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Ajouter le résultat dans votre panierPhilippine porphyry and epithermal deposits / David R. Cooke in Economic geology, Vol. 106 N° 8 (Décembre 2011)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1253-1256
Titre : Philippine porphyry and epithermal deposits : an introduction Type de document : texte imprimé Auteurs : David R. Cooke, Auteur ; Pete Hollings, Auteur ; Zhaoshan Chang, Auteur Année de publication : 2011 Article en page(s) : pp. 1253-1256 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Gold mining Silver mining Philippine Epithermal 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/106/8/1253.extract [article] Philippine porphyry and epithermal deposits : an introduction [texte imprimé] / David R. Cooke, Auteur ; Pete Hollings, Auteur ; Zhaoshan Chang, Auteur . - 2011 . - pp. 1253-1256.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1253-1256
Mots-clés : Gold mining Silver mining Philippine Epithermal 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/106/8/1253.extract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1257-1277
Titre : Geochemistry of tertiary igneous rocks of northern Luzon, Philippines : evidence for a back-arc setting for alkalic porphyry copper-gold deposits and a case for slab roll-back? Type de document : texte imprimé Auteurs : Pete Hollings, Auteur ; Rohan Wolfe, Auteur ; David R. Cooke, Auteur Année de publication : 2011 Article en page(s) : pp. 1257-1277 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Tertiary igneous rocks Alcalic prophyry Copper deposits Gold deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex. In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio area range from 25.7 to 24.8 Ma.
The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the main-arc sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back-arc rifting.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1257.abstract [article] Geochemistry of tertiary igneous rocks of northern Luzon, Philippines : evidence for a back-arc setting for alkalic porphyry copper-gold deposits and a case for slab roll-back? [texte imprimé] / Pete Hollings, Auteur ; Rohan Wolfe, Auteur ; David R. Cooke, Auteur . - 2011 . - pp. 1257-1277.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1257-1277
Mots-clés : Tertiary igneous rocks Alcalic prophyry Copper deposits Gold deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex. In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio area range from 25.7 to 24.8 Ma.
The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the main-arc sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back-arc rifting.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1257.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1279-1315
Titre : Geology of the Didipio region and genesis of the dinkidi alkalic porphyry Cu-Au deposit and related pegmatites, northern Luzon, Philippines Type de document : texte imprimé Auteurs : Rohan C. Wolfe, Auteur ; David R. Cooke, Auteur Année de publication : 2011 Article en page(s) : pp. 1279-1315 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Alcalic prophyry Pegmatite Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Dinkidi Cu-Au porphyry deposit, northern Luzon, Philippines, formed at the tip of the southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The 110 Mt ore deposit is characterized by elevated hypogene grades (avg 1.2 g/t Au and 0.5% Cu) and is one of only a few alkalic porphyry deposits to have been discovered outside of British Columbia and eastern Australia.
Dinkidi is hosted by the Didipio intrusive complex, which intruded calc-alkalic to shoshonitic volcanic rocks of the Mamparang and Upper Mamparang formations in the late Oligocene. An early, composite diorite-monzodiorite pluton was intruded by the Surong monzonite and the Dinkidi stock. The initial phases of the Cu-Au mineralized Dinkidi stock were the equigranular biotite-amphibole–bearing Tunja monzonite and the Balut dike—a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases of the Dinkidi stock are a plagioclase-phyric monzonite-syenite (Quan porphyry) and the Bufu syenite, a crystal-crowded leucocratic quartz syenite. Postmineralization andesite dikes cut the intrusive complex.
Emplacement of the Tunja monzonite was temporally and spatially associated with pervasive stage 1 biotite-magnetite-K silicate alteration, which primarily affected the diorite-monzodiorite pluton. Emplacement of the diopside-phyric Balut dike produced the stage 2 calc-potassic diopside-actinolite-K feldspar-bornite vein stockwork and a calc-potassic alteration assemblage typical of silica-undersaturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2–8 g/t Au) and its sulfides have δ34S values of −3.5 to −0.7 per mil. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidized (sulfate-predominant) Na-K-Ca-Fe–rich brine.
Intrusion of the quartz-saturated Quan porphyry and Bufu syenite led to the formation of the stage 3 quartz-illite-calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stockwork hosts most of the lower-grade (1–2 g/t Au) mineralization at Dinkidi and is typical of silica-saturated alkalic porphyry systems. A coarse-grained assemblage of quartz-actinolite-perthite (the Bugoy pegmatite) formed as an apophysis on the Bufu syenite and was subsequently brecciated by faulting late in stage 3. The stage 3 quartz stockwork was emplaced at high temperatures (mostly >600°C) from a quartz-saturated, oxidized (sulfate-predominant) Na-K-Fe brine (>68 wt % NaCl equiv) that contained up to 0.6 wt percent Cu and 4 wt percent Fe. Cooling to ~430°C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with δ34S values of −4.2 to −0.2 per mil in the quartz stockwork. The quartz-bearing assemblage formed at paleodepths of 2.9 to 3.5 km. Periods of quartz growth from overpressured brines were interrupted episodically by brittle failure events that caused the system to depressurize to near-hydrostatic conditions, triggering vapor generation via boiling. Mineralization was followed by intermediate argillic and high-level advanced argillic alteration (stage 4), and by late-stage, fault-related zeolite-calcite alteration and veins (stage 5).
The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of fractionation of the associated intrusions. Extensive fractionation in a silica-undersaturated dioritic magma chamber is interpreted to have ultimately caused quartz saturation and the development of the late-stage syenite intrusions and related quartz stockwork mineralization. The calcic, silica-undersaturated pegmatitic Balut dike, which is associated with the calc-potassic stockwork, does not fit this fractionation trend and is interpreted to have formed by interaction between the late-stage syenitic melt and a comagmatic mafic melt that underplated the siliceous magma chamber prior to formation of the Balut dike. A reversion to fractionation-dominated magmatic processes in the silicic magma chamber then led to the intrusion of the quartz-saturated Quan porphyry and Bufu syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage andesite dikes.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1279.abstract [article] Geology of the Didipio region and genesis of the dinkidi alkalic porphyry Cu-Au deposit and related pegmatites, northern Luzon, Philippines [texte imprimé] / Rohan C. Wolfe, Auteur ; David R. Cooke, Auteur . - 2011 . - pp. 1279-1315.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1279-1315
Mots-clés : Alcalic prophyry Pegmatite Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Dinkidi Cu-Au porphyry deposit, northern Luzon, Philippines, formed at the tip of the southward-propagating Cagayan Valley basin in a late Oligocene to early Miocene back-arc setting. The 110 Mt ore deposit is characterized by elevated hypogene grades (avg 1.2 g/t Au and 0.5% Cu) and is one of only a few alkalic porphyry deposits to have been discovered outside of British Columbia and eastern Australia.
Dinkidi is hosted by the Didipio intrusive complex, which intruded calc-alkalic to shoshonitic volcanic rocks of the Mamparang and Upper Mamparang formations in the late Oligocene. An early, composite diorite-monzodiorite pluton was intruded by the Surong monzonite and the Dinkidi stock. The initial phases of the Cu-Au mineralized Dinkidi stock were the equigranular biotite-amphibole–bearing Tunja monzonite and the Balut dike—a thin, variably textured and strongly mineralized clinopyroxene syenite pegmatite dike. The youngest phases of the Dinkidi stock are a plagioclase-phyric monzonite-syenite (Quan porphyry) and the Bufu syenite, a crystal-crowded leucocratic quartz syenite. Postmineralization andesite dikes cut the intrusive complex.
Emplacement of the Tunja monzonite was temporally and spatially associated with pervasive stage 1 biotite-magnetite-K silicate alteration, which primarily affected the diorite-monzodiorite pluton. Emplacement of the diopside-phyric Balut dike produced the stage 2 calc-potassic diopside-actinolite-K feldspar-bornite vein stockwork and a calc-potassic alteration assemblage typical of silica-undersaturated alkalic porphyry deposits. Stage 2 lacks quartz, contains high gold grades (2–8 g/t Au) and its sulfides have δ34S values of −3.5 to −0.7 per mil. The stage 2 calc-potassic assemblage is inferred to have formed at temperatures in excess of 600°C from an oxidized (sulfate-predominant) Na-K-Ca-Fe–rich brine.
Intrusion of the quartz-saturated Quan porphyry and Bufu syenite led to the formation of the stage 3 quartz-illite-calcite-chalcopyrite stockwork vein and alteration assemblage. The quartz stockwork hosts most of the lower-grade (1–2 g/t Au) mineralization at Dinkidi and is typical of silica-saturated alkalic porphyry systems. A coarse-grained assemblage of quartz-actinolite-perthite (the Bugoy pegmatite) formed as an apophysis on the Bufu syenite and was subsequently brecciated by faulting late in stage 3. The stage 3 quartz stockwork was emplaced at high temperatures (mostly >600°C) from a quartz-saturated, oxidized (sulfate-predominant) Na-K-Fe brine (>68 wt % NaCl equiv) that contained up to 0.6 wt percent Cu and 4 wt percent Fe. Cooling to ~430°C and sulfate reduction by wall-rock interaction led to the precipitation of stage 3 sulfides with δ34S values of −4.2 to −0.2 per mil in the quartz stockwork. The quartz-bearing assemblage formed at paleodepths of 2.9 to 3.5 km. Periods of quartz growth from overpressured brines were interrupted episodically by brittle failure events that caused the system to depressurize to near-hydrostatic conditions, triggering vapor generation via boiling. Mineralization was followed by intermediate argillic and high-level advanced argillic alteration (stage 4), and by late-stage, fault-related zeolite-calcite alteration and veins (stage 5).
The hydrothermal mineral assemblages at Dinkidi reflect the composition and degree of fractionation of the associated intrusions. Extensive fractionation in a silica-undersaturated dioritic magma chamber is interpreted to have ultimately caused quartz saturation and the development of the late-stage syenite intrusions and related quartz stockwork mineralization. The calcic, silica-undersaturated pegmatitic Balut dike, which is associated with the calc-potassic stockwork, does not fit this fractionation trend and is interpreted to have formed by interaction between the late-stage syenitic melt and a comagmatic mafic melt that underplated the siliceous magma chamber prior to formation of the Balut dike. A reversion to fractionation-dominated magmatic processes in the silicic magma chamber then led to the intrusion of the quartz-saturated Quan porphyry and Bufu syenite. Ultimately, the residual mafic melt was emplaced as a series of late-stage andesite dikes.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1279.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1317-1333
Titre : Igneous geochemistry of mineralized rocks of the Baguio district, Philippines : implications for tectonic evolution and the genesis of porphyry-style mineralization Type de document : texte imprimé Auteurs : Pete Hollings, Auteur ; David R. Cooke, Auteur ; Patrick J. Waters, Auteur Année de publication : 2011 Article en page(s) : pp. 1317-1333 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Tectonic evolution Mineralized rocks Porphyry-style mineralization Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing >35 million ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers.
The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough Ridge would have been younger than the downgoing plate and consequently more susceptible to melting. These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive mafic melts and the more felsic calc-alkaline rocks has generated fertile melts that were highly productive for porphyry copper and epithermal gold mineralization.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1317.abstract [article] Igneous geochemistry of mineralized rocks of the Baguio district, Philippines : implications for tectonic evolution and the genesis of porphyry-style mineralization [texte imprimé] / Pete Hollings, Auteur ; David R. Cooke, Auteur ; Patrick J. Waters, Auteur . - 2011 . - pp. 1317-1333.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1317-1333
Mots-clés : Tectonic evolution Mineralized rocks Porphyry-style mineralization Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing >35 million ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers.
The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough Ridge would have been younger than the downgoing plate and consequently more susceptible to melting. These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive mafic melts and the more felsic calc-alkaline rocks has generated fertile melts that were highly productive for porphyry copper and epithermal gold mineralization.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1317.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1335-1363
Titre : Porphyry and epithermal deposits and 40Ar/39Ar geochronology of the Baguio district, Philippines Type de document : texte imprimé Auteurs : Patrick J. Waters, Auteur ; David R. Cooke, Auteur ; Rene I. Gonzales, Auteur Année de publication : 2011 Article en page(s) : pp. 1335-1363 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Porphyry deposits Epithermal deposits Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the Baguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determinations for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma).
Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15 and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47 ± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively.
Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone (the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based on previous K-Ar dating.
Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted the development of northwest-trending faults in the upper plate that interacted with major arc-parallel north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz of gold and several million tonnes of copper over approximately 3 m.y.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1335.abstract [article] Porphyry and epithermal deposits and 40Ar/39Ar geochronology of the Baguio district, Philippines [texte imprimé] / Patrick J. Waters, Auteur ; David R. Cooke, Auteur ; Rene I. Gonzales, Auteur . - 2011 . - pp. 1335-1363.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1335-1363
Mots-clés : Porphyry deposits Epithermal deposits Philippine Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the Baguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determinations for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma).
Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15 and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47 ± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively.
Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone (the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based on previous K-Ar dating.
Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted the development of northwest-trending faults in the upper plate that interacted with major arc-parallel north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz of gold and several million tonnes of copper over approximately 3 m.y.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1335.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1365-1398
Titre : Exploration tools for linked porphyry and epithermal deposits : example from the Mankayan intrusion-centered Cu-Au district, Luzon, Philippines Type de document : texte imprimé Auteurs : Zhaoshan Chang, Auteur ; Jeffrey W. Hedenquist, Auteur ; Noel C. White, Auteur Année de publication : 2011 Article en page(s) : pp. 1365-1398 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Linked porphyry Epithermal deposits Cu-Au district Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of ~25 km2, including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki and Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y., from ~3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks and volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, ~2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including the ~1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the ~0.02 Ma Lapangan tuff.
Extensive advanced argillic alteration crops out for ~7 km along the unconformity between the basement rocks and the Imbanguila dacite formation and consists of quartz-alunite ± pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite ± kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is ~4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the ~1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches ~600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore.
This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion: The alunite absorption peak at ~1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates that the Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Pb, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu <0.1wt % and Au <0.1 ppm) and alunite-bearing samples, show that Pb and Ag/Au, plus Hg and Ag, decrease toward the intrusive center, and Sr/Pb and La/Pb ratios increase. Normalizing whole-rock Pb to the (Na + K) molal content produces a proxy for the alunite mineral composition, and this ratio provides the same indications as the LA-ICP-MS analyses of alunite. The concealed Victoria epithermal veins consist of intermediate sulfidation mineralization on the southwest flank of the porphyry. The veins are not exposed, but their presence at depth is indicated by subtle alteration (illite or interstratified illite and/or smectite or smectite + pyrite) and geochemical (As, Se) anomalies at the surface. The anomalies are strongly dependent on erosion level; no anomalies were found where the surface is >~350 m above the upper extent of the veins. An airborne geophysics survey indicates that the Far Southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite-destructive phyllic alteration. Such low magnetic anomalies on the margin of a large lithocap elsewhere may deserve attention. The directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry-epithermal districts.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1365.abstract [article] Exploration tools for linked porphyry and epithermal deposits : example from the Mankayan intrusion-centered Cu-Au district, Luzon, Philippines [texte imprimé] / Zhaoshan Chang, Auteur ; Jeffrey W. Hedenquist, Auteur ; Noel C. White, Auteur . - 2011 . - pp. 1365-1398.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1365-1398
Mots-clés : Linked porphyry Epithermal deposits Cu-Au district Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Mankayan mineral district of northern Luzon, Philippines, hosts several significant ore deposits and prospects of various types within an area of ~25 km2, including the Far Southeast porphyry Cu-Au deposit, the Lepanto high sulfidation epithermal Cu-Au deposit, the Victoria intermediate sulfidation epithermal Au-Ag vein deposit, the Teresa epithermal Au-Ag vein deposit, the Guinaoang porphyry Cu-Au deposit, and the Buaki and Palidan porphyry Cu-Au prospects, all having formed in a period of about 2 m.y., from ~3 Ma. The geologic units include (1) a basement composed of Late Cretaceous to middle Miocene metavolcanic rocks and volcaniclastic rocks; (2) the Miocene 12 to 13 Ma tonalitic Bagon intrusive complex; (3) the Pliocene, ~2.2 to 1.8 Ma, Imbanguila dacite porphyry and pyroclastic rocks; and (4) postmineralization cover rocks, including the ~1.2 to 1.0 Ma Bato dacite porphyry and pyroclastic rocks and the ~0.02 Ma Lapangan tuff.
Extensive advanced argillic alteration crops out for ~7 km along the unconformity between the basement rocks and the Imbanguila dacite formation and consists of quartz-alunite ± pyrophyllite or diaspore, with local zones of silicic alteration and a halo of dickite ± kaolinite. The alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps. The northwest-striking portion is ~4 km long and hosts the Lepanto enargite Au ore deposit, also controlled by the Lepanto fault. The Lepanto epithermal deposit is related to the underlying Far Southeast porphyry; the quartz-alunite alteration halo of Lepanto is contemporaneous with the ~1.4 Ma potassic alteration of the porphyry. There are also silicic-advanced argillic alteration patches ~600 m above the Far Southeast orebody at the present surface; these are interpreted to be perched alteration. There is no systematic mineralogical or textural zoning in the Lepanto lithocap that indicates direction to the intrusive source. Most surface samples of the lithocap contain less than 50 ppb Au, despite many being less than a few hundred meters from underground Cu-Au ore.
This study found that several characteristics of the Lepanto lithocap change systematically with distance from the causative intrusion: The alunite absorption peak at ~1,480 nm in the short wavelength infrared (SWIR) spectrum shifts to higher wavelengths where the sample is closer to the intrusive center, due to higher Na and lower K content in the alunite; published experimental studies indicate that high Na/(Na + K) is related to higher formation temperature. High Ca alunite, including huangite, also occurs at locations proximal to the intrusive center. Alunite mineral composition analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates that the Pb content decreases toward the intrusive center, whereas Sr, La, Sr/Pb, and La/Pb increase markedly. Whole-rock compositions, using only nonmineralized (taken as Cu <0.1wt % and Au <0.1 ppm) and alunite-bearing samples, show that Pb and Ag/Au, plus Hg and Ag, decrease toward the intrusive center, and Sr/Pb and La/Pb ratios increase. Normalizing whole-rock Pb to the (Na + K) molal content produces a proxy for the alunite mineral composition, and this ratio provides the same indications as the LA-ICP-MS analyses of alunite. The concealed Victoria epithermal veins consist of intermediate sulfidation mineralization on the southwest flank of the porphyry. The veins are not exposed, but their presence at depth is indicated by subtle alteration (illite or interstratified illite and/or smectite or smectite + pyrite) and geochemical (As, Se) anomalies at the surface. The anomalies are strongly dependent on erosion level; no anomalies were found where the surface is >~350 m above the upper extent of the veins. An airborne geophysics survey indicates that the Far Southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite-destructive phyllic alteration. Such low magnetic anomalies on the margin of a large lithocap elsewhere may deserve attention. The directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry-epithermal districts.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1365.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1399-1424
Titre : Evidence for magmatic-hydrothermal fluids and ore-forming processes in epithermal and porphyry deposits of the Baguio district, Philippines Type de document : texte imprimé Auteurs : David R. Cooke, Auteur ; Cari L. Deyell, Auteur ; Patrick J. Waters, Auteur Année de publication : 2011 Article en page(s) : pp. 1399-1424 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Magmatic-hydrothermal fluids Mineral deposits Epithermal deposits Porphyry deposits Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration.
Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, <2 wt % NaCl equiv) and had moderate temperatures (<300°C). Porphyry-style mineralization was associated with high temperature (300° to >600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (−6.9 to +0.8‰), consistent with oxidizing (SO42−-predominant) mineralizing fluids.
Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured δ34Ssulfide (+1.1 to +6.6‰), δ34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite (0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle-derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metal endowment.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1399.abstract [article] Evidence for magmatic-hydrothermal fluids and ore-forming processes in epithermal and porphyry deposits of the Baguio district, Philippines [texte imprimé] / David R. Cooke, Auteur ; Cari L. Deyell, Auteur ; Patrick J. Waters, Auteur . - 2011 . - pp. 1399-1424.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1399-1424
Mots-clés : Magmatic-hydrothermal fluids Mineral deposits Epithermal deposits Porphyry deposits Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large, broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic alteration.
Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type. The epithermal mineralizing fluids were dilute (generally, <2 wt % NaCl equiv) and had moderate temperatures (<300°C). Porphyry-style mineralization was associated with high temperature (300° to >600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (−6.9 to +0.8‰), consistent with oxidizing (SO42−-predominant) mineralizing fluids.
Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations (Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured δ34Ssulfide (+1.1 to +6.6‰), δ34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite (0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle-derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metal endowment.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1399.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1425-1446
Titre : The trace metal chemistry of deep geothermal water, palinpinon geothermal field, negros island, Philippines : implications for precious metal deposition in epithermal gold deposits Type de document : texte imprimé Auteurs : Andrew J. Rae, Auteur ; David R. Cooke, Auteur ; Kevin L. Brown, Auteur Année de publication : 2011 Article en page(s) : pp. 1425-1446 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Metal chemistry Geothermal water Epithermal gold deposits Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Palinpinon geothermal field, Negros Island, Philippines, is a high-temperature, liquid-dominated geothermal system in an active volcanic island-arc setting. Parallels have been drawn between the geologic setting and hydrothermal alteration styles recognized at Palinpinon with those that characterize several types of magmatic-hydrothermal ore deposits (e.g., porphyry, skarn, and high and intermediate sulfidation epithermal systems).
The neutral chloride Palinpinon reservoir water chemistry is affected by boiling, mixing, and conductive cooling. The occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the deep reservoir fluid is capable of transporting and depositing base and precious metals. However, trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold (1–4 μg/kg). Chemical modeling of Palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition, producing sulfide assemblages similar to those present in well scale deposits. Mixing with acid-bearing sulfate waters can also produce these sulfide assemblages, but less efficiently and with gold and silver deposition occurring at lower temperatures (i.e., <110°C).
Despite indications that modern hydrothermal fluids are capable of metal transportation, base and precious metal deposition is not strongly developed at Palinpinon. Base and precious metal concentrations in drill core and drill cuttings are one to two orders of magnitude below ore grade (<0.02 wt % Cu, <0.03 wt % Pb, <0.01 wt % Zn, <0.01 wt % Mo, <8 g/t Ag, and <0.05 g/t Au), and important features that characterize high-grade ore zones of porphyry (i.e., quartz stockwork veining) and epithermal deposits (i.e., residual vuggy quartz, massive crustiform veins, and/or hydrothermal breccia complexes) are apparently absent at Palinpinon. Such features intimate that high-grade ore deposition is promoted by large volumes of fluid and highly permeable wall rock. Their absence at Palinpinon indicates that permeability, fluid flux, and/or metal transport have been insufficient to form significant zones of base or precious metal enrichment.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1425.abstract [article] The trace metal chemistry of deep geothermal water, palinpinon geothermal field, negros island, Philippines : implications for precious metal deposition in epithermal gold deposits [texte imprimé] / Andrew J. Rae, Auteur ; David R. Cooke, Auteur ; Kevin L. Brown, Auteur . - 2011 . - pp. 1425-1446.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1425-1446
Mots-clés : Metal chemistry Geothermal water Epithermal gold deposits Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Palinpinon geothermal field, Negros Island, Philippines, is a high-temperature, liquid-dominated geothermal system in an active volcanic island-arc setting. Parallels have been drawn between the geologic setting and hydrothermal alteration styles recognized at Palinpinon with those that characterize several types of magmatic-hydrothermal ore deposits (e.g., porphyry, skarn, and high and intermediate sulfidation epithermal systems).
The neutral chloride Palinpinon reservoir water chemistry is affected by boiling, mixing, and conductive cooling. The occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the deep reservoir fluid is capable of transporting and depositing base and precious metals. However, trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold (1–4 μg/kg). Chemical modeling of Palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition, producing sulfide assemblages similar to those present in well scale deposits. Mixing with acid-bearing sulfate waters can also produce these sulfide assemblages, but less efficiently and with gold and silver deposition occurring at lower temperatures (i.e., <110°C).
Despite indications that modern hydrothermal fluids are capable of metal transportation, base and precious metal deposition is not strongly developed at Palinpinon. Base and precious metal concentrations in drill core and drill cuttings are one to two orders of magnitude below ore grade (<0.02 wt % Cu, <0.03 wt % Pb, <0.01 wt % Zn, <0.01 wt % Mo, <8 g/t Ag, and <0.05 g/t Au), and important features that characterize high-grade ore zones of porphyry (i.e., quartz stockwork veining) and epithermal deposits (i.e., residual vuggy quartz, massive crustiform veins, and/or hydrothermal breccia complexes) are apparently absent at Palinpinon. Such features intimate that high-grade ore deposition is promoted by large volumes of fluid and highly permeable wall rock. Their absence at Palinpinon indicates that permeability, fluid flux, and/or metal transport have been insufficient to form significant zones of base or precious metal enrichment.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1425.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1447-1463
Titre : Exploration applications of copper isotopes in the supergene environment : a case study of the bayugo porphyry copper-gold deposit, southern Philippines Type de document : texte imprimé Auteurs : David Braxton, Auteur ; Ryan Mathur, Auteur Année de publication : 2011 Article en page(s) : pp. 1447-1463 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Copper isothrope Copper deposit Gold deposit Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper isotope compositions of secondary phases from the supergene profile of the Bayugo porphyry copper-gold deposit (Philippines) show significant isotopic fractionation relative to hypogene sulfide phases and reveal systematic patterns of isotopic enrichment in the leached cap iron oxides and supergene copper sulfides. Thirty-nine iron oxide, copper oxide, and copper sulfide minerals yielded δ65Cu values between −3.4 and +6.9 per mil. Samples of hypogene sulfides have a narrow range of values near zero per mil (−0.6 to +1.0‰). In contrast, samples of supergene sulfides (chalcocite and djurleite) collected from the exotic and enriched copper zones define a trend of decreasing δ65Cu from the enriched source area and proximal exotic zone (>3‰) downgradient to the distal portions (<1‰) of the exotic zone. The higher isotopic values in the source area and proximal zones are consistent with Rayleigh fractionation models depicting multiple cycles of oxidation, leaching, and enrichment of copper. Large (1.8–2.7‰) copper isotope ratios measured for leached cap iron oxides provide evidence for a preexisting sulfide enrichment zone that was removed in subsequent cycles of oxidation, leaching, and enrichment.
These findings are significant, because within exotic copper zones the copper isotope values of supergene sulfides can provide an indication both of proximity and direction to the source area. The technique could be applied in the search for unrecognized porphyry centers upgradient from headless exotic copper occurrences, by exploring for the supergene copper phases with the highest δ65Cu values. Independently, copper isotope ratios of iron oxides in surface samples or fossil leached caps might be used to rank prospects and to focus drilling in areas with the greatest potential for mature enrichment profiles, by mapping the distribution of isotopically enriched leached cap iron oxides. The leached cap screening and targeting tool could enhance the iron oxide mapping techniques traditionally applied in leached cap interpretation and exploration.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1447.abstract [article] Exploration applications of copper isotopes in the supergene environment : a case study of the bayugo porphyry copper-gold deposit, southern Philippines [texte imprimé] / David Braxton, Auteur ; Ryan Mathur, Auteur . - 2011 . - pp. 1447-1463.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1447-1463
Mots-clés : Copper isothrope Copper deposit Gold deposit Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Copper isotope compositions of secondary phases from the supergene profile of the Bayugo porphyry copper-gold deposit (Philippines) show significant isotopic fractionation relative to hypogene sulfide phases and reveal systematic patterns of isotopic enrichment in the leached cap iron oxides and supergene copper sulfides. Thirty-nine iron oxide, copper oxide, and copper sulfide minerals yielded δ65Cu values between −3.4 and +6.9 per mil. Samples of hypogene sulfides have a narrow range of values near zero per mil (−0.6 to +1.0‰). In contrast, samples of supergene sulfides (chalcocite and djurleite) collected from the exotic and enriched copper zones define a trend of decreasing δ65Cu from the enriched source area and proximal exotic zone (>3‰) downgradient to the distal portions (<1‰) of the exotic zone. The higher isotopic values in the source area and proximal zones are consistent with Rayleigh fractionation models depicting multiple cycles of oxidation, leaching, and enrichment of copper. Large (1.8–2.7‰) copper isotope ratios measured for leached cap iron oxides provide evidence for a preexisting sulfide enrichment zone that was removed in subsequent cycles of oxidation, leaching, and enrichment.
These findings are significant, because within exotic copper zones the copper isotope values of supergene sulfides can provide an indication both of proximity and direction to the source area. The technique could be applied in the search for unrecognized porphyry centers upgradient from headless exotic copper occurrences, by exploring for the supergene copper phases with the highest δ65Cu values. Independently, copper isotope ratios of iron oxides in surface samples or fossil leached caps might be used to rank prospects and to focus drilling in areas with the greatest potential for mature enrichment profiles, by mapping the distribution of isotopically enriched leached cap iron oxides. The leached cap screening and targeting tool could enhance the iron oxide mapping techniques traditionally applied in leached cap interpretation and exploration.DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1447.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1465-1478
Titre : Trace element geochemistry of enargite in the Mankayan district, Philippines Type de document : texte imprimé Auteurs : C. L. Deyell, Auteur ; J. W. Hedenquist, Auteur Année de publication : 2011 Article en page(s) : pp. 1465-1478 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Element chemistry Ore deposits Enargite Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : We report the first laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) study of trace element substitution in enargite. Results indicate significant variability in the composition of enargite samples from a single ore system. Samples come from the Mankayan district, Philippines, which hosts the Lepanto high-sulfidation Cu-Au deposit, now mined out, and the adjacent Far Southeast porphyry Cu-Au deposit; the genetic relationship between these deposits has been documented in previous studies. LA-ICP-MS analyses indicate significant incorporation of Sb and Fe in enargite (locally exceeding 1 wt %). Other elements such as Bi, Sn, Se, Te, Ag, and Zn occur at concentrations exceeding 0.1 wt percent. The distribution of selected trace elements in enargite correlates with previously published variations in fluid inclusion homogenization and melting temperatures and gas compositions. The spatial distribution of data indicates enargite is enriched in Au and Te close to the Far Southeast porphyry. Enargite is enriched in Silver, Fe, and Pb in the center of Lepanto, and Zn (± Cd) is enrich the ore distal to the Far Southeast porphyry. Locally elevated Sb contents occur in samples from subsidiary branch vein structures and may indicate that mineralizing fluids along these structures were modified compared to those that formed the main orebody. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1465.abstract [article] Trace element geochemistry of enargite in the Mankayan district, Philippines [texte imprimé] / C. L. Deyell, Auteur ; J. W. Hedenquist, Auteur . - 2011 . - pp. 1465-1478.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1465-1478
Mots-clés : Element chemistry Ore deposits Enargite Philippines Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : We report the first laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) study of trace element substitution in enargite. Results indicate significant variability in the composition of enargite samples from a single ore system. Samples come from the Mankayan district, Philippines, which hosts the Lepanto high-sulfidation Cu-Au deposit, now mined out, and the adjacent Far Southeast porphyry Cu-Au deposit; the genetic relationship between these deposits has been documented in previous studies. LA-ICP-MS analyses indicate significant incorporation of Sb and Fe in enargite (locally exceeding 1 wt %). Other elements such as Bi, Sn, Se, Te, Ag, and Zn occur at concentrations exceeding 0.1 wt percent. The distribution of selected trace elements in enargite correlates with previously published variations in fluid inclusion homogenization and melting temperatures and gas compositions. The spatial distribution of data indicates enargite is enriched in Au and Te close to the Far Southeast porphyry. Enargite is enriched in Silver, Fe, and Pb in the center of Lepanto, and Zn (± Cd) is enrich the ore distal to the Far Southeast porphyry. Locally elevated Sb contents occur in samples from subsidiary branch vein structures and may indicate that mineralizing fluids along these structures were modified compared to those that formed the main orebody. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1465.abstract
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1479-1480
Titre : Diamonds in nature : a guide to rough diamonds Type de document : texte imprimé Auteurs : John Gurney, Auteur Année de publication : 2011 Article en page(s) : pp. 1479-1480 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Diamonds Rough diamonds Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1479.extract [article] Diamonds in nature : a guide to rough diamonds [texte imprimé] / John Gurney, Auteur . - 2011 . - pp. 1479-1480.
Economic geology
Langues : Anglais (eng)
in Economic geology > Vol. 106 N° 8 (Décembre 2011) . - pp. 1479-1480
Mots-clés : Diamonds Rough diamonds Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/8/1479.extract
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