Artigas amethyst

The geodes here started as bubbles. Basalt of the Arapey formation came up carrying CO2-rich fluid, the gas formed cavities as the lava cooled, and those empty pockets, called protogeodes, sat waiting. What filled them came later and came from below: artesian water from the Guarani aquifer, percolating up through the basalt pile. First a rim of green celadonite, then agate, then quartz, and finally amethyst.

How warm that water was is an open question, and the two published answers matter. Morteani and colleagues, working from fluid inclusions in 2009, put amethyst formation between 50 and 120 degrees Celsius, with oxygen isotopes suggesting calcite grew at only about 24. A 2024 study by Arduin-Rode and colleagues, using nucleation-assisted microthermometry on single-phase inclusions together with triple oxygen isotope analysis, came out lower: 15 to 60 degrees. That is not a rounding difference. Temperatures that low mean the amethyst crystallised only after the entire basalt sequence was already emplaced, from ordinary meteoric groundwater rather than anything volcanic, with salinity from pure water to rarely more than 3 percent.

The mining district is Los Catalanes, about 60 kilometres south of the city of Artigas near the Brazilian border, and it is one of the main deposits of its kind in the world. The geometry is awkward for miners: the productive zone is a horizontal band in the upper part of the lava flows, but the geodes are scattered irregularly through it and vary sharply in abundance, quality and shape. The 2024 paper is blunt that reliable exploration guides are scarce, which makes reserve estimates unreliable and the work closer to prospecting than quarrying.

The sizes are the reason the district is famous. Geodes from these flows run up to about four metres long.