The role of Te-Bi melts in controlling gold enrichment: Insights from the Yuerya gold deposit in the North China Craton

The Yuerya gold deposit, located in the northern North China Craton, contains abundant Te-Bi minerals that coexist with gold and therefore provides an ideal natural laboratory for addressing the mechanisms by which Te-Bi elements control the migration, enrichment, and precipitation of gold, particularly the processes by which Te-Bi efficiently extract gold and promote precipitation of native gold. Based on mineral assemblages, ore textures, and vein crosscutting relationships, the deposit is divided into three stages: (I) K-feldspar–quartz–pyrite, (II) quartz–pyrite–native gold, and (III) quartz-calcite. Native gold commonly coexists with a variety of Te-Bi minerals in stages I and II. In each stage, pyrite is further subdivided into two generations, with native gold and Te-Bi minerals precipitated within pyrite rims, particularly along rims and fractures. Geochemically, Au shows strong positive correlations with Te and Bi, whereas its correlation with As is weak, in contrast to most orogenic gold deposits in China. Droplet-shaped Te-Bi mineral aggregates and continuous compositional variations within their solid solutions were observed. These features are interpreted to suggest the former existence of Te-Bi melts, which likely played a role as efficient scavengers of Au in the complex Au-Ag-Te-Bi-Pb system. Pyrite δ34S values are consistent with a magmatic sulfur source. Thermodynamic simulations indicate an evolution of an ore-forming system from a weakly reducing, slightly acidic environment to a strongly reducing, near-neutral setting. A zircon U-Pb age obtained from biotite-bearing monzogranite (169.6 ± 0.7 Ma) is consistent with gold crystallization ages (169.4 ± 1.0 Ma). Gold enrichment in the Yuerya gold deposit was controlled by coupled magmatic and hydrothermal processes. Elevated magma oxygen fugacity favored the retention of Te-Bi in the residual magma. The extensive partitioning of Te-Bi into magmatic-hydrothermal fluids occurred during late-stage magma crystallization. Subsequently, immiscibility may have led to the formation of Te-Bi melts, which effectively scavenged Au from the hydrothermal fluid. Overall, the magma supplied the ore-forming metals, whereas the Te-Bi melt played a role in concentrating and precipitating native gold.