Geochemistry of a fossil hydrothermal system at Barton Peninsula, King George Island

Abstract

Afossil hydrothermal system on Barton Peninsula, King George Island, Antarctica, formed a series of lead-zinc-and pyrite t 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 t 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. NaC1. 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 cf oxygen and sulphur than the lead-zinc-depositing fluids at depth. The 634S values of sulphide minerals fiom the lead-zinc veins (634S = -4.6 to 0.7°/00> are much higher than the values of pyrite and native sulphur from the pyrite + native sulphur veins (634S = -12.9 to -20.1°/w). This indicates that the fluids depositing native sulphur had higher sulphate/H,S ratios under higher fo, conditions. Sulphur isotope compositions indicate an igneous source of sulphur with a 634S,s value near Oo/w, probably the Noel Hill Granodiorite. Measured and calculated 6l80 and 6D values of the epithermal fluids (6180water = -6.0 to 2.7°/w, 6Dwat,, = -87 to -75°/00) indicate that local meteoric water played an important role for formation of lead-zinc and native sulphur-bearing quartz veins.

Hydrothermal alteration and isotopic variations of igneous rocks in Barton Peninsula, King George Island, Antarctica

“…Quartz-veined volcaniclastic rocks and dyke rocks, which intruded basaltic andesite, have lower δD values than other rock types ( Table 1). The variation implies that the decrease in δ 18 O and δD values, consecutively from altered basaltic andesite to altered dyke and quartz-veined volcaniclastic rocks, could be related to the growing effect of meteoric water in the hydrothermal fluid system (So et al, 1995;Hwang and Lee, 1998;Hur et al, 2001;Willan and Armstrong, 2002). During the process the pluton may have acted as a passive heat source driving groundwater circulation through nearby faults.…”

“…However, this explanation could be wrong if the overall positive correlation is ascribed to such magmatic processes. Instead, for this trend, the effect of post-eruptive hydrothermal processes should be considered, as evidenced by mineralogical and geochemical characteristics indicating extensive hydrothermal alteration in the volcanic rocks (So et al, 1995;Hur et al, 2001;Willan and Armstrong, 2002). As given in Table 1, the SiO 2 content of less altered basaltic andesites is between 54 and 58 wt%, while that of the altered basaltic andesite is mostly much higher, even up to 78 wt%.…”

“…Numerous studies on hydrothermal alteration of the volcanic and intrusive rocks in the South Shetland Islands have also been carried out (Littlefair, 1978;Smellie et al, 1984;Birkenmajer et al, 1986;Park, 1991;So et al, 1995;Hur et al, 2001;Willan and Armstrong, 2002). Four types of wallrock alteration were recognized and described by Littlefair (1978) and Park (1991): propylitic, phyllic (sericitic), argillic, and advanced argillic.…”

“…Hur et al (2001) described the mineral assemblage of altered rocks, and published age data for the hydrothermal alteration of different rock types. Based on petrographic evidence, most workers have related the alteration to the nearby Noel Hill pluton (e.g., Davies, 1982;Tokarski, 1988;So et al, 1995;Hur et al, 2001), and they regarded the alteration event as a post-granodiorite product. Willan and Armstrong (2002) suggested the additional possible role of a hidden igneous body, preceding the Noel Hill pluton found by petrographic and geochemical studies.…”

“…Though the nature of hydrothermal fluids has been discussed in previous studies (So et al, 1995;Hwang and Lee, 1998;Willan and Armstrong, 2002), it still remains unclear what types of igneous activities were related to hydrothermal fluids, and how the fluids evolved through fluid-rock interaction during hydrothermal processes. No systematic isotope study has yet been performed to discuss these problems.…”