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 degrees and 370 degrees C (sphalerite deposition formed at 123-211 degrees 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 delta(34)S values of sulphide minerals from the lead-zinc veins (delta(34)S = -4.6 to 0.7 parts per thousand are much higher than the values of pyrite and native sulphur from the pyrite + native sulphur veins (delta(34)S = -12.9 to -20.1 parts per thousand. This indicates that the fluids depositing native sulphur had higher sulphate/H2S ratios under higher fo(2) conditions. Sulphur isotope compositions indicate an igneous source of sulphur with a delta(34)S(Sigma S) value near 0 parts per thousand probably the Noel Hill Granodiorite. Measured and calculated delta(18)O and delta D values of the epithermal fluids (delta(18)O(water) = -6.0 to 2 7 parts per thousand) delta D-water = -87 to -75 parts per thousand) indicate that local meteoric water played an important role for formation of lead-zinc and native sulphur-bearing quartz veins.