Burial and exhumation of late paleozoic arc-related rocks in the Tulasu basin, Western Chinese Tianshan: Implication for the preservation of epithermal deposits in old orogenic belts

Abstract

Epithermal deposits are susceptible to erosion owing to their shallow emplacement depth of 1 to 2 km, resulting in the observation that the majority of epithermal deposits worldwide are generally Cenozoic in age. Several large and super-large epithermal deposits likely associated with Late-Devonian to Carboniferous arc magmatism were recognized in the Tulasu volcanic-clastic sedimentary basin, western Chinese Tianshan. In order to further clarify the preservation processes of these old epithermal orebodies, apatite fission track (AFT) analysis has been undertaken in this study. Our AFT results gave a range of ages from 54.7 ± 6.3 Ma to 272.3 ± 8.7 Ma, and the mean confined fission track lengths are between 12.1 ± 0.2 μm and 13.5 ± 0.1 μm. Our new data, in combination with the 1:50,000 geological mapping, suggest that the ore-hosting Tulasu basin may have undergone multiple burial and exhumation episodes postdating the mineralization. The main burial scenario, probably occurring in the late Early Carboniferous (350–330 Ma) and Permian (300–250 Ma), may be related to subsidence on syn-volcanic faults concurrent with retreat of the Junggar Ocean and strong uplift of the neighboring mountain range induced by final closure of the Paleo-Asian Ocean, respectively. The calculated thickness of ca. 4.83 km of sedimentary rocks deposited during the burial events form a critical shield for the epithermal ore bodies that have been protected from substantial erosion by subsequent geological processes. Exhumation occurred in two episodes: Late Triassic-Cretaceous (ca. 240–100 Ma) and Oligocene (ca. 35 Ma). The Late Triassic-Cretaceous erosion is characterized by a slow exhumation rate (ca. 0.012 mm/y), corresponding to a total unroofing thickness of ca. 1.67 km. In contrast, the Oligocene event exhibits as a higher exhumation rate (up to ca. 0.057 mm/y), with removal of ca. 3.83 km of sedimentary cover. Therefore, the epithermal deposits have undergone a complex geological path from formation to final exposure. Initial burial and the following sluggish exhumation are two fundamental factors for the preservation of epithermal deposits in the old orogenic belts.