Documents disponibles écrits par cet auteur

Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: / Katharina Wulff in Economic geology, Vol. 105 N° 2 (Mars/Avril 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 285-302 Titre : Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: : constraints from stable (O, H, C, S) isotopes Type de document : texte imprimé Auteurs : Katharina Wulff, Auteur ; Annika Dziggel, Auteur ; Jochen Kolb, Auteur Année de publication : 2011 Article en page(s) : pp. 285-302 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Sediment  hosted  Mineralization  Isotopes  Gold  deposits  Namibia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ18O values of 14 to 15 per mil (V-SMOW). The total variations in δ18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ18O and δ13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid (δ18O = 12–14‰ δD = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/285.abstract [article] Origin of mineralizing fluids of the sediment-hosted Navachab gold mine, Namibia: : constraints from stable (O, H, C, S) isotopes [texte imprimé] / Katharina Wulff, Auteur ; Annika Dziggel, Auteur ; Jochen Kolb, Auteur . - 2011 . - pp. 285-302. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 2 (Mars/Avril 2010) . - pp. 285-302 Mots-clés : Sediment  hosted  Mineralization  Isotopes  Gold  deposits  Namibia Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ18O values of 14 to 15 per mil (V-SMOW). The total variations in δ18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ18O and δ13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid (δ18O = 12–14‰ δD = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/2/285.abstract