Magmatic-hydrothermal constraints on the decoupling of Cu-Au mineralization: Petrological and mineralogical insights from the Chang’an ore-concentration region, Southeast Tibet

The properties of magma origin, evolution, and fluid exsolution collectively govern the ultimate formation of Cu-Au deposits. However, the differential control mechanisms of Cu and Au mineralization remain inadequately explained. The pronounced decoupling of Cu and Au mineralization within the Chang’an-Chang’anchong-Tongchang Cu-Au system of Southeast Tibetan Plateau provides a valuable example for addressing this issue. The Chang’an Au deposit, Tongchang Cu deposit, and Chang’anchong Cu deposit are predominantly developed within the Chang’an ore-concentration region, situated in the southern segment of the Ailaoshan orogenic belt. In this study, we conducted a comprehensive suite of whole-rock and mineral geochemical analyses on three groups of K-rich porphyries from this region. The K-rich porphyries have average εNd(t) values of −5.1, 87Sr/86Sr(t) ratios of 0.70756, and εHf(t) values of −2.8. Collectively, these data indicate that the magma originates from the partial melting of the juvenile, thickened lower crust. With an increase in SiO2 content and decreases in FeO, CaO, and Y, etc., the parental magma successively generated the barren Chang’an syenite porphyry, the Cu-fertile Chang’anchong-Tongchang quartz-syenite porphyry, and the Au-fertile Chang’an syenogranite porphyry. The volatile composition of apatite demonstrates that barren melt is primarily in a fluid-undersaturated stage (melt-XCl/XOH = 0.012). In contrast, fertile melts become fluid-saturated at an XCl/XOH ratio of 0.003, which leads to significant fluid exsolution. During the initial fluid-saturation phase, high oxygen fugacity (ΔFMQ = +1.53) fluids transported significant amounts of Cu, leading to the Cu mineralization at Chang’anchong-Tongchang. In a subsequent stage of magmatic evolution, the reduction in oxygen fugacity (ΔFMQ = +1.26) enhanced the fluid’s capacity to transport Au, which led to the Au mineralization at Chang’an. Therefore, we suggest that concomitant with magmatic evolution and declining oxygen fugacity, the metallogenic system transitioned from Cu to Au mineralization. This system is defined by early-stage Cu mineralization and late-stage Au mineralization.