Thursday, September 12, 2019

Gold and Diamond Indicator Minerals Discovered in Wyoming's Seminoe Mountains

The Bradly Peak Hilton in the Seminoe Mountains, Wyoming
I met Charlie and Donna Kortes at my front door step of my tent on Bradley Peak in the Seminoe Mountains greenstone belt in Wyoming. I was awakened by a knock at my door (of my tent). Usually, people don't find me in the middle of nowhere, and the only one's who ever knock before were coyotes wanting to share some songs around the campfire.

So, I unzipped my door, and there were two people standing there I had never seen before. They introduced themselves as Charlie and Donna Kortes - and yes, much of the state of Wyoming in this region has place names using their namesake, such as the Kortes Dam on the North Platte River, or Charlie's prospect in the Seminoe Mountains. We, these two lovely people were ranchers and rock hounds - a great combination, and they wanted me out of my sleeping bag so they could first show me the Sunday Morning mine (often referred to as the Sunday Morning prospect). We went to the mine adit (entrance) and Charlie used bailing wire to put two ladders together to lower me down into the workings. For some reason, the old miners who dug this tunnel, went straight in for a few yards and then decided to continue the tunnel about 10 to 12 feet lower, requiring the use of a ladder. After that, he sent me in by myself. My thoughts included - did he know that there was something dangerous in here? Or was he going to steal my prized field office and leave me here to die? Actually, it was neither, he was just being very neighborly and gave me an opportunity to see this mine. It didn't have much of anything in it, other than a milky quartz vein with some cuprite - and in two samples, I believe I got most of the ore body.

Massive red cuprite with green malachite and black tenorite, Sunday
Morning prospect, Seminoe Mountains
Charlie and Donna next wanted to show me a gold deposit in the valley to the north of the Seminoe Mountains near 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 later wrote about in my Gold Book, and called it the Kortes placer in honor of my two new friends. 

We left Bradley Peak and drove under a power line (42°11'54"N; 106°52'43"W) built into an old alluvial surface that originated from the adjacent Seminoe Mountains. We dug out some dirt and took it to the bank of the North Platte River and panned tiny gold flakes out of the dirt. It didn't matter where we dug in this old alluvial fan, it had gold! My gold pan also retained some tiny, purple, pyrope garnets!

Red cuprite in milky quartz from Sunday Morning prospect
I was interested in the gold as well as the new found garnets. Over the years, I had been working not only in gold, but also in diamonds. And sometime earlier, I had met a contact from Russia who had access to an electron microprobe research facility in Russia that could be used to test chemistry (the Wyoming Geological Survey had such a tiny budget, we couldn't afford to do this ourselves). So I gave my Russian colleague the garnets and $20 from my pocket, and weeks later received news that all of the garnets were G10 pyropes. G10 pyropes have the same chemistry as pyrope garnets found as diamond inclusions, and thus likely came from the diamond-stability field where the earth makes diamonds.

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.
Some years later, I was able to get funding 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. This is unheard of and suggests there is one heck of a rich diamond deposit in this region. Typically, garnets collected from diamond-bearing kimberlites contain many G9 garnets derived from too shallow depth for diamond, and lesser G10 garnets. But never nothing but G10s. But I suspect this may have been a result of selective sampling, but in my opinion, the Wyoming Geological Survey in Laramie should wake up and conduct research on this dry placer and determine where the gold and garnets are coming from and how extensive is this placer?

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. And need to search for diamonds, gold, jade, and banded iron formation.

After searching the area, we 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).

Well, then there is the Bradley Peak gold deposit. Right 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 to the area 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 an soon the Seminoe Mountains were staked by hundreds of mining claims! 

In the area of the Penn mines, 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 area is mostly in metamorphosed basalt and includes some banded iron formation, and could potentially host a large disseminated gold deposit with high-grade veins.


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

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