Exhumation and Preservation of the Yangchuling Porphyry W-Mo Deposit in the Jiangnan Tungsten Ore Belt, South China: Insights from (U-Th)/He and Fission Track Dating

The Yangchuling porphyry W-Mo deposit(YPWD), located in the Jiangnan porphyryskarn tungsten ore belt, is one of the most important and large-scale porphyry W-Mo deposits in South China. While previous zircon U-Pb and molybdenite Re-Os data suggest that Yangchuling WMo ore bodies formed almost simultaneously with granodiorite and monzogranitic porphyry at ～150–144 Ma, their post emplacement history remains poorly understood, making their preservation status at depth uncertain. In this paper, new zircon and apatite(U-Th)/He and apatite fission track(ZHe, AHe and AFT, respectively) data of one hornfels and five intrusive rocks from a 1 000-meter borehole are presented. These, together with new inverse thermal history models and previous geochronological data, help elucidate the post-diagenetic exhumation history and preservation status of the Yangchuling porphyry W-Mo deposit. In general, ZHe and AHe ages decrease gradually from the near surface downwards and have relatively little intra-sample variation, ranging from 133 to 73Ma and 67 to 25 Ma, respectively. All four granodiorites yield similar AFT ages that range from 63 to 55 Ma with mean track lengths varying from 12.2 ± 0.7 to 12.6 ± 0.5 μm. Thermal history modelling indicates that the Yangchuling ore district experienced slow, monotonic cooling since the Cretaceous. Age-depth relationships are interpreted as recording ～3.7 ± 0.8 km of Cretaceous-recent exhumation in response to regional extension throughout South China thought to have been driven by subduction retreat of the Paleo-Pacific Plate. Comparison of estimated net exhumation and previous metallogenic depth of ～4–5 km suggests that W-Mo ore bodies could still exist at depths of up to ～1.3 ± 0.8 km relative to Earth surface in the YPWD region. Preservation of the YPWD is attributed to the limited amount of regional denudation during the Late Cretaceous and Cenozoic.