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

摘要

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 SO42--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 (delta(34)S = 8.5 parts per thousand to 31.7 parts per thousand; delta(18)O(SO4) = 4.9 parts per thousand to 16.5 parts per thousand; delta(18)O(OH) = 2.2 parts per thousand to 14.4 parts per thousand; delta D = -97 parts per thousand to -39 parts per thousand), sulfides (delta(34)S = -3.0 parts per thousand to 4.3 parts per thousand), sulfur (delta(34)S = - 1.0 parts per thousand, to 1.1 parts per thousand), and clay minerals (delta(18)O = 4.3 parts per thousand to 12.5 parts per thousand; delta D = - 126 parts per thousand to -81 parts per thousand) 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 delta(34)S(Sigma S) signature of - 1 parts per thousand, (3) SO2 disproportionation to HSO4- and H2S between 320 and 180 degrees 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 delta(34)S (8.5 parts per thousand to 23.2 parts per thousand) and generally lower delta(18)O(SO4) (1.0 to 11.5 parts per thousand), delta(18)O(OH) (-3.4 to 5.9 parts per thousand), and delta D (- 93 to - 77 parts per thousand) 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 S02 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 delta D values may also partly reflect continuous degassing of the mineralizing magma. Similarly, data for steam-heated alunite (delta(34)S = 12.3 parts per thousand to 27.2 parts per thousand; delta(18)O(SO4) = 11.7 parts per thousand, to 13.0 parts per thousand; delta(18)O(OH) = 6.6 parts per thousand to 9.4 parts per thousand; delta D = -59 parts per thousand to -42 parts per thousand) are unusual and indicate a strong magmatic influence, relatively high temperatures (140 to 180 degrees C, based on Delta(18) O-SO4 - 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 the high-temperature low-pH conditions that permitted unusually rapid 4 -H,S isotopic equilibration (50-300 days) and (or) long sulfate residence times for this environment. Late void-filling barite (delta(34)S = 7.4 parts per thousand to 29.7 parts per thousand; delta(18)O(SO4) = -0.4 parts per thousand to 15.1 parts per thousand) and later void-filling goethite (delta(18)O = - 11.8 parts per thousand to 0.2 parts per thousand) docume