Abstract

Re-Os analyses of pyrite from the Pueblo Viejo high-sulfidation epithermal deposit (both Monte Negro and Moore ore zones) yield an isochron age of 111.9 ± 3.7 Ma and initial 187Os/188Os ratio of 0.22 ± 0.18 (mean square of weighted deviates = 84, n = 21). Isochrons grouped by pyrite setting (layered versus vein) or locations yield ages within uncertainty of the overall isochron and with each other and, as such, do not yield temporally distinguishable ages. Sphalerite vein samples as well as shale source rocks show evidence of open system behavior and loss of Re and therefore are not included in the above regression. These results confirm that the Pueblo Viejo deposit is Early Cretaceous in age and coeval with the volcanic Los Ranchos Formation, which hosts the deposit. Recent paleontological age assignments for the Hatillo Formation, which overlies the Los Ranchos Formation, show that it is only slightly younger, opening the possibility that mineralization in the Hatillo Formation could also be related to late stages of Los Ranchos magmatism. Whether these revised Los Ranchos-Hatillo relations provide sufficient cover to account for the paragenetically late formation of pyrophyllite at Pueblo Viejo remains unclear. Recognition that Pueblo Viejo is coeval with the Los Ranchos Formation shows that large-scale, high-sulfidation epithermal mineralization can form from basal island-arc tholeiitic magmatism. A review of available experimental data confirms that these magmas can generate hydrothermal solutions of the necessary composition; favorability of the Los Ranchos Formation might have been enhanced by its bimodal composition. These results suggest that exploration for epithermal precious metal deposits should include basal island-arc tholeiite terranes.

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