Documents disponibles écrits par cet auteur

Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States / Patrick Nadoll in Economic geology, Vol. 107 N° 6 (Septembre/Octobre 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - p p. 1275-1292 Titre : Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States Type de document : texte imprimé Auteurs : Patrick Nadoll, Auteur ; Jeffrey L. Mauk, Auteur ; Timothy S. Hayes, Auteur Année de publication : 2012 Article en page(s) : p p. 1275-1292 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : magnetite;  mineral  deposits;  geochemistry;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magnetite (Fe3O4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe3O4, but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1275.short [article] Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic belt supergroup, United States [texte imprimé] / Patrick Nadoll, Auteur ; Jeffrey L. Mauk, Auteur ; Timothy S. Hayes, Auteur . - 2012 . - p p. 1275-1292. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 6 (Septembre/Octobre 2012) . - p p. 1275-1292 Mots-clés : magnetite;  mineral  deposits;  geochemistry;  United  States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Magnetite (Fe3O4) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe3O4, but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/6/1275.short

Hydrothermal alteration and veins at the epithermal Au-Ag deposits and prospects of the waitekauri area, Hauraki Goldfield, New Zealand / Mark P. Simpson in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 945-973 Titre : Hydrothermal alteration and veins at the epithermal Au-Ag deposits and prospects of the waitekauri area, Hauraki Goldfield, New Zealand Type de document : texte imprimé Auteurs : Mark P. Simpson, Auteur ; Jeffrey L. Mauk, Auteur Année de publication : 2011 Article en page(s) : pp. 945-973 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Epithermal  Au-Ag  deposits  Hydrothermal  alteration  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects. From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits. Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area. Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east. Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote. Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek. Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee. Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek. Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions. Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek. Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic. Fluid inclusions in quartz and calcite homogenized between 132° and 310°C and trapped a dilute solution with an apparent salinity of less than 2.6 wt percent NaCl equiv. Homogenization temperatures are highest at Sovereign (avg. 241°C), Jubilee (avg 239°C), and Scimitar (avg 236°C), lower at Scotia (avg 204°C) and lowest at Teutonic (avg 168 °C) and Jasper Creek (avg 162°C). Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval. Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west. Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures. Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10° to the east. Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin. Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/945.abstract [article] Hydrothermal alteration and veins at the epithermal Au-Ag deposits and prospects of the waitekauri area, Hauraki Goldfield, New Zealand [texte imprimé] / Mark P. Simpson, Auteur ; Jeffrey L. Mauk, Auteur . - 2011 . - pp. 945-973. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 945-973 Mots-clés : Epithermal  Au-Ag  deposits  Hydrothermal  alteration  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects. From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits. Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area. Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east. Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote. Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek. Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee. Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek. Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions. Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek. Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic. Fluid inclusions in quartz and calcite homogenized between 132° and 310°C and trapped a dilute solution with an apparent salinity of less than 2.6 wt percent NaCl equiv. Homogenization temperatures are highest at Sovereign (avg. 241°C), Jubilee (avg 239°C), and Scimitar (avg 236°C), lower at Scotia (avg 204°C) and lowest at Teutonic (avg 168 °C) and Jasper Creek (avg 162°C). Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval. Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west. Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures. Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10° to the east. Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin. Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/945.abstract

Punctuated evolution of a large epithermal province / Jeffrey L. Mauk in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 921-943 Titre : Punctuated evolution of a large epithermal province : The Hauraki Goldfield, New Zealand Type de document : texte imprimé Auteurs : Jeffrey L. Mauk, Auteur ; Chris M. Hall, Auteur ; John T. Chesley, Auteur Année de publication : 2011 Article en page(s) : pp. 921-943 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Hauraki  goldfield  Mineralization  Epithermal  province  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Hauraki goldfield in the Coromandel volcanic zone contains approximately 50 adularia-sericite epithermal Au-Ag deposits in a 200-km-long by 40-km-wide north-south-trending belt. These deposits have produced approximately 320,000 kg Au and 1.5 Mkg Ag and formed from Miocene to Pliocene subaerial hydrothermal systems. The goldfield has been divided into three provinces (northern, eastern, and southern), based on the host rocks and geologic setting of the deposits (Christie et al., 2007). In the northern province of the goldfield, adularia from Paritu yields a single 40Ar/39Ar plateau date of 16.32 ± 0.13 Ma, and adularia from Opitonui yields a preferred 40Ar/39Ar age of 13.15 ± 0.03 Ma. Two Re-Os dates of molybdenite from porphyry-style mineralization at Ohio Creek overlap within error and yield dates of 11.87 ± 0.06 and 11.97± 0.08 Ma; geologic relationships suggest that this is the likely age of mineralization in the nearby Thames epithermal deposits. In the eastern province, adularia from the Ohui deposit gives a preferred 40Ar/39Ar age of 8.29 ± 0.25 Ma, adularia from the Broken Hills deposit gives a preferred 40Ar/39Ar age of 7.12 ± 0.02 Ma, and adularia from the Wharekirauponga prospect yields a preferred 40Ar/39Ar age of 6.32 ± 0.12 Ma. In the southern province, adularia from quartz veins at the Maratoto deposit provide a preferred 40Ar/39Ar age of 6.41 ± 0.04 Ma, and adularia from a quartz vein at the Sovereign deposit yields a preferred 40Ar/39Ar age of 6.70 ± 0.16 Ma. Two dates from vein adularia at the world-class Martha deposit overlap within error, and we interpret a preferred age for the deposit of 6.16 ± 0.06 Ma. Two samples of molybdenite from veins in the Martha deposit yield discrete Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma. Adularia from one quartz vein from the Favona deposit yields a 40Ar/39Ar date of 6.05 ± 0.04 Ma. Host rock and vein adularia from the Karangahake deposit yield 40Ar/39Ar plateau dates that range from 6.90 ± 0.20 to 5.71 ± 0.13 Ma, which may reflect more than one stage of mineralization or protracted fluid flow. Adularia from veins at the Waiorongomai deposit yields a preferred 40Ar/39Ar age of 5.71 ± 0.03 Ma, and adularia from a vein at the Eliza deposit yields a preferred age of 4.47 ± 0.06 Ma. The southernmost deposit in the Hauraki goldfield, Muirs Reef, has adularia in quartz veins that yield 40Ar/39Ar plateaus dates of 2.12 ± 0.11 to 1.78 ± 0.16 Ma. Combined with previous work, these results indicate that mineralization in the Hauraki goldfield ranges from 16.3 Ma in the north to 2 Ma in the south, and clusters into two distinct groups that correlate with location, volcanic stratigraphy, and mineralization style. The first group, from ~16.3 to ~10.8 Ma contains epithermal veins, including bonanza-style veins, and porphyry-style mineralization that formed in the northern province in an arc that was dominated by andesitic volcanism. The second period of mineralization occurs primarily from 6.9 to 6.0 Ma in the eastern and southern provinces, when precious metals were deposited into thicker colloform-crustiform banded veins that formed in extensional settings in an arc that was erupting bimodal andesite-rhyolite compositions. Therefore, even though volcanism in the Coromandel volcanic zone was active from 18 to 2 Ma, Au-Ag mineralization was focused into two discrete periods of this arc formation, and the style of mineralization changed through time, coinciding with a change in style of volcanism. In addition, while Hauraki goldfield mineralization discontinuously lasted more than 11 m.y., greater than 80 percent of the known gold endowment was deposited in a relatively brief 0.9 Ma window between 6.0 and 6.9 Ma. These changes through time likely reflect, at least in part, reorganization of the Miocene Northland and Colvil DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/921.abstract [article] Punctuated evolution of a large epithermal province : The Hauraki Goldfield, New Zealand [texte imprimé] / Jeffrey L. Mauk, Auteur ; Chris M. Hall, Auteur ; John T. Chesley, Auteur . - 2011 . - pp. 921-943. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 921-943 Mots-clés : Hauraki  goldfield  Mineralization  Epithermal  province  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Hauraki goldfield in the Coromandel volcanic zone contains approximately 50 adularia-sericite epithermal Au-Ag deposits in a 200-km-long by 40-km-wide north-south-trending belt. These deposits have produced approximately 320,000 kg Au and 1.5 Mkg Ag and formed from Miocene to Pliocene subaerial hydrothermal systems. The goldfield has been divided into three provinces (northern, eastern, and southern), based on the host rocks and geologic setting of the deposits (Christie et al., 2007). In the northern province of the goldfield, adularia from Paritu yields a single 40Ar/39Ar plateau date of 16.32 ± 0.13 Ma, and adularia from Opitonui yields a preferred 40Ar/39Ar age of 13.15 ± 0.03 Ma. Two Re-Os dates of molybdenite from porphyry-style mineralization at Ohio Creek overlap within error and yield dates of 11.87 ± 0.06 and 11.97± 0.08 Ma; geologic relationships suggest that this is the likely age of mineralization in the nearby Thames epithermal deposits. In the eastern province, adularia from the Ohui deposit gives a preferred 40Ar/39Ar age of 8.29 ± 0.25 Ma, adularia from the Broken Hills deposit gives a preferred 40Ar/39Ar age of 7.12 ± 0.02 Ma, and adularia from the Wharekirauponga prospect yields a preferred 40Ar/39Ar age of 6.32 ± 0.12 Ma. In the southern province, adularia from quartz veins at the Maratoto deposit provide a preferred 40Ar/39Ar age of 6.41 ± 0.04 Ma, and adularia from a quartz vein at the Sovereign deposit yields a preferred 40Ar/39Ar age of 6.70 ± 0.16 Ma. Two dates from vein adularia at the world-class Martha deposit overlap within error, and we interpret a preferred age for the deposit of 6.16 ± 0.06 Ma. Two samples of molybdenite from veins in the Martha deposit yield discrete Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma. Adularia from one quartz vein from the Favona deposit yields a 40Ar/39Ar date of 6.05 ± 0.04 Ma. Host rock and vein adularia from the Karangahake deposit yield 40Ar/39Ar plateau dates that range from 6.90 ± 0.20 to 5.71 ± 0.13 Ma, which may reflect more than one stage of mineralization or protracted fluid flow. Adularia from veins at the Waiorongomai deposit yields a preferred 40Ar/39Ar age of 5.71 ± 0.03 Ma, and adularia from a vein at the Eliza deposit yields a preferred age of 4.47 ± 0.06 Ma. The southernmost deposit in the Hauraki goldfield, Muirs Reef, has adularia in quartz veins that yield 40Ar/39Ar plateaus dates of 2.12 ± 0.11 to 1.78 ± 0.16 Ma. Combined with previous work, these results indicate that mineralization in the Hauraki goldfield ranges from 16.3 Ma in the north to 2 Ma in the south, and clusters into two distinct groups that correlate with location, volcanic stratigraphy, and mineralization style. The first group, from ~16.3 to ~10.8 Ma contains epithermal veins, including bonanza-style veins, and porphyry-style mineralization that formed in the northern province in an arc that was dominated by andesitic volcanism. The second period of mineralization occurs primarily from 6.9 to 6.0 Ma in the eastern and southern provinces, when precious metals were deposited into thicker colloform-crustiform banded veins that formed in extensional settings in an arc that was erupting bimodal andesite-rhyolite compositions. Therefore, even though volcanism in the Coromandel volcanic zone was active from 18 to 2 Ma, Au-Ag mineralization was focused into two discrete periods of this arc formation, and the style of mineralization changed through time, coinciding with a change in style of volcanism. In addition, while Hauraki goldfield mineralization discontinuously lasted more than 11 m.y., greater than 80 percent of the known gold endowment was deposited in a relatively brief 0.9 Ma window between 6.0 and 6.9 Ma. These changes through time likely reflect, at least in part, reorganization of the Miocene Northland and Colvil DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/921.abstract

Quantifying metasomatism in epithermal Au-Ag deposits / Mathijs A. Booden in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 999-1030 Titre : Quantifying metasomatism in epithermal Au-Ag deposits : A case study from the Waitekauri Area, New Zealand Type de document : texte imprimé Auteurs : Mathijs A. Booden, Auteur ; Jeffrey L. Mauk, Auteur ; Mark P. Simpson, Auteur Année de publication : 2011 Article en page(s) : pp. 999-1030 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Epithermal  deposits  Goldfield  Geochemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Major element geochemical exploration for epithermal deposits can extend the range of traditional pathfinder elements to a 1- to 10-km scale, and with knowledge of protolith composition, mass changes associated with hydrothermal alteration can be quantified. In the Hauraki goldfield of New Zealand, altered andesites and dacites host epithermal Au-Ag deposits and prospects. The major element compositions of equivalent unaltered rocks correlate with whole-rock Zr/TiO2, an immobile element ratio that is preserved during K metasomatism. We used this feature to estimate the initial composition and calculate a mass balance for veinless altered rocks in the Waitekauri area along a 3-km-wide section that extends from the central Waitekauri fault to the periphery of the alteration zone. The total transferred mass is equal to approximately 11 percent of rock mass in illite-dominated altered rocks, and 24 percent of rock mass in adularia-dominated altered rocks. On average mass losses exceed gains. Potassium was gained in most altered rocks, which contain illite and/or adularia as K-bearing hydrothermal minerals. Silica was gained in adularia-quartz–rich rocks close to the Waitekauri fault. Other major elements are preferentially lost (Ca, Na, Fe, Mg) or effectively immobile (Al, Ti). The greatest K and Si gains occur in adularia-rich rocks that surround Au deposits along the Waitekauri fault, whereas K gains are progressively lower and Si gains are mostly insignificant in deposits and prospects farther east where illite or interstratified illite-smectite is the dominant K-bearing mineral. In contrast, Na and Ca losses do not increase significantly from the periphery to the core of the Waitekauri area, because losses are commonly complete and therefore limited by the initial concentration. However, the K and Si gains correlate with other measures of K metasomatism including K/Sr and Rb/Sr values and molar (M) K/(K + Na + 2Ca) values, and together these parameters vector from the barren periphery to the orebody-hosting center of the Waitekauri area. In contrast to major element trends, the pathfinder elements As, Sb, and Hg define more local hydrothermal alteration cells within the larger Waitekauri area, some of which surround Au deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/999.abstract [article] Quantifying metasomatism in epithermal Au-Ag deposits : A case study from the Waitekauri Area, New Zealand [texte imprimé] / Mathijs A. Booden, Auteur ; Jeffrey L. Mauk, Auteur ; Mark P. Simpson, Auteur . - 2011 . - pp. 999-1030. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 999-1030 Mots-clés : Epithermal  deposits  Goldfield  Geochemistry Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Major element geochemical exploration for epithermal deposits can extend the range of traditional pathfinder elements to a 1- to 10-km scale, and with knowledge of protolith composition, mass changes associated with hydrothermal alteration can be quantified. In the Hauraki goldfield of New Zealand, altered andesites and dacites host epithermal Au-Ag deposits and prospects. The major element compositions of equivalent unaltered rocks correlate with whole-rock Zr/TiO2, an immobile element ratio that is preserved during K metasomatism. We used this feature to estimate the initial composition and calculate a mass balance for veinless altered rocks in the Waitekauri area along a 3-km-wide section that extends from the central Waitekauri fault to the periphery of the alteration zone. The total transferred mass is equal to approximately 11 percent of rock mass in illite-dominated altered rocks, and 24 percent of rock mass in adularia-dominated altered rocks. On average mass losses exceed gains. Potassium was gained in most altered rocks, which contain illite and/or adularia as K-bearing hydrothermal minerals. Silica was gained in adularia-quartz–rich rocks close to the Waitekauri fault. Other major elements are preferentially lost (Ca, Na, Fe, Mg) or effectively immobile (Al, Ti). The greatest K and Si gains occur in adularia-rich rocks that surround Au deposits along the Waitekauri fault, whereas K gains are progressively lower and Si gains are mostly insignificant in deposits and prospects farther east where illite or interstratified illite-smectite is the dominant K-bearing mineral. In contrast, Na and Ca losses do not increase significantly from the periphery to the core of the Waitekauri area, because losses are commonly complete and therefore limited by the initial concentration. However, the K and Si gains correlate with other measures of K metasomatism including K/Sr and Rb/Sr values and molar (M) K/(K + Na + 2Ca) values, and together these parameters vector from the barren periphery to the orebody-hosting center of the Waitekauri area. In contrast to major element trends, the pathfinder elements As, Sb, and Hg define more local hydrothermal alteration cells within the larger Waitekauri area, some of which surround Au deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/999.abstract