Geology, geochemistry and 40Ar/39Ar ages of the Cerro Millo epithermal high-sulfidation gold prospect, southern Peru

The high-sulfidation system of Cerro Millo is hosted in a Late Miocene andesitic paleo-stratovolcano in the High Andes of southern Peru. Very pronounced advanced-argillic (alunite), silicic, and argillic (kaolinite and smectite) alteration characterizes the central part of the hydrothermal system. Propylitic alteration is developed in a 3 to 4 km wide outer halo. Abundant alunite occurs as hypogene, acicular crystals, and very fine-grained aggregates; the latter formed during near-surface steam-heated overprinting. Hypogene alunite has an Ar–Ar plateau age of 10.8 ± 0.9 Ma (2σ), and is synchronous with the andesitic volcanism (Ar–Ar on biotite: 11.0 ± 0.5 Ma). A second ill-defined alunite age plateau of 8.0 ± 0.9 Ma is probably related to steam-heated overprint and points to major erosion in between both hydrothermal events. Telescoping is also evident by a series of silicified horizons which mark the paleo-groundwater table. These units have elevated mercury, antimony and arsenic levels. Late-phase barite occurs in some structurally controlled advanced-argillic altered envelopes. The hypogene alteration mineralogy points to temperatures at ≤ 250 °C. Hydraulic fracturing and steam-heated overprinting suggest a shallow boiling environment. Slight gold enrichment is observed in the lowermost exposed parts of the system.