The origin of large gypsum crystals in the geode of pulpi (Almeria, Spain)

Abstract

The Geode of Pulpi (Almeria, Spain) is an ~11 m3 ovoid cavity, the walls of which are covered with meter-sized idiomorphic and highly transparent gypsum (CaSO4•2H2O) crystals. We performed a thorough study based on field work, and petrographic and geochemical data collection, which aimed to reconstruct the geological history leading to the formation of this geode. The geode is hosted in mineralized Triassic carbonate rocks with a discontinuous mineral sequence from iron-carbonates and barite to celestine and finally gypsum (microcrystalline and selenite). Data from fluid inclusions show that barite precipitated above 100 °C, celestine at ~70 °C, and gypsum below 25 °C. All δ34S sulfate phases fall between Triassic and Tertiary evaporite values. Barite and gypsum, either microcrystalline or large selenite crystals, show variable δ34S and δ18O compositions, whereas celestine and centimetric selenite gypsum have homogeneous values. We propose that the growth of the large selenite crystals in the Geode of Pulpi was the result of a self-feeding mechanism consisting of isovolumetric anhydrite replacement by gypsum at a temperature of 20 ± 5 °C, episodically contributed by a ripening process enhanced by temperature oscillations due to climatic change.