Sulphur sources for epithermal and mesothermal veins in Cretaceous-Tertiary magmatic-arc rocks, Livingston Island, South Shetland Islands
Epithermal veins in early Triassic turbidites on Hurd Peninsula are isotopically homogeneous over an area of 13 km × 2 km (mean δ34S = +2.2‰, 1σ= 1.7, n = 65) suggesting derivation from a deep circulating, neutral-chloride hydrothermal plume containing magmatic sulphur. The sense of fractionation (pyrite = +3.5, arsenopyrite = +3.2, sphalerite = +2.9, chalcopyrite = +2.5, galena = +0.9‰) and isotopic temperatures between 250 and 345°C suggest partial isotopic equilibration. Sulphides in massive dark carbonate/peperitic dyke breccias, with abundant magnetite, hematite and titanite and traces of barite, garnet, zircon and monazite are isotopically heavy (+7 to +14.9‰, n = 12) due either to dyke intrusion into the hydrothermal system resulting in degassing of H2S, or to a second, isotopically heavy and sulphate-bearing solution in fault zones along which dykes were subsequently intruded. Vein sulphides in nearby Cretaceous volcanic rocks are similar to the epithermal system (mean = + 1.8‰, 1σ = 0.9, n = 6, pyrite = +2.0, chalcopyrite = +1.5), suggesting sulphur-derivation by degassing of sub-volcanic magma, or remobilization of disseminated sulphides during plutonism. Sulphide in an Eocene tonalite pluton is slightly 32S-enriched (mean = 0‰, 1σ = 1.9, n =7, molybdenite = +0.7, pyrite = +1.5, chalcopyrite = –2.6) and was exsolved directly from the cooling tonalitic magma.