Hydrothermal alteration in volcanic arcs occurs in many settings and may involve magmatic,
marine, lacustrine or groundwaters, driven by magmatic, tectonic or thermal events. King
George Island, part of the South Shetland Island Cenozoic volcanic arc, contains an 80 km long zone
of propylitized volcanic rocks, with numerous occurrences of quartz veining, silicic, sericitic, argillic
and advanced-argillic alteration. On Barton Peninsula, a basaltic lava sequence (49–44 Ma) intruded
by a small, high-level granodiorite pluton (∼42 Ma), contains these alteration types, previously interpreted
as a single porphyry-copper system. In this study, we report three, possibly four, distinct fossil
hydrothermal episodes. (1) Banded chalcedonic quartz, quartz-sericite and propylitic alteration occurs
along ESE faults and as reworked clasts in nearby tuffs. Drusy quartz + calcite veins with
silicic/sericitic, argillic and propylitic wallrocks may represent feeders to the near-surface silicification.
These characteristics, and anomalous Ag + Pb + Sb + Au plus Te + Se + Zn + As, suggest a neutral-pH
geothermal system that was active during volcanism. (2) The lavas and banded-quartz rocks were
brecciated, veined and replaced by alunite+native sulphur+pyrite, and pyrophyllite + quartz +
pyrite + zunyite + diaspore assemblages with anomalous Hg + Se + As + Bi + Au + Tl + Sb + Cu.
Such advanced-argillic alteration is diagnostic of degassing of a felsic magma into shallow (<500 m)
meteoric groundwaters. Rhyolite tuffs, previously not reported on King George Island, may represent
leakage of this magma to the surface. (3) Subsequent burial to ∼3 km was followed by emplacement of
a granodiorite pluton and formation of a silicic contact-metasomatic aureole containing muscovite,
biotite, actinolite, magnetite, K-feldspar and tourmaline. Disseminated andalusite + corundum also
formed in areas previously affected by the advanced-argillic alteration. Iron/copper-sulphide veinlets
are locally abundant, but a porphyry-style geochemical signature is not present. Early Cretaceous
Ar–Ar ages near the intrusive contact indicate flow of an excess Ar-bearing hydrothermal plume up
the contact. Finally, isolated areas of propylitic alteration in the lavas nearby may be related either to
quartz veins of episode 1 at depth or to (4) continued circulation of heated groundwaters around the
cooling pluton.