3D geochemical modeling of the Qujia gold deposit, China: Implications for ore genesis and geochemical exploration of deep concealed ore bodies

Abstract

Phase separation is a crucial controlling factor of gold deposition in shallow and low-medium temperature orogenic gold systems in the Jiaodong gold province. In this study, spatial distributions by three-dimensional (3D) modeling and correlation analysis of Au, S, Cl, Hg, Fe2O3, Cu, Pb, and Bi were conducted to demonstrate the occurrence of phase separation and its significance in geochemical exploration. The modeling and analysis were based on a geochemical dataset of 1130 borehole samples from the Qujia gold deposit. The ore bodies developed in deep traps (e.g., fault intersections and dilational cracks). Gold and S are positively correlated and mainly distributed along the Jiaojia Fault (JJF). Scatter plots show little correlation between Au and Cl. However, the Au and Cl distributions presented by the ore-bearing boreholes and 3D modeling suggest that Cl escaped from the ore-forming fluid and formed enrichment zones above the ore bodies during the Au mineralization which is consistent with the salinity decrease from the early and main ore-forming stage to the late ore-forming stage of many gold deposits in the JJF and other areas of the Jiaodong gold province. The spatial distribution of Au and Hg are indicative and compelling although they have little correlation in the scatter analysis. Mercury depletion zones and subsequent funnel-like Hg anomalies are present above the ore bodies and strong Hg anomalies appear in the deep traps. The results suggest that phase separation is responsible for Au precipitation in the Qujia gold deposit. Mercury, Cl, and Pb partitioned into a gas phase forming medium-to-high anomalies along the secondary faults whereas Au, S, Fe2O3, Cu, and Bi partitioned into a liquid phase forming ore bodies and medium-to-high anomalies along the JJF during the phase separation. Specifically, the pH increase induced by the partition of CO2, HCl and minor amounts of H2S into the gas phase, and S consumption by the abundant metals (Fe2O3, Cu, Bi, etc.) in the ore-forming fluid, rather than H2S escape, are the dominant factors governing Au precipitation. Finally, potential indicators of geochemical exploration are proposed including composite anomalies of Au and Hg in the near surface medium, high Hg anomaly accompanied by no or low Au anomaly in the fault zones, and extremely high Cl or S anomalies in deep depths. Given the similarity of the ore-forming fluid and metallogenic mechanism at the shallow depths, these indicators are potentially useful for Au exploration in the Jiaodong gold province.