Organic geochemistry of the San Vicente zinc–lead district, eastern Pucará Basin, Peru

Isotopic and chemical studies of organic matter of the San Vicente Mississippi Valley-type Zn–Pb district, hosted in the dolomitic units of the eastern Pucará Basin (Peru) provide insight into the source of organic matter and of the organic matter–ore relationship. A large scatter exists in the δ13C and δ15N values of the highly mature kerogens. These bulk data were combined with δ13C covariations of the saturated and aromatic fractions of the rock extracts to indicate a contribution of continental higher-plant lipids to the marine algal source of the organic matter disseminated in the host carbonate rocks. An enrichment in 15N of the kerogens of mineralized samples compared to barren samples suggests that isotopically heavy nitrogen compounds were introduced to the ore fluid. The fluid likely leached these compounds from the underlying clastic rocks and other detrital units of the Pucará. The extracts from mineralized samples are enriched in 13C compared to the associated kerogens. This suggests that the rocks in the mineralized areas were stained by a mixture of extrinsic and indigenous bitumens. By the use of novel instrumentation, such as a gas chromatograph (GC) coupled to a stable isotope ratio mass spectrometer (IRMS) through a combustion (C) interface (GC/C/IRMS), stable carbon isotope analyses of individual hydrocarbons were also obtained. Both the molecular and isotopic signatures of the individual hydrocarbons are related to mineralization. The variability in the organic molecular parameters and the δ13C values of the individual n-alkanes and isoprenoids indicate mixing of indigenous hydrocarbons with extrinsic 13C-depleted microbially derived hydrocarbons. The extrinsic-source hydrocarbons were introduced to the mineralization site by the ore fluid or later fluids using the same porosity network. Oxidative alteration of the migrating hydrocarbons was involved in the thermochemical reduction of sulfate to H2S and precipitation of pyrobitumen. Large differences (>12‰) in the carbon isotope compositions of long-chain (C20+) n-alkanes can be utilized in distinguishing sources of organics in the barren and mineralized samples.