HOW BLUE IS BLUE GROUND?

Dry placer (?) at Cedar Mountain, Wyoming. These rounded
boulders and cobbles are part of a lamprophyre breccia
pipe that has abundant mantle eclogite fragments, chromian
diopside and pyrope garnet, as well as diamonds
(photo by Dr. Richard Kuchera).

Revised 3/4/2022. 

"How blue is blue ground?" Before you answer, I guess we should learn a little about blue-ground. Blue-ground is in reference to altered clay- and carbonate-rich soils that often form over weathered, diamondiferous-kimberlite that was initially described as 'yellow ground' and 'blue ground' by South African diamond prospectors in the 19th century.

After alluvial diamonds were discovered in South Africa in the 1860s, prospectors searched for the source of these gems. Rich diamond placers were found along the Orange River in 1870 and these gems were traced to what early prospectors thought were dry placers. These ‘dry placers’ were small and filled with all different kinds of rocks that were mostly rounded boulders and cobbles, as if they had been stream polished and deposited by a creek or river, but these were localized in rounded to elliptical areas.

It wasn’t until shafts were sunk into the so-called alluvial deposits that the prospectors hit clay beneath the so-called ‘dry placers’. This clay zone was mistakenly thought to be the bottom of the diamond placers. As a result, some prospectors quickly sold interest in their claims. 

In reality, the miners dug into weathered kimberlite. As they dug into the clays beneath the dry placers, they discovered the blue clays contained diamonds. When they dug deeper, they intersected hard-rock that also contained diamonds that was christened ‘kimberlite’. 

Drill core from the Cedar Mountain pipes showing
breccia  (photo by the author).

Kimberlite is formed primarily of the mineral  olivine. When olivine alters at high temperature and pressure in the presence of water vapor, it produces green serpentine. All kimberlites are essentially serpentine breccias or serpentine porphyries. As the kimberlite sits on the surface of the earth for some time, the serpentine will breaks down into a blue clay and carbonate. The clay is montmorillonite and looks light-gray blue, and when wet, it looks blue. Carbonate is so abundant, that the rock, clay and just about anything to do with the kimberlite will yield carbon-dioxide when dilute hydrochloric acid is placed on then. When wet, the clay looks blue – hence the name, ‘blue ground’. 

Abundant chromian diopside, chromian enstatite,
pyrope garnet & almandine garnet picked from a
Cedar Mountain pipe (photo by Richard Kuchera).

Kimberlite initially erupts as magma in rare volcanic pipes. The magma drills its way through the earth’s upper mantle and crust starting at depths as great as 120 miles. As the magma gets close to the earth’s surface, it is under great pressure and erupts with tremendous force and acceleration. Imagine that bottle of diet coke and mentos. When it erupts, it probably looks similar to a kimberlite eruption. 

As kimberlite arrives at the earth’s surface, the magma is rich in carbon dioxide under pressure. The CO2 is released just like in the diet coke and momentos experiment, and the gaseous emplacement of the magma is so energetic, that it erupts at 3 times the speed of sound with emplacement temperatures of only 32oF! 

Why so cold? It is because of expanding CO2 gas that adiabatically cools. So now you know of one volcano where you can actually get frost bite if you happened to be nearby. But, you don’t want to be nearby as rocks and boulders are flung out like a shotgun blast erupting from the earth. 

Blue ground (weathered kimberlite) in
the Colorado-Wyoming State Line
district exposed in dozen trench (photo
by the author).

The kimberlite volcano is basically a carrot-shape pipe that tapers down to a dike at depth. Rocks trapped in the magma are polished as they are brought up from depth. This polishing effect results in rounded boulders that appear as a dry placer at the surface. The volcano also does not produce a cone (like most volcanoes) and instead produces a depression known as maar volcano, which often is later filled with a shallow, circular pond. More than 300 of these ponds and depressions containing carbonate rich soil and rounded boulders were found by the GemHunter prior to 2008. To date, only one was ever drilled, so, there are likely dozens, if not hundreds of unexplored diamond pipes in Colorado and Wyoming.

So when prospectors search for kimberlite, they look for depressions that give an impression of a impact crater. In fact, one kimberlite in northeastern Kansas was for many years known as the Winkler crater as it was thought to have been the site of an impact. We now know this structure to be kimberlite due to work by the late Dr. Doug Brookins. 

Many of these maar volcanoes remain distinct depressions throughout much of geological time as a result of the host rock (kimberlite) being softer and more easily eroded than the surrounding country rock. Serpentine is soft, as a result, the kimberlite erodes more rapidly. The chemistry of the serpentine and resulting montmorillonite clays also support different kinds of vegetation than surrounding granites, gneisses and schists. 

Some of the first blue ground that I (the GemHunter) came across in 1977 was exposed in badger diggings in kimberlites in the State Line district in Wyoming.  One of these I later called the Aultman 2 kimberlite and was the first kimberlite I had discovered in a district with several other kimberlites discovered by other geologists including David Eggler, M.E. McCallum, and Chuck Mabarak.

Blue ground exposed in highwall of the Kelsey
Lake diamond mine, Colorado-Wyoming State
Line district (photo by the author). This
diamond pipe was initially recognized by David
Eggler and then later explored and developed into
a commercial diamond mine by Howard
Coopersmith.

Exposed blue ground breccia pipe, Kelsey Lake diamond mine. (photo by the author)

Diamond mill at the Kelsey Lake diamond mine
(photo by the author).

Cross-section cartoon of kimberlite pipe.

Blue ground exposed in badger hole (photo by the author).

Jay Roberts in exploration trench in the search for diamonds in Wyoming. The dozer trench dug through sheared Sherman
Granite (pinkish orange) and intersected blue ground containing diamonds (photo by the author).

Backhoe trench in the search for blue ground, Aultman 2 diamondiferous kimberlite, Wyoming
(photo by the author)

The Maxwell diamondiferous kimberlite near Kelsey Lake in Colorado. This pipe shows
as a small, circular depression containing no trees (photo by the author).

Aerial view over the Sloan 5 kimberlite pipe in Colorado showing distinct
depression, open park with no trees, and a controlling fracture along the
right side that is outlined by the linear edge of pine trees. Such fractures often
support more than one kimberlite (photo by the author).

Why are diamonds rarely mentioned in the Bible? 

The reason is simple, diamonds were extremely rare during the writing of the Bible. It was rare for anyone too know anything about diamonds since the primary source was in India. And the few diamonds that were mined, were found in placers possibly by the 4th century BC, and the host rock was unknown and no lode mines were developed. Much later, some lodes were discovered in India, in the host rock known as olivine lamproite. Today's modern market and interest in diamonds began with discovery of detrital diamonds in the Kimberly region of South Africa in 1866. Soon, the source rock was discovered to be kimberlite pipes. DeBeers promoted their South African gems, and established a market for diamonds by 1900.