Abstract

Chilean manto-type (CMT) Cu(-Ag) hydrothermal deposits share a characteristic association of volcano-sedimentary Jurassic to Lower Cretaceous host rocks, style of mineralization, ore and associated mineralogy and geochemistry, with ore grades typically >1%Cu, that make this family of deposits significant and interesting, both academically and economically. Although often stratabound, geological evidence supports an epigenetic origin for these deposits. We present a detailed stable isotope study of La Serena and Melipilla-Naltahua Lower Cretaceous deposits, central Chile, which reveals extremely negative delta S-34 values, to -50 parts per thousand., which are among the lowest values found in any ore deposit. In addition, the range of delta S-34 values from sulfides in the two areas is very wide: -38.3 to -6.9 parts per thousand in La Serena, and -50.4 to -0.6 parts per thousand in Melipilla-Naltahua. These new data significantly extended the reported range of delta S-34 data for CMT deposits. Co-existing sulfates range from 7.9 to 14.3 parts per thousand, and are exclusive to La Serena deposit. The wide sulfide isotopic range occurs at deposit and hand specimen scale, and suggests a polygenic sulfur source for these deposits, where bacteriogenic sulfide dominates. While sulfur isotope data for the bulk of Jurassic CMT deposits, northern Chile, suggests a predominant magmatic source in their origin (mean = 2.7 +/- 1.9 parts per thousand, 1 sigma), contributions of a magmatic component is only likely to be involved at Melipilla-Naltahua deposit. The delta C-13 values obtained for calcites associated with the mineralization range from -20.1 to 0.2 parts per thousand 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 delta S-34, 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 -50.4 parts per thousand, together with sulfides directly precipitated from a hydrothermal fluid with delta S-34 close to 0 parts per thousand. 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. (C) 2013 Elsevier B.V. All rights reserved.