Abstract | dc.description.abstract | Chilean manto-type (CMT) Cu(–Ag) hydrothermal deposits share a characteristic association of volcanosedimentary
Jurassic to Lower Cretaceous host rocks, style of mineralization, ore and associated mineralogy
and geochemistry, with ore grades typically N1%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 δ34S values, to −50‰, which are
among the lowest values found in any ore deposit. In addition, the range of δ34S values from sulfides in the
two areas is very wide: −38.3 to −6.9‰ in La Serena, and −50.4 to −0.6‰ in Melipilla–Naltahua. These
new data significantly extended the reported range of δ34S data for CMT deposits. Co-existing sulfates
range from 7.9 to 14.3‰, 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‰, 1σ), contributions of a magmatic
component is only likely to be involved at Melipilla–Naltahua deposit.
The δ13C values obtained for calcites associated with the mineralization range from −20.1 to 0.2‰ 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 δ34S,
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‰, together with sulfides directly precipitated from a hydrothermal fluid with δ34S close to
0‰. 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. | en_US |