Magmatic Cl-H2O contents, fluid extraction and porphyry fertility: Evidence from zircon and its apatite inclusions

Abstract

The formation of giant porphyry deposits requires the accumulation of up to billions of tons of volatiles (H2O, Cl, and S) in an evolving trans-crustal magma system, followed by their focused release to the hydrothermal system via volatile exsolution in the upper crustal magma chamber. However, it remains unclear whether ore-forming and ore-barren magmatic pulses derived from the same parental magma system differ in initial volatile compositions or exsolution conditions. This uncertainty hinders our comprehension of how massive volatiles enrich and deliver the key drivers of porphyry mineralization. In this work, we investigate zircon water contents and zircon-hosted apatite volatile compositions of the pre-ore (109.7 ± 0.8 Ma), syn-ore (103.1 ± 1.1 Ma), and post-ore (98.1 ± 1.1 Ma) intrusions of the Zijinshan ore field (ZOF). These data, along with numerical modelling, allow us to illustrate the volatile evolution of the magmatic system which sourced the largest Cretaceous porphyry Cu-Mo deposit in South China. Although geochemically similar, these co-sourced magmas exhibit distinct volatile compositions and evolution patterns. Zircon-hosted apatite inclusions from the syn-ore porphyry show highly variable Cl (0.14–2.94 wt%) and F (0.04–3.37 wt%) contents, whereas apatite from the pre-ore and post-ore intrusions displays more restricted halogen variation (0.62–2.70 wt% Cl, and 0.41–1.56 wt% F). Numerical modelling of apatite evolutionary trends suggests that the syn-ore magma had the highest initial melt H2O (∼5.0 wt%) and Cl (∼1300 ppm) contents, exsolving fluids with average aggregated salinity of 3.24 ± 0.57 wt% NaCleq at the deepest level. These results align with the zircon OH– contents and biotite Al-thermobarometers, which indicate the highest saturation pressure (∼211 MPa) for the syn-ore porphyry. We suggest that fluid exsolution from magma with moderate Cl content at > 200 MPa optimises Cl-Cu extraction efficiency and promotes the formation of connected fluid networks throughout the magma chamber, facilitating upward fluid migration and subsequent mineralisation. In contrast, modelling indicates that the pre-ore magma had an intermediate initial melt H2O (∼ 4.0 wt%) but the lowest Cl (∼700 ppm) contents, generating less saline fluids (< 3.0 wt% NaCleq), which hampered metal extraction. The post-ore magma, despite relatively high initial Cl (∼1100 ppm) and comparable fluid salinity (3.36 ± 1.40 wt% NaCleq), had the lowest H2O (3.72 ± 0.70 wt%) and saturated at the lowest pressure (∼78 MPa), limiting Cu extraction, exsolved fluid amount, and focused fluid flux. Our findings indicate that the syn-ore volatile enrichment occurred primarily within the lower-crustal magma reservoir. In the absence of isotopic evidence for direct mafic recharge, we propose that fluids derived from underplating mafic magmas, rather than direct magma mixing, provided the critical volatile budget required for the ZOF porphyry mineralisation.