The Preservation Mechanism of the Duolong Ore District in Northwest Tibet: Evidence from the Low Temperature Thermochronological Study

The Early Cretaceous Duolong porphyry-epithermal copper ore district is one of the largest copper ore districts in the central Tibetan Plateau. In order to investigate the preservation mechanism of Early Cretaceous porphyryepithermal mineralization, new apatite and zircon fission track dating and (U-Th)/He dating of pre-ore sandstone, ore-related and barren intrusive rocks, and post-ore igneous rock, thermal modeling together with previous thermochronological data of the Duolong ore district were combined to study the burial and exhumation processes. Apatite fission track ages range from 68 +/- 4 to 40 +/- 4 Ma, with mean track lengths varying from 12.1 +/- 2.0 to 12.8 +/- 2.4 mu m. Zircon fission track ages range from 106 +/- 4 to 86 +/- 4 Ma. In addition, apatite (U-Th)/ He yields average age ranging from 62.7 +/- 4.2 to 22.6 +/- 1.2 Ma, and zircon (U-Th)/He yields average age of 112.3 +/- 5.1 Ma. The thermal modeling shows that the Duolong ore district experienced two periods of exhumation and two stages of burial. The first period of exhumation with rate of 0.14 km/m.y. took place immediately after the ore formation which caused the erosion of orebody. Two periods of burial are thought to be related to northeast-southwest striking fault and thrust nappe structure induced by Lhasa-Qiangtang collision between 110 and 65 Ma when the overlying rocks of orebodies were thickened to 4 km. The last period of exhumation with rate of 0.062 km/m.y. (since 65 Ma) is attributed to lithospheric delamination related to IndiaAsia collision. The preservation of the Duolong ore district mainly benefited from the burial related to the LhasaQiangtang collision. The exhumation and uplift of Duolong ore district was mainly attributed to the India-Asia collision since Paleocene.