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The trace metal chemistry of deep geothermal water, palinpinon geothermal field, negros island, Philippines / Andrew J. Rae in Economic geology, Vol. 106 N° 8 (Décembre 2011) 

Public ISBD [article]  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