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.
Kimberlite with pyrope garnet.
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Kimberlite with chromian diopside.
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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.
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.
Diamonds in Wyoming.
(Click image to enlarge)
Exploration
Diamond exploration often begins with a sampling program to locate indicator
minerals, such as pyrope garnets and chromian diopside.
Pyrope garnets, chromian diopside (green) and ilmenite (gray).
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.
Diamond exploration in Wyoming.
(Click image to enlarge)
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.
Vigorous grass growth on an Iron Mountain kimberlite (foreground) with blue-gray
clay excavated from trench; less vigorous vegetation on granitic soil in
background. Photo by W. Dan Hausel.
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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).
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.
Related Links
Please visit
WSGS Gemstones group page for more information.
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.
References
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:
<www.theage.com.au/news/world/prices -up-diamonds-not-forever>, 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.