The 未13C values obtained for calcites associated with the mineralization range from 鈭?#xA0;20.1 to 0.2鈥?also suggesting polygenic carbon sources, with the likely strong involvement of degradation of organic matter and leaching of limestone.
Two different genetic models, with involvement of hydrocarbon, are proposed for both areas. For Melipilla-Naltahua, a two-step model can be developed as follows: 1) Framboidal pyrite growth, with very low 未34S, formed by bacterial sulfate reduction in an open system, and with diagenetic degradation of oil-related brines, leaving pyrobitumen. 2) Cu-bearing stage, replacing of framboidal pyrite, inheriting depleted sulfur as low as 鈭?#xA0;50.4鈥? together with sulfides directly precipitated from a hydrothermal fluid with 未34S close to 0鈥? For La Serena, a single step model fits best, without framboidal pyrite generation. Cu-bearing sulfides were precipitated mainly in veins where Cu plus base metal-bearing hydrothermal fluids mixed with H2S generated by bacterial sulfate reduction in the host rocks. Isotopic evidence clearly illustrates that bacterial activity, perhaps enhanced by hydrothermal activity, was fed by hydrocarbon brines and sulfate remobilized from continental evaporites. It is possible that variable ecological conditions led to different extents of isotopic fractionation, adding to the typical sulfur isotopic heterogeneity of such bacterial systems. For both areas, the Cu-bearing stage occurred during the peak to waning stages of the very low-grade metamorphism that affected the Lower Cretaceous sequence.