Source and evolution of fluids in the Shihu gold deposit, Taihang Mountains, China: evidence from microthermometry, chemical composition and noble gas isotope of fluid inclusions

The well-known Shihu quartz vein-type Au deposit in the Taihang Mountains in the North China Craton (NCC) is hosted by ductile-brittle faults within the Neo-archean–Palaeo-proterozoic Fuping Group metamorphic complex. The deposit exhibit four stages of quartz veins: (1) quartz-K-feldspar-sericite; (2) quartzpyrite; (3) quartz-polymetallic sulphide; and (4) quartz-carbonate ± pyrite. Three types of fluid inclusions in quartz are recognized in this study. Solid-bearing high-salinity (Type I) fluid inclusions have homogenization temperatures of up to 390 ºC and high salinities of 33–47 wt% NaCl equivalent. Two-phase H₂O-rich (Type II) fluid inclusions homogenized between 129–396 ºC and have salinities of 1.7–12.5 wt% NaCl equivalent. Two/three-phase CO₂-rich (Type III) fluid inclusions, which are abundant in auriferous quartz veins, have homogenization temperatures of 205–358 ºC and salinities of 1.63–7.64 wt% NaCl equivalent. Laser Raman spectroscopy and quadrupole mass spectrometry confirmed that the vapour phase in the fluid inclusions is dominated by CO₂, H₂S, CH₄, C₂H₄, N₂ and Ar besides H₂O, whereas the liquid phase is composed of Cl-, SO₄²⁻, Na⁺, and K⁺ with minor Mg²⁺ and Ca²⁺. Fluid inclusions in pyrite associated with quartz ore-stages (2) and (3) have ΣREE values of 0.61–342.17 ppm with negative Ce, Eu and Y anomalies and LREE enrichment relative to HREE, generally indicating a crustal source of ore fluids. Helium isotope studies of fluid inclusions in pyrites associated with quartz ore-stages (2) and (3) yielded ³He/⁴He ratios of 0.12–0.93 Ra (Ra=1.4×10-6 for air) and ⁴⁰Ar/³⁶Ar ratios of 3690–23678. The noble gas data suggest c. 10–20% mantle-derived ore fluids, reflecting an increased interaction of ore fluids with surrounding crustal rocks contributing additional 4He to the fluids. The present data and various information from published works suggest that the Shihu Au deposit was formed during lithospheric thinning or decratonization beneath the Taihang Mountains. The fluids derived from the lower crust, formed due to partial melting and dehydration of the lithospheric mantle, were mixed with magmatic and meteoric waters, and finally precipitated Au and associated metals in the Shihu deposit