In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada / T. Ulrich in Economic geology, Vol. 106 N° 4 (Juin/Juillet 2011)  

Public ISBD [article]  inEconomic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 667-686 Titre : In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada Type de document : texte imprimé Auteurs : T. Ulrich, Auteur ; D. G. F. Long, Auteur ; B. S. Kamber, Auteur Année de publication : 2011 Article en page(s) : pp. 667-686 Note générale : Géologie économique Langues : Anglais (eng) Mots-clés : Sulfur analysis Pyrite Paleoproterozoic gold Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at ~1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as “invisible” Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive δ34S values (0.97–9.26‰). The δ34S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative Δ33S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/667.abstract [article] In situ trace element and sulfur isotope analysis of pyrite in a paleoproterozoic gold placer deposit, pardo and clement townships, Ontario, Canada [texte imprimé] / T. Ulrich, Auteur ; D. G. F. Long, Auteur ; B. S. Kamber, Auteur . - 2011 . - pp. 667-686. Géologie économique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 4 (Juin/Juillet 2011) . - pp. 667-686 Mots-clés : Sulfur analysis Pyrite Paleoproterozoic gold Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at ~1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as “invisible” Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive δ34S values (0.97–9.26‰). The δ34S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative Δ33S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/106/4/667.abstract

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