Apatite fission track thermochronology of Cenozoic igneous rocks from porphyry copper deposits in the Gangdese belt of southern Tibet: Implications for cooling history and ore preservation

Abstract

Gangdese of southern Tibet is the largest porphyry copper belt in China. Although ore-forming processes have been extensively concerned, the post-mineralization ore preservation state was poorly focused on. Here, we reported apatite fission track (AFT) data for three porphyry copper deposits in the belt (Zhunuo, Jiru and Dabu) to reveal the cooling and exhumation history and evaluate ore preservation. The AFT ages are of 13.5 ± 2.2 to 9.4 ± 1.2 Ma (2σ) for the Miocene monzogranite and granite porphyry, 5.9 ± 1.4 to 3.5 ± 0.8 Ma (2σ) for the Eocene quartz porphyry and rhyolite, and 32.6 ± 8.8 Ma (2σ) for the Eocene rhyolite at Zhunuo. Combined with the thermal history modelling results, these AFT ages and the elevations of samples revealed two rapid cooling episodes at ~10 Ma and ~4 Ma, respectively, in the Zhunuo deposit. Available AFT data from both sides of a nearly N-trending fault at Zhunuo indicated the eastern part would likely have been uplifted relative to the western part where orebodies occurred. The missing portion of orebodies in the eastern part should be located at ca. 600–1200 m above the present surface, all or part of the displaced orebodies would have been eroded at Zhunuo. Our new AFT ages at Jiru (15.4 ± 3.4 to 8.4 ± 1.4 Ma; 2σ) and Dabu (16.4 ± 2.4 to 11.9 ± 3.4 Ma; 2σ), together with the previous low-temperature geochronological data, indicated that Gangdese porphyry copper belt may have undergone similar cooling and exhumation history during ~15–10 Ma.