Characteristics of deep ore-forming fluids in the Jiaojia gold belt, Jiaodong gold province (China)

Abstract

Genesis of the Jiaodong gold deposits, the largest gold producer in China, remains debated due to uncertainties in ore-forming fluid source and characteristics, particularly at depth (>−600 m). This study investigates deep mineralization (−1000 to −2000 m) in the Qujia gold deposit through integrated fluid inclusion, trace elements and sulfur isotopes of pyrite, with comparisons to Xincheng, Qianchen, and Zhaoxian deposits in Jiaojia gold belt. Fluid inclusions in Qujia evolve from early H₂O-CO₂-NaCl (type I, 315–370 ℃), through CO₂-rich (type II, 245–310 ℃) and H₂O-NaCl (type III, 187–305 ℃) to late aqueous H₂O-NaCl (type III, 136–175 ℃), with salinities of 1.62 to 9.74 wt% NaCl equiv., defining a low-medium-temperature, low-salinity H₂O-CO₂-NaCl system. Deep fluids in Qujia, Qianchen and Zhaoxian show consistent evolutionary trends of decreasing temperatures and salinities from early to late stages. They have less CH₄ than shallow fluids, suggesting CH₄ incorporation by fluid-rock interaction. Vertical analysis reveals uniform fluid properties across −1800 m to −200 m interval, except for late-stage cooling towards shallower levels due to meteoric water mixing. Pyrite (Py1, Py2 to Py3) trace elements display decreasing Co/Ni from 0.2 to 3.9 to 0.01–1.1, and increasing As and Au concentrations from early to late stages, which suggest magmatic-hydrothermal origin of fluids, with later crustal input through fluid-rock interaction. Fluid inclusions and pyrite trace elements reveal that fluid immiscibility and fluid-rock interaction, as potential gold-precipitation mechanisms, occurred during main mineralization stages. Sulfur isotopes in pyrite (δ³⁴S = +9.3 to + 12.3 ‰ in Qujia) are homogeneous across depths and deposits in the Jiaojia gold belt, implying a subcontinental lithospheric mantle source for the ore-forming fluids.