Mesozoic highly oxidized and hydrous adakitic magmatism controlling porphyry Cu mineralization in NE China: Evidence from the Baoquan deposit

Abstract

In northeastern China, Mesozoic porphyry Mo-dominated deposits are abundant but porphyry Cu deposits are rare. Investigating the mechanisms controlling the formation of the limited number of the porphyry Cu deposits is critical in understanding the regional Cu metallogeny. Here, we present geochronological and geochemical data for the Cu-mineralized and barren intrusions in the newly discovered Baoquan porphyry deposit in the northern Great Xing'an Range. Zircon U-Pb dating indicates that the mineralized quartz diorite porphyry and barren syenogranite were emplaced at ∼115 Ma and ∼312 Ma, respectively. The quartz diorite porphyry (SiO₂ = 62.7–63.9 wt%) has high alkaline contents (Na₂O + K₂O = 6.82–8.28 wt%), is enriched in LILEs (K, Rb, Ba, and Sr) and depleted in HFSEs (Th, Nb, Ta, Zr and Ti), and exhibits low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7049–0.7067) and positive ε_Nd(t) values (+1.9 to +2.0), as well as positive zircon ε_Hf(t) values (+4.9 to +13.6), indicating that the parental magma was predominantly derived from the mantle, likely metasomatized by fluids derived from the subducted Paleo-Pacific slab. In contrast, the barren syenogranite (SiO₂ = 76.4–78.0 wt%) is depleted in Sr (40.6–82.4 ppm) and Ba (348–498 ppm), with high Zr + Y + Ce + Nb values (206–277 ppm), showing A-type granite affinities. The Baoquan ore-related porphyry is oxidized (ΔFMQ = +1.2 in average) as calculated using magmatic oxybarometer using trace elements in zircon, in contrast with that of the barren syenogranite with lower magmatic ΔFMQ values (+0.6 in average). The porphyry also has low Zr contents (average 151 ppm) and Sr/Zr ratios (average 7.96), consistent well with other Cu-fertile magmas. Additionally, it also displays adakitic signatures with relatively high Sr/Y ratios (78–130), low Dy/Yb ratios (1.86–2.03) and negligible Eu anomalies (Eu/Eu* = 0.79–0.96). These findings suggest that zircon and hornblende fractionation, rather than plagioclase separation, has dominated magma crystallization, further indicating a high magmatic water content. Together with previously published data for the other Cu-mineralized intrusions in NE China, it is proposed that the magmas with high oxygen fugacities and H₂O contents that have experienced high-pressure fractionation are favorable for Cu mineralization. Notably, the intrusions with such characteristics are mainly distributed in the northern Great Xing'an Range characterized by thickened Mesozoic crust, where should be prioritized for future Cu exploration.