Kimberlite pipes erupt as violent volcanoes - the magma, as it weathers, releases diamonds and tracer minerals such as chromian diopside, pyrope garnet, picroilmenite, chromite, etc. into streams where they can be panned and traced back to the source rock. While panning for these indicator minerals, sometimes other valuable minerals are found including diamond, gold, ruby, sapphire.
EXPLORATION
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| Raw diamonds from Arkansas (photo from Glenn Worthington). If you are interested in digging for diamonds, get a copy of Glenn's book and video. |
EXPLORATION
Cost figures for annual diamond exploration amounts to tens of millions of dollars. Capitalization costs for the development of the Ekati diamond mine in the Northwest Territories Ga (billion years).
Following selection of a favorable terrain, topographic and geological maps, aerial and satellite imagery, and aerial geophysical data are examined. Unusual circular depressions, circular drainage patterns, noteworthy structural trends and vegetation anomalies are noted. For example, Hausel (2009a,b) identified several targets using available software on the Internet including Google Earth, Virtual Earth and others. Geophysics is used to search for distinct (“bull’s eye”) conductors and magnetic anomalies. Geochemical data are examined for Cr, Ni, Mg, and Nb anomalies.Stream sediment sampling. One of the primary methods used in diamond exploration is stream sediment sampling programs designed to search for ‘kimberlitic indicator minerals’ (pyrope garnet, chromian diopside, chromian enstatite, picroilmenite, chromian spinel, and of course diamond). Diamond targets are small and may range from diatremes of several acres to narrow dikes and sills. Diamond-bearing kimberlites and lamproites typically contain abundant soft serpentine with resistant mantle-derived xenocrysts and xenoliths. The serpentine matrix tends to decompose releasing distinct mantle-derived ‘kimberlitic indicator minerals’ into the surrounding environment. The indicator minerals may be carried downstream for hundreds of yards, or a few or many miles depending on the climatic and geomorphic history of the region. Diamonds however, are thought to be carried considerable distances – in some cases, hundreds of miles. The indicator minerals may provide a trail leading back to the source.
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| Panning for diamonds at undisclosed location in Wyoming |
The traditional kimberlitic indicator minerals are rare to non-existent in lamproite, thus other minerals (zircon, phlogopite, K-richterite, armalcolite, priderite) may be considered that unfortunately have low specific gravity, poor resistance, and are potentially difficult to identify. The better indicators for diamondiferous lamproite have been diamond and magnesiochromite.
To take advantage of the dispersion of kimberlitic indicator minerals, the size of samples are determined based on the environment. For example, where there is a general lack of active streams, much larger samples are taken compared to regions with active drainages. In areas with juvenile streams, samples are often panned on site to recover a few pounds of sample concentrates. Recovered indicator minerals are tested for chemistry using an electron microprobe to identify those that have higher probability of originating from the diamond stability field. The data are plotted on maps to facilitate evaluation.
Geomorphology. Kimberlite and olivine lamproite are often pervasively serpentinized, making outcrops the exception rather than the rule. In many cases, geomorphic expressions of pipes are subtle to unrecognizable. The Kimberley South Africa Wyoming Colorado
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| Depressioin over Maxwell diamondiferous kimberlite, one of a few hundred untested diamond pipes in Colorado, Wyoming and Montana. |
Lineaments. Many kimberlites and lamproites are structurally controlled (Hausel and others, 1979; 1981; Macnae, 1979, 1995; Nixon, 1981; Atkinson, 1989; and Erlich and Hausel, 2002). Controlling lineaments and fractures may be indicated by alignment of a cluster of intrusives or by the elongation of a pipe. In Lesotho South Africa
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| During recent exploration, I was able to identify more than 300 cryptovolcanic structures in and surrounding the State Line district. Many of these are likely kimberlites, but remain untouched, such as these depressions in Colorado that sit on distinct lineaments and adjacent to diamondiferous kimberlites. |
Remote Sensing. Kingston
Many pipes and dikes possess distinct structural qualities or vegetation anomalies that may allow detection on aerial photographs. Mannard (1968) reported kimberlites in southern and central Africa were identified on aerial photographs on the basis of vegetation anomalies, circular depressions or mounds, and/or tonal differences. Low-level aerial photographs (both conventional and false color infrared) have been used to locate kimberlite in the USSR US
Geophysical Surveys. Geophysical exploration has been successful in the search for hidden kimberlite and lamproite (Litinskii 1963a, b; Gerryts 1967; Burley and Greenwood 1972; Hausel and others, 1979, 1981; Patterson and MacFadyen 1984; Woodzick, 1980), particularly in districts where kimberlites have previously been discovered. Contrasting geophysical properties are often favorable for distinguishing kimberlite, lamproite and minette from country rock.
INPUT™ airborne surveys are effective in identifying both serpentinized and weathered kimberlite owing to the combination of conductivity and magnetics used in INPUT™. Rock exposures of kimberlite may yield magnetic signatures but are poorly conductive, while deeply weathered kimberlites are conductive but poorly magnetic.
Because of the relatively small size of the diamond host rock, close flight-line spacing is necessary. In an airborne INPUT™ survey over the State Line district, Wyoming Kansas Antioch Australia
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| Geonics EM31 worked very well over buried and exposed weathered conductive kimberlite. |
In the Yakutia province, Russia Mali West Africa , the magnetic contrast between kimberlite and schist and sandstone country rock resulted in 2,400-gamma anomalies over kimberlite (Gerryts 1967). In Lesotho
Fipke and others (1995) indicated that barren peridotite phases in Arkansas Kansas
Most kimberlites in the Colorado–Wyoming State Line district yielded small complex dipolar anomalies in the range of 25 to 150 gammas, with some isolated anomalies of 250 and 1,000 gammas (Hausel and others, 1979). Blue ground kimberlite tends to mask magnetic anomalies. In the Iron Mountain
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| Gem-quality diamonds recovered from Wyoming kimberlite in 1979. Largest stone is about 1 carat. |
Resistivity of weathered lamproite may be lower than that of country rock, owing to the conductive nature of smectitic clay relative to illite, kaolinite and other clay minerals (Gerryts 1967; Janke 1983). However, the Argyle olivine lamproite yielded moderate to strong resistivity anomalies (40-100 ohm/m) compared to the surrounding country rock (200 ohm/m) (Drew 1986).
Biogeochemical and Geochemical Surveys. Kimberlite and lamproite are potassic alkalic ultrabasic igneous rocks with elevated Ba, Co, Cr, Cs, K, Mg, Nb, Ni, P, Pb, Rb, Sr, Ta, Th, U, V and light rare earth elements (LREE). The high Cr, Nb, Ni, and Ta may show up in nearby soils (Jaques 1998), but dispersion of these metals in soils is not extensive. Stream-sediment geochemistry generally is not useful due to efficient dispersion of most metals in streams. In the Colorado–Wyoming State Line district, Cominco American outlined several known kimberlite intrusives on the basis of Cr, Nb, and Ni soil geochemical anomalies. However, dispersion patterns were restricted and of little use in exploration in this terrain.
Gregory and Tooms (1969) found that Mg, Ni, and Nb anomalies did not extend farther than 0.36 mile (0.6 km) from the Prairie Creek lamproite, Arkansas West Kimberley province, Australia
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| Classical indicator minerals used to find kimberlite include picroilmenite, chromite, chrome diopside, and spessartine and pyrope garnets. The purple garnets are typical G10 (diamond-stability) peridotitic garnets and the yellow orange are characteristic eclogitic garnet. |
Bergman (1987) suggested that olivine lamproites are generally enriched in compatible elements relative to leucite lamproites as a result of the abundance of xenocrystal olivine in the former. Barren lamproites contain elevated alkali and lithophile contents (K, Na, Th, U, Y, and Zr) relative to diamondiferous (olivine) lamproites. Diamondiferous lamproites possess twice the Co, Cr, Mg, Nb, and Ni, and half the Al, K, Na and as barren lamproites (Mitchell and Bergman, 1991), and lamproites have anomalous Ti, K, Ba, Zr, and Nb compared to most other rocks. These components may favor the growth of specific flora or may stress local vegetation (Jaques 1998). The Big Spring West Kimberley , Australia
Many kimberlites in the Colorado–Wyoming State Line district will not support growth of woody vegetation resulting in open parks over kimberlite in otherwise forested areas. These same kimberlites may support a lush stand of grass delineating the limit of the intrusive. Distinct grassy vegetation anomalies over kimberlites in the Iron Mountain
Some Siberian kimberlites support denser stands of larch (Larix dahurica) and abundant undergrowth of shrub willow (Salix) and alder (Alnus) compared to surrounding Cambrian carbonates. In central India
Vegetation over the Sturgeon Lake Saskatchewan
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| Vegetation anomaly followed over kimberlite at Iron Mountain, Wyoming. The kimberlite underlies the thicker vegetation to the left and granite to the right. We also noticed the presence of carbonate in the soil (left), with periodic indicator minerals and also abundant diamondbacks (rattlers). |
MINING & MILLING
Economic diamond deposits depend on the average price of stones, the amount of waste material removed, mining methods, company politics, socioeconomics of the area, and many other factors. For example, a diamond deposit may be mined at a comparatively lower cost in a third world country due to the availability of an inexpensive labor force, although constructing an infrastructure in such an area could offset some of these benefits. Whereas in the US
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| Lost Lake volcanoclastic structure (circular depression with structural control and white carbonate-rich soil in center. |
Commercial deposits include narrow dikes to pipes of 100 to 5000 feet (30-1,500 m) across. Pipes range in surface area from 2.5 to 370 acres (1-150 ha) averaging about 30 acres (12 ha) (Jaques 1998). Diamond mines possess resources in the neighborhood of >10 million to 350 million tonnes of ore and the richest deposits contain reserves measured in the hundreds of millions of carats that are valued in the billions of dollars.
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| One of the first grease tables we constructed at the Wyoming Geological Survey for my diamond projects. Diamonds are non-wettable and will stick to grease. Other minerals will wash over the grease. We constructed this using an old Wilfley table (we actually found in the trash on the UW Campus) drive because the state was too cheap to give us any money even though they were taking in $100s of millions in mineral taxes. |
Even so, the average weight of diamonds from the Argyle lamproite was small (only <0.1 carat), and those from Ellendale lamproites are only 0.1 to 0.2 carat (Mitchell and Bergman 1991). The largest reported diamond from the Prairie Creek lamproite is 40.42 carats (Hausel 1998). However, diamonds from some kimberlites are extraordinary: the largest diamond ever recovered was fist size and was mined from the Premier kimberlite, South Africa
To evaluate a potential commercial diamond deposit, they must first be bulk sampled. If favorable, additional bulk samples are used to assist in establishing ore grade maps to assist in a mine planning. Samples are taken on the surface and from drilling in order to achieve a three dimensional view of ore grades. If the pipe is considered to be economic, planning is completed for an initial open pit design and a mill placed near the pipe. Open pit mining typically proceeds from a spiral road developed from the rim of the pit toward the center of the pipe. As mining proceeds, the country rock is cut back in steps to aid in supporting the highwalls of the open pit. Mining in the pit may occur in an oval pattern, or in a polygonal pattern (Bruton 1979).
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| Diamond Extraction mills were constructed in the Colorado Wyoming State Line district at the Sloan kimberlite and on the Kelsey Lake kimberlite (above). Another portable mill was constructed on a trailer used at Kelsey Lake and a fourth was built along the northern edge of Ft. Collins by Cominco American. None of these were well designed and all rejected many diamonds. This problem was documented at Kelsey Lake when a company interested in purchasing the mine tested mill rejects. The first sample processed yielded several diamonds (including a 6.2-ct stone). This problem was serious as it basically resulted in questions as to what the actual diamond grades of kimberlites were. How many macrodiamonds were lost? Other gemstones (chrome diopside and pyrope were all rejected at all four mills. |
As mining continues and the pipe narrows at depth, the open pit will shrink to smaller and smaller diameters. Mining operations may ultimately continue underground using bulk recovery by block caving. However, less than 30% of diamond mines are continued underground. And to do so, the diamond ore must be relatively high value, because the cost of underground mining is considerably higher and the amount of ore recovered is considerably lower. Some kimberlites in Siberia and South Africa
Following recovery of rock mined from open pit operations, the ore is crushed and screened. Screening separates mid-size from larger material rejects and from material too small to contain commercial diamonds. Decisions on the maximum screen size must weigh the cost of processing additional material with the loss of potentially priceless large diamonds.
The typical diamond mill has a basic flow sheet that begins with primary milling and continues to primary gravity concentration, secondary concentration, magnetic separation, attrition milling. The final diamond extraction stage uses grease table, electrostatic separation, and/or x-ray fluorescence extraction (Bruton 1979).
Placer mines are different. The size of a placer mine will vary from a small one-man operation to a full-scale mine using bulldozers, scrapers and/or dredges. Paystreaks are identified in streams or beaches: mining is then completed using small-scale or large-scale earth moving equipment (Bruton 1979).
GEMOLOGY
The primary monetary value for diamond is as gemstones. Diamond prices vary considerably. There are approximately 5,000 diamond categories with prices that vary from $0.5/carat up to several tens of thousands of dollars/carat (for large uncut or colored “fancy” diamonds) (Miller 1995). Many faceted diamonds are worth many times an equivalent weight in gold or platinum. Rough gemstone diamonds have values as high as 100 or more times that of industrial diamonds. After the diamonds are faceted, the value of the gem can increase another 10 to 100 fold, and the final placement of a stone in jewelry will again add another increase in the value of the stone. Thus any mining operation should consider not only recovery of the gems, but also the fashioning of the gems and marketing.
Diamonds include some of the more valuable gemstones on earth, and arguably are the most valuable of all commodities based on weight. For example, some Argyle pink diamonds have sold for as much as $1 million (US) per carat (one carat weighs only 0.2 grams [0.007 ounce]). Thus, an equivalent weight in gold would only be worth $2.80 (at $400/ounce)! The extreme value of diamond is due to its mystique, rarity, extreme hardness, high refractive index and dispersion that can result in brilliant gems with distinctive “fire” when faceted and polished.
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| 14.2 carat diamond recovered at Kelsey Lake (Photo courtesy of Howard Coopersmith. |
Four general types of natural commercial diamonds are recognized. These are gem (well-crystallized and transparent), bort (poorly crystallized, gray, brown translucent to opaque), ballas (spherical aggregates formed of many small diamonds), and carbonado (opaque, black to gray, tough, and compact). Gem diamonds are further subdivided into gem and near-gem (low-quality gemstones).
The fashioning of diamond “rough” into a finished gem may require up to six steps that include marking, grooving, cleaving, sawing, girdling, and faceting (Hurlbut and Switzer 1979). Whether or not all of these steps are used depends on the size, shape, and quality of the rough stone. There are three traditional types of cuts: step-, rose-, and brilliant-cut (Milashev 1989).
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| 620 carat diamond from African Craton. |
Diamonds may be graded using the Gemological Institute of America’s color grading system. This ranges from D (colorless) to X (light yellow). Each letter of the alphabet from D to X shows a slight increase in yellow tinge that is generally not apparent to the untrained eye (Hurlbut and Switzer 1979). Fancy diamonds are separated from colorless diamonds into groups based in color and intensity (Bruton 1978). Clarity is determined by the presence or absence of blemishes, flaws, and inclusions. One typical grading system ranges from Fl (flawless) to I3 (imperfect) with intermediate grades of VVS1 (very, very slightly imperfect), VVS2, VS1, VS2, SI1, SI2, I1, and I2.
USE
The diamond industry is a multi-billion dollar mega-industry. The unique physical and optical properties of diamond also make it indispensable and irreplaceable for many industrial uses in addition to personal adornment in jewelry.
Due to its extreme hardness, industrial and synthetic diamonds are used extensively as abrasives in grinding, drilling, cutting and polishing. Diamond also has chemical, electrical, optical and thermal characteristics that make it the best material available for wear and corrosion resistant coatings, special lenses, heat sinks in electrical circuits, wire drawing, drilling, and many other advanced technologies. One significant future application will be in computer chips due to their unmatched thermal conductivity and resistance to heat, since a tremendous amount of heat can pass through diamond without causing damage.
Today's speedy microprocessors run hot - upwards to 200oF, and microprocessors can't run much faster without failing. Diamond microchips would be able to handle much higher temperatures allowing them to run at speeds that would liquefy ordinary silicon. But manufacturers have not considered using the precious stone, because it has never been possible to produce large diamond wafers affordably. The Florida-based company Gemesis and the Boston company Apollo Diamond plan to use the diamond jewelry business to finance attempts to reshape the semiconducting world.
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| Wyoming diamond with distinct trigons on its surface. |
At room temperature, diamond is the hardest known material with the highest thermal conductivity of any material. Even though diamond is more expensive than competing abrasive materials such as garnet, corundum, and carborundum, diamond has proven to be cost effective in several industrial processes as it cuts faster and lasts longer than rival material. Synthetic industrial diamond is superior to natural industrial diamond in that it can be produced in unlimited quantities and tailored to meet specific applications. Consequently, manufactured diamond accounts for more than 90% of the industrial diamonds used in the US
According to the US Geological Survey, much synthetic industrial diamond produced domestically was used as grit and powder. The major use was in machinery (27%), mineral services (18%), stone and ceramic products (17%), abrasives (16%), contract construction (13%), transportation equipment (6%), and miscellaneous uses (3%). Industrial diamonds are consumed in the production of computer chips, in construction, in the manufacture of machinery, for mineral and energy exploration and mining, stone cutting and polishing, in transportation (infrastructure and vehicles). Stone cutting along with highway construction and repair are some of the largest users of industrial diamond.
Diamond has one significant limitation in industrial use: it reacts with iron at high temperature causing the diamond to revert to graphite resulting in high rates of wear. In an iron rich environment, diamond may be uneconomical to use in comparison to other conventional abrasives, i.e., aluminum oxide, silicon carbide, and boron nitride. Even though these are considerably softer than diamond, they are suitable as high performance abrasives on ferrous work-pieces.
Diamond use has increased in both jewelry and industrial applications. One reason for the increase is due to the development of diamond synthesis technology making it possible to produce diamond abrasives for specific applications. In the past the only option was to use natural diamond, which had to be sorted by size and crushed, or by surface treatment such as rounding. However, synthetic diamond abrasives can now be produced under a controlled environment such that the shape of the crystal can be made irregular and sharp.
Diamond has many potential exotic applications. For example, the Venus probe was fitted with a transparent diamond window since diamond was the only material transparent to infrared light which could withstand the extreme cold and vacuum of space and the extreme high temperatures and atmospheric pressures of Venus’s atmospheric (temperatures as high as 920°F, and pressures a hundred times that of earth) (Ward, 1979). Another exotic use gives a whole new meaning to the family jewels. LifeGem in Illinois
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| Arkansas diamonds (photo from Glenn Worthington). |
Diamonds are also used in dental drills and surgical blades, and provide cutting edges that are many times sharper than the best steel blades. Since diamond has the greatest thermal conductivity of any material, pinhead size gold-coated diamonds are used in high capacity miniature transmitters that carry television and telephone signals.
Synthetics. Synthetic gem diamonds and simulants are becoming more common on the marketplace. These include cubic zirconia and mossainite. Mossainite has twice the fire of natural diamond, is doubly refracting (unlike diamond and cubic zirconia which are singly refractive) and has a hardness of 9.25 –thus both mossainite and cubic zirconia can easily be scratched by diamond. Double refraction is detectable in mossainite when viewing the front of the stone. The back facets will appear to be duplicated due to the double refraction –except when viewing down the optic axis where light is singly refractive. The optic axis is usually perpendicular to the table of mossianite, thus one must observe the back facets through another facet to see evidence of double refraction.
Synthetic gem-quality diamonds may be produced in about 24 hours. Some stones weighing up to 3 carats have been produced for a few hundred dollars (uncut). Most are yellow, but some Russian stones are clear. In 1971, facet quality synthetic diamonds were grown by General Electric that is nearly colorless (0.3 and 0.26 carats).
The colorless gemstones caused concern in the jewelry trade. Diamond simulants can be detected by a simple thermal conductivity test, but most jewelers were unprepared to distinguish faceted synthetic diamond from natural faceted diamond. Thus, DeBeers developed a diamond verification instrument known as DiamondView which uses ultraviolet fluorescence to distinguish colorless natural diamond from synthetic diamond. In addition, many synthetic diamonds examined by GIA contain metallic inclusions in high enough abundance that they are able to attract a magnet. Non-faceted synthetic diamonds exhibit a unique crystal habit of a cuboctohedron with a flat base. Synthetic diamonds also exhibit unusual dendritic and striated surface patterns. According to Shigley and others (1997), because of the technological challenges and high cost of production, it is unlikely that fashioned gem-quality diamonds larger than 25 points will impact the gemstone industry in commercial quantities.
FUTURE OF THE DIAMOND INDUSTRY
Diamonds have intrinsic value because of unique hardness, transparency and thermal conductivity. Diamonds will be needed as long as we have industrialized nations. Without any foreseeable major economic disasters, the future of the diamond industry should remain strong.
As science and industry advance, additional applications will likely be found for diamond in the electronics industry. Demand for diamonds for drilling in exploration for oil, gas, and minerals, as well as in the construction industries is anticipated to increase. Some technological advances will demand both natural and synthetic diamond in the future.
However, the continual decline in new mines and decline in mineral and oil and gas exploration in the US China
For many years, the gem diamond industry was controlled by DeBeers: a monopoly so powerful that the diamond industry and DeBeers were thought by many to be the same. But the discovery of significant diamond resources outside of Africa has diminished DeBeers’s monopoly.
The first real threat to the monopoly occurred with the discovery of significant gem-diamond deposits in the USSR South Africa Western Australia
The next major diamond discoveries were made on the North American Craton. This is the largest Craton with the largest Archon core in the world. Based on the shear size of the craton, and the many finds of detrital diamonds in glacial moraines, this craton should have been a high-priority target for diamond exploration groups. But for many years, the North American craton was ignored.
The discovery of economic diamond deposits in this craton was the result of unrelenting prospecting by geologist Chuck Fipke. The discovery set off the greatest rush in modern history, and resulted in the development of a diamond industry in Canada
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| Part of a day's diamond recovery from Argyle, Austraila in 1986 |
The North American craton extends across the Canadian border into the United States US US Murfreesburo , Arkansas US
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| Diamond in matrix of Chinese kimberlite (GemHunter collection). |
One concern that has risen is the production of relatively inexpensive gem-quality diamonds. However, gem-quality natural diamonds are also relatively inexpensive until they are faceted and mounted in jewelry. Overall, the synthetic gemstones may cost less, but the price difference may not great. And it is human nature to want an original, or the real thing, rather than an imitation. Gem-quality synthetic diamonds will probably not affect the jewelry market.
CONCLUSION
With the current trend of investment, exploration and progressive pro-mining atmosphere, it is anticipated that Canada Canada US US US Superior Wyoming Provinces North America .