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Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids


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Harris, AC, Golding, SD and White, NC 2005 , 'Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids' , Economic Geology, vol. 100, no. 5 , pp. 863-886 , doi: 10.2113/100.5.863.

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Alteration zones at the gold-rich Bajo de la Alumbrera porphyry copper deposit in NW Argentina are centred on several porphyritic intrusions. They are zoned from a central copper-iron sulfide and gold-mineralized potassic (biotite-K-feldspar +/- quartz) core outwards to propylitic (chlorite-illite-epidote-calcite) 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 outwards into phyllic (quartz-muscovite-illite +/- pyrite) alteration. This study contributes new data to previously reported *18O and *D 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 *18O and *D values of fluids (+8.3 to +10.2 degrees and 33 to 81 degrees, respectively) confirm a primary magmatic origin for the earliest potassic alteration phase. Lower temperature potassic alteration formed from magmatic fluids with lower *D values (down to 123 degrees). 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 *^18O and *D values of fluids associated with this alteration range from +4.8 to +8.1 per mil and 31 to 71 per mil, respectively. Compositions determined for fluids associated with phyllic alteration (0.8 to +10.2 degrees and 31 to 119 degrees) 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 degrees to 300 degrees C) stage with D-depleted water (*D = -66 to 119 degrees). 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 degrees C), and variations in the modelled 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 post-date the bulk of mineralization and occurred as the system at Bajo de la Alumbrera waned.

Item Type: Article
Authors/Creators:Harris, AC and Golding, SD and White, NC
Keywords: Porphyry, Copper, Gold, Hydrothermal Fluids, Magmatic, Meteoric, Phyllic, Oxygen and Hydrogen Isotopes, Bajo de la Alumbrera
Journal or Publication Title: Economic Geology
ISSN: 0361-0128
DOI / ID Number: 10.2113/100.5.863
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