Peridot - Gem-Quality Olivine
In 1997, I decided to search the Leucite Hills 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 any diamonds, but mark my words - "Diamonds will be found". This prediction is based on geology and on chemistry of some chromites recovered from the Leucite Hills lamproites. And somewhere in the Leucite Hills, there could easily be a very olivine-rich lamproite hidden under a few inches or feet of soil similar to what was found in the diamond-rich olivine lamproites in Ellendale and Argyle, Australia. What happened in Australia was the olivine poor lamproite flows and volcanoes are relatively resistant to erosion, so they stood out as positive geomorphic features. However, the diamond-rich olivine lamproites serpentinize producing a very soft rock that is easily erode-able and quickly was 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 started its journey from 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 this region, I collected grab samples in the northeastern part of the field & two of the samples yielded chromites with similar geochemistry to chromites found as mineral inclusions in diamond - this means that some of the lamproites began their journey 90 to 120 miles beneath the surface where diamonds crystallize under high pressures and temperatures.
Diamondiferous lamproites are found at Murfreesboro, Arkansas; Argyle & Ellendale, Australia; Majhgwan-Chelima, India; Kaplamp, Zambia; Aldan, Russia & Bobi, Ivory Coast. The richest deposits are typically found in olivine lamproites which often alter to serpentinite (a very soft material) when exposed to the atmosphere. The serpentinite will erode quickly, thus such deposits are usually hidden within a field of leucite lamproites (which are much harder rock). Thus, this suggests a few diamonds may be found in the distinct leucite lamproite volcanoes that have diamond-stability chromites, but the place to search is for the hidden, diamond-rich olivine lamproites under a few feet of soil (after being eroded for a million or so years) or under the sand dunes in the Kilpecker dune field. Along the northern edge of this field, a large dune field marks the location of a major continental shear - a favorable structure for hidden lamproites. Lamproites are known for colored stones - brown, yellow & the beautiful, extremely rare Argyle 'Pinks'. Some have sold for >$US1 million/carat!
While searching for diamonds in this area, I started looking for olivine. In 1997, I came across two green anthills at Black Rock - the ants had collected olivine from the immediate area & decorated their hills. So I collected the 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 was found in the soil. So where did the ants get the olivine? They picked it up in the adjacent soil, so take a screen and shovel with your to the Leucite Hills and start looking in the soil near the olivine-bearing lamproites - such as Black Rock and remember the ants are good miners, but they were limited in the size of material they could carry.
Do ants drink beer? Of course they do - nearly every critter with good taste likes beer. In one research project by Dr. Tom McCandless, Tom drank some beer (hopefully he shared some with the ants) broke the bottle and scattered the shards all around an anthill in the Green River Basin. The 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 of all sizes to their hill. In some cases, the larger pieces were transported by two or more ants working together.
This is an amazing discovery. The area had been investigated by several geologists in the past, some very prominent individuals. For instance, the deposit was looked at by one of the members of the Clarance King Survey, the olivine was described by I. Carmichael of Berkeley, it was investigated in several MS theses and PhD dissertations, but no one paid any attention to the quality of the stones. I showed some material to the Wyoming State Geologist (who reportedly was a mineral collector and certified gemologist) and was told this was not gem-quality. I knew better, so I sent some into the gem cutter in Sri Lanka, and low and behold, most of the material yielded very high quality gemstones.
Some of the first peridots cut from the Leucite Hills lamproites. These were donated to the Wyoming Geological Survey and displayed in the publications section on the first floor of the Daniel N. Miller Building on the University of Wyoming campus.
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.
HOW TO FIND PERIDOT
Peridot is typically found in deep seated mafic and ultramafic volcanic and plutonic rocks. Olivine basalts are good sources of the material. Most ultramafic rocks contain some olivine, but if they often are replaced by serpentine and thus good olivine in ultramafic rocks is hard to find. Search the old metamorphic terrains as well as some area that have had considerable basaltic (mafic) volcanic activity in the past. Some good olivine has been reported in Arizona and at Kilbourne Hole in southern New Mexico.
Below is a group of faceted & raw peridot from the Leucite Hills anthills (now if we can just teach ants how to facet gemstones, we can eliminate the middleman). These specimens were donated to the Wyoming Geological Survey and displayed on the first floor of the Daniel N. Miller, Jr Building on the University of Wyoming campus. Imagine this rock was once nothing but peridotite and the peridotite melted in the earth's mantle producing a dark-gray basaltic rock (lamproite). Now you will have a clear picture of where most olivine (peridot) comes from. Most olivine is a part of our earth's mantle (peridotite) and in rare cases, it can be brought to the earth's surface if it does not all all melt. But when it does melt, it produces magma enriched in magnesium (which is the primary metal in olivine).
Such magnesium-rich magmas can include basalt as well as lamprophyre, lamproite and kimberlite. So where are the diamonds? Diamonds have actually been reported associated with rare basalts in Australia and have also been found in many lamprophyres, lamproites and kimberlites. In the US, diamonds have been found in kimberlites in Colorado, Montana, Michigan, Wyoming and possibly Kansas. Diamonds have also been found in lamproites in Arkansas (and will likely be found one day in Wyoming and Montana lamproites) and have also been found in some Wyoming lamprophyres.