Stable Isotope Evidence for Magmatic Fluids in the
Pueblo Viejo Epithermal Acid Sulfate Au-Ag
Deposit, Dominican Republic
Torsten W. Vennemann, John L. Muntean, Stephen
E. Kesler, James R. O'Neil, John W. Valley, Norman
Russell
(Economic Geology, Vol. 88, 1993, pp. 55-
74)
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 (~250*C) to stage II (~250-300*C) is
indicated by mineral stabilities and sulfur isotope
thermometry of coexisting sulfide-sulfate pairs.
Stage I quartz-alunte-kaolinite-pyrite alteration and
associated disseminated Au-Ag mineralization are
interpreted to have precipitated in equilibrium with
magmatic discharge fluids (*D = -35 to -15%o *18O
= 6.5 to 11%o) that experienced only minor (< =
25%) dilution by meteoric water (*D = -30%o; *18O
= -5%o). 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 depostion
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 (< = 25%)
proportions of nonmagmatic fluids, that is, from
meteoric or seawater sources, may, however, have
been important in causing precipitiation of the ore
components.