Abstract

The magmatic-hydrothermal evolution of the El Teniente porphyry Cu-Mo deposit in the Central Andes in Chile is reconstructed based on field relationships, scanning electron microscopy cathodoluminescence, petrography, and fluid inclusion analysis by microthermometry and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Three major stages of Cu-Mo mineralization are observed. Following the barren hydrothermal stage 1, the stage 2 mineralization is characterized by quartz-anhydrite stockwork veins and breccias with chalcopyrite, bornite, and molybdenite. Both stages 1 and 2 are associated with pervasive potassic alteration. Quartz-anhydrite veins with chalcopyrite, bornite, and molybdenite associated with phyllic alteration represent stage 3 mineralization. Stage 4 mineralization, linked to the formation of the large Braden diatreme, is characterized by breccias and rare veins containing a lower temperature assemblage with tourmaline, sericite, and lesser tennantite, bornite, and chalcopyrite with late gypsum in local vugs.

Ten fluid types are distinguished in this study based on petrographic and microthermometric criteria, such as phase proportions, daughter minerals, homogenization behavior, and salinity. The hydrothermal evolution across the stages of Cu deposition is characterized by the contraction of a vapor phase originating by phase separation during stage 2. Overall cooling of the system at pressures fluctuating around the two-phase fluid surface led to a transition from a two-phase fluid state dominated by vapor at ~410°C and 300 bars in stage 2, to a single-phase low-salinity fluid derived from cooling and contraction of magmatic vapor to a liquid, which dominates during stage 3 mineralization at <350°C and 200 bars. Copper mineralization mainly formed from the vapor phase and its low-salinity liquid derivatives, representing a large volume of fluid with an initially high Cu content (1.2 ± 0.4 wt % Cu). Copper sulfides precipitated upon cooling between 410° and 320°C, indicated by a drop of Cu/(Na + K + Mn + Fe) ratios of over four orders of magnitude through the evolution of the deposit from stage 2 to stage 4. The highest Mo concentrations occur in residual brines resulting from extreme boiling, as indicated by concurrent halite saturation.

Recent geochronology (Cannell, 2005; Maksaev et al., 2004) suggests a relatively long-lived magmatic-hydrothermal system at El Teniente. Our fluid chemical data show no evidence for major crystal fractionation in a large fluid-generating upper-crustal magma chamber, because Cs/(Na + K + Mn + Fe) is constant from pre-ore to all syn-ore fluids. However, the initiation of copper mineralization was associated with a 4- to 10-fold increase in the concentration of Cu, Mo, Li, and Fe in the inferred main ore-forming input fluid, compared with pre-ore fluids of intermediate salinity and otherwise very similar major and trace-element ratios. These data indicate that injection from depth of an exceptionally Cu, Mo, Li, and, probably, also S-rich volatile phase into an already actively evolving upper-crustal magmatic-hydrothermal system triggered the formation of this unusually large and rich copper deposit.

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