Diamonds form under immense pressure and heat far beneath the Earth’s surface. Their extraction involves complex engineering and geological understanding, requiring specialized equipment and techniques tailored to specific geological settings. Understanding this journey provides insight into the intricate operations that bring diamonds to light.
Where Diamonds Are Found
Diamonds originate deep within the Earth’s mantle, at depths ranging from 150 to 200 kilometers, under extreme temperature and pressure. They are brought closer to the surface through volcanic eruptions, forming primary deposits, predominantly found within ancient continental crustal blocks called cratons.
The most common primary deposits are kimberlite pipes, carrot-shaped volcanic conduits extending from the mantle to the surface. Lamproite pipes also serve as primary sources, though less common than kimberlite formations.
Over geological timescales, natural forces like erosion, rivers, and glaciers break down these primary pipes, transporting diamonds away. These transported diamonds accumulate in secondary, or alluvial, deposits, commonly found in riverbeds, ancient floodplains, and coastal areas where water currents concentrate diamond-bearing gravels. Marine deposits, including those offshore on the continental shelf, are another type of secondary deposit where diamonds settle on the seabed.
Mining Methods for Primary Deposits
Extracting diamonds from primary sources, such as kimberlite and lamproite pipes, demands large-scale mining operations due to the hard rock nature of these deposits. Open-pit mining involves excavating a large, conical pit from the surface downwards to access the diamond-bearing rock. This process begins with drilling and blasting to break up the ore, followed by large excavators and haul trucks removing the material. As the pit deepens, haul roads spiral down the sides, allowing continuous access for equipment to transport the ore to the processing plant.
When open-pit mining becomes economically unfeasible due to increasing depth, often around 400 to 600 meters, operations transition to underground methods. This involves constructing a network of shafts and tunnels to access the ore body from below. Block caving, a common technique, undercuts large sections of the ore body, causing rock above to fracture and collapse into collection points. Sub-level caving creates horizontal slices within the ore body, which are systematically blasted and drawn down. These underground techniques allow for continued diamond extraction from deep pipes while minimizing surface disturbance.
Mining Methods for Secondary Deposits
Extracting diamonds from secondary deposits requires approaches tailored to unconsolidated sediments. Alluvial mining targets diamond concentrations in riverbeds, ancient floodplains, and terraces. In wet alluvial environments, dredging uses specialized vessels or floating dredges to scoop up diamond-bearing gravels from river bottoms or submerged areas. For dry alluvial deposits, found in ancient river channels or desert environments, dry mining methods employ excavators and front-end loaders to transport gravels to processing plants.
Marine mining retrieves diamonds from the seabed, primarily off coastal areas where ancient river systems once flowed into the ocean. This mining utilizes large, purpose-built vessels. Some vessels employ crawler-mounted excavators that move along the seafloor, collecting gravels. Other techniques involve powerful suction systems, similar to giant underwater vacuum cleaners, which draw up sediments to the vessel for processing. These marine operations are complex, requiring precise navigation and geological mapping to identify diamond-rich areas.
Processing and Recovery
Once diamond-bearing material is extracted, it undergoes processing steps to liberate the diamonds. The initial stage involves crushing the ore, if from a primary deposit, followed by scrubbing to break down clay and remove lighter waste materials. This preparation is important for efficient separation.
Dense Media Separation (DMS) is a primary technique where the crushed or scrubbed material is introduced into a ferro-silicon slurry with a specific density. Diamonds, being denser than most other minerals, sink to the bottom of the DMS cyclone, while lighter waste materials float and are removed. This process significantly reduces the volume of material that needs further processing.
Following DMS, the diamond-rich concentrate undergoes further refinement using X-ray sorting technology. Diamonds exhibit fluorescence when exposed to X-rays, a property machines can detect. As material passes through an X-ray sorter, the X-ray beam identifies diamonds, triggering precisely timed air jets that eject the diamond-containing particles from the waste stream. This automated process is effective in separating diamonds from other minerals that may have similar densities.
The final stages of recovery involve hand sorting and cleaning of the diamond concentrate. Even after advanced mechanical and optical sorting, human eyes are necessary to identify and pick out the rough diamonds from any remaining non-diamond material. The recovered rough diamonds are then cleaned and prepared for further evaluation, completing the initial recovery process.