Dépouillements

Epithermal gold-silver deposits of the Hauraki Goldfield, New Zealand / David A. John in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 915-919 Titre : Epithermal gold-silver deposits of the Hauraki Goldfield, New Zealand : an introduction Type de document : texte imprimé Auteurs : David A. John, Auteur Année de publication : 2011 Article en page(s) : pp. 915-919 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Epithermal  gold-silver  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The five papers that follow describe diverse aspects of epithermal gold-silver deposits and genetically related hydrothermal systems in the Hauraki goldfield on the Coromandel peninsula of the North Island of New Zealand (Fig. 1). The Hauraki goldfield contains more than 50 Miocene and Pliocene epithermal Au-Ag deposits and several porphyry Cu-Au-Mo occurrences in a 200-km-long by 40-km-wide north-south–trending belt (Fig. 1C; Christie et al., 2007). Production from these deposits between 1862 and 2009 totaled approx. 335,000 kg (10.8 Moz) of Au and 1.6 million kg (51.4 Moz) of Ag (Mauk et al., 2011). The epithermal deposits are related to subaerial hydrothermal systems hosted in rocks of the early Miocene to late Pliocene (~18–1.9 Ma) Coromandel volcanic zone (Skinner, 1986). DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/915.extract [article] Epithermal gold-silver deposits of the Hauraki Goldfield, New Zealand : an introduction [texte imprimé] / David A. John, Auteur . - 2011 . - pp. 915-919. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 915-919 Mots-clés : Epithermal  gold-silver  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The five papers that follow describe diverse aspects of epithermal gold-silver deposits and genetically related hydrothermal systems in the Hauraki goldfield on the Coromandel peninsula of the North Island of New Zealand (Fig. 1). The Hauraki goldfield contains more than 50 Miocene and Pliocene epithermal Au-Ag deposits and several porphyry Cu-Au-Mo occurrences in a 200-km-long by 40-km-wide north-south–trending belt (Fig. 1C; Christie et al., 2007). Production from these deposits between 1862 and 2009 totaled approx. 335,000 kg (10.8 Moz) of Au and 1.6 million kg (51.4 Moz) of Ag (Mauk et al., 2011). The epithermal deposits are related to subaerial hydrothermal systems hosted in rocks of the early Miocene to late Pliocene (~18–1.9 Ma) Coromandel volcanic zone (Skinner, 1986). DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/915.extract

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

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

Structural evolution of a world-class epithermal orebody / K. Bernard Spörli in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 975-998 Titre : Structural evolution of a world-class epithermal orebody : The Martha Hill deposit, Waihi, New Zealand Type de document : texte imprimé Auteurs : K. Bernard Spörli, Auteur ; Hugh Cargill, Auteur Année de publication : 2011 Article en page(s) : pp. 975-998 Note générale : Géologie économiqe Langues : Anglais (eng) Mots-clés : Orebody  Gold  Silver  Epithermal  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : We describe the complex structure in the highest (now mined-out) levels of the world-class Martha Hill gold-silver deposit, Coromandel peninsula, which is hosted in late Miocene andesitic rocks. A northeast-striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast-striking gold-silver lodes up to 30 m thick. Gold production was entirely from quartz veins. The mineralization is embedded in the following geological history: (1) deposition of the host volcanic rocks, (2) initial tectonic fracturing, (3) intrusion of clastic dikes and sills, (4) start of hydrothermal alteration, (5) main phase of faulting, (6) main phase of veining, (7) erosion and deep oxidation, (8–11) deposition of further volcanic material, including prominent rhyolites, and formation of present topography. Veins occupied fractures but also followed a network of preexisting, mostly normal faults with a small to medium amount of displacement. Faulting involved northwest-southeast, north-south, and east-west extension, implying complex 3-D strain. Clastic dikes of carbonaceous sandstone intruded during the faulting phase. Fault rocks were mostly brittle cataclasites or breccias, but some clay-rich zones displayed ductile structures. Many veins overprinted faults, with a change from shear-mode to opening-mode deformation. Stockwork veins formed a mesh composed of two orthogonal sets: one with northeast-southwest– and northwest-southeast–striking veins, including the dominant mineralized lodes, and the other with north-south– and east-west–striking veins. Although northwest-striking veins were earliest, during the main vein phase, opening oscillated between all the vein directions, with the two main lodes in the open pit area, the Martha and the Welcome, capturing most of the strain. A variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid. A special concentration of structural features facilitated focusing of fluid flow to produce this world-class deposit, which contrasts with the less well endowed vein systems around it. The overall tectonic control was dominated by northwest-southeast extension and dip-slip deformation. The 3-D strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north-northwest and northeast trends associated with migration of a subduction zone past the Coromandel peninsula. This demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/975.abstract [article] Structural evolution of a world-class epithermal orebody : The Martha Hill deposit, Waihi, New Zealand [texte imprimé] / K. Bernard Spörli, Auteur ; Hugh Cargill, Auteur . - 2011 . - pp. 975-998. Géologie économiqe Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 975-998 Mots-clés : Orebody  Gold  Silver  Epithermal  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : We describe the complex structure in the highest (now mined-out) levels of the world-class Martha Hill gold-silver deposit, Coromandel peninsula, which is hosted in late Miocene andesitic rocks. A northeast-striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast-striking gold-silver lodes up to 30 m thick. Gold production was entirely from quartz veins. The mineralization is embedded in the following geological history: (1) deposition of the host volcanic rocks, (2) initial tectonic fracturing, (3) intrusion of clastic dikes and sills, (4) start of hydrothermal alteration, (5) main phase of faulting, (6) main phase of veining, (7) erosion and deep oxidation, (8–11) deposition of further volcanic material, including prominent rhyolites, and formation of present topography. Veins occupied fractures but also followed a network of preexisting, mostly normal faults with a small to medium amount of displacement. Faulting involved northwest-southeast, north-south, and east-west extension, implying complex 3-D strain. Clastic dikes of carbonaceous sandstone intruded during the faulting phase. Fault rocks were mostly brittle cataclasites or breccias, but some clay-rich zones displayed ductile structures. Many veins overprinted faults, with a change from shear-mode to opening-mode deformation. Stockwork veins formed a mesh composed of two orthogonal sets: one with northeast-southwest– and northwest-southeast–striking veins, including the dominant mineralized lodes, and the other with north-south– and east-west–striking veins. Although northwest-striking veins were earliest, during the main vein phase, opening oscillated between all the vein directions, with the two main lodes in the open pit area, the Martha and the Welcome, capturing most of the strain. A variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid. A special concentration of structural features facilitated focusing of fluid flow to produce this world-class deposit, which contrasts with the less well endowed vein systems around it. The overall tectonic control was dominated by northwest-southeast extension and dip-slip deformation. The 3-D strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north-northwest and northeast trends associated with migration of a subduction zone past the Coromandel peninsula. This demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/975.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

Geophysical characteristics of adularia-sericite epithermal gold-silver deposits in the Waihi-Waitekauri region, New Zealand / Anne E. Morrell in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1031-1041 Titre : Geophysical characteristics of adularia-sericite epithermal gold-silver deposits in the Waihi-Waitekauri region, New Zealand Type de document : texte imprimé Auteurs : Anne E. Morrell, Auteur ; Corinne A. Locke, Auteur ; John Cassidy, Auteur Année de publication : 2011 Article en page(s) : pp. 1031-1041 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Adularia-sericite  Gold  Silver  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Aeromagnetic, airborne radiometric, and surface gravity data from the adularia-sericite epithermal province of the Hauraki goldfield have delineated distinct anomalies associated with pervasive hydrothermal alteration and gold-silver mineralization. Aeromagnetic derivative images, particularly the analytic signal, define the boundaries of several magnetic quiet zones that result from magnetite destruction in the volcanic host rocks. Six discrete zones, each of ≤10-km2 areal extent are evident and are comparable in size to modern-day geothermal systems in the Taupo volcanic zone. An extensive (>20-km2) zone in the Waitekauri Valley is likely to be the result of multiple overlapping systems, for which there are also analogues in the Taupo volcanic zone that form single large-scale alteration zones up to 100 km2. Low-pass filtering of the analytic signal data reveals still wider zones of relatively low magnetic intensity interpreted as areas of hydrothermally altered rock extending below younger, unaltered cover. Many of these magnetic quiet zones are aligned along a north-northeast–south-southwest structural corridor, indicating that regional-scale structures may have controlled the location of these geophysical features. Radiometric data delineate local high potassium anomalies that reflect up to 12/100 g potassium enrichment in the core of alteration zones in the form of adularia and illite. A broad K/Th anomaly correlates with the extent of the magnetic quiet zones and indicates widespread potassium enrichment in the Waitekauri-Maratoto area. Gravity data in the Waihi district define a unique, double-peaked, 50-gravity unit positive residual anomaly that correlates closely with the extent of the magnetic quiet zone and the locations of the Waihi and Favona deposits. Preliminary modeling indicates that the anomaly source body, beneath the near-surface, low-density altered andesite, has a volume of up to ~11 km3 and a minimum density of 2,900 kg m−3. The nature of this source body is enigmatic; possible causes include a dense intrusion, uplifted anomalously dense basement, dense sulfide mineralization, or some combination of these. In addition, the correlation of each of the two gravity peaks with a north-northeast–trending fault may indicate some structural focusing process active at the deposit scale. This integration of geophysical data provides an outstanding case study of the district- to regional-scale geophysical characteristics of a classic epithermal province and demonstrates the strengths of geophysical surveys in the exploration for epithermal mineral deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1031.abstract [article] Geophysical characteristics of adularia-sericite epithermal gold-silver deposits in the Waihi-Waitekauri region, New Zealand [texte imprimé] / Anne E. Morrell, Auteur ; Corinne A. Locke, Auteur ; John Cassidy, Auteur . - 2011 . - pp. 1031-1041. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1031-1041 Mots-clés : Adularia-sericite  Gold  Silver  deposits  New  Zeland Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Aeromagnetic, airborne radiometric, and surface gravity data from the adularia-sericite epithermal province of the Hauraki goldfield have delineated distinct anomalies associated with pervasive hydrothermal alteration and gold-silver mineralization. Aeromagnetic derivative images, particularly the analytic signal, define the boundaries of several magnetic quiet zones that result from magnetite destruction in the volcanic host rocks. Six discrete zones, each of ≤10-km2 areal extent are evident and are comparable in size to modern-day geothermal systems in the Taupo volcanic zone. An extensive (>20-km2) zone in the Waitekauri Valley is likely to be the result of multiple overlapping systems, for which there are also analogues in the Taupo volcanic zone that form single large-scale alteration zones up to 100 km2. Low-pass filtering of the analytic signal data reveals still wider zones of relatively low magnetic intensity interpreted as areas of hydrothermally altered rock extending below younger, unaltered cover. Many of these magnetic quiet zones are aligned along a north-northeast–south-southwest structural corridor, indicating that regional-scale structures may have controlled the location of these geophysical features. Radiometric data delineate local high potassium anomalies that reflect up to 12/100 g potassium enrichment in the core of alteration zones in the form of adularia and illite. A broad K/Th anomaly correlates with the extent of the magnetic quiet zones and indicates widespread potassium enrichment in the Waitekauri-Maratoto area. Gravity data in the Waihi district define a unique, double-peaked, 50-gravity unit positive residual anomaly that correlates closely with the extent of the magnetic quiet zone and the locations of the Waihi and Favona deposits. Preliminary modeling indicates that the anomaly source body, beneath the near-surface, low-density altered andesite, has a volume of up to ~11 km3 and a minimum density of 2,900 kg m−3. The nature of this source body is enigmatic; possible causes include a dense intrusion, uplifted anomalously dense basement, dense sulfide mineralization, or some combination of these. In addition, the correlation of each of the two gravity peaks with a north-northeast–trending fault may indicate some structural focusing process active at the deposit scale. This integration of geophysical data provides an outstanding case study of the district- to regional-scale geophysical characteristics of a classic epithermal province and demonstrates the strengths of geophysical surveys in the exploration for epithermal mineral deposits. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1031.abstract

The origin of mineralized fractures at the bluebell mine site, Riondel, British Columbia / David P. Moynihan in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1043-1058 Titre : The origin of mineralized fractures at the bluebell mine site, Riondel, British Columbia Type de document : texte imprimé Auteurs : David P. Moynihan, Auteur ; David R. M. Pattison, Auteur Année de publication : 2011 Article en page(s) : pp. 1043-1058 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Mineralized  fracture  Pb-Zn  deposit  Hypothermal  ore  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bluebell Pb-Zn deposit is located on the east side of Kootenay Lake in southeastern British Columbia. It is a fracture-controlled replacement deposit hosted in Lower Cambrian marble of the Badshot and Mohican formations. The orebodies trend west-northwest, parallel to a prominent set of fractures that controlled mineralization. These fractures are interpreted to have formed as part of a conjugate set. Contrary to some earlier suggestions, folding played no role in fracture formation. The youngest generation of folds are reinterpreted as having formed through development of shear bands during extension of the layering prior to fracture development. Formation of the fractures also postdated motion on a number of early Tertiary normal faults that are associated with regional extension and alkaline magmatism. West-northwest–trending fractures, which formed as part of a conjugate set, were reactivated and controlled the intrusion of mafic dikes, mineralizing fluids, and minor late faulting. Fracturing, faulting, mafic dike intrusion, and mineralization may be associated with a long-lived basement structure that runs below the area, which could have facilitated the ascent of deep crustal or mantle-derived magma and mineralizing fluids. The results of this study provide a new perspective on exploration strategies for hydrothermal ore deposits in this part of southeastern British Columbia. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1043.abstract [article] The origin of mineralized fractures at the bluebell mine site, Riondel, British Columbia [texte imprimé] / David P. Moynihan, Auteur ; David R. M. Pattison, Auteur . - 2011 . - pp. 1043-1058. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1043-1058 Mots-clés : Mineralized  fracture  Pb-Zn  deposit  Hypothermal  ore  British  Columbia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bluebell Pb-Zn deposit is located on the east side of Kootenay Lake in southeastern British Columbia. It is a fracture-controlled replacement deposit hosted in Lower Cambrian marble of the Badshot and Mohican formations. The orebodies trend west-northwest, parallel to a prominent set of fractures that controlled mineralization. These fractures are interpreted to have formed as part of a conjugate set. Contrary to some earlier suggestions, folding played no role in fracture formation. The youngest generation of folds are reinterpreted as having formed through development of shear bands during extension of the layering prior to fracture development. Formation of the fractures also postdated motion on a number of early Tertiary normal faults that are associated with regional extension and alkaline magmatism. West-northwest–trending fractures, which formed as part of a conjugate set, were reactivated and controlled the intrusion of mafic dikes, mineralizing fluids, and minor late faulting. Fracturing, faulting, mafic dike intrusion, and mineralization may be associated with a long-lived basement structure that runs below the area, which could have facilitated the ascent of deep crustal or mantle-derived magma and mineralizing fluids. The results of this study provide a new perspective on exploration strategies for hydrothermal ore deposits in this part of southeastern British Columbia. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1043.abstract

40Ar/39Ar dating of alunite from the pueblo viejo gold-silver district, Dominican Republic / Antonio Arribas in Economic geology, Vol. 106 N° 6 (Septembre/Octobre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1059-1070 Titre : 40Ar/39Ar dating of alunite from the pueblo viejo gold-silver district, Dominican Republic Type de document : texte imprimé Auteurs : Antonio Arribas, Auteur ; Isabel Arribas, Auteur ; Grenville Draper, Auteur Année de publication : 2011 Article en page(s) : pp. 1059-1070 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Alunite  Gold-silver  district  Dominican  republic Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : New 40Ar/39Ar ages for alunite from the Moore and Monte Negro deposits in the Pueblo Viejo district, as well as from a newly discovered alunite-bearing zone on Loma la Cuaba west of the known deposits, are reported here. The ages range from about 80 to 40 Ma, with closely adjacent samples exhibiting very different ages. Interpretation of these results in the context of estimated closure temperatures for alunite and the geologic and tectonic evolution of Hispaniola does not lead to a simple conclusion about the age of mineralization. The simplest interpretation, that mineralization was caused by a buried Late Cretaceous (~80 Ma) intrusion, is complicated by lack of intrusions of this age in the area and absence of alteration in overlying limestone. The alternative interpretation, that mineralization was formed during Early Cretaceous (~110 Ma) magmatism and that the 40Ar/39Ar ages were completely reset by Late Cretaceous thrusting, is complicated by a lack of information on the timing and thermal effects of thrusting in central Hispaniola. Alunite studies have yielded similar unclear results in other pre-Cenozoic ore systems, notably those of the Lachlan fold belt in Australia. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1059.abstract [article] 40Ar/39Ar dating of alunite from the pueblo viejo gold-silver district, Dominican Republic [texte imprimé] / Antonio Arribas, Auteur ; Isabel Arribas, Auteur ; Grenville Draper, Auteur . - 2011 . - pp. 1059-1070. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 6 (Septembre/Octobre 2011) . - pp. 1059-1070 Mots-clés : Alunite  Gold-silver  district  Dominican  republic Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : New 40Ar/39Ar ages for alunite from the Moore and Monte Negro deposits in the Pueblo Viejo district, as well as from a newly discovered alunite-bearing zone on Loma la Cuaba west of the known deposits, are reported here. The ages range from about 80 to 40 Ma, with closely adjacent samples exhibiting very different ages. Interpretation of these results in the context of estimated closure temperatures for alunite and the geologic and tectonic evolution of Hispaniola does not lead to a simple conclusion about the age of mineralization. The simplest interpretation, that mineralization was caused by a buried Late Cretaceous (~80 Ma) intrusion, is complicated by lack of intrusions of this age in the area and absence of alteration in overlying limestone. The alternative interpretation, that mineralization was formed during Early Cretaceous (~110 Ma) magmatism and that the 40Ar/39Ar ages were completely reset by Late Cretaceous thrusting, is complicated by a lack of information on the timing and thermal effects of thrusting in central Hispaniola. Alunite studies have yielded similar unclear results in other pre-Cenozoic ore systems, notably those of the Lachlan fold belt in Australia. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/6/1059.abstract

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