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Ajouter le résultat dans votre panierAre iron-rich sedimentary rocks the key to the spike in orogenic gold mineralization in the paleoproterozoic? / Alexis Lambeck in Economic geology, Vol. 106 N° 3 (Mai 2011)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 321-330
Titre : Are iron-rich sedimentary rocks the key to the spike in orogenic gold mineralization in the paleoproterozoic? Type de document : texte imprimé Auteurs : Alexis Lambeck, Auteur ; Terrence P. Mernagh, Auteur ; Lesley Wyborn, Auteur Année de publication : 2011 Article en page(s) : pp. 321-330 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Paleoproterozoic rocks Sedimentary rocks Orogenic mineralization Iron Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Paleoproterozoic rocks with ages from 2100 to 1800 Ma are recognized as belonging to one of the three main periods in Earth history that host orogenic gold deposits. In contrast to older Archean orogenic gold deposits, which are hosted predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Paleoproterozoic. Unusually iron rich 1840 Ma marine mudstones in the Tanami region host one world-class gold deposit and many other large gold deposits. Fluid-rock modelling at 300° and 325°C suggests a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region. These northern Australian Paleoproterozoic iron-rich sedimentary rocks could be linked globally to iron-rich and gold-bearing sedimentary rocks deposited between 2100 to 1800 Ma in Homestake, South Dakota (United States), and in Ghana, and Brazil. Worldwide, from about 2400 to 1800 Ma, the Paleoproterozoic is also marked by the deposition of mainly Superior style BIFs, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in northern Australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic Au deposits hosted in rocks aged from 2100 to 1800 Ma.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/321?maxtoshow=&hi [...] [article] Are iron-rich sedimentary rocks the key to the spike in orogenic gold mineralization in the paleoproterozoic? [texte imprimé] / Alexis Lambeck, Auteur ; Terrence P. Mernagh, Auteur ; Lesley Wyborn, Auteur . - 2011 . - pp. 321-330.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 321-330
Mots-clés : Paleoproterozoic rocks Sedimentary rocks Orogenic mineralization Iron Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Paleoproterozoic rocks with ages from 2100 to 1800 Ma are recognized as belonging to one of the three main periods in Earth history that host orogenic gold deposits. In contrast to older Archean orogenic gold deposits, which are hosted predominantly in greenstone terranes, supracrustal sedimentary rocks became increasingly important as hosts in the Paleoproterozoic. Unusually iron rich 1840 Ma marine mudstones in the Tanami region host one world-class gold deposit and many other large gold deposits. Fluid-rock modelling at 300° and 325°C suggests a strong correlation between gold grade and these iron-rich, fine-grained sedimentary rocks. New regional stratigraphic correlations for similar iron-rich rocks to those in the Tanami region are suggested with ~1860 Ma gold-bearing stratigraphy in the Pine Creek region. These northern Australian Paleoproterozoic iron-rich sedimentary rocks could be linked globally to iron-rich and gold-bearing sedimentary rocks deposited between 2100 to 1800 Ma in Homestake, South Dakota (United States), and in Ghana, and Brazil. Worldwide, from about 2400 to 1800 Ma, the Paleoproterozoic is also marked by the deposition of mainly Superior style BIFs, which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans. The high iron concentrations noted in pre-1800 Ma marine sediments in northern Australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic Au deposits hosted in rocks aged from 2100 to 1800 Ma.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/321?maxtoshow=&hi [...]
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 331-358
Titre : A carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits Type de document : texte imprimé Auteurs : Ross R. Large, Auteur ; Stuart W. Bull, Auteur ; Valeriy V. Maslennikov, Auteur Année de publication : 2011 Article en page(s) : pp. 331-358 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Carbonaceous sedimentary Genetic model Gold deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold.
During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence.
LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/331 [article] A carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits [texte imprimé] / Ross R. Large, Auteur ; Stuart W. Bull, Auteur ; Valeriy V. Maslennikov, Auteur . - 2011 . - pp. 331-358.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 331-358
Mots-clés : Carbonaceous sedimentary Genetic model Gold deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold.
During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence.
LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/331
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 359-380
Titre : Atypical gold mineralization in an orogenic setting : the Bohaun deposit, western Irish caledonides Type de document : texte imprimé Auteurs : Paul A. Lusty, Auteur ; Jon Naden, Auteur ; John J. Bouch, Auteur Année de publication : 2011 Article en page(s) : pp. 359-380 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Gold Mineralization Bohaun deposit Ireland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bohaun gold deposit in County Galway, western Ireland, is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks that have only reached the late diagenetic zone of low-grade metamorphism.
The mineralization consists of undeformed veins, stockworks, and breccias. Quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures, including comb, plumose, and banded, indicative of low confining pressures. The veins have a simple mineralogy made up of quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite.
Vein quartz is associated with two fluids compositions: type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126° to 257°C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175° and 245°C.
The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks.
The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/359 [article] Atypical gold mineralization in an orogenic setting : the Bohaun deposit, western Irish caledonides [texte imprimé] / Paul A. Lusty, Auteur ; Jon Naden, Auteur ; John J. Bouch, Auteur . - 2011 . - pp. 359-380.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 359-380
Mots-clés : Gold Mineralization Bohaun deposit Ireland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bohaun gold deposit in County Galway, western Ireland, is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks that have only reached the late diagenetic zone of low-grade metamorphism.
The mineralization consists of undeformed veins, stockworks, and breccias. Quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures, including comb, plumose, and banded, indicative of low confining pressures. The veins have a simple mineralogy made up of quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite.
Vein quartz is associated with two fluids compositions: type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126° to 257°C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175° and 245°C.
The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks.
The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/359
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 381-412
Titre : Ages and sources of components of Zn-Pb, Cu, precious metal, and platinum group element deposits in the goodsprings district, Clark county, Nevada Type de document : texte imprimé Auteurs : Peter Vikre, Auteur ; Quentin J. Browne, Auteur ; Robert Fleck, Auteur Année de publication : 2011 Article en page(s) : pp. 381-412 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Precious metal Zinc Platinum group Silver Gold Cuivre Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper ± precious metal-platinum group element (PGE) deposits, and gold ± silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses ~500 km2 although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was ~160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs—Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U—were also recovered.
Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag ± Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions ({delta}34S values range from 2.5–13{per thousand}), and highly radiogenic Pb isotope compositions (206Pb/204Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimentary rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag ± Cu ± Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper ± precious metal-PGE and gold ± silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values ({delta}34S values vary from 0–~4{per thousand}; 206Pb/204Pb <18.5). Copper ± precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold ± silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper ± precious metal-PGE, gold ± silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement.
Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO2 and variable K2O/Na2O (0.14–5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous.
Uranium-lead ages of igneous zircons vary inconsistently from ~ 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 ± 1 Ma. K-Ar and 40Ar/39Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183–227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions.
Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold ± silver mineralization.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/381 [article] Ages and sources of components of Zn-Pb, Cu, precious metal, and platinum group element deposits in the goodsprings district, Clark county, Nevada [texte imprimé] / Peter Vikre, Auteur ; Quentin J. Browne, Auteur ; Robert Fleck, Auteur . - 2011 . - pp. 381-412.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 381-412
Mots-clés : Precious metal Zinc Platinum group Silver Gold Cuivre Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper ± precious metal-platinum group element (PGE) deposits, and gold ± silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses ~500 km2 although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was ~160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs—Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U—were also recovered.
Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag ± Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions ({delta}34S values range from 2.5–13{per thousand}), and highly radiogenic Pb isotope compositions (206Pb/204Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimentary rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag ± Cu ± Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper ± precious metal-PGE and gold ± silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values ({delta}34S values vary from 0–~4{per thousand}; 206Pb/204Pb <18.5). Copper ± precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold ± silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper ± precious metal-PGE, gold ± silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement.
Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO2 and variable K2O/Na2O (0.14–5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous.
Uranium-lead ages of igneous zircons vary inconsistently from ~ 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 ± 1 Ma. K-Ar and 40Ar/39Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183–227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions.
Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold ± silver mineralization.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/381
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 413-436
Titre : Physicochemical evolution of a porphyry-breccia system : a laser ablation ICP-MS study of fluid inclusions in the Mount Leyshon au deposit, Queensland, Australia Type de document : texte imprimé Auteurs : Murray M. Allan, Auteur ; Gregg W. Morrison, Auteur ; Bruce W. D. Yardley, Auteur Année de publication : 2011 Article en page(s) : pp. 413-436 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Porphyry-breccia system Fluid inclusions Laser ablation ICP-MS Mount leyshon au deposit Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The evolution of magmatism, brecciation, alteration, and mineralization is described for the Mount Leyshon intrusive complex, Queensland, Australia, using P-T-V-X analysis and major to trace element analysis of single fluid inclusions using laser ablation ICP-MS. The ~290 Ma Mount Leyshon intrusive complex is a subvolcanic porphyry-breccia system mined for ~97 metric tons (t) Au (70 Mt at ~1.4 g/t Au) between 1986 and 2000. The Mount Leyshon intrusive complex is characterized by several generations of genetically related breccias and magmatic intrusions. Both alteration and mineralization can be traced to specific intrusive events. The earliest phase of brecciation was followed by widespread biotite-magnetite alteration (stage 1). Fluid inclusions associated with stage 1 were trapped at >350°C and >145 bars and were anomalously K and Fe rich. This stage was followed by the emplacement of rhyolite to dacite stocks and coeval breccia pipes (stage 2). Fluids exsolved from these magmas simultaneously exsolved Na-K-Fe-Mn-rich brine (~60 wt % NaCl equiv) and dilute vapor at ~500 bars (~2-km paleodepth) and >600°C. Pressures likely oscillated between lithostatic and sublithostatic conditions in response to the transient rupture and sealing of intrusions and wall rocks. Hydrothermal fluids infiltrating breccia pore space produced base metal mineralization (stage 3). Stage 3 fluid inclusions contain 2 to 10 wt percent NaCl equiv and were trapped at 300° to 400°C below 500 bars. Stage 3 fluids were depleted in K, Fe, and Mn, and enriched in Ca, Sr, and Ba relative to stage 2 brines. This trend is consistent with the transition from potassic alteration in stage 2 to feldspar-destructive phyllic alteration in stage 3. Cu, Zn, and Pb were conservative elements above 350°C and precipitated nonselectively as sulfides below 350°C in a bulk ratio very similar to that measured in fluid inclusions. Subsequent emplacement of andesite to rhyodacite dikes (stage 4) was marked by Au and additional base metal sulfide mineralization in veins and breccia cavities. Stage 4 fluids are chemically similar to stage 3 but contain several mol percent CO2, perhaps suggesting a more mafic, CO2-charged magma source relative to earlier intrusive pulses. In addition to fluid cooling, the destabilization of Au bisulfide complexes attending phase separation of H2O-CO2-NaCl fluids below 400°C and 400 bars was the likely cause for Au precipitation. Mount Leyshon appears to indicate that CO2 can play a crucial role in the transport of gold-bearing solutions from a magmatic source at depth into much shallower hydrothermal environments.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/413 [article] Physicochemical evolution of a porphyry-breccia system : a laser ablation ICP-MS study of fluid inclusions in the Mount Leyshon au deposit, Queensland, Australia [texte imprimé] / Murray M. Allan, Auteur ; Gregg W. Morrison, Auteur ; Bruce W. D. Yardley, Auteur . - 2011 . - pp. 413-436.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 413-436
Mots-clés : Porphyry-breccia system Fluid inclusions Laser ablation ICP-MS Mount leyshon au deposit Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The evolution of magmatism, brecciation, alteration, and mineralization is described for the Mount Leyshon intrusive complex, Queensland, Australia, using P-T-V-X analysis and major to trace element analysis of single fluid inclusions using laser ablation ICP-MS. The ~290 Ma Mount Leyshon intrusive complex is a subvolcanic porphyry-breccia system mined for ~97 metric tons (t) Au (70 Mt at ~1.4 g/t Au) between 1986 and 2000. The Mount Leyshon intrusive complex is characterized by several generations of genetically related breccias and magmatic intrusions. Both alteration and mineralization can be traced to specific intrusive events. The earliest phase of brecciation was followed by widespread biotite-magnetite alteration (stage 1). Fluid inclusions associated with stage 1 were trapped at >350°C and >145 bars and were anomalously K and Fe rich. This stage was followed by the emplacement of rhyolite to dacite stocks and coeval breccia pipes (stage 2). Fluids exsolved from these magmas simultaneously exsolved Na-K-Fe-Mn-rich brine (~60 wt % NaCl equiv) and dilute vapor at ~500 bars (~2-km paleodepth) and >600°C. Pressures likely oscillated between lithostatic and sublithostatic conditions in response to the transient rupture and sealing of intrusions and wall rocks. Hydrothermal fluids infiltrating breccia pore space produced base metal mineralization (stage 3). Stage 3 fluid inclusions contain 2 to 10 wt percent NaCl equiv and were trapped at 300° to 400°C below 500 bars. Stage 3 fluids were depleted in K, Fe, and Mn, and enriched in Ca, Sr, and Ba relative to stage 2 brines. This trend is consistent with the transition from potassic alteration in stage 2 to feldspar-destructive phyllic alteration in stage 3. Cu, Zn, and Pb were conservative elements above 350°C and precipitated nonselectively as sulfides below 350°C in a bulk ratio very similar to that measured in fluid inclusions. Subsequent emplacement of andesite to rhyodacite dikes (stage 4) was marked by Au and additional base metal sulfide mineralization in veins and breccia cavities. Stage 4 fluids are chemically similar to stage 3 but contain several mol percent CO2, perhaps suggesting a more mafic, CO2-charged magma source relative to earlier intrusive pulses. In addition to fluid cooling, the destabilization of Au bisulfide complexes attending phase separation of H2O-CO2-NaCl fluids below 400°C and 400 bars was the likely cause for Au precipitation. Mount Leyshon appears to indicate that CO2 can play a crucial role in the transport of gold-bearing solutions from a magmatic source at depth into much shallower hydrothermal environments.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/413
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 437-450
Titre : Hydrothermal breccia textures and processes : lisca Bianca Islet, Panarea Volcano, Aeolian Islands, Italy Type de document : texte imprimé Auteurs : R. Cas, Auteur ; G. Giordano, Auteur ; F. Balsamo, Auteur Année de publication : 2011 Article en page(s) : pp. 437-450 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Hydrothermal system Panarea volcano Italy Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Panarea is a largely submarine to partly subaerial Quaternary lava dome complex-stratovolcano with a long-lived, active, shallow hydrothermal system, located in the Aeolian Islands volcanic arc of southern Italy. The emergent top of the volcano forms a small archipelago, made up of calc-alkaline basaltic andesite to rhyolite lava domes (ca. 150–20 ka). We document the facies outcropping on Lisca Bianca islet, Panarea archipelago, based on grain size, clast fabric, and degree of hydrothermal alteration, identifying coherent facies, boulder breccia facies, cobble breccia facies, pebble breccia facies, and pervasively altered andesite facies (alunite-marcasite-sulfur). The breccias all have ubiquitous jigsaw-fit clast textures, and are variably hydrothermally altered. The breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics. The breccias formed through a cyclical process, involving the following: stage 1: progressive build-up of fluid pressure toward the level of the tensile strength of the host andesite; stage 2: incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions; stage 3: pervasive fracturing of the host andesite, leading to an increase in permeability as a network of fractures develops; stage 4: declining pressure, with fluid flow rates that lead to infilling and sealing of fractures by natroalunite, thereby reducing permeability, leading to progressive build-up of fluid pressure again, and the beginning of a new cycle.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/437 [article] Hydrothermal breccia textures and processes : lisca Bianca Islet, Panarea Volcano, Aeolian Islands, Italy [texte imprimé] / R. Cas, Auteur ; G. Giordano, Auteur ; F. Balsamo, Auteur . - 2011 . - pp. 437-450.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 437-450
Mots-clés : Hydrothermal system Panarea volcano Italy Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Panarea is a largely submarine to partly subaerial Quaternary lava dome complex-stratovolcano with a long-lived, active, shallow hydrothermal system, located in the Aeolian Islands volcanic arc of southern Italy. The emergent top of the volcano forms a small archipelago, made up of calc-alkaline basaltic andesite to rhyolite lava domes (ca. 150–20 ka). We document the facies outcropping on Lisca Bianca islet, Panarea archipelago, based on grain size, clast fabric, and degree of hydrothermal alteration, identifying coherent facies, boulder breccia facies, cobble breccia facies, pebble breccia facies, and pervasively altered andesite facies (alunite-marcasite-sulfur). The breccias all have ubiquitous jigsaw-fit clast textures, and are variably hydrothermally altered. The breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics. The breccias formed through a cyclical process, involving the following: stage 1: progressive build-up of fluid pressure toward the level of the tensile strength of the host andesite; stage 2: incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions; stage 3: pervasive fracturing of the host andesite, leading to an increase in permeability as a network of fractures develops; stage 4: declining pressure, with fluid flow rates that lead to infilling and sealing of fractures by natroalunite, thereby reducing permeability, leading to progressive build-up of fluid pressure again, and the beginning of a new cycle.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/437
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 451-480
Titre : Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada : evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems Type de document : texte imprimé Auteurs : John L. Mair, Auteur ; G. Lang Farmer, Auteur ; David I. Groves, Auteur Année de publication : 2011 Article en page(s) : pp. 451-480 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Postcollisional magmatism Lithospheric mantle Scheelite Dome Gold systems Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits.
Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87Sr/86Sr from 0.711-0.714) and Pb isotope ratios (206Pb/204Pb from 19.2-19.7), and negative initial {varepsilon}Nd values (–8.06 to –11.26).
Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/451 [article] Petrogenesis of postcollisional magmatism at Scheelite Dome, Yukon, Canada : evidence for a lithospheric mantle source for magmas associated with intrusion-related gold systems [texte imprimé] / John L. Mair, Auteur ; G. Lang Farmer, Auteur ; David I. Groves, Auteur . - 2011 . - pp. 451-480.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 451-480
Mots-clés : Postcollisional magmatism Lithospheric mantle Scheelite Dome Gold systems Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits.
Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87Sr/86Sr from 0.711-0.714) and Pb isotope ratios (206Pb/204Pb from 19.2-19.7), and negative initial {varepsilon}Nd values (–8.06 to –11.26).
Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/451
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 481-509
Titre : Cu-Ni-PGE mineralization in the south filson creek area, south Kawishiwi intrusion, Duluth complex : mineralization styles and magmatic and hydrothermal processes Type de document : texte imprimé Auteurs : Benedek Gal, Auteur ; Ferenc Molnar, Auteur ; Dean M. Peterson, Auteur Année de publication : 2011 Article en page(s) : pp. 481-509 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Mineralization Duluth Complex Magmatic processes Hydrothermal processes Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The South Filson Creek mineralization occurs above the basal units of the South Kawishiwi intrusion and represents an uncommon geologic setting of Cu-Ni-platinum-group element (PGE) mineralization within the Duluth Complex. Unlike other mineralization of economic interest in the basal heterogeneous troctolite units along the lower contact, it is located at an approximate stratigraphic height of 1,000 m above the contact with the footwall granitoids in the usually unmineralized homogeneous troctolite unit.
Magmatic sulfide mineralization forms disseminated fine-grained patches and pockets in an area of approximately 1/2 km2 of homogeneous troctolitic rocks in the northeastern part of the South Kawishiwi intrusion. Pyrrhotite, chalcopyrite, pentlandite, and cubanite are the main sulfide minerals with subordinate amounts of other copper-bearing phases (bornite, covellite, talnakhite). Cu/Pd ratios for unaltered mineralization indicate that sulfides were formed from multiple injections of magma that segregated sulfides with different R factors or having different initial compositions.
Variation in the halogen contents of apatite from pegmatoids within troctolitic rocks revealed that a fluid has segregated from the crystallizing melt into which fluid Cl and rare earth elements partitioned.
Amphibole, chlorite, sericite, prehnite, pumpellyite, and carbonate are associated with alteration along brittle structures and isolated patches in the area. The hydrothermal fluid-rock interaction has affected the composition of primary magmatic sulfide mineralization and is also responsible for mineralization in the anorthositic hanging wall of the South Kawishiwi intrusion. Hydrothermal alteration resulted in a slight Pd depletion in magmatic sulfide mineralization, which might indicate remobilization of precious metals to an unknown location. The hanging-wall mineralization is characterized by relative Cu enrichment in comparison to the magmatic sulfide mineralization with no PGE enrichment.
Partial remobilization of platinum group elements (PGE) has also occurred as a result of a separate alteration event, on the evidence that platinum group minerals (PGM) are also associated with serpentinization of olivine and Ca enrichment of plagioclase along grain boundaries with sulfides. This alteration was probably effective on a very local scale.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/481 [article] Cu-Ni-PGE mineralization in the south filson creek area, south Kawishiwi intrusion, Duluth complex : mineralization styles and magmatic and hydrothermal processes [texte imprimé] / Benedek Gal, Auteur ; Ferenc Molnar, Auteur ; Dean M. Peterson, Auteur . - 2011 . - pp. 481-509.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 481-509
Mots-clés : Mineralization Duluth Complex Magmatic processes Hydrothermal processes Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The South Filson Creek mineralization occurs above the basal units of the South Kawishiwi intrusion and represents an uncommon geologic setting of Cu-Ni-platinum-group element (PGE) mineralization within the Duluth Complex. Unlike other mineralization of economic interest in the basal heterogeneous troctolite units along the lower contact, it is located at an approximate stratigraphic height of 1,000 m above the contact with the footwall granitoids in the usually unmineralized homogeneous troctolite unit.
Magmatic sulfide mineralization forms disseminated fine-grained patches and pockets in an area of approximately 1/2 km2 of homogeneous troctolitic rocks in the northeastern part of the South Kawishiwi intrusion. Pyrrhotite, chalcopyrite, pentlandite, and cubanite are the main sulfide minerals with subordinate amounts of other copper-bearing phases (bornite, covellite, talnakhite). Cu/Pd ratios for unaltered mineralization indicate that sulfides were formed from multiple injections of magma that segregated sulfides with different R factors or having different initial compositions.
Variation in the halogen contents of apatite from pegmatoids within troctolitic rocks revealed that a fluid has segregated from the crystallizing melt into which fluid Cl and rare earth elements partitioned.
Amphibole, chlorite, sericite, prehnite, pumpellyite, and carbonate are associated with alteration along brittle structures and isolated patches in the area. The hydrothermal fluid-rock interaction has affected the composition of primary magmatic sulfide mineralization and is also responsible for mineralization in the anorthositic hanging wall of the South Kawishiwi intrusion. Hydrothermal alteration resulted in a slight Pd depletion in magmatic sulfide mineralization, which might indicate remobilization of precious metals to an unknown location. The hanging-wall mineralization is characterized by relative Cu enrichment in comparison to the magmatic sulfide mineralization with no PGE enrichment.
Partial remobilization of platinum group elements (PGE) has also occurred as a result of a separate alteration event, on the evidence that platinum group minerals (PGM) are also associated with serpentinization of olivine and Ca enrichment of plagioclase along grain boundaries with sulfides. This alteration was probably effective on a very local scale.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/481
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 511-522
Titre : Re-Os age of polymetallic Ni-Mo-PGE-Au mineralization in early cambrian black shales of south China : a reassessment Type de document : texte imprimé Auteurs : Xu Lingang, Auteur ; Bernd Lehmann, Auteur ; Mao Jingwen, Auteur Année de publication : 2011 Article en page(s) : pp. 511-522 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Mineralization Polymetallic Ni-Mo-PGE-Au Black shales China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Early Cambrian Niutitang Formation in South China consists of a black shale sequence with a synsedimentary organic carbon-rich polymetallic sulfide layer with extreme metal enrichment locally mined as Ni-Mo-PGE-Au ore. We report a new composite Re-Os isochron age of the sulfide ore layer of 521 ± 5 Ma (n = 14) from three mine sites (Dazhuliushui and Maluhe in the Guizhou province and Sancha in the Hunan province) several hundred kilometers apart. This age corresponds to the biostratigraphic Tommotian age, and is in agreement with recent interpolated Pb-Pb and SHRIMP zircon U-Pb ages from an underlying tuff unit. The initial 187Os/188Os value of the sulfide ore is 0.87 ± 0.07, which is equal to the initial 187Os/188Os ratio of ~0.80 of the black shale host rock, consistent with the seawater origin of the metals under depositional conditions of very low clastic input.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/511 [article] Re-Os age of polymetallic Ni-Mo-PGE-Au mineralization in early cambrian black shales of south China : a reassessment [texte imprimé] / Xu Lingang, Auteur ; Bernd Lehmann, Auteur ; Mao Jingwen, Auteur . - 2011 . - pp. 511-522.
Géologie Economique
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 511-522
Mots-clés : Mineralization Polymetallic Ni-Mo-PGE-Au Black shales China Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Early Cambrian Niutitang Formation in South China consists of a black shale sequence with a synsedimentary organic carbon-rich polymetallic sulfide layer with extreme metal enrichment locally mined as Ni-Mo-PGE-Au ore. We report a new composite Re-Os isochron age of the sulfide ore layer of 521 ± 5 Ma (n = 14) from three mine sites (Dazhuliushui and Maluhe in the Guizhou province and Sancha in the Hunan province) several hundred kilometers apart. This age corresponds to the biostratigraphic Tommotian age, and is in agreement with recent interpolated Pb-Pb and SHRIMP zircon U-Pb ages from an underlying tuff unit. The initial 187Os/188Os value of the sulfide ore is 0.87 ± 0.07, which is equal to the initial 187Os/188Os ratio of ~0.80 of the black shale host rock, consistent with the seawater origin of the metals under depositional conditions of very low clastic input.
DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/511
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