Sulfide inclusions reveal deep Au pre-enrichment and efficient metal transfer to hydrothermal systems

The Au-rich sulfide saturation during deep magmatic evolution of hydrothermal ore-forming systems has emerged as a frontier in recent research. However, the underlying formation mechanisms and their coupling with shallow mineralization remain controversial. The North China Craton (NCC), one of the most significant gold province globally, exhibits characteristics of short-term and explosive mineralization, providing an ideal natural laboratory to investigate gold enrichment processes. Here, we present an LA–ICP–MS study of sulfide inclusions (SIs) and CO2-bearing compound droplets from mantle xenoliths in Fangcheng basalts and intermediate-mafic dikes in Guocheng gold deposit.
We discovered SIs in both mantle xenoliths and dikes from different depths within the same magmatic system. The SIs in mantle xenoliths contain Au contents (0.01–0.98 ppm, mean 0.15 ppm, n=102) that are 2-5 orders of magnitude higher than whole-rock mantle peridotites from the eastern NCC, indicating significant Au pre-enrichment during the sulfide saturation stage. Furthermore, the prevalent occurrence of SIs in phenocrysts at the Mg-rich and Fe-rich zones of hornblende phenocrysts exhibit higher Au contents (0.03–1.90 ppm, mean 0.36 ppm, n=49), suggesting that multi-stage crust-mantle mixing triggered sulfide melting and further promoted gold enrichment. Element ratios of the SIs largely correspond to those of unaltered bulk ore, indicating that the dikes and ore share a common origin, with the magmatic sulfides having served as a key transport medium of Au and ore-forming elements. In addition, by simulating the diffusion of olivine to record the constraints on the rapid ascent of magma, we suggest that CO2-bearing compound droplets inhibited sulfide precipitation and redissolution, allowing Au-rich SIs to retain their metal enrichment during ascent.
We propose a continuous metal-enrichment model for deep magmatic systems, providing new insight into the efficient transfer of metals from the mantle and lower crust to shallow hydrothermal systems.