地质科技通报 (Jan 2023)
Sources and evolution of ore-forming fluids from the Saxi tungsten-beryllium deposit in southeastern Yunnan Province: Evidence from fluid inclusions and H-O isotopes
Abstract
The Saxi tungsten-beryllium deposit is located in the Laojunshan tungsten-tin polymetallic metallogenic area in southeastern Yunnan. Determining the fluid nature, source, and evolution process of early layered orebodies and late vein-like orebodies and constraining their fluid geochemistry and isotopes can effectively explore the metallogenic mechanism of the Saxi tungsten-beryllium deposit, which is a collection of Laojunshan deposits. This study provides theoretical thinking for future prospecting work in the district. The formation of vein-like ore bodies in this deposit experienced a silicate stage, an oxide-sulfide stage, and a carbonate-fluorite stage. Scheelite and beryl were mainly formed in the first two stages. This article studies the fluid inclusions in quartz at different stages of the orebodies of the Saxi tungsten-beryllium deposit, including petrographic characteristics, microscopic temperature measurements, and laser Raman spectroscopy. The quartz sample's hydrogen-oxygen isotope composition in the ore-like ore body was tested. Three primary types of fluid inclusions (FIs) in vein-like ore bodies are detected, i.e.: liquid-rich FIs, gas-rich FIs, and aughter minerals bearing FIs. The gas phase composition of FIs is mainly H2O, with a small amount of reducing gas such as C2H2, and the liquid phase composition is also mainly H2O. From the silicate stage to the carbonate-fluorite stage, the peak homogenization temperature and salinity (NaCleq) of FIs are 240-360℃, 2.35%-13.81%; 220-310℃, 4.03%-9.86%, and 190-270℃, 2.41%-6.88%. From early to late, both homogenization temperature and salinity of the ore-forming fluid decreased obviously. The ore-forming fluid is generally a medium temperature, low salinity, CO2-poor NaCl-H2O fluid system. The δD and δ18O values of the quartz samples from early quasi-layered ore body are -102.8‰ to -99.0‰ and 11.7‰-13.0‰ respectively, and the corresponding δ18O value of the H2O value is 3.16‰-6.46‰. The δD and δ18O values of the quartz samplesfrom bulk oxide-sulfide section are -99.6‰ to -69.5‰ and 11.2‰-14.1‰ resoectively, and the corresponding δ18O value of H2O value is 3.08‰-6.73‰. This indicates that the ore-forming fluid mainly sourced from magmatic water mixed with a small amount of atmospheric precipitation or organic water.The fluid may boil, forming the precipitation of the main ore-forming minerals in the late vein-like ore body oxide-sulfide stage. The Saxi tungsten-beryllium deposit belong to a medium-to high-temperature hydrothermal system.
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