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UBC Theses and Dissertations

The geology, alteration and timing of porphyry intrusions and breccias associated with the development of Los Helados porphyry copper-gold deposit, Chile Guitart, Agustin


The Los Helados porphyry Cu-Au deposit is part of the Miocene belt of northern Chile, it is located at -28.34 Lat and -69.58 Long, between the Maricunga and El Indio metallogenic belts. Multiple intrusion and brecciation events occurred during the late Miocene in the Los Helados that gave rise to the current distribution of hydrothermal alteration assemblages and associated sulfide mineralization. The Miocene magmatic-hydrothermal history is divided in 6 stages grouped in pre-mineralization stage 1 (Quartz Feldspar Porphyry), inter-mineralization stages 2-3-4-5 (Fine Grained Plagioclase Crowded Porphyry, Matrix Rich Breccia, Cement Rich Breccia, Bimodal Plagioclase Porphyry) of which stages 3 and 4 are the main mineralizing events, and late- mineralization stage 6 (Biotite Plagioclase Porphyry). Based on U-Pb zircon ages, the duration of the magmatic processes is estimated between 13.97+/- 0.1 Ma (the age obtained for the precursor Quartz Feldspar Porphyry) and 13.75+/-0.13 (the age obtained for the youngest porphyry, the Biotite Plagioclase Porphyry). This estimation gives a minimum duration of 0.22 +/- 0.16 Ma for the magmatic processes at Los Helados. White micas are a common alteration component in all alteration assemblages at Los Helados, and changes in the chemistry of those micas can be detected using short-wave infrared spectra (SWIR). The economic ore metal (Cu, Au, Ag) at Los Helados is associated with the phengite composition indicating the copper deposition in less acidic conditions. Los Helados is located in a zone with a complex tectonic segmentation. On the one hand, it is situated near the present-day northern limit of the Chilean-Pampean flat-slab segment of the central Andes. On the other hand, translithospheric faults that can be traced from the Pacific coast of Chile, across the Cordillera Occidental, and into the Puna play an important role in the segmentation of the Andes at this latitude. As a result of this segmentation, Los Helados shows differences in terms of host rock, style of mineralization, depth of emplacement and alteration that distinguished it from the classic Au (Cu) porphyry deposits of the Maricunga metallogenetic belt.

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