Apatite and zircon compositions as petrogenetic and metallogenic indicators for late Ordovician porphyries in the Songshunangou gold district, North Qilian orogenic belt (China)

Abstract

To elucidate the factors that control Au mineralization in porphyry systems, we present new whole-rock and mineral (apatite, zircon) chemical data on fertile quartz-dioritic porphyries (QDP) and barren granodioritic porphyries (GDP) from the Songshunangou gold district in the North Qilian orogenic belt, China. High Sr/Y ratios and the presence of ubiquitous hornblende phenocrysts, as well as “gull-wing” whole-rock rare earth element (REE) patterns, indicate that magmatic fractionation of hornblende was the main process that accounted for the formation of fertile and barren magma batches. Mineral chemistry of zircon (i.e., Ce/Sm, Yb/Gd, Th/U, and Hf/Y ratios) indicates that the development of the GDP magma was likely controlled by hornblende, apatite, and titanite fractionation, whereas hornblende-dominated fractional crystallization explains the origin of the QDP magma. Application of zircon oxy-barometry yielded moderate oxidation states for both magmas, with ΔFMQ − 0.25 to ΔFMQ + 2.34 (av. ΔFMQ + 0.33) for the GDP, and ΔFMQ − 0.25 to ΔFMQ + 0.72 (av. ΔFMQ + 0.27) for the QDP, which are favorable for the mobilization and transport of Au. Whole-rock and apatite Sr/Y ratios, combined with the (Ce/Nd)/Y and (10000 × Eu/Eu*)/Y ratios of zircon, support that higher H₂O contents were associated with the QDP magma if compared to the GDP. Apatite in the GDP has low X_F/X_Cl ratios of 2.99–11.33, indicating crystallization from a volatile-undersaturated magma, with average melt Cl concentrations of 0.35 wt%. Apatite in the QDP with low X_F/X_Cl ratios (≤7.78) is interpreted to have crystallized from volatile-undersaturated magma with higher melt Cl concentrations (av. 0.42 wt%), whereas apatite with high X_F/X_Cl ratios (11.53–59.80) is interpreted to have crystallized after fluid exsolution. Our results confirm that moderate oxygen fugacity, high H₂O and Cl as well as early fluid exsolution, are prerequisites for porphyry Au mineralization.