Cooling and exhumation of the Late Paleozoic Tulasu epithermal gold system, Western Tianshan, NW China: implications for preservation of Pre-Mesozoic epithermal deposits

Abstract

Epithermal gold deposits are rarely well preserved in pre-Mesozoic terranes because of their low-temperature mineralization at shallow crust levels, and they are easily destroyed by subsequent erosion or depleted by tectonic events. However, several significant Late Paleozoic epithermal gold deposits have been found in the Tulasu volcanic basin in NW China, forming one of the largest gold districts in the western Tianshan Orogen. Here, we report a new 40Ar/39Ar age from a monzonite porphyry enclave hosted in andesite and apatite fission track data for 10 volcanic rocks from the Tulasu basin. These data, combined with the previous dataset, are used to perform inverse thermal modelling to quantify the district's cooling and exhumation history. Our modelling indicates a phase of burial reheating during Late Paleozoic sedimentation following mineralization, a subsequent rapid exhumation in the Jurassic to Early Cretaceous (c. 196–128 Ma), and a slow exhumation to the present. The Mesozoic exhumation is likely related to the far-field effects of the Cimmerian orogeny along the southern Eurasian margin. Therefore, we suggest that the rapid burial by thick sediments and the slow protracted exhumation after mineralization were crucial for the preservation of the Paleozoic epithermal gold system at Tulasu.