Mystery of Uruguay's amethyst geodes: New model to explain amethyst formation

Amethyst has been mined for over 150 years in the Los Catalanes District of Uruguay, where the research was carried out. This is an area renowned for the deep violet colour and high quality of its gems, as well as magnificent giant geodes sometimes over 5 m high. The deposits here have been recognised as one of the top 100 geological heritage sites in the world, highlighting their scientific and natural value. However, limited knowledge of how these geodes formed has made locating them challenging, relying largely on miners' experience. To address this, researchers conducted extensive geological surveys across more than 30 active mines, analysing geode minerals, geode-hosted water, and groundwater. Using advanced techniques like nucleation-assisted microthermometry of initial one-phase fluid inclusion and triple-oxygen-isotope geochemistry, the team uncovered new insights into how these prized geodes formed. As well as finding that the amethyst geodes formed at unexpectedly low crystallisation temperatures, the researchers also showed that the mineralising fluids had the low levels of salinity and proportion of isotopes consistent with water originating from the natural weather cycle, which probably came from groundwater held in nearby rocks.

"The precision and accuracy of these new techniques, allowed us to estimate with confidence the temperature and composition of the mineralizing fluids," said Fiorella Arduin Rode, lead author and PhD researcher at Göttingen University's Geoscience Centre. "Our findings support the idea that these amethysts crystallised at low temperatures from groundwater-like fluids." The study proposes a model where mineral phases like amethyst crystallise within volcanic cavities in a dark rock known as basalt, influenced by regional variations in temperature in the Earth's crust. Arduin Rode adds, "Understanding the conditions for amethyst formation -- such as the temperature and composition of the mineralising fluid, as well as the silica source, the timing of the mineralisation, and its relationship with the host rocks -- is crucial for unravelling the process. This could significantly improve exploration techniques and lead to sustainable mining strategies in the future."

Funding for this research was provided by Research Grants -- Doctoral Programmes in Germany, 2021/22 -- 57552340 -- Deutscher Akademischer Austauschdienst (DAAD)