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New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au-Ag and Cordilleran polymetallic ores

Bendezú, Ronner ; Page, Laurence ; Spikings, Richard ; Pecskay, Zoltan ; Fontboté, Lluís

In: Mineralium Deposita, 2008, vol. 43, no. 7, p. 777-789

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    Titre
    • New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au-Ag and Cordilleran polymetallic ores
    Auteur
    • Bendezú, Ronner. Section des Sciences de la Terre, Université de Genève, Geneva, Switzerland
    • Page, Laurence. Department of Geology, Lund University, Sölvegatan 12, 22362, Lund, Sweden
    • Spikings, Richard. Section des Sciences de la Terre, Université de Genève, Geneva, Switzerland
    • Pecskay, Zoltan. Institute of Nuclear Research, Hungarian Academy of Sciences, Bemtér 18/C, 4026, Debrecen, Hungary
    • Fontboté, Lluís. Section des Sciences de la Terre, Université de Genève, Geneva, Switzerland
    Type de document
    • Postprint
    Identifiant OAI-PMH
    • oai:doc.rero.ch:322508
    Summary
    We present 40Ar/39Ar data acquired by infra-red (CO2) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au-(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO2-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9Ma and lasted until ∼10.6Ma. Four alunite samples associated with high-sulfidation epithermal Au-(Ag) ore gave 40Ar/39Ar plateau ages between ∼11.9 and ∼11.1Ma (compared to the previously documented ∼11.6 to ∼11.3Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au-(Ag) deposition period can be estimated to at least 0.4My. Three new 40Ar/39Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au-(Ag) and the base-metal Cordilleran ores may suggest that SO2-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO2-bearing vapor degassing longer than 11.9 to 10.6Ma may be recognized with further more detailed work