Ore deposit models

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Overview of Ore Deposit Models

An ore deposit is a geological anomaly in which metals have been concentrated in a higher abundance than that of the neighboring parent rock. Ore is a classification of rock that describes an economically viable block, from which metal or minerals can be extracted profitably.

An ore deposit model is an extensive organization of information required to adequately describe the state of a mineral deposit that has been transitioned into an economically mineable category. Different from a mineral model that has shown the presence of a mineral concentrated from its country host rock, an ore deposit model must include information that can assist project developers in determining the economic status of a mineral deposit. It is imperative to identify and understand the fields of interest required to model the economics of a project. The nuance in the field of a mineral deposit’s transition to an orebody involve the economics for the development of a project. This process occurs through the gathering of information while a deposit is explored. The relationship between reserves and resources is depicted in Figure 1 showing how as more information is acquired on a mineral deposit it can transition from a resource classification to a reserve classification and simultaneously an ore deposit.

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Figure 1: Relationship between Mineral Reserves and Mineral Resources [1]

An economic analysis requires the development of an extraction plan, which is dependent on; geography, hydrogeology, mining technology, regional policy, tax, global metal markets, land restrictions and other considerations that may affect the mineral economics of the project directly or impact the viability of the project in the region of interest.

Types of Ore Deposit Models

Types of Ore Deposit Models

A descriptive model is a refined, site specific classification method for ore bodies. They identify the volume of ground needed to describe the extent of pertinent geological phenomena. There is further subdivision of the model into spatial sub-regions that allow for geological variability to be shown at manageable scale to provide project developers with a better understanding for the orebody under study. After the geometric boundaries of the deposit model have been defined the phenomena of interest for the site must be evaluated, including; geology, hydrogeology, hydrochemistry, geotechnical, transport properties, thermal properties and biosphere. This gives context to formation and a potential development platform for economic analysis of the mineral body under consideration. The purpose of a descriptive model is to give site specific insight for further description of a particular site’s geology. In particular the information gathered throughout the process will be foundational in a progressive project.

Genetic Models

The differentiation of deposit type by genetic origin is a method of empirically relating geological bodies by the general phenomena that governed their formation. The genetic classification of deposit models is categorical dependent on mechanism of its formation, which is subdivided into four major classifications shown in Figure 2.

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Figure 2: Geological Body Genetics

Formations that involve magma flow are classified within the igneous classification and sub-classed based on the environment that the magma flows mineralized. For deposits that formed gradually, through the accumulation of particles or layers, they are classified within the sedimentary. This includes limestone formations which is a marine formation occurring through the gradual accumulation of shelled animals in seas, which contain calcium carbonate in their shells. Deposits that involve the combination of two or more different formation type are classified as metamorphic. Finally, deposits that occur due to phenomena near the surface of the earth are categorized as surficial. These deposit types are caused by meteorological forces such as erosion and oxidation. Deposition deposits include placer gold deposits, and a residual branch of deposits including iron ring formations caused by the oxidation of iron by atmospheric oxygen.

Grade-Tonnage Models

Grade-Tonnage deposit models become necessary once enough descriptive models have been identified for a particular morphology. They give an indication to the expected distribution of grade and ore tonnage for a particular deposit type. As seen in Figure 3 the majority of ore bodies in Canadian Archean strata are concentrated around the 10M tonnes of ore, with a grade marginally below 10 grams per tonne.

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Figure 3: Zinc grades and tonnage by deposit type. Median grade and tonnage of each type located at center of ellipse [2]

As a tool, a grade tonnage model is thought to assist the mining industry during the exploration planning stage of a mines development by providing expected ranges and variability in tonnage and grades for commodities based on deposit types. In addition to assisting the mining industry directly, grade tonnage modelling has use in evaluation of mineral deposits from a research perspective as it provides relationships that provide data for further research [3].


Classification Systems

There are several systems of ore deposit model classification, but two primary systems are used today by geoscientist and mining engineers in the industry. The first is a classification system that is thoroughly based on geologic setting. The second system is based on the categorization of the different temperatures and pressures (hydraulics) of ore formation. Both systems overlap each other in many of the same geological characteristics considered during classification [4].Deposit types are sometimes classified into their respective commodities groups. The most frequent commodity groups are: energy (natural gas, petrol), metals (steel, copper, zinc), industrial minerals (aggregates, granulars) [5].

By Lithological-Tectonic Environment

The US Geological Survey published Mineral Deposit Models [6] in 1986 with the aim of providing an organized compilation set of mineral deposit models that could be used and deemed effective for mineral resource assessment, exploration, and valuation. Their classification system is primarily based on the lithologic-tectonic environment of the deposit – that is, the host rock lithology and tectonic setting. Both descriptive and genetic deposit model types are included in the system. The system primarily constitutes of metallic deposits formed under magmatic processes. Each deposit model in the classification system has a 1-page summary of geological characteristics in regard to their lithological and tectonic characteristics. Figure 4 shows an example of a descriptive model of Fe seam deposit models.

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Figure 4: Descriptive model of Fe seam deposits - example of 1 page deposit description [3]

The most commonly found ore deposit models classified in this system include porphyry Cu and hot-spring Au-Ag. As previously mentioned, Figure 2 shows a high level outline of the US Geological Survey classification system.

By Inferred Pressures and Temperatures of Ore Formation

Variations of the Lindgren mineral deposit model classification system are popularly used in North America by academic institutions. Their classification system is primarily based on the inferred pressure and temperature conditions of mineral formation. Both descriptive and genetic deposit model types are included in the system. The system includes all mineral types, not only those formed in magmatic processes. The first level of classification is into the chemical and mechanical processes of mineral concentration. Further levels break down into temperature and pressure conditions. Figure 5 shows some of the deposit type classifications with their respective formation conditions [7].

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Figure 5: Some of the categories in Lindgren's classification system

Relevance in the Mining Industry

An understanding of ore deposit models is very important in deposit valuation and evaluating the viability of a mining project. The ability to identify specific deposit models allows geologists and engineers to more accurately estimate where significant concentration of ore occurs. Understanding ore deposit model types provides a foundation for ore deposit exploration and consistent assessment. With an adequate understanding of ore deposit models, a mining operation increases its certainty of discovering ore occurrences. Insufficient understanding of deposit models can result in wasting resources in mineral exploration.

Characteristics of Ore Deposit Types

In this section of the article, major ore deposit types are summarized based on their formation as well as their mineralogical, geological, and geotechnical characteristics. Additionally, relevant mineral camps and mines are identified in each case.

Volcanic Massive Sulfides (VMS)

Volcanic massive sulfides are composed of primarily iron sulfides and varying proportions of lead, copper, and zinc sulfides [8]. The deposits are normally stratiform within volcanic rocks, laminated, large in size, and formed in the Early Proterozoic era. In common theme with many ore deposit categories, defining VMS deposits is onerous due to the number of deposits which marginally depart from one another. For instance, the lead-zinc rich MacArthur River deposit is known to be VMS; however, the deposit occurs in sedimentary successions with relatively minor proportions of volcanic rocks. VMS deposits are base metal-rich; however, they also contain lesser amounts of precious metals. The deposits form in lens-like or tabular bodies parallel to bedding or stratigraphy on, or below, the ocean floor [9]. The sulfides are exhaled into the ocean as brines from white or black smokers/chimneys. Recently, five classification schemes have been proposed based on setting and host rock association [10]:

1. Bimodal-mafic: hosted by volcanic sequences with greater abundances of mafic than felsic volcanics. Mineralization is associated with felsic strata. Examples of bimodal-mafic VMS deposits include Flin Flon and Kidd Creek camps.

2. Mafic Associated: dominated by mafic volcanic rocks. A prominent example of this type of deposit are those found in the Newfoundland Appalachians.

3. Mafic-siliciclastic: hosted within sequences of siliciclastic and mafic volcanic rocks. Often mafic and ultra-mafic intrusive rocks are associated and felsic rocks can be a minor component. An example of this type of deposit is found at Windy-Craggy in British Columbia which was considered “world-class”.

4. Felsic-siliciclastic: hosted within siliciclastic sediment-dominant settings with an abundance of felsic volcanics and some mafics. An example of this case of deposit is the Bathurst camp in New Brunswick

5. Bimodal-felsic: hosted within bimodal volcanic sequences with higher felsic rock content than mafic and minor sedimentation. An example includes the Buchans deposits in Newfoundland.

Mafic Igneous Intrusions and Complexes

Mafic igneous intrusions are often very large in size and lens shaped. This is due to the high initial heat in which they are formed and slow cooling at depth [11]. Through this process, alternating layers of course-grained mafic and ultra-mafic rocks are formed. The majority of nickel platinum group elements and chrome are hosted in mafic igneous complexes. These metals are crystallized out of magma impulses which results in layered concentrations at high grades. The Bushveld complex in South Africa is the most notable, spanning 400km by 800km with a volume greater than 1,000,000km3. Figure 6 illustrates the notable layering associated with many mafic igneous complexes.

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Figure 6: Mafic layering at the bushveld complex [12]

Additionally, copper, nickel massive sulfides are particularly relevant in the mafic intrusion models. They are formed when copper and nickel rich magma assimilates sulfur-rich host rocks. This results in a melt with high density that collects and at the bottom of magma chambers. Approximately 36% of nickel production is associated with these types of deposits. Notable examples include Voisey’s Bay in northern Labrador and the Sudbury deposit in Ontario.

Porphyry

In comparison to the previous two orebody types, porphyries develop at much shallower depths. They form around magma chambers that feed volcanoes [13]. The process involves high pressured steam rupturing intrusions and condensating as a metal-rich brine which makes its way outwards. Metals precipitate out of the solution as the brine cools and moves away from the heat source. Figure 7 illustrates the forming process of a copper porphyry.

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Figure 7: Schematic of copper porphyry formation

Due to this process, porphyry deposits are highly disseminated or occur in small veinlets within a large mass of hydrothermally altered igneous rock. Furthermore, porphyries are often massive in size and contain low grades of copper, molybdenum, and gold. Therefore, with reference to their depth and grade, they are almost always mined out of open pits. Porphyries account for over half of the world’s copper, over 90% of the world’s molybdenum, and almost a quarter of the world’s gold [14]. Most porphyry deposits are located on the west coast of North and South America. Notable deposits include Highland Valley Copper in British Columbia, Morenci in Arizona, and Chuquicamata in Chile.

Sedimentary Exhalative (SedEx)

SedEx deposits are formed in a variety of manners; however, they are generally characterized by ore bearing fluids being discharged onto the seafloor where they react with water and precipitate out [15]. Concentrations often occur in depressed areas of the ocean floor or in the faults and feeder systems. Therefore, they are very similar to VMS deposits, with the main differences being the formation of SedEx deposits occurring in the continental crust with host rocks that are usually shales. Furthermore, SedEx deposits are quite scarce; however, they are the largest source of the world’s zinc and lead. Additionally, the deposits are often high grade and contain variable amounts of copper, gold, and silver by-products. Teck Resources’ Red Dog mine is an example of a high grade SedEx deposit that is currently in operation producing a significant amount of zinc concentrate.

Kimberlite

Kimberlite deposits most commonly form pipe shaped intrusives with small diameter but they are also known to form dykes or beds of volcanic plastics surrounding the intruded pipes [16]. Kimberlite is composed primarily of olivine which is a form of peridotite and a chrome rich granite. Kimberlite in the earth’s crust is hard and a dark green/blue colour; however, upon oxidization, kimberlites become soft and change to a yellowish-brown colour. The kimberlite originates in or below the mantle and diamonds are deposited in the melt as the fluid moves towards surface. Upon reaching the surface, a volcanic eruption occurs and material is washed away by rain or washed back into the crater. All diamonds, including those found in placid deposits, have originated in kimberlite pipes or dykes. Furthermore, diamonds are formed under continental plates where specific temperature and pressure conditions exist. Diamonds are generally found under old Archean crusts due to their old age of formation (approximately 1-3.3 billion years old) [16]. Although much of the world’s diamonds are recovered from kimberlite pipes, only 1% of kimberlite pipes host economic diamond deposits. In terms of significant kimberlite mineral camps, 80% of the world’s diamonds are produced from Sub-Sahara Africa and Russia. The Jubilee Diamond Mine located in Sakha, Russia is the largest diamond mine in the world with estimated reserves of 153 million carats [17].

Mississippi Valley Type (MVT)

MVT deposits are hydrothermal and characterized by lead-zinc mineralization. They form at low temperatures and after emplacement within dolstone or limestone strata of sedimentary basins (epigenetic) [18]. Additionally, the deposits are precipitated from saline brines with the presence of barite or fluorite gangue mineralization. The name is derived from the Mississippi River drainage basin as this is where the mineralization is largest and first studied; however, MVT deposits can be found all over the world. Furthermore, the deposits are composed primarily of sphalerite and galena ores which host zinc and lead respectively. They are often formed in clusters over multiple precipitation and dissolution events. The hydrothermal systems are believed to have traveled hundreds of kilometers in which some debate that tectonic events were the driving force exists. Similar to VMS deposits, MVT deposits are diverse and have been grouped by some scientist into subcategories based on their ratio of lead to zinc, gangue materials present, and the style of emplacement. Examples of significant MVT deposits include the Pine Pont District in the Northwest Territories, the Cornwallis district in the Yukon, and the Upper Mississippi Valley district which spans over Iowa, Wisconsin, and Illinois.

Placer

Placers are known as secondary deposits as they are a result of minerals being freed from their original matrix by weathering process and redepositing [19]. Often they are washed downslope by gravity and concentrated in streams, waterbeds, and residual gravels. Therefore, the minerals which form placer deposits are resistant to weathering and have high specific gravities. Examples include gold, platinum, magnetite, cassiterite, ilmenite, chromite, rutile, native copper, and various other gemstones. Furthermore, placers can be subcategorized as stream placers/alluvial placers, eluvial placers, beach placers, and eolian placers; of which stream placers are most prominent. The deposits are often on surface or near surface and hosted in sedimentary environments. Thus, they have contributed to the majority of early gold and diamond rushes due to their ability to be mined using primitive methods. The aforementioned types of placer deposits are summarized below with relevant examples:

1. Stream Placers: Formed by swiftly flowing water in streams. The Klondike gold rush and the diamond placers of the Congo are examples of historic stream placers which have had significant impacts on mining.

2. Beach Placers: Beach placers form on seafloors where currents and wave action shift the materials depositing the heaviest at the bottom to lightest at the top. Examples of beach placers include the magnetite sands of Oregon and the Nome gold deposits in Alaska.

3. Eluvial Placers: Eluvial placers are often an earlier stage of stream placers as they are formed by wind and rain carrying away lighter materials on hillslopes. Examples include the cassiterite placers of Malaysia and gold deposits in Otago, New Zealand

4. Eolian Placers: Eolian placers deposit where winds are the only concentrating agent and act by carrying away lighter particles. Therefore, they are often found in arid climates. One example, is the gold deposits in the deserts of Australia.

Mesothermal/Greenstone

Mesothermal deposits form from the rise of gold rich melts from the earth’s crust depositing at depths ranging from 1km to 10km in fissures, fractures, and veins [20]. The temperature at these depths typically ranges between 4500C and 2500C. Greenstone deposits were formed in the late Archean era to the early Proterozoic era. Further, they typically host high grade gold mineralization ranging from six to ten grams per tonne. The deposits often display multiple shears which host mineralized veins as depicted in Figure 8.

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Figure 8: Example of mesothermal/greenstone deposition [21]

Examples of greenstone deposits include Red Lake and Hemlo located in Ontario. Both of these mines have sustained years of production as greenstone deposits are often quite large. Overall, the vast majority of gold mined in Canada is produced from greenstone belt deposits.

Limitations of Using Ore Deposit Models

There are inherent difficulties faced when considering classification systems for mineral or ore deposit models. This is primarily due to the large number of variables and geological characteristics to be considered for categorization, including lithological, chemical, and tectonic setting and/or formation and the resulting infrequency of groups of deposit models to perfectly fit inside specific categories. If deposits were categorized as detailed as possible, there would be a category for almost every ore deposit. Furthermore classification mechanisms may change over time due to newly obtained information of ore deposit types from a different site, or different deposit of the same ore type [6]. Many descriptive ore models are incomplete with information (not a consistent amount of information provided). Also, it is noted that genetic models are speculative in nature and are not constructed as a precise science [6]. The difficulties mentioned regarding the use of ore deposit models for the purposes of deposit valuation indicate that while a significant understanding of mineral deposit model is useful, estimations should always be checked using other means and not be the sole basis for a valuation. There are also human faults that lead to bias in use of ore deposit models. Typically, situations arise where professionals tend to use more simple models to increase a sense of certainty and decrease confusion in valuation results [22].

Case Study - Kimberlite Deposits

Kimberlite-hosted diamond ore deposits are those kimberlite pipes, or diatremes, which are diamond bearing, or diamondiferous. The minerals etymology relates to its place of discovery in the late 19th century, in Kimberly, South Africa, where the first commercial extraction of the gems occurred. The products of these orebodies are industrial and gem-quality diamonds, whose type is dictated by the color, purity, and size of the diamond. Industrial diamonds are primarily used as a friction material in processes involving crushing, grinding, cutting, etc. One notable example is their use in the construction of diamond drill bits, and associated exploration drilling programs. The property of value in these industrial uses is the hardness of the material. Diamonds are one of the hardest known minerals or materials, and as such, are used as the highest reference in the well-known Mohs’ scale of mineral hardness, at a value of 10. Gem-quality diamonds are used in jewellery, and their demand is associated with their brilliance and lustre, in addition to effective marketing campaigns by diamond producers. Diamondiferous pipes are rare, and it is estimated that only 1% of all 5000 known kimberlite pipes globally are economic [23].

Formation

Diamond-bearing kimberlite deposits are created through two distinct processes: 1. The formation of the diamonds within the earth’s mantle, and 2. The intrusion of the kimberlite igneous rock through the earth’s crust. This explains the relative rarity of diamond bearing kimberlite pipes, as the environments both for diamond formation and magmatic intrusion must coincide spatially. The formation of diamonds occurs predominantly within a thick section of the lithosphere that coincides with the innermost layer of the earth’s crust and the outermost layer of the earth’s mantle. This environment offered the temperature and pressure conditions necessary to form diamonds, and the lack of convection in this zone allowed the diamonds to reside undisturbed until transportation to through the crust during a magmatic intrusion. The diamonds being mined from kimberlite ore today were formed between 1.0 – 3.5 billion years ago and as such, their deposits are only found within continental plates whose age aligns with the formation period of the diamonds. These are known as Archean cratons, and are the home of all economic kimberlite deposits [23].

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Figure 9: Block model of typical kimberlite deposit formation [24]

The formation of kimberlite pipes is the result of volcanic activity whereby rock melt from a layer of the earth’s mantle known as the asthenosphere is violently released through the crust and up to the surface. The energy levels associated with these eruptions are known to be high, due to the fact that kimberlite pipes have been found in very strong rock, such as granite [25]. Through the melting of water and carbonate-rich rock from the crust, and the subsequent formation of CO2 gas, pressure builds deep within the mantle until being released through dykes and fissures. The decreasing pressure near surface results in the expansion of some constituents of the magma and ultimately leads to the conical shape of the kimberlite pipe, while the associated gases cause cratering through explosive release and expansion [26]. Known kimberlite pipes have been determined to range in age from 52 million to 1.2 billion years old [23], postdating the age of diamond formation by at least 1 billion years. In addition to the primary kimberlite deposit, secondary placer deposits can also exist. These are the result of rapid weathering of the exposed kimberlite diatreme. The diamonds can be transported great distances and may end up being deposited in drainage systems or the marine environment, and at one point, these alluvial deposits constituted 80% of global diamond production [27].

Geology & Mineralogy

As described in the formation section, kimberlite pipes are typically found within Archean cratons. These describe the crust type and age that would contain the kimberlite deposit. As these have varying compositions across the globe, no definitive descriptor of host rock characteristics can be provided. The kimberlite itself, does show an archetypal composition. This intrusive igneous rock typically has a porphyritic texture, whereby larger crystals are suspended within a mass of much smaller crystals. In addition, due to the nature of formation, this breccia typically contains inclusions from the existing geological structures in the mantle and crust. These intrusions are considered xenoliths, as their formation is completely separate to the kimberlite formation. Through the processes of explosion that results from the kimberlite pipe reaching the surface, fine airborne matter called tuffs, or rock ash, are produced. These are typically found on or around the crater area generated through the explosion. The kimberlite ore itself, is composed primarily of the silicate minerals olivine, phlogopite, pyroxene, and garnet, in addition to potentially being home to ilmenite, chromite, perovskite, and apatite [25]. The valuable minerals within the kimberlite ore deposit are diamond and non-gem diamond subtypes such as bort, carbonado, ballas, and amorphous carbonado. These are irregularly distributed and sparsely disseminated within the greater kimberlite breccia pipe and crater structures [28]. These diamond minerals, which were brought from their formation location to the kimberlite orebody through geological transport processes, are considered xenocrysts, as they were produced separately from the kimberlite ore [25]. A closely related mineral to kimberlite is lamproite. Lamproite can be diamondiferous, however the only known economic diamond mine from this mineral is the Argyle Mine, owned by Rio Tinto and situated in Western Australia. This lamproite deposit produces diamonds that are of much lower value than kimberlite diamonds, due to their smaller average size, however the high grade within the orebody makes up for this shortcoming.

Mine Design

A number of kimberlite deposit characteristics have a significant influence on the design and operation of a diamond mine. Some major considerations have been determined by Jakubec in 2008, and have been provided in the figure below:

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Figure 10: Jakubec's Kimberlite Diamond Mine Design Process [29]

Additional detail has been provided to further explain the influence of these factors on mine design parameters:

1. Kimberlite orebody size and geometry

o Open pit, underground, or a combination of both

o Bulk or selective mining methods underground mining method

o Open pit stripping ratio depending on thickness and pipe shape (conical or cylindrical)

2. Kimberlite rocks competency

o Underground ground support requirements based on particular UCS

o Associated weathering characteristics and strength deterioration

3. Kimberlite treatability

o Processing plant design

o Clay content of ore

4. Contact zone characteristics:

o Geotechnical zones of weakness for open pit and underground mines

o Water conductivity relative to country rock and kimberlite pipe

o Mixing of ore and waste and impacts to dilution and economics

5. Diamond grade and quality

o Overall economic value of deposit from average carat size, colour, purity, etc.

o Diamond recovery as function of average carat size

6. Hydrogeological characteristics

o Dewatering capacity requirements from intrusive alteration structures

An additional consideration for mine design, is the geochemistry of both the kimberlite ore and country rock. Due to the rapid nature of pipe formation, it is rare for alteration to occur within the country rock and as such, it is unlikely that sulphide minerals would exist that would necessitate significantly more complex and expensive effluent management. In addition, the kimberlite ore itself has a relatively low sulphide content, and as such, neither the tailings nor waste rock from typical diamond mining operations are particularly concerning from an environmental management perspective [30].

Exploration

Due to the lack of obvious surface features at a kimberlite orebody, indicator minerals present in placer deposition are typically the best source for primary exploration activities. These minerals, including pyrope garnet, ilmenite, and clinopyroxene, are resilient to alteration from surface conditions [31], and can therefore be considered a reliable indication of potential kimberlite deposits. Once these indicators have been found, additional work should be undertaken to locate the source deposit(s), as well as determine whether the kimberlite is diamondiferous through additional placer sampling. If a major pipe is discovered, work can then begin on drilling the orebody to understand its economic potential. An important shortcut to exploration exists in the fact that kimberlite deposits typically exist in clusters [32]. This means that deposits are likely to be found near one another, and if one is found in a novel geography, others many exist nearby. This clustering has led to the formation of significant diamond camps, two of which are described in further detail later in this article.

Mineral Camps

Economic kimberlite diamond deposits are only found in a select number of continental cratons with sufficient age and thickness to support and align with the age of formation of both diamonds and kimberlite pipes. Two notable examples are the Slave Craton in North America, and the Kaapvaal Craton in Southern Africa.

Slave Craton

The Slave Craton is located in the Northwest Territories of Canada composed of granite and greenstone bedrock [32]. Notable diamond mines in this region are Rio Tinto’s Diavik, Dominion Diamond’s Ekati, and De Beer’s Gahcho Kué. These mines contribute significantly to Canada’s position as the 3rd largest diamond producer (by carats) globally in 2017 [33].

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Figure 11: Diavik diamond mine, courtesy of Rio Tinto [34]

Kaapvaal Craton

The Kaapvaal Craton is located in Southern Africa, and spans across South Africa, Botswana, and Zimbabwe [35] . Notable diamond mines in this region are Debswana’s Jwaneng and Orapa in Botswana, and De Beer’s historical Kimberly Mine in South Africa. The currently producing Debswana mines contribute significantly to Botswana’s position as the 2nd largest diamond producer (by carats) globally in 2017 [33].

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Figure 12: Jwaneng diamond mine, courtesy of Debswana [36]













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