Stable isotope (H, C, O, and S) systematics of the Pueblo Viejo acid
sulfate system, the world's largest bulk mineable acid sulfate gold
deposit, were investigated to determine the importance of magmatic
fluids in hydrothermal systems of this type. Mineralization at Pueblo
Viejo is hosted by a maar-diatreme system that cuts sedimentary rocks
and spilites of the Late Cretaceous Los Ranchos Formation. The deposit
is characterized by two stages of advanced argillic alteration: stage I
fluids produced deep alunite + quartz and shallower kaolinite + quartz,
both associated with disseminated pyrite; stage II fluids overprinted
stage I and produced deep pyrophyllite + diaspore and an overlying
silica cap. An increase in temperature from stage I (approximately
250-degrees-C) to stage II (approximately 250-degrees-300-degrees-C) is
indicated by mineral stabilities and sulfur isotope thermometry of
coexisting sulfide-sulfate pairs. Stage I
quartz-alunite-kaolinite-pyrite alteration and associated disseminated
Au-Ag mineralization are interpreted to have precipitated in equilibrium
with magmatic discharge fluids (deltaD = -35 to -15 parts per thousand;
deltaO-18 = 6.5 to 11 parts per thousand) that experienced only minor
(less-than-or-equal-to 25%) dilution by meteoric water (deltaD = -30
parts per thousand; deltaO-18 = -5 parts per thousand). Stage II
advanced argillic alteration resulted from a renewed influx of
essentially pure magmatic discharge fluid. The shallow silica cap
generated during stage II resulted from significant ground-water
dilution of magmatic fluids.
Circulation of conductively heated seawater adjacent to the acid sulfate
system caused deposition of a carbonate-sulfate zone that sealed the
acid sulfate mineralization from ground-water and seawater infiltration.
It is possible that the regional greenschist facies alteration of Los
Ranchos Formation rocks was also related to such seawater circulation.
These results suggest that the dominant proportion of metals and fluids
in acid sulfate systems are derived from bulk condensates of magmatic
vapors. Relatively small (less-than-or-equal-to 25%) proportions of
nonmagmatic fluids, that is, fluids from meteoric or seawater sources,
may, however, have been important in causing precipitation of the ore
components.