DIAMONDS – ORIGIN & OCCURRENCE
- The origin of diamonds
- Where does the carbon come from?
- When did diamonds form?
- How did diamonds reach the Earth’s surface?
- What rock types contain diamonds?
- The shapes and nature of kimberlitic volcanic systems
The origin of diamonds
Diamond, a high-density crystalline polymorph of carbon, is formed by subjection of other low-density carbon (graphite and organic compounds) to very high pressures and temperatures. At high pressures and temperatures the carbon atoms are forced closer together to form the denser crystalline material - diamond. Under the crusts of continents, diamonds can form below a depth of ~150 kilometers (90 miles), where pressure is roughly 5 gigapascals (49,350 atmospheres) and the temperature is ~1200° Celsius (2200° Fahrenheit). Diamond can form under the crust below oceans, but at much greater depths, due to higher rock temperatures, which require concomitantly higher pressures to form diamonds. Diamonds are virtually ubiquitous through the earth’s mantle and occur in some parts of the lithosphere.
Where does this carbon come from?
By measuring the ratios of the stable isotopes C12 and C13 it has been demonstrated that the carbon of diamonds is possibly derived from both organic and inorganic sources. Some diamonds, which are associated with a rock known as harzburgite, are clearly formed from inorganic “primitive” carbon, which exists deep in the planet’s mantle and in meteorites. In contrast, diamonds occasionally found in a rock termed eclogite seem to contain organic carbon, which might have been carried down from the Earth’s surface during the process of tectonic plate subduction. It is fascinating to think that the diamond you are wearing might contain traces of the earliest life, which existed on the evolving surface of our planet!
When did diamonds form?
We can determine the age of diamonds by analyzing the various isotope ratios of tiny mineral grains from the mantle and lower lithosphere, which were trapped inside the diamond crystals as they grew. Such studies reveal that most diamonds are very old and range from under 1 billion to more than 4 billion years old. They have thus been resting in the depths awaiting transport to the surface for a very long time. It is interesting to think that we now adorn ourselves with some of the records of our Earth's early history!
How did diamonds reach the Earth's surface?
Diamonds are accidentally swept to the surface and near-surface during highly explosive volcanic eruptions of deep-origin. Diamond-bearing volcanic craters and pyroclastic deposits are underlain by volcanic pipes (diatremes) or by intrusive sills and dikes. The diamond-bearing craters and volcanic pipes are often less than 1 hectare (10,000 m2) in area and normally less than 20 hectares. The magma for such diamond-bearing volcanic eruptions must originate at the profound depths where diamonds are formed, i.e. usually 150 km to 250 km (but rarely as deep as 600 km) below surface. Such volcanic events have been relatively rare throughout geological time, for it is seldom that faults fracture so deep into the lower lithosphere and upper mantle to tap partly molten rocks. As diamonds are unstable at normal surface temperatures and pressures (given an opportunity they will slowly revert to the stable carbon polymorph graphite) they have to be rapidly brought to the surface by erupting magma and gases and quickly chilled to survive. Thus we know that the highly explosive eruptions rocketed through the crust at speeds sometimes in excess of 100km/hr. and showered the crater and its surroundings with ash, broken rocks and occasionally diamonds.
What rock types carry diamonds?
Diamonds can occur in volcanic rock suites of two characteristic types, namely: kimberlite or lamproite. The kimberlite and lamproite magmas themselves do not precipitate the crystalline diamond; instead, they act as transporters of samples of minerals and rocks which were stable in the Earth's lower lithosphere. Thus we have to think of diamonds and the other minerals, which we commonly find with them (indicator minerals), as “accidental passengers” swept up in the escaping explosive magmas. Many kimberlites and lamproites do not carry diamonds, or contain very few; indeed on average only 1% of kimberlites have brought diamonds with them. The lack of diamonds in such volcanic rocks can be explained either by the fact that the melts originated in sectors of the mantle or lithosphere, which were depleted in diamonds, or that their diamonds have been later destroyed and converted to graphite or even carbon dioxide. Kimberlites and lamproites belong to the “ultramafic” class of rocks i.e. they are rich in Magnesium- and Iron-minerals such as olivine and pyroxene (these are often altered to serpentine minerals or clay minerals, due to the amount of wet, carbon dioxide-rich gas in the erupting magmas). Thus many kimberlites and lamproites are relatively soft and easily eroded. Kimberlites are also characterized by a high concentration of potassium, which is mostly contained in the bronze-coloured mica Phlogopite. Certain of the aforementioned indicator minerals in diamondiferous kimberlites can be used as mineralogical tracers when geologists search for diamond deposits. These minerals are rich in Chromium (Cr) or Titanium (Ti), elements that often impart bright colors to the minerals. The most common kimberlite indicator minerals (KIMs) are Chromian garnets (usually bright red or purple Chromian-pyrope, and occasionally green Ugrandite-series garnets), orange or brown eclogitic garnets, orange Titanian-pyrope, red Chromian spinels, black Chromite, emerald green Chromian-diopside, glassy green or pale yellow olivine (Peridote), grey picroilmenite, and black magnetite. As many of the KIMs survive during the weathering of rocks, and can be transported from the actual eruptive site by rivers, streams and glaciers, sampling of surface sediments in an area can provide important clues as to the potentials for the discovery of diamond-bearing kimberlites.
The volcanic system containing diamonds is referred to as a “primary” source of diamonds. “Secondary” sources of diamonds include all areas where a significant number of diamonds, eroded out of their kimberlite or lamproite matrices, accumulate because of weathering in-situ, water transport or ice transport. Such secondary deposits, which are often referred to as placers, include alluvial deposits (river gravels), eluvial deposits (debris moved only a short distance from the source by gravity) and colluvial deposits (mass downhill transport of debris).
The shapes and appearance of kimberlitic volcanic systems
Kimberlites are complex volcanic systems with hypabyssal intrusions, which grade upwards into diatreme breccias near surface and to pyroclastic rocks in a crater environment at surface. The kimberlites commonly occur as steep-sided, downward-tapering, cone-shaped diatremes (pipes), which may have complex root zones with multiple dikes. Hypabyssal kimberlites commonly form dikes and sills which are the feeder systems for the eruption. Most so-called “pipes” are actually “blows” i.e. eruptions on dike filled fault systems. The contacts of pipes are sharp. Surface exposures of diamond-bearing pipes range from less than 2 hectares up to 146 hectares (Mwadui in Tanzania). In some instances the associated crater fill debris and tuff ring (volcanic ash and debris) may be preserved. Kimberlites typically occur in fields comprising up to 100 individual intrusions, which often group in those fields as “clusters”. Each kimberlite field can exhibit considerable diversity with respect to the composition, mineralogy and diamond content of individual kimberlites. Economically viable diamondiferous and barren kimberlites can occur in close proximity. Controls on the differences in diamond content between kimberlites are possibly due to the depths of origin of the kimberlite magmas (above or below the diamond stability field), to differences in the diamond content of the mantle sampled by the kimberlitic magma, or to the degree of resorption of diamonds during upward transport or cooling of the kimberlite.
DIAMONDS – HOW WE FIND THEM
- Where do geologists search for kimberlites?
- What methods do we use to find kimberlites?
- What is the course of action after a kimberlite is found?
Where do geologists search for kimberlites?
Primary diamond deposits of economic interest usually occur within the ancient stable Precambrian Shield areas (Cratons) of the major continents, especially where the continental crust is thick and is pushed down into the pressure/temperature domains where diamonds are formed and have been present for billions of years. Such brittle domains of hard continental crust can, during the rigors of Plate Tectonic activity, develop profound and deeply penetrating faults, which can penetrate down into the lower lithosphere and affect even the upper mantle regions. Such deep faults can thus sudden pressure drops and partial melting of otherwise crystalline solid or semi-solid rocks in the mantle. Thus kimberlite or lamproite magmas may be generated which rush up the faults and carry with them broken samples of the lower lithosphere and mantle rocks, plus indicator minerals and perhaps diamonds. Thus if we want to find diamonds we travel to areas underlain by thick Precambrian Shield rocks which are cut by major fault systems. These major fault systems are often expressed as major linear topographic features on satellite imagery referred to as “lineaments”. Within these domains we then start to search for KIMs in surficial sediments (soils, sands, gravels or glacial tills). If we then find that KIMs are present, we know that potentials exist for the discovery of diamonds. It is undeniable that the best indicator for potential diamond deposits are diamonds themselves!
What methods do we use to find kimberlites?
Geophysical surveys are normally the second phase of exploration in the search for diamond-bearing kimberlites, after indicator mineral (KIM) surveys and geological mapping. Such geophysical surveys are based upon the precept that a kimberlite or its weathered products might have detectable magnetic (Mag), electromagnetic (EM), density or light reflectance properties, which contrast dramatically with the surrounding host rocks. This is often true, but not always the case. Modern miniaturized electronics and computer hardware/software have enabled the design of relatively small and light magnetic- and electromagnetic-detectors, which may be mounted in light aircraft or in helicopters. In this way, the performance of rapid airborne Mag and EM surveys has become the preferred second-phase exploration method of searching for the potentially diamondiferous kimberlites. The airborne surveys are carried out by flying over the region along controlled survey lines spaced anywhere between 200 metres to 50 metres apart. Such closely spaced survey lines are logical when one realizes that many kimberlites pipes are less than 100 metres in diameter. Airborne Mag and EM surveys are also assisted by the relatively recent development of Global Positioning Systems (GPS) and Laser Altimeters. This has resulted in the ability to attain very accurate location of Mag and EM Geophysical Anomalies augmented by detailed high-resolution satellite images. The third phase of kimberlite exploration usually follows a thorough computerized analysis of the digital data from the airborne Mag and EM surveys. This is somewhat similar to the everyday computer treatment of digital photographs to enhance their contrast and sharpness. Thus the geophysical crew spends a considerable effort in revealing and enhancing relatively subtle, but often significant, geophysical anomalies. An alternative second or fourth phase of exploration may involve ground-based Mag or EM surveys. These ground-based permit a greater accuracy to be placed upon the mapping of the geophysical anomalies and thus a more confident location of the sites for the next phase of exploration, namely auger-drilling, diamond-drilling or surface-sampling or pit/trench-sampling of the potential kimberlite. At this stage the data of the first phase indicator mineral surveys are used to focus more upon those geophysical anomalies that hold higher potentials for the discovery of diamonds.
WHAT HAPPENS WHEN WE FIND A KIMBERLITE?
What is the course of action after a kimberlite is found?
When a kimberlite is discovered samples are taken either from drill core or from pit- or outcrop-samples. These may then be submitted for a number of scientific tests, such as petrographic analysis (microscopic analysis of thin sections) and caustic dissolution (destruction of the silicate rock matrix to free any caustic resistant components such as diamonds). With luck a caustic dissolution process will produce some diamonds, but in most cases these are of miniscule size, well below 0.5 mm. In much rarer instances the recovered diamonds are macroscopic. In very rare instances the diamonds may be of sizes in the carat range (1 carat = 0.2 grams). In extremely rare instances the geologist may be lucky to see and sample diamonds on surface or in the drill core.
Following the initial assessment stages of exploration and discovery of diamonds in a kimberlite there is usually a protracted period of pitting, drilling and bulk sampling that ensues. The samples are then put through a plant which mechanically separates and concentrates the diamonds from a measured volume or tonnage of the excavated soil, saprolite or rock. This plant usually relies on the separation of a dense mineral concentrate that will carry minerals like diamond, garnet, ilmenite, chromite etc. This heavy mineral concentrate can then be further reduced (to concentrate diamonds) via the use of various machines such as a magnetic separator, an X-ray “Sortex” machine or a grease-table. These processes have to be extremely efficient to recover of all sizes of diamonds.
The bulk sampling and diamond separation process facilitates the accurate economic viability evaluation of the deposit. Unlike exploration for gold and base metals, where simple chemical assays permit an easy and rapid assessment of the economic viability of the metalliferous deposits, the feasibility of mining diamonds really depends upon the question of an ability to market the final manufactured products - the gem quality diamond and the lower-quality industrial diamonds. Thus, in order to assess the economic feasibility of mining a diamond deposit we have to consider not only the amount of diamonds present, but also their sizes, their shapes, their colours and their transparency (clarity) and their market values. All of these factors, together with the dimensions, tonnage and geographical/political location of the deposit have to be combined in a complex matrix which forms the foundations of a Scoping Study, a Pre-Feasibility Report and the Feasibility Report - the final judgment of whether or not it will be profitable to mine that kimberlite under current regulatory, fiscal, economic and political climates.