Fluid Inclusion Characteristics And Genesis Of The Zaozigou Gold Deposit In Gansu Province

The Zaozigou gold deposit is one of the large-size gold deposits discovered in recent years in the western part of Western Qinling Mountains.The deposit occurs in the Gulangdi Formation of Middle Triassic,and the occurrence of ore bodies is closely related to intermediate-acid dikes.The ore bodies are controlled by faults,showing vein-like and strip-like shapes,with a few layered occurrences.The metallogenic stages are divided into quartz-sulfide stage,natural gold-polymetallic stage,carbonation stage,oxidation-weathering stage,and the main metal sulfides are stibnite,pyrite,arsenopyrite and so on.According to the petrographic characteristics of inclusions,phase transformation during temperature measurement and laser Raman analysis of inclusions,fluid inclusions in each metallogenic stage of the Zaozigou gold deposit can be divided into three types:gas-liquid-water inclusions,CO₂-H₂O fluid inclusions and pure H₂O fluid inclusions.The liquid-rich gas-liquid-water inclusions account for most of the inclusions,accounting for about 70%of the total;the CO₂-H₂O fluid inclusions account for about 25%of the total,and can be divided into two types according to the phase state at room temperature?20??:three-phase CO₂-H₂O fluid inclusions and two-phase CO₂-H₂O fluid inclusions;pure H₂O fluid inclusions are less,with a content of about 5%.In this paper,gas-liquid-water inclusions and CO₂-H₂O fluid inclusions are mainly studied.Early stage homogenization temperature is173³46?,with an average temperature of 268?.Medium-stage homogenization temperature is 165³01?,with an average temperature of 242?.The average temperature of fluid inclusions in the late stage is 121²17?,with an average of 169?.Laser Raman spectra of single fluid inclusions show that the main gas components of fluid inclusions are H₂O,CO₂ and CH₄,and the ore-forming fluid is NaCl-H₂O-CO₂ system.The fluid inclusion study reveals that the fluid is low salinity and low density.The calculation results of thermodynamic parameters show that the pressure of ore-forming fluid decreases,pH increases,Eh increases,oxygen fugacity and sulfur fugacity decreases from early to late,indicating that the ore-forming fluid of Zaozigou gold deposit is characterized by medium-low temperature,low salinity,low density,low oxygen fugacity,low sulfur fugacity and neutral-weak weakening.In this study,sulfur isotopes of four pyrite samples and four antimony samples from Zaozigou Gold Mine were tested.The measured values of?³⁴S were negative,ranging from-25.4‰^-3.5‰with an average of-12.1‰and the variation range of?³⁴S was large.It is concluded that the ore-forming materials of the Zaozigou gold deposit are likely to be provided by magmatism or metamorphism in the region and mixed with sulfur in some strata in the later stage.The results of lead isotope test show that ²⁰⁶Pb/²⁰⁴Pb ranges from18.393¹8.872,²⁰⁷Pb/²⁰⁴Pb ranges from 15.503¹5.700,and ²⁰⁸Pb/²⁰⁴Pb ranges from38.167³8.976.It can be seen that the Pb isotope composition of sulfide in Zaozigou gold deposit has little change.It shows that lead in Zaozigou gold deposit is a mixture of crustal and deep-source lead.The total sulfur activity of Zaozigou gold deposit shows a decreasing trend from early to late.The dissolution type of sulfur in ore-forming hydrothermal solution is mainly H₂S,followed by HS^-.The migration form of gold element in metallogenic hydrothermal solution is AuHS⁰ and Au?HS?2^-.The precipitation mechanism of gold is that the ore-bearing fluid migrates from deep to shallow along the structural channel.Because of the changes of pressure,temperature,pH,f_O2 and other conditions,the gold-bearing complex decomposes and the gold element precipitates into ore.