Exhumation Degree of Intrusions in the Kangguer–Huangshan Area and Its Constraint on Ore-Preservation in the Jueluotage Tectonic Belt, Eastern Tianshan

Abstract

The Kangguer–Huangshan ductile shear zone lies in the Eastern Tianshan area, southeast of the Central Asian Orogenic Belt (CAOB), it is a complex and strong tectonic deformation zone that is oriented from east to west. The area in Northwest China contains important polymetallic mineral resources, with intensive structural and magmatic activities, superior ore-forming geological conditions, especially porphyry copper deposits, ductile shear-type gold deposits, Cu–Ni sulfide deposits, etc. In this work, the intermediate–acid intrusive rocks in the Kangguer–Huangshan ductile shear zone are used as the research objects, and detailed petrographic observations and electron microprobe analysis (EMPA) of the chemical compositions of amphibole and biotite are carried out to determine the temperature and pressure conditions, the oxygen fugacity and water content during magmatic emplacement and to calculate the emplacement depth of intrusive rocks to estimate the metallogenic potential of polymetallic deposits in the area. Apatite fission track (AFT) thermochronology have been conducted to obtain information about the uplift–exhumation history in the Kangguer–Huangshan area since the Mesozoic–Cenozoic. Combined with the mineralisation depth of typical deposits in the area, the constraints of the uplift–exhumation degree on the post-ore modification and preservation of deposits in the area are analysed. The crystallisation temperatures of the amphiboles and biotites are 665.1°C–919.5°C, the crystallisation pressures are 85.2–355.2 MPa, corresponding the magma emplacement depths are 2.4–10.9 km. The oxygen fugacity logf(O₂) values of magma characterised by amphibole and biotite elements vary from −13.1 to −9.1, and H₂O in coexisting melts is greater when amphiboles crystallised. Apatite fission track analysis performed on 10 samples from 8 granites in the area reveals that the apatite fission track pooled ages of granitoids from the Kangguer–Huangshan ductile shear zone range from 120 ± 9 to 55 ± 4 Ma, with mean track lengths varying from 11.5 ± 1.9 to 13.0 ± 1.9 μm. Further temperature–time modelling of the fission tracks indicates that there exists two exhumation events in the Kangguer–Huangshan ductile shear zone that occurred mainly in the Cretaceous (120–60 Ma) and Miocene (20–0 Ma). The Meso–Cenozoic exhumation and rates in this area are calculated by the best fitting line, and the values are 3.1–4.0 km and 0.013–0.032 mm/a, respectively. A comparison of the emplacement depth and exhumation depth of granites with the regional metallogenic depth reveals that the Permian Cu–Ni sulfide deposits with deep mineralisation, while little regional tectonic uplift influence for the orebodies, and that most of the orebodies are still preserved in deep underground and have great potential for prospecting. The metallogenic depth of porphyry Cu (Mo) deposits is similar to the emplacement depth of the granitoids, with a middle exhumation degree during Meso–Cenozoic. The deposits uplifted to the surface after formation, and some were eroded, but the deep orebody is retained to some extent. The depth of gold mineralisation is the shallowest in the area, but due to intense exhumation, almost all the orebodies are exposed to the shallow ground surface.