Thursday, September 15, 2011

The Search for Diamonds In the Leucite Hills

Note that all the members on our field excursion to the outback
found the only two boab trees for hundreds of kilometers around
and at temperatures of 120 degrees F and above, shade was
at a premium.
I ended up looking for diamonds in the Leucite Hills: one indicator mineral for diamonds in lamproites is olivine. There is a correlation between the amount of olivine in lamproite and the possibility of finding diamonds that was recognized by Australian geologists several years ago.

When I visited the Ellendale lamproite field in Western Australia in 1986, some things were notable - first, it was hotter than Phoenix (but not by much). And the second was that there were a lot of leucite lamproites in the district that were similar to those in the Leucite Hills.

But then there were a couple of other interesting rocks in the Ellendale field that were not exposed at the surface. This was because these particular rocks had considerable olivine. The olivine serpentinized over geological time resulting in the breakdown of the rock. These olivine rich rocks were also diamond-bearing rocks that were buried under thin layers of soil in the outback.

These hidden lamproites were found under a few inches to a few feet of dirt because of the serpentinization process (serpentinization produces some magnetite that can be detected by magnetic geophysical surveys). Over the years, when I was at the Wyoming Geological Survey, I periodically had prospectors stop in my office with possible apple- and emerald-green jade specimens. They were curious as to why their jade was so soft. I would get out a magnet and show them that the samples contained pockets of magnetite (the rock was magnetic), and this was due to serpentinization that results in some iron separating from the magnesium silicates and reducing to magnetite. The remaining magnesium silicates are relatively soft.
Black Rock butte lamproite plug seen in the background.

But this magnetite is strong enough to result in a magnetic signature over a hidden lamproite pipe. And soon, an Australia company searched the Ellendale field and found hidden lamproites. The hidden lamproites all contained diamonds, unlike the lamproites exposed on the surface, and this was because of the correlation of the olivine, serpentinzation and diamonds.

So when I returned to the Leucite Hills, I started searching for olivine. This is how I made the discovery of more than 13,000 carats of peridot in just two anthills at Black Rock.

Then I started sampling some of the olivine leucite lamproites in the area as well as volcaniclastics for diamonds and indicator minerals. AND we recovered diamond-stability chromites from lamproites in the northeastern portion of the Leucite Hills. Diamond-stability minerals have the same chemistry as mineral inclusions found in diamonds - thus, it is thought that such minerals formed at depths in the earth where diamonds exist. Finding such indicator minerals suggests these rocks originated within a portion of the earth's mantle that has diamonds - so it is very likely one will find diamonds in the Leucite Hills (if only someone will seriously look).

But let's use Ellendale as an example. The real rich olivine lamproites in this area are also going to be serpentinized and the host rocks hidden. Where? If I were to search this area, I would start with aerial magnetic/conductivity surveys and start mapping out geophysical anomalies in the northeastern portion of the Leucite Hills. I would bet that there are hidden lamproites in this portion of the field and possibly further north under some of the dune fields.
Ellendale field with termite mound in foreground

So why doesn't somebody look? Good question. I tried to get the Director of the WGS to ask the legislature for funding for an airborne survey before I realized this guy was not in this position for the science. So, it never got done. Over the years while I sampled and mapped this region, I did recover one excellent, transparent micro-octohedron with surface trigons. Was it a diamond? Who knows?

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