Wyoming Diamonds
Diamonds were discovered in southeastern Wyoming in 1975 about 25 miles south of Laramie. Since then, more than 130,000 diamonds have been found in the Colorado-Wyoming State Line kimberlite district. These vary from microdiamonds to high-quality gems, with one as large as a 28.3-carat diamond found in Colorado. The largest diamond found in Wyoming to date weighed 6.2 carats.
Diamond, composed of pure carbon, is the hardest naturally occurring mineral found on Earth. Diamonds form under extreme pressure and high temperature deep within the earth’s mantle. They arrive at the surface through volcanic processes that carry them upward in rare magmas (melted rocks) known as kimberlites and lamproites. A few other rock types may transport diamonds from the mantle, but have not yet demonstrated commercial diamond production. Both kimberlites and lamproites occur in Wyoming, as do some less well-known potential diamond host-rocks.
Most of Wyoming has high potential for the occurrence of diamonds. The ancient core of the North American continent, the Archean craton, extends southward from Canada and lies beneath most of Wyoming where it is known as the Wyoming craton or Wyoming Province. This stable part of the continent is more than 2.5 billion years old (2.5 Ga) and is believed to have a high potential for diamond deposits. Slightly younger 1.6 to 2.5 Ga accreted parts of the continent have a moderate potential for diamonds. However, the accreted terrain of the Proterozoic Colorado Province includes numerous diamond-bearing kimberlites located in Colorado and along the Wyoming-Colorado border in the State Line kimberlite district.
Although most of Wyoming has not been explored for diamonds, several diamondiferous kimberlite and related host rocks have been discovered in the state, along with some unrelated placer diamonds. The sources of these placer diamonds remain unknown.
Diamond-bearing rocks crop out in the Wyoming-Colorado State Line kimberlite district, in the Iron Mountain District in the east-central part of the Laramie Mountains, and in the Cedar Mountain area southwest of Green River. The diamond-bearing rocks outside of the State Line district have not been fully evaluated, and some have not been completely mapped.
Exploration
Diamond exploration often begins with a sampling program to locate indicator minerals, such as pyrope garnets and chromian diopside.
Concentrations of indicator minerals point to the potential presence of nearby hidden kimberlites and diamond deposits. During the last 30 years, the WSGS has identified several hundred concentrations of kimberlite indicator minerals, indicative of possible nearby hidden diamond deposits. Further exploration may involve geophysical surveys and drilling to identify and determine the size of kimberlites or other potential diamond-hosting rocks.
Because kimberlites and related diamond host rocks tend to be deeply weathered, they often occupy areas of low relief or are covered by deep soil and debris from adjacent rocks. This makes them quite difficult to find in most areas, although local conditions may allow some to stand out in relief. Soils derived from weathered kimberlite contain abundant montmorillonite clay, often support more vigorous growth of grasses than do surrounding areas, and may show a marked absence of woody plants. These vegetative anomalies and topographic depressions are clues that may point to the existence of hidden kimberlitic intrusions. Summaries of kimberlites and diamond exploration in Wyoming can be found in WSGS Reports of Investigations 53 and 54.
Mine Development
Development of a diamond mine begins with extensive exploration followed by claim staking where the minerals are federally owned or by leasing of state- or privately-owned minerals. However, finding economic diamond deposits is much more difficult than locating deposits of other minerals. Even in a world-class diamond mine, diamonds account for less than one part per million in the host rock. Once a diamond-bearing rock has been located, sampling to evaluate the diamond content of the deposit begins. Sampling progresses in stages beginning with a few tons. With favorable results, sampling increases to a few hundred tons and then to a few thousand tons. Continued favorable diamond showings at each step will eventually lead to full-scale mining.
A small Colorado diamond mine operated along the Wyoming-Colorado border from 1996 to 2003. The ore grade varied from 5 to 15 carats per 100 tons, and the mine produced many high-quality gems larger than one carat in size. The largest diamond extracted there weighed 28.3 carats. However, the mine closed due to legal problems rather than a lack of diamonds. The State Line kimberlite district has never been fully evaluated.
Economics
Diamonds are valued primarily as gemstones. Uncut diamond prices climbed nearly 50 percent between 2002 and 2006 due to growing worldwide markets. Prices retreated substantially following the 2008 recession, but recovered by 2011. Since 2012, diamond prices have fluctuated in response to various factors in the world economy and emerging competition from synthetic diamonds, according to Spektorov and others (2012). Recently, several prospectors, and at least one company, were actively exploring diverse locations within Wyoming for kimberlites and diamonds during 2017 and 2018.
The four Cs of color, clarity, carat weight, and cut determine the value of individual diamonds. A slight increase in carat weight can dramatically increase the value of a diamond. Exceptional stones command much higher than average prices, and cutting rough stones may increase their value by 10 times or more.
Diamond mines are based on the presence of gem-quality diamonds (larger is always better). The smallest size of recoverable diamonds, when defining ore reserves, is specific to each mine and mill. A mine’s reserves represent the economic material around which it is designed. Some large mines in Canada (the world’s third largest diamond producer) include diamonds as small as 2 to 3 millimeters in their ore reserve calculations. Low-quality and extremely small diamonds are used as abrasives but are not profitable to mine in the absence of gems.
The potential for new diamond discoveries in Wyoming is great, as is the possibility for one or more diamond mines in Wyoming’s future. Untapped prospecting opportunities for placer diamonds downstream from known kimberlites, and in areas where placer diamonds have been reported in the past, also abound in Wyoming. WSGS Information Pamphlet 12, Searching for Placer Diamonds by W. Dan Hausel (2004) gives detailed information for prospectors interested in hunting for placer diamonds.
Recommended Diamond References
Several WSGS publications describe Wyoming’s known kimberlite districts and address many aspects of kimberlite exploration. WSGS Report of Investigations 53 gives an overall summary of diamond exploration in Wyoming and the rest of the United States. WSGS Report of Investigations 18 includes a detailed map of the State Line kimberlite district, and WSGS Report of Investigations 54 contains maps and details on the Iron Mountain kimberlite district. WSGS Report of Investigations 56 addresses the geology and geochemistry of the Leucite Hills lamproite volcanic field. Additional detailed information concerning kimberlites and diamond exploration in Wyoming can be found in WSGS Report of Investigations 12, 18, 19, and 31.
For a complete listing of WSGS materials, go to the Online Catalog.
Hausel, W.D., Sutherland, W.M., and Gregory, R.W., 1995, Lamproites, diamond indicator minerals, and related anomalies in the Green River Basin, Wyoming: Wyoming Geological Association, 46th annual field conference, Guidebook, p. 137–151.
The Northern Miner, 2006, Mining explained—Diamond markets: The Northern Miner, August 4-10, 2006, v. 92, no. 24, p. 3.
Contact:
Christopher Doorn, christopher.doorn@wyo.gov