A geological oxygen switch
The sandstone layers of the Savannakhet Basin hold a chemical record of Earth's past. These deposits are a specific type known as sandstone-hosted or "roll-front" uranium deposits. They formed within the porous, medium-to-coarse-grained sandstones of the Jurassic-aged Phu Kradung Formation. This formation, which is between 800 and 1200 meters thick, was deposited by ancient river systems on a vast floodplain.
The process began when groundwater, rich in dissolved oxygen from the atmosphere, percolated through the sandstone. This oxygenated water carried uranium in its soluble hexavalent state (U6+). As the water seeped deeper, it encountered zones containing reducing agents like plant debris, pyrite, or other carbonaceous materials. This created a sharp chemical boundary, known as a reduction-oxidation (redox) front. At this front, the chemical environment flipped from oxidizing to reducing. The change caused the uranium to be reduced to its insoluble tetravalent state (U4+), forcing it to precipitate out of the water. Over millions of years, this process concentrated the uranium, creating ore bodies of uraninite and coffinite that trace the precise location of this ancient chemical switch.