The origin of large gypsum crystals in the Geode of Pulpi (Almeria, Spain)

The Geode of Pulpi (Almeria, Spain) is an similar to 11 m(3) ovoid cavity, the walls of which are covered with meter-sized idiomorphic and highly transparent gypsum (CaSO4 center dot 2H(2)O) 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 degrees C, celestine at -70 degrees C, and gypsum below 25 degrees C. All delta S-34 sulfate phases fall between Triassic and Tertiary evaporite values. Barite and gypsum, either microcrystalline or large selenite crystals, show variable delta S-34 and delta O-18 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 degrees C, episodically contributed by a ripening process enhanced by temperature oscillations due to climatic change.