Stable-isotope geochemistry of the Pierina high-sulfidation Au-Ag deposit, Peru: influence of hydrodynamics on SO42−−H2S sulfur isotopic exchange in magmatic-steam and steam-heated environments

Magmatic-steam alunite has higher δ34S (8.5‰ to 23.2‰) and generally lower δ18OSO4 (1.0 to 11.5‰), δ18OOH (−3.4 to 5.9‰), and δD (−93 to −77‰) values than predicted on the basis of data from similar occurrences. These data and supporting fluid-inclusion gas chemistry imply that the rate of vapor ascent for this environment was unusually slow, which provided sufficient time for the uptake of groundwater and partial to complete SO42−-H2S isotopic exchange. The slow steam velocities were likely related to the dispersal of the steam column as it entered the tuffs and possibly to intermediate exsolution rates from magmatic brine. The low δD values may also partly reflect continuous degassing of the mineralizing magma. Similarly, data for steam-heated alunite (δ34S=12.3‰ to 27.2‰; δ18OSO4=11.7‰ to 13.0‰; δ18OOH=6.6‰ to 9.4‰; δD=−59‰ to −42‰) are unusual and indicate a strong magmatic influence, relatively high temperatures (140 to 180 °C, based on Δ18 OSO4-OH fractionations), and partial to complete sulfur isotopic exchange between steam-heated sulfate and H2S. Restricted lithologically controlled fluid flow in the host tuffs allowed magmatic condensate to supplant meteoric groundwater at the water table and create the high-temperature low-pH conditions that permitted unusually rapid SO42−-H2S isotopic equilibration (50-300 days) and (or) long sulfate residence times for this environment. Late void-filling barite (δ34S=7.4‰ to 29.7‰; δ18OSO4=−0.4‰ to 15.1‰) and later void-filling goethite (δ18O=−11.8‰ to 0.2‰) document a transition from magmatic condensate to dominantly meteoric water in steam-heated fluids during cooling and collapse of the hydrothermal system. These steam-heated fluids oxidized the top ∼300 m of the deposit by leaching sulfides, redistributing metals, and precipitating barite±acanthite±gold and goethite-hematite±gold. Steam-heated oxidation, rather than weathering, was critical to forming the orebody in that it not only released encapsulated gold but likely enriched the deposit to ore-grade Au concentrations.