Exhumation history and preservation of the Changjiang uranium ore field, South China, revealed by (U-Th)/He and fission track thermochronology

Abstract

The Changjiang uranium ore field (CUOF) is one of the most important and representative granite-type uranium ore fields in South China. Previous work on the CUOF has primarily concerned uranium ore genesis, focusing on aspects such as timing of metallogenesis and uranium mineralization mechanisms. However, the exhumation history and preservation of the ore deposits in the CUOF are poorly constrained. This study reports multi-system low-temperature thermochronology data that constrain the thermal history of the CUOF in the south-central Zhuguangshan complex, providing important insights into its tectonic evolution and the degree of ore preservation. Zircon (U-Th)/He (ZHe) ages of the mineralized and non-mineralized samples from fault-bounded Triassic-Jurassic granitic basement blocks range from 123 to 24 Ma and negatively correlate to effective uranium (eU) content (335–6611 ppm). Apatite fission track (AFT) ages of granite samples vary from 63 to 58 Ma with relatively short to moderate track lengths, and apatite (U-Th-Sm)/He (AHe) ages range from 39 to 24 Ma. Inverse thermal history models from individual and grouped samples produce similar time–temperature reconstructions exhibiting a Cretaceous-Cenozoic three-stage thermal history involving an initial Late Cretaceous-Early Paleocene exhumation phase (~80–60 Ma) and subsequent Middle Paleocene to Middle-Late Eocene burial phase (~60–40 Ma), interpreted to be primarily controlled by temporal, spatial and velocity variabilities in Pacific Plate subduction dynamics. A final Late Eocene-Quaternary stage (~40–0 Ma) of exhumation was likely caused by the uplift associated with India-Asia collision and the eastward extrusion of the Tibetan Plateau. The ~ 1.5–3.6 km of post-mineralization regional exhumation estimated from thermal history reconstructions, combined with existing uranium mineralization data (timing, temperature and pressure conditions) consistent with an ore-forming depth of ~ 3.2–5.6 km, suggest that the bulk of CUOF uranium ore bodies may still be preserved at depth.