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Re-Os isotope systematics of sulfides from felsic igneous rocks: Application to base metal porphyry mineralization in Chile

¹ Department of Geosciences, University of Arizona, Tucson, Arizona 85721
² Departamento de Geologia, Universidad de Chile, Santiago, Chile

Dating sulfides and determining the source of ore-forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits. Here we analyze sulfides from base metal porphyry deposits from Chile to demonstrate the potential of the Re-Os system to determine both the age and the source of metals for hydrothermal mineralization. Cogenetic chalcopyrite, bornite, pyrite, and sphalerite from El Teniente (ca. 5 Ma) have ¹⁸⁷Re/¹⁸⁸Os ratios from 0.3 to 21.8 and initial ¹⁸⁷Os/¹⁸⁸Os ratios from 0.17 to 0.22. A paragenetically late pyrite that has an initial ¹⁸⁷Os/¹⁸⁸Os of 0.88 indicates that the ore deposit is not isotopically homogeneous in Os throughout the formation of the deposit. Pyrite samples from Andacollo (ca. 100 Ma) have ¹⁸⁷Re/¹⁸⁸Os ratios from 15 to 3600, and different isochrons yield ages between 87 and 103 Ma, consistent with different sericite K-Ar ages. Initial ¹⁸⁷Os/¹⁸⁸Os ratios of 0.2 to 1.1 are similar to those of El Teniente. The relatively homogeneous ¹⁸⁷Os/¹⁸⁸Os ratios ( 0.19) for cogenetic sulfides from El Teniente suggest that most of the Os in the ore was from the causal intrusive. The late-stage pyrite probably incorporated Os leached from the country rock by meteoric fluids, which mixed with magmatic fluids at the periphery of the hydrothermal system. We demonstrate that the Re-Os isotope system can be a powerful geochronological tool in hydrothermal ore deposits. The large range in Re/Os ratios of sulfides permits the age of mineralization to be well constrained, despite isotopic heterogeneities of the hydrothermal fluids.

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