Uranium concentrated by ancient bacteria. The ore follows 2-billion-year-old microbial mat structures, shows life can create economic mineral deposits.
Life as a geological force
The Kayragach uranium deposits in Uzbekistan's Kyzylkum Desert are not typical mineral formations. They are the direct result of biological activity that took place two billion years ago. These deposits are primarily found within black shales, which are sedimentary rocks with high organic carbon. The dark color comes from the remains of ancient microbial mats, vast communities of microorganisms that thrived in a primordial sea.
The process of ore formation began when uranium, dissolved in its oxidized state (U6+) in seawater, washed over these microbial mats. The mats, teeming with decaying organic matter, created a powerful anoxic and reducing environment. When the uranium-rich water encountered this zone, a chemical reaction was triggered. The uranium was reduced to its insoluble state (U4+) and precipitated out of the water as the mineral uraninite. Over immense spans of geological time, this continuous process concentrated the uranium along the fossilized structures of the microbial mats, creating the economically viable ore bodies mined today. This type of formation is classified as a "black-shale" type deposit, distinct from the more common sandstone-hosted deposits also found in the region.
Mining ancient life
The state-owned enterprise Navoiyuran operates the uranium mines in this region. Uranium exploration in the Kyzylkum Desert began in the late 1940s, with the first major mine established in 1958. While many of Uzbekistan's deposits are of the sandstone "roll-front" type, the black-shale deposits like Kayragach are a different and complex geological target.
Because of the way the uranium is finely disseminated within the shale, a specific mining technique is often employed. Since 1994, all uranium in Uzbekistan is mined using a method called in-situ leaching (ISL), also known as in-situ recovery (ISR). This process involves drilling wells into the ore body and injecting a solution, called a lixiviant, to dissolve the uranium directly from the rock underground. This uranium-bearing solution is then pumped to the surface for processing. This technique avoids large open pits or underground tunnels, and is considered to have environmental and safety advantages over conventional mining. Navoiyuran has even developed a low-reagent ISL technology that uses oxygen to increase recovery and reduce costs. Uzbekistan is a major player in the global uranium market, ranking among the top producers worldwide.
💡Fun Facts
The host rocks for the deposit date back to the Paleoproterozoic Eon, a period from 2.5 billion to 1.6 billion years ago.
The carbon-rich shales containing the uranium can have an organic content of up to 10%.
In-situ leaching was first used in Uzbekistan in 1962 and has been the exclusive method for all uranium mining in the country since 1994.
Besides uranium, these black shales can also be enriched in other valuable elements like molybdenum, vanadium, and rare earth elements.