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 El Mochito are compatible with a high-temperature (350 degrees -500 degrees ) 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 of stable isotope compositions of the host limestone around the El Mochito deposit indicate a gradual depletion in 13 C and 18 O 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 18 O of up to 18 per mil relative to background limestone compositions. 18 O depletion is easily detected for distances of up to 4 kilometers from the orebody. The depletion of 13 C 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 [asymp] 5.0ppm, delta 18 O SMOW [asymp] 23.5ppm), 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.

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