Vein carbonates in the low sulfidation epithermal Au-Ag District of El Peñón, II Región, Chile: environment of formation and exploration implications

Abstract

Carbonate minerals are common gangue minerals in the low sulfidation epithermal Au-Ag District of El Peñón, II Región, 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% PbC0₃ and up to 4.1 wt% ZnC0₃. In contrast, the paragenetically late carbonates are relatively pure calcite, although dolomite has been recognized locally. Late calcites contain less than 3.5 wt% MnC0₃, less than 0.5 wt% ZnC0₃ and less than 2.7 wt% FeC0₃ 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 C0₂-rich steam-heated fluids and are not directly associated with ore. 8¹⁸0 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 8¹⁸0 values of 3 and 5%o, respectively. The calculated composition of water in equilibrium with calcite at the estimated temperature of deposition of 230°C yielded 8¹⁸0 values between 6 and 13%o, but negative values of -3.1 and -5.1%o for two samples of late calcite. Although the low 8¹⁸0 values for two of the late calcitesand 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. 8¹³C values range between -8.9%o and +1.5%o 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 Peñón 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.