Diamonds

Diamonds from the State Line kimberlite district.

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 or 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 diamondiferous and potentially diamondiferous rocks.

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 and is believed to have a high potential for diamond deposits. Slightly younger (1.6 to 2.5 billion years old) accreted parts of the continent have a moderate potential for diamonds (Hausel, 1998). However, the accreted terrain of the Colorado Province includes numerous diamond-bearing kimberlites located in Colorado and along the Wyoming-Colorado border in the State Line kimberlite district. Most of Wyoming has high diamond potential.

Kimberlite with pyrope garnet. Kimberlite with chromian diopside.

Several occurrences of diamondiferous kimberlite and related host rocks have been discovered in Wyoming, along with some unrelated placer diamonds. The sources of these placer diamonds remain unknown. Most of Wyoming has not been explored for diamonds.

Exploration

Pyrope garnets, chromian diopside (green) and ilmenite (gray).

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 20 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 diamond host 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 structurally controlled topographic depressions are clues that may point to the existence of hidden kimberlitic intrusions.


Diamond exploration in Wyoming
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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 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, 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 23.8 carats. However, the mine closed due to legal problems rather than a lack of diamonds. Since diamonds were first discovered in the State Line district in 1975, more than 130,000 diamonds have been recovered, including several very large gemstones. The State Line kimberlite district has never been fully evaluated, although recent exploration across the area may change this.

Diamond-bearing rocks also crop out in the Laramie Range in the Iron Mountain District, and in the Cedar Mountain area southwest of Green River. These diamond-bearing rocks also have not been fully evaluated, and some have not even been completely mapped.

Economics

Diamonds are valued primarily as gemstones. Uncut diamond prices climbed nearly 50 percent between 2002 and 2006 due to growing world-wide markets. Prices of rough diamonds from active mines around the world recently averaged from as little as US$20 per carat to more than US$100 per carat ($2,835 to $14,175 per ounce).

Common Measures for Gemstones

This is, of course, a gross generalization in a market with thousands of diamond price categories based on the four Cs of color, clarity, carat weight, and cut. 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 individual 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 very 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 Reference Material

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 Preliminary Report 18 includes a detailed map of the State Line kimberlite district, and 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 Store.

Hausel, W. D., 1998, Diamonds and mantle source rocks in the Wyoming craton with a discussion of other U.S. occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p.

Hausel, W. D., 2004, Searching for Placer Diamonds: Wyoming State Geological Survey Information Pamphlet 12, 7 p.

Hausel, W.D., 2006, Geology and geochemistry of the Leucite Hills volcanic field: Wyoming State Geological Survey Report of Investigations 56, 71 p.

Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological and geophysical investigations of kimberlites in the Laramie Range of southeastern Wyoming: Wyoming State Geological Survey Preliminary Report 18, 13 p., 2 plates (scale 1:24,000).

Hausel, W.D., Gregory, R.W., Motten, R.H., and Sutherland, W.M., 2003, Geology of the Iron Mountain kimberlite district (with a summary of investigations of nearby kimberlitic indicator mineral anomalies in southeastern Wyoming): Wyoming State Geological Survey Report of Investigations 54, 42 p.

Hausel, W.D., McCallum, M.E., and Woodzick, T.L., 1979, Evaluation for diamonds-bearing kimberlite in Colorado and Wyoming – an evaluation of exploration techniques: Wyoming State Geological Survey Report of Investigations 19, 29 p.

Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological investigations of kimberlite in the Laramie Range of southeastern Wyoming: Wyoming State Geological Survey Report of Investigations 18, 13 p., 2 maps, scale 1:24,000.

Hausel, W.D., McCallum, M.E., and Roberts, J.T., 1985, The geology, diamond testing procedures, and economic potential of the Colorado-Wyoming kimberlite province – A review: Wyoming State Geological Survey Report of Investigations 31, 22 p.

Hausel, W.D., and Sutherland, W.M., 2000, Gemstones, and other unique minerals and rocks of Wyoming – A field guide for collectors: Wyoming State Geological Survey Bulletin 71, 268 p.

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream-sediment sample results in search of kimberlite intrusives in southeastern Wyoming: Wyoming State Geological Survey Open-File Report 88-11, 11 p. (5 plates) (revised 1993).

Hausel, W.D., Sutherland, W.M., and Gregory, R.W., 1995, Lamproites, diamond indicator minerals, and related anomalies in the Green River Basin, Wyoming: WGA 1995 Field Conference Guidebook, p. 137-151.

McCallum, M.E., and Mabarak, C.D., 1979, Diamond in state-line kimberlite diatremes, Albany County, Wyoming and Larimer County, Colorado: Wyoming State Geological Survey Report of Investigations 12, 36 p. The Age Company Ltd., 2005, Prices up – Diamonds not forever, miners find: , Accessed Dec.24, 2005.

The Northern Miner, 2006, Mining Explained - Diamond markets: The Northern Miner, August 4-10, 2006, Vol.92, No.24, p.3.