Abstract

Alteration zones at the gold-rich Bajo de la Alumbrera porphyry copper deposit in northwestern Argentina are centered on several porphyritic intrusions. They are zoned from a central copper-iron sulfide and gold-mineralized potassic (biotite-K-feldspar ± quartz) core outward to propylitic (chlorite-illite-epidotecalcite) assemblages. A mineralized intermediate argillic alteration assemblage (chlorite-illite ± pyrite) has overprinted the potassic alteration zone across the top and sides of the deposit and is itself zoned outward into phyllic (quartz-muscovite-illite ± pyrite) alteration. This study contributes new data to previously reported δ18O and δD compositions of fluids responsible for the alteration at Bajo de la Alumbrera, and the data are used to infer likely ore-forming processes.

Measured and calculated δ18O and δD values of fluids (+8.3 to +10.2 and −33 to −81‰, respectively) confirm a primary magmatic origin for the earliest potassic alteration phase. Lower temperature potassic alteration formed from magmatic fluids with lower δD values (down to −123‰). These depleted compositions are distinct from meteoric water and consistent with degassing and volatile exsolution of magmatic fluids derived from an underlying magma. Variability in the calculated composition of fluid associated with potassic alteration is explained in terms of phase separation (or boiling). If copper-iron sulfide deposition occurred during cooling (as proposed elsewhere), this cooling was largely a result of phase separation.

Magmatic water was directly involved in the formation of overprinting intermediate argillic alteration assemblages at Bajo de la Alumbrera. Calculated δ18O and δD values of fluids associated with this alteration range from +4.8 to +8.1 and −31 to −71 per mil, respectively. Compositions determined for fluids associated with phyllic alteration (−0.8 to +10.2 and −31 to −119‰) overlap with the values determined for the intermediate argillic alteration. We infer that phyllic alteration assemblages developed during two stages; the first was a high-temperature (400°–300°C) stage with D-depleted water (δD = −66 to −119‰). This compositional range may have resulted from magma degassing and/or the injection of new magmatic water into a compositionally evolved hydrothermal system. The isotopic variations also can be explained by increased fluid-rock interaction. The second stage of phyllic alteration occurred at a lower temperature (~200°C), and variations in the modeled isotopic compositions imply mixing of magmatic and meteoric waters. Ore deposition that occurred late in the evolution of the hydrothermal system was probably associated with further cooling of the magmatic fluid, in part caused by fluid-rock interaction and phase separation. Changing pH and/or oxygen fugacity may have caused additional ore deposition. The ingress of meteoric water appears to postdate the bulk of mineralization and occurred as the system at Bajo de la Alumbrera waned.

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