Atypical  tectonic  Pipe  rocks  Uzbekistan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The diamondiferous Karashoho pipe was discovered in the Bukantau Mountains, Uzbekistan, in the 1990s and evaluated from 2002 to 2007. It is an elongate, twinned, subvertical-dipping body with an approximate surface expression of 530 × 65–155 m (~45,600 m2). Its northern part is composed of eruptive breccias and shoshonite, whereas the southern part is composed of micaceous shoshonite-absarokite-picritic rocks and shonkinite. The pipe is located within the southern Tian-Shan Hercynian fold system enclosed between the Karakum-Tajik Massif in the south and the middle Tian-Shan system to the north. The Rb-Sr isochron age of the pipe is 353 ± 13 Ma. By petrographic, mineralogical, and chemical compositions the Karashoho rocks belong to a potassic shoshonite-absarokite-picrite rock series, similar to Permian shoshonitic series rocks from the Tian-Shan volcanic belt in north Xinjang, China. However, they differ significantly from common shoshonites and absarokites because of the high magnesian index of rock-forming minerals in the pipe and the presence of some high-pressure mineral phases, such as chromian spinel (including high-chromian varieties of the diamond association) and manganoilmenite. This indicates that the rocks originated under high-pressure conditions. Bulk sampling of the Karashoho rocks yielded a diamond grade in the range of 0.1 to 5 cpht (carats per hundred tons), with the weighted-mean for different rock varieties ranging from 0.87 to 2.55 cpht. The highest grades are in eruptive breccias of the first generation and shoshonitic-absarokitic rocks. The Karashoho pipe is the first example of diamond potential in a shoshonite-absarokite-picrite rock series; and the first primary diamond deposit located within a Phanerozoic fold system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/825.abstract

The petrogenesis of platinum-group element reefs in the upper main zone of the northern lobe of the Bushveld complex on the farm Moorddrift, South Africa / W. D. Maier in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 841-854 Titre : The petrogenesis of platinum-group element reefs in the upper main zone of the northern lobe of the Bushveld complex on the farm Moorddrift, South Africa Type de document : texte imprimé Auteurs : W. D. Maier, Auteur ; S.-J. Barnes, Auteur Année de publication : 2011 Article en page(s) : pp. 841-854 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Platinum  group  Petrogenesis  Bushveld  rocks  Mineralization Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Boreholes have intersected the interval hosting the Pyroxenite Marker at the base of the Upper Main zone on the farm Moorddrift in the northern Bushveld Complex. The concentrations of the major and compatible trace elements in the rocks are in the range of other Pyroxenite Marker intervals elsewhere in the Bushveld Complex (MgO up to 22 wt %, Cr up to 300 ppm). Over a stratigraphic height of some 300 m the interval is characterized by a broad, but irregular reversal in Mg number and Cr/V, analogous to stratigraphically equivalent horizons elsewhere in the Bushveld Complex. The rocks have distinctly higher concentrations in incompatible trace elements, particularly large ion lithophile elements (LILE) and light rare earth elements (LREE), than other Main zone rocks and thus appear to contain an elevated crustal component. The interval hosts several thick (meters to 10s of meters), but mostly relatively low grade and laterally discontinous PGE reefs that have been delineated over a strike length of ca 1 km. PGE contents locally reach ~16 ppm Pt + Pd + Au in rocks containing up to ~4 to 5 percent sulfides. The PGE tenors of most sulfides are >100 ppm, Pt/Pd ratios are above unity, and Pd/Ir is >200. Based on well-defined positive correlations between Pt and Pd with Ir we interpret the mineralization as being largely of primary magmatic origin. Most previous models interpreted the Pyroxenite Marker to have formed from a major replenishment to the Bushveld magma chamber with relatively primitive magma. The new magma would have been emplaced on top of an ~4-km-thick cumulate package consisting of the ultramafic-mafic Lower and Critical zones and the lower to central portions of the gabronoritic Main zone. Here, we propose that the Upper Main zone including the Pyroxenite Marker did not result from a magma replenishment. Instead, it crystallized from the S-saturated residual magma of the upper Critical zone cumulates and originally directly overlaid the latter. The ~1-to 2-km-thick lower-central Main zone package formed from a crystal-charged magma emplaced at a relatively late stage in the solidification history of the Bushveld chamber, between the largely consolidated Upper Critical and Upper Main zones. We envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite, perhaps in response to the evacuation of a putative lower staging chamber from which the Bushveld Complex was fed. Semiconsolidated portions of the cumulate package sagged and slumped toward the center of the Complex, and the flowing crystal mushes underwent mechanical unmixing of light, relatively buoyant plagioclase from denser pyroxenes, oxides, and sulfides. This resulted in the formation of an interlayered sequence of relatively melanocratic sulfide-enriched with relatively leucocratic sulfide-poor layers. reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the Main zone magma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/841.abstract [article] The petrogenesis of platinum-group element reefs in the upper main zone of the northern lobe of the Bushveld complex on the farm Moorddrift, South Africa [texte imprimé] / W. D. Maier, Auteur ; S.-J. Barnes, Auteur . - 2011 . - pp. 841-854. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 841-854 Mots-clés : Platinum  group  Petrogenesis  Bushveld  rocks  Mineralization Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Boreholes have intersected the interval hosting the Pyroxenite Marker at the base of the Upper Main zone on the farm Moorddrift in the northern Bushveld Complex. The concentrations of the major and compatible trace elements in the rocks are in the range of other Pyroxenite Marker intervals elsewhere in the Bushveld Complex (MgO up to 22 wt %, Cr up to 300 ppm). Over a stratigraphic height of some 300 m the interval is characterized by a broad, but irregular reversal in Mg number and Cr/V, analogous to stratigraphically equivalent horizons elsewhere in the Bushveld Complex. The rocks have distinctly higher concentrations in incompatible trace elements, particularly large ion lithophile elements (LILE) and light rare earth elements (LREE), than other Main zone rocks and thus appear to contain an elevated crustal component. The interval hosts several thick (meters to 10s of meters), but mostly relatively low grade and laterally discontinous PGE reefs that have been delineated over a strike length of ca 1 km. PGE contents locally reach ~16 ppm Pt + Pd + Au in rocks containing up to ~4 to 5 percent sulfides. The PGE tenors of most sulfides are >100 ppm, Pt/Pd ratios are above unity, and Pd/Ir is >200. Based on well-defined positive correlations between Pt and Pd with Ir we interpret the mineralization as being largely of primary magmatic origin. Most previous models interpreted the Pyroxenite Marker to have formed from a major replenishment to the Bushveld magma chamber with relatively primitive magma. The new magma would have been emplaced on top of an ~4-km-thick cumulate package consisting of the ultramafic-mafic Lower and Critical zones and the lower to central portions of the gabronoritic Main zone. Here, we propose that the Upper Main zone including the Pyroxenite Marker did not result from a magma replenishment. Instead, it crystallized from the S-saturated residual magma of the upper Critical zone cumulates and originally directly overlaid the latter. The ~1-to 2-km-thick lower-central Main zone package formed from a crystal-charged magma emplaced at a relatively late stage in the solidification history of the Bushveld chamber, between the largely consolidated Upper Critical and Upper Main zones. We envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite, perhaps in response to the evacuation of a putative lower staging chamber from which the Bushveld Complex was fed. Semiconsolidated portions of the cumulate package sagged and slumped toward the center of the Complex, and the flowing crystal mushes underwent mechanical unmixing of light, relatively buoyant plagioclase from denser pyroxenes, oxides, and sulfides. This resulted in the formation of an interlayered sequence of relatively melanocratic sulfide-enriched with relatively leucocratic sulfide-poor layers. reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the Main zone magma. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/841.abstract

Copper mineralization prevented by arc-root delamination during Alpine-Himalayan collision in central Iran / Michael Haschke in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 855-865 Titre : Copper mineralization prevented by arc-root delamination during Alpine-Himalayan collision in central Iran Type de document : texte imprimé Auteurs : Michael Haschke, Auteur ; Jamshid Ahmadian, Auteur ; Mamoru Murata, Auteur Année de publication : 2011 Article en page(s) : pp. 855-865 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Copper  mineralization  Arc-root  delamination  Iran Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Contrasting geochemical signatures of copper ore hosting Eocene and some barren undeformed Miocene diorites to granites in central Iran temporally overlap with the Alpine-Himalayan collision and provide key implications on the existence and lack of Cu mineralization during collisional magmatism. High Sr and low Y (and Yb) contents of Eocene arc rocks in the Natanz arc segment reflect thickened, Andean-type orogenic arc crust (~45 km), whereas barren Miocene Natanz arc rocks (21–19 Ma) indicate thin arc crust similar to collisional volcanism in Anatolia. Geochemical modeling indicates a change in the mineralogy of the melt residual, from precollisional Eocene basaltic garnet-bearing (5–30%) amphibolite to syn- or postcollisional Miocene metasomatized mantle peridotite, which can be explained by collision-induced delamination of the arc lithospheric root. Subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet-bearing arc crustal keel explain the low Sr and high Y (and Yb) contents, the relatively enriched initial Sr isotope ratios of postcollisional Miocene Natanz rocks, and the lack of copper mineralization in postcollisional Miocene Natanz arc rocks. Arc-root delamination removes the copper- and sulfur-enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits. Lack of the dense melt residues also provides an alternative explanation for the elevated, thin crustal Iranian back-arc plateau (38 km) as a result of uplift by isostatic rebound rather than uplift by anomalous shortening. Miocene arc-root delamination implies a minimum age of >21 Ma for the Alpine-Himalayan collision in central Iran. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/855.abstract [article] Copper mineralization prevented by arc-root delamination during Alpine-Himalayan collision in central Iran [texte imprimé] / Michael Haschke, Auteur ; Jamshid Ahmadian, Auteur ; Mamoru Murata, Auteur . - 2011 . - pp. 855-865. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 855-865 Mots-clés : Copper  mineralization  Arc-root  delamination  Iran Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Contrasting geochemical signatures of copper ore hosting Eocene and some barren undeformed Miocene diorites to granites in central Iran temporally overlap with the Alpine-Himalayan collision and provide key implications on the existence and lack of Cu mineralization during collisional magmatism. High Sr and low Y (and Yb) contents of Eocene arc rocks in the Natanz arc segment reflect thickened, Andean-type orogenic arc crust (~45 km), whereas barren Miocene Natanz arc rocks (21–19 Ma) indicate thin arc crust similar to collisional volcanism in Anatolia. Geochemical modeling indicates a change in the mineralogy of the melt residual, from precollisional Eocene basaltic garnet-bearing (5–30%) amphibolite to syn- or postcollisional Miocene metasomatized mantle peridotite, which can be explained by collision-induced delamination of the arc lithospheric root. Subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet-bearing arc crustal keel explain the low Sr and high Y (and Yb) contents, the relatively enriched initial Sr isotope ratios of postcollisional Miocene Natanz rocks, and the lack of copper mineralization in postcollisional Miocene Natanz arc rocks. Arc-root delamination removes the copper- and sulfur-enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits. Lack of the dense melt residues also provides an alternative explanation for the elevated, thin crustal Iranian back-arc plateau (38 km) as a result of uplift by isostatic rebound rather than uplift by anomalous shortening. Miocene arc-root delamination implies a minimum age of >21 Ma for the Alpine-Himalayan collision in central Iran. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/855.abstract

Quantitative mineral resource assessments / Paul G. Spry in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - p. 867 Titre : Quantitative mineral resource assessments : an integrated approach Type de document : texte imprimé Auteurs : Paul G. Spry, Auteur Année de publication : 2011 Article en page(s) : p. 867 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Mineral  ressource Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/867.extract [article] Quantitative mineral resource assessments : an integrated approach [texte imprimé] / Paul G. Spry, Auteur . - 2011 . - p. 867. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - p. 867 Mots-clés : Mineral  ressource Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/867.extract

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