Differential exhumation histories between Qulong and Xiongcun porphyry copper deposits in the Gangdese copper metallogenic Belt: Insights from low temperature thermochronology

Abstract

The Qulong and Xiongcun porphyry deposits in the Gangdese Copper (Molybdenum) Metallogenic Belt (GCMB) are both giant porphyry deposits. They were located in the eastern and the central GCMB (about 400 km apart), and were generated in the Miocene and the Jurassic, respectively. In order to reveal why the two asynchronous deposits could occur in the same metallogenic belt, the in-situ zircon U-Pb, zircon and apatite fission track (ZFT and AFT), and apatite (U-Th)/He (AHe) dating were conducted. Their associated exhumation amounts and histories were compared, and finally the prospecting significance was evaluated. AHe ages of the Qulong deposit indicate it underwent an amount of at least 1.3–2.8 km exhumation since 13.8–13.1 Ma. However, AFT data and modeling results reveal that the Xiongcun deposit experienced two stages of rapid cooling at 20–13 Ma and 13–8 Ma, respectively. The temperature decreases from 160 °C to 40 °C after ∼20 Ma and the corresponding exhumation amount is about 4–6 km. Integrated the regional thermochronological data, exhumation amounts of 4–6 km and <2 km in the southern and northern parts of the central Gangdese Magmatic Arc (GMA) and up to 6–8 km in the eastern GMA during 20–13 Ma were assumed, while exhumation amounts of both the eastern and central GMA post ∼13 Ma were <2 km, except for north–south rifting zone and river valley. These observations imply that porphyry deposits formed prior to 20 Ma should sparsely preserve, whereas those generated after Early Miocene should preserve well and thus have preservation potential. However, the occurrence of Xiongcun porphyry deposit with intensive exhumation indicates it had been buried by post-mineralization volcano-sedimentary rocks. Therefore, the temporal-spatial distribution of the porphyry deposit in the GCMB is largely controlled by the combination of burial, uplift, and exhumation. The two stages of exhumation in the Xiongcun deposit resemble with those of the entire GCMB. The Early Miocene rapid exhumation might be regulated by Xietongmen-Lhasa-Oiga brittle-ductile shear (XLOBDS), while the Late Miocene rapid exhumation might be associated with incision of Yarlung River.