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Carbonate minerals are common gangue minerals in the low sulfidation epithermal Au-Ag District of El Penon, II Region, Chile. They can be subdivided into two principal groups on the basis of the paragenetic relationships. Paragenetically early carbonates occur together with quartz and sulfide minerals in banded veins and are closely associated with precious metal mineralization. These carbonates are compositionally complex, consisting of calcite, finely intermixed with ankerite, kutnohorite, rhodochrosite and dolomite, but also contain up to 0.8 wt% PbCO3 and up to 4.1 wt% ZnCO3.. in contrast, the paragenetically late carbonates are relatively pure calcite, although dolomite has been recognized locally. Late calcites contain less than 3.5 wt% MnCO,, less than 0.5 wt% ZnCO3, and less than 2.7 wt% FeCO3 and are also distinct from the early carbonates in that they exhibit moderate to intense red fluorescence under short-wave ultraviolet light. The red fluorescence color is likely related to moderate Mn contents in calcite, combined with trace amounts of Pb, but low Fe and Mg concentrations. Late carbonates are interpreted to have precipitated from CO2 -rich steam-heated fluids and are not directly associated with ore. delta O-18 compositions of both, early and late carbonates generally range between 21.1 and 14.1%o relative to the Standard Mean Ocean Water (VSMOW). However, two samples of late calcite yielded delta O-18 values of 3 and 5 parts per thousand, respectively. The calculated composition of water in equilibrium with calcite at the estimated temperature of deposition of 230 degrees C yielded delta O-18 values between 6 and 13 parts per thousand, but negative values of -3.1 and -5.1 parts per thousand. for two samples of late calcite. Although the low delta O-13 values for two of the late calcites and oxygen isotopic data of vein quartz from earlier studies suggest precipitation from a meteoric fluid, the heavy isotopic signature of most carbonates indicates that the mineralizing fluid was overall heterogeneous. delta C-13 values range between -8.9 parts per thousand. and +1.5 parts per thousand. relative to the Pee Dee Belemnite (VPDB) standard and are consistent with a meteoric or magmatic origin, in some samples probably with a minor contribution of organic carbon. Periodic involvement of a fluid with a strong magmatic component or, alternatively, meteoric water which has undergone extensive evaporation probably played an important role in the hydrothermal system. The complex stable isotope signature of El Penon is interpreted as indicative for arid climatic conditions as a meteoric origin of the fluid is not as evident as in similar deposits emplaced in more pluvial regions.