El Mochito, the largest operating mine in Central America, is a typical
skarn sulfide-oxide, limestone-replacement lead-zinc-silver deposit.
Fluid inclusion and stable isotope geochemistry of the skarn and sulfide
mineralization at Fl Mochito are compatible with a high-temperature (350
degrees-500 degrees C) origin from hydrothermal fluids with delta D
values of -40 to -90 per mil and delta(18)O values between 10 and -1 per
mil which were probably of mixed magmatic-meteoric origin. A detailed
study crf stable isotope compositions of the host limestone Is around
the Fl Mochito deposit indicate a gradual depletion in C-13 and O-18
with approach to the ore body, indicating that the changes in isotopic
composition were imposed on the limestone during the hydrothermal
activity related to the mineralization. Oxygen isotope exchange between
the hydrothermal fluid and limestone produced a strong depletion in O-18
of up to 18 per mil relative to background limestone compositions. O-18
depletion is easily detected for distances of up to 4 kilometers from
the orebody. The depletion of C-13 is much more restricted, with changes
of up to 4 per mil in delta(13)C Values relative to the inferred
original limestone, extending only 30 m of the ore deposit. The original
stable isotope composition of the limestone, as inferred from distal,
unaltered samples (delta(13)C(PDB) approximate to 5.0 parts per
thousand, delta(18)O(SMOW) approximate to 23.5 parts per thousand), is
similar to that of other Cretaceous limestones. Small-scale stable
isotope analyses of the matrix, relict fossils, and late-stage Veins
within the limestones, and comparison to the isotopic composition of the
bulk-rock samples suggests that the extent of stable isotope alteration
was a function of the initial grain size, the permeability of the rocks,
the fluid/rock ratio, and the temperature of the hydrothermal system, as
well as the isotopic composition and dominant C-bearing species within
the hydrothermal fluid. Both extensive recrystallization of lime-mud
matrices and the presence of faults and fractures may have enhanced the
secondary porosity and permeability of the limestones, allowing for
effective fluid infiltration and increasingly pervasive fluid flow with
approach to the ore deposit. The mechanisms responsible for C and O
isotope alteration of the limestone were calcite recrystallization and
late calcite veining. Large stable isotope alteration halos within
limestones surrounding ore deposits of this type provide useful guides
to exploration.