The Pierina epithermal Au–Ag deposit, Ancash, Peru: paragenetic relationships, alunite textures, and stable-isotope geochemistry

Abstract

The Pierina high-sulfidation epithermal Au–Ag deposit (9°26.5′S; 77°35′W) was emplaced in the Middle Miocene into a hypabyssal-to-extrusive “pumice-tuff” and an underlying, older, dacitic flow-dome complex, both of which are cut by hydrothermal breccias and small dacitic domes. Stage I advanced argillic alteration generated a core of vuggy silica, focused in the tuff, and surrounded successively by zones of quartz–alunite, dickite±kaolinite±pyrophyllite, and illite–montmorillonite±kaolinite. Laser-ablation ICP-MS analysis of the sulfide minerals of the succeeding, Stage II, Cu (–Pb, Bi, Sb, Zn, As)–barite mineralization, largely confined to the vuggy-silica zone, reveals that both Au and Ag were introduced at that time. This assemblage was almost entirely obliterated during Stage III, when oxidation by low-temperature meteoric waters generated botryoidal hematite–goethite assemblages, which are now the main precious-metal hosts. Stage IV barite–acanthite mineralization shows limited temporal overlap with Stage III, but dominantly overprints the hematite–goethite assemblage. The deposit incorporates alunite exhibiting a wide range of modes of occurrence, grain size, and morphology. Disseminated alunite dominates Stage I alteration, which replaced phenocrysts and fragments, and locally hosts corroded, ≤40 μm alumino-phosphate-sulfate (APS) inclusions. The alunite shows limited Na substitution [molar Na/(Na+K)=<0.2], but alunite–natroalunite assemblages occur sporadically throughout the quartz–alunite alteration zone. Isotopic analysis yields δ 34S values of 16.6–31.0‰, consistent with a magmatic–hydrothermal origin. Rare occurrences of disseminated alunite and pyrite in textural equilibrium occur in unoxidized areas of the deposit, and yieldΔ 34S alu-py precipitation temperatures of 179 to 250 °C, with the majority below 200 °C. Disseminated alunite in shallower, oxidized portions of the deposit, where pyrite has been destroyed, yield δ 18O SO 4 values of 6.3‰ to 14.4‰. The lighter compositions occur along the axis of the quartz–alunite alteration zone, and imply precipitation at higher temperatures, albeit with some meteoric water involvement. Porcelaneous alunite (10–100 μm) forms the matrix of breccias that cut Stage I alteration. Sodium contents are low [Na/(Na+K molar≃0.1) and APS inclusions have not been identified. The δ 34S values of 21.8–27.1‰ are consistent with a magmatic–hydrothermal origin, but the δ 18O SO 4 values of 11.4‰ to 14.6‰ indicate deposition under cooler temperatures and the involvement of meteoric water that interacted substantially with igneous country-rocks. Coarse, open-space-filling alunite, which occurs as veins or coatings on breccia fragments, exhibits limited Na substitution, is not associated with natroalunite, and does not host APS inclusions. Its δ 34S and δ 18O SO 4 values of 12.9‰ to 26.2‰ and 6.5‰ to 8.6‰, respectively, imply a magmatic–hydrothermal origin and deposition at higher temperatures. Dike-like bodies and mantos of friable alunite have distinctive δ 34S (16.2–19.3‰) and δ 18O SO 4 (7.6–9.2‰) values, and may record a steam-heated environment. However, these compositions may also indicate loss of H 2S gas during oxidation, or overprinting of steam-heated alunite by magmatic–hydrothermal alunite (or vice versa). All forms of alunite exhibit higher δ 18O SO 4 values in the central part of the deposit, and the coherent isotopic chemistry casts doubt on the reliability of the widely accepted textural criteria for the origin of this mineral in hydrothermal systems.