Application of multiple chronometers (including U–Pb and 40Ar/39Ar 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 40Ar/39Ar 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, 40Ar/39Ar 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 (~100–200°C) reheating
events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part
by our new 40Ar/39Ar 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°C, with a more protracted history down to 150°C.
We conclude that the late-stage low-temperature (150 to 200°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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
The effect of tidal flooding on survival of juvenile muskrats (Ondatra zibethicus) was investigated in a brackish marsh in Louisiana by examining 50 muskrat lodges each month from July 1984 to June 1985 and tidal data over a 19-yr period. Tide levels increased at a rate of 1.58 cm yr?1 during the 19-yr period prior to the study, and during the study nest chambers in muskrat lodges were flooded on 43 d. Seventy-seven captured litters averaged 2.2±0.3 young per litter. older litters were less common than younger litters, but the number of young per litter did not differ among 5-d age classes, suggesting that mortality factors usually affected entire litters. The frequency of tidal flooding prior to opening of lodges each month was associated negatively with the number of litters and number of young per litter. If marsh subsidence and sea level rise continue, tidal flooding will become more prevalent and litter mortality will likely increase. 相似文献
The islands of the Habomai group, Shikotan, Kunashir and Iturup are claimed by both the Soviet Union and Japan. These islands are each entitled to an Exclusive Economic Zone. This paper examines the mineral and energy potential on these islands and in their maritime zones and the implications for resource allocation and management of different solutions to the islands question. Return of all four islands would mean the transfer of substantial titanium sand, sulfur and sulfide deposits, as well as extensive fishing grounds. Return of only the southern two would still entail a major transfer of fishing grounds but not known mineral deposits. Return of the southern two and joint development of the other two would give Japan significant fishing grounds and access to several economic mineral deposits. 相似文献
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