Thursday, September 12, 2019

Diamonds, Gold, Fish, and Friends in the Seminoe Mountains of Wyoming

The Bradly Peak Hilton in the Seminoe Mountains, Wyoming
I met Charlie and Donna Kortes at my front door of the Seminoe Hilton (my tent) on Bradley Peak in the Seminoe Mountains greenstone belt in southern Wyoming. I was awakened by a knock on the canvas door. Rarely do people find me in the middle of nowhere, and the only one who ever knocked prior to this visit was Wiley Coyote with his acapello sharing songs around the campfire. So yes, I was curious as to who was knocking on a door?

I climbed out of my sleeping bag and unzipped my tent flap: there were two people standing outside, who I'd never seen before. They introduced themselves as Charlie and Donna Kortes! 

All kinds of structures and geographic features in this part of Wyoming include place names with their namesake, such as the Kortes Dam on the North Platte River and Charlie's Prospect in the Seminoe Mountains. These two wonderful people spent much of their lives as ranchers and rock hounds - a great combination. They wanted me out of my sleeping bag so they could show me the nearby Sunday Morning mine (often referred to as the Sunday Morning prospect). So, after yawning and clearing my eyes of the rising sun, we talked a little about rocks, about their ranch, and about how they found me. Apparently, they ran into a prospector in Sinclair Wyoming, who mentioned I was in the area - and knowing the layout of the land, they knew the best (and only) place to camp at Bradley Peak.

After sharing some stories, they took me over to the nearby Sunday Morning mine. Charlie and Donna grabbed a couple of short aluminum ladders with some bailing wire. At the entrance, we sat down while Charlie wired the ladders together and explained there was a drop-off a short distance into the old mine. Next we walked into the mine adit (portal) to the drop off. Charlie lowered the constructed ladder down to the next level - about 8 to 10 feet, but just enough to make it challenging to get out of the mine without the ladder. For some reason, the previous miners dug the tunnel, went straight in for a few yards, then decided to continue the tunnel 10 feet lower than the entrance. After placing the ladder, he told me to go ahead and he and Donna would wait outside for me. My thoughts ran wild - did he know something about this mine I should know? Were they going to steal my prized field 1975 Ford Bronco field vehicle with 300,000 miles and tent and leave me here to die? Thank goodness, it was neither, the Kortes' were just being neighborly and gave me an opportunity to see this mine while they sat outside. The mine didn't have much of anything in it, and the workings followed a narrow milky-quartz-vein containing minor cuprite. After taking two samples from the mine, I believe I got most of the ore body.

Massive red cuprite with green malachite and black tenorite,
Sunday Morning mine, Seminoe Mountains
Charlie and Donna next wanted to show me a gold deposit they found in the valley to the north of the Seminoe Mountains near the Miracle Mile bridge west of the North Platte River. In this area, they claimed was nothing more than dirt and sagebrush with lots of gold - so they proceeded to show me an extensive, previously unknown, dry gold placer, which I wrote about in my 2011 book on gold, and named it the Kortes placer in honor of my two new friends. 

We left Bradley Peak and drove through the sand dunes on the south side, turned north through scenic Seminoe Canyon and broke out on the north side of the range to see a very tired, worn out, desert valley (not sure if it is officially named) driving under a power line (42°11'54"N; 106°52'43"W) from Kortes Dam. The towers of the power line were located on an old alluvial surface that mostly originated from the Seminoe Mountains. We dug some dirt and took it to the bank of the North Platte River and panned tiny gold flakes from the dirt. It didn't matter where we dug in this alluvial fan, it had gold everywhere we sampled! My pan also retained some tiny, purple, pyrope garnets! I just was not expecting this!

Red cuprite in milky quartz from Sunday Morning mine
I was interested in the gold and new-found garnets. In the past, I had researched gold and diamonds deposits. During my research, I met a former Russian geoscientist living in Denver, who had access to an electron microprobe research facility in Russia  (the Wyoming Geological Survey had such a tiny budget, we couldn't afford to do this ourselves). So I gave my colleague (Dr. Erlich) the garnets and $20 from my paycheck, and weeks later,  we received news that all of the garnets from the Seminoes were 'G10' pyropes (high-Cr and -Mg garnets). G10 pyropes are chemically equivalent to pyrope garnets found as mineral inclusions within diamond! Some years ago, Dr. John Gurney from the University of Cape Town, South Africa, identified which types of garnets were associated with commercial diamond deposits - and this was one of these types of garnets! It most likely came from the diamond-stability field where the earth makes diamonds. So, if we could find the source area for the garnets, we likely would found a diamond deposit.

Gold in milky quartz (circled) from the Penn mines. A rule of thumb - whenever you see a hand specimen like these with visible gold, they will assay more than 1.0 ounce per tonne.

A few years later, I received grants for a mapping project in the Seminoes and did additional tests on other garnets I collected from the Kortes placer. Every garnet tested as G10. Even though this is was a tiny sampling of garnet, it suggests there is a diamond deposit somewhere in the area. 

Outcrop of folded banded iron formation
near the Sunday morning prospect.
As rock hounds, we should plan to spend time in the area looking for garnets in anthills. When I did a cursory survey of the area, I couldn't find any anthills. But how much gold and actual diamonds does this dirt and gravel from the alluvial fan contain? I'm not sure of the ownership of the land, but there must be some public land in the area - actually a lot of public land because it is mostly, worthless desert. And no one other than Charlie Kortes (RIP) may know the extent of the gold paleoplacer. We didn't even check on the east side of the North Platte River, but it likely extends into that area.

My recommendation to anyone who needs exercise, take a gold pan, dig some dirt, pan it in the North Platte River, and you will be a little richer in gold, health and you might even catch a fish or two. And look for diamonds, jade, jasper, agates, and banded iron formation. You might even start in places like Sunday Morning Creek, Deweese Creek, and other drainages. Even though they call these creeks in Wyoming, they are pretty much dry most of the year, but Deweese Creek would be my choice as it drains off Bradley Peak (where there are known gold deposits).

After searching the area using existing maps and aerial photography, my staff found a very nice, circular, depression in the Seminoe Mountains on the east side of the North Platte River. This depression was designated as the Seminoe Mountains cryptovolcanic anomaly (42°9'42"N; 106°52'22"W), and was later drilled by a diamond exploration company. The company later told me that they were puzzled by the anomaly as it appeared as a piston-like graben filled with soil, that bottomed out in granite (Howard Coopersmith, personal communication). Personally, I think they should have continued drilling a little deeper, as kimberlite pipes (a primary source of diamonds) often capture large blocks of foreign rock, known as xenoliths, when they erupt. Even a group of diamond-bearing kimberlites in Colorado, known as the Sloan ranch kimberlites, have some impressive granitic xenoliths that were intersected in underground workings in the 1980s. Then, there is the Kelsey Lake diamond mine in Colorado, portions of the kimberlite are covered by a layer of granite.

Well, then there is the Bradley Peak gold deposit. At the base of Bradley Peak are some old gold mines, known as the Penn mines. These all have small, quartz veins with visible gold! In fact, in 1981, I accidentally started a gold rush after finding gold in quartz and in one sample of quartz cutting banded iron formation. After mentioning it to a reporter, the article showed up in the newspaper and soon the Seminoe Mountains were staked by dozens of mining claims! 

In this same region, some years ago, Dr. Terry Klein of the US Geological Survey identified a large, circular, propylitic alteration zone that also surrounds the gold-bearing veins at the Penn mines. This is mostly in metamorphosed basalt and includes some banded iron formation, and could potentially host a large disseminated gold deposit with high-grade veins.

Oh, and one last note - when I camped at Bradley Peak for the summer, it was one of two places where my tent was surrounded by some kind of critter, alien, skinwalker, or who knows what?! In the middle of the night, it ran circles around my tent at a very high speed. It woke me up, and I searched outside with a flashlight and my bear gun, and found no evidence (or tracks) of anything. It happened a couple of times in the Seminoes, and also happened in the Lewiston district of South Pass south of Lander. It both cases, it ran in a clockwise direction, right next to my tent. Based on the sound of the critter, I would guess it was about the size of a mid-size dog, but no panting, and no other sounds other than the sound of its feet, paws moving as fast as they could go. To this day, I still have no idea what it could have been?

Polished banded iron formation sample from Seminoe Mountains.
Spinifex-textured komatiite, Seminoe Mountains, Wyoming.

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