Stable-isotope geochemistry of the Pierina high-sulfidation Au–Ag deposit, Peru: influence of hydrodynamics on SO₄^2-–H₂S sulfur isotopic exchange in magmatic-steam and steam-heated environments

Authors

Richard H. Fifarek, Southern Illinois University
Robert O. Rye, U.S. Geological SurveyFollow

Date of this Version

2005

Comments

Published in Chemical Geology 215 (2005)

Abstract

The Pierina high-sulfidation Au–Ag deposit formed 14.5 my ago in rhyolite ash flow tuffs that overlie porphyritic andesite and dacite lavas and are adjacent to a crosscutting and interfingering dacite flow dome complex. The distribution of alteration zones indicates that fluid flow in the lavas was largely confined to structures but was dispersed laterally in the tuffs because of a high primary and alteration-induced permeability. The lithologically controlled hydrodynamics created unusual fluid, temperature, and pH conditions that led to complete SO₄ ^2‒–H₂S isotopic equilibration during the formation of some magmatic-steam and steam-heated alunite, a phenomenon not previously recognized in similar deposits. Isotopic data for early magmatic hydrothermal and main-stage alunite (δ³⁴S=8.5%◦ to 31.7%◦; δ¹⁸O_SO4 =4.9%◦ to 16.5%◦; δ¹⁸O_OH=2.2%◦ to 14.4%◦ ; δD=‒97%◦ to ‒39%◦), sulfides (δ³⁴S=‒3.0%◦ to 4.3%◦), sulfur (δ³⁴S=‒1.0%◦ to 1.1%◦), and clay minerals (δ¹⁸O=4.3%◦ to 12.5%◦; δD=‒126%◦ to ‒81%◦) are typical of high-sulfidation epithermal deposits. The data imply the following genetic elements for Pierina alteration–mineralization: (1) fluid and vapor exsolution from an I-type magma, (2) wallrock buffering and cooling of slowing rising vapors to generate a reduced (H₂S/SO₄≈6) highly acidic condensate that mixed with meteoric water but retained a magmatic δ³⁴S_ΣS signature of ~1%◦, (3) SO₂ disproportionation to HSO^‒₄and H₂S between 320 and 180 °C, and (4) progressive neutralization of laterally migrating acid fluids to form a vuggy quartz→alunite– quartz±clay→intermediate argillic→propylitic alteration zoning. Magmatic-steam alunite has higher δ³⁴S (8.5%◦ to 23.2%◦) and generally lower δ¹⁸O_SO4 (1.0 to 11.5%◦), δ¹⁸O_OH (‒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 SO₄ ^2‒–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