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Fluid inclusions and stable isotopes of giant transparent gypsum crystals in a huge geode recently discovered in Pulpi (SE Spain) are useful tools for explaining its geological formation. Fluid inclusions suggest a mixture of fluids, starting with freshwater and including seawater in more advanced stages of the gypsum crystal growth. The isotopic composition of the hydration water in the gypsum crystals agrees with a meteoric fluid. The gypsum is enriched in [sup.34]S, which denotes genetic links with marine sulphates via freshwater dissolution-recrystallization of earlier marine evaporites. Keywords: gypsum, geodes, fluid inclusions, sulphur isotopes.

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides. A review of modern marine sediments suggests that near-surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction. Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hunsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis. Keywords: pyritization, fossils, sulphur, isotopes.

Epithermal veins in early Triassic turbidites on Hurd Peninsula are isotopically homogeneous over an area of 13 km x 2 km (mean [[Delta].sup.34]S = +2.2[per thousand], 1[Sigma] = 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%o) and isotopic temperatures between 250 and 345 [degrees] 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[per thousand], n = 12) due either to dyke intrusion into the hydrothermal system resulting in degassing of [H.sub.2]S, 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%o, 1[Sigma] = 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 tonalire pluton is slightly 32S-enriched (mean = 0[per thousand], 1[Sigma] = 1.9, n = 7, molybdenite = +0.7, pyrite = + 1.5, chalcopyrite = -2.6) and was exsolved directly from the cooling tonalitic magma. Keywords: Andean Orogeny, South Shetland Islands, veins, sulphur, isotopes.