Pyrite textures and compositions in Jiangshan gold deposit, Bengbu Uplift, southeastern North China Craton: Implications for ore genesis

Abstract

Many Phanerozoic gold deposits in East China, e.g., those in the Jiaodong Peninsula, hosted either in granitoids of the same age or Precambrian basement rocks of the North China Craton, have been well documented. The Bengbu Uplift along the southeastern margin of the North China Craton is believed to be the western extension of the Jiaodong gold province, separated by the large-scale sinistral Tan-Lu Fault Zone. However, no deposits with more than 10 t gold resources have been discovered in the Bengbu Uplift, and it is unclear whether the timing and genesis of gold deposits in the Bengbu Uplift are similar to those in the Jiaodong gold province. In this study, laser ablation inductively coupled plasma mass spectrometry trace element analyses, mapping, and in situ sulfur isotope analyses were used to determine the source and process of gold mineralization in Jiangshan gold deposit, the largest in the region. The wall rocks in the Jiangshan gold deposit are metamorphic rocks of the Neo-Archaean Wuhe Group, and the ore bodies are controlled by the eastern part of the Guangou–Xiajijia Fault. Four stages were identified from cross-cutting relationships and textural and mineralogical characteristics: pre-ore stage (stage I), quartz–pyrite–muscovite alteration (stage II), pyrite–quartz veins (stage III), and quartz–pyrite veinlets (stage IV). Each of the four stages is characterized by a different type of pyrite, here named Py1, Py2, Py3 and Py4. Py1 formes the core of Py2, has elliptical and irregular shapes, contains low gold and arsenic concentrations, and has δ34S values of 0.86–2.86‰. Py2 is hosted in quartz–- pyrite–muscovite-altered rocks with disseminated and aggregated euhedral to subhedral textures. It has elevated arsenic content and the lowest gold content, with some rhythmic zones and fractures. Stage II was crosscut by stage III. Py3 has anhedral textures with brittle deformation and occupies fractures in stage III pyrite–quartz veins. The gold content of Py3 is slightly higher than that of Py1 and Py2, but its arsenic content is extremely low. Py4 in the quartz–pyrite veinlets has cataclastic textures with the highest gold and chalcophile element contents. Native gold and electrum also formed in stage IV, which is the main gold stage in the Jiangshan deposit. The δ34S values of Py2–Py4 are broadly similar (5.79–7.98‰), suggesting a common sulfur source. Based on previous studies and higher δ34S values of hydrothermal pyrite, the fluid sulfur isotopes are interpreted to relate to degassing of the mantle wedge metasomatized by slab fluids during the westward subduction of the palaeo-Pacific plate beneath the North China Craton during the early Cretaceous. Based on the geological characteristics of the deposit, we classify the Jiangshan gold deposit as a Jiaodong fracture-disseminated-type gold deposit which occurs as disseminated veinlets and disseminated mineralization in wallrocks along fracture zones. Tectonic events may be the most important factor in gold mineralization in fracture-disseminated type gold deposits.