Abstract

Stratabound Cu-(Ag) deposits in the Coastal Cordillera of northern Chile were emplaced under an extensional setting during the Late Jurassic and Early Cretaceous. Las Luces and Altamira are two stratabound Cu-(Ag) deposits located approximately at the same latitude (similar to 25 degrees 45'S), but the former is hosted by Jurassic volcanic and volcaniclastic rocks, and the latter by Cretaceous volcano-sedimentary sequences. Both deposits show similar hydrothermal alteration types with albitization and hematite-chlorite superimposed on low-grade regional metamorphism. Sulfide mineralization is represented mainly by pyrite, chalcopyrite, and a bornite- "chalcocite" assemblage. Chalcopyrite is relatively minor and can replace early pyrite. In addition, framboidal pyrite of possible diagenetic origin was observed in Altamira. Copper mineralization is dominated by a bornite- "chalcocite" assemblage; however, electron probe analyses show that "chalcocite" has a composition ranging from geerite to djurleite. The typical mymekitic-like exsolution texture observed in the bornite-Cu sulfides assemblage is interpreted as caused by sub-solidus re-equilibration on cooling of the bornite-digenite solid solution. Silver, the main byproduct in these deposits, is probably incorporated in solid solution in Cu sulfides and bornite, although Ag-sulfide microparticles were occasionally observed within sulfides in Altamira. Copper sulfides of the geerite-djurleite series can contain high amounts of Ag, ranging between 202 and 789 ppm, whereas in bornite from Las Luces Ag can reach up to 270 ppm. The presence of low-temperature (similar to 100 degrees C) hydrothermal Cu sulfides is consistent with formation temperatures of < 300 degrees C, based on previous fluid inclusion studies. Bulk stable isotope data shows that sulfur in these deposits have different sources. In Las Luces delta S-34 values for bornite and pyrite (-2.5 to + 2.9 parts per thousand) indicate a magmatic source, whereas in Altamira the negative values for "chalcocite" (delta S-34: - 38.7 to -10.7 parts per thousand) are interpreted as sulfur derived by bacterial reduction of marine sulfate. The Las Luces and Altamira deposits were possibly formed by high water/rock ratios where basin-derived fluids leached metals from the volcanic/volcano-sedimentary host rocks. However, extensive leaching of the volcanic host rocks necessary to extract the Cu contained in silicate minerals is not consistent with the relatively small volume of hydrothermal alteration associated with these deposits, suggesting an additional magmatic contribution. In the revised genetic model, variable contributions of a magmatic and non-magmatic source are needed to form these stratabound Cu-(Ag) deposits.