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

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

Sulfosalt melts / John Mavrogenes in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 257-262 Titre : Sulfosalt melts : evidence of high-temperature vapor transport of metals in the formation of high-sulfidation lode gold deposits Type de document : texte imprimé Auteurs : John Mavrogenes, Auteur ; Richard W. Henley, Auteur ; Agnes G. Reyes, Auteur Année de publication : 2011 Article en page(s) : pp. 257-262 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sulfosalt  Metals  Vapor  transport  Sulfidation  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In enargite-gold (high-sulfidation) vein deposits, magmatic fluid is considered responsible for the transport of metals and sulfur into the depositional regime. New data from Field-Emission SEM analyses of sulfosalt mineral assemblages (primarily enargite and tennantite) from El Indio, Chile, and Summitville, Colorado, provide direct evidence of high-temperature deposition, including the following: (1) the preservation of delicate euhedral quartz assemblages in sulfosalts, (2) a range of discrete Sb-rich sulfosalt, quartz, feldspar, and flourapatite vug-filling minerals, and (3) symplectic sulfosalt-chalcopyrite textures that are arguably quenched melts. Together, these features indicate formation from the vapor phase at high temperatures. Furthermore, euhedral quartz crystals from El Indio contain high-temperature, vapor-rich fluid inclusions. Combined, these observations are interpreted as suggestive of deposition in response to vapor-phase decompression within fracture arrays that may be considered the analogues of the feeder fractures beneath large low-grade silver-gold deposits such as Yanacocha, Peru. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/257.abstract [article] Sulfosalt melts : evidence of high-temperature vapor transport of metals in the formation of high-sulfidation lode gold deposits [texte imprimé] / John Mavrogenes, Auteur ; Richard W. Henley, Auteur ; Agnes G. Reyes, Auteur . - 2011 . - pp. 257-262. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 257-262 Mots-clés : Sulfosalt  Metals  Vapor  transport  Sulfidation  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : In enargite-gold (high-sulfidation) vein deposits, magmatic fluid is considered responsible for the transport of metals and sulfur into the depositional regime. New data from Field-Emission SEM analyses of sulfosalt mineral assemblages (primarily enargite and tennantite) from El Indio, Chile, and Summitville, Colorado, provide direct evidence of high-temperature deposition, including the following: (1) the preservation of delicate euhedral quartz assemblages in sulfosalts, (2) a range of discrete Sb-rich sulfosalt, quartz, feldspar, and flourapatite vug-filling minerals, and (3) symplectic sulfosalt-chalcopyrite textures that are arguably quenched melts. Together, these features indicate formation from the vapor phase at high temperatures. Furthermore, euhedral quartz crystals from El Indio contain high-temperature, vapor-rich fluid inclusions. Combined, these observations are interpreted as suggestive of deposition in response to vapor-phase decompression within fracture arrays that may be considered the analogues of the feeder fractures beneath large low-grade silver-gold deposits such as Yanacocha, Peru. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/257.abstract

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

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