Trace Element and In Situ O–Nd Isotope Signatures of Apatite from the Bilihe Porphyry Au-Only Deposit: Magmatic Constraints on Au Mineralization

Abstract The magmatic controls on the formation of porphyry Au-only deposits are still poorly understood. The Bilihe deposit (> 30 t at 2.7 g/t Au) located at the northern margin of the North China Craton is such an example, where apatite chemistry from a causative granodiorite intrusion constrains the physicochemical conditions and ore-forming potential of the porphyry-related magmas. Apatite can be subdivided by distinct CL intensities into early CL-bright and late CL-gray/dark sub-types both occurring in fresh (Ap-f), as well as in potassic (Ap-k) and phyllic (Ap-p) altered granodiorite. The mostly euhedral CL-bright apatite shows a homogeneous texture and is enriched in LREE and depleted in Fe, Mg, Mn, Y, and U compared to CL-gray/dark apatite, which develops patchy or oscillatory zoning. The CL-gray/dark apatite exhibits decreasing Fe and Mg contents, as well as δ18O values from Ap-f to Ap-k and Ap-p due to consecutive fluid alteration during porphyry evolution, which was generally not observed in CL-bright apatite. We thus conclude that all types of CL-bright apatite but only CL-gray/dark Ap-f inherit the original magma composition. Binary O–Nd isotope mixing simulations suggest that both magmatic CL-bright apatite and CL-gray/dark Ap-f were influenced by crustal sediment assimilation. Our results further indicate that the magma parental to the Bilihe porphyry Au-only deposit was H2O-poor (< 2 wt %), causing plagioclase fractionation in the absence of hornblende, as evidenced by relatively low Sr/Y (< 0.1) and significantly negative Eu anomalies in magmatic apatite. Pressure estimates by magmatic biotite that record the late crystallization stage yielded 48.9 ± 8.4 MPa, indicating a shallow final emplacement depth of 1.85 ± 0.32 km. The studied apatites formed under volatile-saturated conditions from S-deficient magmas at relatively reduced conditions (ΔFMQ +0.5) compared to porphyry Cu deposits. We thus conclude that these physicochemical parameters together with the low-pressure conditions result in a fertile magmatic-hydrothermal environment for the formation of Au-only porphyry deposits.