DIAMONDS (Ice) & THEIR HOST ROCKS

Ever wonder why diamonds are called 'ice'?  They kind of look like ice, but this is not the reason. Diamonds are heat conductors and if you ever have the opportunity to feel a large diamond pressed against your lips, you will notice the gemstone feels cool. This is because diamond is a good conductor and will conduct heat away from your lips, making it feel cool to the touch.

Diamonds are the most valuable commodity on earth based on dollar value per weight. Some diamonds have sold for many times the value of an equivalent weight in gold - such as the Hancock Red that weighed only 0.95 carat. The purplish-red diamond & sold for nearly US$1 million. To put this in perspective, one carat weighs 0.2 gram, which is equivalent to 0.007 ounce. Thus this diamond was valued at >300,000 times an equal weight in gold [1]! Several other pink to red diamonds have also been valued at nearly $1 million per carat.

Other priceless diamonds have been purchased by Royalty or donated to Royal treasuries. Most notable were those cut from the Cullinun rough, the largest diamond ever found that weighed 3,106 carats in the rough. The extraordinary gems faceted from this huge rough are now proudly displayed in the crown jewels of England. 

Diamonds are found with other gems; however companies ever bother with these associated gems even though many are extraordary stones in their own right and out-shine ruby & emerald. These other gems are known as Cape Ruby (pyrope garnet) and Cape Emerald (chromian diopside and enstatite).

Diamonds are found in several rock types, but the primary commercial host rocks are kimberlite and lamproite [2].

Kimberlite erupts as a small maar-like volcanoes and dikes. At the surface, the pipes (maars) appear as depressions: several have been mistaken for impact craters. Many occur as circular to elongate depressions with vegetation anomalies. They are controlled by fractures, have blue ground (montmorillonite clay) and are so carbonate rich their soils react with dilute hydrochloric acid and fizz as they emit carbon-dioxide gas. Most kimberlites pipes are <0.5 mile in diameter.

Gem diamond (14.2 carats) from Kelsey Lake, Colorado.
 
Several hundred cryptovolcanic structures discovered in northern Colorado and southeastern Wyoming likely include many undiscovered diamond deposits.
 
During mapping of the Iron Mountain ('fly to Farthing, WY, 82009' on Google Earth where you will be only 4 miles south of the district), State Line (fly to 'Tie Siding, WY' where you are now located 3 miles north of the State Line diamond district & if you search due south of Tie Siding along the State Line, you will see an elongated scar immediately south of Fish Creek, this is the former Kelsey Lake diamond mine - how many diamonds are in Fish Creek?), Sheep Rock districts hidden kimberlites were found in these areas [3,4,5].
 
Areas that remain unmapped include Indian Guide (fly to 'Indian Guide, East Albany, WY 82201' & you will be 3 miles due east of the Indian Guide cryptovolcanic structures and due west of the Iron Mountain diamond district), Twin Mountain, Happy Jack & others where there are many cryptovolcanic structures that could be kimberlites. One of these areas (Twin Mountain Lakes) a major district of 50+ cryptovolcanic anomalies were found sitting within view of Interstate 80 west of Cheyenne! Some may be the largest kimberlites in North America.
 
Lamproites were also mapped in the Leucite Hills in SW Wyoming: a few of these yielded diamond-stability chromites and could contain diamond. In one area, more than 13,000 carats of gem-quality peridot was recovered from just two anthills adjacent to one of these lamproites. This is exciting in that there is a strong correlation in diamond content and olivine (peridot) content of lamproites.
 
Commercial deposits occur in placers, kimberlite and/or lamproite [6]. Several lamprophyres also have diamond. The deposits south of Laramie are kimberlite & placers. Kimberlites were deeply eroded & spilled millions of diamonds into the surrounding streams, but no one has ever systematically looked for diamonds in the creeks!

Kimberlite is a ultrabasic, potassic igneous rock that erupts along fractures from 90 to 120 mi depths. They typically occur in very old cratons (basically ancient continental cores that consist of >1.5 billion year old granite, gneiss & schist). The kimberlite magma rises rapidly from the mantle with considerable water vapor & carbon dioxide in the magma. Some suggest gaseous emplacement velocities are on the order of Mach 3. The eruption is relatively cool: CO2 gas expands to cool the magma such that emplacement temperatures of 32 degrees F are not uncommon. This collection of unusual characteristics results in small, circular maar-like volcanoes (without cones) & dikes that are structurally controlled.

Keep in mind:
kimberlite will serpentinize because of water vapor, this produces a soft rock that erodes faster than surrounding country rock & usually results in a depressions with different vegetation than surrounding rocks. These depressions may contain shallow ponds. They are structurally-controlled such that more than one anomaly is often found in a line. Because of calcium carbonate in kimberlite, carbonate will leach into a pond staining the soil white. I
n the craton basement (i.e., mountain ranges of Colorado, Montana, Wyoming) carbonate sources are rare, so if you spot a structurally-controlled lake surrounded by salt in old Precambrian rock, you might want to find out why.
 
Diamonds found in Colorado & Wyoming ranged from microdiamonds to 28.3 cts & included one chip from a 80- to 90-ct stone. Some believe there are no commerical deposits in this area, but all 4 mills constructed in the past were so poorly designed they rejected many diamonds of all sizes. For example, the Kelsey Lake mill was designed to reject anything weighing >40 cts! It also rejected a very large number of diamonds under 40 cts such that when the waste tailings were later tested in 1997, the first sample yielded a 6.2-ct stone!
 
The grades of several kimberlites were high, the gem:industrial ratios were good & diamond values were reasonable. The biggest problem with the State Line district was good diamond companies with diamond expertise were in short supply.
 
Diamond is relatively easy to recognize, but there are very few prospectors, geologists or jewelers with experience in recognizing uncut (rough) diamonds. One of the easiest methods is to use a relatively inexpensive device marketed as a 'Diamond Detector' or 'Diamond Detective'. This measures the unique surface conductivity of a diamond. It is so easy to use that its rumored that some policicans can be taught to operate this push one-button instrument.  
 
Diamonds are the hardest minerals on earth - the only other minerals that have similar hardness include lonsdaleite and carborondo, both considered diamond polymorphs. Thus diamond will scratch essentially anything. But don't fall for that old trick of scratching glass.
 
Glass has a moh's hardness of only about 5.5 while diamond is 10. There are a lot of minerals inbetween these two (including quartz, the commonest mineral on the earth's surface) that will also scratch glass. This is why I thought it humorous when I received a call from a diamond prospector who claimed to have found thousands of diamonds - his method of testing was scratching his truck's windshield. Poor guy, I hope he can still see out his window. Ever hear someone call diamond 'ice'? This is because diamond has such excellent thermal conductivity that it attracts heat. Thus when touched on the lips, it will conduct heat away from your lips and give a sensation of feeling cool.
 
Diamonds also appear greasy and attract grease, and they have very distinct growth plates on the crystal surfaces.
 
Note the range of colors in the 'kimberlitic indicator minerals' (left).

CAPE RUBIES, CAPE EMERALDS, GARNETS

Associated with diamonds are a host of rare mantle nodules & gemstones known as Cape Ruby (pyrope garnet), Cape Emerald (chromian diopside & enstatite) that are always overlooked by mining companies. Yet these are very attractive, value-added gemstones. With some marketing , these could capture parts of the colored gemstone market. For example, many Cape Emeralds are more saturated & beautiful than emerald.
 

 

Photos (above) Faceted pyropes from Green River Basin, Wyoming.

 HOW TO FIND DIAMONDS

Over the years of looking for kimberlite and lamproite, I was amazed at how people continue to overlook these rocks and gemstones. On one field trip, I led a group of 50 geologists & prospectors to the Chicken Park kimberlites. They were told we would walk over kimberlites and they were to watch for the kimberlites, as I would not tell them when we walked over the deposits.  We walked over the kimberlites - not one person saw them until I took them back over the rocks to show them what they had missed (and these were considered relatively obvious kimberlites).  There are literally hundreds of kimberlites in Colorado, Montana and Wyoming that remain unexplored! So you as a prospector have an opportunity to find a major diamond deposit.  Here's how to find the obvious pipes:

Go to a good website such as Google Earth or Virtual Earth which has areal photography [7].  A good place to start would be the Kimberly Region of South Africa. Look for some diamond mines - now look in the areas surrounding the diamond mines (I found several probable kimberlites in this area). Do the same for the NW Territories of Canada (search the Ekati mine). The Ekati has about 120 kimberites in the area surrounding the mine, most are under shallow lakes.

False-color infrared aerial photo over Indian Guide district, Laramie Range. Note the (white) haulage road, and the several, structurally controlled depressions, some filled with water (black).

Kimberlites can be recognized by looking for fractures (lineaments).  Look at the above image (this is a false color IR image which means the colors are not natural). First, this is located in the craton (all of the rocks under the blue area are Precambrian and >1.5 billion years in age). This is known as the Indian Guide district in Wyoming west of Chugwater. I found this many years ago, but these remain unexplored and unsampled. Are they just ponds or are they kimberlites? Note all of the depressions - some form small ponds (water shows up as black in this image). All of these are structurally controlled (note all of the lines (lineaments) that project through the various depressions). Note that a couple of ponds have white rings (kind of like the bathtub rings we use to leave as kids for our mothers to clean up). On the ground, these will react to weak hydrochloric acid. There is no known source for calcium carbonate in these Precambrian rocks that we know of, so these may be kimberlites.

If you were to visit these on the ground, you would want to look for rounded bounders and cobbles in the depressions (characteristic of kimberlite), and look for blue ground clays, as well as look for the kimberlitic indicator minerals. Also look for diamonds. So how many kimberlites (or cryptovolcanic depressions) can you find in this one photo? At least 25.

What surprises me the most - Wyoming has one of the largest kimberlite-lamproite-diamond provinces in the world that extends south into Colorado. Montana has a separate province as does Kansas. A potential multi-billion dollar industry that the state's are ignoring.

References Cited

  1. Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.
  2. Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits - Origin, Exploration and History of Discovery. Society of SME. 374 p.
  3. Hausel, W.D., McCallum, M.E., and Woodzick, T.L., 1979, Exploration for diamond-bearing kimberlite in Colorado and Wyoming: an evaluation of exploration techniques: Geological Survey of Wyoming Report of Investigations 19, 29 p.
  4. Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological and geophysical investigations of kimberlites in the Laramie Range of southeastern Wyoming: Geological Survey of Wyoming Preliminary Report 18, 13 p., 2 plates (scale 1:24,000).
  5. Hausel, W.D., Gregory, R.W., Motten, R.H., and Sutherland, W.M., 2003, Geology of the Iron Mountain Kimberlite District & Nearby Kimberlitic Indicator Mineral Anomalies in Southeastern Wyoming: Wyoming State Geological Survey Report of Investigations 54, 42 p.
  6. Hausel, W.D., 1998, Diamonds & Mantle Source Rocks in the Wyoming Craton with Discussions of Other US Occurrences. WSGS Report of Investigations 53, 93 p.
  7. Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.

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Information Pamphlet 12 (Searching for Placer Diamonds)