DIAMONDS, DIAMONDS & MORE DIAMONDS

Gem diamonds from Kelsey Lake Colorado.

My first experience in diamond hunting occurred back when I mapped the Wyoming State Line kimberlite district. I  later mapped the Iron Mountain and Sheep Rock kimberlite districts and the Leucite Hills lamproite field. And when I had time, I  explored for diamonds as a consultant for US, Australian, and some Canadian companies. 

I searched for kimberlite, lamproite and lamprophrye in California, Kansas, Montana & Wyoming & identified, a few hundred cryptovolcanic structures within & surrounding the State Line in Colorado and Wyoming - some of which are likely diamond deposits (nearly all remain unexplored)! A few  include Indian Guide, Twin Mountain, Happy Jack & others. I expanded my research & found cyptovolcanic structures in Canada & even in the Kimberley region of South Africa, and found a major anomaly of 50+ anomalies sitting along Interstate-80 in Wyoming!

16.8 faceted gem diamond, Kelsey Lake, Colorado

Diamond deposits south of Laramie are in kimberlite pipes & placers. The kimberlites are deeply eroded & spilled millions of diamonds into the surrounding streams, but no one ever systematically looked for diamond in the creeks (even so, diamonds were accidentally recovered in Rabbit Creek along with a 5-carat diamond, and hundreds were recovered in George Creek, and several including a 6.2 carat diamond were recovered in Fish Creek, but the rest of the streams are mostly UNPROSPECTED!

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

Gem diamond with excellent characteristic
trigons on surface

Things to keep in mind: kimberlite will serpentinize because of water vapor, this produces a relatively soft rock that erodes faster than surrounding country rocks & usually results in a depression with different vegetation than the surrounding rocks. These depressions may contain shallow ponds. They are structurally-controlled such that >one anomaly is often found in a line. Because of calcium carbonate in kimberlite, carbonate will leach out into the pond staining the soil white. Keep in mind that salts are not all that uncommon in basins where lots of young sedimentary rocks occur with considerable carbonate. But in the craton basement (i.e., mountain ranges of Wyoming) there is no known source for carbonate, so if you spot a structurally-controlled lake surrounded by salt in old Precambrian rock, you might want to find out why? When you find one, typically, you will find others along the same structure.

Diamonds found in Colorado & Wyoming ranged from microdiamonds to 28.3-cts & included one chip from a 80- to 90-carat stone. Many people erroneously assume there are no commercial diamond deposits in this region - but all four diamond mills that were constructed in the district, were so poorly designed that they likely rejected as many diamonds as they recovered. Even so, commercial diamond deposits were encountered at Kelsey Lake and mined for only about 6-months before lawsuits shut down the operation. Thus, only the very top of the diamond pipe was touched by mining and thousands (if not many millions) of gem-quality diamonds lie in the host kimberlite and remain unmined!  And this doesn't even include the thousands of placer diamonds eroded from the kimberlite and carried downstream over geological time. 

A flawless, 14.2-carat octahedral diamond recovered from
the Kelsey Lake diamond mine. Photo courtesy of Howard
Coopersmith.

Then there is the Sloan 1 and 2 kimberlites in Colorado. DiamonEx from Australia was developing this property for commercial tests when the economy crashed in 2008 and put most diamond operations out of business including DiamonEx Ltd. Up to that point, the property appeared to be commercial based on the diamond ore grades.

All of the mills in the State Line district were so poorly designed they rejected diamonds of all sizes. The Kelsey Lake mill rejected anything weighing >40 cts! It also rejected many diamonds under 40-cts such that when the tailings were tested in 1997, the first sample yielded a 6.2-ct gemstone along with other diamonds!

High wall of Kelsey Lake kimberlite, State Line district, Colorado-Wyoming. Nearly all of the 
diamondiferous kimberlite was abandoned and never mined because of lawsuits. To this day (2022) 
this diamond deposit remains mostly unmined.

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 that diamond mill expertise was in short supply.

PERIDOT

In 1997, I decided to search the Leucite Hills in western Wyoming for diamonds (see photo at left). I didn't find diamonds, but mark my words - "DIAMONDS WILL BE FOUND HERE". This is an easy prediction to make based on geology.

The Leucite Hills consist of several lamproite volcanoes & flows that erupted 900,000 to 3.1 million years ago. They started their journey under one of the thickest parts of the Wyoming Craton. Cratons are very old, cool, continental cores necessary to have melting deep within the earth's upper mantle (where diamonds are formed). While exploring this region, I collected grab samples from lamproite in the northeastern part of the field & two yielded chromites with similar geochemistry to chromites found as mineral inclusions in diamond - thus this tells me some of the volcanoes began their journey at 90 to 120 miles beneath the surface where diamonds are common.

Diamondiferous lamproites are found at Murfreesburo, Arkansas; Argyle & Ellendale, Australia; Majhgwan-Chelima, India; Kapamba, Zambia; Aldan, Russia & Bobi, Ivory Coast. The richest deposits are typically found in olivine lamproites which often alter to serpentinite (a very soft material) that erodes quickly, thus such deposits are usually hidden within a field of leucite lamproites (which are much harder rock). Thus, this all suggests a few diamonds are likely to occur in the lamproites with diamond-stability chromites, but the locations for rich diamond deposits are hidden. Along the northern edge of this field, a large sand dune field that marks the location of a major continental shear - a favorable structure for hidden lamproites. One can almost guarantee there are diamondiferous olivine lamproites in this area hidden under a few feet of sand. Lamproites are also well known for colored stones - brown, yellow & the beautiful, extremely rare Argyle 'Pinks'. Some have sold for >$US1 million/carat!

While searching for diamonds, I started looking for olivine. In 1997, I came across two green anthills at Black Rock - the ants had collected all of the available olivine in the immediate area & decorated their hills. So I collected the hills & processed them for diamonds - but all we found were 13,000 carats of flawless peridot (gem-quality olivine). Some were 12 millimeters in length. I also found peridot in place that were nearly 0.5 inch across.
Even though olivine had been recognized in this area >100 years ago, no one had ever looked at the quality of the olivine. Later, I mapped the Leucite Hills and identified all of the olivine bearing volcanoes.