Abstract
The Triassic Pulang porphyry Cu-Au deposit, located in the South Yidun terrane, is the oldest and one of the largest porphyry deposits in the southeastern Tibetan Plateau. The mineralization occurs mostly in the potassic alteration zone of the Pulang intrusive complex. U-Pb-He triple dating, namely apatite (U-Th)/He, zircon U-Pb, and zircon (U-Th)/He dating, together with inverse thermal modeling, reveals that the Pulang complex was emplaced at a paleodepth of ˜5.0 to 6.5 km at 215 ± 2 Ma. The deep-level emplacement of the complex, coupled with the episodic replenishment of the magma chamber, gave rise to the establishment of a prolonged magmatic-hydrothermal system at Pulang. Although a range of single-grain zircon and apatite (U-Th)/He ages were obtained on each sample, the weighted mean zircon and apatite (U-Th)/He ages vary systematically with elevation, defining a multistage cooling/denudation history at Pulang. Specifically, three phases of cooling were recognized from inverse thermal modeling, including rapid cooling (80°–120°C/m.y.) in the Late Triassic, moderate cooling (3°–5°C/m.y.) from the Late Triassic to Early Cretaceous, and a protracted slow cooling period (50 m.y.) of extremely slow cooling and erosion at Pulang could be related to the formation and preservation of a peneplain on the southeastern Tibetan Plateau since the Late Cretaceous. A relict peneplain thus signifies a favorable tectonic environment for the preservation of ancient porphyry systems worldwide.
Highlights
Porphyry ore deposits are products of complex magmatic and hydrothermal activity at convergent plate margins (Richards, 2011; Cooke et al, 2014) and commonly form at paleodepths of 1 to 6 km
Previous compilations of geochronological data revealed that porphyry deposits worldwide typically have Phanerozoic ages, with a strong Cenozoic age population (e.g., Singer et al, 2008)
There is no positive correlation between equivalent spherical radius (Req) and (U-Th)/He ages, suggesting that grain size is not the dominant factor accounting for the intrasample dispersion in single-grain ages (Fig. 7a, b)
Summary
Porphyry ore deposits are products of complex magmatic and hydrothermal activity at convergent plate margins (Richards, 2011; Cooke et al, 2014) and commonly form at paleodepths of 1 to 6 km (mode average of ~2 km; Wilkinson and Kesler, 2007; Yanites and Kesler, 2015). We have applied the vertical transect sampling strategy, U-Pb-He dating, and numerical modeling to study the cooling and erosion history of the giant Pulang porphyry Cu-Au deposit, which formed during the Late Triassic in the Yidun terrane, at the southeastern margin of the Tibetan Plateau. The single-grain ages are relatively scattered, the weighted mean zircon and apatite (U-Th)/He ages generally display a positive correlation with elevation (Table 2; Fig. 4), allowing us to estimate long-term erosion rates of the Pulang complex. These rates are roughly commensurate with the values derived from the age-elevation relationships of the apatite and zircon (U-Th)/He data (see above). It is estimated that the Pulang complex was exposed to paleosurface at ~74.3 to 48.9 Ma, and the eroded thicknesses of the complex vary from 558 to 1,099 m
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