Multimillion year thermal history of a porphyry copper deposit: application of U-Pb, ~(40)Ar/~(39)Ar and (U-Th)/He chronometers, Bajo de la Alumbrera copper-gold deposit, Argentina

摘要

Application of multiple chronometers (including U-Pb and ~(40)Ar/~(39)Ar geochronology and zircon and apatite (U-Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry copper-gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U-Pb geochronology, combined with our new ~(40)Ar/~(39)Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries). Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, ~(40)Ar/~(39)Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12 +- 0.13 Ma; biotite) to the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated hydrothermal alkali feldspar (6.82+- 0.05 Ma and 6.64 +- 0.09 Ma). The temperature-sensitive Ar data also record an unexpected prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (approx 100-200 deg C) reheating events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part by our new ~(40)Ar/~(39)Ar data (including multidomain diffusion modeling) and (U-Th)/He ages. Single-grain (U-Th)/He ages (n=5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent with the deposit cooling rapidly from magmatic temperatures to below about 300 deg C, with a more protracted history down to 150 deg C. We conclude that the late-stage low-temperature (150 to 200 deg C) thermal anomaly localized at Bajo de la Alumbrera resulted from radiation of heat and/or fluids sourced from deeper-seated magma bodies, emplaced beneath the deposit. To produce the observed thermal longevity of the porphyry system, magma bodies underlying the Bajo de la Alumbrera deposit must have been repeatedly replenished by new magma batches. Without replenishment, crystallization of the source magma will occur, and heat release will stop, leading to rapid cooling (in less than ten thousand years). The influx of deep-seated magma may have caused the development of late low-temperature hydrothermal alteration assemblages at Bajo de la Alumbrera, at the same time that mineralization formed at Agua Rica, some 25 km away. All available chronologic data for the Bajo de la Alumbrera deposit suggest that the hydrothermal system was active episodically over at least a three-million and possibly up to a four-million-year period.