is a method of scientifically classifying diamonds by the level and type of their chemical impurities. Diamonds are separated into four types: Type Ia, Type Ib, Type IIa, and Type IIb. The impurities measured are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect.
Different diamond types react in different ways to diamond enhancement techniques. Different types can coexist within a single stone; natural diamonds are often mixes of Type Ia and Ib, which can be determined by their infrared absorption spectrum.
- ♦ Type I diamonds, the most common class, contain nitrogen atoms as their main impurity, commonly at a concentration of 0.1%. Type I diamonds absorb in both the infrared and ultraviolet region, from 320 nm. They also have a characteristic fluorescence and visible absorption spectrum.
- Type Ia diamonds make up about 98% of all natural diamonds. The nitrogen impurities, up to 0.3% (3000 ppm), are clustered within the carbon lattice, and are relatively widespread. The absorption spectrum of the nitrogen clusters can cause the diamond to absorb blue light, making it appear pale yellow or almost colorless. Most Ia diamonds are a mixture of IaA and IaB material; these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. Type Ia diamonds often show sharp absorption bands with the main band at 415.5 nm (N3) and weaker lines at 478 nm (N2), 465 nm, 452 nm, 435 nm, and 423 nm (the "Cape lines"), caused by the N2 and N3 nitrogen centers. They also show blue fluorescence to long-wave ultraviolet radiation due to the N3 nitrogen centers (the N3 centers do not impair visible color, but are always accompanied by the N2 centers which do). Brown, green, or yellow diamonds show a band in the green at 504 nm (H3 center), sometimes accompanied by two additional weak bands at 537 nm and 495 nm (H4 center, a large complex presumably involving 4 substitutional nitrogen atoms and 2 lattice vacancies).
- Type IaA, where the nitrogen atoms are in pairs; these do not affect the diamond's color.
- Type IaB, where the nitrogen atoms are in large even-numbered aggregates; these impart a yellow to brown tint.
- Type Ib make up about 0.1% of all natural diamonds. They contain up to 0.05% (500 ppm) of nitrogen, but the impurities are more diffuse, the atoms are dispersed throughout the crystal in isolated sites. Type Ib diamonds absorb green light in addition to blue, and have a more intense or darker yellow or brown colour than Type Ia diamonds. The stones have an intense yellow or occasionally brown tint; the rare canary diamonds belong to this type, which represents only 0.1% of known natural diamonds. The visible absorption spectrum is gradual, without sharp absorption bands. Most blue-gray diamonds coming from the Argyle mine of Australia are not of type IIb, but of Ia type; those diamonds contain large concentrations of defects and impurities (especially hydrogen and nitrogen) and the origin of their color is yet uncertain. Almost all HPHT synthetic diamonds are of Type Ib.
- ♦ Type II diamonds have no measurable nitrogen impurities. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225 nm, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum. The crystals as found tend to be large and irregular in shape. Type II diamonds were formed under extremely high pressure for longer time periods.
- Type IIa diamonds make up 1--2% of all natural diamonds (1.8% of gem diamonds). These diamonds are almost or entirely devoid of impurities, and consequently are usually colourless and have the highest thermal conductivity. They are very transparent in ultraviolet, down to 230 nm. Occasionally, while Type IIa diamonds are being extruded towards the surface of the Earth, the pressure and tension can cause structural anomalies arising through plastic deformation during the growth of the tetrahedral crystal structure, leading to imperfections. These imperfections can confer a yellow, brown, orange, pink, red, or purple colour to the gem. Many large famous diamonds, e.g. Cullinan and Koh-i-Noor, are Type IIa.
- Type IIb diamonds make up about 0.1% of all natural diamonds, making them one of the rarest natural diamonds and very valuable. In addition to having very low levels of nitrogen impurities comparable to Type IIa diamonds, Type IIb diamonds contain significant boron impurities. The absorption spectrum of boron causes these gems to absorb red, orange, and yellow light, lending Type IIb diamonds a light blue or grey color, though examples with low levels of boron impurities can also be colorless. These diamonds are also p-type semiconductors, unlike other diamond types, due to uncompensated electron holes; as little as 1 ppm of boron is enough for this effect. However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron. Type IIb diamonds show distinctive infrared absorption spectrum and show gradually increasing absorption towards the red side of visible spectrum.
- ♦ Type III diamonds refers to those diamonds that receive their colors by other means that are known, but not understood very well. A great example is green colored diamonds, who derive their color from exposure to ionizing radiation.
Botswana Diamond Projects
Tsodilo Resources Ltd. ("Tsodilo" or the "Company") was granted a prospecting license (PL369/2014) over the BK16 kimberlite pipe through its 100% owned Botswana subsidiary, Bosoto Pty (Ltd), in October 2014. The BK16 kimberlite pipe is located within the Orapa Kimberlite cluster in Botswana and is 37 kilometers ("km") east-southeast of the Orapa Diamond Mine AK01, 25 km southeast of the Damshtaa Diamond Mine, and 13 km north-northeast of the Letlhakane Diamond Mine, all operated by Debswana and 28 km East-northeast from Lucara Diamond Corporation's Karowe mine (F/K/A AK6).
BK16 is one of the known diamondiferous kimberlites of the Orapa Kimberlite Field ("OKF") in Botswana and will be evaluated for its economic potential by Bosoto. The OKF lies on the northern edge of the Central Kalahari Karoo Basin along which the Karoo Supergroup dips very gently to the SSW and off-laps against an irregular pre-Karoo topography of Precambrian rocks which outcrops within the Makgadikgadi Depression.
The OKF includes at least 83 kimberlite bodies, varying in size from insignificant dykes to the 110 hectare AK01 kimberlite pipe. The AK01 pipe has been dated at 93.1 Ma and it is presumed that all the kimberlite intrusions in the OKF are of similar and post-Karoo age. Of the 83 known kimberlite bodies, nine (9), AK01 (Orapa, Debswana); AK06 (Karowe, Lucara Diamond Corporation); BK01, BK09, BK12 and BK15 (Damtshaa, Debswana); DK01 and DK02 (Letlhakane, Debswana); BK11 (Firestone Diamonds), are currently being or have been mined.
The BK16 kimberlite was initially discovered by De Beers in the 1970's using soil sampling techniques, airborne magnetics, and ground magnetic surveys. This initial work was followed up by some initial drilling and the sinking of a shallow shaft to 36 meters in the central part of the pipe. Initial indications were that the kimberlite was diamondiferous albeit low grade and no further work was done by De Beers.
Over the period 1994 to 2010, several companies held the prospecting rights over the area containing the BK16 kimberlite and various forms of surveying and sampling were employed all in an attempt to ascertain whether BK16 was economically viable. However, none of those efforts systematically evaluated the kimberlite to answer the question as to BK16's merits. Tsodilo believes that much of the above described sampling was done in the upper part of the kimberlite which is characterized by a basalt breccia. Like several of the other Orapa kimberlites, this upper zone of basalt diluted kimberlite is of low grade but the underlying 'cleaner' kimberlite, as is the case at BK16, is known to be of higher grade.
The first work Bosoto undertook on BK16 was to complete 51 line kilometers of high resolution (closely spaced) ground magnetic survey and 441 survey stations of detailed gravity survey over the license. These two surveys are more detailed than any previous geophysical survey over the BK16 kimberlite, this detail was needed to more accurately estimate the size of the pipe. The magnetic data was surveyed on lines 20 meters apart and readings were taken every 5 seconds. The Gravity survey meter for the gravity survey took readings every 50 meters on lines 50 meters apart, these gravity stations were measured for elevation using a differential GPS. All data was processed and modelled using in-house Geosoft and PotentQ software.
The magnetic and gravity data had excellent overlaps showing the extent of the kimberlite pipe to be between 4.8 and 5 hectares in size with a high level of confidence due to the high precision nature of our surveys. These geophysical surveys also highlighted different zones within the kimberlite, which are most likely associated with different kimberlite facies.
Figure 1 shows the ground magnetic data and the gravity data. The gravity data suggests there is a possible extension of the main kimberlite pipe towards the southeast which is around 2.5 hectares in size. The magnetic results over this area are not conclusive and this area will have to be tested by drilling.
Figure 1. Left is the magnetic first vertical derivative ground magnetic data and the right is the Bouger gravity first vertical derivative data for the BK16 license area.
BK16 CONCEPTUAL GEOLOGICAL MODEL
Bosoto reviewed all of the available historical drill data including geological logs to create a conceptual geological model for BK16, see Figure 2. Also incorporated into this conceptual model was the Company's high resolution ground magnetic and gravity survey results.
The historical drill data used for this project includes 27 boreholes to a cumulative depth of 3,553.25 meters. Which were a combination of 12-inch reverse circulation drilling (5 holes, 641 meters), 6.5-inch percussion holes (19 holes, 2,290 meters), and HQ diamond drilling (3 holes, 622.25 meters).
Along with the main kimberlite pipe the geophysical model supports the presence of a smaller satellite pipe to the south-east of the main pipe where kimberlite has been intersected by two of the historical exploration drill holes. The main pipe and this second pipe have been interpreted to be linked by a north-west trending dyke based on the geophysics. This second intrusion has not penetrated the Karoo basalts and if therefor interpreted as a blind pipe, see Figure 3.
NEXT PHASE - EVALUATION DRILLING
The conceptual model above has been used as a platform to design the upcoming drill program. There have been 12 diamond drill core holes planned to a cumulative depth of 4,200 meters, these will be drilled using Tsodilo's drill rigs. The planned drill holes are shown on Figure 3. This drill plan will enable the company to establish a more robust geological model that will be used to site the planned 38 Large Diameter Drill (LDD - 24-inch) holes for bulk samplings to obtain the sample grades of the various kimberlite facies and variation thereof across the pipe. Once this has been established the next phase will be to site the next phase of drilling to extract sufficient diamond for valuation purposes.
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History of Diamond Exploration in Botswana
De Beers began prospecting for diamonds in the then Bechuanaland Protectorate in 1954. The legendary Gavin Lamont joined Kimberlitic Searches, the local De Beers Company in 1955, after working for the Botswana Geological Survey. His first exploration programme was in the Tuli Block along the Limpopo River.
Before De Beers began this work, no systematic diamond prospecting had been conducted in Botswana. Central African Selection Trust (CAST) was the only other company exploring for diamonds at that time and in 1959 they found three small diamonds (0.2, 0.14 and 0.02 carat) in gravels associated with the Motloutse River close to Foley Siding. CAST believed that these stones had been derived from the disintegration of the basal Karoo conglomerates and relinquished the ground. De Beers then applied for the Bamangwato tribal territory and began work there in 1962. They confirmed the CAST's results when they found a 0.50 carat stone and a 2.39 carat octahedron diamond as well as kimberlitic garnets along the same river.
South Africa's well known geologist Alex du Toit had published the idea of crustal warping in 1933. Lamont, who had known Du Toit from his University days, recognised the importance of this concept and applied this to the diamond finds along the Motloutse River in search for the primary sources. He suggested that the source of these diamonds could lie far to the west, having been transported from such a source by a palaeo-Motloutse River now decapitated by a one of Du Toit' crustal up warps.
At more or less the same time Lamont changed from using gold pans, as a means of concentrating soil samples, to gravitating screens. The process of using classifying screens to size sand grains from about 2 mm down to 0.5 mm before the gravitating process was much more effective that using gold pans on unclassified material. Sand grains from the Kalahari sediments generally range in size from about 2 mm down to 0.3 mm, and an experienced hand-gravitating operator can recover about 90% of the ilmenites and 80% of the garnets in the 0.5 to 2.0 mm size range.
In 1966, after eleven years of prospecting, De Beers found the first two 'kimberlites' near Mochudi in the Kgatleng District. Although not true kimberlites but rather lamprophyres (para-kimberlites) they contain ilmenite and it proved that the sampling technique was effective.
After this initial technical success Lamont targeted an area around Letlhakane south of the Makgadikgadi Pan where the Kalahari sand cover is fairly thin. A rapid reconnaissance scoop sampling programme was undertaken later in 1966, taking sampling along as many roads and tracks as possible. They covered the area in about three days and then returned to a small pan which had some water left from the summer rains to hand-gravitate these samples and found the first kimberlitic garnets and ilmenites of the Letlhakane-Orapa kimberlite province in practically all them.
Several north-south baselines and access roads were cut across map sheets 2125A and 2125B. Geologist Manfred Marx, fresh out of university, began a programme of reconnaissance soil sampling, working from east to west. By January 1967, he was finding abundant ilmenites in his reconnaissance soil samples, and on the 1st
March, he discovered the first true kimberlite in Botswana - 2125-B/K1 or BK1. This was followed on 17th
March by BK2 and then on 25th
April the Orapa pipe -- AK1. It was a massive intrusive body, which showed up as an oval feature of over 100 hectares on the aerial photographs. It was said that South African Airways pilots used it as navigation landmark on their flights to Europe (Fig. 1).
Figure 1. Aerial photo of the Orapa kimberlite cluster prior its discovery.
Letlhakane, or DK1, was discovered in June 1969, followed by AK6 (now Karowe), BK09, BK12, BK15 and part of BK01 (all four now part of the Damtchaa Mine) in 1970 and BK11 in 1974.
De Beers continued with a programme of reconnaissance soil-sampling using scoop sampling between baselines. By the end of 1969, De Beers reported the first definite and 'probable' kimberlitic ilmenites from reconnaissance soil samples taken over 150 square miles across the Kweneng\Ngwaketse boundary, and are considered to be the first indication of the Jwaneng kimberlite province.
The Orapa pipe outcropped on surface and the garnet and ilmenite anomalies there were clear and confined to the kimberlite pipes, while at Jwaneng with approximately 50 to 60 m of Kalahari cover the anomalies were much more diffuse and spread over much larger areas. Stuart Vercoe and Norman Lock were involved in follow-up sampling to try to define drill targets below the Kalahari sediments. Ground magnetics was not effective since the Jwaneng pipes were in general weakly magnetic. So with the use of a small Vole drill they intersected kimberlite and this culminated in the discovery of the Jwaneng kimberlite in 1972.
In the side wall of the mine it is possible to see small burrows in the weathered kimberlite and the overlying 50 metres of Kalahari sediments that were made by termites. These insects burrowed down to get water and moist clay with which they build their large termite mounds on surface and at the same time bringing up the small garnets and ilmenites from the kimberlite. It is these mineral grains, especially the more weather resistant ilmenite, that are picked-up in the loam samples which ultimately leads the geologists to the buried kimberlite pipes.
Other companies started to explore in Botswana and in 1977 Falconbridge used airborne geophysical technology over areas that had proven to be positive with regard to kimberlitic indicator minerals from previous De Beers sampling programs. They covered two such areas near Tshabong and Kukong and found many aeromagnetic targets to investigate. Under the guidance of Chris Jennings Falconbridge discovered some 40 kimberlites in the Tshabong area and 21 in the Kukong area. Based on those early technical successes Falconbridge, with their joint venture partner Superior Oil, started regional soil sampling programmes themselves. Based on the recovery of two ilmenites in the Central Kalahari Game Reserve, they flew an airborne magnetic survey and in 1981 found Go25 (Gope or Ghaghoo as it is now called). Construction of a mine over this kimberlite is presently in process by Gem Diamonds.
In July 1988, De Beers covered the Lerala area in east Botswana with reconnaissance loam sampling. After the initial positive results from the reconnaissance program, Grid Loam Sampling (DGL) and Detailed Grid Loam Sampling (DGLS) led to the discovery of two kimberlites clusters in 1991: one comprising three pipes (K1, K7 and K8) close to the Lotsane River, and the other five pipes (K2, K3, K4, K5 and K6) near the Susulela River. It is the latter group of kimberlites that proved diamondiferous and has been subjected to several trial mining operations.
The former cluster produced very few garnets, is diamond-poor but dominated by ilmenite. The diamondiferous kimberlites are garnet-bearing and ilmenite-poor. Interestingly the samples, which eventually led to the discovery of the two groups of kimberlites, were rich in ilmenite that came from the first cluster.
Since the discovery of the first kimberlite in Botswana in 1967, a total of 386 kimberlites have been found mainly associated with 12 kimberlite clusters (Fig. 2). Of these 16 kimberlites from four clusters have produced or are producing diamonds, and these are: Orapa (AK1, Karowe-AK6, Letlhakane-DK1 & DK2, BK11, Damtshaa-BK1, BK9, BK12 & BK15), Ghaghoo (Go-25), Jwaneng (DK2), and Lerala (K2, K3, K4, K5 and K6).
Figure 2. Distribution of diamond mines and other kimberlites in Botswana
Typically, these would produce around 28.8 million carats per annum of which 26 million would come from the two big Orapa AK1 and Jwaneng DK2 mines. The Letlhakane mine produces approximately 1 million carats per annum and the remaining 1.8 million carats from the other 12 kimberlites combined. This emphasises the importance of the two large producing kimberlites not only in Botswana but also globally.
Globally the first kimberlites to be mined were the Jagersfontein and Kimberley Mines in South Africa, in 1870 and 1871 respectively. The latter is probably better known as the Big Hole.
Between 1870 and 2012, the world has produced some 4,898 million carats, mostly from kimberlites. Botswana, which has only been producing diamonds since 1971, one hundred years after the Big Hole, has provided 665 million carats of this global figure which represents 14% of all diamonds ever produced, and this is almost the same as what South Africa has produced (680 million carats) since 1870. However, the Botswana figure will increase significantly in the future as many of the other global producers are reaching the end of their lives and no new major discoveries have been made in the last two decades.
Finally, since the economic downturn most producers have reduced their output and in 2011 the world generated 124 million carats valued at 14.4 billion US$. Botswana was responsible for 23 million of those carats (18.5 %) which were valued at 3.9 billion US$ or 27% of the total value.
In 2011, De Beers announced that it would shift its rough diamond trading operation from London to Gaborone. The move which was completed in 2013 is expected to bring an extra $6 billion of diamond sales into the country establishing Botswana as one of the world's key diamond centers.