Abstract
New geochronological data from the Los Bronces cluster of the Río Blanco-Los Bronces mega-porphyry Cu-Mo district establish a wide range of magmatism, hydrothermal alteration, and mineralization ages, both in terms of areal extent and time. The northern El Plomo and southernmost Los Piches exploration areas contain the oldest barren porphyritic intrusions with U-Pb ages of 10.8 ± 0.1 Ma and 13.4 ± 0.1 Ma, respectively. A hypabyssal barren intrusion adjacent northwesterly to the main pit area yields a slightly younger age of 10.2 ± 0.3 Ma (San Manuel sector, U-Pb), whereas in the Los Bronces (LB) open-pit area, the present day mineral extraction zone, porphyries range from 8.49 to 6.02 Ma (U-Pb). Hydrothermal biotite and sericite ages are up to 0.5 Ma younger but consistent with the cooling of the corresponding intrusion events of each area. Two quartz-molybdenite B-type veins from the LB open pit have Re-Os molybdenite ages of 5.65 ± 0.03 Ma and 5.35 ± 0.03 Ma consistent with published data for the contiguous Río Blanco cluster. The San Manuel exploration area within the Los Bronces cluster, located about 1.5–2 km southeast of the open-pit extraction zone, shows both the oldest hydrothermal biotite (7.70 ± 0.07 Ma; 40Ar/39Ar) and breccia cement molybdenite ages (8.36 ± 0.06 Ma; Re-Os) registered in the entire Río Blanco-Los Bronces district. These are also older than those reported from the El Teniente porphyry Cu(-Mo) deposit, suggesting that mineralization in the late Miocene to early Pliocene porphyry belt of Central Chile commenced 2 Ma before the previously accepted age of 6.3 Ma.
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Acknowledgments
This study has been financed by Anglo American Chile. The authors thank V. Irarrazaval for allowing the publication of the geochronological data. We are grateful for the mineral separation work by J. Vargas (Universidad de Chile). A. Saeed (GEMOC), F. Barra (University of Arizona), and T. Ullrich (UBC) are thanked for the analytical U-Pb, Re-Os, and Ar-Ar data, respectively. We are also grateful to J. Zamorano for his support during the first sample collection, J.C. Toro for his comments about this manuscript, J. Bravo and E. Silva for their friendship and assistance in drafting Figs. 2 and 3, and J. Le Roux for helping to improve the English of the text. We acknowledge and thank T. Bissig for his constructive comments and suggestions throughout the different versions of the text, B. Lehmann for his help in English grammar and in edition. Finally, we recognize that the present knowledge of the geology of this district has resulted from the work of a number of anonymous geologists that have contributed merely out of a scientific interest: to all of them, many thanks.
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Deckart, K., Silva, W., Spröhnle, C. et al. Timing and duration of hydrothermal activity at the Los Bronces porphyry cluster: an update. Miner Deposita 49, 535–546 (2014). https://doi.org/10.1007/s00126-014-0512-9
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DOI: https://doi.org/10.1007/s00126-014-0512-9