Prospecting for Gemstones

Some years back. The author in his lab
at the University of Wyoming

I searched for gemstone deposits in Wyoming, something that was nearly unknown in the cowboy state other than jade and some agates. The more I looked, the more I found, and then I began to uncover geological clues.

Found more gemstone deposits and previously unknown minerals in Wyoming than any other person. More than anyone should have been able to do. After developing some good prospecting models for finding gold and gemstone deposits - I went searching for more mineral deposits. 

Gemstones have unique qualities that make them attractive. If they are translucent to transparent, or have bright colors or color combinations, they could potentially be used in jewelry. Others just have unique shapes that make them attractive, such as some staurolite or cordierite (iolite) specimens. Sounds simple, but gemstones were almost completely ignored in Wyoming prior to 1977, and because some geologists stated that there were none (other than jade),  people believed them. But, I wasn't so sure, and after I began searching, I even found gemstones adjacent to highways and the interstate. The only other person to look for gem characteristics in minerals was Dr. J.D. Love of the US Geological Survey! Why? Well it likely goes back to education. In geology, students are not taught to look for gems - it's that simple. And instead of calling peridot "peridot", geologists call it "olivine". Instead of calling iolite "iolite", we are taught to call it "cordierite".

Then there are gemologists - they can recognize gems, but most have no idea how to find them in the field and few have any idea what they look like in nature, since they rarely see raw gemstones. So, the education system for geologists and gemologists has this one major flaw that worked in my favor. So, do you think universities bothered to correct this flaw? Not yet.

One of many tpublications
by the author

Take for instance the discovery of gem-quality peridot in Wyoming. Some years ago, while searching the Leucite Hills near Rock Springs for evidence of near-surface, buried, olivine-rich lamproite similar to those in Western Australia at Ellendale and Argyle, where some of the more attractive fancy diamonds in the world are found and mined. Nope, didn't find any diamonds associated with those lamproites, but did find some diamond-indicator minerals with good chemistry that suggest a diamond or two might be found in the Leucite Hills, while searching, I recovered over 1,300-carats of gem-quality peridot! Some famous geologists from the past looked at those same olivine crystals and saw nothing much of interest. I looked at them, and I saw gemstones!

In 1986, the Wyoming Geological Survey had a competent director who requested a special appropriation from the legislature to send me to Western Australia to study the newly discovered diamond pipes for about a month. Three things I brought back from Australia with me - (1) they have jack rabbits that are much taller than the Wyoming rabbits and the Aussie jack rabbits also have pockets, (2) the people speak funny, (3) and the newly discovered diamond deposits occurred in olivine lamproite - not kimberlite. And guess who had the largest lamproite field in North America? That's right - Wyoming! So, I had another project for my 'to do' list.

One of my finest achievements - I nearly drank
as much beer as half of the prospectors in the
Atlantic City Mercantile - and also published
a book on South Pass

So, after I returned from the International Kimberlite Conference in Australia, I went back to work mapping Wyoming's principal gold district - the 500-square mile South Pass greenstone belt near Lander. I don't want to get into a discussion on greenstone belts at this point, but you can read about them at wikipedia. 

In 1991, I finished the South Pass greenstone belt, published numerous papers, eight 7.5 minute quadrangles with geology, wore out a few pairs of field boots, tipped several beers with locals at the Atlantic City Mercantile, mapped 3 dozen underground mines, and published a summary of the district with a 1:50,000 scale map. I was proud of those accomplishments and even found some gemstones in the district: native gold (of course), helidor beryl (which led a prospector from Lander, by the name of Elmer Winters, Jr., to find a fabulous aquamarine gemstone in the Louis Lake pegmatites along the western edge of the greenstone belt). Elmer (RIP), was a good friend of mine and part owner of the Duncan mine. He dug on a solid pegmatite over one weekend using an old method known as single jacking from working in a mine. Good thing he dug it out, if it would have been me, it would have been in 10,000 pieces - but Elmer got it out in two pieces!

Then, I had another project, another, and another. I mapped the Seminoe Mountains greenstone belt north of Sinclair Wyoming, searching for gold and gemstones. The project was very positive - and at the end, I published several papers on the area, stepped on a rattlesnake, learned I could jump 10 feet high without a pole-vault, and found lapidary material in the banded iron formation, some fuchsitic quartzite, cuprite, malachite, chrysocolla, visible gold specimens at the Penn mines. Likely, the two most valuable discoveries were the occurrence of a potentially giant, Tertiary to Recent gold and pyrope garnet paleoplacer along the northern flank of the Seminoe Mountains. Actually, I did not discover gold in the paleoplacer. This was made by two wonderful people - Donna and Charlie Kortes! Yes, the Kortes Dam and everything else out there with the Kortes namesake, is named after Charlie and his family tree, but, if it wasn't for that mapping project, I would never have met the Kortes's, and we would most likely not know about that paleoplacer.

I still remember meeting them at the Sunday Morning mine. They wired together a couple of step ladders and lowered me into the mine. While I was exploring the extension of the mine tunnel, the thought occurred to me - "Hey, I don't even know these two. They could easily pull out the ladder and no one would find me for weeks". But, these two were absolutely wonderful, and they did let me out of the tunnel. Thank God!

Anyway, Charlie and Donna took me out near the Miracle Mile on the North Platte River which I wrote about in one a Gemstone Book. We started dry panning some of the dirt from the paleoplacer and finished panning at the North Platte - it all contained a few colors. While panning, I was more impressed by all of the pyrope garnets I found. Later, I was able to get some of the garnets tested for chemistry at a lab in Russia due to my connection with a co-author (Dr. Ed Erlich) on a diamond book we wrote. Later, we tested other pyrope garnets at the UW microprobe lab - and every garnet tested (not many as I didn't have any budget to speak of), tested to have a favorable combination of magnesium and chromian enrichment comparable to diamond inclusion garnets, that indicate somewhere in that region, there are some diamond deposits! So, when you are out in this paleoplacer searching for gold, there is a good chance you may pan out diamonds.

After finding significant gold in the Rattlesnake
Hills, I published the above book and several other
papers.

Anyway, I next went on to map the Rattlesnake Hills greenstone belt and found evidence of gold. In addition to gold, there was a jasperoid deposit with some uncommon fossil leaf imprints. After I finished reconnaissance mapping and exploring of the Rattlesnake Hills, I published several more papers on the geology and mineralization of that district.

So, how did I find so many gold deposits in the Seminoe, South Pass, Rattlesnake Hills greenstone belts and other mining districts in Wyoming? It was easy. I found hundreds of gold anomalies by using the following prospecting method. I went to places that had already been prospected! Then I lined up the prospect pits on aerial photos, and looked for evidence for additional mineralization, such as vegetation anomalies, quartz outcroppings, narrow resistant ridges (indicating silicification, i.e., veins), lineaments, and gossans. And I went searching the Rattlesnake Hills primarily because it was a greenstone belt with rocks enriched in gold, with several alkalic volcanoes that intruded the greenstone belt, providing heat engines and breccias necessary to mobilize gold from the rocks and concentrate the gold.

A 1.1-carat, ruby, from Palmer Canyon
Wyoming. Photo courtesy of Chuck
Mabarak.

The old prospectors found a lot of gold deposits in the 19th century. But, they only mined what they could find that was minable at a profit when gold prices were only $20.67 per ounce. Last I looked, the gold price was just under $1,300 per ounce. Because of the high gold prices of today, we can mine a lot of material that the old timers ignored and threw away. So, if you want to find a gold deposit, examine old gold districts, figure out the controlling structure(s) (where do all of the old mine dumps, head frames and prospect pits line up), and start walking along that structure. In between each pair of mines and prospect pits, the same gold structure is still under your feet, maybe just below a few inches of dirt, but its there. But also keep an eye out for low-grade gold. At the Carissa mine at South Pass, I found evidence of a giant gold deposit - a shear zone that was as much as 1,000 feet wide and more than 1,000 feet long, with a minimum depth of 970 feet deep and likely continues a thousand feet or more at depth. That is a lot of gold! And then, how much more gold has been missed from the Carissa to the Duncan and the Tabor Grand mines?  Wow, there is a real gold mine out there! Now, look at these gold structures and where are they cut by streams and gullies - yep, there are gold placers down slope and stream from these. Now this is a simple prospecting method.

A 1.4-carat iolite from Palmer
Canyon. Photo courtesy of Chuck
Mabarak

But these were not the only ones. I have to laugh at this next one because it is soooooo simple. After I found gem-quality iolite at Palmer Canyon, it occurred in association with other gemstones including ruby, sapphire and kyanite. The close association of the corundum minerals with a rock type known as vermiculite schist led me to five new ruby discoveries almost over-night. Arthur Hagner wrote a book on vermiculite in Wyoming in 1944. I had a copy, so I read through it and after a short time it was clear to me that the aluminum-rich vermiculite was an alteration product related to relatively high-grade metamorphism and such deposits will have aluminum-rich minerals - and hopefully some of those aluminum-rich minerals will have good color and clarity and yield gemstones. Corundum (ruby and sapphire) is aluminum oxide. Some other minerals such as kyanite, cordierite (iolite), andalusite, sillimanite and staurolite are all aluminum-rich silicates created at high metamorphic pressures and temperatures. So, all I had to do was to take Hagner's book and visit all of the vermiculite deposits I could in Wyoming. I couldn't get access to all, but five yielded rubies. For those of you who would like to find a ruby deposit for yourself, try doing the same in Montana and you will likely find some ruby deposits.

The author in Arizona.

Job 28:16