Carbon and oxygen isotope studies of the host and gangue carbonates of
Mississippi Valley-type zinc-lead deposits in the San Vicente District
hosted in the Upper Triassic to Lower Jurassic dolostones of the Pucara
basin (central Peru) were used to constrain models of the ore formation.
A mixing model between an incoming hot saline slightly acidic radiogenic
(Pb, Sr) fluid and the native formation water explains the overall
isotopic variation (delta(13)C = - 11.5 to + 2.5 parts per thousand
relative to PDB and delta(18)O = + 18.0 to + 24.3 parts per thousand
relative to SMOW) of the carbonate generations. The dolomites formed
during the main ore stage show a narrower range (delta(13)C = - 0.1 to +
1.7 parts per thousand and delta(18)O = + 18.7 to + 23.4 parts per
thousand) which is explained by exchange between the mineralizing fluids
and the host carbonates combined with changes in temperature and
pressure. This model of fluid-rock interaction explains the pervasive
alteration of the host dolomite I and precipitation of sphalerite I. The
open-space filling hydrothermal white sparry dolomite and the coexisting
sphalerite II formed by prolonged fluid-host dolomite interaction and
limited CO2 degassing. Late void-filling dolomite III (or calcite) and
the associated sphalerite III formed as the consequence of CO2 degassing
and concomitant pH increase of a slightly acidic ore fluid. Widespread
brecciation is associated to CO2 outgassing. Consequently, pressure
variability plays a major role in the ore precipitation during the late
hydrothermal events in San Vicente.
The presence of native sulfur associated with extremely carbon-light
calcites replacing evaporitic sulfates (e.g., delta(13)C = - 11.5 parts
per thousand), altered native organic matter and heavier hydrothermal
bitumen (from - 27.0 to - 23.0 parts per thousand delta(13)C) points to
thermochemical reduction of sulfate and/or thiosulfate.
The delta(13)C- and delta(18)O-values of the altered host dolostone and
hydrothermal carbonates, and the carbon isotope composition of the
associated organic matter show a strong regional homogeneity. These
results coupled with the strong mineralogical and petrographic
similarities of the different MVT occurrences perhaps reflects the fact
that the mineralizing processes were similar in the whole San Vicente
belt, suggesting the existence of a common regional mineralizing
hydrothermal system with interconnected plumbing.