Evolution of ore-forming fluids in the Sawayaerdun gold deposit in the Southwestern Chinese Tianshan metallogenic belt, Northwest China

摘要

The Sawayaerdun gold deposit was discovered in the early 1990s and is the largest orogenic-gold deposit in the Chinese Tianshan. Gold mineralization occurs in quartz veins, which are hosted in carbonaceous metasediments and controlled by faults and fractures. Three major hydrothermal events can be identified at Sawayaerdun: An early barren quartz vein stage, middle stage quartz veins with pyrite and gold mineralization and late carbonate (-quartz) veins.Fluid inclusion studies of quartz, pyrite and carbonate formed during the major hydrothermal stages show distinct changes in fluid, temperature, composition and redox conditions occurred during fluid evolution. Three types of fluid inclusions have been identified at Sawayaerdun: H_2O + NaCI (L + V), H_2O + NaCl + CO_2 (L + V) and CO_2 (L + V). Fluids trapped in early barren quartz have high temperatures (280-420 ℃), low salinities (<3 wt.% NaCl equiv.), high Cl and are CO_2-rich. Middle-stage quartz contains fluids with lower temperatures (210-310℃) and variable salinities (0.8-17.5 wt.% NaCl equiv.). They are commonly S-rich, CO_2-containing and relatively reduced compared with early stage fluids, in contrast, hydrothermal pyrite hosted in middle-stage quartz veins trapped fluids formed over a narrower temperature range (220-270 ℃), more oxidized (high CO_2/CH_4 ratio) and have higher Ca, K and Mg than fluids in quartz. Late stage carbonate is dominated by fluids with low temperatures (<200 ℃) and low contents of Cl, S and K. Fluid immiscibility probably commenced at the onset of mineralization along with middle-stage quartz formation, resulting in the generation of coexisting fluid inclusions with distinguishable vapor/liquid ratios and salinities. The compositional difference between fluids trapped in pyrite and quartz indicate fluid mixing may have occurred during the mineralization stage. The oxidized fluids, probably a mixture of meteoric water and fluids generated during water-host rock interaction, emerged into the ore-forming system to generate extensive metals precipitation. The fluid immiscibility and fluid mixing are most likely caused by pressure decrease during Late Paleozoic uplift and erosion in the Chinese Southern Tianshan area. Comparison of fluids between four major mineralization zones at Sawayaerdun indicate that zone II is similar to the most important zone IV and the order of similarity is probably zone Ⅱ > Ⅺ >Ⅰ, which could be applied to the future mineral exploration in this area..