The Diamonds of the Wyoming Craton

Bedrock map of North America   USGS

This is a tale about how riches were overlooked in Colorado, Montana and Wyoming by gold seekers in days of yore.  In these three states a whole diamond province has been overlooked in the kimberlites by the hundreds that underlie these states.  The diamonds can also be found in lamproites, lamprophyres and other cryptovolcanic structures that are associated by the Wyoming Craton underlying these three states.  Many of the larger targets lie right next to I-80 between Cheyenne and Laramie that remain unexplored.  This area was originally discovered by early gold miners who discovered it contained little or no gold so they passed it on.

Subsequent discoveries indicate this may be one of the most unexplored diamond provinces in North America where the diamonds can be found in the carbonate rich sediments found in a Archean gneiss.  More then fifty depressions have been discovered here suggesting cryptovolcanic activity that lie east of the State Line diamond district.  It is thought these depressions could be kimberlites containing possibly large amounts of diamonds. 

Ongoing exploration suggests that the Wyoming Craton contains a world-class diamond province and like the diamond province near Marathon, Ontario also contains large deposits of colored gemstones as rubies and sapphires.  This craton is found covering a large portion of Canada.

The Wyoming Craton aside from covering several western states also includes exposures in Alberta and Saskatchewan. Diamonds have been discovered in both the kimberlites and lamprophyres in Montana, Wyoming, Colorado, Kansas and Nebraska as well as Canada.

Another potential place where you can look is in any conglomerates that are derived from the lamprophyres and kimberlites of the Wyoming Craton.  One of my prospecting buddies has found a diamond deposit like this near Marathon, Ontario.  Another Canadian prospecting buddy that lives in western Ontario has also found probable diamonds in his part of the province.

It isn’t just diamonds that can be found there; recently one of the world’s largest deposits of rare earth metals was found in Nebraska,  When you start poking around in this ancient craton you never know what you’re likely to find.

The Wyoming Craton has the greatest southwestern outcrops of Archean rocks in North America.  Theoretically terranes like this have deep roots in the lithosphere that according to Clifford’s rule this deep keel is necessary for the formation of kimberlites with their association with diamonds.  Although this terrane wasn’t deformed during the

Ancestral Rockies Orogeny it was pervasively deformed during the Laramide Orogeny and one theory has it that its deep keel was removed partially or wholly allowing it to be deformed during the Cretaceous or later.  

Finding Diamonds in California

Uncut diamonds

From the earliest days of the California Gold Rush diamonds were found in the tailraces of sluice boxes that were used for gold washing.  Diamonds were found not only in recent stream gravels but much older gravel that was buried under layers of lave and tuff from ancient volcanic activity as fossilized stream channels that ran along the mountains from north to south.  These stream channels indicated an ancient stream flow that was almost at right angles to the present day flow.

The biggest problem with California diamonds is there just are not enough to make going after them profitable. Most of the stones have been found as a result of gold washing operations in both present day streams and in the fossilized beds of earlier streams. Many of the diamonds were destroyed by the use of stamp mills used the breakup the gold ore retrieved from the fossilized streambeds. Their presence was noted as fragments seen in the gold ore. The diamonds that were recovered from ore were for the most part small or industrial grade diamonds.

As early as 1853 diamonds were discovered on top of Table Mtn. about thirteen miles north of Oroville on state Highway 70 close to the ghost town of Cherokee.  It was in the gold diggings surrounding the town that reportedly more then 300 diamonds were found most of them being of industrial grade.  Claims have been made that at the time this was the largest discovery of diamonds made in North America.  This has been exceeded with the discovery of several diamond producing areas in Canada.

Cherokee was the site of the only diamond mine in California, and it was here the first diamonds were found.  Mike Maher discovered a perfect blue diamond when he was cleaning out his sluice box in 1866.  Other diamonds were found here with one of them weighing six carets, but because gold was the primary concern it is unknown how many millions of dollars worth were discarded with the mine tailings produced by the hydraulicing for gold with high pressure jets of water that washed away the gold ore.  

The fact that diamonds do occur in California is well attested to in newspapers and scientific journals.  In some cases however it is readily apparent that many of these so-called diamonds were in fact quartz crystals that had become rounded because of stream action.  One such quartz crystal had been used as a marker in a game of marbles where it show several percussion marks that were the result of being hit by marbles.  A diamond would not show such percussion marks, but would rather cleave into pieces instead waiting to be found in the sluice boxes at work in the area.

Kimberlite the source for Diamonds

Kimberlite is a potassic volcanic rock best known for sometimes containing diamonds.  It was originally discovered in Kimberly, South Africa in 1871 hence its name.  The miners digging in the “big hole” found a large diamond weighing 83.5 carets that spawned the South African Diamond Rush.  There have been literally thousands of kimberlites that have been discovered since the initial discovery in 1871.  Of all the discoveries however only about one kimberlite in three hundred actually become diamond mines.  Diamondiferous kimberlites are actually quite rare.

In cross section a kimberlite is a vertically standing feature called a diatreme in the earth’s crust that is shaped like a big carrot.  Where in the earth’s mantle kimberlites originate has been a matter of controversy for decades the one thing we are sure of however that they come from a depth greater then the so-called diamond window at more then 140 Km below the earth’s surface. 

The highly complex volcanics that solidify into kimberlite or lamproite are not the source of diamonds they are the elevator that brings them to the surface.  The volcanics rise through a complicated set of cracks and fissures until they near the surface where their extreme pressure causes them to blast an opening through the surface called a pipe.  In the case of kimberlites the pipe is carrot shaped, and in lamproites it is shaped like a champagne glass.

Cross section of a kimberlite chimney
By Heriberto Arribas Abato

Kimberlite and lamproite are similar magmas full of debris from the mantle of the earth as xenoliths with the actual magma acting as glue to stick the mass together.  Diamonds found in kimberlites or lamproites are part of the debris brought to the surface.  The magmas are rich in magnesium and volatile components such as water and carbon dioxide as these volatiles near the earth’s surface they explode into the characteristic carrot or champagne craters.  At the time of eruption they also form small conelike features on the surface of the earth that are quickly eroded away by the process of weathering.  Both types of eruption are quite rare and small in size.  They may also occur as dikes or sills in the country rock which undergoes considerable fracturing as the magma reaches towards the surface.  The rocks in the lithosphere are very brittle making it easy for kimberlites to reach the surface.

There are several different compositions related to kimberlites and they are named after the predominant mineral.  Normally the most abundant mineral associated with the primary magma is olivine making the kimberlite with olivine known as a periodotic kimberlite.  Other types are eclogitic, websteritic, sublithospheric and uncertain.  Any of these may contain diamonds.    

References:

Mineralogical Association of Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series, Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits, http://amonline.net.au/geoscience/earth/gem.htm
Geology of Gem Deposits, Mineralogical Association of Canada, Editor Lee A. Groat, http://www.mineralogicalassociation.ca/doc/promo_SC37.pdf  Volume 37 © 2007
Industrial Minerals and Rocks, Page 418, http://books.google.com/books?id=zNicdkuulE4C&pg=PA417&lpg=PA417&dq=kimberlites+and+the+occurance+of+diamonds&source=bl&ots=Nhqev_Ebqc&sig=WQyRiIxdpBMtnaZWwhGwKC5o1sY&hl=en&ei=csyvSbDZAojWnQfAmpTNBQ&sa=X&oi=book_result&resnum=9&ct=result#PPA418,M1
Kimberlite, Wikipedia the free encyclopedia, http://en.wikipedia.org/wiki/Kimberlite
The Nature of diamonds, Kimberlites and Lamproites, American Museum of Natoual History, http://www.amnh.org/exhibitions/diamonds/kimberlite.html

Inclusions found in Diamonds tell quite a Tale

A flaw seen near the center of a diamond crystal - Wikipedia

The inclusions found in a natural diamond are a window on the interior of the earth.  Basically there are two types of inclusions found in a diamond.  The first type is a Syngenetic inclusion that was formed in the natural process of crystallization was ongoing.  The second type is the Epigenetic inclusion that was formed after the diamond was made.

The most serious flaw is a cleavage that can compromise the stability of a diamond.  These cleavage cracks are straight.  If they are serious enough they can split a diamond along the cleavage crack.  The diamond cutter often takes advantage of this flaw to cleave a diamond into two or more pieces.  They will also cause a diamond to split if accidentally subjected to a sudden high pressure event.

Bearding is hair-like cracks that often happen during the cutting of a diamond usually occurring around the girdle.  This is the area in the Anatomy of a Diamond where the setting prongs grasp the diamond.  Sometimes this flaw is also called “girdle fringes.”

Growth lines are sometimes called grain lines they typically form while the diamond crystal is growing.  If irregular crystallization occurs this is often a reason why these lines form.  If they are colorless they don’t affect the diamond’s clarity.  Sometimes though they are white or colored they are more visible and do affect the value of the diamond.

The next group of inclusions is man made that called ‘laser lines’ and can come from several different operations on the diamond.  The most common type is where the diamond cutter has used a laser for removing dark or prominent inclusions from the diamond.  The second most common is from the use of a microprobe used to analyze the tiny crystals of other minerals that are Syngenetic inclusions in the diamond. 

Feathers derive their name because they actually look like small feathers inside a diamond.  These seem to be Syngenetic tiny cracks that formed with the diamond.  Small feathers seem to be harmless, but if they reach the surface of the diamond crystal then can increase the risk of breakage. 

The last group of inclusions is definitely Syngenetic in nature; they are called pinpoints.  These are crystals of other minerals that were trapped inside the diamond crystal as it formed.  These are true windows into the interior of the earth as they were formed billions of years ago.  These same crystals are used by geochemists using a microprobe to determine their composition, and learn about the interior of the earth.  The most common inclusions are pyrope garnet, pyrite, zircon, eclogite and other rare minerals.  Sometimes an inclusion inside a diamond is what is termed a negative crystal which is just a void looking like a diamond crystal.  Other inclusions are minute drops of liquid usually water or liquid carbon dioxide.

Flaws are the characteristic inclusions found in a diamond.  They are called flaws because their presence means that the diamond is not perfect.  These inclusions act like fingerprints because no two of them are the same they are used to identify individual diamonds if they are lost or stolen.  Other inclusions affect a diamond’s clarity causing it to be less brilliant as they interfere with the light as it passes through a diamond.  There are other inclusions that can cause a diamond to fracture if it is subjected to a sudden sharp blow.

References:

Mineralogical Association of Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series, Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits, http://amonline.net.au/geoscience/earth/gem.htm
Geology of Gem Deposits, Mineralogical Association of Canada, Editor Lee A. Groat, http://www.mineralogicalassociation.ca/doc/promo_SC37.pdf  Volume 37 © 2007
Industrial Minerals and Rocks, Page 418, http://books.google.com/books?id=zNicdkuulE4C&pg=PA417&lpg=PA417&dq=kimberlites+and+the+occurance+of+diamonds&source=bl&ots=Nhqev_Ebqc&sig=WQyRiIxdpBMtnaZWwhGwKC5o1sY&hl=en&ei=csyvSbDZAojWnQfAmpTNBQ&sa=X&oi=book_result&resnum=9&ct=result#PPA418,M1

Kimberlite, Wikipedia the free encyclopedia, http://en.wikipedia.org/wiki/Kimberlite

Diamond Inclusions, http://jewelry.about.com/cs/diamondclarity/a/inclusions.htm

How Diamonds are Formed

This shows the Earth's mantle where diamonds are formed.
Courtesy of the Debeers Group.

A vast amount of knowledge has been developed since the 1980s about how diamonds were formed.  It has been through the inclusions found in diamonds that much of the information about how they were formed and their origin was discovered,  Inclusions are all natural materials that are found in diamonds including some that are gaseous, liquid and solid inclusions.  Although theoretically it is possible to form diamonds by squeezing a lump of coal in nature that hasn’t happened although in the lab this had been done under conditions of extreme heat and pressure in giant hydraulic presses,

Some of the diamonds that have been found in meteorites that are more then 4.25 billion years old far exceeding the age of life on Earth.  However natural diamonds on earth were formed at least 990 million years ago.  One of the discoveries made since the 1980s shows that diamond is formed whenever you burn acetylene gas that forms as a coating on metal that has been exposed to burning acetylene that is often deposited during the course of oxy-acetylene welding.  This type of diamond coating has since been made use of industrially for imparting a hard coating onto metals.

How a diamond bearing pipe is formed.
Courtesy of the Debeers Group 

It takes extreme pressure that ranges from 40 to 50 thousand times higher then the pressure found at the earth’s surface of 14,7 pounds or 1,000 millimeters that is termed one bar.  This is the pressure encountered at depths ranging from 125 to 200 kilometers below the surface   Most diamonds are formed from kerogene that was deposited in vast quantities on earth during its formation.  There are other diamonds that were formed from carbonaceous material like fossil coral or shells that have been subducted deep into the earth’s crust where they were also exposed to extreme heat and pressure.  A few diamonds were even formed at depths ranging from 300 to 400 kilometers deep or even further, but these diamonds are extremely rare.  Most diamonds were formed at temperatures that exceeded 900⁰ to 1,300^o Celsius.

Formation and Emplacement of a Kimberlite Pipe

At about 125 kilometers beneath the surface of the earth the pressure of the magma cracks the surrounding rock until the cracks reach the earth’s surface.  Much of this cracking is due to contained carbon dioxide in the magma that causes the magma to bubble and expand.  This is caused either by heat from below or reduced pressure as the magma column reaches towards the surface.  It is this expansion that eventually causes the magma to erupt explosively at the surface.

An eruption like this has never been observed by mankind because it has been millions of years since one occurred on earth predating mankind.  It has been theorized that an eruption of this type rather then being hot like most volcanic eruptions are; it is emplaced under freezing temperatures because of the rapid expansion of the volcanic gases.  In the final few kilometers these gases rise at the speed of sound where they form a pipe following the path of least resistance through the surrounding rock finally reaching the surface.

It is because of this rapid ascent of magma that allows the diamonds to reach the surface of the earth rather then being converted into graphite a far more stable substance at the earth’s surface.  The pressure him pardoned by dissolved gases in the magma eject the diamonds they don't have time to be degraded into graphite.  The diamonds are actually xenoliths, particles caught up in the magma train as foreign objects.  Once the volcanic cone cools it starts too erode releasing the diamonds into the soil. The diamonds that are not eroded state trapped in a kimberlite or lamproite where they are mined.

Different Types of Diamond Deposits

How diamonds are mined.
Courtesy of the Debeers Group.

Diamonds are found in two different types of deposits of which primary deposits come first.  These are deposits found in kimberlite, lamproite or komatiitef pipes and are mined lake any other mineral found in hard rock.  Secondary diamonds are found in river gravel deposits where they are concentrated after traveling for some distance from their primary source.  The gravel found in the Orange River in South Africa is an important source of diamonds.

Often overlooked by many is a third source of diamonds found in a rock type called conglomerate that are really fossilized river gravel.  One important formation like this is found near Marathon, Ontario that not only carries diamonds, but rubies and sapphires as well..

Appalachian Diamonds

A diamond crystal in matrix
Photo by Rob Lavinsky

Look out, alluvial diamonds have been found scattered along the backbone of the Appalachians for years making it likely that sometime in the future a lucky prospector is likely to find a kimberlite or lamproite the two types of rock where diamonds are apt to be formed.  Diamonds have been found in Virginia, West Virginia, North and South Carolina, eastern Kentucky and Tennessee, Georgia and Alabama.  All of these states are below the area that was affected by the continental glaciers of the past million years making it likely that their host rocks are in place.

At the time of its discovery in April 1928 the famous “Punch” Jones at 34.43 carets was the second largest diamond ever found in the United States.  The Uncle Sam diamond was found in 1924 at Murfreesboro Arkansas in the Prairie Pipe Mine weighing at the time of its discovery 40.23 carets is the largest diamond ever found in North America.  The Punch Jones diamond is presumed to have washed Rich Creek from neighboring Virginia.   There are five sites in or near to Virginia where diamonds have been found one in kimberlite and one in peridotite; it is presumed all of these diamonds came from similar rock types.

Brilliant Cut Diamonds

North Carolina produced 13 diamonds in the mid-1800s.  The North Carolina Geological Survey has recently undertaken studies of lamproites in the Charlotte area to better understand where these diamonds originated and to discover clues as to their origin for other exploration projects in the Piedmont.  The other Southern States produced one or two additional diamonds.  The lack of glaciation in all these states suggests a local origin for theses diamonds.  Lamproites and kimberlites have been discovered in the immediate areas where most of these diamonds have been found, but none have been found since WW II when gold mining operations ceased in the southern Appalachians.

Resumption of placer mining operations make the discovery of diamonds likely.  Most of the diamonds discovered in the past were from placer mines with a few exceptions.  The Punch Jones diamond is one of these exceptions it was discovered when Punch Jones and his father were playing horseshoes.  Although this diamond was discovered in 1928 it took until 1942 before its true identity was established.

Fortunately diamonds have some distinct minerals associated with them making it possible to find diamond deposits by tracing the diamonds and other associated minerals to their source.  These minerals include type “G” pyrope garnets, chrome diopside, phlogophite mica and serpentine.  The mineral illmanite is frequently found associated with diamonds too.  Tracing these minerals is often done with soil samples taken below the “A” horizon that contains organic matter.  

Diamond the Hardest of all Gems

Kimberlite
Photo by Subarcticmike

The very name for the diamond is “adamant” meaning roughly that it is indestructible.  The gem we call diamond was known to the ancients when practically the entire earth’s supply came from the region around Golconda, India.  This region in India supplied most of the diamonds in the world until the discovery of alluvial diamonds in Brazil in 1729.  The diamond fields of Brazil produced most of the world’s diamonds for almost a century until diamonds were discovered in South Africa in 1867.  This led to the discovery of diamonds in an ultramafic rock that was named kimberlite after the South African town of Kimberly where the new rock was discovered.

A diamond crystal in a matrix of kimberlite. USGS

The other source of diamonds during antiquity was from western Borneo where they are found in alluvial deposits of Eocene age.  Most of the stones found in these deposits are small and of poor quality.  These diamonds are still being mined mainly by artesianal methods under primitive methods.  To this day nobody has ever discovered the primary source of these diamonds.

During the post World War II period extensive deposits of diamonds were discovered in the Soviet Union by tracing a special type of pyrope garnet that is in high in chromium to its source.  The source was found in kimberlite pipes in Siberia.  Several mines were opened causing the Soviet Union to become one of the world’s largest producers of diamonds.

A princess cut diamond mounted in a ring.
Photo by Stephen Durham

After the discoveries in the Soviet Union the scene shifted to Australia where a new type of deposit was discovered in a type of rock called lamproite.  This has resulted in the development of the largest diamond mine on earth, the Argylle Mine.  Some pf the diamonds found in this mine are colored purple an extremely rare type of diamond.

The diamond discoveries in the Soviet Union and Australia led to the discovery of kimberlite deposits in Canada, Brazil and India.  There are also deposits producing diamonds in Arkansas and eastern Colorado and Wyoming.