Diamond Deposits FAQs
What is kimberlite?
Kimberlite is a special type of igneous rock that sometimes contains diamond. It is named after the town of Kimberley in South Africa, where kimberlite was first recognized (in the late eighteen hundreds) as a primary source of diamond. Kimberlites are relatively rare and occur in continental areas as small intrusions (dykes, sills and plugs) and as volcanic pipes. They range in age from about 45 million to in excess of 2 billion years. Depending on their mode of formation, kimberlites can be highly variable in appearance and mineral content. However, they are usually dark rocks that contain high concentrations of olivine and an unusual suite of minerals that are very important for their exploration and evaluation.
How are kimberlites formed?
Kimberlite magmas form by melting of the Earth’s mantle at great depths, between 200 and 670 kilometers below surface. As the kimberlite magma ascends towards the surface, it interacts with and incorporates large amounts of upper mantle material. Ascending kimberlite magma can either solidify below the surface as dykes (vertical sheets), sills (horizontal sheets) and plugs (small pipe-shaped intrusions) or erupt at surface to form pipe-like bodies and / or craters filled with volcanic kimberlite.
How are diamonds formed?
Diamond is a very high pressure mineral that can only form and be preserved in significant quantities at depths of > 120 km within the Earth’s mantle below the thick stable parts of continents. It forms locally within mantle rock types such as peridotite and eclogite, most likely as a result of the influx of carbon-rich fluid. Most gem diamonds are apparently very old, having formed between approximately 1 and 3.6 billion years ago.
Why are diamonds found in kimberlite?
Because kimberlites are generated at great depths below continents and interact with mantle material, they sometimes incorporate pre-existing diamond-bearing rocks. These can be retained within the magma during its ascent through the Earth’s upper mantle and crust, resulting in trace quantities of diamonds in the final kimberlite rocks preserved at surface. Diamonds may occur as inclusions contained within fragments of mantle rocks in the kimberlite or, more commonly, are liberated from their original host rock and dispersed within the kimberlite. About one in 100 kimberlite pipes contain gem-quality diamonds and only a very small proportion of these contain sufficient quantities of diamond to support economic extraction. Other rock types can be diamondiferous, but kimberlites are by far the most important source for economic concentrations of diamonds.
What are primary diamond deposits?
Diamond deposits are termed primary when diamonds are found in the host igneous or volcanic rocks. The host rock types in which significant quantities of diamond can occur include kimberlite and, more rarely, orangeite and lamproite, igneous rocks with similar characteristics to kimberlite. Primary diamond deposits occur as intrusions (dykes, sills, plugs) and volcanic pipes.
What are alluvial diamond deposits?
Alluvial diamond deposits are formed as a result of surface weathering and erosion of primary diamond deposits over millions of years. The diamonds are transported by rivers, and deposited and concentrated in a new environment such as certain portions of the river bed, a shoreline or an ocean floor. Diamond, being the hardest known natural mineral, survives weathering, erosion and transport. Alluvial diamond mining is the process by which diamonds are recovered from such deposits. Around 10% of the world’s rough diamonds are sourced through industrial alluvial mining and 14% through artisanal or small-scale informal alluvial diamond digging.
What are kimberlite dykes and blows?
Some kimberlites are emplaced as steeply-dipping sheet-like bodies known as dykes. These can form complex systems of multiple dykes that extend for many kilometers and may be exposed at surface. The dykes consist of coherent kimberlite (i.e. that formed by direct cooling and crystallisation from a magma) and can vary in width from less than a centimeter to more than a meter. Small pipe-like features known as blows are occasionally formed along such dyke systems. These can be cylindrical to irregular in shape and may consist of coherent kimberlite and volcaniclastic kimberlite (i.e. kimberlite formed following explosive fragmentation of the magma by volcanic processes).
What are kimberlite pipes?
Some kimberlites occur as steep-sided pipe-like bodies that are the erosional remnants of kimberlite volcanoes. These generally occur as clusters of two or more pipes. Kimberlite pipes are commonly filled with volcaniclastic kimberlite, a mixture of fragmented kimberlite and country rock material. Kimberlite pipes are sometimes associated with previously-formed kimberlite dykes and the pipes themselves may be cross-cut by later kimberlite dykes. The surrounding host rock is commonly brecciated and altered as a result of pipe formation.