Magmatic evolution and metallogenic diversity of the late Cretaceous granites in the Yidun terrane: Constraints from zircon and apatite geochemistry

Abstract

Porphyry-hydrothermal type Cu-Mo(−W) deposits in the southern Yidun terrane (SYT) and hydrothermal-skarn type Sn-Pb-Zn-Ag deposits in the northern Yidun terrane (NYT) are closely related to late Cretaceous granitic intrusions in time and space. However, the genetic mechanisms and controlling factors responsible for the metallogenic diversity remain unclear. We investigated zircon and apatite compositions of the Sn-Pb-Zn-Ag-associated granites in northern Yidun terrane and the Mo-W-Cu and Mo-Cu-associated granites in southern Yidun terrane. The Cretaceous granites are distributed along a N–S-trending belt having ages ranging from 106 to 73 Ma, with a transition from predominant plagioclase crystallization to amphibole-dominated crystallization. Our data show that the granites from the NYT have lower zircon Eu/Eu* (0.06 ± 0.04), Ce4+/Ce3+, and ΔFMQ (FMQ is fayalite-magnetite-quartz) values compared to the granites in the SYT, indicating that the granites from the NYT are relatively reduced (ΔFMQ = -1.03 ± 1.02) in contrast to the oxidized granites in the SYT (ΔFMQ = 1.10 ± 1.05). Furthermore, apatite can record the volatile compositions of magma before fluid exsolution. Apatites in granites from the NYT with low XF/XCl ratios (≤120) are interpreted to have crystallized from volatile-undersaturated magma. These apatites are characterized by low Cl contents ranging from 0.04 to 0.12 wt% (0.07 ± 0.02 wt%; n = 21) and low SO3 contents of 0.01–0.06 wt%. In contrast, apatites in granite from the SYT with low XF/XCl ratios (≤80) have high Cl contents of 0.05 to 0.34 wt% (0.16 ± 0.07 wt%; n = 52) and SO3 contents of 0.01–0.46 wt%. The chemical compositions of zircon and apatite indicate that the Mo-W-Cu and Mo-Cu-associated granites in the SYT have higher fO2 and melt S contents before fluid exsolution than the Sn-Pb-Zn-Ag-associated granites in the NYT. Therefore, the contrasting magmatic oxidation states and S contents are critical in controlling the metallogenic diversity of the Cretaceous granites in the Yidun terrane. The Xingguolongba and Cuopu granites, first reported in this article, have U-Pb ages consistent with the Rongyicuo and Cuomolong granites. Furthermore, they exhibit similar geochemical properties, such as oxygen fugacity and volatiles, suggesting the possibility of forming Sn-Pb-Zn-Ag deposits.