Dépouillements

A group of papers on porphyry Cu deposits / Lawrence D. Meinert in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 1-2 Titre : A group of papers on porphyry Cu deposits : introduction Type de document : texte imprimé Auteurs : Lawrence D. Meinert, Auteur Année de publication : 2011 Article en page(s) : pp. 1-2 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Porphyry  Cu  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/1.extract [article] A group of papers on porphyry Cu deposits : introduction [texte imprimé] / Lawrence D. Meinert, Auteur . - 2011 . - pp. 1-2. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 1-2 Mots-clés : Porphyry  Cu  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/1.extract

Porphyry copper systems / Richard H. Sillitoe in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 3-41 Titre : Porphyry copper systems Type de document : texte imprimé Auteurs : Richard H. Sillitoe, Auteur Année de publication : 2011 Article en page(s) : pp. 3-41 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Copper  Volcanic  connections Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/3.abstract [article] Porphyry copper systems [texte imprimé] / Richard H. Sillitoe, Auteur . - 2011 . - pp. 3-41. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 3-41 Mots-clés : Copper  Volcanic  connections Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/3.abstract

The Bingham canyon porphyry Cu-Mo-Au deposit. I. Sequence of intrusions, vein formation, and sulfide deposition / Patrick B. Redmond in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 43-68 Titre : The Bingham canyon porphyry Cu-Mo-Au deposit. I. Sequence of intrusions, vein formation, and sulfide deposition Type de document : texte imprimé Auteurs : Patrick B. Redmond, Auteur ; Marco T. Einaudi, Auteur Année de publication : 2011 Article en page(s) : pp. 43-68 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Mo-Au  deposit  Sulfide  mineralization  Igneous  rocks Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz mon-zonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/43.abstract [article] The Bingham canyon porphyry Cu-Mo-Au deposit. I. Sequence of intrusions, vein formation, and sulfide deposition [texte imprimé] / Patrick B. Redmond, Auteur ; Marco T. Einaudi, Auteur . - 2011 . - pp. 43-68. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 43-68 Mots-clés : Cu-Mo-Au  deposit  Sulfide  mineralization  Igneous  rocks Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz mon-zonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast–trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast– and northwest-striking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/43.abstract

The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension / Gillian Gruen in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 69-90 Titre : The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension Type de document : texte imprimé Auteurs : Gillian Gruen, Auteur ; Christoph A. Heinrich, Auteur ; Kim Schroeder, Auteur Année de publication : 2011 Article en page(s) : pp. 69-90 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Mo-Au  deposit  Vein  geometry  Porphyry  ore  shell Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/69.abstract [article] The Bingham canyon porphyry Cu-Mo-Au deposit. II. vein geometry and ore shell formation by pressure-driven rock extension [texte imprimé] / Gillian Gruen, Auteur ; Christoph A. Heinrich, Auteur ; Kim Schroeder, Auteur . - 2011 . - pp. 69-90. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 69-90 Mots-clés : Cu-Mo-Au  deposit  Vein  geometry  Porphyry  ore  shell Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/69.abstract

The Bingham canyon porphyry Cu-Mo-Au deposit. III. Zoned copper-gold ore deposition by magmatic vapor expansion / Marianne R. Landtwing in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 91-118 Titre : The Bingham canyon porphyry Cu-Mo-Au deposit. III. Zoned copper-gold ore deposition by magmatic vapor expansion Type de document : texte imprimé Auteurs : Marianne R. Landtwing, Auteur ; Caroline Furrer, Auteur ; Patrick B. Redmond, Auteur Année de publication : 2011 Article en page(s) : pp. 91-118 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Cu-Mo-Au  deposit  Gold  deposit  Magmatic  vapor  Fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/91.abstract [article] The Bingham canyon porphyry Cu-Mo-Au deposit. III. Zoned copper-gold ore deposition by magmatic vapor expansion [texte imprimé] / Marianne R. Landtwing, Auteur ; Caroline Furrer, Auteur ; Patrick B. Redmond, Auteur . - 2011 . - pp. 91-118. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 91-118 Mots-clés : Cu-Mo-Au  deposit  Gold  deposit  Magmatic  vapor  Fluid  inclusion Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/91.abstract

Multistage intrusion, brecciation, and veining at El Teniente, Chile / V. H. Vry in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 119-153 Titre : Multistage intrusion, brecciation, and veining at El Teniente, Chile : evolution of a nested porphyry system Type de document : texte imprimé Auteurs : V. H. Vry, Auteur ; J. J. Wilkinson, Auteur ; J. Seguel, Auteur Année de publication : 2011 Article en page(s) : pp. 119-153 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Porphyry  system  Copper-molybdenum  deposit Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Mafic Complex. These are spatially associated with magmatic-hydrothermal breccias and multiple vein types that form individual mineralized complexes. Here we present detailed observations on mineralogy, textures, and intrusion, breccias, and vein crosscut-ting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided into three main stages: (1) premineralization biotite and/or K-feldspar ± quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins ± potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (1) igneous-cemented breccias, (2) K-feldspar-cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display a similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1–2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vein types (types 3–8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9–10) that represent a single event linked to the emplacement of the Braden Breccia Pipe. The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow level, felsic-intermediate pipelike intrusions and the development of igneous and mineralized magmatic-hydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in that the Teniente Mafic Complex provided a particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and as an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/119.abstract [article] Multistage intrusion, brecciation, and veining at El Teniente, Chile : evolution of a nested porphyry system [texte imprimé] / V. H. Vry, Auteur ; J. J. Wilkinson, Auteur ; J. Seguel, Auteur . - 2011 . - pp. 119-153. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 119-153 Mots-clés : Porphyry  system  Copper-molybdenum  deposit Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Mafic Complex. These are spatially associated with magmatic-hydrothermal breccias and multiple vein types that form individual mineralized complexes. Here we present detailed observations on mineralogy, textures, and intrusion, breccias, and vein crosscut-ting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided into three main stages: (1) premineralization biotite and/or K-feldspar ± quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins ± potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (1) igneous-cemented breccias, (2) K-feldspar-cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display a similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1–2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vein types (types 3–8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9–10) that represent a single event linked to the emplacement of the Braden Breccia Pipe. The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow level, felsic-intermediate pipelike intrusions and the development of igneous and mineralized magmatic-hydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in that the Teniente Mafic Complex provided a particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and as an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/119.abstract

Evolution of the giant marcona-Mina Justa iron oxide-copper-gold district, south-central Peru / Huayong Chen in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 155-185 Titre : Evolution of the giant marcona-Mina Justa iron oxide-copper-gold district, south-central Peru Type de document : texte imprimé Auteurs : Huayong Chen, Auteur ; Alan H. Clark, Auteur ; T. Kurtis Kyser, Auteur Année de publication : 2011 Article en page(s) : pp. 155-185 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Iron  oxide  Copper  district  Gold  district  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Perú, centered at latitude 15°11′ S, longitude 75°6′ W, incorporates Marcona, the preeminent central Andean iron oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one of the few major Andean IOCG deposits with economic copper grades (346.6 Mt @ 0.71% Cu). The emplacement of magnetite orebodies with uneconomic Cu grades (avg 0.12%) at Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine Middle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene Cu sulfide (~15 g/t Ag, 0.12 g/t Au) mineralization at Mina Justa was emplaced along reactivated listric-normal detachment faults during the mid-Cretaceous inversion of the contiguous, plate boundary-parallel, Aptian to Albian Cañete basin, accompanied by the earliest, largely granodioritic-dioritic, stocks of the Coastal batholith. Alteration and mineralization assemblages, supported by 40Ar/39Ar geochronology of biotite, phlogopite, actinolite, cummingtonite, and K-feldspars, reveal a history of magmatic and hydrothermal processes extending episodically for at least 80 m.y., from ca. 177 to 95 Ma, wherein metal-rich mineralization events were preceded and separated by episodes of barren alteration. At Marcona, precursor, subocean-floor hydrothermal activity in the Aalenian (177 Ma) and Bajocian (171 Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperature Mg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocks and minor carbonate units underlying the nascent Río Grande Formation arc. Subsequent areally widespread, albite-marialite alteration (Na-Cl metasomatism) largely predated but overlapped with the emplacement of an en echelon swarm of massive magnetite orebodies, in turn overprinted by subordinate magnetite-sulfide assemblages. Magnetite and weak Cu and Zn sulfide mineralization coincided with a 156 to 162 Ma episode of andesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely by quartz-rich metaclastic rocks. From 162 to 159 Ma, iron oxide mineralization evolved from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite assemblages. These were overprinted at 156 to 159 Ma by chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, locally resulting in grades of 0.45 percent Cu and 0.5 percent Zn. Hydrothermal activity was thereafter focused in the Mina Justa area, 3 to 4 km to the northeast of Marcona, where Middle Jurassic andesites experienced intense albite-actinolite alteration at ca. 157 Ma, i.e., contemporaneous with sulfide mineralization at Marcona, and magnetite-microcline alteration (K-Fe metasomatism) at ca. 142 Ma. Development of the Mina Justa Cu (-Ag) deposit proper, however, began much later, with, successively, actinolitization at ca. 109 Ma, the deposition of calcite and specular hematite, now entirely pseudo-morphed by magnetite, and the metasomatic emplacement of bodies of barren, massive magnetite and pyrite at 101 to 104 Ma. Finally, at 95 to 99 Ma, chalcopyrite-bornite-digenite-chalcocite mineralization, with abundant calcite and hematite, was emplaced as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays of breccia and stockwork, cored by preexisting magnetite-pyrite lenses. Supergene oxidation generated a chryso-colla-atacamite-covellite blanket, hosting ~40 percent of the Cu reserve, prior to the eruption of a 9.13 ± 0.25 Ma rhyodacitic ignimbrite flow. Although areally contiguous, the major magnetite and copper-rich centers of the Marcona district record independent metallogenic episodes widely separated in age. Further, whereas the Cu-poor magnetite mineralization at Marcona was integral to the terminal eruptions of the Middle Jurassic arc, representing a shallow-marine analog of the Pliocene El Laco magnetite deposits of northern Chile, the Mina Justa Cu sulfide orebodies—like the other economic, mid-Cretaceous, Cu-rich IOCG deposits of the central Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, and Rául-Condestable—was the product of brines released during the inversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the Mesozoic Andes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration and mineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCG deposit clan. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/155.abstract [article] Evolution of the giant marcona-Mina Justa iron oxide-copper-gold district, south-central Peru [texte imprimé] / Huayong Chen, Auteur ; Alan H. Clark, Auteur ; T. Kurtis Kyser, Auteur . - 2011 . - pp. 155-185. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 155-185 Mots-clés : Iron  oxide  Copper  district  Gold  district  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Perú, centered at latitude 15°11′ S, longitude 75°6′ W, incorporates Marcona, the preeminent central Andean iron oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one of the few major Andean IOCG deposits with economic copper grades (346.6 Mt @ 0.71% Cu). The emplacement of magnetite orebodies with uneconomic Cu grades (avg 0.12%) at Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine Middle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene Cu sulfide (~15 g/t Ag, 0.12 g/t Au) mineralization at Mina Justa was emplaced along reactivated listric-normal detachment faults during the mid-Cretaceous inversion of the contiguous, plate boundary-parallel, Aptian to Albian Cañete basin, accompanied by the earliest, largely granodioritic-dioritic, stocks of the Coastal batholith. Alteration and mineralization assemblages, supported by 40Ar/39Ar geochronology of biotite, phlogopite, actinolite, cummingtonite, and K-feldspars, reveal a history of magmatic and hydrothermal processes extending episodically for at least 80 m.y., from ca. 177 to 95 Ma, wherein metal-rich mineralization events were preceded and separated by episodes of barren alteration. At Marcona, precursor, subocean-floor hydrothermal activity in the Aalenian (177 Ma) and Bajocian (171 Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperature Mg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocks and minor carbonate units underlying the nascent Río Grande Formation arc. Subsequent areally widespread, albite-marialite alteration (Na-Cl metasomatism) largely predated but overlapped with the emplacement of an en echelon swarm of massive magnetite orebodies, in turn overprinted by subordinate magnetite-sulfide assemblages. Magnetite and weak Cu and Zn sulfide mineralization coincided with a 156 to 162 Ma episode of andesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely by quartz-rich metaclastic rocks. From 162 to 159 Ma, iron oxide mineralization evolved from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite assemblages. These were overprinted at 156 to 159 Ma by chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, locally resulting in grades of 0.45 percent Cu and 0.5 percent Zn. Hydrothermal activity was thereafter focused in the Mina Justa area, 3 to 4 km to the northeast of Marcona, where Middle Jurassic andesites experienced intense albite-actinolite alteration at ca. 157 Ma, i.e., contemporaneous with sulfide mineralization at Marcona, and magnetite-microcline alteration (K-Fe metasomatism) at ca. 142 Ma. Development of the Mina Justa Cu (-Ag) deposit proper, however, began much later, with, successively, actinolitization at ca. 109 Ma, the deposition of calcite and specular hematite, now entirely pseudo-morphed by magnetite, and the metasomatic emplacement of bodies of barren, massive magnetite and pyrite at 101 to 104 Ma. Finally, at 95 to 99 Ma, chalcopyrite-bornite-digenite-chalcocite mineralization, with abundant calcite and hematite, was emplaced as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays of breccia and stockwork, cored by preexisting magnetite-pyrite lenses. Supergene oxidation generated a chryso-colla-atacamite-covellite blanket, hosting ~40 percent of the Cu reserve, prior to the eruption of a 9.13 ± 0.25 Ma rhyodacitic ignimbrite flow. Although areally contiguous, the major magnetite and copper-rich centers of the Marcona district record independent metallogenic episodes widely separated in age. Further, whereas the Cu-poor magnetite mineralization at Marcona was integral to the terminal eruptions of the Middle Jurassic arc, representing a shallow-marine analog of the Pliocene El Laco magnetite deposits of northern Chile, the Mina Justa Cu sulfide orebodies—like the other economic, mid-Cretaceous, Cu-rich IOCG deposits of the central Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, and Rául-Condestable—was the product of brines released during the inversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the Mesozoic Andes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration and mineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCG deposit clan. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/155.abstract

Broad synchroneity of three gold mineralization styles in the kalgoorlie gold field / Noreen M. Vielreicher in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 187-227 Titre : Broad synchroneity of three gold mineralization styles in the kalgoorlie gold field : SHRIMP, U-Pb, and 40Ar/39Ar geochronological evidence Type de document : texte imprimé Auteurs : Noreen M. Vielreicher, Auteur ; David I. Groves, Auteur ; Lawrence W. Snee, Auteur Année de publication : 2011 Article en page(s) : pp. 187-227 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold  mineralization  Uranium  Plomb Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : There has been a long-standing controversy regarding the timing and number of gold mineralization events at Kalgoorlie. Uranium-Pb dating of zircons and hydrothermal monazite and xenotime, as well as 40Ar/39Ar analysis of metasomatic fuchsite and white mica, are used to date pre- to synore dikes, alteration, and orebodies in order to resolve this issue. The majority of gold mineralization at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, are different expressions of a complex mineralizing system that was active at broadly the same time at ca. 2.64 Ga. Gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time, under similar P-T conditions. This giant ore system formed after ca. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at 2680 ± 9 Ma, and intrusion of calc-alkaline feldspar-quartz porphyry dikes at 2670 ± 5 Ma. Gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 ± 6 Ma and overlapped the waning stages of hornblende- and albite-bearing porphyry dike emplacement at 2650 ± 6 Ma and regional metamorphism. Subsequent brittle deformation in the Kalgoorlie gold field was accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the Golden Mile between about 2.61 and 2.60 Ga. This late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks at Kalgoorlie and may be related to uplift and final cooling of the terrane. Despite this late event, the geometry of the Kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/187.abstract [article] Broad synchroneity of three gold mineralization styles in the kalgoorlie gold field : SHRIMP, U-Pb, and 40Ar/39Ar geochronological evidence [texte imprimé] / Noreen M. Vielreicher, Auteur ; David I. Groves, Auteur ; Lawrence W. Snee, Auteur . - 2011 . - pp. 187-227. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 187-227 Mots-clés : Gold  mineralization  Uranium  Plomb Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : There has been a long-standing controversy regarding the timing and number of gold mineralization events at Kalgoorlie. Uranium-Pb dating of zircons and hydrothermal monazite and xenotime, as well as 40Ar/39Ar analysis of metasomatic fuchsite and white mica, are used to date pre- to synore dikes, alteration, and orebodies in order to resolve this issue. The majority of gold mineralization at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, are different expressions of a complex mineralizing system that was active at broadly the same time at ca. 2.64 Ga. Gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time, under similar P-T conditions. This giant ore system formed after ca. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at 2680 ± 9 Ma, and intrusion of calc-alkaline feldspar-quartz porphyry dikes at 2670 ± 5 Ma. Gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 ± 6 Ma and overlapped the waning stages of hornblende- and albite-bearing porphyry dike emplacement at 2650 ± 6 Ma and regional metamorphism. Subsequent brittle deformation in the Kalgoorlie gold field was accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the Golden Mile between about 2.61 and 2.60 Ga. This late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks at Kalgoorlie and may be related to uplift and final cooling of the terrane. Despite this late event, the geometry of the Kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/187.abstract

Spatial statistical analysis of the distribution of komatite-hosted nickel sulfide deposits in the Kalgoorlie terrane, western Australia / Antony Mamuse in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 229-242 Titre : Spatial statistical analysis of the distribution of komatite-hosted nickel sulfide deposits in the Kalgoorlie terrane, western Australia : clustered or not? Type de document : texte imprimé Auteurs : Antony Mamuse, Auteur ; Alok Porwal, Auteur ; Oliver Kreuzer, Auteur Année de publication : 2011 Article en page(s) : pp. 229-242 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Nickel  deposits  Spatial  analysis  Statistical  analysis  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The spatial distribution of mineral deposits is a critical component of predictive estimation of undiscovered mineral resources. Nickel sulfide deposits in the Kalgoorlie terrane of Western Australia, the world’s premier province for komatiite-hosted nickel sulfide deposits, are generally perceived to be clustered. We apply distance-based spatial analysis methods (nearest neighbor and K function) to determine the spatial distribution pattern of nickel sulfide deposits in the Kalgoorlie terrane. Results of these spatial analyses indicate the komatiite bodies that contain the nickel sulfide deposits in the terrane are clustered. In contrast, nickel sulfide deposits within komatiite bodies are either randomly distributed or dispersed and not clustered. Therefore, the apparent clustering of nickel sulfide deposits within the Kalgoorlie terrane may be a mere expression of the underlying clustering of the host komatiite bodies. These findings have two main implications: (1) nickel exploration models implemented through area selection based on localization controls of favorable komatiite bodies, followed by direct detection of deposits within komatiite bodies have spatio-statistical validity, and (2) a Poisson distribution could be a plausible initial model for predicting the number of nickel sulfide deposits within a komatiite body. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/229.abstract [article] Spatial statistical analysis of the distribution of komatite-hosted nickel sulfide deposits in the Kalgoorlie terrane, western Australia : clustered or not? [texte imprimé] / Antony Mamuse, Auteur ; Alok Porwal, Auteur ; Oliver Kreuzer, Auteur . - 2011 . - pp. 229-242. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - pp. 229-242 Mots-clés : Nickel  deposits  Spatial  analysis  Statistical  analysis  Australia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The spatial distribution of mineral deposits is a critical component of predictive estimation of undiscovered mineral resources. Nickel sulfide deposits in the Kalgoorlie terrane of Western Australia, the world’s premier province for komatiite-hosted nickel sulfide deposits, are generally perceived to be clustered. We apply distance-based spatial analysis methods (nearest neighbor and K function) to determine the spatial distribution pattern of nickel sulfide deposits in the Kalgoorlie terrane. Results of these spatial analyses indicate the komatiite bodies that contain the nickel sulfide deposits in the terrane are clustered. In contrast, nickel sulfide deposits within komatiite bodies are either randomly distributed or dispersed and not clustered. Therefore, the apparent clustering of nickel sulfide deposits within the Kalgoorlie terrane may be a mere expression of the underlying clustering of the host komatiite bodies. These findings have two main implications: (1) nickel exploration models implemented through area selection based on localization controls of favorable komatiite bodies, followed by direct detection of deposits within komatiite bodies have spatio-statistical validity, and (2) a Poisson distribution could be a plausible initial model for predicting the number of nickel sulfide deposits within a komatiite body. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/229.abstract

Handbook of mineral exploration and ore petrology / R. Dahana Raju in Economic geology, Vol. 105 N° 1 (Janvier/Fevrier 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - p. 243 Titre : Handbook of mineral exploration and ore petrology : techniques and applications Type de document : texte imprimé Auteurs : R. Dahana Raju, Auteur Année de publication : 2011 Article en page(s) : p. 243 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Ore  petrology  Mineral  exploration Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/243.extract [article] Handbook of mineral exploration and ore petrology : techniques and applications [texte imprimé] / R. Dahana Raju, Auteur . - 2011 . - p. 243. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 1 (Janvier/Fevrier 2010) . - p. 243 Mots-clés : Ore  petrology  Mineral  exploration Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/1/243.extract

Exemplaires

Bibliothèque Ecole Nationale Polytechnique d'Alger

Accueil

Se connecter

Météo

Adresse