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Geology and intrusion-related affinity of the morila gold mine, Southeast Mali / Christopher R.M. McFarlane in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 727-750 Titre : Geology and intrusion-related affinity of the morila gold mine, Southeast Mali Type de document : texte imprimé Auteurs : Christopher R.M. McFarlane, Auteur ; John Mavrogenes, Auteur ; Dave Lentz, Auteur Année de publication : 2011 Article en page(s) : pp. 727-750 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Gold  deposits  Mali Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The ~8 Moz Morila gold mine, hosted within Paleoproterozoic Birimian volcano-sedimentary rocks of southeast Mali, is spatially and temporally associated with prolonged (2098–2065 Ma) arc magmatism during the late stages of the Eburnean orogeny. Visible gold at Morila is associated with variably deformed polymineralic veins containing native bismuth, maldonite, aurostibite, rare tellurobismuthite, and löllingite, suggesting a proximal intrusion-related source for this period of gold mineralization. This early formed mineralization is contained within a zone of hornblende hornfels contact metamorphism and is spatially associated with syn- to post-D2 emplacement of 2098 to 2091 Ma quartz-diorite, granodiorite, and leucogranite magmas. The occurrence of immiscible Au-Sb-Bi-Te blebs within sills or dikes associated with gold mineralization at the Morila deposit explicitly links granitic magmatism with gold mineralization This early intrusion-related gold system was over-printed by a younger post-D2 stage of hydrothermal alteration recorded by sulfidation along a north-north-east–trending zone characterized by disseminated idioblastic arsenopyrite porphyroblasts that contain polygonal gold blebs. Silicate alteration during this stage includes albitization of plagioclase and the growth of randomly distributed biotite and titanite, the latter typically surrounding ilmenite. Uranium-Pb dating of this generation of titanite yields a preliminary age for late-stage sulfidation of 2074 ± 14 Ma, which brackets mineralization to the interval 2098 ± 4 to 2074 ± 14 Ma. The geochemistry and isotope systematics of syn- to post-tectonic intermediate intrusions at the Morila deposit point to their derivation in a suprasubduction zone setting and emplacement into tectonically thickened crust. Based on these observations, it is suggested that the Morila gold deposit formed during late-stage collisional orogenesis involving the accretion of juvenile volcanic arc terranes against the Archean Man (Liberian) cratonic nucleus. This setting is analogous to younger Phanerozoic active continental margin settings which host the best-described examples of intrusion-related gold systems. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/727.abstract [article] Geology and intrusion-related affinity of the morila gold mine, Southeast Mali [texte imprimé] / Christopher R.M. McFarlane, Auteur ; John Mavrogenes, Auteur ; Dave Lentz, Auteur . - 2011 . - pp. 727-750. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 727-750 Mots-clés : Gold  deposits  Mali Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The ~8 Moz Morila gold mine, hosted within Paleoproterozoic Birimian volcano-sedimentary rocks of southeast Mali, is spatially and temporally associated with prolonged (2098–2065 Ma) arc magmatism during the late stages of the Eburnean orogeny. Visible gold at Morila is associated with variably deformed polymineralic veins containing native bismuth, maldonite, aurostibite, rare tellurobismuthite, and löllingite, suggesting a proximal intrusion-related source for this period of gold mineralization. This early formed mineralization is contained within a zone of hornblende hornfels contact metamorphism and is spatially associated with syn- to post-D2 emplacement of 2098 to 2091 Ma quartz-diorite, granodiorite, and leucogranite magmas. The occurrence of immiscible Au-Sb-Bi-Te blebs within sills or dikes associated with gold mineralization at the Morila deposit explicitly links granitic magmatism with gold mineralization This early intrusion-related gold system was over-printed by a younger post-D2 stage of hydrothermal alteration recorded by sulfidation along a north-north-east–trending zone characterized by disseminated idioblastic arsenopyrite porphyroblasts that contain polygonal gold blebs. Silicate alteration during this stage includes albitization of plagioclase and the growth of randomly distributed biotite and titanite, the latter typically surrounding ilmenite. Uranium-Pb dating of this generation of titanite yields a preliminary age for late-stage sulfidation of 2074 ± 14 Ma, which brackets mineralization to the interval 2098 ± 4 to 2074 ± 14 Ma. The geochemistry and isotope systematics of syn- to post-tectonic intermediate intrusions at the Morila deposit point to their derivation in a suprasubduction zone setting and emplacement into tectonically thickened crust. Based on these observations, it is suggested that the Morila gold deposit formed during late-stage collisional orogenesis involving the accretion of juvenile volcanic arc terranes against the Archean Man (Liberian) cratonic nucleus. This setting is analogous to younger Phanerozoic active continental margin settings which host the best-described examples of intrusion-related gold systems. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/727.abstract

Marble-hosted submicroscopic gold mineralization at asimotrypes area, mount Pangeon, southern Rhodope core complex, Greece / Demetrios G. Eliopoulos in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 751-780 Titre : Marble-hosted submicroscopic gold mineralization at asimotrypes area, mount Pangeon, southern Rhodope core complex, Greece Type de document : texte imprimé Auteurs : Demetrios G. Eliopoulos, Auteur ; Stephanos P. Kilias, Auteur Année de publication : 2011 Article en page(s) : pp. 751-780 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Marble  Gold  mineralization  Greece Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Gold mineralization at Asimotrypes, Mount Pangeon, Greece, occurs within amphibolite facies rocks of the Southern Rhodope Core Complex, one of the largest metamorphic core complexes in the world. Exhumation of the complex resulted from middle Eocene to middle Miocene northeast-southwest–oriented extension in the northern Aegean and was controlled by the Kerdylion detachment zone. Host rocks are mylonitic, impure dolomite marbles of marine provenance (δ13C = 1.9 – 2.9‰), which are intercalated with paramica schists, and amphibolites, and intruded by early Miocene syntectonic granitoids. In the Asimotrypes area, metamorphic rocks and granitoids exhibit flat mylonite-type ductile fabrics with consistent top-to-the-southwest sense of shear, as does the entire Complex. Two mineralogically similar and spatially coincident gold mineralization styles, with a supergene overprint, exist: (1) strata-bound replacement bodies that contain up to 17 ppm gold, concentrated at and controlled by the intersections of several hydrothermally altered top-to-the-southwest marble mylonites, with crosscutting northwest-southeast to east-west–trending high-angle brittle normal faults and fractures, and (2) structurally controlled quartz veins, pods, and lenses with 4 to 13 ppm gold, which occur along the northwest-oriented subvertical brittle structures; the latter crosscut the ductile shear foliation of the host rocks, together with the granitoids, and dip steeply to the southwest. Gold-bearing replacement bodies typically occur as discordant wide halo adjacent to the centrally located brittle structures. Ductile top-to-the-southwest shear zones and cross-cutting brittle structures are considered contemporaneous within the regional extensional deformation and exhumation history of the core complex host rocks. The distribution of gold mineralization is related to the geometry of the brittle structures, strongly suggesting that faults acted as major fluid feeder conduits during gold mineralization. Hydrothermal alteration associated with the auriferous bodies and veins, overprinting the metamorphic rocks at Asimotrypes, consists of quartz, muscovite (sericite), chlorite, calcite (dedolomite), and sulfide minerals (locally, as much as 35 vol %). Gangue quartz occurs as (1) peripheral banded quartz with mylonitic texture, (2) dominant gold-bearing jasperoidal quartz, and (3) late, fine drusy quartz. The gold assemblage consists of arsenopyrite (41.7–43 wt % As; electron microprobe analyses) and arsenian pyrite encompassed by jasperoidal quartz; chalcopyrite, galena, tetrahedrite-tennantite, and sphalerite are trace phases. Secondary ion mass spectrometry (SIMS) spot analyses revealed that gold in replacement ore is submicroscopic and occurs in arsenopyrite and arsenian pyrite (As = 0.01–3.8 wt %), ranging from 0.5 to 29 ppm (mean = 11.6 ppm), and 0.14 to 11 ppm (mean = 2.3 ppm), respectively. Supergene overprint includes colloidal goethite + hematite containing native gold grains (size range, 20–40 μm) and lesser covellite, malachite, cerrussite, and chrysocolla. SIMS analysis also shows a primary, inhomogeneous distribution pattern for gold that appears to mimic finely banded growth twinning lamellae or growth zones. The form of the vein-related gold is unknown. Fluid inclusions trapped during gold mineralization are low to medium salinity (<10 wt % NaCl equiv), aqueous-carbonic H2O-CO2-NaCl inclusions with highly variable carbonic to aqueous contents and naturally decrepitated or reequilibrated inclusions, interpreted to represent decompression by concurrent exhumation of the Southern Rhodope Core Complex. Trail-bound aqueous H2O-NaCl inclusions are interpreted as a late extensional brittle deformation-related hydrothermal event unrelated to gold-bearing sulfide precipitation. Trapping temperatures of the H2O-CO2-NaCl inclusions and oxygen isotope equilibrium temperatures between quartz and muscovite (sericite) indicate formation of gold-bearing arsenopyrite, arsenian pyrite, and quartz bodies at ~270°C, with pressures of 1,800 to 2,000 bars corresponding to depths between 6 and 9 km under lithostatic load. Measured hydrogen and calculated oxygen isotope compositions of the aqueous-carbonic mineralizing fluids (δDwater = −125 to −105‰, δ18Owater = 11.9 to 13.7 ‰ at 250°C, and 13.9 and 15.7 ‰ at 300°C) indicate an evolved meteoric origin. Water/rock calculations indicate these values could derive from heated meteoric water that had undergone isotope exchange during circulation through marbles at low water/rock ratio. Gold-associated sulfide minerals have δ34S values between 2.2 and 3.1 per mil, interpreted as leaching of the core complex Paleozoic marine metasedimentary (marble), and interlayered mafic metavolcanic (amphibolites) and Miocene granitic rocks through fluid/rock interaction. Gold was probably precipitated from an H2S-rich fluid when the host sulfide minerals were stabilized by pyritization of marble wall rock. Asimotrypes gold mineralization is interpreted as postmetamorphic and postmagmatic, and linked to late stages of exhumation of the Complex by middle Eocene to middle Miocene extension in the northern Aegean. The location and shape of gold orebodies are controlled by interactive ductile-then-brittle deformation, fluid flow and fluid-marble interaction processes. High paleogeothermal gradients associated with extension may have driven the dominantly meteoric fluid circulation downward within the exhuming Complex. Asimotrypes may represent a potential new gold mineralization style for the Rhodope region. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/751.abstract [article] Marble-hosted submicroscopic gold mineralization at asimotrypes area, mount Pangeon, southern Rhodope core complex, Greece [texte imprimé] / Demetrios G. Eliopoulos, Auteur ; Stephanos P. Kilias, Auteur . - 2011 . - pp. 751-780. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 751-780 Mots-clés : Marble  Gold  mineralization  Greece Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Gold mineralization at Asimotrypes, Mount Pangeon, Greece, occurs within amphibolite facies rocks of the Southern Rhodope Core Complex, one of the largest metamorphic core complexes in the world. Exhumation of the complex resulted from middle Eocene to middle Miocene northeast-southwest–oriented extension in the northern Aegean and was controlled by the Kerdylion detachment zone. Host rocks are mylonitic, impure dolomite marbles of marine provenance (δ13C = 1.9 – 2.9‰), which are intercalated with paramica schists, and amphibolites, and intruded by early Miocene syntectonic granitoids. In the Asimotrypes area, metamorphic rocks and granitoids exhibit flat mylonite-type ductile fabrics with consistent top-to-the-southwest sense of shear, as does the entire Complex. Two mineralogically similar and spatially coincident gold mineralization styles, with a supergene overprint, exist: (1) strata-bound replacement bodies that contain up to 17 ppm gold, concentrated at and controlled by the intersections of several hydrothermally altered top-to-the-southwest marble mylonites, with crosscutting northwest-southeast to east-west–trending high-angle brittle normal faults and fractures, and (2) structurally controlled quartz veins, pods, and lenses with 4 to 13 ppm gold, which occur along the northwest-oriented subvertical brittle structures; the latter crosscut the ductile shear foliation of the host rocks, together with the granitoids, and dip steeply to the southwest. Gold-bearing replacement bodies typically occur as discordant wide halo adjacent to the centrally located brittle structures. Ductile top-to-the-southwest shear zones and cross-cutting brittle structures are considered contemporaneous within the regional extensional deformation and exhumation history of the core complex host rocks. The distribution of gold mineralization is related to the geometry of the brittle structures, strongly suggesting that faults acted as major fluid feeder conduits during gold mineralization. Hydrothermal alteration associated with the auriferous bodies and veins, overprinting the metamorphic rocks at Asimotrypes, consists of quartz, muscovite (sericite), chlorite, calcite (dedolomite), and sulfide minerals (locally, as much as 35 vol %). Gangue quartz occurs as (1) peripheral banded quartz with mylonitic texture, (2) dominant gold-bearing jasperoidal quartz, and (3) late, fine drusy quartz. The gold assemblage consists of arsenopyrite (41.7–43 wt % As; electron microprobe analyses) and arsenian pyrite encompassed by jasperoidal quartz; chalcopyrite, galena, tetrahedrite-tennantite, and sphalerite are trace phases. Secondary ion mass spectrometry (SIMS) spot analyses revealed that gold in replacement ore is submicroscopic and occurs in arsenopyrite and arsenian pyrite (As = 0.01–3.8 wt %), ranging from 0.5 to 29 ppm (mean = 11.6 ppm), and 0.14 to 11 ppm (mean = 2.3 ppm), respectively. Supergene overprint includes colloidal goethite + hematite containing native gold grains (size range, 20–40 μm) and lesser covellite, malachite, cerrussite, and chrysocolla. SIMS analysis also shows a primary, inhomogeneous distribution pattern for gold that appears to mimic finely banded growth twinning lamellae or growth zones. The form of the vein-related gold is unknown. Fluid inclusions trapped during gold mineralization are low to medium salinity (<10 wt % NaCl equiv), aqueous-carbonic H2O-CO2-NaCl inclusions with highly variable carbonic to aqueous contents and naturally decrepitated or reequilibrated inclusions, interpreted to represent decompression by concurrent exhumation of the Southern Rhodope Core Complex. Trail-bound aqueous H2O-NaCl inclusions are interpreted as a late extensional brittle deformation-related hydrothermal event unrelated to gold-bearing sulfide precipitation. Trapping temperatures of the H2O-CO2-NaCl inclusions and oxygen isotope equilibrium temperatures between quartz and muscovite (sericite) indicate formation of gold-bearing arsenopyrite, arsenian pyrite, and quartz bodies at ~270°C, with pressures of 1,800 to 2,000 bars corresponding to depths between 6 and 9 km under lithostatic load. Measured hydrogen and calculated oxygen isotope compositions of the aqueous-carbonic mineralizing fluids (δDwater = −125 to −105‰, δ18Owater = 11.9 to 13.7 ‰ at 250°C, and 13.9 and 15.7 ‰ at 300°C) indicate an evolved meteoric origin. Water/rock calculations indicate these values could derive from heated meteoric water that had undergone isotope exchange during circulation through marbles at low water/rock ratio. Gold-associated sulfide minerals have δ34S values between 2.2 and 3.1 per mil, interpreted as leaching of the core complex Paleozoic marine metasedimentary (marble), and interlayered mafic metavolcanic (amphibolites) and Miocene granitic rocks through fluid/rock interaction. Gold was probably precipitated from an H2S-rich fluid when the host sulfide minerals were stabilized by pyritization of marble wall rock. Asimotrypes gold mineralization is interpreted as postmetamorphic and postmagmatic, and linked to late stages of exhumation of the Complex by middle Eocene to middle Miocene extension in the northern Aegean. The location and shape of gold orebodies are controlled by interactive ductile-then-brittle deformation, fluid flow and fluid-marble interaction processes. High paleogeothermal gradients associated with extension may have driven the dominantly meteoric fluid circulation downward within the exhuming Complex. Asimotrypes may represent a potential new gold mineralization style for the Rhodope region. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/751.abstract

Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada / Darren LeFort in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 781-808 Titre : Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada Type de document : texte imprimé Auteurs : Darren LeFort, Auteur ; Jacob Hanley, Auteur ; Marcel Guillong, Auteur Année de publication : 2011 Article en page(s) : pp. 781-808 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Prophyry  deposits  Alkalic  porphyry  Cu-Au  mineralization  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu (<740 ppm; rarely > 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/781.abstract [article] Subepithermal Au-Pd mineralization associated with an alkalic porphyry Cu-Au deposit, mount Milligan, Quesnel Terrane, British Columbia, Canada [texte imprimé] / Darren LeFort, Auteur ; Jacob Hanley, Auteur ; Marcel Guillong, Auteur . - 2011 . - pp. 781-808. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 781-808 Mots-clés : Prophyry  deposits  Alkalic  porphyry  Cu-Au  mineralization  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu (<740 ppm; rarely > 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/781.abstract

Geochemical exploration for gold through transported alluvial cover in Nevada / John Muntean in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 809-833 Titre : Geochemical exploration for gold through transported alluvial cover in Nevada : examples from the cortez mine Type de document : texte imprimé Auteurs : John Muntean, Auteur ; Paul Taufen, Auteur Année de publication : 2011 Article en page(s) : pp. 809-833 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Gold  mining  Geochemical  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and “footprints” associated with deposits at the district scale. Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6- to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the −80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore. Assays from 1,859 drill holes show a 4- to 5-km2 “footprint” at the basin gravel-bedrock unconformity centered on the Pipeline Carlin-type gold deposit, where samples of basal alluvium provide a large, coherent >50 ppb Au anomaly. Enriched As, Tl, K, and F in alkaline groundwater sampled from monitoring wells surrounding the Pipeline open pit provide a ≥5-km2 hydrogeochemical footprint. Higher As and Tl concentrations occur down gradient from Pipeline indicating Carlin-type Au mineralization is the source of the enrichment. The surface metal anomalies are consistent with upward migration of metals through fractured alluvial cover. Likely metal transport mechanisms include barometric pumping of gases or seismic pumping of groundwater. Upward diffusion of metals through the thick vadose zones in northern Nevada is not a viable process. Surface anomalies over Gap and other deposits appear to be mature and may have developed over millions of years. In such mature anomalies, much of the metal that migrated from underlying bedrock is probably hosted in resistant secondary minerals, which are more readily dissolved by aqua regia than by various selective leaches. Sampling of soil gas and soils is appropriate at the drill target scale. Vegetation should be sampled when consistent soils are not available. Sample spacing should be adjusted according to “real-time” soil gas readings in order to increase sampling density over fracture zones. The patterns of >50 ppb Au for both the top of bedrock and base of alluvium at Pipeline provide useful templates for comparison with other drilling programs through cover. Groundwater is an effective and under-utilized reconnaissance-scale sample medium. Gold is likely to be soluble in neutral to alkaline groundwater in Nevada, and could provide a direct indicator of blind covered ore. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/809.abstract [article] Geochemical exploration for gold through transported alluvial cover in Nevada : examples from the cortez mine [texte imprimé] / John Muntean, Auteur ; Paul Taufen, Auteur . - 2011 . - pp. 809-833. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 809-833 Mots-clés : Gold  mining  Geochemical  exploration  Nevada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and “footprints” associated with deposits at the district scale. Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6- to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the −80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore. Assays from 1,859 drill holes show a 4- to 5-km2 “footprint” at the basin gravel-bedrock unconformity centered on the Pipeline Carlin-type gold deposit, where samples of basal alluvium provide a large, coherent >50 ppb Au anomaly. Enriched As, Tl, K, and F in alkaline groundwater sampled from monitoring wells surrounding the Pipeline open pit provide a ≥5-km2 hydrogeochemical footprint. Higher As and Tl concentrations occur down gradient from Pipeline indicating Carlin-type Au mineralization is the source of the enrichment. The surface metal anomalies are consistent with upward migration of metals through fractured alluvial cover. Likely metal transport mechanisms include barometric pumping of gases or seismic pumping of groundwater. Upward diffusion of metals through the thick vadose zones in northern Nevada is not a viable process. Surface anomalies over Gap and other deposits appear to be mature and may have developed over millions of years. In such mature anomalies, much of the metal that migrated from underlying bedrock is probably hosted in resistant secondary minerals, which are more readily dissolved by aqua regia than by various selective leaches. Sampling of soil gas and soils is appropriate at the drill target scale. Vegetation should be sampled when consistent soils are not available. Sample spacing should be adjusted according to “real-time” soil gas readings in order to increase sampling density over fracture zones. The patterns of >50 ppb Au for both the top of bedrock and base of alluvium at Pipeline provide useful templates for comparison with other drilling programs through cover. Groundwater is an effective and under-utilized reconnaissance-scale sample medium. Gold is likely to be soluble in neutral to alkaline groundwater in Nevada, and could provide a direct indicator of blind covered ore. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/809.abstract

The sandstone-hosted beverley uranium deposit, lake frome basin, south Australia / Pierre-Alain Wülser in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 835-867 Titre : The sandstone-hosted beverley uranium deposit, lake frome basin, south Australia : mineralogy, geochemistry, and a time-constrained model for Its genesis Type de document : texte imprimé Auteurs : Pierre-Alain Wülser, Auteur ; Joël Brugger, Auteur ; John Foden, Auteur Année de publication : 2011 Article en page(s) : pp. 835-867 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Uranium  deposits  Mineralogy  Geochemistry  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The sandstone-hosted Beverley uranium deposit is located in terrestrial sediments in the Lake Frome basin in the North Flinders Ranges, South Australia. The deposit is 13 km from the U-rich Mesoproterozoic basement of the Mount Painter inlier, which is being uplifted 100 to 200 m above the basin by neotectonic activity that probably initiated in the early Pliocene. The mineralization was deposited mainly in organic matter-poor Miocene lacustrine sands and partly in the underlying reductive strata comprising organic matter-rich clays and silts. The bulk of the mineralization consists of coffinite and/or uraninite nodules, growing around Co-rich pyrite with an S isotope composition (δ34S = 1.0 ± 0.3‰), suggestive of an early diagenetic lacustrine origin. In contrast, authigenic sulfides in the bulk of the sediments have a negative S isotope signature (δ34S ranges from −26.2 to −35.5‰), indicative of an origin via bacterially mediated sulfate reduction. Minor amounts of Zn-bearing native copper and native lead also support the presence of specific, reducing microenvironments in the ore zone. Small amounts of carnotite are associated with the coffinite ore and also occur beneath a paleosoil horizon overlying the uranium deposit. Provenance studies suggest that the host Miocene sediments were derived from the reworking of Early Cretaceous glacial or glaciolacustrine sediments ultimately derived from Paleozoic terranes in eastern Australia. In contrast, the overlying Pliocene strata were in part derived from the Mesoproterozoic basement inlier. Mass-balance and geochemical data confirm that granites of the Mount Painter domain were the ultimate source of U and REE at Beverley. U-Pb dating of coffinite and carnotite suggest that the U mineralization is Pliocene (6.7-3.4 Ma). The suitability of the Beverley deposit for efficient mining via in situ leaching, and hence its economic value, are determined by the nature of the hosting sand unit, which provides the permeability and low reactivity required for high fluid flow and low chemical consumption. These favorable sedimentologic and geometrical features result from a complex conjunction of factors, including deposition in lacustrine shore environment, reworking of angular sands of glacial origin, deep Pliocene weathering, and proximity to an active fault exposing extremely U rich rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/835.abstract [article] The sandstone-hosted beverley uranium deposit, lake frome basin, south Australia : mineralogy, geochemistry, and a time-constrained model for Its genesis [texte imprimé] / Pierre-Alain Wülser, Auteur ; Joël Brugger, Auteur ; John Foden, Auteur . - 2011 . - pp. 835-867. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 835-867 Mots-clés : Uranium  deposits  Mineralogy  Geochemistry  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The sandstone-hosted Beverley uranium deposit is located in terrestrial sediments in the Lake Frome basin in the North Flinders Ranges, South Australia. The deposit is 13 km from the U-rich Mesoproterozoic basement of the Mount Painter inlier, which is being uplifted 100 to 200 m above the basin by neotectonic activity that probably initiated in the early Pliocene. The mineralization was deposited mainly in organic matter-poor Miocene lacustrine sands and partly in the underlying reductive strata comprising organic matter-rich clays and silts. The bulk of the mineralization consists of coffinite and/or uraninite nodules, growing around Co-rich pyrite with an S isotope composition (δ34S = 1.0 ± 0.3‰), suggestive of an early diagenetic lacustrine origin. In contrast, authigenic sulfides in the bulk of the sediments have a negative S isotope signature (δ34S ranges from −26.2 to −35.5‰), indicative of an origin via bacterially mediated sulfate reduction. Minor amounts of Zn-bearing native copper and native lead also support the presence of specific, reducing microenvironments in the ore zone. Small amounts of carnotite are associated with the coffinite ore and also occur beneath a paleosoil horizon overlying the uranium deposit. Provenance studies suggest that the host Miocene sediments were derived from the reworking of Early Cretaceous glacial or glaciolacustrine sediments ultimately derived from Paleozoic terranes in eastern Australia. In contrast, the overlying Pliocene strata were in part derived from the Mesoproterozoic basement inlier. Mass-balance and geochemical data confirm that granites of the Mount Painter domain were the ultimate source of U and REE at Beverley. U-Pb dating of coffinite and carnotite suggest that the U mineralization is Pliocene (6.7-3.4 Ma). The suitability of the Beverley deposit for efficient mining via in situ leaching, and hence its economic value, are determined by the nature of the hosting sand unit, which provides the permeability and low reactivity required for high fluid flow and low chemical consumption. These favorable sedimentologic and geometrical features result from a complex conjunction of factors, including deposition in lacustrine shore environment, reworking of angular sands of glacial origin, deep Pliocene weathering, and proximity to an active fault exposing extremely U rich rocks. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/835.abstract

Textural evidence for extensive melting of the broken hill orebody / B. Ronald Frost in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 869-882 Titre : Textural evidence for extensive melting of the broken hill orebody Type de document : texte imprimé Auteurs : B. Ronald Frost, Auteur ; Susan M. Swapp, Auteur ; John Mavrogenes, Auteur Année de publication : 2011 Article en page(s) : pp. 869-882 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Broken  hill  orebody Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Galena, pyrrhotite, and chalcopyrite in samples of Zn-rich ore from the Broken Hill orebody show textures similar to those in partially melted metamorphic rocks, including strings of low melting phases separating lithons of minerals likely to be restite, low dihedral angles between low melting-temperature minerals and restitic minerals, and enlargement of these dihedral angles due to partial annealing. Analysis of existing experimental work from the system Fe-Pb-Zn-S indicates that at 1 bar, galena and sphalerite coexisting with pyrrhotite of the compositions found at Broken Hill would have been partially melted at temperatures of about 750°C; at 5 kbars, this melting would occur at 780°C. This temperature is well within the range of metamorphic temperatures reported from the area. From these data we conclude that the Broken Hill orebody was partially molten at peak metamorphism, with much of the Pb, Cu, Ag, Sb, and As in the melt and Fe and Zn residing mostly in the restite. Differentiation during cooling enriched the residual melt in Sb, As, Ag, and other low melting-temperature chalcophile elements. The main sulfide melt froze after the last penetrative deformation to affect the orebody, at temperatures below 720°C and possibly as low as 650°C. Sulfosalts crystallized out of the remaining melt at temperatures of 600° to 450°C, and the final melt crystallized native bismuth at ca. 250°C. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/869.abstract [article] Textural evidence for extensive melting of the broken hill orebody [texte imprimé] / B. Ronald Frost, Auteur ; Susan M. Swapp, Auteur ; John Mavrogenes, Auteur . - 2011 . - pp. 869-882. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 869-882 Mots-clés : Broken  hill  orebody Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Galena, pyrrhotite, and chalcopyrite in samples of Zn-rich ore from the Broken Hill orebody show textures similar to those in partially melted metamorphic rocks, including strings of low melting phases separating lithons of minerals likely to be restite, low dihedral angles between low melting-temperature minerals and restitic minerals, and enlargement of these dihedral angles due to partial annealing. Analysis of existing experimental work from the system Fe-Pb-Zn-S indicates that at 1 bar, galena and sphalerite coexisting with pyrrhotite of the compositions found at Broken Hill would have been partially melted at temperatures of about 750°C; at 5 kbars, this melting would occur at 780°C. This temperature is well within the range of metamorphic temperatures reported from the area. From these data we conclude that the Broken Hill orebody was partially molten at peak metamorphism, with much of the Pb, Cu, Ag, Sb, and As in the melt and Fe and Zn residing mostly in the restite. Differentiation during cooling enriched the residual melt in Sb, As, Ag, and other low melting-temperature chalcophile elements. The main sulfide melt froze after the last penetrative deformation to affect the orebody, at temperatures below 720°C and possibly as low as 650°C. Sulfosalts crystallized out of the remaining melt at temperatures of 600° to 450°C, and the final melt crystallized native bismuth at ca. 250°C. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/869.abstract

Derivation of S and Pb in phanerozoic intrusion-related metal deposits from neoproterozoic sedimentary pyrite, Great Basin, United States / Peter G. Vikre in Economic geology, Vol. 106 N° 5 (Août 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 883-912 Titre : Derivation of S and Pb in phanerozoic intrusion-related metal deposits from neoproterozoic sedimentary pyrite, Great Basin, United States Type de document : texte imprimé Auteurs : Peter G. Vikre, Auteur ; Simon R. Poulson, Auteur ; Alan E. Koenig, Auteur Année de publication : 2011 Article en page(s) : pp. 883-912 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Neoproterozoic  intrusion  Isotrope  Sedimentary  pyrite  Metal  deposits  United-States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The thick (≤8 km), regionally extensive section of Neoproterozoic siliciclastic strata (terrigenous detrital succession, TDS) in the central and eastern Great Basin contains sedimentary pyrite characterized by mostly high δ34S values (−11.6 to 40.8‰, >70% exceed 10‰; 51 analyses) derived from reduction of seawater sulfate, and by markedly radiogenic Pb isotopes (207Pb/204Pb >19.2; 15 analyses) acquired from clastic detritus eroded from Precambrian cratonal rocks to the east-southeast. In the overlying Paleozoic section, Pb-Zn-Cu-Ag-Au deposits associated with Jurassic, Cretaceous, and Tertiary granitic intrusions (intrusion-related metal deposits) contain galena and other sulfide minerals with S and Pb isotope compositions similar to those of TDS sedimentary pyrite, consistent with derivation of deposit S and Pb from TDS pyrite. Minor element abundances in TDS pyrite (e.g., Pb, Zn, Cu, Ag, and Au) compared to sedimentary and hydrothermal pyrite elsewhere are not noticeably elevated, implying that enrichment in source minerals is not a precondition for intrusion-related metal deposits. Three mechanisms for transferring components of TDS sedimentary pyrite to intrusion-related metal deposits are qualitatively evaluated. One mechanism involves (1) decomposition of TDS pyrite in thermal aureoles of intruding magmas, and (2) aqueous transport and precipitation in thermal or fluid mixing gradients of isotopically heavy S, radiogenic Pb, and possibly other sedimentary pyrite and detrital mineral components, as sulfide minerals in intrusion-related metal deposits. A second mechanism invokes mixing and S isotope exchange in thermal aureoles of Pb and S exsolved from magma and derived from decomposition of sedimentary pyrite. A third mechanism entails melting of TDS strata or assimilation of TDS strata by crustal or mantle magmas. TDS-derived or assimilated magmas ascend, decompress, and exsolve a mixture of TDS volatiles, including isotopically heavy S and radiogenic Pb from sedimentary pyrite, and volatiles acquired from deeper crustal or mantle sources. In the central and eastern Great Basin, the wide distribution and high density of small to mid-sized vein, replacement, and skarn intrusion-related metal deposits in lower Paleozoic rocks that contain TDS sedimentary pyrite S and Pb reflect (1) prolific Jurassic, Cretaceous, and Tertiary magmatism, (2) a regional, substrate reservoir of S and Pb in permeable and homogeneous siliciclastic strata, and (3) relatively small scale concentration of substrate and magmatic components. Large intrusion-related metal deposits in the central and eastern Great Basin acquired S and most Pb from thicker lithospheric sections. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/883.abstract [article] Derivation of S and Pb in phanerozoic intrusion-related metal deposits from neoproterozoic sedimentary pyrite, Great Basin, United States [texte imprimé] / Peter G. Vikre, Auteur ; Simon R. Poulson, Auteur ; Alan E. Koenig, Auteur . - 2011 . - pp. 883-912. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 5 (Août 2011) . - pp. 883-912 Mots-clés : Neoproterozoic  intrusion  Isotrope  Sedimentary  pyrite  Metal  deposits  United-States Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The thick (≤8 km), regionally extensive section of Neoproterozoic siliciclastic strata (terrigenous detrital succession, TDS) in the central and eastern Great Basin contains sedimentary pyrite characterized by mostly high δ34S values (−11.6 to 40.8‰, >70% exceed 10‰; 51 analyses) derived from reduction of seawater sulfate, and by markedly radiogenic Pb isotopes (207Pb/204Pb >19.2; 15 analyses) acquired from clastic detritus eroded from Precambrian cratonal rocks to the east-southeast. In the overlying Paleozoic section, Pb-Zn-Cu-Ag-Au deposits associated with Jurassic, Cretaceous, and Tertiary granitic intrusions (intrusion-related metal deposits) contain galena and other sulfide minerals with S and Pb isotope compositions similar to those of TDS sedimentary pyrite, consistent with derivation of deposit S and Pb from TDS pyrite. Minor element abundances in TDS pyrite (e.g., Pb, Zn, Cu, Ag, and Au) compared to sedimentary and hydrothermal pyrite elsewhere are not noticeably elevated, implying that enrichment in source minerals is not a precondition for intrusion-related metal deposits. Three mechanisms for transferring components of TDS sedimentary pyrite to intrusion-related metal deposits are qualitatively evaluated. One mechanism involves (1) decomposition of TDS pyrite in thermal aureoles of intruding magmas, and (2) aqueous transport and precipitation in thermal or fluid mixing gradients of isotopically heavy S, radiogenic Pb, and possibly other sedimentary pyrite and detrital mineral components, as sulfide minerals in intrusion-related metal deposits. A second mechanism invokes mixing and S isotope exchange in thermal aureoles of Pb and S exsolved from magma and derived from decomposition of sedimentary pyrite. A third mechanism entails melting of TDS strata or assimilation of TDS strata by crustal or mantle magmas. TDS-derived or assimilated magmas ascend, decompress, and exsolve a mixture of TDS volatiles, including isotopically heavy S and radiogenic Pb from sedimentary pyrite, and volatiles acquired from deeper crustal or mantle sources. In the central and eastern Great Basin, the wide distribution and high density of small to mid-sized vein, replacement, and skarn intrusion-related metal deposits in lower Paleozoic rocks that contain TDS sedimentary pyrite S and Pb reflect (1) prolific Jurassic, Cretaceous, and Tertiary magmatism, (2) a regional, substrate reservoir of S and Pb in permeable and homogeneous siliciclastic strata, and (3) relatively small scale concentration of substrate and magmatic components. Large intrusion-related metal deposits in the central and eastern Great Basin acquired S and most Pb from thicker lithospheric sections. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/5/883.abstract

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