Peridot - Gem-Quality Olivine
"Grab your shovels boys, this is war! If these ants won't share their peridot, we'll just take it all!"
- W. Dan Hausel
In 1997, I decided to search the Leucite Hills lamproites in western Wyoming for diamonds. Diamonds had already been found in similar rocks in Australia, so I thought I should have a look in Wyoming. I didn't find diamonds, but mark my words - "Diamonds will be found". This is based on geology and geochemistry of some chromites from the Leucite Hills lamproites. And somewhere in the Leucite Hills, there could be some olivine-rich lamproites hidden under a few inches or feet of soil, similar to that in the diamond-rich olivine lamproites in Ellendale and Argyle, Australia. What happened in Australia was lamproite flows without olivine are relatively resistant to erosion, so they stand out as positive geomorphic features. However, diamond-rich olivine lamproites serpentinize to produce very soft rock that easily erodes and is quickly covered by blowing dirt and sand.
The Leucite Hills include several, geologically young lamproite volcanoes & flows that erupted 900,000 to 3.1 million years ago. This magma began its journey deep in the earth's mantle under one of the thickest parts of the Wyoming Craton. Cratons are very old, cool, continental cores necessary for melting deep within the earth's upper mantle (where diamonds are formed).
While exploring, I collected grab samples in the northeastern part of the field & two samples yielded chromites with similar geochemistry to those found as mineral inclusions in diamond - this means, some of these lamproites began their journey 90 to 120 miles beneath the surface where diamonds crystallize under high pressure and temperature.
Diamondiferous lamproites are found at Murfreesboro, Arkansas; Argyle & Ellendale, Australia; Majhgwan-Chelima, India; Kaplamp, Zambia; Aldan, Russia & Bobi, Ivory Coast. Diamonds are pretty much restricted to the olivine lamproites, which often alter to serpentinite (soft, easily-eroded material) when exposed to the atmosphere. The serpentine erodes quickly, thus such deposits are usually hidden in a field of leucite lamproites (which are much harder). This suggests a few diamonds may be found in lamproite volcanoes that have diamond-stability chromite. But the best place to search, is for hidden, diamond-rich olivine lamproites under a few feet of soil or under sand dunes in the Kilpecker dune field. Along the northern edge of this field, sand dunes mark the location of a major continental shear (Gordon Marlatt, pers. comm). Lamproites are known for colored stones - brown, yellow & the beautiful, extremely rare Argyle 'pinks' that have sold for as much as $US1 million/carat!
While searching for diamonds in this area, I began looking for olivine, and came across two green anthills at Black Rock - the ants collected olivine from the immediate area & decorated their hills. So I collected their hills & processed them for diamond - 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 in rock outcrops that was 0.5 inch across. Other material occurs in the soil. So where did the ants get the olivine? They picked it up from the adjacent soil - so take a screen and shovel to the Leucite Hills and start looking in the soil near the olivine-bearing lamproites - such as Black Rock. Remember, the ants are good miners, but they are limited in the size of material they can carry.
Do ants drink beer? Of course they do - nearly every critter with good taste likes beer. In one project by Dr. Tom McCandless who was at the time at the University of Utah, Tom may have drank some beer (hopefully shared some with the ants) broke the bottle and scattered shards all around an anthill in the Green River Basin. Bottle fragments were mapped on a grid to see what the ants carried to their hill. And yes, the little buggers loved their Bud Light and carried all kinds of shards to their hill. In some cases, the larger pieces were transported by two or many ants working together.
The peridot at the Leucite Hills is an amazing discovery. The area had been investigated by several geologists in the past, some very prominent individuals. For instance, the deposit was examined by one of the members of the Clarance King Survey, olivine was described by Dr. I. Carmichael of Berkeley, it was investigated in several MS theses and PhD dissertations, but no one paid attention to the quality of the stones. I sent some to the gem cutter in Sri Lanka, and low and behold, most nearly all yielded very high quality gemstones.
Even though olivine had been recognized in this area >100 years ago, no one had ever looked at the quality. Later, I mapped the Leucite Hills and identified all of the olivine bearing volcanoes.
HOW TO FIND PERIDOT
Peridot is typically found in deep-seated mafic and ultramafic volcanic and plutonic rocks. Olivine basalts are good sources. Most ultramafic rocks contain some olivine, but it is often replaced by serpentine; thus, good olivine in ultramafic rocks is hard to find. Search areas that have had considerable basaltic (mafic) volcanic activity in the geological past. Some good olivine has been reported near Peridot, Arizona, as well as at Kilbourne Hole in southern New Mexico.
Now if we can just teach ants to facet olivine, we can eliminate the middleman. Most olivine is a part of our earth's mantle (peridotite) and in rare cases, it can be brought to the earth's surface in volcanic rock. Such magnesium-rich magmas can include basalt as well as lamprophyre, lamproite and kimberlite. So where are the diamonds? Diamonds have actually been reported in rare basalts in Australia and found in lamprophyres, lamproites and kimberlites. Diamonds are found in kimberlite in Colorado, Montana, Michigan, Wyoming and possibly Kansas. Diamonds are also found in lamproite in Arkansas (and will likely be found one day in Wyoming and Montana lamproites) and found in some Wyoming lamprophyre dikes on Cedar Mountain.