Uruguay’s giant amethyst geodes may have grown from groundwater

Amethyst geodes—some stretching 5 m long—dot the ancient lava flows of Uruguay’s Los Catalanes mining area. The geodes contain calcite, colorless quartz, and agate, another quartz mineral. Large violet crystals of amethyst pierce the center of the geodes. Researchers have debated whether the gem-studded geodes formed from hot magmatic fluids or cooler liquids seeping into the rock. A new analysis suggests the geodes grew from groundwater and crystallized at low temperatures (Miner. Deposita 2024, DOI: 10.1007/s00126-024-01310-2).

It’s challenging to investigate how such geodes formed, says Jens Götze, a mineralogist at Freiberg University of Mining and Technology who was not part of the work. “You cannot observe the formation of these amethyst or agates in nature. It’s also very difficult to produce them in the laboratory.” Instead, researchers look to the surrounding rock and the geodes themselves for clues to their origin.

Fiorella Arduin Rode, a geologist studying mineral resources at the University of Göttingen, visited the deposits in Uruguay. She saw how small geodes speckled the surface in an open pit mine, and she examined massive geodes lodged in rock in an underground mine. She also collected samples for her team to analyze.

The researchers investigated pockets of fluid trapped in the geode minerals using a technique called nucleation-assisted microthermometry. Using a laser, scientists created a bubble of vapor in the fluid inclusion. Then they heated the fluid inclusion, and the vapor coalesced with the liquid. Measuring the temperature at which the vapor in the bubble shrank in volume and coalesced with the liquid provided an estimate of the crystallization temperature. That measurement suggested the crystals formed between 15 and 60 °C.

The team also took a suite of measurements for three oxygen isotopes in the geode minerals. These findings supported the crystallization temperatures they had estimated. The relative amounts of isotopes revealed that the source water for the geodes’ minerals likely came from groundwater that infiltrated the volcanic rocks after they had solidified.

“It was surprising to find these very low temperatures,” Arduin Rode says. Researchers studying amethyst geode formation in Brazil’s lava flows have proposed temperatures below 100 °C. The new study uses similar techniques and includes an additional oxygen isotope. “This is state of the art—in particular, the oxygen isotope studies,” Götze says. The study demonstrates how modern analytical tools can help solve questions about how mineral and ore deposits form, he says.

While the lava flows are some 130 million years old, the geodes may be much younger, Arduin Rode says. Because local groundwater has a low concentration of silica, her team estimates that the geodes could have taken several millions of years to form.

These amethyst geodes are important to the local economy, Arduin Rode says. Understanding the process may help reduce the impact of mining the geodes, which are displayed in museums and used for jewelry, decoration, and spiritual purposes, she says.