Geochemistry of a Fossil Hydrothermal System at Barton Peninsula, King George Island

A fossil hydrothermal system on Barton Peninsula, King George Island, Antarctica, formed a series of lead-zinc- and pyrite + native sulphur-bearing epithermal quartz± calcite veins, filling fault-related fractures in hydrothermally altered volcanic rocks of Eocene age. The lead-zinc veins occur within argillic hydrothermal alteration zones, whereas the pyrite + native sulphur veins are found within advanced argillic alteration zones. Fluid inclusion data indicate that the vein formation occurred at temperatures between about 125° and 370°C (sphalerite deposition formed at 123-211 °C) from fluids with salinities of 0.5-4.6 wt. % eq. NaCl. Equilibrium thermodynamic interpretation of mineral assemblages indicates that the deposition of native sulphur in the upper and central portions of the hydrothermal system was a result of the mixing of condensates of ascending magmatic gases and meteoric water giving rise to fluids which had lower pH (<3.5) and higher fugacities of oxygen and sulphur than the lead-zinc-depositing fluids at depth. The S³⁴S values of sulphide minerals from the lead-zinc veins (δ³⁴S = -4.6 to 0.7°/_oo) are much higher than the values of pyrite and native sulphur from the pyrite + native sulphur veins (δ³⁴S = -12.9 to -20.1°/₀₀). This indicates that the fluids depositing native sulphur had higher sulphate/H₂S ratios under higher fo₂ conditions. Sulphur isotope compositions indicate an igneous source of sulphur with a δ³⁴S_2S value near 0°/₀₀, probably the Noel Hill Granodiorite. Measured and calculated δ¹⁸O and δD values of the epithermal fluids (δ¹⁸O_water = -6.0 to 2.7°/₀₀, δD_water = -87 to -75°/₀₀) indicate that local meteoric water played an important role for formation of lead-zinc and native sulphur-bearing quartz veins.