Dépouillements

Sulfide-bearing veinlets throughout the stratiform mineralization of the central african copperbelt: / Richard H. Sillitoe in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1361-1368 Titre : Sulfide-bearing veinlets throughout the stratiform mineralization of the central african copperbelt: : temporal and genetic implications Type de document : texte imprimé Auteurs : Richard H. Sillitoe, Auteur ; José Perelló, Auteur ; Alfredo García, Auteur Année de publication : 2011 Article en page(s) : pp. 1361-1368 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Sulfiride-bearing  Statform  mineralization  Congo Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world’s largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either metamorphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (~765–<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (~530 Ma) and continuing in places until at least ~500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1361.abstract [article] Sulfide-bearing veinlets throughout the stratiform mineralization of the central african copperbelt: : temporal and genetic implications [texte imprimé] / Richard H. Sillitoe, Auteur ; José Perelló, Auteur ; Alfredo García, Auteur . - 2011 . - pp. 1361-1368. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1361-1368 Mots-clés : Sulfiride-bearing  Statform  mineralization  Congo Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world’s largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either metamorphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (~765–<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (~530 Ma) and continuing in places until at least ~500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1361.abstract

Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada / R. J. Chapman in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1369-1392 Titre : Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada : 1. Evidence for a small, gold-rich, orogenic hydrothermal system in the Bonanza and Eldorado Creek area Type de document : texte imprimé Auteurs : R. J. Chapman, Auteur ; J. K. Mortensen, Auteur ; E. C. Crawford, Auteur Année de publication : 2011 Article en page(s) : pp. 1369-1392 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Lode  gold  Microchemical  studies  Orogenic  hydrothermal  system  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear. Placer gold deposits in Bonanza and Eldorado Creeks in the northern Klondike district goldfield, Yukon, Canada, probably yielded more than 2 million ounces (Moz) from a small geographic area, but known occurrences of gold-bearing orogenic-style veins from which the placers were derived accounted for only a few thousand ounces. The alloy chemistry and mineralogy of opaque mineral microinclusions (microchemical signatures) within these gold grains were compared to that of samples collected during a detailed placer gold sampling program. Collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization. A total of 1,163 gold grains from 18 lode sources and 2,777 grains from 27 placer localities within the Bonanza and Eldorado Creek drainages were characterized in terms of the Ag and Hg content of the alloy together with the associated inclusion suite, and two main compositional ranges, along with several subtypes, were identified. Type 1 gold is a simple Au-Ag alloy but various subtypes have been identified on the basis of different ranges of Ag and/or specific inclusion assemblages. Type 2 gold is an Au-Ag-Hg alloy composition. The spatial arrangement of the gold types shows a systematic change in microchemical signature. The lowest Ag gold occurs in the Lone Star Ridge area, and the Ag content increases both in the northeast and southwest. The appearance of Te-bearing mineral inclusions in populations coincides with increasing Ag, and argentite inclusions are present only in high Ag gold toward the periphery of study area. Type 2 gold is absent at Lone Star Ridge and all localities to the east but is progressively more abundant in sample localities to the west. The geographic distribution of gold types correlates well with placer abundance (inferred from the production from previous mining activity) and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction. We conclude that the Lone Star Ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation. Variation in Ag is ascribed to decrease in Au/Ag(aq) following preferential Au deposition in the center of the system. Controls on Hg contents within Au alloy are unclear but may be a consequence of mineralization temperature. The complex but consistent signatures of placer gold samples from the Eldorado Creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences. The different proportions of types 1 and 2 gold observed in placer and lode samples from west of Lone Star Ridge reflect both temporal and spatial evolution of the hydrothermal system. The main Eldorado Creek placers exhibit a signature most similar to lode gold from the Buckland and Nugget zones on Lone Star Ridge. Placer gold in upper Bonanza Creek is distinguished by a relatively narrow Ag range and the small but consistent presence of hessite in the inclusion suite. No corresponding lode occurrence for this gold type has been identified to date. This is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail, and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present, while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1369.abstract [article] Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada : 1. Evidence for a small, gold-rich, orogenic hydrothermal system in the Bonanza and Eldorado Creek area [texte imprimé] / R. J. Chapman, Auteur ; J. K. Mortensen, Auteur ; E. C. Crawford, Auteur . - 2011 . - pp. 1369-1392. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1369-1392 Mots-clés : Lode  gold  Microchemical  studies  Orogenic  hydrothermal  system  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear. Placer gold deposits in Bonanza and Eldorado Creeks in the northern Klondike district goldfield, Yukon, Canada, probably yielded more than 2 million ounces (Moz) from a small geographic area, but known occurrences of gold-bearing orogenic-style veins from which the placers were derived accounted for only a few thousand ounces. The alloy chemistry and mineralogy of opaque mineral microinclusions (microchemical signatures) within these gold grains were compared to that of samples collected during a detailed placer gold sampling program. Collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization. A total of 1,163 gold grains from 18 lode sources and 2,777 grains from 27 placer localities within the Bonanza and Eldorado Creek drainages were characterized in terms of the Ag and Hg content of the alloy together with the associated inclusion suite, and two main compositional ranges, along with several subtypes, were identified. Type 1 gold is a simple Au-Ag alloy but various subtypes have been identified on the basis of different ranges of Ag and/or specific inclusion assemblages. Type 2 gold is an Au-Ag-Hg alloy composition. The spatial arrangement of the gold types shows a systematic change in microchemical signature. The lowest Ag gold occurs in the Lone Star Ridge area, and the Ag content increases both in the northeast and southwest. The appearance of Te-bearing mineral inclusions in populations coincides with increasing Ag, and argentite inclusions are present only in high Ag gold toward the periphery of study area. Type 2 gold is absent at Lone Star Ridge and all localities to the east but is progressively more abundant in sample localities to the west. The geographic distribution of gold types correlates well with placer abundance (inferred from the production from previous mining activity) and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction. We conclude that the Lone Star Ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation. Variation in Ag is ascribed to decrease in Au/Ag(aq) following preferential Au deposition in the center of the system. Controls on Hg contents within Au alloy are unclear but may be a consequence of mineralization temperature. The complex but consistent signatures of placer gold samples from the Eldorado Creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences. The different proportions of types 1 and 2 gold observed in placer and lode samples from west of Lone Star Ridge reflect both temporal and spatial evolution of the hydrothermal system. The main Eldorado Creek placers exhibit a signature most similar to lode gold from the Buckland and Nugget zones on Lone Star Ridge. Placer gold in upper Bonanza Creek is distinguished by a relatively narrow Ag range and the small but consistent presence of hessite in the inclusion suite. No corresponding lode occurrence for this gold type has been identified to date. This is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail, and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present, while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1369.abstract

Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada / R. J. Chapman in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1393-1410 Titre : Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada : 2. Constraints on the nature and location of regional lode sources Type de document : texte imprimé Auteurs : R. J. Chapman, Auteur ; J. K. Mortensen, Auteur ; E. C. Crawford, Auteur Année de publication : 2011 Article en page(s) : pp. 1393-1410 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Lode  gold  Klondike  district  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro-chemical signature of 3,050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern, central, and southern Klondike gold district, Yukon, Canada. These data augment those from a detailed parallel study of placer-lode gold relationships in the western part of the Klondike goldfield (see Chapman et al., 2010). Gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization. Three main gold types have been identified each of which exhibits subtypes. Type 1 gold is generally a simple Au-Ag alloy with low Ag (12–25%) and an inclusion suite dominated by pyrite and simple base metal sulfides. Type 2 gold is generally Ag rich (18–55%) and contains Hg (≤9%). Type 3 gold (10–40% Ag ± Hg ≤ 4%) is prevalent in the south of the region. Analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations. Type 1 gold is the major component of placer deposits in Bonanza, Hunker, Dominion, and upper Sulphur Creeks. Type 2 gold is relatively trivial but contributes to placer gold in Bear Creek and some of the paleoplacers in White Channel Gravel in the north of the study area. Type 3 gold is actively augmenting the placer inventory in the south of the region. This study builds upon a detailed parallel study in the western Klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent, centered on the Lone Star Ridge. Here we propose that small, rich hydrothermal systems were also present in the upper reaches of Bear and Last Chance Creeks and Hunker Dome. If correct, this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized. In addition, type 3 gold is widespread in the south of the study area where it appears to be present at the surface. Consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization. We propose that this type offers the best target for exploration in the northern and central Klondike. In the southern Klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of Gold Run Creek. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1393.abstract [article] Microchemical studies of placer and lode gold in the klondike district, Yukon, Canada : 2. Constraints on the nature and location of regional lode sources [texte imprimé] / R. J. Chapman, Auteur ; J. K. Mortensen, Auteur ; E. C. Crawford, Auteur . - 2011 . - pp. 1393-1410. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1393-1410 Mots-clés : Lode  gold  Klondike  district  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro-chemical signature of 3,050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern, central, and southern Klondike gold district, Yukon, Canada. These data augment those from a detailed parallel study of placer-lode gold relationships in the western part of the Klondike goldfield (see Chapman et al., 2010). Gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization. Three main gold types have been identified each of which exhibits subtypes. Type 1 gold is generally a simple Au-Ag alloy with low Ag (12–25%) and an inclusion suite dominated by pyrite and simple base metal sulfides. Type 2 gold is generally Ag rich (18–55%) and contains Hg (≤9%). Type 3 gold (10–40% Ag ± Hg ≤ 4%) is prevalent in the south of the region. Analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations. Type 1 gold is the major component of placer deposits in Bonanza, Hunker, Dominion, and upper Sulphur Creeks. Type 2 gold is relatively trivial but contributes to placer gold in Bear Creek and some of the paleoplacers in White Channel Gravel in the north of the study area. Type 3 gold is actively augmenting the placer inventory in the south of the region. This study builds upon a detailed parallel study in the western Klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent, centered on the Lone Star Ridge. Here we propose that small, rich hydrothermal systems were also present in the upper reaches of Bear and Last Chance Creeks and Hunker Dome. If correct, this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized. In addition, type 3 gold is widespread in the south of the study area where it appears to be present at the surface. Consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization. We propose that this type offers the best target for exploration in the northern and central Klondike. In the southern Klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of Gold Run Creek. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1393.abstract

The genesis of distal zinc skarns / A. E. Williams-Jones in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1411-1440 Titre : The genesis of distal zinc skarns : evidence from the Mochito deposit, Honduras Type de document : texte imprimé Auteurs : A. E. Williams-Jones, Auteur ; I. M. Samson, Auteur ; K. M. Ault, Auteur Année de publication : 2011 Article en page(s) : pp. 1411-1440 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Distal  zinc  skarns  Mochito  deposits  Honduras Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log fO2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower fO2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1411.abstract [article] The genesis of distal zinc skarns : evidence from the Mochito deposit, Honduras [texte imprimé] / A. E. Williams-Jones, Auteur ; I. M. Samson, Auteur ; K. M. Ault, Auteur . - 2011 . - pp. 1411-1440. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1411-1440 Mots-clés : Distal  zinc  skarns  Mochito  deposits  Honduras Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log fO2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower fO2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1411.abstract

The Marcona magnetite deposit, ica, South-Central Peru / Huayong Chen in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1441-1456 Titre : The Marcona magnetite deposit, ica, South-Central Peru : a product of hydrous, iron oxide-rich melts? Type de document : texte imprimé Auteurs : Huayong Chen, Auteur ; Alan H. Clark, Auteur ; T. Kurtis Kyser, Auteur Année de publication : 2011 Article en page(s) : pp. 1441-1456 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Magnetite  deposit  Product  of  hydrous  Iron  oxide-rich  melts  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Marcona, the preeminent Andean magnetite deposit (1.9 Gt @ 55.4% Fe and 0.12% Cu), is located in the iron oxide copper-gold (IOCG) subprovince of littoral south-central Perú. Fe oxide and Cu (-Zn-Pb) sulfide mineralization was controlled by northeast-striking faults transecting a Middle Jurassic (Aalenian-to-Oxfordian) andesitic, shallow-marine arc and a succession of contiguous, plate boundary-parallel, Late Jurassic to mid-Cretaceous volcanosedimentary basins. At Marcona, hydrothermal activity was initiated in the earliest Middle Jurassic (161–177 Ma) by high-temperature Mg-Fe metasomatism represented by cummingtonite and phlogopite-magnetite assemblages. Subsequently, during the terminal eruptions (156–162 Ma) of the arc, widespread albite-marialite alteration (Na-Cl metasomatism) was followed by the emplacement of an en echelon swarm of massive magnetite ore-bodies with subordinate, overprinted magnetite-sulfide assemblages, hosted largely by Paleozoic metasilici-clastics. The magnetite orebodies exhibit abrupt, smoothly curving contacts, dike-like to tubular apophyses, and intricate, amoeboid interfingering with dacite porphyry intrusions. There is no convincing megascopic or microscopic evidence for large-scale Fe metasomatism associated with the main, sulfide-poor mineralization. The largest, 400 Mt Minas 2-3-4 orebody is interpreted as a bimodal magnetite-dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma. Oxygen and sulfur stable-isotope geothermometry indicates that the evolution at ca. 159 Ma from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphi-bole-pyrrhotite-pyrite assemblages coincided with quenching from above 800° C to below 450°C and the concomitant exsolution of dilute aqueous brines. Subsequently, chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, in part metasomatic, were deposited from lower temperature (≤360°C) brines. The Cu-poor Marcona (“Kiruna-type”) magnetite and Cu-rich IOCG deposits in the district, therefore, although spatially contiguous, represent contrasting ore deposit types. The former are interpreted as the product of Fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back-arc basin. The weak associated magmatic-hydrothermal Cu sulfide mineralization at Marcona was generated through melt vesiculation and contrasts with the considerably higher grade Cu- and Ag-rich orebodies of the major Cu-rich IOCG deposits in the Central Andes, e.g., La Candelaria-Punta del Cobre, Mantoverde, Raúl-Condestable, and Mina Justa, which were the products of cool, oxidized, hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1441.abstract [article] The Marcona magnetite deposit, ica, South-Central Peru : a product of hydrous, iron oxide-rich melts? [texte imprimé] / Huayong Chen, Auteur ; Alan H. Clark, Auteur ; T. Kurtis Kyser, Auteur . - 2011 . - pp. 1441-1456. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1441-1456 Mots-clés : Magnetite  deposit  Product  of  hydrous  Iron  oxide-rich  melts  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Marcona, the preeminent Andean magnetite deposit (1.9 Gt @ 55.4% Fe and 0.12% Cu), is located in the iron oxide copper-gold (IOCG) subprovince of littoral south-central Perú. Fe oxide and Cu (-Zn-Pb) sulfide mineralization was controlled by northeast-striking faults transecting a Middle Jurassic (Aalenian-to-Oxfordian) andesitic, shallow-marine arc and a succession of contiguous, plate boundary-parallel, Late Jurassic to mid-Cretaceous volcanosedimentary basins. At Marcona, hydrothermal activity was initiated in the earliest Middle Jurassic (161–177 Ma) by high-temperature Mg-Fe metasomatism represented by cummingtonite and phlogopite-magnetite assemblages. Subsequently, during the terminal eruptions (156–162 Ma) of the arc, widespread albite-marialite alteration (Na-Cl metasomatism) was followed by the emplacement of an en echelon swarm of massive magnetite ore-bodies with subordinate, overprinted magnetite-sulfide assemblages, hosted largely by Paleozoic metasilici-clastics. The magnetite orebodies exhibit abrupt, smoothly curving contacts, dike-like to tubular apophyses, and intricate, amoeboid interfingering with dacite porphyry intrusions. There is no convincing megascopic or microscopic evidence for large-scale Fe metasomatism associated with the main, sulfide-poor mineralization. The largest, 400 Mt Minas 2-3-4 orebody is interpreted as a bimodal magnetite-dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma. Oxygen and sulfur stable-isotope geothermometry indicates that the evolution at ca. 159 Ma from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphi-bole-pyrrhotite-pyrite assemblages coincided with quenching from above 800° C to below 450°C and the concomitant exsolution of dilute aqueous brines. Subsequently, chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, in part metasomatic, were deposited from lower temperature (≤360°C) brines. The Cu-poor Marcona (“Kiruna-type”) magnetite and Cu-rich IOCG deposits in the district, therefore, although spatially contiguous, represent contrasting ore deposit types. The former are interpreted as the product of Fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back-arc basin. The weak associated magmatic-hydrothermal Cu sulfide mineralization at Marcona was generated through melt vesiculation and contrasts with the considerably higher grade Cu- and Ag-rich orebodies of the major Cu-rich IOCG deposits in the Central Andes, e.g., La Candelaria-Punta del Cobre, Mantoverde, Raúl-Condestable, and Mina Justa, which were the products of cool, oxidized, hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1441.abstract

Patterns in the copper isotope composition of minerals in porphyry copper deposits in southwestern United States / Ryan Mathur in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1457-1467 Titre : Patterns in the copper isotope composition of minerals in porphyry copper deposits in southwestern United States Type de document : texte imprimé Auteurs : Ryan Mathur, Auteur ; Molly Dendas, Auteur ; Spencer Titley, Auteur Année de publication : 2011 Article en page(s) : pp. 1457-1467 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Porphyry  copper  deposits  Isotope  composition  United-States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cu isotope measurements of minerals from American southwest porphyry Cu deposits reveal a positive relationship with the grade of Cu and display spatial patterns. We measured the Cu isotope composition of 87 minerals contained within veins from drill core and hand specimen samples from three deposits (the Silver Bell, Ray, and Morenci mines). The overall range of Cu isotope ratios varies from −13.5 to +8.3 per mil. The deposits possess three different Cu reservoirs in which each has unique Cu isotope signatures, where δ65Cu for leach cap minerals < hypogene minerals < supergene enrichment minerals. The pattern exists in all three deposits and resulted from oxidative weathering of Cu at surface and reprecipitation of Cu at depth. A Rayleigh distillation model mimics the Cu isotope patterns found in the leach cap and quantifies the degree of leaching. A comparison of the isotopic composition of the three reservoirs among the three deposits reveals distinct differences—the Silver Bell mine possesses leach cap minerals that are more depleted in 65Cu and enrichment minerals that are more enriched in 65Cu than both the Morenci and Ray mines. A possible explanation for the difference among the deposits could indicate that the Silver Bell deposit experienced a greater degree of leaching than the Ray and Morenci deposits. The comparative analysis of the Cu isotope data reveals the exploration potential of using Cu isotope fractionation in the supergene environments. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1457.abstract [article] Patterns in the copper isotope composition of minerals in porphyry copper deposits in southwestern United States [texte imprimé] / Ryan Mathur, Auteur ; Molly Dendas, Auteur ; Spencer Titley, Auteur . - 2011 . - pp. 1457-1467. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1457-1467 Mots-clés : Porphyry  copper  deposits  Isotope  composition  United-States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Cu isotope measurements of minerals from American southwest porphyry Cu deposits reveal a positive relationship with the grade of Cu and display spatial patterns. We measured the Cu isotope composition of 87 minerals contained within veins from drill core and hand specimen samples from three deposits (the Silver Bell, Ray, and Morenci mines). The overall range of Cu isotope ratios varies from −13.5 to +8.3 per mil. The deposits possess three different Cu reservoirs in which each has unique Cu isotope signatures, where δ65Cu for leach cap minerals < hypogene minerals < supergene enrichment minerals. The pattern exists in all three deposits and resulted from oxidative weathering of Cu at surface and reprecipitation of Cu at depth. A Rayleigh distillation model mimics the Cu isotope patterns found in the leach cap and quantifies the degree of leaching. A comparison of the isotopic composition of the three reservoirs among the three deposits reveals distinct differences—the Silver Bell mine possesses leach cap minerals that are more depleted in 65Cu and enrichment minerals that are more enriched in 65Cu than both the Morenci and Ray mines. A possible explanation for the difference among the deposits could indicate that the Silver Bell deposit experienced a greater degree of leaching than the Ray and Morenci deposits. The comparative analysis of the Cu isotope data reveals the exploration potential of using Cu isotope fractionation in the supergene environments. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1457.abstract

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

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

Composition of the marginal rocks and sills of the rustenburg layered suite, Bushveld complex, South Africa / Sarah-Jane Barnes in Economic geology, Vol. 105 N° 8 (Décembre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1491-1511 Titre : Composition of the marginal rocks and sills of the rustenburg layered suite, Bushveld complex, South Africa : implications for the formation of the platinum-group element deposits Type de document : texte imprimé Auteurs : Sarah-Jane Barnes, Auteur ; Wolfgang D. Maier, Auteur ; Edward A. Curl, Auteur Année de publication : 2011 Article en page(s) : pp. 1491-1511 Note générale : éologie economique Langues : Anglais (eng) Mots-clés : Platinum-group  element  deposits  Marginal  rocks  South  Africa Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1491.abstract [article] Composition of the marginal rocks and sills of the rustenburg layered suite, Bushveld complex, South Africa : implications for the formation of the platinum-group element deposits [texte imprimé] / Sarah-Jane Barnes, Auteur ; Wolfgang D. Maier, Auteur ; Edward A. Curl, Auteur . - 2011 . - pp. 1491-1511. éologie economique Langues : Anglais (eng) in Economic geology > Vol. 105 N° 8 (Décembre 2010) . - pp. 1491-1511 Mots-clés : Platinum-group  element  deposits  Marginal  rocks  South  Africa Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/8/1491.abstract

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