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El Soldado, Chile, is a giant strata-bound copper deposit hosted in Lower Cretaceous basalt and rhyodacite. Previous work suggested that copper was concentrated preferentially where hydrothermal copper-rich solutions replaced pre-existing, low-temperature, diagenetic pyrite, which is generally associated with bitumen (solidified petroleum). Doubt remains on whether some deep zones with massive crystalline pyrite veins, and massive chalcopyrite, bornite and chalcocite ores, could represent a net input of sulphur from hydrothermal, magmatically-derived sources. Magmatic related sulphur has a d34S value close to zero ‰. Conversely, diagenetic, low-temperature crystallization of pyrite, especially with the aid of sulphur-reducing bacteria in a degrading petroleum reservoir, would have led to extreme fractionation of sulphur and a wide range of d34S values, which would be locally available to form Cu sulphides during the hydrothermal phase. There is textural evidence of pre-existing diagenetic pyrite, as well as textures indicative of new hydrothermal growth. Diagenetic pyrite is characterized by framboidal structures of ca. 16µm diameter or smaller; colloform textures found in pyrite also suggest a low-temperature genesis. Although controversial, the general consensus is that framboids may grow with bacterial involvement. A range of stages of development of massive crystalline aggregates is observed in the samples: individual microcrysts, framboids, framboid clusters, recrystallized megacryst overgrowths, and banded concentric zones. Hydrothermal or high-temperature textures are characterized by idioblastic pyrite cubes or pyritohedra suspended in late calcite matrix. Temperatures from fluid inclusions in calcite indicate maximum (pressure corrected) temperatures of ca. 300 °C, and minimum temperatures of over 100 °C.

d34S values from analysis in this study have a range of 24.5‰, from -7.4‰ to + 17.1‰. This variation is characteristic of a compartmentalized system that has been incompletely fractionated, and compatible with bacterial interaction. Several samples yielded d34S values overlapping with that characteristic of magmatic sulphur, thus allowing for the possibility of some degree of input of homogenized sulphur, perhaps (but not necessarily) from a magmatic source.

Microprobe analyses indicate the presence of local concentrations of arsenic within the core of framboidal structures and also in fresh overgrowths on idiomorphic pyrite, probably inherited from the diagenetic phase.

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Pages: 174
Supervisor: Marcos Zentilli