Predominant fracture gold in the Jiaodong gold province, China: Constraints on gold mineralization processes

The Jiaodong Peninsula, the largest gold province in China, hosts proven gold resources of similar to 5,500 t. Knowledge of the accurate occurrence of gold and its formation mechanism is key to a better understanding of the mineralization process of these deposits. In this study, we combined field-emission scanning electron microscopy, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), electron backscattering diffraction (EBSD), and high-resolution X-ray computed tomography (micro-CT) to investigate the predominant gold occurrence type and its formation processes in the Sanshandao and Xincheng giant gold deposits. Microscopic observations confirmed that visible gold in the two-dimensional mode occurred as fracture gold and inclusions in the pyrite. However, high-resolution micro-CT revealed that almost all visible gold in the three-dimensional mode was connected to the fracture, suggesting that the previously so-called gold inclusions were actually fracture gold. The LA-ICP-MS results showed that the pyrite contained low concentrations of invisible gold (<1 ppm). Thus, fracture gold is the predominant type in the Jiaodong gold province. The EBSD analyses showed that the neighboring pyrite fragments shared a similar crystallographic orientation, and these fragments could be assembled into individual grains. The in-situ fracturing texture resulted from structural activities and/or hydraulic fracturing. By combining petrographic, crystallographic, and geochemical analyses with geological information, the mineralization process of the Jiaodong gold deposits can be summarized as follows. When the gold-bearing fluid reacted with the wall rocks, intensive fluid-rock interactions led to the precipitation of massive amounts of pyrite. Subsequent structural activities and/or hydraulic fracturing broke the pyrite and triggered fluid boiling. Boiling induced gold saturation and precipitation in the cracks of the fractured pyrite. Thus, massive ore fluids with low gold concentrations can also form world-class gold deposits via the synergistic mechanisms of intensive fluid-rock interaction and fluid boiling.