Deciphering ore-forming fluid evolution processes of the Yuerya gold deposit, eastern Hebei, China: Insights from pyrite texture, trace element and sulfur isotope compositions

The Yuerya gold deposit, a representative gold deposit associated with Mesozoic granites in the eastern Hebei region, is located in the eastern Hebei-western Liaoning gold ore-concentrated area, the northern margin of the North China Plate. The orebody is mainly preserved in the Yanshanian granite and its contact zone with the carbonate rocks of the Gaoyuzhuang Formation, and the main ore mineral is pyrite. Based on the pyrite texture, geochemical characteristics of the in-situ trace element analysis, and sulfur isotope, the pyrite in the mining area is classified into five generations (Py1, Py2, Py3, Py4a, and Py4b). Five generations of pyrite all have an average Te/Se ratio > 1, and their average Co/Ni ratios are 1087.27, 43.97, 4.17, 12.62, and 4.99, respectively, indicating that the ore-forming fluid predominantly originates from magmatic-hydrothermal. In-situ sulfur isotopic analysis using LA-MC-ICP-MS shows that the delta S-34 values of the five generations of pyrites ranged from 0.3 parts per thousand to 9.6 parts per thousand; the mean values are 3.8 parts per thousand (Py1, n = 4), 4.2 parts per thousand (Py2, n = 10), 2.6 parts per thousand (Py3, n = 18), 4.0 parts per thousand (Py4a, n = 20) and 4.6 parts per thousand (Py4b, n = 29), respectively. Approximately 32 % of the results exceed 5.0 parts per thousand, surpassing the values indicative of a magmatic-hydrothermal source. Thermodynamic simulations reveal that the fluctuations in the oxidation state of the ore-forming fluid is not the predominant cause of delta S-34 enrichment in the pyrite. Our studies suggest that sulfur is primarily derived from mantle-crust mixtures, with some contributions from the wallrock of the Gaoyuzhuang Formation. The results from LA-ICP-MS trace element analysis of pyrites reveal a significant enrichment of As-Bi-Te-Tl alongside Au in the ore-forming fluid. Additionally, the results of ore-forming elements in three typical cross sections of different wallrocks in the mining area also show that Au, Ag, As, and Pb are closely related. Therefore, it is considered that the over-enrichment of As could be an important factor leading to the precipitation of Au. Pyrite undergoes expansion of the mineral lattice parameters or lattice dislocation due to the substitution of S with As, creating space for the growth of solid-solution Au. Subsequently, some of the solid-solution Au within pyrite is liberated through internal oscillations, forming the visible gold particles.