Role of Fluid Inclusion Analysis in Understanding Gigantic Selenite Crystal Growth in a Deep Karst Cave (Naica, Mexico)

The largest natural crystals in the world were discovered in a Ag, Pb and Zn mine in Naica, Chihuahua, Mexico. These selenite crystals (up to 11 m in length, corrected U/Th age of 155±47 ka) are as white as the moon when illuminated from the back. Due to the mining activities, the ground water is constantly being pumped away which by chance lead to the discovery of the crystal caves at two levels down to -290 m. The caves are now drained for as long as the pumping is maintained for mining, but conditions inside the chambers are lethal for the unprotected person, because of the high temperature (>50° C) and nearly 100% relative humidity. Nevertheless, formation of such impressive natural objects is intensively studied. The tiny amounts of liquid trapped in the crystal (Fluid Inclusions, FI, tens of µm) record composition of the liquid at the time the crystal of growth at that position. The FI were analyzed with Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry and trace elements quantified via the total salt content of the FI which was obtained prior to ablation with microthermometric measurements. Samples from las espadas (-130 m below entrance) and cristales and ojo de la reina (-290 m b.e.) were taken from the crystals and the trace element concentrations were determined. Only Na, Mg, K and Sr were found, other elements were not present or below limit of detection. The trace element composition is different for deeper two caves and the shallower one. However, the composition is similar for various FI assemblages of a crystal. A favorable geological setting must have controlled the formation of such large crystals. Among other factors, our results indicate that mixing of two different water bodies may have lead to the supersaturated liquid necessary for crystallization.