The Role of Hydrocarbons in the Genesis of the Sediment-Hosted Stratiform Copper Deposits, Neuquen Basin (Argentina)

The Cu deposits of Neuquen Basin are an example of sediment-hosted deposits generated by interaction of hydrocarbons with host red-beds and metal-charged basinal brines. After deposition and diagenesis of red beds, the incoming of hydrocarbons promoted the reduction of hematite to Fe2+ resulting in bleaching of the sandstones. The presence of bitumen is evidence that hydrocarbons were the reducing agents. Three calcite events are: i) calcite I, with heaviest delta C-13(V-PDB) values, precipitated from evolved meteoric waters before hydrocarbons migration, ii) calcite II, with lighter delta C-13(V-PDB) values, precipitated during bleaching probably supplied by the organic acids that migrated together with hydrocarbons and, iii) calcite Ill precipitated with the mineralization, showing even more negative delta C-13(V-PDB) values. Cu-sulfides have highly negative delta S-34(V-CDT) values (-45.2 parts per thousand to -41.7 parts per thousand), indicating the involvement of bacterial sulfate reduction, which may also be linked to low delta C-13(V-PDB) in calcite Ill. Fluid inclusions in calcite II and Ill fluoresce under UV light, present immiscibility and adherence phenomena and generates clathrates during cooling experiments suggesting the presence of organic fluids.