Diamond discoveries can take various forms, mainly kimberlites, but also lamproites and alluvial/paleo-placer deposits.
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush, eventually creating the Big Hole. Kimberlite occurs in the Earth’s crust in vertical structures known as kimberlite pipes. Kimberlite pipes are the most important source of mined diamonds today. However, it is estimated that only 1 in 200 pipes will have diamonds.
In general they are formed in the mantle at depths between 150 and 450 kilometres (93 and 280 miles). They are formed by anomalously enriched exotic mantle compositions that erupted rapidly and violently. Kimberlites are predominantly found in regions underlain by stable Archean cratons. The kimberlites rise quickly from the mantle and are emplaced as multi-stage, high-level diatremes, tuff-cones and rings, hypabyssal dikes and sills. Economic deposits occur in kimberlites from Proterozoic to Tertiary in age.
How to identify kimberlites: Kimberlites commonly have high titanium, chromium, nickel and magnesium values in overlying residual soils. Mineral chemistry is used extensively to help determine whether the kimberlite source is diamondiferous or barren. For further information see http://www.minelinks.com/alluvial/diamondExploration3.html Geophysical techniques are used to locate kimberlites, but these give no indication as to their diamond content. Ground and airborne magnetometer surveys are commonly used. Kimberlites can show as either magnetic highs or lows.
Indicator minerals are also used extensively in the search for kimberlites, as well as bulk sampling, to assess the diamond content of a particular pipe. Diamonds may also indicate a kimberlitic or lamproitic. However they are rarely encountered in surficial sediments.
Lamproites are ultrapotassic mantle-derived volcanic and subvolcanic rocks. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively highMgO content and extreme enrichment in incompatible elements.
Lamproites are geographically widespread yet are volumetrically insignificant. Unlike kimberlites which are found exclusively in Archaean cratons, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They also vary widely in age, from Proterozoic to Pleistocene, the youngest known example being 56,000 ± 5,000 years old.
Lamproite volcanology is varied, with both diatreme styles and cinder cone or cone edifices known.
Alluvial diamonds is the term used to describe diamonds that have been removed from the primary source (Kimberlite) by natural erosive action over millions of years, and eventually deposited in a new environment such as a river bed, an ocean floor or a shoreline. Alluvial diamond deposits are found on the Atlantic coast of South Africa and Namibia, as well as in some riverbeds in Angola, Sierra Leone, Democratic Republic of Congo, Central African Republic, Cote d’Ivoire, Guinea, Ghana, Liberia, Tanzania, Togo, Brazil, Venezuela, Guyana and South Africa. 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.
How to identify alluvial diamonds: Anomalous concentrations of gold, diamonds or other elements in stream sediments. The geophysical identification is through ground penetrating radar especially useful for delineating the geometry, structure and thickness of deposits with low clay contents, especially fluvial terrace placers. Shallow seismic, electromagnetic, induced polarization, resistivity and magnetometer surveys are locally useful. Geophysical logging of drill holes with apparent conductivity, naturally occurring gamma radiation and magnetic susceptibility tools can supplement stratigraphic data. Otherwise the more traditional method of panning is used which accounts for nearly all the informal alluvial diamonds.
Paleo-placer diamonds: Paleo-placers have been mostly ignored and overlooked as a serious source of diamonds as just about all miners and explorers have been fixated on kimberlites and alluvial sources of diamonds. Nobody has really spent much time thinking about paleo-placers or bothered to look for ones that have diamonds. This of course means that they are a new target for explorers and it will be first come first served. The whole paleo-placer interest was kicked off by the Marange paleo-placer deposits which are staggeringly rich in diamonds but they are of low value, which is probably why De Beers originally walked away from them. But since then they have been shown also have good stones and as a deposit it is relatively rich as a result. The extraction of paleo-placer diamonds is a fairly simple process, very similar to that of kimberlites. The only drawback is the volume of ore. The placers tend to be thin, laterally extensive, and to extract meaningful amounts of diamonds a large earthmoving operation is required.
Evaluation Of Diamonds
When assessing diamond deposits, grade, tonnage and the average value ($/carat) of the diamonds must be considered. Diamonds, unlike commodities such as gold, do not have a set value. They can be worth from a few $/carat to thousands of $/carat depending on their quality (evaluated on the size, color and clarity of the stone). Some deposits have higher grades at the surface due to residual concentration. Typically alluvial diamonds are of high tonnage but are of low value, as with the Marange find in Zimbabwe. Some estimates for African producers are as follows:
|Pipe||Tonnage (Mt)||Grade (carats*/100 tonne)|
* one carat of diamonds weighs 0.2 grams