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1 INTRODUCTION Diamond is an extraordinary mineral with extreme hardness and inherent beauty that is sought for personal adornment and industrial use. Because the genesis of this unique mineral requires extreme temperature and pressure, natural diamond is rare—so rare that some diamonds are the most valuable commodity on earth, based on weight. Diamonds are mined on several continents. The value of the raw production has resulted in a multi-billion-dollar industry. Natural diamond production annually averages more than 110 mil- lion carats, valued at more than $7 billion for the raw stones. Dia- mond values dramatically increase following the fashioning of the stones, and the value again dramatically increases with their dress- ing in jewelry, such that diamond jewelry typically sells for 10 to 100 times the value of the raw stone. Industrial diamonds, which are of considerably lower value, include synthetic industrial dia- monds. Synthetic industrial diamond production has an average annual value of about $1 billion. MINERALOGY Diamond consists simply of carbon. In nature, native carbon may occur as one of the following polymorphs: diamond, graphite, or lonsdaleite (Erlich and Hausel 2002). The physical differences among these polymorphs reflect the different bonds between the carbon atoms in the crystal structure. In diamond, the coordination of the carbon atoms is tetrahedral with each atom held to four others by strong covalent bonds, resulting in a mineral with extreme hardness. In contrast, graphite has six-member hexagonal carbon rings that resonate between single- and double-shared electron bonds. These graphite sheets are very strong, but the hexagonal rings are stacked and do not share electrons between adjacent sheets, only a residual electrical charge—thus, no chemical bonds occur between the sheets, resulting in graphite being soft and the sheets easily separated. The hexagonal modification of diamond, known as lonsdale- ite, has a closer-packed arrangement of atoms than diamond or graphite, resulting in a rare mineral of extreme hardness (Lonsdale 1971). Lonsdaleite was initially synthesized at temperatures greater than 1,000°C (1,830° F) under static pressures exceeding 130 kbar (Bundy and Kasper 1967). DuPont deNemours and Co. obtained the same transformation by intense shock compression and thermal quenching. Lonsdaleite has since been identified in meteorites and in rare unconventional host rocks, the most notable being the Popi- gay Depression in Siberia (Erlich and Hausel 2002). The extreme hardness of lonsdaleite makes it ideal for industrial grinding, but its rarity makes it unattractive for commercial use. Crystal Habit Diamonds are isometric with cubic, octahedral, hexoctohedral, dodecahedral, trisoctahedral, and related habits. Twinning along the octahedral {111} plane is common, and many crystals are often flattened parallel to this plane, producing a habit that appears as flattened, triangular-shaped diamond known as a macle. Cube Cubes are a relatively uncommon habit for diamond, and when found are primarily frosted industrial stones. Many have been found in placers in Brazil, and a significant percentage of diamonds in the Snap Lake kimberlites of Canada have cubic habit (Pokhilenko et al. 2003). Crystal faces of a cube often exhibit square-shaped pyramidal depressions rotated 45° diagonally to the edge of the crystal face. The cube may also include scattered trigons mixed with pyramidal and other depressions of hexagonal morphology visible with a microscope. Octahedron The octahedron is an eight-sided crystal that has the appearance of 2 four-sided pyramids attached at a common base. Each pyramid con- tains four equilateral triangles known as octahedral faces. In nature, an octahedral face will often have positive or negative trigons—small equilateral triangles that are visible under a microscope. These are growths or etches on the crystal surface that represent disequilib- rium during transport to the earth’s surface from the initial stable conditions at depth within the mantle. Partial resorption of the octahedron will result in different crystal habits, including a rounded dodecahedron (12-sided) with rhombic faces. Further resorption may result in ridges on the rhom- bic faces, yielding a 24-sided crystal known as a trishexahedron. Many diamonds from Argyle, Australia; Murfreesburo, Arkansas; and the Colorado–Wyoming State Line District exhibit resorbed crystal habits. Four-sided tetrahedral diamonds are sometimes encountered that are distorted octahedrons (Orlov 1977; Bruton 1979). Diamonds W. Dan Hausel

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INTRODUCTIONDiamond is an extraordinary mineral with extreme hardness andinherent beauty that is sought for personal adornment and industrialuse. Because the genesis of this unique mineral requires extremetemperature and pressure, natural diamond is rare—so rare thatsome diamonds are the most valuable commodity on earth, basedon weight.

Diamonds are mined on several continents. The value of theraw production has resulted in a multi-billion-dollar industry.Natural diamond production annually averages more than 110 mil-lion carats, valued at more than $7 billion for the raw stones. Dia-mond values dramatically increase following the fashioning of thestones, and the value again dramatically increases with their dress-ing in jewelry, such that diamond jewelry typically sells for 10 to100 times the value of the raw stone. Industrial diamonds, whichare of considerably lower value, include synthetic industrial dia-monds. Synthetic industrial diamond production has an averageannual value of about $1 billion.

MINERALOGYDiamond consists simply of carbon. In nature, native carbon mayoccur as one of the following polymorphs: diamond, graphite, orlonsdaleite (Erlich and Hausel 2002). The physical differencesamong these polymorphs reflect the different bonds between thecarbon atoms in the crystal structure. In diamond, the coordinationof the carbon atoms is tetrahedral with each atom held to fourothers by strong covalent bonds, resulting in a mineral with extremehardness.

In contrast, graphite has six-member hexagonal carbon ringsthat resonate between single- and double-shared electron bonds.These graphite sheets are very strong, but the hexagonal rings arestacked and do not share electrons between adjacent sheets, only aresidual electrical charge—thus, no chemical bonds occur betweenthe sheets, resulting in graphite being soft and the sheets easilyseparated.

The hexagonal modification of diamond, known as lonsdale-ite, has a closer-packed arrangement of atoms than diamond orgraphite, resulting in a rare mineral of extreme hardness (Lonsdale1971). Lonsdaleite was initially synthesized at temperatures greaterthan 1,000°C (1,830° F) under static pressures exceeding 130 kbar(Bundy and Kasper 1967). DuPont deNemours and Co. obtainedthe same transformation by intense shock compression and thermal

quenching. Lonsdaleite has since been identified in meteorites andin rare unconventional host rocks, the most notable being the Popi-gay Depression in Siberia (Erlich and Hausel 2002). The extremehardness of lonsdaleite makes it ideal for industrial grinding, but itsrarity makes it unattractive for commercial use.

Crystal HabitDiamonds are isometric with cubic, octahedral, hexoctohedral,dodecahedral, trisoctahedral, and related habits. Twinning along theoctahedral {111} plane is common, and many crystals are oftenflattened parallel to this plane, producing a habit that appears asflattened, triangular-shaped diamond known as a macle.

Cube

Cubes are a relatively uncommon habit for diamond, and whenfound are primarily frosted industrial stones. Many have been foundin placers in Brazil, and a significant percentage of diamonds in theSnap Lake kimberlites of Canada have cubic habit (Pokhilenko et al.2003). Crystal faces of a cube often exhibit square-shaped pyramidaldepressions rotated 45° diagonally to the edge of the crystal face.The cube may also include scattered trigons mixed with pyramidaland other depressions of hexagonal morphology visible with amicroscope.

Octahedron

The octahedron is an eight-sided crystal that has the appearance of 2four-sided pyramids attached at a common base. Each pyramid con-tains four equilateral triangles known as octahedral faces. In nature,an octahedral face will often have positive or negative trigons—smallequilateral triangles that are visible under a microscope. These aregrowths or etches on the crystal surface that represent disequilib-rium during transport to the earth’s surface from the initial stableconditions at depth within the mantle.

Partial resorption of the octahedron will result in differentcrystal habits, including a rounded dodecahedron (12-sided) withrhombic faces. Further resorption may result in ridges on the rhom-bic faces, yielding a 24-sided crystal known as a trishexahedron.Many diamonds from Argyle, Australia; Murfreesburo, Arkansas;and the Colorado–Wyoming State Line District exhibit resorbedcrystal habits. Four-sided tetrahedral diamonds are sometimesencountered that are distorted octahedrons (Orlov 1977; Bruton1979).

DiamondsW. Dan Hausel

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2 Industrial Minerals and Rocks

Diamonds commonly enclose mineral inclusions along cleav-age planes. These tiny inclusions provide important data on the ori-gin of diamond and can be used to determine the age of the stoneor to identify the unique chemistry associated with the genesis ofdiamond.

Bort

Bort is poor-grade diamond used as an industrial abrasive. It formsrounded grains with a rough exterior and has a radiating crystalhabit. The term is also applied to diamonds of inferior quality andto small diamond fragments.

Carbonado

Carbonado is a black to grayish, opaque, fine-grained aggregate ofmicroscopic diamond, graphite, and amorphous carbon with or with-out accessory minerals. The material is hard, occurs mainly as irregu-lar porous concretions and dendritic aggregates of minute octahedra,and sometimes forms regular, globular concretions. Carbonado ischaracterized by large aggregates (averaging 8 to 12 mm in diameter)that commonly weigh as much as 20 carats. Specimens of severalhundred carats are not uncommon. The density for carbonado is lessthan that for diamond and varies from 3.13 to 3.46 gm/cm3.

Although carbonado had been found in placers in Brazil andRussia, it was not until the 1990s that it was found in situ. Twenty-six grains of carbonado ranging in size from 0.1 to 1 mm wererecovered from a 330-lb sample taken from avachite (a specific typeof basalt from the Avacha volcano of eastern Kamchatka) (Smish-lyaev 1999; Erlich and Hausel 2002).

Physical Properties of DiamondDiamond exhibits perfect octahedral cleavage with conchoidal frac-ture. The mineral is brittle and will easily break with a mild strikeof a hammer. Even so, it is the hardest of all naturally occurringminerals and is assigned a hardness of 10 on the Mohs scale andnearly 8,000 kg/mm2 on the Knoop scale. Corundum, the next hard-est naturally occurring mineral, has a Mohs hardness of 9. Even so,corundum does not even compare to diamond and only has a Knoopscale hardness of 1,370 kg/mm2. Because of diamond’s extremehardness and excellent transparency, diamond is extensively used injewelry and has a variety of industrial uses.

Diamond’s hardness varies in different crystallographic direc-tions. This allows for the mineral to be polished with less difficultyin specific directions using diamond powder. For example, it is lessdifficult to grind the octahedral corners off the diamond, whereasgrinding parallel to the octahedral face is nearly impossible.

With perfect cleavage in four directions parallel to the octa-hedral faces, an octahedron can be fashioned from an irregulardiamond by cleaving (Orlov 1977). The specific gravity of dia-mond (3.516 to 3.525) is high enough that the gem will concen-trate in placers with “black sand.” This density is surprisinglyhigh, given that it is composed of such a light element. Comparedto graphite, diamond is twice as dense because of the close pack-ing of atoms.

Color

Diamonds occur in a variety of colors, including white to colorlessand shades of yellow, red, pink, orange, green, blue, brown, gray,and black. Those that are strongly colored are termed fancies. Col-ored diamonds have included some spectacular stones. For example,at the 1989 Christie’s Auction in New York, a 3.14-carat Argylepink sold for $1.5 million. More recently, a 0.95-carat fancy purplishred Argyle diamond sold for nearly $1 million. The world’s largestfaceted diamond, a yellow-brown fancy known as the 545.7-carat

Golden Jubilee (Harlow 1998), is considered priceless. Possibly themost famous diamond in the world, the 45-carat Hope, is a bluefancy.

In most other gemstones, color is the result of minor transitionelement impurities; however, this is not the case for diamond. Colorin many diamonds is related to nitrogen and boron impurities or isthe result of structural defects. Diamonds with dispersed nitrogenmay produce yellow (canary) gemstones. If the diamond containssome boron, it may be blue, such as the Hope diamond. The Hopewas found in India; however, many natural blue diamonds havecome from the Premier mine in South Africa. Blue diamonds withtraces of boron are referred to as type IIb diamonds and are semi-conductors. Natural irradiation may also result in blue coloration insome diamonds (Harlow 1998).

The most common color for diamond is brown. Before thedevelopment of the Argyle mine in Australia in 1986, brown dia-monds were considered industrial stones. But because of Australianmarketing strategies, brown diamonds are now highly prized gems.The lighter brown stones are labeled champagne and the darkerbrown referred to as cognac. Yellow is the second most commoncolor, and such stones are referred to as Cape diamonds in referenceto the Cape Province of South Africa. When the yellow color isintense, the stone is referred to as canary.

Pink, red, and purple diamonds are rare. The color in these isconcentrated in tiny lamellae (referred to as pink graining) in anotherwise colorless diamond. The color lamellae are thought to bethe result of deformation of the diamond structure.

Even though there are many green diamonds, few are fac-eted, primarily because most have a thin green surface layer cov-ering clear diamond that is removed during faceting. Facetedgreen diamonds are so rare that only one is relatively well known(the 41-carat Dresden Green), and is thought to have either origi-nated in India or Brazil. The color in most green diamonds is theresult of natural irradiation. Other green diamonds may resultfrom hydrogen impurities. Another variety, known as a greentransmitter, produces strong fluorescence that tends to mask theyellow color of the stone. Other colors include rare orange andviolet diamonds (Harlow 1998).

One of the better-known black diamonds is the 67.5-caratOrlov. Black diamonds are colored by numerous graphite inclu-sions, which also make the diamond an electrical conductor. Theseare difficult to polish because of abundant soft graphite, so blackgem diamonds are uncommon. Opalescent, or fancy milky whitediamond is the result of numerous mineral inclusions and possiblynitrogen defects in the crystal (Harlow 1998).

Dispersion, Transparency, Conductivity, Wet Ability

Diamond has a high coefficient of dispersion (0.044), the coeffi-cient is the difference in refractive index of two visible lightwavelengths at the opposite ends of the spectrum (one blue-violetand the other red), resulting in the distinct fire seen in faceted dia-mond caused by its high dispersion. Diamond is completely trans-parent to a broad segment of the electromagnetic spectrum,making it useful in a variety of industrial, electrical, and scientificapplications. It is also transparent to radio and microwaves. Col-orless diamonds are also transparent to visible light wavelengthsextending into the ultraviolet (UV), and a few rare diamonds (typeII) are transparent over much of the UV spectrum (Harlow 1998).

Diamond has a luster described as greasy to adamantine that isrelated to its high refractive index (IR = 2.4195) and density. Suchhigh density greatly diminishes the speed of light. For example, thespeed of light in a vacuum is 186,000 miles/sec (300,000 km/sec),but in diamond, it is only 77,000 miles/sec (Harlow 1998).

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Diamonds 3

Many diamonds are luminescent: approximately one third ofall diamonds luminesce blue when placed in UV light. In mostcases, luminescence will stop when the UV light is turned off(known as fluorescence). Diamonds fluoresce in both long- andshort-wave UV light. The fluorescence is usually greater in long-wave light, and diamond may appear blue, green, yellow, or occa-sionally red. Fluorescence is generally weak, however, and it maynot be readily apparent to the naked eye. In some cases, light emis-sion is still visible for a brief interval after the UV light source isturned off (known as phosphorescence). Some diamonds may alsoshow brilliant phosphorescence when rubbed or exposed to theelectric charge in a vacuum tube, or when exposed to UV light(Dana and Ford 1951).

At room temperature, diamond is four times as thermally con-ductive as copper, even though it is not electrically conductive.Because of the ability to conduct heat, diamond has a tendency tofeel cool to the lips when touched, since the gemstone conductsheat away from the lips. This is why diamonds have been referredto as ice. Gem testers (about the size of a pen) are designed to iden-tify the unique thermal conductivity of diamond and distinguish itfrom other gems and imitations.

Diamonds are hydrophobic (nonwettable). Even though dia-mond is 3.5 times heavier than water, it can be induced to float onwater. Because it is hydrophobic, diamond will attract grease, thusproviding an efficient method for extracting diamond from oreconcentrates (i.e., grease table).

Diamonds are unaffected by heat except at high temperatures.When heated in oxygen, diamond will burn to carbon dioxide(CO2). Without oxygen, diamond will transform to graphite atmuch higher temperatures (1,900°C [3,450°F]). Diamonds areunaffected by acids.

ORIGIN AND OCCURRENCEThere are literally thousands of known kimberlites and many hun-dreds of lamproites and lamprophyres, but only a handful containcommercial amounts of diamond. One estimate made many yearsago suggested that less than 1% of all kimberlites are commer-cially mineralized (Lampietti and Sutherland 1978). Althoughmany hundreds of new discoveries have been made since thatpaper was published, this statistic remains essentially valid.

Diamondiferous kimberlites and lamproites are essentiallyrestricted to cratons and cratonized terrains. These include stableArchean cratonic cores (known as Archons) as well as cratonizedProterozoic margins (referred to as Protons) (Figure 1). Someunconventional diamondiferous host rocks have also been identifiedin cratons and outside cratonic terrains within tectonically activeregions along the margins of cratons. Because high ore grades havebeen detected in some of these, unconventional commercial hostrocks are anticipated to be found in the future (Erlich and Hausel2002). Current diamond exploration programs are designed tosearch for conventional host rocks (i.e., kimberlite and lamproite)or for placers presumably derived from these.

Most diamonds are considered xenocrysts that separated fromdisaggregated mantle peridotite and eclogite during transportation tothe earth’s surface in kimberlitic, lamproitic, and some lamprophyricmagmas. Kimberlites, lamproites, and lamprophyres tend to occur inclusters of a few to more than 100 occurrences. Structural control isthought to be important in the emplacement of these, and severalstructural orientations are often recognized within each district.

KimberliteThe majority of diamond mines are developed in kimberlite such asthe Wesselton, DeBeers, Kimberley, Dutoitspan, and Ekati, or in

placers, particularly beach placers along the west coast of Africa.Lampietti and Sutherland (1978) reported that only about 10% ofthe known kimberlites were mineralized with diamond. This statis-tic may no longer be valid in that as many as 50% of kimberlitesfound in Canada and Wyoming in recent years, and possibly asmany as 90% in Colorado, have yielded diamond. Even so, only avery small portion is commercially mineralized. When economic,kimberlites may contain hundreds of millions to billions of dollarsworth of stones; thus kimberlite should be a priority target in anyexploration program.

Kimberlites are essentially carbonated alkali peridotites thatexsolve CO2 during ascent to the surface from the earth’s uppermantle, resulting in diatremes with considerable brecciation anddissolution-rounded xenoliths and cognate nodules. The diatremesappear as subvertical to vertical pipes that taper down at depth,forming steeply inclined cylindrical bodies. The average angle ofinclination of the walls of various pipes in the Kimberley region ofSouth Africa (Wesselton, DeBeers, Kimberley, and Dutoitspan) is82° to 85°. Ideally, the pipes have rounded to ellipsoidal horizontalcross-sections filled with kimberlitic tuff or tuff-breccia. Many con-tinue from the surface to depths of 2 to 2.4 km, where they pinchdown to narrow root zones emanating from a feeder dike.

The Kimberley pipe, which was mined out by 1915 (about20 years after discovery), contracted sharply at depth. At the low-est level of mining (1,056 m), it was no longer pipe shaped butrather had the appearance of three intersecting dikes (Kennedyand Nordlie 1968). Combined with the estimated 1,600 m of ero-sion since the time of emplacement, the depth to the original pointof expansion was probably 2.4 km.

Adapted from Levinson, Gurney, and Kirkley 1992.Figure 1. The North American craton showing regions of favorability for conventional diamondiferous host rocks. Major Archean provinces are in all capital letters.

SomersetIsland

RAELac de Gras

SLAV

E

HEARNESturgeonLake

WYOMING

Tran

s-H

udso

n

Oregon

Fort a la Corne SUPERIOR

NAIN

EXPLANATION

Prairie Creek

State Linedistrict

LakeEllen

KirklandLake

Noranda

Grenv

ille

Archon

Proton

Tecton

Diamond-bearingoccurrences

0 500 1,000 km

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4 Industrial Minerals and Rocks

Kimberlitic magmas are interpreted to originate from depthsas great as 200 km and travel to the earth’s surface in a matter ofhours (O’Hara, Richardson, and Wilson 1971). The magma isthought to rise rapidly, possibly 10 to 30 km/hr in order to transporthigh-density ultramafic xenoliths. Within the last few kilometers ofthe surface, emplacement rates are thought to increase dramaticallyto several hundred kilometers per hour. Such velocities could bringdiamonds from the mantle to the surface in less than a day.McGretchin (1968) estimated that the speed of the fluidized mate-rial near the surface increased to as much as 400 m/sec, or about thespeed of sound (Mach 1 or 331 m/sec). Some estimates have evensuggested kimberlite emplacement at the earth’s surface may haveachieved velocities exceeding Mach 3 (Hughes 1982).

The temperature of the magma at the point of eruption is rela-tively cool (Figure 2). Watson (1967) indicated a magma tempera-ture of less than 600°C (1,110°F) on the basis of the coking effectson coal intruded by kimberlite. A low temperature of emplacementis also supported by the absence of any visible thermal effects oncountry rock adjacent to most kimberlite contacts. Davidson (1967)suggested the temperature of emplacement may have been as low as200°C based on the retention of argon. Hughes (1982) pointed outthat the near-surface temperatures of the gas-charged kimberlitemelt may be as low as 0°C (32°F) because of the adiabatic expan-sion of CO2 gas as kimberlite erupts at the surface.

Kimberlites typically transport xenoliths and xenocrysts to thesurface. Many of these are derived from mantle depths and someform a distinct suite of minerals that are referred to as kimberliticindicator minerals. The traditional indicator minerals used toexplore for kimberlite include pyrope garnet, chromian diopside,chromian enstatite, picroilmenite, chromite, and diamond.

LamproiteSerious interest in lamproite intensified following the discovery ofa world-class diamond deposit in olivine lamproite in 1979 in theKimberley region at Argyle, Western Australia. The discovery ledto the recognition of other diamondiferous lamproites in Australia,

Brazil, China, Gabon, Zambia, Ivory Coast, India, Russia, and theUnited States.

Scott-Smith (1996) subdivides lamproites into two generalgroups: phlogopite-leucite lamproites (~60% SiO2 [silicon dioxide])and olivine lamproites (>20% MgO [magnesium oxide], 35% to45% SiO2, and 7% K2O [potassium oxide]) with abundant serpen-tine pseudomorphs after olivine. Instead of pipes with steep wallsthat slowly diminish in diameter with increasing depth, lamproitesare characterized by “champagne-glass” vents filled by tuffaceousrocks, often with massive volcanic rocks in the core.

In some cases, lamproites appear to have formed in the dia-mond stability field (Nixon 1995). A qualitative correlationbetween diamond and olivine in lamproite is confirmed in both theEllendale, Australia, and Kapamba, Zambia, provinces in whichdiamond grades are consistently higher in olivine lamproites thanleucite lamproites. When found, diamonds occur primarily in pyro-clastic rocks; the magmatic phases are notoriously diamond poor,owing to the high temperatures sustained in the flows during erup-tion, which are antipathetic to diamond preservation; that is, thediamonds will burn (Scott-Smith 1986).

Where vents flare out, a potential for substantial tonnagesexists in larger craters. At Argyle, Western Australia, past reserveestimates of 94 Mt of ore at an average grade of 750 carats/100 t ledto its classification as a world-class deposit. Some of the richer por-tions of this deposit yielded grades as high as 2,000 carats/100 t.Large numbers of the Argyle diamonds, however, are graphitizedand partially resorbed; more than 60% are irregular in shape andinclude macles, polycrystalline forms, and rounded dodecahedrons.The largest Argyle diamond weighed 42.6 carats; the overall size ofdiamonds is quite small (average <0.1 carat). Nearly 80% arebrown, and the remaining stones are dominantly yellow or color-less. Rare but economically important pink to red diamonds bringArgyle fame (Shigley, Chapman, and Ellison 2001).

Many lamproitic diamonds are relatively small and includecommon fancy yellow to brown stones. For example, macro dia-monds (>1 mm) from the Ellendale field in Western Australia aredominantly yellow dodecahedra, and many micro diamonds arecolorless or pale-brown, frosted, step-layered octahedral (Shigley,Chapman, and Ellison 2001).

PlacersBecause of a relatively high specific gravity (3.5) and extreme hard-ness, diamonds are often found in secondary stream or marine plac-ers with other minerals of relatively high specific gravity such asmagnetite, spinel, ilmenite, rutile, garnet, and gold. Some of themore productive deposits include stream and marine placers wherea large percentage of diamonds are gem quality, owing to fracturingand disaggregation of imperfect industrial diamonds during streamtransport. Considerable numbers of diamonds have been minedfrom stream sediments along the Orange River basin in southernAfrica and continuing in beach sands downcurrent from the mouthof the Orange River along the Atlantic coast. Historically, therehave been many reports of gold prospectors finding diamonds whilesearching for placer gold. Examples include California, Colorado,Georgia, North Carolina, and Wyoming in the United States, andNew South Wales in Australia (Hausel 1998).

Placer diamond deposits formed throughout geological his-tory as is evident by diamonds in ancient Proterozoic paleoplacersin the Witwatersrand metaconglomerates of South Africa and theSnowy Range Group in Wyoming, United States, as well as modernplacers along the Atlantic coast of Africa and Smoke Creek near theArgyle mine, Australia.

Courtesy of W.D. Hausel.Figure 2. Exposed contact of a Schaffer diamondiferous kimberlite, Wyoming, showing the knife sharp contact between the kimberlite (left) and granite (right), explained by adiabatic cooling of the kimberlite magma during eruption

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Diamonds 5

DISTRIBUTION AND PRODUCTIONDiamond ProductionDiamonds are mined from at least 20 countries, and the leading pro-ducers of natural diamond are Australia, Botswana, Canada, SouthAfrica, Russia, and Zaire. The World Diamond Council estimatedthat natural diamond production in 1999 was more than 111 millioncarats, valued at US$7.4 billion. In 2000, diamond production wasestimated at more than 110 million carats, valued at US$7.9 billion.In 2001, the U.S. Geological Survey (USGS) estimated that 119 mil-lion carats were mined, with an estimated value of US$7.3 billion,and in 2003, diamond production estimates stood at 132 million car-ats (Olson 2003). The Northern Miner (Anon. 2005) reported thatrough diamond sales in 2003 for the Diamond Trading Company(DeBeers marketing arm) were $5.52 billion.

Canada ranked sixth in diamond production during the sameperiod, but in the second quarter of 2004, it surpassed South Africato become the third largest diamond producer (based on value).This is one of the great exploration success stories of the twentiethcentury because before 1998, Canada did not have a diamondindustry (Krajick 2001).

Industrial diamonds have considerably less value than gem dia-mond, and much of industrial production is now synthetic. In 2001,nearly 70% of the total natural and synthetic industrial diamond pro-duction came from Ireland, Russia, and the United States: 92% wassynthetic (Olson 2001). According to the USGS, world production ofnatural industrial diamond totaled 48 million carats in 2001 and 48.9million carats in 2002. More than one third of the world’s natural dia-mond production was classified as industrial. This represented only avery small percentage (~1%), however, of the total monetary value ofnatural diamond production. Australia led the market in recovery ofnatural industrial diamonds and has averaged 22.1 million carats peryear; however, declining reserves at the Argyle mine resulted in Aus-tralian industrial diamond production of only 13.1 million carats in2001 and in 2002 (Olson 2003).

The World Diamond Council reported that the United Stateswas the largest producer of synthetic industrial diamonds, with 125million carats manufactured in 1999. The USGS reported thatdomestic synthetic industrial diamond production for 2002 was310 million carats. The total industrial output worldwide was esti-mated to be in excess of 800 million carats in 2001, valued at morethan US$600 million (Olson 2001). Domestic synthetic diamondswere produced by two companies: GE Superabrasives in Ohio andMypodiamond Inc. in New Jersey.

Natural diamond production was dominated by southern Afri-can countries with a significant contribution by Russia and Austra-lia. Nearly all of the Australian diamond production was from theArgyle mine, which accounted for more than 20% of world’s dia-monds. The relatively low quality of the Argyle diamonds, how-ever, rendered the production to be less valuable than some smalleroperations elsewhere.

Diamond DistributionAlthough there are hundreds of known diamond occurrences aroundthe world, commercial diamond deposits are rare. In the richest, dia-mond occurs in concentrations of much less than 1 ppm (Lampiettiand Sutherland 1978). The few commercial diamond deposits arehosted by kimberlite, lamproite, and placers derived from these hostrocks. These are all associated with Archean cratons and cratonizedProterozoic belts. The discovery of several unconventional host rocksin recent years, though, some with very high ore grades, suggests thatother rock types and geological environments will become diamondtargets in the future (Hausel 1996; Erlich and Hausel 2002).

The world’s natural diamonds are produced from a smallgroup of deposits, which typically have operating lives of 20 to 30years. A notable exception is the Premier mine (South Africa),which potentially could operate for more than 100 years (Levinson,Gurney, and Kirkley 1992) (see Table 1).

Africa

The Orange River basin with its many tributaries covers a regionwith more than 3,000 known barren and diamondiferous kimberlitepipes that include some of the richest pipes in the world. The prin-cipal diamond-producing countries in Africa are Angola,Botswana, the Central African Republic, the Democratic Republicof Congo (formerly Zaire), Ghana, Guinea, Namibia, Sierra Leone,South Africa, and Zimbabwe. In total, Africa accounts for nearly50% of the world’s diamond production.

Angola. Angola produces 2 million carats of high-quality dia-monds annually derived primarily from alluvial sources. Nearly alldiamond production is derived from alluvial sources in the Andradaand Lucapa areas of northeastern Lunda Norte and the CuangoRiver. Only minor amounts are mined from colluvial and eluvialdeposits overlying kimberlite at the Camafuca–Camazomba intru-sive along the Chicapa River near Calonda. Other kimberlites havebeen identified in Angola (Janse 1995).

Atlantic Coast. Erosion of diamond pipes and dikes in theOrange River basin resulted in the concentration of millions of dia-monds in the basin and along the Atlantic Ocean shoreline. Streamsediments in the basin and beach sands along the west coast ofAfrica extending from Port Nolloth, Namaqualand, to Luderitz,Nambia, contain placer diamonds. The powerful energy generatedby the wave action along this coast has destroyed or broken largenumbers of poor-quality stones while gemstones remain intact.

Table 1. Diamond production of major mines in 2001

CountryCarats,×1000

Amount,kt US$/carat

Value, US$ million

Canada

Ekati 3,685 3,685 144 531

Botswana

Jwaneng 12,339 8,920 110 1,357

Orapa 13,056 15,779 50 653

Letlhakane 1,021 3,625 180 184

South Africa

Venetia 4,977 4,602 85 423

Namaqualand 808 6,083 180 145

Finsch 2,465 4,768 70 173

Premier 1,637 3,102 75 123

Kimberley 550 3,766 110 61

Baken 65 5,835 400 26

Koffiefontein 145 2,299 225 33

Russia

Udachnaya 11,500 9,000 85 978

Jubilee 5,500 9,100 65 358

Australia

Argyle 26,000 15,100 11 286

Merlin 70 270 110 8

Namibia

Namdeb Onshore 1,385 21,867 220 305

Adapted from Mining Journal 2002.

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6 Industrial Minerals and Rocks

Botswana. DeBeers discovered three world-class kimberlitepipes (Orapa, Letlhakane, and Jwaneng) in Botswana between 1967and 1973. The Orapa pipe was found in 1967 and production beganin 1972. It is the second largest producer of diamonds in the worldand yielded more than 13 million carats in 2001 (Table 1). TheJwaneng pipe was discovered in 1973 under the sands of the KalahariDesert, and mining began on the property in 1982. It has been thethird most productive diamond mine by weight and first by value.Two smaller pipes known as the Letlhakane 1 and 2 were discoveredin 1968.

Botswana’s diamond reserves are immense. Total productionin 2001 was a record 26.3 million carats, compared to 21.26 millioncarats in 1999 and 19.8 million carats in 1998. Output from themines was 13 million, 1 million, and 12 million carats from Orapa,Letlhakane, and Jwaneng, respectively (Table 1). A fourth mine,Tswapong, produced 10,100 carats in 1999. An application for afifth mine at Gope in the Central Kgalagadi Game Reserve wasreviewed in 1999, and Debswana Diamond Company Ltd. (formedby the Botswanna government and South Africa’s DeBeers in equalpartnership) applied for a license beginning in 2001 to mine dia-monds from four small kimberlite pipes known as the B/K pipesnear the Orapa mine.

Central African Republic. Diamonds from the Central Afri-can Republic are mined from alluvium. Diamondiferous alluviumhas been found near Bria in the central area of the country; Carnot-Berberati in the southwest; and the Mouka Ouadda plateau in thenortheastern portion of the Central African Republic. To date, thesource rocks for the diamonds have not been identified. Productionamounts to about 500,000 carats per year (Janse 1995).

Democratic Republic of Congo. Formerly known as Zaire, theDemocratic Republic of Congo accounts for about 18% of the worldproduction and, in recent years, has been the second largest producerby weight, next to Australia. Only 6% of the Congo diamonds, how-ever, are gem quality with another 40% near-gem, resulting in theCongo being the fourth-ranked producer based on value. Accordingto the American Museum of Natural History Web site (undated), theMbuji-Mayi mine in the Congo has been a prolific source for dia-monds with recent annual production of about 5 million carats.

Ghana. Most diamonds in Ghana (formerly known as theGold Coast) have been mined from two placers known as theAkwatia and Birim concessions located northwest of the capitalcity of Accra. Annual production peaked at 2,283,000 carats in1975 and has since declined. About 10% of country’s output is clas-sified as gem quality, and most of the remaining stones are microdi-amonds (<2 mm) (Janse 1996). Recoverable resources areestimated to range between 20 and 50 million carats and Ghana’sestimated annual production could well exceed 1.3 million carats(Miller 1995). The Akwatia deposits are nearly depleted, but largenew resources have been identified at the Birim River deposits. Onealtered meta-lamproite was found that is thought to represent a pri-mary source for diamonds.

Guinea. Most Guinean diamonds are mined from exception-ally rich gravel placers. Some of the gravel was traced to theBanankoro kimberlite swarms in eastern Guinea, which consists ofsmall uneconomic dikes and pipes. Rich placers mined downstreamfrom the kimberlite swarms were part of the Aredor placer mine(closed in 1993), and produced a number of large diamonds includ-ing several that weighed more than 100 carats; the largest was theGuinean Star, weighing 255.6 carats (Janse 1995).

Ivory Coast. Also known as Côte d’Ivoire, Ivory Coast pro-duces a small number of diamonds annually from alluvial depositsand dikes in the Seguela area in the western portion of the country.Alluvial deposits in the Toritya field in central Ivory Coast also pro-

duce a limited number. The source of many diamonds for the Seg-uela placers is the Toubabouko olivine lamproite dike. Anotherdike, the Bobi lamproite, has yielded about 400,000 carats from therock and overlying eluvial deposits (Mitchell and Bergman 1991;Janse 1996).

Lesotho. Several kimberlites were found in Lesotho (formerlyBasutoland) but production is limited. Two pipes, the Kao and theLetseng-la-Terai kimberlites, are apparently low grade. The Let-seng pipe, however, operated as a commercial mine from 1977 to1982 and produced some large stones, including a pale-brown 601-carat diamond (Janse 1995).

Liberia. Almost all Liberian diamond production (45% ofwhich is gem quality) comes from small alluvial diggings aroundGbapa.

Mali. Alluvial diamonds and kimberlite pipes occur nearKenieba in western Mali, but no commercial diamond deposits havebeen identified (Janse 1996).

Namibia. Formerly known as South West Africa, Namibia isa source for small high-quality diamonds from placers and allu-vium. Essentially all of Namibia’s production is derived from allu-vial, and coastal and submarine terrace deposits in theNamaqualand coastal region, which includes the coastal regionfrom Luderitz to Bogenfels.

Submerged terrace deposits are mined to depths of 100 malong the coast. These are thought to extend 100 km from shorealong the continental shelf (Janse 1995). The Elizabeth Bay depositalong the coast 30 km south of Luderitz began production in 1991and has yielded many very high-quality small diamonds. Thedeposit was reported to host 38 Mt of ore averaging 0.066 carats/t.The Auchas mine, located on the north bank of the Orange River 45km inland, was reported to contain 12.3 Mt averaging 0.036 carats/t. Kimberlites in Namibia occur in five different fields; all haveproven to be barren (Janse 1995).

Sierra Leone. Diamonds in Sierra Leone are found in streamand river placers, and in terraces, off-shore terraces, and also in afew kimberlites. Many placers have been depleted, although largehigh-grade zones are still mined. Kimberlite dikes and two smallpipes were found on the Tongo deposit. The dikes were relativelyrich, but their narrow width made them unfavorable for mining. Thesmall Koidu pipe (0.4 ha) has an ore grade reported at 1.0 carat/tand is reported to host a very high occurrence (60%) of gem-qualitystones. Sierra Leone is known for its relatively large, high-qualityplacer gemstones, and has produced some very attractive stonesincluding the Woyie River diamond, which weighs 770 carats.

South Africa. Diamonds were initially reported in SouthAfrica in the 1860s, and between 1870 and 1871, a great diamondrush occurred along the Orange River, resulting in discovery ofseveral deposits (Wagner 1914). South Africa is the fifth largestproducer of diamonds (by value) with annual production of 8 to10 million carats. In 2001, the South African mines produced10.65 million carats. The region has produced more than 500 mil-lion carats since the 1860s. A high percentage of these have beengem and near-gem, and South African mines have produced someof the largest diamonds found in history.

The diamonds occur in kimberlite pipes and dikes and in asso-ciated alluvial placers. The largest pipe in South Africa is the 54-haPremier. The Premier mine has been the source of some of theworld’s largest diamonds, including the Cullinan, Premier Rose,Niarchos, Centenary, and Golden Jubilee. The largest diamond everfound, the fist-size (3,106 carats) Cullinan, was recovered from thePremier.

The Finsch mine covers 17.9 ha and lies 160 km northwest ofKimberley. It is one of DeBeers’ seven South African operations.

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Diamonds 7

Discovered in 1961, the deposit was initially developed by open pit.Since 1991, underground mine operations continued beneath theabandoned pit. Production from the mine in 2001 was 2.46 millioncarats from 4.8 Mt of ore (51.7 carats/100 t).

Diamond-bearing gravels were discovered as early as 1903,close to the Limpopo River, 35 km northeast of the present locationof the Venetia mine in South Africa. In 1969, DeBeers launched areconnaissance sampling program to locate the source of the allu-vial deposits, and kimberlite pipes were discovered upstream in1980. Mine construction began in 1990, and the Venetia mineopened in 1992, with full production in 1993. This mine representsone of DeBeers’ single largest investments in South Africa. Situ-ated 80 km from Messina in the Northern Province, the propertyrequired a capital investment of $400 million. The mine produced4.98 million carats from 4.6 Mt of ore in 2001 (108 carats/100 t).

There are 12 kimberlites in the Venetia cluster. Of the 11 pipesand 1 dike, only two kimberlites, K1 and K2, are currently beingmined. Some of the pipes were formed by multiple intrusiveevents, resulting in a variety of kimberlite facies. The kimberlitesare clustered over approximately 3 km, and the total surface areaof kimberlite is 28 ha. Venetia is being mined by conventionalopen pit. Surface mining is expected to continue for 20 years withthe targeted pit depth of 400 m.

Swaziland. One small commercial operation is reported inSwaziland. Janse (1995) describes an alluvial deposit referred to asthe Hlane occurring downstream from the small (2.8 ha) Dokol-wayo kimberlite pipe. The placer has produced about 50,000 carats/year since 1983.

Tanzania. Diamonds were initially found in alluvial depositsand later in eluvium on the Mabuki kimberlite 60 km south of LakeVictoria in northern Tanzania. In 1940 another diamondiferouskimberlite of significance was found 140 km south of Lake Victoriaby an independent Canadian prospector, John Williamson, whoapparently rode a bicycle in his search for diamonds. This pipe, thelargest economic kimberlite ever found, is known as the Mwaduipipe. The Mwadui is 1,500 m in diameter, covering 146 ha of sur-face area. The pipe produced some fine pink stones and averagedmore than 500,000 carats annually in the 1960s, with declining pro-duction in recent years. DeBeers later discovered hundreds of otherkimberlites in this region; none have been productive (Janse 1996).

Zimbabwe. Formerly known as Rhodesia, Zimbabwe has pro-duced minor amounts of diamonds from alluvium. The first kimber-lite found in southern Zimbabwe—named the Colossus pipe—wasdiscovered near Lochard in 1907. The pipe was reported to be 1 kmin diameter (Wagner 1971). Janse (1995), however, indicated thekimberlite to be considerably smaller (900 by 150 m) and not viable.Other kimberlites were found but all proved to be unprofitable withthe exception of the River Ranch kimberlite, discovered in 1975. Amine was officially opened there in 1995, but production was mini-mal and operations ceased in 1998 (Janse 1995).

Australia

New South Wales. Alluvial diamonds were initially reportedin New South Wales (NSW) in 1861, and were later found in Queen-sland (1887), in South Australia (1894), and in Tasmania (1899).From 1884 to 1922, 167,548 diamonds (with stones weighing asmuch as 8 carats) were recovered from alluvium in the Copetonfield, NSW.

The diamonds were found in gravel buried by Tertiary basaltin an active tectonic environment similar to that of the Urals in Rus-sia, the west coast of the United States, and some Archean green-stone terrains in Canada (Erlich and Hausel 2002; Ayer and Wyman2003; Kaminsky, Sablukov, and Sablukova 2003). It is thought that

the diamonds were derived from phreatomagmatic volcaniclasticsand tuffs associated with lamprophyre pipes (Atkinson and Smith1995). Diamonds in such geological terrains provide signatures,suggesting derivation from a relatively shallow mantle (<80 km)(Ayer and Wyman 2003).

Northern Territories. Decades after the diamond discoveriesin NSW, Ashton Exploration discovered diamonds in 1976 near Mt.Percy, West Kimberley, by following a trail of kimberlitic indicatorminerals. The mineral trail led to diamondiferous lamproite in theEllendale field (Tertiary). In August 1979, diamonds were found inSmoke Creek, more than 350 km to the northeast. In October 1979,the 1.2-billion-year-old (Ga) Argyle lamproite was discovered(Atkinson and Smith 1995) (Figure 3). Both lamproite fields arelocated within Proterozoic mobile belts cratonized about 1.8 Gaand were tectonically active until the Devonian or later (Jaqueset al. 1982, 1983; Atkinson, Smith, and Boxer 1984) (Figure 4).

Courtesy of W.D. Hausel. Figure 3. The Argyle mine

Source: Skinner et al. 1985; reprinted with permission of the Geological Society of South Africa.Figure 4. The Kimberley block, Western Australia, showing locations of kimberlites, lamproites, diamonds, the Ellendale field (E), Calwynyardah field (C), and Noonkanbah field (N)

124˚ 128˚

18˚

20˚

KimberleyBlock

Fitzroy

Trough

Indian Ocean

Argyle

TimorSea

Lennard

Shelf

King Leopold

Mobile Zone

Hal

ls C

reek

Mob

ile Z

one

KingSound

Halls Creek

Kununurra

EC

NCanning Basin

Proterozoic Mobile BeltAnticlineSynclineFaultConcealed Fault

KimberliteOlivine lamporiteLaucite LamporiteDiamond Occurrence

Derby

3

2

1

4

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8 Industrial Minerals and Rocks

Currently, about 450 lamproites, kimberlites, and lamprophyreshave been identified in Australia, of which more than 180 are dia-mondiferous. Some of the recently discovered kimberlites yieldedminor to significant diamond grades (Berryman et al. 1999).

Production began in the mid-1980s at the Argyle mine, andAustralia became a leading diamond producer. At full production,the mine yielded more than 30% of the world’s annual production.Further development of the open pit continued into 2001, and thecurrent operator (Rio Tinto) reported plans to expand operationsunderground.

Diamonds were recovered from the Normandy Bow Riverplacer mine in the lower reaches of Limestone Creek, 20 km north-east of the Argyle. This deposit was discovered in the early 1980sand mined by Poseidon/Freeport and Normandy from 1988 untillate 1995 (Biggs and Garlick 1987). The plant was inactive at theend of late 1995, after nearly 7 million carats were produced from24 Mt of gravel.

Kimberley Diamond Company acquired the Ellendale leasespreviously held by Argyle Diamond Mines. Initial bulk samplingresults from Ellendale 4 and 9 revealed higher ore grades near thesurface. The company reported the Ellendale 4 resource at morethan 2 million carats to a depth of 140 m (23 Mt at 0.088 carats/t),which included a higher-grade zone (444,000 t at 0.261 carats/t) toa depth of 3 m. The near-surface enrichment zone was part of themining target for 2002. Primary diamond resources of Ellendale 4and 9 were estimated at more than 2.6 million carats.

For the first 3 years of operation, 2.2 Mt of ore was expectedto be mined from the top 3 m of enriched material on both Ellen-dale 4 and 9. The ore was estimated to average 0.15 carat/t. Thecompany also reported the discovery of 11 previously unknownlamproite pipes in the area (Shigley, Chapman, and Ellison 2001).

The Merlin mine, which was developed on a group of 12 dia-mondiferous kimberlites in northern Australia, yielded the country’slargest diamond, the 104.73-carat Jungiila Bunajina (“star meteoritedreaming stone”) white diamond. Merlin is located 80 km south ofBorroloola. After 6 years of production, the mine closed in 2002because of marginal ore.

Australia’s total diamond production in 2001 was 26.2 mil-lion carats, a decrease of 0.4 million carats from the previous year.The Argyle mine (26.1 million carats) accounted for nearly all ofthe Australian production. At one point, Argyle mined nearly 40%of the world’s annual diamonds: by the end of 2000, the mine hadproduced an extraordinary 558 million carats (Shigley, Chapman,and Ellison 2001). The Merlin mine in the Northern Territory pro-duced 55,000 carats, making it the second largest Australian pro-ducer in 2001.

Brazil

A diamond rush occurred in Brazil in 1725, and by the end of 1729,several diamond placers had been found in eastern Brazil in theregion of Diamantina (“diamond city”). Placers were also foundalong the Sao Francisco, Parana, Goyas, and other streams in south-eastern Brazil.

In 1844, rich diamond placers were found in another region ofBrazil—the state of Bahia to the north. During the first 120 years ofmining, about 10 million carats were recovered, including somestones weighing more than 100 carats.

The primary source of the diamonds has not been found, and itwas initially assumed that a rock referred to as itacolumite (mica-ceous sandstone) was the source. This assumption was based on thepresence of middle Proterozoic diamondiferous conglomerates thathave supported some small mining operations in the DiamantinaArea.

The large number of diamonds found in placers suggests thatmajor primary diamond deposits will be found in Brazil some day.Since 1967, a systematic exploration program identified more than300 kimberlite, lamproite, kamafugite, and melilitite intrusives,none of which contain economic amounts of diamond (Bizzi et al.1994; Meyer et al. 1994). A total of about 55 million carats havebeen recovered from Brazil, with annual production averagingabout 1.2 million carats.

China

Diamond deposits in Liaoning Province in China are associatedwith kimberlite. More than 100 kimberlites are found in this region,including the Jingangshi Kimberlite, which contains commercialamounts of diamond (Sunagawa 1990). At another locality, theChangma mine in Shandong Province near Mengyin, about 500 kmsoutheast of Beijing, is China’s largest diamond producer. Thisdeposit was initially mined as an open pit over the past severaldecades and converted to underground mining in 2002, with anexpected life of another 30 years. The Changma deposit consists oftwo kimberlite diatremes and a dike that all merge at 40 m belowthe surface. The kimberlite has been drilled to depths of 600 m.Production from the mine during the past 30 years included 1.6 mil-lion carats recovered from ore that averaged 1.27 carats/t; the larg-est diamond was a 119-carat stone. The property has an indicatedresource of 1.4 Mt of ore at a grade of 0.92 carats/t with an inferredresource of 1.5 Mt of 0.63 carats/t. The Changma property includesnine diamondiferous kimberlites with a total measured and indi-cated resource of 9.7 Mt at an average grade of 0.055 carats/t(Beales 2004).

India

Diamonds were reported in the Golconda region of India frommedieval time to the nineteenth century. Golconda was actually themarketplace, and the source of the diamonds were placers in thePenner, Karnool, Godvari, and Makhnadi rivers in the Krishna Val-ley, and possibly in the Panna diamond field to the north in south-central India (Mathur 1982). Many of the better diamonds ended upin the royal treasuries of sultans and shahs of India and Persia. Totalproduction is estimated at about 12 million carats (Milashev 1989).

The majority of the diamonds was found in placer deposits(Sakuntala and Brahman 1984), although diamonds were alsofound in the Majhagawan lamproite as early as 1827. After kimber-lite was described in South Africa in 1877, intensive exploration inthe ancient diamond-producing areas of India resulted in the dis-covery of what was thought to be kimberlite in areas adjacent tomany placers (e.g., Majhagawan and Hinota near the Panna placerdistrict, and the Wajakurnur and other intrusives in the AnantpurDistrict). Years later, petrographic studies of some Indian kimber-lites confirmed that many were actually olivine lamproite (e.g.,Majhagawan and Chelima) (Scott-Smith 1989; Middlemost andPaul 1984; Rock et al. 1992). Diamonds recovered from the pipesare mostly transparent and flawless, with dominantly octahedraland dodecahedral habits. About 40% are gem quality (ore grade~0.01 carats/t).

Mitchell and Bergman (1991) indicated that there are severalother lamproites, kimberlites, and peridotites in this region, andRock and others (1992) also reported several olivine lamprophyresand minettes of potential economic interest in eastern India. Knownkimberlites in India are primarily Proterozoic in age and include dia-mondiferous kimberlites in the Wajrakarur field in the AndhraPradesh of the southern kimberlite province and kimberlites in theRaipur field in southeastern Madhya Pradesh in the central province(Middlemost and Paul 1984).

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Diamonds 9

Many of the Indian deposits were depleted by the nineteenthcentury and new deposits were discovered in the mid-twentiethcentury, including placers in the Junkel region and Koel Valley,and in the Simla region near the Himalayas (which were originallydescribed in Sanskrit texts). Total historical production is esti-mated to be between 14 and 21 million carats. Currently, about20,000 carats are produced each year.

North America

There is little doubt that Canada, which has become a major dia-mond producer, will remain in the forefront of diamond productionand exploration for decades to come. Recent exploration in Canadahas resulted in the discovery of more than 500 kimberlites (includ-ing some unconventional host rocks), of which nearly half are dia-mondiferous (Kjarsgaard and Levinson 2002). Some of theunconventional host rocks include lamprophyre (includingminette) and actinolite schist at Wawa, Canada, that is interpretedto represent metamorphosed komatiite.

The North American craton is the largest in the world. The cra-tonic basement rocks of Canada continue south into the United Statesand underlie large parts of Montana–Wyoming and the Great Lakesregion. Exploration in the United States, however, has been relativelyminimal. Even so, more than 100 kimberlites, lamproites, and lam-prophyres have been identified in the southern extension of the NorthAmerican craton in Colorado, Wyoming, and Montana. Approxi-mately half of the kimberlites found in Colorado and Wyoming arediamondiferous; only one in Montana has yielded diamonds to date.One mine was developed along the edge of the Wyoming craton in1995–1996. The Kelsey Lake mine in the Colorado–Wyoming StateLine District south of Laramie, Wyoming, contained low-grade ore(about 0.05 carats/t) and yielded some high-quality diamonds weigh-ing as much as 28.3 carats (Coopersmith, Mitchell, and Hausel2003). Mine operations ended because of legal problems.

The presence of several hundred kimberlitic indicator mineralanomalies, several diamonds, and some geophysical and remotesensing anomalies support the concept that the Wyoming craton hasbeen intruded by a major swarm of kimberlitic and related intru-sives, most of which remain undiscovered. Because a large part ofthe Wyoming craton remains unexplored for diamonds, additionaldiscoveries are expected. In the Great Lakes region, a group ofabout 30 kimberlites are reported in the Michigan–Illinois Area(eight of which contain trace amounts of diamond) (Hausel 1998).

One of the great exploration success stories of the twentiethcentury was the discovery of diamonds in the Northwest Territoriesof Canada, which sparked the largest claim-staking rush in history(Krajick 2001). A group of diamondiferous kimberlites were foundnearly 300 km northeast of Yellowknife under a group of shallowlakes in the Lac de Gras region. Within a few years following thediscovery, BHP commissioned Canada’s first diamond mine in late1998 (Figure 5). This mine, known as Ekati, is a world-class mine.The mine property includes a group of 121 kimberlite intrusives, andto date, commercial mineralization has been identified and reservesestablished for the Fox, Leslie, Misery, Koala, Koala North, Panda,Beartooth, Sable, and Pigeon kimberlites on the Ekati property; theother kimberlites are being evaluated for reserves. The mine is antic-ipated to have a minimum life of at least 25 years.

In 2001, Ekati produced 3.7 million carats, totaling about 6%of the world’s diamond value. In 2003, production increased to6.96 million carats (Anon. 2004). The open-pit operation on thePanda kimberlite reached its maximum economic depth in 2003,5 years after mining was initiated. The declining production fromthe Panda open pit, however, was replaced by production from thenearby Misery and Koala open pits. Evaluation showed that the

Panda kimberlite mine life could be extended using undergroundmining techniques; thus, the remaining kimberlite is being devel-oped using sublevel retreat mining. Underground mining was pre-viously initiated at the adjacent Koala North pipe in 2002. ThePanda underground mine is expected to produce 4.7 million caratsover an operating period of 6 years, with production scheduled tobegin in 2005, followed by full production in 2006. The Ekati pro-duction for the first quarter of 2004 totaled 1.27 million carats ofdiamonds, which was a 40% decline from the previous quarter. Forthe first 9 months of fiscal year 2004, the Ekati mine producedmore than 5.3 million carats.

Ore reserves at the Ekati mine are substantial. On June 30,2003, the Ekati mine reported 47.7 Mt of ore reserves graded at0.8 carats/t (36.6 million carats of recoverable diamonds) based ona 2-mm cutoff size. Measured, indicated, and inferred kimberliteresources stood at 127.9 Mt of ore containing an estimated171.2 million carats (Robertson 2004). As exploration continues onthe property, these reserves will increase.

A few other commercial properties have been identified in theNorthwest Territories, and several other properties are beingexplored or evaluated for reserves. These include Snap Lake, Dia-vik, and Jericho.

Production at the Diavik mine began in 2003. The Diavikpipes located in the Lac de Gras region east of Ekati are beingmined by Diavik Diamond Mines based at Yellowknife (Figure 5).Diavik Diamond Mines is a subsidiary of London-based Rio Tinto,and the mine is a joint venture between Rio Tinto (60%) and Tor-onto-based Aber Diamond Mines (40%). Rio Tinto assumed oper-ating responsibility from their subsidiary, Kennecott CanadaExploration. The deposit is estimated to contain 138 million caratsof diamond and includes four kimberlites (A154S, A154N, A418,and A21). The A154S kimberlite is one of the richest kimberlites inthe world and contains a reserve of 11.7 million carats at an aver-age grade of 5.2 carats/t. The property is anticipated to yield 6 to

Source: Goepel McDermid Securities 1999; reprinted with permission from Robert W. Klassen. Figure 5. Important diamond localities in Canada

68˚N

64˚N

62˚

64˚N

68˚N

120˚W 116˚W 112˚ 108˚

CoronationGulf

VictoriaIsland

BathurstInlet

GreatBearLake

TakijuoLake

Artic

Circle

ConwoytoLake

PointLake Lac

de Gras

MacKayLake

AylmerLake

Yellowknife

GreatSlaveLake

Saskatchewan

Northwest Territories

Kennedy Lake

Snap Lake

Diavik

Ekati

Jericho

AdvancedDiamond Project

Kilometers

0 100 200

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10 Industrial Minerals and Rocks

8 million carats/year when in full production and has reserves thatwill sustain the operation for 16 to 22 years. The property lies on a20-km2 island known as East Island, 300 km northeast of Yel-lowknife. The Diavik kimberlites (55 million years old [Ma])intruded the Precambrian basement complex (2.5 to 2.7 Ga).

The Snap Lake mine is located in a kimberlite dike about100 km south-southeast of Ekati and 220 km northeast of Yel-lowknife. Snap Lake will be DeBeers’ first mine developed outsideof southern Africa and is anticipated to begin production in 2006,or possibly as late as 2008. The kimberlite will be mined entirelyunderground. The kimberlite is estimated to contain 38.8 millioncarats with an average ore grade of 1.46 carats/t.

Toronto-based Tahera Diamond Corporation is the operator ofthe Jericho project, located about 170 km north of Ekati near EchoBay’s Lupin gold mine. This property includes six diamondiferouskimberlites within the Nunavut Territory. When placed into produc-tion, the property will produce about 6 million carats over a minelife of 8 years. Reserves of 2.6 Mt of ore averaging 1.2 carats/t havebeen established. The mine is expected to begin development in2004 and production is scheduled for 2005 (Anon. 2004).

Another project of DeBeers Canada—the Victor Project—liesin the James Bay Lowlands. Victor is one of 18 kimberlite pipesdiscovered on the property, 16 of which are diamondiferous. TheVictor kimberlite has a surface area of 15 ha and consists of twopipes, known as the Victror Main pipe and Victor Southwest pipe,that coalesce at the surface. The Victor kimberlite is a complex pipeconsisting of pyroclastic crater and hypabyssal facies kimberliteand has highly variable diamond grades. If a decision is made to putthe property in production, the open-pit mine will have a life of12 years and total project life of 17 years. The proposed mine wouldbe supported by a processing plant designed to process 2.5 Mtpy.

DeBeers is also involved in the Kennady (Gahcho Kue) Lakeproject, about 100 km east of Snap Lake near Ft. Defiance andsoutheast of Ekati. Kennady Lake is under exploration by a jointventure between Mountain Lake Resources and DeBeers. The prop-erty includes the 5034, Hearne, and Tuzo kimberlites. Initial sam-pling of the 5034 and Hearne pipes yielded an average ore grade of1.67 carats/t. If this project receives a go-ahead, it is expected thatpermitting will require 2 to 3 years followed by another 3 years ofmine development (Anon. 2004).

Since the 1990s, many other deposits have been found inCanada in the Northwest Territories, Nunavut, Alberta, Ontario,Quebec, and Saskatchewan (Olson 2001).

According to Engineering and Mining Journal (Anon. 2004),Canada is currently supplying about 15% of the world’s diamondsand is expected to show dramatic increases in the future. In 2002,the Canadian diamond industry produced nearly 5 million carats. In2003, production increased to 11.2 million carats, and it is esti-mated that essentially 50% of the world diamond exploration fund-ing is focused on Canada.

Russia

The official discovery of kimberlite in Russia occurred in 1954 atwhat later became known as the Mir pipe (Erlich and Hausel 2002).In 1957, development began on placers associated with the Mirpipe and was followed by open-pit operations in the kimberlite.Years later, operations ceased at a depth of 340 m. The average oregrade was high in the upper mine levels (4.0 carats/t) but decreasednear the bottom of the pit (1.50 to 2.0 carats/t). The Mir had highgem to industrial diamond content and was the source of severallarge gems, including the Star of Yakutia (232 carats) and the Dia-mond of 26th Party Congress (342.57 carats). Annual output fromthe mine was 6.0 million carats (Miller 1995).

The Udachnaya pipe was found in 1955, and mining began onthe associated placers in 1957, followed by open-pit operations inthe pipe. Udachnaya has been the most productive diamond mine inRussia with more than 14.4 million carats mined, of which 80%were gems. By 1956, over 500 kimberlites had been discovered inthe former U.S.S.R. During the next 30 years, Russia became thethird largest producer in the world: nearly all its production camefrom mines within the northern Siberian platform.

In 1960, the Aikhal pipe was discovered in Yakutia, wheremining began in 1962 and ceased sometime between 1981 and1988, presumably because of overproduction from other sources.Production resumed after 1988, and by 1995 the pit reached a finaldepth of 240 m. Annual production at the peak of mining was600,000 carats at an average grade of 1.0 carat/t (Erlich and Hausel2002).

Another commercial pipe, known as the Sytykanskaya, was dis-covered in 1955. Open-pit mining began in 1979, and 600,000 carats/year were produced (average grade of 0.60 carat/t). Another commer-cial diamond mine, the Internatsional’naya pipe, was found in 1969.Mining began in 1971 and the open pit was developed to a depth of280 m by 1980. Open-pit operations ceased, but plans were made toresume mining underground.

The 23rd Party Congress pipe was discovered in 1959, andmining on this very rich pipe began in 1966. The ore averaged6.0 carats/t and the open pit reached a depth of 124 m after 15 yearsof operation. The Jubilee (Yubileinaya) pipe was discovered in1975. Following the removal of 70 to 100 m of basalt overburden,open-pit mining began. The Jubilee was anticipated to replace pro-duction from the declining Udachnaya pipe.

During the 1970s, other diamondiferous kimberlites were dis-covered within the Russian platform. At about the same time, sev-eral kimberlitic pipes were discovered northeast of the city ofArkhangel’sk, which included the Lomonosov diamond deposit.Currently, Russia is the fourth largest producer of diamonds in theworld (by weight). The American Museum of Natural HistoryNature of Diamonds exhibit in New York City reported that thecountry has produced a total of 332 million carats and currently hasan annual production of 10 to 12.5 million carats.

Venezuela

In 1890 and 1901, secondary placer deposits were discovered inVenezuela and Guyana, and near the end of the 1960s, a placerdeposit was found on Caroni River in southeastern Venezuela. Tomid-1969, 1.3 million carats had been mined; the largest stoneweighed 12 carats. In September 1971, near the town of Salvacionin the state of Bolivar, another significant placer was discovered.Within a short period, monthly diamond production from the Salva-cion region reached 50,000 carats, but the source of the diamondsremains unknown.

Other Cratons

Several diamondiferous pipes have been reported in other cratonssuch as in the Greenland region, and also in Kazakhstan. Kazakh-stan also has the added attraction of having some very unusual andvery rich unconventional metamorphic diamond deposits—butmost of the diamonds are small, low-value industrial micro-diamonds (Erlich and Hausel 2002).

EXPLORATIONCost figures for annual diamond exploration amounts to tens ofmillions of dollars. Capitalization costs for the development of theEkati diamond mine in the Northwest Territories alone were morethan US$800 million. Regional circumstances will dictate which

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Diamonds 11

exploration method will need to be used; however, when an explo-ration program begins, priority is given to areas of favorability forfinding “traditional” diamondiferous host rocks. For example, com-mercial diamondiferous kimberlites are considered to be restrictedto cratonic regions that have been relatively stable for about 1.5 Ga.Janse (1984, 1994) suggested that cratons be separated into areas offavorability known as Archons, Protons, and Tectons. This methodfor outlining regions of favorability provides an excellent firstoption priority list.

Archons (Archean basement stabilized more than 2.5 Ga ago)are considered to have high potential for discovery of commercialdiamond deposits hosted by kimberlite and possibly by lamproite andlamprophyre. Protons (Early to Middle Proterozoic [2.5 to 1.6 Ga]basement terrains) have moderate potential for commercial dia-mond deposits in kimberlite and high potential for similar depositsin lamproite and possibly lamprophyre. Tectons (Late Proterozoic[1.6 Ga to 600 Ma] basement terrains) are considered to have lowpotential for commercial diamondiferous host rock. Unconven-tional diamond deposits (such as high-pressure metamorphic com-plexes, astroblemes, subduction-related complexes, and volcani-clastics) may occur in tectonically active terrains, but the methodsfor exploration for these are not well defined.

Following selection of a favorable terrain, topographic andgeological maps, aerial and satellite imagery, and aerial geophysi-cal data are examined. Unusual circular depressions, circular drain-age patterns, noteworthy structural trends, and vegetationanomalies are noted. Geophysics is used to search for distinct(“bull’s eye”) conductors and magnetic anomalies. Geochemicaldata are examined for chromium (Cr), nickel (Ni), magnesium(Mg), and niobium (Nb) anomalies.

Stream-Sediment SamplingOne of the primary methods used in diamond exploration is astream-sediment sampling program designed to search for “kimber-litic indicator minerals” (pyrope garnet, chromian diopside, chro-mian enstatite, picroilmenite, chromian spinel, and of coursediamond). Diamond targets are small and can range from diatremesof several hectares to narrow dikes and sills. Diamond-bearing kim-berlites and lamproites typically contain abundant soft serpentinewith resistant mantle-derived xenocrysts and xenoliths. The serpen-tine matrix tends to decompose, releasing distinct, mantle-derived,kimberlitic indicator minerals into the surrounding environment.The indicator minerals may be carried downstream for hundreds ofmeters or several kilometers, depending on the climatic and geo-morphic history of the region. Diamonds, however, are thought tobe carried considerable distances—in some cases, hundreds of kilo-meters. The indicator minerals can provide a trail leading back tothe source.

In the planning stages of stream-sediment sampling, proposedsample sites are initially marked in prominent drainages on a topo-graphic map using a sample spacing designed to take advantage ofthe region. In arid regions, sample spacing should take advantage ofrelatively short transport distances of the indicator minerals. In sub-arctic to arctic areas (i.e., Canada, Sweden, Russia, etc.), sampledensity may be considerably lower, owing to the greater transportdistance and the logistical difficulties of collecting samples. Anom-alous areas are then resampled at a greater sample density.

The usual kimberlitic indicator minerals are rare to nonexistentin lamproite; thus other minerals (zircon, phlogopite, K-richterite,armalcolite, priderite) may be considered that unfortunately havelow specific gravity and poor resistance to abrasion, and are poten-tially difficult to identify. The better indicators for diamondiferouslamproite are diamond and magnesiochromite.

To take advantage of the dispersion of kimberlitic indicatorminerals, the size of samples are determined based on the environ-ment. For example, much larger samples are taken where there is ageneral lack of active streams compared to regions with activedrainages. In areas with juvenile streams, samples are often pannedon site to recover a few pounds of sample concentrate. Recoveredindicator minerals are tested for chemistry using an electron micro-probe to identify those that have higher probability of originatingfrom the diamond stability field. The data are plotted on maps tofacilitate evaluation.

GeomorphologyKimberlite and olivine lamproite are often pervasively serpenti-nized, making outcrops the exception rather than the rule. In manycases, geomorphic expressions of pipes are subtle to unrecogniz-able. The Kimberley pipe in South Africa was expressed as a slightmound, but nearby pipes (i.e., Wesselton pipe) were expressed assubtle depressions. Others produced subtle modifications of drain-age patterns (Mannard 1968). In the subarctic, where glaciation hasscoured the landscape, some kimberlites produce noticeabledepressions filled by lakes. In the semiarid region of Wyoming andColorado, a few kimberlites are expressed as slight depressions, butmost blend into the surrounding topography and may or may nothave a subtle vegetation anomaly.

In the Ellendale field, in Western Australia, serpentinized dia-mondiferous olivine lamproites lie hidden under a thin layer of soilin a field of well-exposed leucite lamproite volcanoes. The Argylelamproite and diamondiferous lamproites in the Murfreesburo Areaof Arkansas were also hidden by a thin soil cover.

LineamentsMany kimberlites and lamproites are structurally controlled (Hau-sel, McCallum, and Woodzick 1979; Hausel, Glahn, and Woodzick1981; Macnae 1979, 1995; Nixon 1981; Atkinson 1989; Erlich andHausel 2002). Controlling lineaments and fractures may be indi-cated by alignment of a cluster of intrusives or by the elongation ofa pipe. In Lesotho, South Africa, Dempster and Richard (1973)reported a close association of kimberlite with lineaments: 96% ofkimberlites were found along west–northwest trends, and manypipes were located where the west–northwest trends intersectedwest–southwest fractures.

Lamproites in the Leucite Hills, Wyoming, are found on theflank of the Rock Springs uplift, where distinct east–west fractureslie perpendicular to the axis of the uplift (Hausel, Gregory, andSutherland 1995). In the West Kimberley Province of WesternAustralia, some lamproites are spatially associated with the SandyCreek shear zone, a Proterozoic fault. In the Ellendale field, severallamproites lie near cross faults perpendicular to the Oscar Rangetrend, even though the intrusions do not appear to be directlyrelated to any known fault. The Argyle lamproite to the east has anelongated morphology suggestive of fault control and intrudes asplay on the Glenhill fault (Jaques, Lewis, and Smith 1986).

Remote SensingKingston (1984) reported that remote-sensing techniques are widelyused to search for kimberlite; these include conventional and falsecolor aerial photography, Landsat multispectral scanner satellitedata, and airborne multispectral scanning. Multispectral scanningdata are used to identify spectral anomalies related to magnesium-rich clays (i.e., montmorillonite), carbonate, and other material withlow silica content. Image enhancement techniques (contrastenhancements, ratios, principal components, and clustering) pro-duce images that are optimum for discrimination of kimberlite and

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12 Industrial Minerals and Rocks

olivine lamproite soils. These and other photo images can be used tosearch for vegetation and structural anomalies. Airborne multispec-tral scanning provides higher resolution than Landsat and can alsobe used to measure reflectance qualities of clay in soil.

Many pipes and dikes possess distinct structural qualities orvegetation anomalies that may allow detection on aerial photo-graphs. Mannard (1968) reported that kimberlites in southern andcentral Africa were identified on aerial photographs on the basis ofvegetation anomalies, circular depressions or mounds, and tonaldifferences. Low-level aerial photographs (both conventional andfalse color infrared) have been used to locate kimberlite in theformer U.S.S.R. (Barygin 1962) and in the United States (Hausel,McCallum, and Woodzick 1979; Hausel et al. 2000, 2003).

Geophysical SurveysGeophysical exploration has been successful in the search for hid-den kimberlite and lamproite (Litinskii 1963a, 1963b; Gerryts 1967;Burley and Greenwood 1972; Hausel, McCallum, and Woodzick1979; Hausel, Glahn, and Woodzick 1981; Paterson and MacFadyen1984; Woodzick 1980), particularly in districts where kimberliteshave previously been discovered. Contrasting geophysical propertiesare often favorable for distinguishing kimberlite, lamproite, andminette from country rock.

INPUT airborne surveys are effective in identifying both ser-pentinized and weathered kimberlite, owing to the combination ofelectromagnetics and magnetics used in the survey. Rock exposuresof kimberlite may yield magnetic signatures but are poorly conduc-tive, whereas deeply weathered kimberlites are conductive butpoorly magnetic.

Because of the relatively small size of the diamond host rock,close flight-line spacing is necessary. In an airborne INPUT surveyover the State Line District, Wyoming, a flight-line spacing of200 m effectively detected several kimberlites and identified dis-tinct magnetic anomalies interpreted as blind diatremes (Patersonand MacFadyen 1984). An aeromagnetic (200- to 400-m line spac-ing) survey flown over parts of northeastern Kansas identified sev-eral anomalies, some of which were drilled, resulting in thediscovery of previously unknown kimberlites (i.e., Baldwin Creek,Tuttle, and Antioch kimberlites) (Berendsen and Weis 2001).Flight-line spacings of 50 to 100 m were used for INPUT, mag-netic, and radiometric surveys in the Ellendale field in Australia(Atkinson 1989; Janke 1983; Jaques, Lewis, and Smith 1986). Theolivine lamproites yielded distinct dipolar magnetic anomalies.

In Yakutia Province, Russia, ground magnetic surveys wereused where differences between the magnetic susceptibility of kim-berlite and the carbonate sedimentary country rock were high. Air-borne surveys also successfully detected anomalies as great as5,000 gammas (Litinskii 1963b). In Mali, West Africa, the mag-netic contrast between kimberlite and schist and sandstone countryrock resulted in 2,400-gamma anomalies over kimberlite (Gerryts1967). In Lesotho, anomalies over kimberlite were comparablewith those in Yakutia Province (Burley and Greenwood 1972).

Fipke and colleagues (1995) indicated that barren peridotitephases in Arkansas yielded magnetic highs, but the diamondifer-ous phases were not detected. In northeastern Kansas, Brookins(1970) reported large positive (550 to 5,000 gamma) and negative(0 to –2,800 gamma) anomalies over some kimberlites emplacedin regional sedimentary rocks. The sedimentary rocks had rela-tively low magnetic susceptibility, making magnetic surveys aneffective method for exploration.

Most kimberlites in the Colorado–Wyoming State Line Dis-trict yielded small complex dipolar anomalies in the range of 25 to150 gammas, with some isolated anomalies of 250 and 1,000 gam-

mas (Hausel, McCallum, and Woodzick 1979). Blue ground(weathered) kimberlite tends to mask magnetic anomalies. In theIron Mountain District, where much of the kimberlite is relativelyhomogeneous, massive hypabyssal-facies kimberlite, only weak toindistinct magnetic anomalies were detected (Hausel et al. 2000).

Magnetite is replaced by hematite during weathering, maskingnear-surface magnetic affinity. Clay produced during weatheringpromotes water retention, thus weathered blue ground over kimber-lite may produce vegetation anomalies that are susceptible to detec-tion by electrical methods. For example, resistivity surveys in theColorado–Wyoming State Line District detected apparent resistiv-ity of 25 to 75 ohm-m over weathered kimberlite, compared with150 to 2,250 ohm-m in the country rock granite (Hausel, McCal-lum, and Woodzick 1979).

Resistivity of weathered lamproite may be lower than that ofcountry rock, owing to the conductive nature of smectitic clay rela-tive to illite, kaolinite, and other clay minerals (Gerryts 1967; Janke1983). The Argyle olivine lamproite, however, yielded moderate tostrong resistivity anomalies (40 to 100 ohm-m) compared to thesurrounding country rock (200 ohm-m) (Drew 1986).

Biogeochemical and Geochemical SurveysKimberlite and lamproite are potassic alkalic ultrabasic igneousrocks with elevated barium (Ba), cobalt (Co), Cr, cesium (Cs),phosphorus (P), lead (Pb), rubidium (Rb), strontium (Sr), tantalum(Ta), thorium (Th), uranium (U), vanadium (V), and light rare earthelements (LREE). The elevated Cr, Nb, Ni, and Ta may show up innearby soils (Jaques 1998), but dispersion of these metals in soils isnot extensive. Stream-sediment geochemistry generally is not use-ful because of efficient dispersion of most metals in streams. In theColorado–Wyoming State Line District, Cominco American out-lined several known kimberlite intrusives on the basis of Cr, Nb,and Ni soil geochemical anomalies. Dispersion patterns wererestricted, however, and of little use in exploration in this terrain.

Gregory and Tooms (1969) found that Mg, Ni, and Nb anoma-lies did not extend farther than 0.6 km from the Prairie Creek lam-proite, Arkansas. Haebid and Jackson (1986) noted that soilgeochemical anomalies (Co, Cr, Nb, Ni) were detected in sand andsoil immediately above lamproite vents in West Kimberley, Austra-lia. Such anomalies could prove useful in the search for hidden oli-vine lamproites. Gregory (1984) used lithochemistry to distinguisholivine lamproite from leucite lamproite on the basis of Mg, Ni, Cr,and Co ratios.

Bergman (1987) suggested that olivine lamproites are gener-ally enriched in compatible elements relative to leucite lamproitesas a result of the abundance of xenocrystal olivine in the former.Barren lamproites contain elevated alkali and lithophile contents(K, sodium [Na], Th, U, yttrium [Y], and zirconium [Zr]) relative todiamondiferous (olivine) lamproites. Diamondiferous lamproitespossess twice the Co, Cr, Mg, Nb, and Ni and half the aluminum(Al), K, and Na as barren lamproites (Mitchell and Bergman 1991),and lamproites have anomalous titanium (Ti), K, Ba, Zr, and Nbcompared to most other rocks. These components may favor thegrowth of specific flora or may stress local vegetation (Jaques1998). The Big Spring vent, West Kimberley, Australia, is charac-terized by anomalous faint pink tones that reflect the growth patternof grass on the vent (Jaques, Lewis, and Smith 1986).

Many kimberlites in the Colorado–Wyoming State Line Dis-trict will not support growth of woody vegetation, resulting in openparks over kimberlite in otherwise forested areas. These same kim-berlites may support a lush stand of grass delineating the limit of theintrusive. Distinct grassy vegetation anomalies over kimberlites inthe Iron Mountain District, Wyoming, were used successfully to map

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Diamonds 13

many intrusives (Hausel et al. 2000). The anomalies are especiallydistinct after a few days of rain in the late spring.

Some Siberian kimberlites support denser stands of larch(Larix dahurica) and abundant undergrowth of shrub willow (Salix)and alder (Alnus) compared to surrounding Cambrian carbonates.In central India, trees over the Hinota pipe are healthier, taller, anddenser than those in the surrounding quartz arenite. This may beattributed to greater availability of K, P, micronutrients, and water.

Vegetation over the Sturgeon Lake kimberlite inSaskatchewan was tested for 48 elements; the kimberlite showed aconsistent spatial relationship with Ni, Sr, Rb, Cr, manganese(Mn), and Nb, and to a lesser extent with Mg, P, and Ba; relativelyhigh Ni concentrations occurred in dogwood twigs. In hazelnuttwigs, Cr levels were greater than 15 ppm near the kimberlite butonly 5 to 8 ppm elsewhere, and Nb was higher in hazelnut twigs.Sr and particularly Rb were relatively enriched in some plant spe-cies on kimberlite. The Sr was probably derived from the carbon-ates associated with the kimberlite, whereas the Rb was derivedfrom phlogopite. Ni, Rb, and Sr distribution and Cr enrichmentassociated with Mn depletion in the twigs could be used to identifynearby kimberlite.

MINING AND MILLINGEconomic diamond deposits depend on the average price of stones,the amount of waste material removed, mining methods, companypolitics, socioeconomics of the area, and many other factors. Forexample, a diamond deposit may be mined at a comparatively lowercost in a developing country because of the availability of an inex-pensive labor force, although constructing an infrastructure in suchan area could offset some of these benefits. In the United States,high labor and mining costs require higher-value ore for commer-cial operation; however, an infrastructure may already be available.

More than half of the world’s natural diamonds are minedfrom kimberlite and lamproite and the rest are mined from placers.Economic cutoff grades are typically >0.10 carat/t (Jaques 1998),but the grade is highly dependent on mining costs and the value ofthe recovered diamonds. Thus the economic cutoff grade will varydepending on these factors. Average ore grades range from a highof 6.8 carats/t for Argyle to a low of about 0.15 carat/t for PrairieCreek, Arkansas. Some of the rich crater facies lamproite mined atArgyle yielded grades as high as 20 carats/t. Most economic depos-its yield >30% gem-quality diamonds.

Commercial deposits include narrow dikes to pipes of 30 to1,500 m across. Pipes range in surface area from 1 to 150 ha, aver-aging about 12 ha (Jaques 1998). Diamond mines possess resourcesin the neighborhood of more than 10 Mt to 350 Mt of ore, and therichest deposits contain reserves measured in the hundreds of mil-lions of carats that are valued in the billions of dollars.

Open-pit diamond mines are typically designed to recover aslittle as 100,000 t to more than 10 Mt of ore per year. Annual dia-mond production may range from several thousand carats to a fewmillion carats. For example, the Finsch mine, South Africa, pro-duced about 5 million carats annually between 1981 and 1991,whereas annual diamond production for the extremely rich Argylelamproite reached a record 39 million carats during the height ofoperation.

Diamond quality and size must also be considered in commer-cial operations. Lamproites appear to produce small diamonds withlarge percentages of colored stones. Many kimberlites yield a largerange in diamonds, including some very large stones. For example,the average diamond from the Argyle lamproite is small (only<0.1 carat), and those from Ellendale lamproites are only 0.1 to0.2 carat (Mitchell and Bergman 1991). The largest reported dia-

mond from the Prairie Creek lamproite is 40.42 carats (Hausel1998). Diamonds from some kimberlites, however, are extraordi-nary. The largest diamond ever recovered was the size of a humanfist; it was mined from the Premier kimberlite, South Africa, andweighed 3,106 carats.

Bulk sampling is the initial step in evaluation of a commercialdiamond deposit. If favorable, additional bulk samples are used toassist in establishing ore grade maps to aid in mine planning. Sam-ples are taken on the surface and from drilling in order to achieve athree-dimensional view of ore grades. If the pipe is considered to beeconomic, planning is completed for an initial open-pit design and amill placed near the pipe. Open-pit mining typically proceeds froma spiral road developed from the rim of the pit toward the center ofthe pipe. As mining proceeds, the country rock is cut back in stepsto aid in supporting the highwalls of the open pit. Mining in the pitmay occur in an oval pattern or in a polygonal pattern (Bruton1979).

As mining continues and the pipe narrows at depth, the openpit will shrink to smaller and smaller diameters. Mining operationsmay ultimately continue underground using bulk recovery by blockcaving. Fewer than 30% of diamond mines, however, are continuedunderground. And to do so, the diamond ore must be of relativelyhigh value, because the cost of underground mining is considerablyhigher and the amount of ore recovered is considerably lower.Some kimberlites in Siberia and South Africa have been mined todepths of 1,080 m. Open pits may have mine lives of 2 to 50 years(Jaques 1998).

Following recovery of rock mined from open-pit operations,the ore is crushed and screened. Screening separates midsize fromlarger material rejects and from material too small to contain com-mercial diamonds. Decisions on the maximum screen size mustweigh the cost of processing additional material with the loss ofpotentially priceless large diamonds.

The typical diamond mill has a basic flowsheet that beginswith primary milling and continues to primary gravity concentra-tion, secondary concentration, magnetic separation, and attritionmilling. The final diamond extraction stage uses grease tables, elec-trostatic separation, or x-ray fluorescence extraction (Bruton 1979).

Placer mines are different. The size of a placer mine will varyfrom a small, one-person operation to a full-scale mine using bull-dozers, scrapers, and dredges. Paystreaks are identified in streamsor beaches; mining is then completed using small-scale or large-scale earth-moving equipment (Bruton 1979).

GEMOLOGYThe primary monetary value for diamond is as gemstones. Dia-mond prices vary considerably. There are approximately 5,000 dia-mond categories with prices that vary from $0.5/carat up to severaltens of thousands of dollars per carat (for large uncut or coloredfancy diamonds) (Miller 1995). Many faceted diamonds are worthmany times an equivalent weight in gold or platinum. Rough gem-stone diamonds have values as high as 100 or more times that ofindustrial diamonds. After the diamonds are faceted, the value ofthe gem can increase another 10- to 100-fold, and the final place-ment of a stone in jewelry will again add another increase in thevalue of the stone. Thus, any mining operation should consider notonly recovery of the gems but also the fashioning of the gems andmarketing.

Diamonds are one of the more valuable commodities on earth,and arguably are the most valuable of all commodities based onweight. For example, some Argyle pink diamonds have sold foras much as $1 million per carat (one carat weighs only 0.2 g[0.007 oz]). Thus, an equivalent weight in gold would be worth

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14 Industrial Minerals and Rocks

only $2.80 (at $400/oz). The extreme value of diamond is due to itsmystique, rarity, extreme hardness, high refractive index, and dis-persion that can result in brilliant gems with distinctive “fire” whenfaceted and polished.

Four general types of natural commercial diamonds are recog-nized. These are gem, bort (poorly crystallized, gray, brown trans-lucent to opaque), ballas (spherical aggregates formed of manysmall diamonds), and carbonado (opaque, black to gray, tough, andcompact). Gem diamonds are further subdivided into gem and near-gem (low-quality gemstones).

The fashioning of diamond “rough” into a finished gem mayrequire up to six steps that include marking, grooving, cleaving,sawing, girdling, and faceting (Hurlbut and Switzer 1979). Whetheror not all of these steps are used depends on the size, shape, andquality of the rough stone.

The value of finished gem diamonds is judged by the “fourCs”: cut, clarity, carat weight, and color. The cut of a diamond canincrease its value tremendously, and the better proportioned, pol-ished, and faceted, the greater its value. When the girdle (base) andtable of the diamond are proportioned correctly, the diamond willexhibit greater fire and brilliance.

Diamonds can be graded using the Gemological Institute ofAmerica’s color-grading system. This ranges from D (colorless) toX (light yellow). Each letter of the alphabet from D to X shows aslight increase in yellow tinge that is generally not apparent to theuntrained eye (Hurlbut and Switzer 1979). Fancy diamonds are sep-arated from colorless diamonds into groups based on color andintensity (Bruton 1978). Clarity is determined by the presence orabsence of blemishes, flaws, and inclusions. One typical gradingsystem ranges from Fl (flawless) to I3 (imperfect) with intermediategrades of VVS1 (very, very slightly imperfect), VVS2, VS1, VS2,SI1, SI2, I1, and I2.

USEThe diamond industry is a multi-billion-dollar mega-industry. Theunique physical and optical properties of diamond also make itindispensable and irreplaceable for many industrial uses in additionto personal adornment in jewelry.

Harlow (1998) and Olson (2001, 2003) describe many usesfor industrial diamonds. Because of the mineral’s extreme hardness,industrial and synthetic diamonds are used extensively as abrasivesin grinding, drilling, cutting, and polishing. Diamond also haschemical, electrical, optical, and thermal characteristics that makeit the best material available for wear- and corrosion-resistant coat-ings, special lenses, heat sinks in electrical circuits, wire drawing,drilling, and many other advanced technologies. One significantfuture application will be in computer chips because of the dia-mond’s unmatched thermal conductivity and resistance to heat. Atremendous amount of heat can pass through diamond withoutcausing damage.

Today’s speedy microprocessors run hot—up to 200°F—andmicroprocessors cannot run much faster without failing. Diamondmicrochips would be able to handle much higher temperatures thatwould liquefy ordinary silicon, allowing them to run at higherspeeds. But manufacturers have not considered using the preciousstone because it has never been possible to produce large diamondwafers affordably. The Florida-based Gemesis and the Boston-based Apollo Diamond Company plan to use the diamond jewelrybusiness to finance attempts to introduce diamonds into thesemiconducting world.

At room temperature, diamond is the hardest known materialwith the highest thermal conductivity of any substance. Eventhough diamond is more expensive than competing abrasive materi-

als such as garnet, corundum, and carborundum, diamond hasproven to be cost-effective in several industrial processes because itcuts faster and lasts longer than rival material. Synthetic industrialdiamond is superior to natural industrial diamond in that it can beproduced in unlimited quantities and tailored to meet specific appli-cations. Consequently, manufactured diamond accounts for morethan 90% of the industrial diamonds used in the United States.

According to the USGS, much of the synthetic industrial dia-mond produced domestically has been used as grit and powder(Olson 2002, 2003). The major uses were in machinery (27%),mineral services (18%), stone and ceramic products (17%), abra-sives (16%), contract construction (13%), transportation equipment(6%), and miscellaneous uses (3%) (Olson 2002, 2003). Industrialdiamonds are used in the production of computer chips; in con-struction; in the manufacture of machinery; for mineral and energyexploration and mining, stone cutting and polishing; and in trans-portation (infrastructure and vehicles). Stone cutting, along withhighway construction and repair, are some of the largest users ofindustrial diamond.

Diamond has one significant limitation in industrial use: itreacts with iron at high temperature, causing the diamond to revertto graphite, which results in high rates of wear. In an iron-rich envi-ronment, diamond may be uneconomical to use in comparison toother conventional abrasives (i.e., aluminum oxide, silicon carbide,and boron nitride). Even though these are considerably softer thandiamond, they are suitable as high-performance abrasives on fer-rous work-pieces.

Diamond use has increased in both jewelry and industrial appli-cations. One reason for the increase is the development of diamondsynthesis technology, making it possible to produce diamond abra-sives for specific applications. In the past, the only option was to usenatural diamond, which had to be sorted by size and crushed, or bysurface treatment such as rounding. Synthetic diamond abrasives,however, can now be produced under a controlled environment suchthat the shape of the crystal can be made irregular and sharp.

Diamond has many potential exotic applications. For exam-ple, the Venus probe was fitted with a transparent diamond windowbecause diamond was the only material transparent to infrared lightthat could withstand the extreme cold and vacuum of space and theextreme high temperatures and atmospheric pressures of Venus’satmosphere (up to 920°F, and pressures a hundred times that ofEarth) (Ward 1979). Another exotic use gives new meaning to thefamily jewels. LifeGem in Illinois started manufacturing diamondsfrom cremated human ashes for jewelry for surviving relatives. Thecost for a “family jewel” is reported to be more than $2,000 for a0.25-carat stone.

Diamond has applications in high-energy physics. Diamondwindows are used in high-power lasers because of the high thermalconductivity, low absorption coefficient, and low value of tempera-ture coefficient of refractive index. Diamond anvils are used inhigh-pressure research, where pressures greater than 4 megabarsare needed. Such ultra-high-pressure research can simulate condi-tions in the core of the earth and on other planets.

Diamonds are also used in dental drills and surgical blades,and provide cutting edges that are many times sharper than the beststeel blades. Since diamond has the greatest thermal conductivity ofany material, pinhead-size gold-coated diamonds are used in high-capacity miniature transmitters that carry television and telephonesignals.

SyntheticsSynthetic gem diamonds and simulants are becoming more com-mon in the marketplace. These include cubic zirconia and moissan-

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Diamonds 15

ite. Moissanite has twice the fire of natural diamond, is doublyrefracting (unlike diamond and cubic zirconia, which are singlyrefractive), and has a hardness of 9.25—thus, both moissanite andcubic zirconia can easily be scratched by diamond. Double refrac-tion is detectable in moissanite when viewing the front of the stone.The back facets will appear to be duplicated because of the doublerefraction—except when viewing down the optic axis where light issingly refractive. The optic axis is usually perpendicular to the tableof moissanite; thus, one must observe the back facets throughanother facet to see evidence of double refraction.

Synthetic gem-quality diamonds can be produced in about24 hours. Some stones weighing up to 3 carats have been producedfor a few hundred dollars (uncut). Most are yellow, but some Rus-sian stones are clear. In 1971, General Electric grew facet-qualitysynthetic diamonds that were nearly colorless (0.3 and 0.26 carats).

The colorless gemstones caused concern in the jewelry trade.Diamond simulants can be detected by a simple thermal conductiv-ity test, but most jewelers were unprepared to distinguish facetedsynthetic diamond from natural faceted diamond. Thus, DeBeersdeveloped a diamond verification instrument known as Diamond-View, which uses UV fluorescence to distinguish colorless naturaldiamond from synthetic diamond. In addition, many synthetic dia-monds examined by the Gemological Institute of America containmetallic inclusions in high enough abundance that they are able toattract a magnet. Nonfaceted synthetic diamonds exhibit a uniquecrystal habit of a cuboctohedron with a flat base. Synthetic dia-monds also exhibit unusual dendritic and striated surface patterns.According to Shigley and others (1997), because of the technologi-cal challenges and high cost of production, it is unlikely that fash-ioned gem-quality diamonds larger than 25 points will affect thegemstone industry in commercial quantities.

FUTURE OF THE DIAMOND INDUSTRYDiamonds have intrinsic value because of unique hardness, trans-parency, and thermal conductivity. Diamonds will be needed aslong as there are industrialized nations. Without any foreseeablemajor economic disasters, the future of the diamond industryshould remain strong.

As science and industry advance, additional applications fordiamond are likely to be found in the electronics industry. Demandfor diamonds for drilling in exploration for oil, gas, and minerals,and in the construction industries, is anticipated to increase. Sometechnological advances will demand both natural and synthetic dia-mond in the future.

For many years, the gem diamond industry was controlled byDeBeers—a monopoly so powerful that the diamond industry andDeBeers were thought by many to be the same. But the discovery ofsignificant diamond resources outside of Africa has diminishedDeBeers’s control over the diamond market.

The first real threat to the monopoly occurred with the discov-ery of significant gem-quality diamond deposits in the formerU.S.S.R. in the 1950s, but communistic bureaucracy could not com-pete with South Africa, and the Soviet diamonds did not greatlyaffect the market (Erlich and Hausel 2002). A major diamond dis-covery (Argyle) in Western Australia in the 1980s started the realfirst erosion of the monopoly. The Argyle deposit, however, thoughrich in diamonds, was dominated by industrial stones, and the gem-stones recovered from the mine were small. Even so, the Australiancompany, Ashton Mining, decided to market their own production.

Some gemstones produced by Argyle included rare pink dia-monds. Marketing strategies by the Australians were brilliant,resulting in the Argyle pinks becoming some of the more valuablegemstones on Earth. A large population of the Argyle diamonds

was also light-brown to brown and had been considered by the jew-elry trade as industrial or near-gem. These were marketed as cham-pagne and cognac diamonds, and the marketing strategy effectivelyresulted in these stones becoming highly sought gemstones. Evenso, many of the Argyle diamonds were small and required the spe-cial cutting skills of gem cutters in India and Sri Lanka.

The next major diamond discoveries were made on the NorthAmerican craton. This is the largest craton, with the largest Archoncore, in the world. Based on the sheer size of the craton, and themany finds of detrital diamonds in glacial moraines, this cratonshould have been a high-priority target for diamond explorationgroups. But for many years, the North American craton wasignored.

The discovery of economic diamond deposits in this cratonwas the result of unrelenting prospecting by a small group of geolo-gists (Krajick 2001). The discovery set off the greatest rush in mod-ern history and resulted in the development of a diamond industryin Canada.

Diamond production began in Canada following the capitali-zation of BHP’s Ekati mine at more than $800 million. A few othermines have now been developed, and in April 2004, the value ofdiamond production from Canada surpassed that of South Africa.This occurred in 6 years. In the future, many more discoveries ofdiamondiferous kimberlite can be expected in the North Americancraton. To date, as many as 500 kimberlites and some unconven-tional host rocks have been identified in Canada—early reports arethat 50% contain diamonds—which could easily make the NorthAmerican craton the primary source of diamonds in the near future.

The North American craton extends across the Canadian bor-der into the United States, where several diamond deposits havebeen found. Even so, much of the terrain in the United States hasnot been prospected, or was only partially explored for diamonds.Many exploration targets remain inexplicably unexplored. To date,only two deposits have been mined for diamonds in the UnitedStates—one in the Colorado–Wyoming State Line District, andanother near Murfreesburo, Arkansas.

Diamond exploration in the near future will continue to focuson Canada, where the geology and political climate are favorable.In addition to discoveries of diamonds in kimberlite and some lam-proites, one might anticipate additional diamond discoveries insome unconventional host rocks such as minettes, alnoites, otherlamprophyres, komatiites, and in particular, subduction-zone-related breccias.

One concern that has arisen is the potential production ofrelatively inexpensive synthetic gem-quality diamonds. Naturalgem-quality diamonds, however, are also relatively inexpensiveuntil they are faceted and mounted in jewelry. And it is humannature to want a natural gem rather than a synthetic stone or imita-tion. Gem-quality synthetic diamonds will probably not greatlyaffect the jewelry market.

CONCLUSIONWith the current trend of investment, exploration, and progressivepro-mining atmosphere, it is anticipated that Canada will be a lead-ing diamond producer for decades to come. The sheer size of theNorth American craton allows one to predict Canada to become theworld’s primary source for diamonds in the future. Unless there is amajor change in attitude of the U.S. government and the population,little is expected to be produced in the United States, even thoughparts of the country are underlain by this craton. The importance ofthe North American craton in the future of the diamond industryhas resulted in investments of hundreds of millions of dollars inexploration in North America.

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16 Industrial Minerals and Rocks

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