Carbon and oxygen isotope study of hydrothermal carbonates in the zinc-lead deposits of the San Vicente district, central Peru: a quantitative modeling on mixing processes and CO2 degassing

Carbon and oxygen isotope studies of the host and gangue carbonates of Mississippi Valley-type zinc-lead deposits in the San Vicente District hosted in the Upper Triassic to Lower Jurassic dolostones of the Pucará basin (central Peru) were used to constrain maiels of the ore formation. ng model between an incoming hot saline slightly acidic radiogenic (Pb, Sr) fluid and the native formation water explains the overall isotopic variation (δ 13C = −11.5 to + 2.5‰ relative to PDB and δ 18O = + 18.0 to + 24.3‰ relative to SMOW) of the carbonate generations. The dolomites formed during the main ore stage show a narrower range (δ 13C = −0.1 to + 1.7‰ and δ 18O = + 18.7 to +23.4‰) which is explained by exchange between the mineralizing fluids and the host carbonates combined with changes in temperature and pressure. This model of fluid-rock interaction explains the pervasive alteration of the host dolomite I and precipitation of sphalerite I. The open-space filling hydrothermal white sparry dolomite and the coexisting sphalerite II formed by prolonged fluid-host dolomite interaction and limited CO2 degassing. Late void-filling dolomite III (or calcite) and the associated sphalerite III formed as the consequence of CO2 degassing and concomitant pH increase of a slightly acidic ore fluid. Widespread brecciation is associated to CO2 outgassing. Consequently, pressure variability plays a major role in the ore precipitation during the late hydrothermal events in San Vicente. esence of native sulfur associated with extremely carbon-light calcites replacing evaporitic sulfates (e.g., δ 13C = −11.5‰), altered native organic matter and heavier hydrothermal bitumen (from −27.0 to −23.0‰ δ 13C) points to thermochemical reduction of sulfate and/or thiosulfate. 13 C- and ° 18O-values of the altered host dolostone and hydrothermal carbonates, and the carbon isotope composition of the associated organic matter show a strong regional homogeneity. These results coupled with the strong mineralogical and petrographic similarities of the different MVT occurrences perhaps reflects the fact that the mineralizing processes were similar in the whole San Vicente belt, suggesting the existence of a common regional mineralizing hydrothermal system with interconnected plumbing.