Volatile-rich and oxidized ore-forming magmas of the Zijinshan porphyry Cu deposit, China: A mineral composition perspective

Abstract

The Zijinshan orefield (Fujian Province) is a representative Cu-Au-Mo-Ag porphyry-epithermal mineralization system in eastern China. It contains both Cu ore-bearing and ore-barren granitoids, which provide an opportunity to study the unique properties of fertile magmas. This study focuses on chemistry of minerals such as zircon, apatite, amphibole, and plagioclase in ore-related (zircon U–Pb ages of 103–101 Ma) and ore-barren (zircon U–Pb ages of 157–156 Ma and ∼105 Ma) granitoids to constrain the characteristics of mineralized magmas. The results of whole-rock geochemistry and zircon Hf isotopes suggest that both ore-barren and ore-related granitoids were derived from partial melting of the lower crust, with the ore-related magma being influenced by mantle materials. The analysis of mineral chemistry reveals that the Zijinshan Cu ore-bearing porphyry magmas have high oxidation states (ΔFMQ +0.23–+2.25), high water contents (4.4–4.9 wt%), and shallow emplacement depths (1.9–4.0 km). These magmatic conditions are favorable for fluid exsolution and mineralization, similar to the global Cu-fertile porphyry magmas, but distinct from regional pre-mineralization ore-barren igneous rocks. Apatites from the ore-related granitoids have significantly lower Cl contents (0.03–0.83 wt%) and higher F/Cl ratios (1.52–377) compared to coeval ore-barren monzogranites (Cl = 0.14–2.42 wt%; F/Cl = 0.07–2.82). Additionally, the apatites’ volatile budgets, including XOH, XCl and XF values, and their covariation relationship, in the ore-related intrusions are consistent with apatite crystallization in H2O-saturated conditions. This suggests that extensive fluid exsolution from the parent magma in the chamber plays a crucial role in the formation of porphyry Cu deposits from a low-Cl magma. These low-Cl-apatite intrusions have the potential to form porphyry deposits. Together, we propose that the high oxygen fugacity, high water contents, and fluid exsolution of the parent magma may have triggered the Zijinshan porphyry mineralization.