NORTH AMERICAN DIAMOND DEPOSITS北美的金刚石矿床.doc

NORTH AMERICAN DIAMOND DEPOSITSWYOMING, USA/pressrel/march25_98.htmHow much are diamonds worth? Many diamonds have such a high value that a person could easily carry a kings ransom in diamonds in a shirt pocket. People tend to think of gold as a very high-value commodity, but it would require an armored truck to carry enough gold of similar value to a handful of gem-quality diamonds.For example, one fingernail-sized, gem-quality diamond recently recovered from the State Line district south of Laramie weighed slightly more than 28 carats and produced a cut stone of 16.8 carats (or about 0.1 ounce) with an estimated value of $300,000. It would take 1,000 ounces of gold to yield a similar value. Imagine trying to carry that in your shirt pocket! Some other diamonds mined in Australia, known as Argyle pinks, have sold for as much as $1,000,000 a carat. An equivalent value of gold would weigh 3,400 ounces!Diamonds are a billion dollar industry. The Economist (12/20/97) recently reported that 1996 sales of gem diamonds amounted to $52 billion. And while most mined commodities have declined in price, diamonds have shown a 50% increase from 1986 to 1996. Thus, when diamonds were recently discovered in the Northwest Territories of Canada, one of the largest claim-staking rushes in history followed, and is continuing today. Capital investments for the mines currently under development total more than $800 million, which will result in a tremendous boost to the Canadian economy. And a major off-shoot to the diamond exploration in Canada was the discovery of a world-class nickel-cobalt deposit in Voisey Bay, Labrador, by a company searching for diamonds.There are dozens of companies searching for diamonds in Canada. According to W. Dan Hausel, Senior Economic Geologist with the Wyoming State Geological Survey, several companies have also contacted his office for information on diamond deposits in Wyoming. Some of these companies have already staked as many as 5 to 6 million acres of land in Alberta and Saskatchewan, and are curious about Wyoming because of its similar geology.According to Hausel, much of Wyoming is underlain by rocks similar to large portions of Canada, as well as to South Africa and Australia (two major diamond producers). The Wyoming State Geological Survey has also discovered and/or mapped several kimberlites and lamproites (the only known host rocks for commercial quantities of diamond), and has identified numerous anomalies that could potentially lead to undiscovered diamond deposits. The Wyoming State Geological Survey also recently discovered a significant nickel-cobalt occurrence in southeastern Wyoming.With declining oil production in Wyoming, a diamond industry could help off-set declining revenues. But it takes considerable time and effort to find, identify, and develop diamond deposits. Unfortunately Wyoming will probably not see a rush on the same scale as Canada. This is not because Wyoming does not have the potential to host major diamond deposits. According to Hausel, Wyoming is every bit as favorable, but the Canadian government is providing exploration incentives, something the U.S. Government is not doing. With so much of the mineral estate in Wyoming owned by the Federal government, incentives could make a difference. However, Hausel is optimistic that significant diamond discoveries in Wyoming will attract the attention of the diamond industry. Already, the Wyoming-Colorado State Line district south of Laramie has produced several of the largest diamonds found in North America, including diamonds of 2.6, 5.51, 6.2, 9.4, 10.5, 11.85, 14.2, 16.9, 28.18, and 28.3 carats. Diamonds have also been found in several other regions of the State.ARKANSAS, USAhttp:/www.state.ar.us/agc/gemstone.htmArkansas Diamond-bearing RocksOne of the few places in North America where diamonds are present in their host rock and the only place tourists may hunt for diamonds is the Prairie Creek pipe in Arkansas. It is roughly triangular in surface outcrop, exposed over 73 acres, and is situated 2.5 miles southeast of Murfreesboro in Pike County. The site has been known to geologists since 1842. It is a breccia-filled volcanic pipe of Cretaceous age, formed by a series of gaseous explosions as are several other pipes nearby.Various rock types are present in the Arkansas diamondiferous pipes. Magmatic lamproite is a dark-colored igneous rock with a texture that has two distinct grain sizes (porphyritic). Some of it was broken explosively as it neared the earths surface. This broken rock material is lamproite breccia tuff. Rarely have diamonds been reported in the soils formed by the weathering of magmatic lamproite; most have been recovered in the lamproite breccia tuff or in the thin residual soils overlying this rock. Microdiamonds, however, have been recovered from the magmatic rock by special techniques. Epiclastic rock, which is a rock formed by the mechanical mixing of tuffaceous volcanic material and local Cretaceous sediments, was recently recognized by geologists.Arkansas HistoryDiamonds were first discovered in Arkansas in 1906 when two stones were picked up by John M. Huddleston near the mouth of Prairie Creek southeast of Murfreesboro. Following this discovery, diamonds were reported from two small areas 2 miles to the northeast of the Prairie Creek pipe. Later, various efforts were made to mine diamonds on a commercial basis, but without sustained success. In 1919, the Arkansas Diamond Corporation was organized and a washing plant was built which processed 18,000 loads of surface-mined diamond-bearing material in 1920. The corporation discontinued operations after 9 months. Early in 1940, the property was taken over by The Diamond Corporation of Arkansas. Their 2,000-ton washing and concentrating plant began operation in 1948, but shut down a year later. Howard A. Millar operated the Crater of Diamonds tourist attraction on part of the Prairie Creek pipe during the 1950s and 1960s. This volcanic pipe and some surrounding acreage became Crater of Diamonds State Park in 1972 when the State of Arkansas purchased the property for $750,000 from General Earth Minerals. From 1972 to 1996 inclusive, 1,895,143 guests had visited the Park.Recovery figures are incomplete, but estimates are that about 100,000 diamonds, thought to average 0.25 carats in weight, have been recovered by commercial efforts and by tourists. Since the Prairie Creek pipe became a state park in 1972, the state has maintained records of the number of diamonds discovered and reported. From 1972 to 1997 inclusive, 21,068 diamonds weighing a total of 4,195.34 carats have been reported (Tom Stolarz, personal communication). It is not known what percentage of the stones from Arkansas are of industrial grade.In the early 1990s, drill-hole data on the Prairie Creek pipe indicated about 78 million tons of diamond-bearing rock are present to a depth of 650 feet. Several nearby diamond-bearing lamproite pipes are present northeast and east of the state park on private property.Arkansas DiamondsThe 1967 Arkansas General Assembly passed legislation which became Act 128 designating diamond as the official gem of the State. To qualify as gem-grade, diamond must meet several criteria. It should be large enough to cut a stone whose value is more than the cost of cutting and the cost of the rough stone. An octahedral-shaped diamond allows the production of the largest brilliant round cut with the least waste. Even when faceted to this ideal shape, which possesses the most fire for the finished weight, a given diamond crystal will lose about 50 percent of its weight. The rough or uncut stone must be clear and not contain major inclusions or flaws. Diamonds not meeting the graders standards are termed industrials. Total lack of color is usually considered to be of the highest value. However, some particularly strong or unusual colors are highly valued, such as canary yellow, pink, or blue. Pale yellow, brown, and green are considered inferior colors for gemstones.A number of diamonds recovered from the volcanic pipes near Murfreesboro qualify as gem-grade material. The largest diamond discovered in the United States came from this site and was recovered by commercial methods. Named the Uncle Sam, this diamond was discovered in 1924. It weighed 40.23 carats in the rough, and was faceted into an elongate emerald-cut gemstone weighing 12.42 carats. It has a faint rose color and is presently owned by Peiken of Fifth Avenue, New York. The best known diamond recovered by a tourist is the Star of Arkansas, a white 15.33-carat crystal which was faceted to a flawless 8.27-carat marquise-shaped gem. This stone was auctioned at Christies of New York in 1994 and brought $145,000. The largest diamond discovered since the beginning of Crater of Diamonds State Park in 1972 is the Amarillo Starlight, weighing 16.37 carats. This stone was faceted to a 7.54-carat marquise-shaped gem, valued at between $150,000 and $175,000.Numerous smaller stones from Arkansas have been cut or left as natural crystals and utilized in jewelry. First Lady, Hillary Clinton, wore a natural diamond crystal set in a gold ring, during the 1992 Presidential Inaugural Ball in Washington, D. C. This diamond crystal weighs 4.25 carats, is strong canary-yellow in color, and possesses a natural brilliant luster.Records of diamonds discovered at Crater of Diamonds State Park for the period of 1972 to 1999, inclusive, indicate that diamonds were reported that weighed over 1 carat each (Tom Stolarz, personal communication). Some of these stones would qualify as cuttable gemstones.A particularly notable recently discovered diamond that was cut and found to be a flawless perfect top quality colorless stone is the Stawn-Wagner diamond. It weighed 3.03 carats rough and was cut to a brilliant round gem of 1.09 carats. Gemologists have graded the stone as grade D-flawless, 0/0/0 (cut, color, clarity) and stated that this diamond is one in a billion! It was purchased by the Crater of Diamonds State Park for $34,500, using part grant money and part private donations in 1999. This flawless gem may be viewed on special occasions at the Park.One notable find of a gem-grade diamond is reported from White County in 1926. A young girl picked up a pretty stone while working in a cotton field. It was later identified as a 27.21-carat gem-quality diamond. In 1946, the finder, Mrs. Pellie Howell, sold this uncut stone to Tiffany & Company of New York for $8,500. Still in Tiffanys possession, it is of a fine cape (pale yellow) color, has not yet been cut, and is the third largest diamond so far discovered in the United States. Some believe that this single stone was originally discovered by a Native American at or near the Prairie Creek pipe and dropped near Searcy where it was later found. There are no geologic or geophysical indications of diamond-bearing pipes in northeast Arkansas.ReferencesHoward, J. M., 1989, Finding diamonds in Arkansas!, Arkansas Geological Commission pamphlet.Krol, L. G., 1988, Prairie Creek kimberlite (lamproite), in Colton, G. W., ed., Proceedings of the 22nd Forum on the Geology of Industrial Minerals: Arkansas Geological Commission Miscellaneous Publication 21, p. 73-75.DIAMOND DEPOSITS OF CANADAhttp:/www.nrcan.gc.ca/gsc/mrd/mdag/pub/diamonds_e.html#intro1Diamonds, though extremely rare, are widely distributed and have been discovered in unconsolidated and consolidated sediments, diverse igneous rocks of upper mantle origin, mantle xenoliths, ophiolites, ultra high pressure metamorphic rocks, meteorites and impact structures. Of these, only diamond-bearing kimberlite and lamproite, and their derived placer and paleoplacer deposits, have proven to be economically viable. Before 1960, 80% of all diamonds were recovered from secondary deposits; by 1990, increased diamond production from kimberlite and lamproite pipes reduced this to 2.5 Ga) cratons. Lamproites, on the other hand, typically intrude demobilized Archean cratons or Proterozoic orogenic belts. For example, the Argyle deposit in Australia, the worlds most productive diamond mine, occurs in a lamproite pipe within the Proterozoic Halls Creek mobile belt. Locations of diamond-bearing rocks in Canada are shown in Figure 1, along with the locations of many commercially important kimberlites and lamproites. This figure also shows the worldwide distribution of Archean and Proterozoic cratons based on a generalized geological map of the world, now available digitally (Kirkham et al., 1994, 1995). Figure 1. Locations of diamond-bearing kimberlites in Canada, and worldwide locations of kimberlite clusters and primary diamond producers and past producers. The distribution of Archean and Proterozoic rocks is derived from a digital geological map of the world compiled by Kirkham et al. (1994, 1995).In Canada, Archean and Proterozoic structural provinces are well known and geological maps provide a rigorous framework for examining the distribution of kimberlites and for testing models of diamond formation. Precambrian rocks of the North American craton have been subdivided into numerous crustal domains with distinctive ages, structural trends, and geophysical characteristics (Stockwell, 1961; Hoffman, 1989). Archean cratons, which underlie a large part of the exposed Canadian Shield, were welded together by Proterozoic orogeny, many of which continue in the sub-surface beneath adjacent Phanerozoic sedimentary cover (e.g. Trans-Hudson orogen - Fig. 1: Percival, 1996). The contrast between exposed Archean fields and adjacent thinly covered Proterozoic orogeny is one characteristic that hints at significant differences between Archean and Proterozoic processes of lithosphere formation and preservation (Durrheim and Mooney, 1994). The continental lithosphere, made up of the crust and part of the upper mantle, is a strong persistent layer relative to the underlying convecting asthenosphere. Seismological studies indicate that the lithosphere is thicker beneath Archean cratons, and contains regions of cool lithospheric mantle within the diamond stability field (i.e. depths 150 km; Durrheim and Mooney, 1994; Grand, 1994; Polet and Anderson, 1995). The first three papers in this volume provide a brief summary of the complex history of Archean cratons and flanking Proterozoic orogeny in Canada, and contain key references to maps and reports on the Precambrian geology of Canada. In this volume, the geology of kimberlites and lamproites is covered in summary papers, accompanied by short papers on analytical methods and specific kimberlite fields in Canada. For comprehensive summaries of the petrology of kimberlites, lamproites, and lamprophyres the reader is referred to books by Mitchell (1986; 1995), Mitchell and Bergman (1991) and Rock (1991). Extensive additional information can be obtained from the Proceedings Volumes for the five International Kimberlite Conferences published up to 1994, and in the Abstracts Volume for the Sixth International Kimberlite Conference (*Proceeding Volumes, 1979-1994; Abstracts Volume, 1995). Kimberlites resemble and can be spatially associated with other alkaline rocks, some of which originate at depths 150 km, and are therefore potentially diamond-bearing. Diamonds have been reportedly recovered from lamprophyre diademe breccias in the Canadian Cordillera, from lamprophyre dykes and diatremes south of Baker Lake, N.W.T., and from the Ile Bizard diatreme breccia in Quebec. These diamond-bearing rocks and several other alkaline intrusions in Canada are described in a series of short Papers. Kimberlite, lamproite and other alkaline magmas originate as small volume melts of deep-seated origin. The triggering mechanism for generation of kimberlites is unknown, and correlation of kimberlite magnetism with mantle plumes, with flexure of the lithosphere, or with specific plate tectonic processes has not been adequately demonstrated worldwide (Mitchell, 1986; Haggerty, 1994). Furthermore, no viable theory has accurately predicted the location of kimberlite fields within a craton, although many fields have preferred orientations that suggest pre-existing structural controls are important. Lamproites and lamptophyres originate from shallower sources than kimberlites, and magmas are derived from partial melting of subcontinental lithospheric mantle that has a long and complex metasomatic history (Mitchell and Bergman, 1991). Kimberlites have chemical and isotopic signatures suggesting they originated from sources in the asthenosphere, although the magmas can be modified by interaction with metasomatized regions in the overlying continental lithosphere (Haggerty, 1994; Ringwood et al., 1992; Tainton and McKenzie, 1994). Much of our knowledge of the deep crust and upper mantle is inferred indirectly from geophysical methods. Mantle and crustal xenoliths and xenocrysts transported to surface by deep-seated magmas are actual samples of the mantle and deep crust, and can be used to test the geophysical models and study the lithosphere beneath Canada. Pilot studies provide a glimpse of the rich potential provided by xenolith suites in kimberlites and related rocks, discovered as a result of the continued success of diamond exploration across Canada. Age determinations on diamond inclusions and on primary minerals in diamond-bearing kimberlites indicate that diamonds are xenocrysts in the kimberlite magma. Kimberlites act only as transporta