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

This article focuses on the magmatic controls and physicochemical conditions underlying the formation of the Bilihe porphyry Au-only deposit in the northern margin of the North China Craton. Using detailed apatite chemistry, including trace elements and in situ oxygen (O) and neodymium (Nd) isotope analyses, the study reveals that the deposit’s parental granodiorite magma was relatively water-poor (< 2 wt % H₂O), sulfur-deficient, and formed under moderately reduced conditions (ΔFMQ +0.5) at shallow crustal depths (~1.85 km). The contrasting cathodoluminescence (CL) intensities and geochemical signatures of apatite indicate magma evolution involving assimilation and fractional crystallization of crustal sediments. These findings suggest that shallow emplacement of reduced, sulfur-poor magmas with low volatile contents may be key factors promoting the formation of Au-only porphyry deposits, distinguishing them from typical porphyry Cu-Au systems.