Halogen characteristics and their implications for geochemical exploration in the Qujia gold deposit, Laizhou, Jiaodong

The ore-forming fluids in the Jiaodong gold deposits are characterized by low to medium salinities and Au-HS complexes are the predominant Au species in the fluid. The role of halogen elements in geochemical exploration for the gold deposits is still unclear. This study takes the Qujia gold deposit as an example to explore the issue. Scatter plots and 3D modeling were conducted based on the geochemical data of 1130 borehole samples. The halogen characteristics are governed by fluid-rock interactions, and rock types and heterogeneity. Nevertheless, the halogen characteristics in the alteration zone along the Jiaojia fault zone are still interesting and appealing. In the mineralized zone along the Jiaojia fault, the F and Cl concentrations generally reduce with a decrease in depth; F anomalies evolved from strong positive anomalies at deep depths to negative anomalies at shallow depths; positive Cl anomalies appeared in the upper part or above the mineralized zone while negative Cl anomalies appear in the middle and lower parts of the mineralized zone. The zonation pattern may be attributed to the preferential replacement of OH- by F- due to the acidic pH and the intense beresitization. The consumption of halogen elements may cause the destabilization of metal-Cl complexes and increase the pH of oreforming fluid, and the deposition of metal sulfides further results in the destabilization of Au-HS complexes and Au deposition. Therefore, F, Cl, and F/Cl ratio can be jointly used as exploration indicators for the geochemical exploration only if rock types, and characteristics of hydrothermal fluid and alteration are examined in advance.