Trace Element and Sulfur Isotopic Analysis of Pyrite from the Luyuangou Gold Deposit, Xiong'ershan Au-Ag Polymetallic District, Central China: Implications for The Origin and Evolution of Ore-Forming Fluids

The Luyuangou gold deposit is located in the eastern section of the Xiong'ershan Au-Ag polymetallic district (XESPMD) and consists of a few gold-bearing veins found in the EW-striking faults located in the Archean Taihua and Mesoproterozoic Xiong'er Groups. The gold deposits contain numerous gold-bearing pyrites in thin quartz veins, representing an ideal tool for explaining the enigmatic genesis of gold deposits in the XESPMD. The distributions of trace elements and the sulfur isotopes of gold-bearing pyrite in the Luyuangou gold deposit were investigated to define the origin and evolution of ore-forming fluids. Five generations of pyrite have been identified: coarse-grained euhedral pyrite cores (Py1-1) and margins (Py1-2) in milky quartz veins, fine-grained pyrite (Py2) in quartz veins and host rocks, pyrite (Py3) in quartz + polymetallic sulfide veins, and pyrites (Py4) in quartz calcite veins. The distributions of trace elements indicated that Py2 and Py3 represented the main gold-bearing minerals and contained high concentrations of As, Au, Ag, Pb, Zn, and Cu, and the distributions were controlled by the micro/nanoinclusions. The delta S-34 values in the five pyrite generations ranged from -19.5 to 3.4 parts per thousand. Py2 (-15.4 to -6.1 parts per thousand) and Py3 (-19.5 to -12.4 parts per thousand) had the lowest delta S-34 values, indicating that the sulfur originated from an oxidizing fluid. Py1 showed delta S-34 values (-0.3 to 1.9 parts per thousand) corresponding to a magmatic origin. Py4 (1.1-3.4 parts per thousand) displayed the highest delta S-34 values, indicating that the sulfur originated from the host rock under the action of meteoric water cycles. Analyses of the pyrite's trace elements and sulfur isotopes, in combination with geological evidence, indicated that magmatic ore-forming fluids contributed to the formation of the Luyuangou gold deposit. The magmatic ore-forming fluids interacted with meteoric water during the main mineralization period. The changing physicochemical conditions of the mineralized fluids caused the precipitation of a large amount of gold and other mineralized elements.