Halloran, Jack2026-05-212026-05-212026-05-21http://hdl.handle.net/10393/51692https://doi.org/10.20381/ruor-31984The Valeriano Cu-Au-Mo deposit is located in the northern end of the El Indio belt in the Frontal Cordillera of Chile. In 2024, drilling campaign intercepted a high-grade zone associated with white mica alteration and subsequent kaolinite-siderite alteration (e.g., ATXD26, 122 m of 1.11 wt% Cu, 0.49 g/t Au, 2.7 g/t Ag, and 348 g/t Mo and ATXD23A, 342 m of 1.01 wt% Cu, 0.47 g/t Au, 3.0 g/t Ag, and 326 g/t Mo, with anomalous Bi, Te and As). The high-grade zone associated with siderite is atypical for porphyry deposits. The deposit is associated with Miocene porphyry and biotite-hornblende granodiorite stocks, that intruded Permian volcaniclastic rocks. A north trending lithocap of advanced argillic alteration outcrops at ~4500 m elevation. The intrusion-hosted porphyry mineralization is overlain by an extensive polyphase, polymictic magmatic-hydrothermal inter-mineral breccia, containing country rock fragments, vein fragments as well as granodiorite porphyry fragments with potassic alteration. The inter-mineral breccia, and fragments within, have been variably overprinted by illite and chlorite + hematite (white mica) with chalcopyrite + bornite + digenite ± covellite, syn- to post-brecciation. Following the white mica alteration, the earlier inter-mineral breccia (1), underwent subsequent brecciation (i.e., inter-mineral breccia 2) which was then locally cemented by siderite with high-grade Cu sulfides and accompanied with high-crystallinity kaolinite alteration. High Cu-Au grade zone hosted by inter-mineral breccia 2 was discovered in 2025. The high-grade zone with kaolinite alteration contains bornite, digenite, chalcocite, luzonite/enargite, covellite and Au-Ag bearing-tellurides, which replace earlier formed bornite and chalcopyrite during the potassic and white mica alteration stages. The fluids for high-grade mineralization are evaluated using S isotope compositions and trace element abundance in quartz. Pairs of anhydrite/gypsum and chalcopyrite/pyrite in the white mica altered high-grade zone yielded S isotope equilibrium temperatures between 285 and 375 ºC, with 1:1 sulfide to sulfate ratios in fluids. The mineral assemblages, combined with Ti-in quartz geothermometry suggest that the high-grade mineralization accompanied by kaolinite alteration formed at 250 ± 50 ºC. Sulfur isotope data indicate that S from the white mica and kaolinite alteration stage were magmatic and the high grade Cu sulfides formed under less oxidized conditions with 10:1 for sulfide to sulfate in fluids. Calculated δ13C and δ18O of the hydrothermal fluids from siderite indicate a dominantly magmatic source, consistent with S isotope data. Similar high-grade zones may be present in the underexplored areas of this portion of the Cordillera, that have previously gone unrecognized. A zone of nodular pyrophyllite alteration was recognized at the base of the lithocap in all 22 drill holes. The zone of nodular pyrophyllite alteration is extensive over a 2 km in NW x 1 km NE. The nodular replacement occurs 600-1000 m above the porphyry intrusions, and ~0-500 m above the 0.3% Cu grade shell. The nodules consist of pyrophyllite, diaspore, dickite/kaolinite and locally alunite with pyrite that replace quartz and muscovite. Pyrophyllite (Si:Al ratio of 2:1) replacing muscovite (1:1 ratio) during cooling requires silica, which causes quartz dissolution, as evidenced by cuspate and embayment textures of quartz. Sulfur isotopic compositions of pyrite alunite pairs (n=18) from the nodules yield equilibrium temperatures of 375-275°C, consistent with muscovite-pyrophyllite coexistence at ~350-280°C at quartz saturation. The zone of nodular alteration represents a paleo-isotherm over the causative intrusions and can be used as a vector for exploration of concealed porphyry Cu deposits elsewhere.enAttribution-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nd/4.0/Mineral explorationGeochemistryPetrologyMineralogyPorphyry copperThesis