Ore-forming process of the Haigou gold deposit in the eastern Central Asian Orogenic Belt, NE China: Constrains from EPMA and LA-ICP-MS analysis of Au-bearing pyrite

The Haigou lode deposit, containing 40 t of gold at an average grade of 3.5 g/t, is one of the largest deposits in the Jiapigou gold belt located along the eastern segment of the northern margin of the North China Craton. The deposit comprises 15 gold-bearing quartz veins hosted in a Carboniferous monzonite-monzogranite stock. Although many studies relating the genesis of this deposit have been carried previously, the understanding of ore-forming process remains controversial. In this study, we combined field geological mapping with petrography, Electron Probe Micro Analysis (EPMA) and in situ LA-ICP-MS analysis of gold-bearing pyrite to provide new insight into the genesis of the gold deposit. Our results show that Au in the Haigou gold deposit mainly exists in the form of native gold. LA-ICP-MS in situ multi-element mapping shows that Au has a significant positive correlation with Ag, Te, Bi and Pb in Au-bearing pyrite, indicating that these elements co-precipitated with Au in pyrite lattice. The ore-forming fluid existed under medium to low temperature, aiding complexing of the gold with sulfur, chlorine, and tellurium. Decoupling of sulfide and telluride complexes facilitated gold precipitation in the Haigou area. The abundance of lead and nickel tellurides in the Haigou gold deposits with general absence of tellurium in native gold is indicative of expulsion of tellurium through boiling during the process of fluid migration and evolution. Also, coexistence of Bi in native gold indicates that they were at Au-Bi eutectic. The gold-bearing pyrite is characterized by dispersed magnetite in and around the fractures. The Au-Te-Bi-Pb element anomalies indicate that fluid oxygen fugacity increases while sulfur fugacity decreases with time, resulting in Au precipitation. Our results provide a new theoretical basis for the establishment of metallogenic model and deep prospecting of the Haigou gold deposit.