The planet's first poison
Deep beneath the red sands of the Kalahari Desert lies a geological formation that records one of the most violent transformations in Earth's history. The Kalahari Manganese Field is the largest land-based deposit of manganese on the planet. The story of its formation begins around 2.4 billion years ago when the oceans were rich in dissolved iron and manganese, and the atmosphere was almost entirely without oxygen. In these ancient seas, the dominant life forms were anaerobic microbes, for whom oxygen was a deadly poison.
Then, a new type of bacterium evolved: cyanobacteria. Through photosynthesis, these microorganisms began releasing vast quantities of oxygen into the oceans and atmosphere. This "Great Oxidation Event" was a biological catastrophe. The newly introduced oxygen was toxic to most existing life, causing a mass extinction that allowed oxygen-breathing organisms to evolve. The event also permanently altered global chemistry. Dissolved iron and manganese in the seawater reacted with the new oxygen, causing them to precipitate and sink to the ocean floor. This process, happening on a global scale, created massive deposits called banded iron formations and manganese formations. The Kalahari field is the most significant of these manganese graveyards.
Layers of a changed world
The manganese deposits are part of the Hotazel Formation, a sequence of sedimentary rocks around 2.2 billion years old. This formation contains three distinct manganese-rich layers, interbedded with banded iron formation. The main ore body, known as the Lower Body, can be up to 45 meters thick. The sheer scale is immense, with estimates suggesting the field contains over 4 billion tonnes of manganese.
The deposit contains two main types of ore. The first is a lower-grade primary sedimentary ore called Mamatwan-type, which makes up most of the resource. The second is a much higher-grade Wessels-type ore, which was altered by later hydrothermal activity. This later alteration, about a billion years after the initial deposition, also created a spectacular variety of rare minerals. The mines in this field, such as the Wessels and N'Chwaning mines, are world-famous among mineral collectors for producing exceptional specimens of rhodochrosite, hausmannite, and other minerals. In total, 182 valid mineral types have been identified here, with 28 of them first discovered at this location.