DIAMOND DEPOSITS OF THE NORTH AMERICAN CRATON & ADJACENT ACCRETED TERRAINS

By

W. Dan Hausel

W Dan Hausel Geological Consulting LLC

 

INTRODUCTION

In the past, many diamonds were found in North America in placers derived from unknown areas and in glacial deposits from a distant land. The number of diamonds found in North America indicated that several diamond localities were to be discovered.

 

Some discoveries initially led to developments of small diamond mines in Murfreesburo, Arkansas (Prairie Creek) and the Colorado-Wyoming State Line district (Kelsey Lake). However, world-class diamond deposits were later found in Canada that led to that nation becoming a major source of gem-quality diamonds over a very short period. Canada now ranks as the third largest diamond producer of gem-quality diamonds in the world, which followed the development of their first mine (Ekati) in 1998 and a second mine (Diavik) a few years later. A third mine was recently place into production (Jericho) and two additional mines are under development. With hundreds of additional discoveries, some that are very promising, some additional diamond mines will be developed in the future. Even though favorable Cratonic (Archon) basement rocks extend south of Canada under large portions of the United States, essentially all meaningful exploration has been confined to Canada over the past 25 years. Reports of hundreds of diamonds along with hundreds of kimberlitic indicator mineral anomalies, kimberlites, lamproites, lamprophyres and some distinct geophysical anomalies in the US, one would anticipate that exploration could lead to significant discoveries in the US – however, the political climate in the US remains unfavorable for exploration.

 

The geology of the North American craton and cratonized margins is favorable for discovery of diamondiferous kimberlite, lamproite, lamprophyre and some unconventional host rocks. This Archon and some unconventional terrains of interest lie under large regions of Canada. These terrains do not stop at the Canadian border but continue southward into the Great Lakes region (Superior Province) as well as under Wyoming and Montana (Wyoming Province). One should anticipate that major swarms of mantle-derived intrusives from Canada to continue southward into Montana, Wyoming, and Colorado and that this terrain could potentially become a major source for diamonds in the future.

 

This craton and cratonized margin has been intruded by widespread swarms of these mantle-derived magmas. The North American craton is predicted to become the principal primary source of gem and industrial quality diamonds in the near future and for decades to come.

 

Commercial diamond deposits are extremely rare. In the richest primary mines, diamond is found in concentrations considerably less than 1 ppm (Lampietti and Sutherland, 1978). Such deposits so far have been hosted by kimberlite, rarely by lamproite, and in placers derived from these primary host rocks. Most primary diamond deposits are found within thermally stable Archean cratons that have thick cratonic keels (termed Archons), and in cratonized Proterozoic belts accreted to the margins of these Archean basements (termed Protons). However, the discovery of several diamondiferous unconventional host rocks in recent years requires a new philosophy in diamond exploration, as some unconventional host rocks potentially contain economic amounts of diamond (Hausel, 1996; Erlich and Hausel, 2002). Most unconventional host rocks are typically cursory sampled or ignored by exploration groups and often little effort has been made to evaluate many of these potentially economic deposits.

 

The world’s natural diamonds are mined from a small group of primary and secondary deposits that mostly have operating lives of 10 to 30 years. A notable exception is the South African Premier mine, the source of some of the largest gem diamonds ever found including the 3,106 carat Cullinun, the largest diamond ever recovered. The Premier mine has operated for more than 100 years. Another exception is the marine placers along the western coast of Africa, which have been productive for many decades.

 

Diamond production statistics show the top world producers (total carats) of natural diamonds (gems plus industrial stones) are: Botswana, Russia, Canada, Congo, South Africa, Australia and Angola (Hausel, 2006). Notably absent from this list is the US, even though large regions of the country are underlain by cratonic basement terrains suitable for the discovery of diamond. Canada, which became a major diamond producer in 1998, will remain in the forefront of diamond production and exploration for decades to come. Not only is the terrain and geology of much of Canada favorable for discovery of significant diamond deposits, but various Provincial and Territorial governments have provided exploration and investment incentives. In the US, there is a perceived negative business climate for exploration and little to no support for research. As a result, nearly all North American exploration activity and investment is taking place in Canada. This philosophy has led to the discovery of more than 500 kimberlites and dozens of unconventional host rocks in Canada over the past 10 to 15 years - nearly half of which are diamondiferous (Kjarsgaard and Levinson, 2002). In Wyoming-Colorado-Montana, with a very limited research budget, more than 100 kimberlites and dozens of lamproites and lamprophyres have been discovered, and many remain unevaluated.

 

The North American Craton is the very old core of the continent that provides a favorable setting for the discovery of diamonds: this is the largest craton in the world (Figure 1). The cratonic basement that underlies large regions of Canada projects south into the US. Some of the more favorable terrain (Archon) extends under large regions of Montana, Idaho and Wyoming and into the Great Lakes region, where the craton has been subdivided into smaller provinces. Even with these favorable extensions of cratonic basement into the US, the political climate has been less than favorable south of the Canadian border. For example, investment in exploration and research in Wyoming Province – the most favorable extension of the craton in the US, has amounted to much less than 0.01% of research and exploration investments in Canada. If one were to compare the research expenditures for Wyoming (approximately $100,000) over the past 15 years to just one province in Canada – for instance Alberta ($68 million), it is little wonder that anything of significance has ever been discovered in Wyoming. With just a ten-fold increase in the research budget in Wyoming, one would anticipate significant discoveries would be made simply because the Wyoming State Geological Survey (WSGS) has identified several very good diamond and colored gemstone targets on little to no funding. However, these discoveries have only been made because of a couple of driven individuals – without these individuals, a large increase in funding would most likely lead to a larger bureaucracy without successes. It is the individuals that have made all of the significant discoveries in the mineral industry. This is very important to recognize. In Canada, hundreds of kimberlites have been discovered in recent years with dozens containing commercial quantities of diamond. Exploration projects in Canada are very well funded and significant discoveries have been made in every province and territory.

 

South of the Canadian border, many detrital diamonds, hundreds of kimberlitic indicator mineral anomalies, dozens of vegetation anomalies, circular geomorphic and vegetation anomalies, geophysical anomalies, known kimberlites and the largest field of lamproites in North America have been identified. Many diamonds and diatremes have been discovered throughout various regions of the US, both within the cratonic environments and also in unconventional terrains (Hausel, 1996; 1998). Some of the more notable terrains in the US include the Appalachian Uplift, the Arkansas Proton, Superior Province, the Wyoming Craton and the California Sierra Nevada and coastal mountain terrains.

 

Both gem and industrial diamond have been created in the laboratory (Hazen, 1999), but the value of synthetic gem diamond falls short of natural gem diamond. Some natural diamonds represent the most valuable commodity on earth based on price per unit weight (Hausel, 2006).

 

Native carbon may occur as one of three polymorphs: diamond, graphite and lonsdaleite (Erlich and Hausel, 2002). The physical differences between these are due to the bonds between carbon atoms. The crystalline cell of diamond approximates a cube with sides of 3.56Å and the coordination of carbon atoms in diamond is tetrahedral such that each atom is held to four others by strong covalent bonds resulting in the extreme hardness, incompressibility and thermal conductivity associated with diamond.

 

In its simplest form, diamond is a cube. Even so, cubic habits are relatively uncommon for diamond, but when found, they are often frosted industrial stones. Many diamond cubes have been found in placers in Brazil and a significant percentage of diamonds recovered from the Snap Lake kimberlites in Canada also have cubic habit (Pokhilenko and others, 2003). One of the more common habits for diamond is that of an octahedron (Figure 2). Partial resorption of the octahedron can result in a rounded (12-sided) dodecahedron with rhombic faces. Many dodecahedrons develop ridges on the rhombic faces and produce a 24-sided crystal known as a trishexahedron. Four-sided tetrahedral diamonds are distorted octahedrons (Bruton, 1979; Orlov, 1976; Shafranovsky, 1964). A tetrahedron by definition is a four-faced polyhedron in which each face forms a triangle (Bates and Jackson, 1980). These have four faces, four edges, and six apexes (Erlich and Hausel, 2002). Twinning in diamond commonly follows the spinel law and yields a flat triangular macle.

 

Diamonds recovered from lamproites often exhibit resorbed habits and octahedrons are typically less common in lamproites than in many kimberlites. The many resorbed diamond textures associated with lamproitic magmas are a result of the instability of diamond in lamproite as compared to many kimberlites. In particular, the slower rate of rise of lamproite magma through the graphite stability field, coupled with high magmatic temperatures in an oxygen-rich environment provides conditions that favor diamond resorption. Similar resorbed habits are found in many diamonds recovered from lamprophyres as well as some kimberlites that exhibit geochemical evidence of eruption as an oxidizing magma. In addition to resorbed textures, known diamondiferous lamproites tend to produce a large percentage of industrial to gem diamonds and often many fancy (colored) diamonds.

 

Industrial stones may be classified as bort (a poor grade diamond that is used as industrial abrasive) and carbonado (an opaque, black to grayish, fine-grained aggregate of microscopic diamond, graphite, and amorphous carbon with or without accessory minerals) (Erlich and Hausel, 2002). Even though diamond is extremely hard, it is brittle and will break to yield a conchoidal to hackly fracture along with smooth cleavage surfaces. Diamonds exhibit perfect cleavage in four directions parallel to the octahedral faces: thus an octahedron can be fashioned from an irregular shaped diamond simply by cleaving (Kukharenko, 1954; Orlov, 1977). Natural diamonds contain tiny mineral inclusions along cleavage planes. These provide important data on the origin of diamond and some inclusions can be used for age determinations. The mineral inclusions typically form assemblages that are characteristic of peridotite or eclogite. Some rare inclusions have been identified that are characteristic of very deep mantle sources and interpreted as ultra-high pressure diamonds that originated within the lower mantle (Erlich and Hausel, 2002).

 

The specific gravity for diamond (3.516 to 3.525) is high enough that it will concentrate in stream, river or marine placers with ‘black sand’ heavy minerals. This density is surprisingly high given the fact that diamond is composed of such a light element (carbon). Compared to graphite (2.2), diamond is twice as dense due to the close packing of atoms from high pressures within the earth’s mantle (Harlow, 1998). The depth of erosion of many diamondiferous kimberlites in Wyoming and Colorado led Hausel (2004) to the conclusion that placer diamonds are likely to be common within and surrounding the Colorado-Wyoming State Line district. Diamonds have already been found in black sand concentrates in at least three drainages in the district, and only a few scattered samples have been collected for diamond.

 

Diamond has greasy to adamantine luster. The luster is due to high refractive index resulting in a gemstone of unparalleled beauty with extraordinary fire. Diamonds also occur in a variety of colors from white to colorless, gray to black, and shades of yellow, red, pink, orange, green, blue, violet and brown. Strongly colored diamonds are termed fancies and many have extraordinary beauty that sell for premium prices. As an example, in 1989 a 3.14-carat Argyle pink diamond sold for $1,510,000. More recently, a 0.95-carat fancy purplish-red Argyle diamond sold for nearly $1 million. Thus, these diamonds are thousands of times more valuable than an equivalent weight of gold.

 

The color in most other gemstones is due to trace impurities of transition metals. However, the color in diamond is often caused by trace nitrogen, boron or by structural defects. Pink diamonds in particular, are thought to result from structural defects. Diamonds may be red, pink, purple, orange, yellow, green, blue, white, black, gray or brown. The most common color is brown. Prior to the development of the Argyle mine in Australia in the 1980s, brown diamonds were considered unattractive and typically classified as industrial stones. But due to Australian marketing strategies, some brown stones are now highly prized gems. Lighter brown diamonds are quite variable and have color tones that range from very light brown, light (champagne) brown, medium brown, dark brown to very dark brown. Color saturation is also variable resulting in bright brown and dark (cognac) brown colors. In particular, champagne and cognac gem diamonds are in high demand due to marketing.

 

Pink, red and purple diamonds are rare and the color in these is concentrated in tiny lamellae in an otherwise colorless diamond (Harlow, 1998). The color lamellae are interpreted as a result of micro-deformation possibly resulting from stresses applied to diamond while crystallizing within an active subduction zone. This might suggest that diamond deposits associated with a paleo- or active Benioff zone could include pink diamonds. Areas such as the Sierra Nevada, California, the State Line district, Colorado-Wyoming, and the Cordillera of British Columbia may be good targets to search for such facies. At any rate, pure pink diamonds are extremely rare. Most green diamonds have only a thin surface coating that is removed during faceting – thus natural faceted green diamonds are rare. The green color results from natural irradiation, while others may result from the presence of hydrogen. The rarest color is orange, for which the coloring agent has yet to be identified in diamond. The range of orange color tone is quite variable in lightness and saturation resulting in pale orange, bright orange, dull orange and deep orange. One of the most exquisite colors for all orange diamonds is a pumpkin orange.

 

Black diamonds result from the presence of graphite inclusions, which not only color diamond, but also make the diamond an electrical conductor. Individual colors can vary from pale charcoal black, dull ink black, to bright gun metal black, all with weak saturation. Gray diamonds are hydrogen rich and their color is related to light absorption by hydrogen defects. Opalescent or fancy milky white diamonds are the result of numerous mineral inclusions (and possibly nitrogen defects) (Harlow, 1998).

 

Diamond’s high index of refraction (IR=2.4195) is a result of density. Such high density diminishes light velocity to only 77,000 mi/sec in diamond, or less than half its velocity in a vacuum (Harlow, 1998). Diamonds are thermal conductors and four times as thermally conductive (5 to 25 watts/cm/oC) as copper at room temperature. Unlike copper, diamond is also an electrical insulator (0 to 100 ohm/cm at 300oK). Because of its high thermal conductivity, diamond feels cool and the gem will conduct heat away from the lips, which is why diamonds are sometimes referred to as “ice”. Diamond detectors are designed to measure this unique thermal conductivity. Diamonds are relatively unaffected by heat except at high temperature. Without the presence of oxygen, diamond will transform to graphite residue at 1900°C. When heated in oxygen, diamond will burn to CO2 at much lower temperatures (>690oC). Diamonds are unaffected by acids.

 

Diamonds repel water and are therefore hydrophobic (non-wetable) and attract grease. Even though they are 3.5 times heavier than water, diamonds can be induced to float. Oxygen atoms in water and in a given material tend to link. Water will adhere to materials that contain oxygen making them wetable, but diamond contains no oxygen. Hydrocarbons such as grease have affinities for material without oxygen. This property is used effectively in grease tables, where such tables are coated with grease to attract non-wetable diamonds, while wetable oxygen-bearing minerals tend to wash over grease plates on the tables (Erlich and Hausel, 2002).

 

There are four general types of commercial natural diamonds: (1) gem (well-crystallized, transparent, flawless to nearly flawless), (2) bort, (3) ballas (spherical aggregates formed by many small diamonds), and (4) carbonado (opaque, black to gray, tough and compact industrial diamond). Gem diamonds may be further subdivided into gem and near-gem (lower-quality gemstones).

 

Rough gem diamonds have values as much as 10 to 100 times greater than industrial diamonds. Gem diamonds, when cut and polished, will fetch values that are 5- to 100- times that of the rough stone particularly when they are dressed in jewelry. The extreme value of diamond as a gemstone is due to its mystique, rarity, extreme hardness, gem preparation, beauty and high refractive index and dispersion that produce brilliant faceted gems with distinctive “fire”, and of course, marketing.

 

Top cutters in the world produce beautiful gems from rough material and may require considerable pragmatic crystallographic research to determine location of cleavage, fractures, pits, curves, protrusions, inclusions, and color inconsistencies. In some cases, valuable diamonds have been studied and mapped by cutters for as much as a year prior to faceting. Preparation for conventional faceting can take place over a considerable amount of time and require mathematical models to calculate the greatest potential yield of dispersion for the gem. Since 1981, lasers, and since 1988, computer modeling and scanning, have become an integral part of diamond fashioning. A rough diamond can now be modeled with a computer and scanner to determine the optimum faceted stone using a virtual 3D model to display positions of mineral inclusions and virtual saw planes.

 

The size and shape of rough diamond, the number and location of imperfections and inclusions, and the direction of cleavage (referred to as “grain” by cutters) are considered prior to creating a gemstone. Large diamonds may be pre-shaped by cleaving to save time. In this case, the cutter selects the octahedral cleavage and cuts a small groove in the octahedral plane with a sharp-edged diamond chip. With the diamond mounted on a dop holder, a steel knife is placed in the groove and the back of the knife is struck to create enough force to cleave the stone (laser kerfing may instead be used to mark a notch that is burned into the stone).

 

If the cleavage is improperly identified, the diamond may shatter into pieces when the knife is struck. Most conventional primary shaping is done by cutting the stone with a diamond saw. After the stone is placed in a clamp and the cleavage is marked, the diamond is either cut parallel to the cube or to the dodecahedron with a rapidly rotating blade impregnated with diamond powder. Because of hardness, it may take 4 to 8 hours to complete a cut through a 1-carat diamond of only 6 to 8 mm in diameter by using conventional cutting methods (Hurlbut and Switzer, 1979)! The process is more rapid using a laser with the diamond mounted in a dop on an x-y platform. With the desired cut preprogrammed in a computer, the platform moves the diamond through the laser. At the point where the beam is focused, the temperature is extremely high and the molecular structure of diamond is converted to graphite on the first pass, and the graphite is then "burned off" on the return pass. Diamond combustion occurs at 690oC to 875oC. Representative cutting time using a laser would be approximately eight hours for a 10-carat crystal (Baker, 1981).

 

Faceting is completed by grinding and polishing the diamond on a revolving horizontal lap impregnated with diamond powder. The diamond is held at various angles and polished. In a standard, round, brilliant diamond, as many as 58 facets may be cut and polished. The optimum directions for conventional polishing are those parallel to the crystallographic axes. Because the cubic faces of the diamond are parallel to axes, they are easiest to polish. Those that lie nearly parallel to an optic axis are also more favorable to polish because of lower hardness.

 

Dodecahedral faces lie parallel to a crystallographic axis, and each face has one optimum conventional polishing direction. The octahedral face is the hardest on the diamond since it lies at the greatest angle from the crystallographic axis. It is nearly impossible to saw or facet using conventional methods if the plane of the cut or facet varies more than a few degrees from that of a cubic face. In this case, a laser is necessary to produce cuts and facets.

 

Tiny inclusions of diamond may be randomly scattered within a host diamond. With conventional methods, the diamond inclusions must be avoided during sawing since vibrations produced when the blade contacts the included diamond can cause the host to shatter. Even if the stone does not shatter, the cutting time may increase 2 to 3 times and extend the cutting into many days or even weeks. With laser technology, this problem is resolved and may take only a matter of hours. The laser also includes the ability to produce new fancy shapes that were not formerly possible with diamond, such as horse-heads, oil wells, stars, butterflies, initials, etc. Many diamonds that had distorted growth, such as twinning, were virtually impossible to cut by conventional means because of changes in cleavage and crystallographic axes. However, these stones can now be cut by a laser without regard for the grain (Baker, 1981).

The value of the finished gem is judged by “four Cs”—cut, clarity, carat weight and color. The cut of a diamond can increase its value enormously - the better proportioned, polished and faceted, the greater the value of the finished stone. For diamonds of similar quality, those of greater size can dramatically increase in value with increased carat weight. When the girdle (base) and table of the diamond are correctly proportioned, the diamond will exhibit greater fire and brilliance. Gem diamonds include fancy (colored) and white (colorless) stones. Colorless diamonds range from colorless (white) and blue-white to pale yellow (Bruton, 1978). One of the more common systems for evaluating diamonds is that of the Gemological Institute of America’s (GIA) color grading system. This system ranges from D (colorless) to X (light yellow). Each letter of the alphabet from D to X shows a slight increase in yellow tinge (Hurlbut and Switzer, 1979).

 

A visual appraisal is completed in a well-lighted room using natural north window light. Such appraisals compare the stone to a master set of instrument-graded diamonds. The instrument used in color grading is a colorimeter, which quantitatively measures the degree of yellowness (Hurlbut and Switzer, 1979). Clarity is determined by the presence or absence of blemishes, flaws and inclusions. Many grading systems in use have descriptive terms such as flawless (F)or imperfect (I) and terms that denote intermediate grades such as very slightly imperfect (VSI).

 

Economic Value

Diamond deposits can provide significant economic boosts to a local economy and even drive the economy for an entire country. For example, diamond mines may host from $500 million to $75 billion in raw stones. Rough diamonds may be valued at only $50 to as much as $400 per carat. Faceted stones are typically valued at 10 to as much as 100 times the raw stone depending on the placement of the stone in jewelry. The diamond mines typically have lives of several years to as much as 100 years.

 

The recent discoveries of commercial deposits near the arctic north in Canada have resulted in dramatic costs for capitalization of mines. Additionally, spring and summer thaw of the ice roads for mine supplies will result in price increases for the diamonds and cost of mining, as much of the materials and fuels for the mines will have to be flown in at a very high cost. The discovery of commercial diamond deposits further south in Alberta, Montana and Wyoming could provide favorable economics for capitalization due to the presence of better infrastructure.

 

Commercial concentrations of diamond have only been found in rare magmatic rocks and placers presumably derived from these igneous rocks. The only host rocks that have been found (to date) that contain commercial deposits of diamond are kimberlite and lamproite. Even so, diamonds have been identified in several other igneous rock types (i.e., alkali basalts, lamprophyres, ultramafics, etc) as well as in some very unusual ultra-high pressure metamorphic rocks (Hausel 1996; Erlich and Hausel, 2002). Commercial host rocks so far have been restricted to ancient stabilized cratons and cratonized margins that include Archons (cratons of Archean age) and Protons (cratonized belts of Early to Middle Proterozoic age). Nearly all modern exploration ventures focus on the search for diamondiferous kimberlite in traditional cratonic terrains.

 

Primary magmatic diamond deposits are limited to a few rock types that originally formed under extreme pressure and temperature at great depth beneath the lithosphere. The most notable magmatic diamond deposits are associated with kimberlite, lamproite and some lamprophyres. Many diamondiferous kimberlites, lamproites and lamprophyres tend to occur in small or large clusters of a few to more than 100 individuals. The emplacement of clusters in most cases can be related to structural control - as a result, more than one intrusive is often found along the same fracture or orientation, or along parallel or cross fractures. Several structural orientations are typically recognized within a given district and many individual structures (faults, shear zones, etc) responsible for control have limited strike lengths. Larger, more distinct structures may occur near some districts that are thought to be related in some way to kimberlite emplacement (Hausel and others, 1979). Even so, the evidence is not always convincing.

 

Detailed mapping of smaller linear structures responsible for the orientation of kimberlites may lead to discovery of additional hidden to poorly exposed kimberlite (Hausel and others, 1979; 1981; 2000). The emplacement of kimberlite in the Iron Mountain district of Wyoming is thought to have had an association with the nearby Cheyenne Belt suture zone (Hausel and others, 2003). This suture is interpreted to represent a paleo-Benioff zone marking the break between the Wyoming (>2.5 Ga) Province to the north, from the Colorado (1.8 -1.6 Ga) Province to the south (R.S. Houston, personal communication, 1996). The suture lies 6 mi north of the known kimberlites at Iron Mountain while the Iron Mountain kimberlites tend to occur along fractures that parallel the projected suture. However, 60 miles further south, kimberlites of the State Line district show primarily north-northwesterly trends with some east-west cross-trends but no evidence of control by major structures (Hausel and others, 1981).

 

Kimberlite magmas tend to erupt as diatremes (pipes) at the earth’s surface. These erupt with considerable latent energy ejecting pyroclastic material into the air (referred to as crater facies kimberlite) and disrupting and incorporating blocks of country rock to produce a volcaniclastic rock (Figure 3). The resulting breccia, referred to as diatreme facies kimberlite, exhibits fragments of kimberlite along with crustal xenoliths and cognate mantle nodules within a serpentinized peridotite matrix. Kimberlite diatremes typically exhibit more than one episode of magma intrusion and often suggest several episodes of intrusion within the same pipe as well as within the same district. For instance six-different kimberlite facies were mapped within the Sloan 1 and 2 kimberlite complex in Colorado (McCallum and Mabarak, 1976).

 

Diatremes appear as vertical pipes that taper at depth to steeply inclined cylindrical bodies. The average angle of wall inclination at the Wesselton, DeBeers, Kimberley and Dutoitspan pipes in South Africa is 82° to 85°. Ideally, the pipes form circular or ellipsoidal cross sections in the horizontal plane that are filled with kimberlitic tuff or tuff-breccia. In a vertical plane, the ideal cross-section is carrot-shaped. Most pipes taper from the surface to depths of 0.6 to 2 miles where they pinch to narrow root zones that originate from a feeder dike beneath the root (Kennedy and Nordlie, 1968). At the feeder dike, the kimberlite is massive porphyritic (root-zone or hypabyssal facies) peridotite rather than a breccia. The porphyry may have considerable olivine or serpentinized olivine phenocrysts with minor pyroxene in a fine-grained serpentine matrix typical of peridotite.

 

Diamondiferous kimberlite was initially identified in 1870 at the Jagersfontein and Dutoitspan pipes in South Africa. The diamonds were recovered from deeply weathered, oxidized kimberlite (referred to as ‘yellow ground’). Less intensely weathered kimberlite (referred to as ‘blue ground’) lying beneath the yellow ground consisted primarily of carbonated montmorillonite clay with scattered rounded boulders of country rock and mantle nodules. As the kimberlite was mined to greater depth, hard, serpentinized rock was intersected. H.C. Lewis introduced the term “kimberlite” in 1887 for diamondiferous rock at the type locality near Kimberley, South Africa that was defined as a porphyritic mica-bearing peridotite.

 

The kimberlitic magma temperature is hot at depth, but at the point of eruption is strikingly cool. Watson (1967) suggested an emplacement temperature of <1100°F was necessary to produce coking effects on coal intruded by kimberlite. A much lower temperature of emplacement is supported by the absence of visible thermal effects on country rock adjacent to most kimberlite contacts. Davidson (1967) suggests that the temperature of emplacement may be as low as 390°F based on the retention of argon. Hughes (1982) argues that near-surface temperatures of the gas-charged kimberlite melt may be as low as 32°F owing to the adiabatic expansion of CO2 gas during eruption at the surface. He also supports that emplacement velocity of gasses and magma which produced the diatreme breccias and crater facies pyroclastics at the surface could have been as high as Mach 3!

 

Lamproite, another important host for diamond, became of major interest following the discovery of a world-class diamond deposit in olivine lamproite in the Kimberley region of Western Australia in 1979. This discovery led to the development of the Argyle mine. Several other diamondiferous lamproites have been identified or recognized in Australia, Canada, Zambia, Ivory Coast, India, Russia and the United States (Mitchell and Bergman, 1991).

 

Lamproites have been identified in more than 25 provinces or fields in the world (Mitchell and Bergman, 1991; Coopersmith and others, 2003). Altered diamondiferous leucite lamproite had been described as early as 1967 near Seguela, Ivory Coast (Dawson, 1967). More than a century earlier (in 1827), diamonds had been recovered from the Majhgawan lamproite in India. Diamonds had also been identified in the Prairie Creek lamproite in Arkansas as early as 1906 (Scott-Smith, 1986, 1989). Olivine lamproites have yielded much higher ore grades than leucite lamproites. But for the most part, these are all very low grade such as the Mahjgawan olivine lamproite (1.14 Ga) (10 carats/100 tonnes) and the Prairie Creek olivine lamproite (11 carats/100 tonnes). The Zhenyuan field lamproites of the Yangtze craton, China, grade at only 25 carats/100 tonnes (Mitchell and Bergman, 1991). However, there is one exception - the extraordinarily rich Argyle olivine lamproite that was reported to have yielded some bulk samples as high as 2,000 carats/100 tonnes!

 

The morphology of lamproite contrasts with the typical kimberlite pipe. Instead of pipes with steep walls that slowly diminish in width with increasing depth like kimberlites, lamproites are characterized by champagne glass-shaped vents filled by tuffaceous rocks often with massive volcanic rocks in the core. Many lamproites form distinct cinder cones, flows, and/or maar-like volcanoes (Mitchell and Bergman, 1991). A qualitative correlation between diamond and olivine in lamproite is supported by the Ellendale and Kapamba districts, where diamond grades are consistently higher in olivine lamproites compared to adjacent leucite lamproites.

 

Because of a relatively slow magma ascent rate compared to kimberlite, diamonds in lamproites often show a variety of morphologies suggestive of resorption - large diamonds are also uncommon. At Argyle, for instance, more than 60% of the recovered diamonds were irregular in shape and include macles, polycrystalline forms and rounded dodecahedrons (Shigley and others, 2001). Some diamonds also exhibit evidence of shearing or deformation. Where found, ore grades are essentially restricted to preserved pyroclastics in a given vent where magma temperatures declined rapidly following eruption (Scott-Smith, 1986).

 

Where lamproitic vents flare into large craters, a potential for substantial ore tonnage exists. At Argyle, early reserve estimates of 94 million tons of ore at an average grade of 750 carats/100 tonnes led to its classification as a world-class deposit. Many fabulous gemstones were recovered, but a large portion of the diamonds were graphitized and/or partially resorbed, while the largest diamond only weighed 42.6 carats. The overall average size of the diamonds was <0.1 carat. Even so, at one point, Argyle’s annual production totaled 40% of the world’s production, and by the end of 2000 the mine had produced an extraordinary 558,400,000 carats (Shigley and others, 2001).

 

Most lamproite-derived diamonds are relatively small and many exhibit ‘fancy’ colors. Overall, diamonds from Argyle and Ellendale lamproites are smaller than those in many kimberlites. Macrodiamonds (>1 mm) from Ellendale lamproites are dominantly yellow dodecahedra, whereas microdiamonds (<1 mm) are colorless to pale-brown, frosted, unresorbed step-layered octahedra. The Argyle diamonds are mostly irregularly shaped, fractured, strongly resorbed dodecahedra or combinations of octahedra and dodecahedra. Almost 80% of Argyle diamonds are brown and many of the remaining 20% are yellow to colorless. Significant, but rare, are the economically important pink diamonds.

 

A variety of lamprophyres have similarities to kimberlite and lamproite. Some of the lamprophyres have yielded diamond and these potassic rocks are becoming of greater and greater interest for diamonds. Erlich and Hausel (2002) predicted that some lamprophyres would most likely be found that contain commercial amounts of diamond. With greater and greater interest in diamondiferous rocks, a large number of diamondiferous lamprophyres are now being found particularly in Canada.

 

Diamond production in North America prior to 1998 was restricted to minor production from two small operations at Murfreesburo, Arkansas and Kelsey Lake, Colorado. But due to extraordinary exploration efforts, Canada is now a world power in diamonds surpassing South Africa, and now ranks as the number 3 producer in the world following only Botswana and Russia. It is likely that Canada will soon become the number 2 source of diamonds based on the number of discoveries and exploration expenditures and investments. For instance, the great Ekati diamond mine opened in 1998, and encloses some of the richest kimberlites in the world. But, just after 3 years of operation, the Diavik mine, which opened about 4 years after the Ekati, became Canada’s top diamond producer after recovering just under 20,000,000 carats of rough (Robertson, 2006). Canada currently has three major diamond mines in operation - Diavik, Ekati and Jericho. Other mines are under construction and permitted that include Snap Lake, Gahcho Kue and Victor, with other properties in the feasibility stage.

 

Even though large regions of the United States have potential to host significant diamond deposits, the US will remain unproductive unless effort is made to devote research funding in the search of diamond deposits. According to Kjarsgaard and Levinson (2002), exploration over the past several years resulted in the discovery of more than 500 kimberlites (including some unconventional host rocks) in Canada, of which nearly half are diamondiferous. Some unconventional diamond-bearing host rocks include actinolite schist (metamorphosed komatiite? or lamprophyre?) at Wawa, Canada, as well as a large number of diamondiferous lamprophyres found elsewhere.

 

Cratonic basement rocks underlie large parts of Canada and continue south into the US under large regions of Montana, Wyoming and also the Great Lakes region (Hausel 1996, 1998). Numerous anomalies have been identified in the US (Figure 5). Within the Wyoming Province (>2.5 Ga) and portions of the Colorado Province (<2.5-1.6 Ga), collectively referred to as the Wyoming Craton, more than 100 kimberlites and several lamproites and lamprophyres have been found surrounded by hundreds of kimberlitic indicator mineral anomalies. The Wyoming Craton underlies nearly all of Montana and Wyoming, and a portion of northern Colorado. A few dozen kimberlites and lamprophyres have also been found in the Superior Craton in the Great Lakes region of Michigan, Wisconsin and Illinois.

 

Approximately 30% of kimberlites found in the Wyoming Craton are diamondiferous, although even though many of the remaining 70% yield favorable geochemistry for diamonds, most have not been tested for diamond. Twenty-two in situ diamond deposits have been identified in Wyoming; and 20 diamondiferous kimberlites have been found in Colorado (Hausel, 1998) with one diamondiferous kimberlite described in Montana (Ellsworth, 2000). Diamondiferous host rocks have also been found in the Great Lakes region where as many as 26 kimberlitic and lamprophyric intrusives were discovered in the Michigan-Wisconsin-Illinois region. Eight, or approximately 30% of the kimberlites yielded diamond (Cannon and Mudrey, 1981; Carlson and Floodstrand, 1994). A diamondiferous lamprophyre was also discovered in southeastern Wisconsin (Carlson and Adams, 1997) and a small group of diamondiferous lamproites have been known in Arkansas for nearly 100 years (Hausel, 1995).

 

Diamonds were mined on a small scale at two US localities – one in Arkansas, and the other along the Colorado-Wyoming border. The pipe at Murfreesboro, Arkansas was initially mined in the early 1900s. The host is an olivine lamproite but results indicate the deposit is too low grade (about 10 carats/100 tonnes) to sustain a commercial operation. Along the Colorado-Wyoming border, a small group of kimberlites were mined in 1995-96 and the pipes produced some attractive diamonds, but the grade was also too low (15 carats/100 tonnes) (Coopersmith and others, 2003). Numerous other detrital diamonds have been found scattered throughout the US with very little to no follow-up studies. Many kimberlites, lamproites and lamprophyres have also been described in the US (Hausel, 1998). Only the more interesting deposits in the US will be described and for a cursory examination of all deposits reported in the US, refer to Hausel (1995, 1998).

 

 

 

Alaska may appear to be an unlikely terrain for diamond. Even so, three detrital macrodiamonds found between 1982 and 1986 were recovered from a placer gold operation on Crooked Creek in the Circle mining district northeast of Fairbanks. The Circle district lies near the fragmented northern margin of the North American craton. No kimberlitic indicator minerals were identified with the diamonds suggesting that the gems originated from lamproite or lamprophyre, or from a distal source. A distal source is supported by the percussion marks and fractures in the diamonds suggesting that the stones had a complex alluvial history (Forbes and others, 1987).

 

The area in which the diamonds were found is a Tertiary basin. The basin fill is derived from Late Proterozoic through Late Paleozoic sedimentary and metamorphic rock from the Crazy Mountains to the north and Paleozoic to Precambrian metamorphic and Late Cretaceous granitic plutonic rocks from the Yukon-Tanana region to the south. Some alkalic igneous rocks are also reported to the south although no kimberlites or lamproites have been identified (Forbes and others, 1987).

Diamonds were also described in situ in a diamond-bearing tuffaceous maar near Shulin Lake north of Anchorage. However, this occurrence remains to be verified (Casselman and Harris, 2002). According to reports, in 2002 Golconda Resources Ltd. and Shear Minerals Ltd. announced the recovery of 15 microdiamonds and one macrodiamond from 22 pounds of drill core from their Shulin Lake property. The interval was described as interbedded volcaniclastic and tuffaceous rocks containing olivine and pyroxene (Shear Minerals Press Release, July 8, 2002). Six of eleven holes intersected the tuffaceous horizon. The property is located 45 miles north of Anchorage (Casselman and Harris, 2002).

 

Portions of the Gulf Coastal region of Arkansas and Texas are underlain by an Early to Middle Proterozoic basement considered to have low to moderate favorability for discovery of diamondiferous lamproite, kimberlite, and/or lamprophyre. Within this terrain are some diamondiferous lamproites that includes the Prairie Creek intrusive, an olivine lamproite located along the edge of the Ouachita Mountains uplift. This lamproite was the site of North America's first diamond mine following the discovery of diamonds in 1906 near the mouth of Prairie Creek, 4 2.5 miles southeast of Murfreesboro. The pipe yielded more than 90,000 diamonds including the largest diamond found in the United States (40.42 carats). The property was later incorporated into the Crater of the Diamonds State Park.

 

Diamonds recovered from Prairie Creek include 30% gems: there has been little attempt to recover microdiamonds (Sinkankas, 1959). Some large diamonds recovered from the property include the Uncle Sam (40.42 carats), the Star of Murfreesboro (34.25 carats), the Amarill Starlight (16.37 carats) and the Star of Arkansas (15.24 carats). Most diamonds are white, yellow or brown, and the most common habit is a distorted hexoctahedron with rounded faces (Bolivar, 1984; Kidwell, 1990). The area is characterized by Cretaceous age sedimentary rocks that dip gently to the south (Meyer and others, 1977) and were intruded by the lamproite during Late Cretaceous (106 Ma) (Gogineni and others, 1978). The pipe covers an area of approximately 73 acres and consists of breccia, tuff and hypabyssal olivine lamproite (Miser and Ross, 1922; Bolivar, 1984). Nearly all diamonds have been recovered from breccia facies lamproite, whereas the other magmatic facies are diamond poor. Gogineni and others (1978) report pyrope compositions to be equivalent to G9 calcic-chrome pyropes and Fipke and others (1995) identified only one sub-calcic G10 pyrope from the lamproite. None of the chromite analyses from the pipe yielded favorable geochemistry for diamonds.

 

Five other lamproites have been reported in this region and due to very thick vegetation and a long history of erosion the probability of other undiscovered and hidden lamproites is likely. Other lamproites are found 2 miles north of Prairie Creek and include the Kimberlite, American, Black Lick, Twin Knobs and Twin Knobs 2 intrusives (Krol, 1988; Mike Howard, written communication, 1996). Both the Kimberlite and American lamproites have yielded some diamonds (Miser, 1914; and Miser and Ross, 1922).

Other ultramafic rocks of lamproitic or lamprophyric affinity have been reported a few miles east of Prairie Creek and about 3 miles south of Corinth. Another intrusive of possible interest is the Blue Ball kimberlite 24 miles discovered southwest of Danville (Salpas and others, 1986). Little information is available on this intrusive.

 

State Line district. Diamonds were found in situ in the Colorado-Wyoming region in 1975 in a Wyoming kimberlite (McCallum and Mabarak 1975). Since 1975, essentially every kimberlite in this district has yielded diamond. Even so, many of the kimberlites still have not been bulk sampled and several geophysical anomalies interpreted as blind diatremes remain inexplicably unexplored and untested. Of the bulk samples taken, ore grades range from <0.5 to 135 carats/100 tonnes with 30 to 50% gemstones and more than 130,000 diamonds have been recovered. The kimberlites (Early Cambrian & Early Devonian) extend 3 miles north into Wyoming and about 10 miles south into Colorado (Hausel, 1998).

 

The Kelsey Lake diamond mine was developed along the Colorado-Wyoming border in 1995-96. Commercial production began in 1996 and the Kelsey Lake mill had a capacity of only 25,000 carats/year. The mine was developed on two Kelsey Lake kimberlites (KL1 and KL2) which had been initially mapped as the Schaffer 1, 2, 6, 7, 8, and 9 by Eggler (1967). Years later, additional kimberlites were apparently discovered, and the new discoveries along with the earlier Schaffer pipes were designated as the Kelsey Lake pipes. The kimberlites are irregular-shaped pipes and fissures containing diatreme facies kimberlite with zones of hypabyssal facies and minor crater facies kimberlite. An apparent Devonian age on the Kelsey Lake kimberlites is in agreement with Early Devonian and/or Cambrian isotopic ages for most other pipes found in the Colorado-Wyoming kimberlite province (Coopersmith, 1993, 1997; Hausel, 1998).

 

 

 

The mine yielded many high-quality diamonds >1 carat in weight. Some of the larger stones included 6.2, 9.4, 10.48, 11.85, 14.2, 16.9, 28.18, and 28.3 carat gemstones. One broken fragment was estimated to have fragmented from a larger stone of 80 to 90 carats (Howard Coopersmith, personal communication, 1999). The diamonds have predominantly octahedral habit and are colorless with some honey-brown gemstones (Coopersmith and Schulze, 1996). The 28.18 carat gem was cut resulting in the largest faceted diamond recovered from the US. The finished stone weighed 16.8 carats and had an estimated value of more than $250,000 (Denver Post, September 25, 1997). A 28.3 carat diamond, also recovered from Kelsey Lake, was cut into a 5.39-carat gemstone that sold for $87,000 (Paydirt, 1996).

 

The mine consisted of two open pits only 125 feet deep. The mine ore averaged only about 5 to 15 carats/100 tonnes (Coopersmith and others, 2003) and operations terminated due to legal problems and the property was reclaimed in 2005. It is important to note that the Kelsey Lake kimberlites are not mined out and considerable unmined ore remains on the property. Resources were established at 16.9 million tonnes to a depth of 320 feet (Coopersmith, 1997). Unfortunately, the Kelsey Lake mill was poorly designed and rejected some diamonds to its tailings as well as everything over 40 carats in weight. Thus the possibility that large diamonds were lost during the operation is likely.

 

Most kimberlites in the State Line district surrounding mine show distinct structural control. Thus exploration for additional kimberlites is enhanced by field mapping of structures and trends. All of the kimberlites in the district have been deeply eroded such that diatreme and hypabyssal facies kimberlites are exposed at the surface. This implies that a very large diamond budget was carried downstream during periods of erosion. The probability that diamond placers have been overlooked is very likely. For example, detrital diamonds including a 6-carat gemstone was found in Fish Creek, Wyoming, and smaller diamonds were found in placers south in Colorado. There has been very little to no exploration of placers or paleoplacers.

 

Some of the many unexplored anomalies in the district include:

(1) A group of distinct INPUT geophysical anomalies identified within the Wyoming portion of the district that is interpreted as blind diamond pipes – these have never been drilled. Similar anomalies identified by the same INPUT survey later became part of the Kelsey Lake diamond mine. The southern portion of the district and adjacent areas has not been explored using airborne geophysics and similar blind diatremes are expected!

(2) Some additional kimberlites were recently found in the district (Pearl Creek and Sand Creek) – these remain unsampled.

(3) Some very strong indicator mineral anomalies have also been found in recent years along the eastern edge of the district that indicate the presence of undiscovered kimberlites.

 

Iron Mountain district. A second major kimberlite district lies 45 miles north of Cheyenne near Chugwater, Wyoming (Figure 6). This district, known as Iron Mountain includes the nearby Indian Guide kimberlites. The district includes a very large cluster of kimberlite dikes, sills, blows, structurally controlled depressions and other anomalies in the Sherman granite (1.4 Ga) and the Laramie anorthosite batholith (1.5 Ga). The kimberlites form continuous anatomizing (Early Devonian) dikes with some blows. Portions of the dike complexes were mapped over a strike length of 5 miles prior to the kimberlites disappearing under Phanerozoic and Quaternary sediments at both ends of the complex. This indicates that the complex extends for an unknown distance along both extremities under younger sedimentary rock (Hausel and others, 2000). In addition, there is considerable cover by Quaternary (Tertiary?) boulder conglomerate within the district and kimberlites were mapped to the edge and continue under the conglomerate based on mapping.

 

A group of circular depressions were found in the district that enclose very distinct vegetation anomalies that contain considerable carbonate-rich clays (Hausel and others, 2003). These remain essentially unexplored for diamonds are interpreted as probable, deeply eroded blows. Much kimberlite is hypabyssal with some diatreme facies. A group of kimberlites in the northwestern portion of the district, known as the Indian Guide kimberlites, yielded diamonds from one pipe (the others apparently have not been tested) from diatreme facies. Much of this kimberlite was hidden under Quaternary-Tertiary cover and discovered by Cominco American Incorporated following a kimberlitic indicator mineral trail, with follow-up geophysics, drilling and trenching.

 

Samples collected from every kimberlite in this district yielded diamond stability minerals. In many cases the geochemical signature for the Iron Mountain kimberlites is essentially the same as that for the Kelsey Lake diamond mine. Thus there is significant potential for diamond discoveries in this district. Due to lack of funding, much of the district as well as the region to the north and west remain unmapped and unexplored. With the exception of possibly 1 to 2%, the remainder of kimberlites and similar anomalies remain untested for diamond. All of the kimberlites (with the exception of carbonated breccia along the southern margin of the district that is interpreted as carbonatite) have yielded a distinct sampling of diamond stability indicator minerals (pyrope and chromite) (Hausel and others, 2003). Bulk sample tests in the district have been very minimal with one sample of kimberlite along the northwestern margin of the district yielding a macrodiamond (0.3 carat) and some microdiamonds (Coopersmith and others, 2003).

 

In addition to the Iron Mountain kimberlites, a group of distinct, structurally-controlled, circular depressions are found west of Iron Mountain. Some of these anomalies, referred to as the Indian Guide anomalies, are occupied by shallow ponds. These lie along trend with the Iron Mountain kimberlites and have high probability of being buried kimberlite.

 

Middle Sybille Creek. Northwest of the Iron Mountain and Indian Guide districts is the Middle Sybille Creek district where a single kimberlite blow (Radichal) was found in 1980 (Hausel and others, 1981). The kimberlite is surrounded by more than 4 dozen very strong kimberlitic indicator mineral trails that provide evidence for several hidden kimberlites in this region. One of the strongest anomalies found to date (referred to as the Grant Creek anomaly) lies along Grant Creek at the eastern edge of the district. A few hundred indicator minerals were recovered from stream sediment samples that suggest a proximal source, particularly since no indicator minerals are found a short distance upstream. Immediately west of the anomaly along Albany County Road 12 is limestone (xenolith or floating reef?) in the Laramie anorthosite complex (1.5 Ga). This limestone is either out-of-place or possibly represents a Paleozoic outlier similar to those found in the State Line district in the early 1960s that were later proven to be kimberlite. The origin of the limestone along Grant Creek remains unknown. This is referred to this as the ‘Grant Creek outlier’.

 

Eagle Rock-Happy Jack district. The Eagle Rock-Happy Jack district was discovered by the WSGS during stream sediment sampling between Laramie and Cheyenne. Dozens of kimberlitic indicator minerals were recovered along several drainages indicating a presence of hidden kimberlites. Some of the more interesting anomalies in this region include the Eagle Rock anomaly (a very distinct vegetation anomaly along a lineament), several indicator mineral anomalies in beaver ponds suggesting a possible presence of hidden kimberlites under the ponds, a mantle nodule that was collected by a local prospector near the Eagle Rock anomaly, a widespread fluorite anomaly associated with the Sherman Granite, and the Bowling Pin anomaly (section 14, T14N, R71W) discovered during reconnaissance.

 

The Bowling Pin anomaly is very interesting in that it is a distinct, oval-shaped depression in Sherman Granite. This intermittent lake has relatively steep sides and the soil in the depression is carbonate-rich. The carbonate is so prevalent, that soils react to dilute hydrochloric acid. A group of other shallow ponds (depressions) lie north of the Happy Jack road on a northeasterly trend from the Bowling Pin anomaly. These are all underlain by Sherman Granite.

 

Indicator mineral anomalies (pyrope garnet, chromian diopside, picroilmenite, chromite, and/or diamonds). Between the Iron Mountain and State Line districts, as well as several miles north and along the eastern flank of the Medicine Bow Mountains, more than 300 kimberlitic indicator mineral anomalies were discovered by the WSGS: very few have ever been traced to their source and little to no follow-up has occurred on these due to lack of funds (Hausel and others, 1988). Numerous other kimberlitic indicator mineral anomalies were identified by Cominco American in the same region (Howard Coopersmith, personal communication, 1990).

 

Only a small number of the indicator minerals were tested for geochemistry. Of those tested, a relatively high percentage yielded diamond-stability geochemistry (Hausel and others, 2003) suggesting that the Wyoming Province could be a significant diamond province. With the total disregard of this project by various directors of the WSGS, it is likely that we may never know the actual diamond potential of Wyoming for decades. Potentially, Wyoming encloses several major untapped mineral resources, but none have potential of being a world-class industry comparable to the State’s Coal and Oil and Gas industry as Wyoming’s diamond potential. Much of the State remains unexplored for diamonds including the core of the craton in the vicinity of the Granite Mountains, Wind River Mountains, Owl Creek Mountains and Bighorn Mountains that likely is underlain by a thicker cratonic keel, as well as the Rock Springs uplift that is reported to be underlain by a thicker part of the cratonic keel.

 

Kimberlitic indicator mineral anomalies are widespread and have been identified in the Laramie, Hartville, Sierra Madre and Seminoe Mountains in southeastern Wyoming, in the Greater Green River Basin in southwestern Wyoming, in the Bighorn Basin and the southern Bighorn and Owl Creek Mountains, the Powder River Basin of northern Wyoming, in the Front Range of northern Colorado, in the Uintah Mountains of northeastern Utah and in the Sweet Grass Hills of Montana. The presence of several hundred kimberlitic indicator mineral anomalies along with geophysical and remote sensing anomalies support that the Wyoming Craton has been intruded by major swarms of kimberlitic and related intrusives. Because a large part of the Craton remains unexplored for diamonds, additional discoveries are anticipated in the future.

 

Another very large indicator mineral anomaly was identified in southwestern Wyoming a few decades ago (McCandless and others, 1995). Five diamonds were also apparently found in the early 1980s in a drainage running from the flank of Cedar Mountain along the western edge of this anomaly. Later, a group of 10 lamprophyric mafic to ultramafic breccia pipes and dikes were discovered in this drainage, and many others were also mapped in the region by Amselco. The pipes lie along a 5- to 10-mile-long, northerly-trending lineament in the Bridger Formation (Eocene). Samples recovered from the pipes yielded some diamonds (Hausel and others, 1999) and Guardian Resources later reported the discovery of two additional breccia pipes nearby (Press Release, Guardian Resources, 1997). Several alluvial diamonds were also recovered from a nearby drainage (Guardian Resources Press Release, Sept. 24, 1996). The Cedar Mountain pipes and dikes contain numerous 'kimberlitic indicator' minerals that are geochemically similar to those found in the Bishop Conglomerate and in anthills to the north, many of which are gem quality. The presence of similar minerals in the pipes provides a source rock for a portion of the detrital minerals found in the anthills and Bishop Conglomerate (Oligocene). However, the pipes only account for a small portion of the indicator minerals in this region. Thus, numerous undiscovered pipes in the basin and the nearby Uintah Mountains are necessary to account for such a widespread anomaly.

 

Leucite Hills lamproites. The largest lamproite field in North America lies northeast of Cedar Mountain, and north of the towns of Superior and Rocks Springs. Twenty-two lamproites were mapped in this area but the field remains unexplored for diamonds even though numerous gem-quality olivines (peridot) were found in this area and two diamond-stability chromites were recovered from two lamproites in the northeastern part of the field (Hausel, 2006). The possibility of hidden, diamondiferous olivine lamproite in the Leucite Hills needs to be considered.

 

 

    

 

Many other strong and distinct anomalies have been identified Wyoming including kimberlitic indicator mineral anomalies in the Seminoe Mountains and the Bighorn Basin, and many vegetation anomalies along with diamonds and indicator minerals in the Medicine Bow Mountains. One very interesting anomaly identified by the WSGS several years ago is a Tertiary-Quaternary conglomerate along the north flank of the Seminoe Mountains. This conglomerate contains occasional pebbles of tawny-colored banded iron formation typical of that found at the western edge of the Seminoe Mountains greenstone belt (Hausel, 1993). Panned samples of the dry conglomerate in the flats near the Miracle Mile yielded gold as well as pyrope garnet. Pyrope garnets from both sides of the North Platte river yielded a small number of lilac to purplish garnets that were tested for geochemistry. All analyses from this area have yielded diamond-stability geochemistry typical of sub-calcic chrome pyropes (G10). This suggests that the possibility of finding diamonds in this region is very high.

 

Detrital diamonds have been found in Montana in the northern portion of the Wyoming craton, along with numerous potential host rocks (alnöite, peridotite, monchiquite, lamproite and kimberlite) (Figure 5). Several potential hosts are found within the central alkalic province in eastern Montana, and a few lamproites are reported in western Montana, including the Ruby Slipper (Pete Ellsworth, personal communication, 1996). Two diamonds were found in gravels of the Etzikom Coulee in the Milk River drainage north of the Sweet Grass Hills in northern Montana. The diamonds weighed 0.14 and 0.17 carats (Lopez, 1995). The occurrence lies near a buried magnetic anomaly aligned with presumed kimberlitic rocks in Alberta.

An extensive field of lamproites, lamprophyres and kimberlites occur in eastern Montana. Some of these have trace amounts of indicator minerals (Fipke and others, 1995) that suggest a sampling of the diamond stability field. Within this field are some interesting targets including a belt of ultramafic lamprophyre and kimberlite diatremes within the Grassrange Field in east-central Montana. The area was highly recommended (Hausel, personal field notes, 1994) as having high potential for diamonds. A few years following this recommendation, the Homestead kimberlite was discovered within this field and proved to be diamondiferous (Ellsworth, 2000). The Homestead kimberlite sits near an extensive breccia pipe known as Yellow Water Butte. This breccia consists of massive to brecciated olivine-phlogopite-diopside-carbonate lamprophyre with massive hypabyssal olivine lamprophyre facies (Doden, 1996). The breccia is considered as a potential host (Hausel, personal field notes, 1994).

 

Hypabyssal facies kimberlites are also found near Landusky north of the Grassrange Field. These include four closely-spaced diatremes in the eastern part of an east-northeasterly trending swarm of ultramafic alkalic diatremes, dikes and plugs (46 to 51 Ma) in the Missouri Breaks area of north-central Montana that are referred to as the Williams kimberlites. Available analyses of peridotitic garnets from the kimberlites indicate compositions equivalent to G-9 (Hearn and McGee, 1983). However, P-T estimates from co-existing orthopyroxene-clinopyroxene pairs in some of the peridotite nodules indicate some nodules may have originated from the diamond stability field (Fred Barnard, written communication, 1994).

 

Many diamonds were found in the past during gold placer mining in California, Oregon and Washington. The source of the diamonds remains unknown, although the discovery of diatremes with diamond-stability minerals suggest a potential source for many of the diamonds. Some historical hydraulic gold placer mines north of Oroville, California in the Round Mountain area downstream from the diatremes yielded diamonds as a by-product of gold mining between 1853 and 1918. About 400 diamonds and 600,000 ounces of gold were recovered from the operations on the Feather River (Hill, 1972). Kunz (1885) also reported diamonds were found in all of the northern counties of California drained by the Trinity River in the vicinity of Coos Bay, Oregon; and on the banks of the Smith River of Del Norte County, California. Five diamonds were also recovered from a tributary of the South Fork of the Trinity River known as Hayfork Creek. One found in 1987 weighed 32.99 carats (Kopf and others, 1990). Countless numbers of small diamonds have also been reported in the black sands of the Trinity River. Sinkankas (1959) reported that microdiamonds were found in the black sands of the Trinity River near its junction with the Klamath River. Pyrope garnet and chromian diopside were also described from the Trinity River (Kopf and others, 1990) and chromian diopside-bearing serpentinites were also later identified in this area (Hausel, personal field notes 1995).

 

The presence of an active Benioff zone along the coastal region provides a mechanism over-pressurized magmas erupted at depth from the Benioff zone. Such breccia pipes have been found at Leek Springs and at an undisclosed locality in the Sierra Nevada of California. Both pipes contain some diamond-stability minerals and the possibility of other breccia pipes in this region needs to be considered.

 

The Leek Springs diatreme was described in the Northern Miner (January 29, 1996). Diadem Resources reported a discovery of a cluster of dikes including a 1,875 by 188 m (6000 x 600-ft-wide) ‘dike’ after following an indicator mineral trail upstream from a historic diamond placer at Leek Springs. Drill cuttings from 120 feet of what was described as a lamproite yielded 235 diamond fragments (Northern Miner, May 20, 1996). At another breccia pipe at an undisclosed location, the diatreme has clasts of serpentinite along with a large number of indicator minerals typical of mantle peridotite. The geochemistry of the indicator minerals include both diamond-stability and graphite-stability minerals (Hausel, personal field notes, 2003). The pipe has not been tested for diamond.

 

A group of kimberlites are reported in the Michigan-Illinois area in the Great Lakes region of the US. Parts of the Great Lakes region are underlain by the Superior Province, which is an Archean craton underlying much of Minnesota and eastern South and North Dakota that continues north into Canada. The Superior Province is bounded on the west by the Trans-Hudson Orogen (which separates the Superior Province from the Wyoming Province) and a Proton of Early to Middle Proterozoic basement rocks to the east and south suggesting that this region has moderate potential for discovery of diamond deposits.

 

The Early to Middle Proterozoic basement along the eastern and southern margin of the Superior Archon underlies much of the Great Lakes region. This basement is bounded by Late Proterozoic rocks of the Grenville Tecton further to the east. The Grenville Tecton extends into eastern Michigan and Indiana.

 

Several diamonds (including some fairly sizable stones) were recovered from the Great Lakes region (Hausel, 1995, 1998). Historically, these were thought to have been transported from Canada by continental glaciers during the last ice age. This assumption is questioned particularly since the discovery of several post-Ordovician kimberlites in Michigan. A few dozen kimberlites and lamprophyres have also been described within the Superior Craton in Michigan, Wisconsin and Illinois. Eight kimberlites in the Michigan area yielded diamond (Cannon and Mudrey, 1981; Carlson and Floodstrand, 1994) and a diamondiferous ultramafics lamprophyre diatreme was also discovered in southeastern Wisconsin (Carlson and Adams, 1997). At least 26 kimberlites have been found in Michigan, Wisconsin and northern Illinois. In addition, eleven magnetic anomalies were detected that are suggestive of buried diatremes. Michigan also hosts Paleozoic outliers that are completely surrounded by Proterozoic rocks. These are interpreted as cryptovolcanic structures potentially related to kimberlite.

 

One diamondiferous kimberlite near Crystal Falls, Michigan, lies one mile west of Lake Ellen near the Wisconsin border. The intrusive, known as the Lake Ellen kimberlite, is poorly exposed but yields a strong positive magnetic anomaly circular in plan that suggests the presence of a 650 to 950 feet diameter kimberlite with a surface area of about 20 acres. The kimberlite was emplaced in Proterozoic age volcanic rocks and contains abundant Ordovician(?) dolomite xenoliths. Diatreme facies kimberlite at Lake Ellen is described as containing olivine, pyroxene, mica, pyrope and magnesian ilmenite in a fine-grained serpentine matrix (Cannon and Mudrey, 1981). Another kimberlite, known as the Michgamme kimberlite, lies a short distance northwest of the Lake Ellen intrusive along the Michgamme Reservoir shoreline (Carlson and Floodstrand, 1994).

 

Northwestern Wisconsin is underlain by basement rocks of the Superior Province while Proterozoic age rocks underlie much of the remainder of the state. Since 1876, 25 diamonds were found in southern and central Wisconsin. All of the diamonds were found in Pleistocene glacial deposits or Holocene river gravel and a diamondiferous ultramafic lamprophyre (described as melnoite) known as the Six-Pak diatreme, was discovered by Ashton Exploration with airborne magnetics. The diatreme was drilled and has an area of approximately 50 acres. The pipe consists of hypabyssal facies lamprophyre with a typical kimberlitic mineral suite including calcic pyrope garnet (G9). Several small diamonds were recovered from the intrusive, which lies in the outskirts of Kenosha in southeastern Wisconsin (Carson and Adams, 1997). Many other kimberlites, lamproites and lamprophyres have been identified in the US. The reader is referred to Hausel (1995, 1995a, 1998) for information on these.

 

Canada.

Canada is undergoing a major evolution – an evolution that is dynamically changing its economy. Many kimberlites and other potential host rocks have been identified over large regions of the North American Craton in Canada. Most of the discoveries have been made since the early 1990s and the number of discoveries and the variety of host rocks as well as the incredible investment of money in exploration in Canada will change fundamental concepts on diamond deposits and their genesis. The entire scenario will also result in one of the greatest economic evolutions in history. Within a very short period, Canada became the world’s number three producer of gem-quality diamonds. Prior to 1998, Canada had not commercially produced a single natural diamond. Today, only Namibia and Russia out-pace Canada. But within the next decade, Canada is expected to become the number two diamond producer in the world and may even surpass Namibia within the foreseeable future.

 

The number of diamond discoveries in Canada since 1990 is remarkable. The discoveries span the North American craton from one end of Canada to the other, from the west to the east, and from the north to the south. One highly significant characteristic of these discoveries is that they essentially end at the border with the United States. This anomaly, however, is a political anomaly, not a geological anomaly.

 

Considerable exploration in Alberta has resulted in the discovery of several kimberlites, lamprophyres along with widespread kimberlitic indicator mineral anomalies and some magnetic anomalies. Both magnetics and electromagnetics have proven invaluable in the search for hidden kimberlite. Widespread kimberlitic indicator mineral anomalies have been identified throughout much of Alberta. The Alberta Geological Survey reports widespread anomalies from the Canadian-Montana border extending all the way northward to the Northwest Territories, with extensive anomalies in the central portion of the Province (Alberta Geological Survey, 2004). The data suggests that several kimberlite and related host rock fields remain hidden.

 

A group of kimberlites and related host rocks were discovered along a northeasterly trend paralleling two major shear structures near the north-central portion of the province. These deposits occur at (1) Mountain Lake northeast of Grande Prairie, (2) Buffalo (Head) Hills northeast of Mountain Lake and (3) in the Birch Mountains further to the northeast and located southwest of Lake Athabasca. Many of the kimberlites in northern Alberta yield 70 to 85 Ma ages (Simandl and others, 2005).

 

(1) The Mountain Lake diatremes are lamprophyres and include two pipes along the southwestern extent of the kimberlite-lamprophyre trend of central Alberta. The Mountain Diatreme was discovered in 1973 and was initially interpreted as kimberlite. However, recent analyses suggest the diatreme is a hybrid that exhibits geochemical affinities for basanite (olivine potassic basalt), olivine minette, alnöite and melilitite. Compared to the Buffalo Hills and Birch Mountains kimberlites, the Mountain Lake diatreme has higher SiO2, Al2O3, Na2O, K2O, Na2O/K2O, Ga, Rb and peralkalinity index, and lower MgO, Nb, LREE, and Sr. This chemistry implies that the rock should be classified as high potassic, alkali, ultramafic intrusive (Eccles, 2002).

 

In 1983, a sample of the lamprophyre collected by Superior Minerals Company yielded two microdiamonds from a 77-pound sample. Superior Minerals also recovered 8 or 9 microdiamonds from crater facies outcrops at the surface (Casselman and Harris, 2002).Two pipes are known in this area but the principal interest has been for the Mountain Lake diatreme. This diatreme is located 118 miles southwest of Norman Wells, Northwest Territories in the Mackenzie Fold Belt. It is a 1,970 foot diameter pipe that intrudes Upper Cambrian to Middle Ordovician limestone of the Rocky Mountain Assemblage. The diatreme has xenocrysts of picroilmenite, pyrope and chrome-diopside as well as microdiamonds very similar to the DK pipes at Cedar Mountain in the Green River Basin of the Wyoming Province. The Mountain diatreme has a central core of dark green, mainly autolithic breccia with lesser xenoliths of country rock. The matrix is composed of chlorite, phlogopite and carbonate with minor serpentine, tremolite and opaque minerals. K-Ar dating of phlogopite returned a date of 445 Ma for the intrusive.

 

The Buffalo Hills cluster to the northeast of the Mountain Lake diatremes are of current interest due to the discovery of 36 diamondiferous pipes within a cluster of 38 kimberlites. Three (K14, K91 and K252) have yielded bulk sample tests of >11 carats/100 tonnes. The K252 kimberlite yielded an initial test grade of 55 carats/100 tonnes and is of potential economic interest (Alberta Geological Survey, 2004; Cummings, 2006). The Buffalo Head Hills are underlain by an Early Proterozoic crystalline basement with possibly some Archean rock of the Buffalo Head Craton. Deep-seated, penetrative structures such as the Peace River Arch could have provided favorable access for kimberlitic magmas. The regional setting was favorable for emplacement of kimberlite during periodic tectonic activity associated with movement along the Peace River Arch (Alberta Geological Survey, 2004; Cummings, 2006).

 

Samples collected in the region yielded significant numbers of indicator minerals including olivine, pyrope garnet, chromite and picroilmenite. Some of these samples were collected well north of the northernmost known Buffalo Hills kimberlite indicating a strong likelihood that undiscovered kimberlites lie to the north. In addition, some geophysical anomalies within the cluster are characteristic of hidden kimberlite. At least three distinctive volcaniclastic units are recognized in the Buffalo Hills kimberlites, two of which are primary pyroclastic deposits that are not normally preserved in most kimberlites. The pipes are distributed over 2,300 mi2 and are unusual in that they intrude Proterozoic Buffalo Head Terrain rather than Archean basement, very similar to the State Line, Iron Mountain, Indian Guide, Middle Sybille Creek, Estes Park and Boulder kimberlites in Wyoming and Colorado. The kimberlites erupted through Proterozoic basement, Devonian sedimentary and Cretaceous sedimentary rock, but were covered by Quaternary till (Boyer and others, 2005).

 

Volcaniclastic crater facies kimberlite was found in the district indicating very little erosion has occurred since emplacement of the diatremes. The crater facies includes well-sorted, ash-sized, fine-grained, olivine-rich layers interbedded with lapilli-sized, fragment-rich layers. Cross-stratified and finely bedded deposits are found that are similar to those formed by basal surge and pyroclastic ash fall. Some accretionary fragments with multiple magmatic rinds thought to have formed during a series of eruptions is typical of proximal crater-fill and pyroclastic ash falls and some poorly-sorted, subtly bedded, crystal-rich kimberlite also occurs that is depleted in fine-grained matrix material (Boyer and others, 2005).

 

The Buffalo Head Hills and Birch Mountain diatremes (to the northeast) are chemically similar to Group I kimberlites. Of the two, the Buffalo Hills kimberlites have the highest MgO, Cr, and Ni, the lowest Al2O3, SiO2, V, Y, Pb, Sr and Ga values, and have geochemical signatures similar to primitive kimberlite in the Northwest Territories. In addition, a high proportion of the Buffalo Head Hills kimberlites are diamondiferous (Eccles, 2002).

 

Diamonds recovered from the K11, K91 and K252 kimberlites in the Buffalo Hills cluster are mainly colorless and transparent: the majority have resorbed octahedral habit. The garnet, olivine and pyroxene inclusions indicate a presence of both eclogitic and peridotitic diamonds and the data suggest that the lithospheric mantle beneath Buffalo Hills is dominated by an eclogitic component, similar to many of the younger diamond-bearing areas around the world. The presence of rare majoritic garnet inclusions in some diamonds suggests that some diamonds were formed at a much deeper mantle source (Eccles, 2002).

 

At least nine kimberlites have been identified in the Birch Hills cluster to the northeast of the Buffalo Head Hills: two of which have yielded diamonds. The Birch Mountains kimberlites are more evolved than the Buffalo Hills kimberlites and have lower SiO2, Ni and MgO and higher Fe2O3, TiO2, Nb, V, Sc, Zr, Hf, Y, Ba, Rb, LREE, Ga and Pb. Hence, whole-rock geochemistry of the Birch Mountains kimberlites is similar to Group IB South African kimberlite. One of the Birch Mountains kimberlite, known as the Legend, is a 1,640 to 2,625 foot diameter multiphase kimberlite. The Legend Kimberlite lies beneath 42 feet of overburden, and was tested by a drill hole that produced 4 microdiamonds from an 896-pound sample (Eccles, 2002).

 

British Columbia. Much of British Columbia is underlain by rocks considered as unconventional terrain for primary diamond deposits. However, recent studies in the extreme northeastern portion of the province suggest that part of that region is underlain by a structurally disturbed fragment of the North American Craton. Even though much of British Columbia is considered unfavorable to host in situ diamond deposits based on traditional diamond exploration concepts, diamonds have been recovered from a group of breccia pipes, many with the classical kimberlitic indicator minerals. These intrude the Cordilleran belt forming a NNW-trend.

 

Lithologies of the host rock for the breccia pipes, or diatremes, include alkalic basalts, and alkalic and ultramafics lamprophyres. Only a few true kimberlites are reported in British Columbia. Microdiamonds have been recovered from several of the pipes.

 

Some intrusives and many anomalies lie near the Alberta border just north of Montana and others are found further to the north in northern British Columbia. The known kimberlites and lamprophyres have yielded age dates of 391 to 410 Ma for the HP pipe to 240 Ma for the Cross diatreme (Simandl and others, 2005). Of 58 samples of alluvium, regolith and bedrock collected in extreme northeastern British Columbia in the Etsho plateau near Ft. Nelson and Dawson Creek, 38 contained kimberlitic indicator minerals supporting the likely presence of hidden pipes in that region. Some indicator minerals yielded diamond-stability geochemistry and one contained a microdiamond.

 

Nearly all of diatremes lie along a north-south 54 by 12 mile trend within the Rocky Mountains Uplift (Figure 9). This region is remote and very rugged suggesting that other discoveries will likely be made with continued exploration (Roberts and others, 1980; Grieve, undated). Many of the diatremes were emplaced in Cambrian to Permian carbonate and clastic sedimentary rocks of the Foreland and Intermontane Belts near the west coast of Canada (Simandl, 2003). This area is characterized by thrusts and associated folding. All of the diatremes were emplaced in Middle Devonian and older strata while the Cross diatreme in the Elkford cluster in the southeastern corner of the province was emplaced in Permian rocks. The terrain is not what would be anticipated for primary conventional diamond models that require cool, stabilized, cratonic cores (Archons) with thick keels. Instead, this region is geologically unstable and has been subjected to considerable deformation with displaced and accreted terrains.

 

Kechika Group. The Kechika River group includes the northernmost diatreme in British Columbia. One breccia pipe in this group, known as the Xeno, lies at the northern end of Dall Lake in the Kechika Range of the Cassiar Mountains. The property was originally acquired for rare earths associated with a mafic alkalic igneous complex underlain by quartzite of the Lower Cambrian Atan Group, chlorite-sericite schist, phyllite, marble and dolomite of the Cambrian-Ordovician Kechika Group, and by siliceous tuff, chert, sandstone and argillite of the Ordovician to Silurian Sandpile Group. The rare earths are hosted by alkalic igneous complex that forms a west-northwest trending belt of cogenetic syenites, trachytic volcanics and carbonatites that have been traced for 12.5 miles along strike and is a few hundred feet to a few miles wide. At the southern end of this belt, a diatreme breccia was discovered with a variety of igneous and sedimentary (mainly quartzite and carbonate) xenoliths and chrome-spinel xenocrysts in a pale green, carbonate-rich tuffaceous matrix. Exploration in 2002 identified a nearby lamprophyre dike that varies in width from a few to over 160 feet exposed intermittently along a 1.6 mile strike length. A 70 pound sample collected from the surface exposure of the dike yielded a transparent, green microdiamond (0.38 x 0.30 x 0.25 mm). The Kechika River diatreme within the Kechika Range lies west of the Rocky Mountain trench has geochemically affinity for alkalic lamprophyre (Ijewliw and Pell, 1996).

 

The Ospika pipe to the south of the Kechika River (north of Mackenzie) is complex breccia with at least 5 intrusive events. The breccias contain xenoliths, cognate nodules and phlogopite, titaniferous augite, rare altered olivine and bright green diopside in aphanitic carbonate matrix. The pipe is classified as an ultramafic lamprophyre (aillikite) based on petrography and whole rock geochemistry (Ijewliw and Pell, 1996).

 

Golden Field. In the Golden field located much further south includes diatreme breccias and dikes identified at five localities. These include the Bush River, Mons Creek, Valenciennes River, Lens Mountain and Campbell. The Bush River breccia and dikes have been classified as olivine kersantites (calc-alkalic lamprophyres) based on mineralogy, although geochemically have affinities for more alkaline chemistry. Diatremes and dikes in the Mons Creek and Valenciennes River are altered and have pseudomorphs of serpentine after olivine, clinopyroxene, biotite and plagioclase and are classified as camptonites (alkalic lamprophyres). The Lens Creek diatreme may be lamproitic, and the HP pipe south of the Campbell Ice field consists of limestone clasts, quartzite, plutonic rock clasts with autoliths, megacrysts and phenocrysts of clinopyroxene (chrome diopside), melanite garnet, biotite spinel and apatite in a groundmass of calcite, chlorite, serpentine, talc and pyrite and is classified as aillikite (Ijewliw and Pell, 1996).

 

Bull-Elk Creek Field. Another 40+ breccia pipes and dikes are found south-southeast of the Golden Field near Cranbrook in what is referred to as the Bull-Elk Creek field. These are primarily tuffaceous intrusives with vesicular lapilli, clinopyroxene, olivine, calcite, and spinel in a groundmass of carbonate, chlorite, talc and minor plagioclase.

 

Summer diatremes. The Summer diatremes 24 miles northeast of Cranbrook, lie near the intersection of Galbraith and Summer Creeks. One known as the Quinn diatreme lies near the head of a tributary of Quinn Creek, 36 miles northeast of Cranbrook. This diatreme intrudes Ordovician-Silurian (Beaverfoot-Brisco Formation) carbonates and has a gray-green matrix with small clasts and phenocrysts of olivine and spinel (<5 mm). In thin section, angular quartz and feldspar, volcanic fragments, carbonate, argillaceous material and serpentine occur in a carbonatized groundmass. Xenoliths in the breccia include well-rounded limestone, argillite, quartzite, granite and rare ultrabasics (Grieve, undated).

 

Elkford kimberlites. One pipe in this region has been known for a few decades that is known as the Cross diatreme, located to the east near Elkford. The Cross was initially reported in 1957 on the north side of Crossing Creek valley. The diatreme (595 Ma; reported as 240 Ma by Simandl and others, 2005) covers a surface area of 225 by 190 feet and is composed of intrusive breccia in a shear zone in Permian Rocky Mountain Group shales, limestones and cherts. No thermal metamorphism is visible along the contact between the sedimentary rocks and breccia pipe and the breccia matrix is a bluish-green, calcareous groundmass that encloses phenocrysts and megacrysts of phlogopite, altered olivine, hematite, calcite, chromian diopside and reddish-brown pyrope-almandine garnet with rounded to subangular xenoliths of limestone, argillite, serpentinite and peridotite. Nearly two decades (1976) after the discovery of the breccia, reconnaissance exploration was not initiated until after the Cross diatreme was described as kimberlite. Geochemical analyses support that it is a kimberlite – four other kimberlites were apparently found in the region (Ijewliw and Pell, 1996).

 

According to (Simandl, 2003), diamonds were recovered from samples from the Jack (Lens Mountain) and Mark (Valenciennes River) diatremes and a microdiamond from a breccia in a carbonatite complex in the Kechika area to the north. Macrodiamonds have also been extracted from the Cranbrook cluster, the Bonus and the Ram 5 and 6 diatremes. The Ram 6 is located north of Elkford and reported to be diamondiferous and possibly kimberlitic (Allan, 1999).

 

Portions of the field are underlain by blueschist and eclogite facies rocks interpreted as subducted-related. Some diatremes are weakly diamondiferous possibly from sampling material from a paleo-subduction zone. The breccia matrix or magma type for the British Columbia breccia pipes is not well defined. The majority of the diatremes in British Columbia are ultramafic lamprophyres while the Cross is kimberlitic (Grieve, Undated). Even so, a large number of the lamprophyres have yielded diamonds.

 

At least four areas in northern Labrador within the Nain Province (Archean) have been identified with rocks of apparent kimberlitic to lamprophyric affinity. These include (1) Capes Aillik-Makkovik dikes and pipe, (2) the Ford’s Bight diatreme, (3) the Saglek dikes and pipes, and (4) the Torngat Mountains dikes. Group 1, 2, and 3 rocks are described as kimberlites, and number 4 includes both kimberlites and ultramafic lamprophyres (melilitites and or aillikites). The Torngat dikes are affiliated with the Abloviak shear zone and several are diamondiferous (Wilton and others, 2002).

 

Exploration in Manitoba resulted in the discovery of the Wekusko kimberlite dike north of Lake Winnipeg. Research by the Canadian Geological Survey resulted in the identification of numerous indicator mineral trains in the Gods Lake-Knee Lake area near the Snow Lake-Flin Flon area. The indicator mineral trains suggest the presence of several hidden, mantle-derived pipes in eastern Manitoba and has led to considerable exploration in recent years.

 

Diamond deposits have been discovered at numerous locations in the Canadian far north. Some of the more important are:

(1) Lac De Gras (NWT & Nunavut),

(2) Thirsty Lake (Nunavut)

(3) Parry Peninsula (Darnley Bay) (NWT),

(4) Victoria Island cluster (NWT- Nunavut),

(5) Somerset Island cluster (Nunavut),

(6) Rankin Inlet cluster (Nunavut)

(7) The Melville Peninsula (Nunavut),

(8) Baffin Island (Nunavut),

(9) Dry Bones Bay (NWT)

(10) Coronation Bay (Nunavut)

 

(1) Lac De Gras region. A major kimberlite district was discovered within the Slave Province northeast of Yellowknife in the Northwest Territories in the early 1990s. Several commercial diamond pipes and a commercial sill have been identified in this region that include the Ekati group, Diavik group, Snap Lake, Gahcho Kue (formerly Kennady Lake) and Jericho (Figure 10). There are a number of other kimberlites in this region such as those at Carp Lake, Hardy Lake and others, but only the commercial deposits are described.

 

One of the great exploration success stories in history was the discovery of diamonds in the Canadian Northwest Territories, which sparked the largest claim staking rush in history (Krajick, 2000). Within a few years, capitalization of the Ekati mine resulted in the first Canadian diamond mine which began production in 1998. Since the mine initiated operations, other commercial properties have been identified that include the Snap Lake dike and the Diavik pipes. A fourth commercial diamond prospect in Canada, Jericho, is located in Nunavut 100 miles north of Ekati and 250 miles NNE of Yellowknife. Of these four commercial operations Ekati is by far the largest operation. Most kimberlites in the Northwest Territories were emplaced at 45 to 75 Ma (Simandl and others, 2005).

 

Kimberlites at Ekati are located nearly 180 miles NE of the town of Yellowknife. Several pipes were discovered lying under a group of shallow lakes in the Lac de Gras region in the early 1990s. A short time following the discovery, Canada’s first diamond mine was commissioned by BHP in late 1998. This is a world-class mine as the property includes a cluster of 121 kimberlite intrusives (52-65 Ma). To date, commercial mineralization has been identified and reserves established at the Fox, Leslie, Misery, Koala, Koala North, Panda, Beartooth, Sable and Pigeon kimberlite pipes on the Ekati property: other kimberlites on the property are being evaluated and the mine has an anticipated minimum life of 25 years.

 

In 2001, three years after the mine opened, the Ekati produced 3.7 million carats. In 2003, production increased to 6.96 million carats (EMJ, 2004). Open pit operations on the Panda pipe reached maximum economic depth in 2003, five years after mining was initiated. Declining production from the Panda open pit has been replaced by production from the nearby Misery and Koala open pits. The Panda mine life will be extended by underground mining, thus the kimberlite is being developed for sublevel retreat mining. Underground mining was previously initiated at the adjacent Koala North pipe in 2002. The Panda underground mine is expected to produce 4.7 million carats over an operating period of 6 years: production was scheduled in 2005 followed by full production in 2006. Ekati production for the first quarter of 2004 totaled 1.27 million carats, which was a 40% decline from the previous quarter. For the first 9 months of fiscal year 2004, the Ekati mine produced more than 5.3 million carats.

 

On June 30th, 2003, it was reported that the Ekati mine had 47.7 million tonnes of ore reserves averaging 80 caras/100 tonnes (36.6 million carats of recoverable diamonds) based on a 2 mm cutoff size (the typical cutoff size that distinguishes macrodiamonds from microdiamonds is 2 mm, although some companies use a 1 mm diameter cutoff size). Measured, indicated, and inferred kimberlite resources stood at 127.9 million tonnes of ore with an estimated 171.2 million carats (Robertson, 2004)! As exploration continues on the property, reserves will increase (Hausel, 2006).

 

The Panda kimberlite is small, steeply-dipping carat-shaped pipe that is 640 feet across and roughly circular in plan covering a surface area of only 7.4 acres. The fault-controlled pipe is slightly asymmetrical in vertical section. Panda has been delineated to depths of 1,800 feet where it tapers to a narrow 64 foot wide ‘blow’. The structure is filled with a complex mixture of volcaniclastic kimberlite containing variably carbonized wood fragments and mudstone that are locally abundant in bedded material at depth. Primary diatreme-facies kimberlite is present in the lower portions of Panda (>1,150 feet below surface) and minor intrusions of hypabyssal-facies kimberlite occur as occasional narrow peripheral dikes (McElroy and others, 2003).

 

The Misery Main pipe is an even smaller intrusive of only 3.7 acres. It is elongated and steep-sided with dimensions of 295 by 574 feet at the surface. According to Mustafa and others (2003), the pipe transects a contact zone between Archean granite and metagreywacke with its location corresponding to the intersection of this contact with a narrow, north-south trending shear. The Misery Main pipe ranges from ash- to mud-rich phases to coarse-grained, olivine-rich volcaniclastic kimberlite. In places, fine-scale bedding is defined by variations in the abundance and grain size of olivine. Numerous other kimberlite intrusives occur in the immediate vicinity of the Misery Main kimberlite. The Misery Main pipe is largest in this local cluster while the others include narrow hypabyssal dikes that radiate out from the pipe, pipe-like hypabyssal intrusions, and small pipes with diatreme-facies kimberlite (Mustafa and others, 2003).

 

Diavik mine. Production at the Diavik mine began in 2003. The Diavik pipes are located in the Lac de Gras region of the Slave Province west of Ekati and are operated by Diavik Diamond Mines: a joint venture between Rio Tinto (60%) and Aber Diamond Mines (40%). Rio Tinto assumed responsibility from their subsidiary Kennecott Canada Exploration. Fifty-five kimberlites occur on the Diavik property of which 25 are diamondiferous and at least four are commercially mineralized. The mine is estimated to contain 138 million carats in four kimberlites (A154S, A154N, A418, A21). Of these, the A154S kimberlite is one of the richest in the world with a reserve of 11.7 million carats at an average ore grade of 520 carats/100 tonnes.

 

Sample of kimberlite from the Victor pipe, Canada.

 

Operations are currently focusing on the A154S and A154N, with production scheduled to reach 6 to 8 million carats/year. The mine has resources to sustain an operation for 16 to 22 years. In 2004, the mine produced 7.6 million carats including some large stones that weighed up to 151 carats. The property lies on a 7.7 mi2 island known as East Island located 180 miles NE of Yellowknife. The Diavik kimberlites (55 Ma) intrude the Precambrian Slave basement complex (2.5 to 2.7 Ga). Several lie beneath lakes. Capitalization costs to initially open the mine were on the order of $1.3 billion, but the mine made up for high capitalization by surpassing the 20,000,000 carat production mark at the end of 2005!

 

Snap Lake. This mine is scheduled to begin diamond recovery from a kimberlite sill located 60 miles SSE of Ekati and 130 miles NE of Yellowknife. The sill dips under adjacent Snap Lake and has a strike length of 2 miles with a dip of 15o and dip length of at least 1.9 miles. DeBeers began mine construction in 2006 and anticipates full production in 2008. The ore will be mined entirely underground from rock estimated to contain 38.8 million carats in 22.8 million tonnes (146 carats/100 tonnes) (Robertson, 2004). The mine life is anticipated for 22 years. Snap Lake is one of three properties under developed by DeBeers. The other two are Gahcho Kue east of Snap Lake and Victor in the James Bay Lowlands of northern Ontario.

 

Gahcho Kue (formerly Kennady Lake). The Gahcho Kue property lies south of Lac de Gras, 50 miles SE of Snap Lake near Ft. Defiance and 186 miles NE of Yellowknife. At least 8 diamondiferous kimberlites are on this property including sills and dikes. Inferred and indicated resources of three pipes are 31.4 million carats averaging 148 carats/100 tonnes and ore reserves are reported to average 167 carats/100 tonnes. Estimates suggest a potential tonnage of at least 20 million tonnes of ore occur in the 5034, Hearne and Tuzo pipes. Gahcho Kue is currently being explored by joint venture (Mountain Lake Resources and DeBeers). Development is expected to take 3 years (EMJ, 2004). The mine is anticipated to have a life of 15 years and to produce 3 million carats annually when in full production.

 

Jericho mine. The Jericho mine includes six diamondiferous kimberlites. During bulk sampling of one of the pipes by Tahera Exploration, a decline was driven to obtain a 9,435 tonne bulk sample which yielded 10,539 carats at a cutoff size of 1 mm. The stones included 44 diamonds that weighed between 5 and 10 carats and 23 stones larger than 10 carats. The largest weighed 40 carats. Production at Jericho began in January of 2006. The mine lies south of Carat Lake within the Nunavut Territory 260 miles NE of Yellowknife, about 106 miles north of the Ekati mine near Echo Bay’s Lupin gold mine. The principal Jericho kimberlite (172 Ma) is a multiphase intrusive measuring 960 by 120 feet found on dry land. The pipe has an indicated and inferred resource of 7.1 million tonnes averaging 84 carats/100 tonnes with an estimated resource of 6 million carats that will be produced over 9 years. Reserves of 2.6 million tonnes of ore averaging 120 carats/ 100 tonnes have been established (EMJ, 2004).

 

About 8 miles west of the Jericho property is the Muskox pipe. This kimberlite has twice the surface area as the Jericho pipe and may considerably extend the life of the Jericho mine. Bulk samples from the Muskox pipe have yielded grades from 26 carats/100 tonnes to 142 carats/100 tonnes (Northern Miner, 2006, v. 92, no. 6., p 1-2).

 

(2) Thirsty Lake lamprophyre. The Thirsty Lake dike is part of the Akluilak dike system in the central Churchill Province of the Northwest Territories along the northwestern margin of Hudson Bay. The dike system lies west of the Rankin Inlet kimberlites. Kaminsky and others (1998) interpreted the dike as a metamorphosed minette (metaminette). Similar to some ultrahigh pressure metamorphosed diamond deposits in the Kazakhstan region, this deposit is very rich in microdiamonds (Hausel, 1996; Erlich and Hausel, 2002). A small 44 pound sample collected from the dike yielded >1,700 diamonds! The sample lacked macrodiamonds (MacRae and others, 1995).

 

The Thirsty Lake North and South dikes comprise a zone with a strike length of over 9 miles. Diamonds recovered from the property are strongly colored yellow and brown diamonds with cubo-octahedral and cubic forms and lesser dodecahedral habits. Nitrogen aggregations of the diamonds are comparable to those of the Kokchetav massif diamonds in Kazakhstan (Chinn and others, 2000). The Kokchetav diamonds are believed to have formed during an ultrahigh metamorphic event (DeCorte and others, 1998; Erlich and Hausel, 2002). However, such an event has not been recognized at Thirsty Lake.

 

The chemistry of mineral inclusions in the microdiamonds suggests the Thirsty Lake stones grew metastably within the graphite-stability field similar to those in Kazakhstan. According to Chinn and others (2000), the diamonds exhibit elevated hydrogen which has been connected to nucleation processes for synthetic diamonds. The presence of the high volatiles (H and N) associated with these diamonds may have been responsible for metastable growth of the microdiamonds at pressures below the diamond-stability field. The high hydrogen abundance is thought to explain the high nucleation rate for microdiamonds and the lack of macrodiamonds in this deposit (Chinn and others, 2000).

 

(3) Parry Peninsula. Along the Parry Peninsula to the NNW of Ekati, adjacent to Darnley Bay in the Amundsen Gulf north of the Arctic Circle, exploration was initiated over the strongest isolated gravity anomaly in North America in a search for ultramafic-hosted nickel and platinum-group mineralization. During exploration in 1997, an aeromagnetic survey flown over the gravity anomaly identified several characteristic ‘bulls-eye’ mag-anomalies that are typically associated with diatremes. Many of these were evaluated and 12 were drilled resulting in the discovery of kimberlite (270 Ma) at 10 of the anomalies: diamonds were recovered from 6 of the intrusives.

 

(4) Victoria Island cluster (Nunavut). The Victoria Island cluster is located east of the Parry Peninsula a considerable distance north of Yellowknife. The Snowy Owl kimberlite within this cluster consists of hypabyssal-, diatreme- and crater-facies kimberlite. Initial samples of 966 pounds yielded 785 microdiamonds with 4 macrodiamonds. The nearby Longspur kimberlite yielded 36 microdiamonds and 3 macrodiamonds from a 198-pound sample, and the Golden Plover kimberlite yielded 41 microdiamonds and 3 macrodiamonds from a 397-pound sample of crater and hypabyssal facies kimberlite.

 

(5) Somerset Island cluster (Nunavut), located nearly 200 miles east of Victoria Island within the Arctic Circle. A cluster of 36 kimberlites, most of which have been extensively studied over the past 35 years, are known. Kimberlites on Somerset Island show strong NE-, NW- and also N-S structural controls that parallel basement foliation. A few kimberlites (88 to 105 Ma) appear to be weakly mineralized and have yielded microdiamonds. The Somerset Island kimberlites lie to the west of the Brodeur Peninsula kimberlites located along the NW extent of Baffin Island. Diapros established a 1-ton/hr processing facility in the vicinity of the Batty kimberlites in the summer of l972 and processed 262.3 tons of kimberlite and an additional 215.1 tonnes were processed from the Diapron, Batty, Nord, Oucat, Ham and Elwin kimberlites. Diamonds were recovered from the Nord.

 

The kimberlites are hypabyssal and diatreme facies (105 Ma). The transition from lithosphere to asthenosphere at depths of 87 miles beneath Somerset Island is suggested by an inflection in the temperature and pressure array defined by mantle peridotites. The presence of diamonds indicates that kimberlites tapped lithosphere within the diamond stability field, although the lithospheric root is believed to be thinner under Somerset Island than in the central Slave craton.

 

Several other notable diamondiferous occurrences are located within the Nunavut Territory. Exploration continues in the Tomgas region of southeastern Nunavut and the north coast of Baffin Island. Further to the southeast, the Torngat dikes (346 Ma) are analogous to the kimberlite dikes of west Greenland. The extent of Proterozoic metasomatism within the lithosphere beneath the western Churchill Province is still poorly documented and its effects on diamond stability are not understood (Armstrong, 2000).

 

(6) Rankin Inlet. Much of this region is underlain by the Churchill Province. These kimberlites were emplaced in the Archean Rankin Inlet group of metamorphics. The kimberlites lie about 75 miles ESE of the Thirsty Lake (Parker Lake) diamondiferous minette dike that is believed to be associated with the magmatic event responsible for the Christopher Island Formation (Proterozoic). Past exploration in the region has been largely for gold and base metals and systematic exploration for diamonds has been limited. Some kimberlite dikes (192-214Ma) were intersected during drilling at the Meliadine gold deposit. In 2003, Cumberland and Comaplex announced the discovery of 11 new kimberlites, and the Geological Survey of Canada reported numerous kimberlite float occurrences along the Meliadine trend. These provide evidence for multiple kimberlitic sources in the Churchill region.

 

Exploration in this region resulted in the recovery of 145 mineral grains that were confirmed as diamond indicator minerals recovered from 183 till samples. About 46% of the pyropes were G10 (garnets that are chemically similar to diamond-inclusion garnets) subcalcic pyropes (Gurney, 1984). The indicator mineral results defined several corridors of interest on the property that were followed up in 2003 with >1,800 till samples. A high resolution airborne magnetic survey identified 226 priority targets. In 2003 a follow-up line was flown which identified more than 100 additional high priority targets including a cluster of 29 magnetic lows. The geophysical evidence suggests that a kimberlite cluster of more than 100 pipes is present at the Churchill Diamond Property!

 

Ground geophysics completed on 58 priority targets resulted in 29 being selected to drill which resulted in the discovery of 16 kimberlite pipes. This cluster of pipes occurs over a spatially large area measuring 37 by 30 miles. The kimberlites were initially recognized as magnetic highs and lows, with some correlating EM signatures. Nine of the kimberlites yielded diamonds (Strand, 2004).

 

(7) Melville Peninsula. The Melville Peninsula is located along the edge of the RAE craton, north of the Arctic Circle adjacent to the Foxe Basin and south of Baffin Island. A group of 9 diamondiferous kimberlites known as the Aviat kimberlites were discovered in this region as well as till samples with diamond-stability (G10) pyrope garnets. These kimberlites lie NE of another group of kimberlites referred to as the Wales Island kimberlites. Based on a 10.4 tonne sample, the AV-1 kimberlite yielded a preliminary ore grade of 83 carats/100 tonnes. The Wales Island kimberlites to the southwest include a group of 10 kimberlites that were recently discovered.

 

(8) Baffin Island. Following an initial study that resulted in a focus on Baffin Island having geology favorable for diamonds, a field program in 2005 consisted of an aeromagnetic survey and prospecting. Results from the aeromagnetic survey led to the staking of approximately 75,000 acres in the south central region of Baffin Island. One of the three claim blocks contains 14 discrete geophysical anomalies that vary in size from 410 to 2,625 feet in diameter. All of the anomalies are found within a single cluster and in an area considered to be structurally favorable for kimberlite intrusion.

 

The Jackson Inlet kimberlite is located on the West Coast of the Brodeur Peninsula of Baffin Island. It is centered 2 miles south of Jackson River. The Brodeur Peninsula is bounded by Admiralty Inlet, Lancaster Sound and Prince Regent Inlet. Flat-lying Ordovician and Silurian carbonates are exposed along the steep coastline of the Brodeur Peninsula and in the deeply incised river gorges. Between these gorges, the land surface is an undulating plateau. Except at the crests of some hills, a thick blanket of glacial till was deposited by a small ice cap centered on the peninsula during the last glaciation and beyond the northern limit of the continental glacier which covered much of Canada.

 

Although isolated gneissic erratics provide evidence of an earlier much more extensive glaciation, the till consists mainly of carbonate blocks in a matrix of pulverized carbonate. It is grey or light brown and supports very sparse vegetation. Annual rainfall is low. From the air and on aerial photographs, evidence of the Jackson Inlet cluster of kimberlite pipes is manifested as three dark brown circular patches within a 1,640 by 1,970 foot halo of tan coloration. Within the halo are patches of darker tan color. The surrounding Ordovician-Silurian limestone is grey and the tan color of the halo is interpreted as a result of limestone weathering, which was dolomitized by introduction of magnesium from the kimberlitic magma.

 

(9) Drybones Bay kimberlites (NWT). At least 3 diamondiferous kimberlites have been found in the Drybones Bay area along the north shore of the Great Slave Lake to the east of Yellowknife.

 

(10) Coronation Gulf. The Coronation Gulf area, located 70 miles NNW of the Jericho mine, includes a group of more than 11 diamondiferous kimberlites. The Knife Pipe, under exploration by DeBeers, is described as being significantly diamondiferous. Ashton Mining reported highly encouraging results from caustic fusion analyses of kimberlite from their nearby Potentilla, Stellaria and Artemesia pipes. Samples of the Potentilla kimberlite include hypabyssal and diatreme facies kimberlite with an estimated grade of only 17.5 carats/100 tonnes.

 

Ontario. North of the Great Lakes in Ontario, a number of kimberlites, some lamprophyres, and a group of diamondiferous actinolite schists are recognized. To date, the most notable appear to be the Attawapiskat cluster and the Wawa dikes. Other discoveries in Ontario include kimberlites at Kyle Lake, Kirkland Lake, Keith Township, New Liskeard and others. Only the principal occurrences are described: (1) The Attawapiskat cluster includes several well-mineralized kimberlites including the Victor pipe in the tundra near Hudson Bay; (2) The Kyle Lake kimberlite cluster about 60 miles west of the Attawapiskat cluster; (3) The Kirkland Lake cluster along the eastern Ontario border adjacent to Quebec; (4) The Keith Township kimberlite; and unconventional host rocks of great interest known as (5) the Wawa cluster on the northeastern shore of Lake Superior.

 

(1) The Attwapiskat cluster includes at least 20 kimberlites (155 to 170 Ma) near the Attwapiskat River in the James Bay lowlands that lie along a distinct NNW trend. Within this field is one commercial property. The project, operated by DeBeers, encloses 18 kimberlite pipes, 16 of which are diamondiferous. The Victor Main and Victor Southwest pipes consist of two pipes that coalesce at the surface and have a combined area of 37 acres. The composite pipe is formed of pyroclastic crater, diatreme and hypabyssal facies kimberlite with highly variable diamond grades. Reports suggest that the pipe averages 33 carats/100 tonnes with an average value of $154/carat. Plans are to develop an open pit with an expected 12-year mine-life and total project life of 17 years. The Victor mine would be supported by a processing plant with a designed capacity of 2.5 million tonnes/year.

 

In 2003 and 2004, airborne magnetic signatures were evaluated within the Attwapiskat cluster and 5 previously unknown kimberlite pipes were found in the MacFayden group. The interpretation of the magnetic total field showed distinct circular magnetic isograds along a very prominent NNW magnetic trend that is interpreted as a buried kimberlite dike. This trend continues for about 11 miles or more with the MacFayden Group. This group is situated along the bank of the Attwapiskat River. The Victor pipe, 5 miles to the SSE, lies along the same trend.

 

The Kyle Lake kimberlite cluster 60 miles west of the Victor pipe, includes a group of 5 Precambrian kimberlites. Preliminary tests of the Kyle Lake #1 kimberlite yielded an average ore grade of 60 carats/100 tonnes. Several phases were mapped in the kimberlite including one that yielded an ore grade as high as 800 carats/100 tonnes! The surface area of the kimberlite is only 6.2 acres, but the kimberlite has an ore resource of 14.5 million tonnes to a depth of 1,670 feet. The recovered diamonds show little resorption and have distinct octahedral habit. The Kyle Lake #3 kimberlite lies near the confluence of the Attwapiskat and Muketei rivers. This is a vertical dike with average width of 82 feet and a blow at one end that increases the width to 410 feet. The dike has been traced by drilling to more than 1475 feet along strike, and has an even greater strike length based on ground magnetic surveys. The average grade based on limited sampling was 92 carats/100 tonnes.

 

The Kirkland Lake group includes two clusters within the Kirkland Lake mining district. The Kirkland Lake district is a well-established gold mining camp underlain by Archean rocks (2.5 to 2.7 Ga) of the Superior Province. Proterozoic rocks of the southern Cobalt Group of the Huron Supergroup (2.5 to 2.2 Ga), Grenville Province metamorphics (1.1 Ga) and Phanerozoic sedimentary rocks also underlie the region. The Kirkland Lake kimberlites (150-159 Ma) include a cluster of 10 pipes in the Kirkland Lake area as well as some to the south near Cobalt in the New Liskeard area near Lake Timiskaming, along with 11 dikes in the Kirkland Lake area. Some of these are weakly mineralized in diamond and the geochemistry of the indicator minerals supports that the kimberlites tested to date should only be weakly mineralized. The kimberlites in the New Liskeard area continue from Ontario into Quebec along a northeasterly trend. These have been known for nearly 50 years (Schulze, 1996).

 

The Wawa cluster. The Wawa diamond deposits are similar to diamond deposits in the Akwatia field, Ghana. The Wawa discovery is significant for many reasons which include that these are the oldest diamond deposits ever found and they occur as stratiform, metamorphosed schists within an Archean greenstone belt. The host rock precursors remain an enigma, but are interpreted as metamorphosed lamprophyre, metamorphosed crater facies kimberlite, lahar, breccia, conglomerate and even komatiite. It may be some time before the origin of these hosts is known. At any rate, the discovery of these deposits provides a whole new concept in exploration for diamonds worldwide.

 

Diamondiferous actinolite schist from the Wawa property.

 

The host rocks are reported as Archean (2.7 Ga) age diamondiferous ultramafics breccias and schists. Limited sampling of some of the breccias have yielded grades ranging from 6 to 262 carats/100 tonnes (Wilson, 2004). The largest diamond found to date is 1.39 carats. The diamond deposits occur within the western Michipicoten greenstone belt of the Wawa subprovince of the Superior craton. The Wawa rocks are unique because they represent a diamondiferous occurrence of 2,674 Ma in age. They have been metamorphosed and deformed during 4 episodes of deformation, and little evidence of their primary precursor is preserved. Two types of diamond-bearing rocks are described - both have been metamorphosed to upper greenschist facies and the data suggest that the rocks had magmatic predecessors interpreted to be a polymictic volcaniclastic breccia and lamprophyre, or komatiite. The lamprophyre(?) is younger than the volcaniclastic breccia, but contains rare fragments of the breccia. Both the lamprophyre and breccia are intercalated with 2.7 Ga felsic to intermediate metavolcanics, intermediate to mafic metavolcanics and mafic intrusive rocks.

 

The matrix- to clast-supported breccia forms ~200 to 230 foot thick units. It contains dominantly angular, granular to large boulder-sized fragments with a wide variety of igneous lithologies, including metamorphosed mantle-derived ultramafic rocks with high Cr and Ni. The breccias are interpreted as volcaniclastic debris flows based on the stratigraphy, the wide range in fragment lithologies, crude bedding, poor sorting, and lack of sedimentary structures. The fine-grained breccia matrix is comprised of upper greenschist to epidote-amphibolite mineral assemblages. This assemblage includes 50-75% actinolite, 1-20% epidote, 1-20% titanite 1-10% quartz/feldspar, 0-20% biotite, 0.5-15% hornblende, and 0-10% chlorite, with minor calcite, albite, opaques and rutile.

 

The rock may occur as dikes and appear to cross-cut the metavolcanic sequences and breccia. In some areas the relationship between the lamprophyre(?) and breccia, and the