Isotopic geochemistry of the Sawayaerdun orogenic-type gold deposit, Tianshan, northwest China: Implications for ore genesis and mineral exploration

摘要

The Sawayaerdun gold deposit is hosted by Carbonaceous metasediments and is considered to be the largest Muruntau-type gold deposit in the Chinese Tianshan metallogenic belt. Gold mineralization at Sawayaerdun occurs in quartz veins associated with three major hydrothermal events: an early, barren quartz vein stage, middle stage mineralized quartz veins with pyrite and late carbonate (-quartz) veins. The isotopic compositions of quartz and sulfides from the Sawayaerdun gold deposit show some variation but are generally comparable to those of other orogenic-type gold deposits. Fluids trapped in early-stage quartz have a δ ~(18)O range of 13.6‰ to 15.4‰, δD of -48‰ to -75‰, δ ~(13)C of 0.5‰ to 4.2‰ and δ ~(30)Si of -0.2‰ to 0‰. In contrast, isotopic compositions of fluids trapped in middle-stage quartz have δ ~(18)O values of 6.7‰ to 14.7‰, δD of -56‰ to -110‰, δ ~(13)C of 0.4‰ to 10.1‰ and δ ~(30)Si of -0.3‰ to 0‰. Diagenetic and hydrothermal pyrite have similar sulfur (-1.8‰ to 0.9‰) and Pb isotopic values that are associated with host rock compositions. The early-stage, ~(18)O and ~(13)C-rich fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth, leading to the precipitation of early barren quartz veins. In the middle stage, a decrease in the regional pressure and temperature regime could have resulted in the incorporation of external fluids into the ore-forming system. These external fluids with isotopic signatures similar to that of the host rock and generally rich in ~(34)S and radiogenic Pb mixed with original ore-forming fluids to generate extensive metal precipitation. Late-stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water, indicating the cessation of hydrothermal fluid circulation at Sawayaerdun occurred at this time. The metallogenetic model illustrated by stable and Pb isotopes is also consistent with fluid inclusion studies in Sawayaerdun. The development of mineralization at Sawayaerdun is strongly linked to fluid mixing, as witnessed by the isotopic signatures of fluids from identified ore-bearing zones. The isotopic compositions of other anomalous zones at Sawayaerdun are similar to those of the mineralized zones, suggesting a high potential for further exploration.