Seminoe Mountains Gold and Diamond Anomaly

Limonite boxworks, produced by natural rusting.
Original rocks contained pyrite (FeS2) which oxidized (rusted). 
The sulfur washed away with rain while the iron (Fe)
remained in-situ staining the rock red to brown.  Pyrite is often
gold-bearing. If the gold occurs as free gold within the pyrite
crystal structure, the pyrite will dissolve while free gold will
 remain in place. Looking in vugs  sometimes reveals visible
gold (Hausel and Hausel, 2011).

In 1981, I was working as the economic geologist for the Wyoming Geological Survey. My goal was to visit every mining district in the State and map all of the major mining districts. One of the first districts I visited was located along the edge of Bradley Peak in the Seminoe Mountains where gold had been found in the late 1800s, but mining ended due to constant battles between miners and Indians.

When I reached Bradley Peak, I found a small number of old mine dumps on the northeastern margin of Bradley Peak in an area that had been known as the Ernst Mining district, and also the Seminoe Mountains Mining district. Here I combed the mine dumps and picked several quartz specimens with limonite boxworks that contained visible gold. If one sees visible gold with the naked eye in a hand sample, a rule of thumb is that the sample will assay at least 1.0 ounce per ton in gold (opt Au) (Hausel and Hausel, 2011).

One of these had considerable visible gold (but someone else decided they liked the sample better than I did and it disappeared from my office). One of the other samples that remained in the office had no obvious visible gold but still assayed 2.87 opt Au (which means that a ton of this material would contain nearly $4900 in gold).

A gold rush followed after the Wyoming Geological Survey released information on the discovery, but unfortunately, a company known as Timberline Minerals staked all of the public land and kept all other companies out of the area (the Seminoe Mountains are surrounded by private land to the south. Along the north, there is public land and I initially accessed the area by way of Sunday Morning Creek on the north - a very rough road). Timberline was more of a promoter than exploration group and tried to sell the property. This effectively locked up the district and kept any serious exploration from occurring. Anyway, I was told that all motels were filled with geologists in Rawlins, Sinclair and Saratoga following the release of our report.

One of the Penn mines in the Seminoe Mountains as it appeared in 1981.

Since that discovery, I revisited the district and mapped the Seminoe Mountains greenstone belt and identified some interesting targets. Not only are the narrow veins of interest, but this area also encloses komatiites, metabasalts and banded iron formation that have been altered to propylitic minerals (calcite, chlorite, epidote, etc) and this altered zone contains anomalous gold. Within this zone I also picked up a sample of banded iron formation with a cross-cutting vein that assayed 1.1 opt Au. To me, this area should be an excellent gold target.

The same mine dump in the 1800s.
(American Heritage Photo,
University of Wyoming).
Note that there has been considerable
forestization during the past 100 years.

Veins on the edge of Bradley Peak have been eroding for millions of years. One drainage (Deweese Creek) is an immature drainage, but there is no evidence it had been prospected for gold or nuggets in modern times. It drains the old Penn mines at Bradley Peak and it should provide some gold (and nuggets) for some industrious prospector. The biggest problem with the drainage is the overall lack of much gravel to mine.

While mapping this area, I met two wonderful prospectors, Donna and Charlie Kortes (the Kortes Dam was named in their honor). They showed me where they had collected some quartz with visible gold at the Sunday Morning mine - I also collected a specimen of milky quartz with gem-quality chrysocolla and cuprite from the adit. Some extraordinary banded iron formation is found in this area, some would make excellent decorative stone.

Charlie and Donna took me out into the basin near the Miracle Mile along the North Platte River where they had been digging up gold in the dry alluvial gravels on both sides of the North Platte River more than a mile from the drainage. I dug gold from this alluvium at several places a mile or more from the river bank. In most of these samples, we also recovered pyrope garnets (diamond indicator mineral). EVERY pyrope we tested with the University of Wyoming's microprobe yielded diamond-stability chemistry. Such data supports there is a rich, hidden, diamond pipe(s) in the area. Although we only tested a few dozen pyropes, I've never encountered a 100% diamond stability anomaly. So when you are looking for gold in this area, keep an eye open for diamonds - there is likely diamonds in this alluvium as well as in the kimberlite pipe(s) somewhere in nearby uplifts.

References Cited

• Hausel, W.D., 1981, Report on selected gold-bearing samples, Seminoe Mountains greenstone belt, Carbon County, Wyoming: Geological Survey 
• Hausel, W.D., 1992, Economic geology of the Seminoe Mountains mining district, Carbon County Wyoming: Geological Survey of Wyoming Mineral Report MR92-6, 32 p.
• , W.D., 1993, Preliminary report on the mining history, geology, geochemistry, and mineralization of the Seminoe Mountains mining district, Carbon County, Wyoming: Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 387-409.
• , W.D., 1994, Economic Geology of the Seminoe Mountains Mining District, Carbon County, Wyoming: Wyoming State Geological Survey Report of Investigations 50, 31 p.
• , W.D., and Hausel, E.J., 2011, GOLD - Field Guide for Prospectors and Geologists (Part 1 Wyoming Examples), 366 p.

Geology & Gold in the Seminoe Mountains greenstone belt, Wyoming

In 1981, I decided to have a look at the Seminoe Mountains north of Sinclair, Wyoming. This region was basically an unknown commodity with reports of some gold in the area. Otherwise, there was little known about the precious metal in this region. After gaining access to the district, I was impressed by the geology - exposures of metamorphosed komatiites and grunerite-rich iron formation that looked like it had been squeezed by unbelievable tectonic forces.

When I reached the top of Bradley Peak at the west end of the Seminoe Mountains, I found the remains of some old gold mines. I sat down and began to examine some of the old mine dumps. Nearly every piece of milky quartz I picked up with evidence of any kind of boxworks, I found visible gold, but with a lab budget from the State of Wyoming that was laughable, I took nearly 50% of my entire annual budget and had two pieces of quartz assayed along with a a couple of pieces of banded iron formation. The assays came back with enough gold to attract the attention of every mining company in the region. Only the State Legislature didn't seem to care and I received no support from the state to speak of. Over the next month or so, every motel room in Rawlins was taken. It was like a geologists convention.

Sample of milky quartz from Penn mines, Seminoe Mountains, Wyoming. All contain visible gold. Note the circle on the one vein sample - this encloses a tiny piece of visible gold. Such a specimen, with only one tiny piece of gold will assay at least 1.0 opt Au (one ounce per ton of gold) (Hausel and Hausel, 2011).

Later, after conducting research on the mining history of the district, it became apparent that this district had been abandoned since the late 1800s.

The Seminoe Mountains were named in honor of one of General John C. Fremont's guides, Basil Cimineau Lajeunesse, a French trapper (Reed, 1872). In 1871, troops under the command of Generals Bradley of Fort Sanders and Thayer of Nebraska, set out on an expedition to the Seminoe Mountains to search for reported rich deposits of argentiferous galena (Ferry, 1871).

Instead of finding silver (which probably came from the Ferris Mountains), gold-quartz veins were discovered by Mr. Ernest, a gold prospector from Laramie who accompanied the 1871 expedition. Ernest's discovery was made along the flank of Bradley Peak (named in honor of General Bradley) about one-half mile west of Deweese Pass. Deweese Pass was named for Captain Deweese who was the first military officer to drive through the pass with wagons (Ferry, 1871).

Several gold prospects were staked following these historic expeditions. Most of the prospects were located on well-defined, gold-quartz veins along the flank of Bradley Peak and included the Ernst, Mammoth, Break of Day, Jesse Murdock, Slattery, and Edward Everett in what was initially known as the Ernst Mining district. In several instances, the ore was reported to assay as high as $100 per ton in gold (5 opt), and in one case, as high as $250 per ton (12 opt) (Morrow, 1871). In 1873, everything appeared propitious following the erection of a stamp mill by the owners of the Ernst gold mine (Reed,1873), but in the following year, all prospecting and mining came to an abrupt halt. According to an 1874 congressional report, many fatalities resulted from an Indian raid on the mining camp, and the few survivors were driven from the district (Reed, 1874). A cavalry expedition to the district reported that all 30 cabins and the stamp mill were vacated (Rawlins Sentinel, Sept. 11, 1874).

Magnesium-rich, ultramafic komatiite from the Seminoe Mountains.
 Note the unusual texture on this rock: it almost looks like fossilized
grass. This is known as spinifex texture & appears to be similar to
spinifex grasses found in Australia & Africa. These rare volcanic
rocks are found in most greenstone belts around the world and
are often associated with gold, nickel, chromium and possibly
diamonds - similar to the actinolite schists in Wawa, Canada.

The Seminoe district was avoided by the miners and prospectors for the next few years. As many as four years after the conflict, an 1878 congressional report stated... "A visit to the Seminoe Mountains found the mining camp for the most part deserted." A sample collected from the Ernst tunnel at this time assayed $106.20 in gold per ton (5.14 opt). The report went on to say, "Other prospects in this locality afford quite good indications; and, now that the Indians are no longer to be feared there, I shall expect a revival of interest in it on the return of more prosperous times " (Reed, 1878).

This optimistic report apparently did not hold true, for in an 1881 congressional report it was written that the "...shafts went down to a little depth and tunnels had been ambitiously started when this camp too was broken up by a band of hostiles..." It's not clear if this report is referring to the earlier 1874 raid or a later raid in 1881.

According to the Engineering and Mining Journal (EMJ) (1885, v. 39, April 18, p. 269), some mines in the district were purchased by the Penn Mining Company in 1885. The Penn Mining Company based out of Wilkes-Barre, Pennsylvania, was incorporated on May 5th, 1885 with a capital stock of $100,000. Five company trustees were named including Samuel Aughey, the Wyoming Territorial Geologist.

Aughey (1886) reported that the company extended the Deserted Treasure tunnel (possibly the original Ernst mine) to a length of 200 feet. The ore from the mine was described as having free milling gold in quartz associated with pyrite and chalcopyrite. The vein averaged 4 feet thick. The company constructed a 10 stamp mill with concentrator which successfully handled about 22 tons of ore per day (Warren, 1885; Aughey, 1886). Plans were made to construct another stamp mill after the Penn Mining Company struck a 6 foot gold-bearing vein (EMJ, 1886, v. 42, Oct. 9, p. 265).

A document published 10 years later in 1895 disagreed as to the success of the mill and reported the 10-stamp mill erected by the Penn Mining Company proved to be a failure due to bad management (possibly also due to the lack of oxidized ore and abundant sulfides). Three clean ups from the mill gave $8, $12, and $16 in gold per ton (EMJ, 1895, v. 59, May 18, p. 472).

Folded sample of banded iron formation from the Seminoe Mountains showing alternating layers of grunerite, magnetite and silica. The magnet sticks to the magnetite layers

Another mine operated by the Penn Mining Company, the King mine, ran almost parallel to the Deserted Treasure, but more southwesterly and northeasterly. In 1886, improvements in the King included a 120 foot drift with a 54 foot deep winze. At the bottom of the winze, the ore was 5 feet thick. Seventy tons of the King ore reduced by the mill yielded $700 in gold, but the sulfides were not saved. The East King mine shaft (an extension of the King property) and crosscut encountered a streak of very high grade, gold-bearing quartz. Other properties in the district included the Jennie, Meager, and Bennett (Aughey, 1886).

Nice sample of folded banded iron formation cobble collected in the
gold-bearing Tertiary Conglomerate paleoplacer to the north of the
Seminoe Mountains near the Miracle Mile along the Platte River.
Both gold and chrome-rich pyrope garnet are also found
 in the same area.

In addition to the lode mines, some placer activity was also reported. The EMJ (1886, v. 42, Oct. 9, p. 265) reported two placer miners (Hanley and Firth) worked a claim that yielded $0.30 to the pan. For several years after 1886, not much was reported about the Seminoe district, although Ricketts (1888) noted the occurrence of iron in the district. Then in 1894, the EMJ (1894, v. 58, Dec. 29, p. 615) reported the Penn Mining Company resumed work on its gold mines in the Seminoe Mountains after a 6 year shut down.

During this period, the mines were extended. The King mine was extended from 120 feet in 1886 to 700 feet in 1896. The vein varied from 1 to 4 feet wide with an average width of 30 inches. Assays of the vein quartz averaged $25 in gold per ton (1.2 opt) (EMJ, 1896, Aug. 8, v. 62, p. 135). The "Penn mine" tunnel was also extended to 165 feet with a 135 foot deep winze on a 3 to 5 foot wide vein. Drifts were driven along the ore body for a distance of 100 feet in each direction. The ore from the mine averaged $20 in gold per ton (1.0 opt) and carried some copper (EMJ, 1896, Aug. 8, v. 62, p. 135).

In 1902, some interest in iron was expressed when Hendricks (1902) examined the high-grade iron deposits in the Patterson Basin area along the southern flank of Bradley Peak for the Lake Superior iron company. The Patterson Basin deposits were estimated to include 1 million tons of ore averaging 60% iron.

Isoclinal, open and box folds in the Seminoe Formation banded iron
formation at Bradley Peak.

In 1906, Dickman (1906) reported some of the iron deposits in the district yielded weak precious metal anomalies. The first detailed description of the iron deposits was made by Lovering (1929). Another detailed investigation was made 37 years later by the U.S. Bureau of Mines (Harrer, 1966). Harrer estimated about 100 million tons of taconite (banded iron formation) occurred in the vicinity of Bradley Peak. In 1951, a geophysical investigation of the iron deposits of the Patterson Basin area was made Wilson Exploration Company for Empire State Oil Company of Thermopolis. Later, the U.S. Geological Survey completed an aeromagnetic survey of the district (Philbin and McCaslin, 1966). The US Bureau of Mines met its demise during the Clinton Administration due to efforts of Al Gore to eliminate this once very productive agency.

In 1979 and 1980, gold prices rose to their highest levels in history. In the following year (1981), I visited the Seminoe district and recovered of several quartz vein samples with visible gold that assayed as high as 2.87 opt (the more highly mineralized samples were not assayed). A sample of iron formation recovered at this time assayed more than 1.0 opt Au (Hausel, 1989b). Following this discovery, a gold rush occurred, and Timberline Minerals Company and Kerr McGee Corporation obtained favorable land positions.

The Seminoe Mountains greenstone in central Wyoming is a fragmented belt of Archean metamorphic rocks cropping out along the western flank of the Seminoe Mountains. The core of the Seminoe Mountains is formed by crystalline rock consisting of an ancient greenstone terrane of metamorphosed volcanic, sedimentary and plutonic rock intruded by Late Archean granodiorite. The metamorphic rocks include amphibolite, mica schist, serpentinite, ultramafic schist, metagreywacke, metapelite, and banded iron formation. The flanks of the Precambrian core are unconformably overlain by Phanerozoic sedimentary rock that form a spectacular steeply dipping precipice along the southern flank of the range.

The district is known for its iron ore and gold deposits, but also hosts some copper, silver, serpentine, asbestos, jasper, jade and leopard rock. Some previously unknown zones of anomalous lead and zinc associated with shear zones were detected during a mapping project by the author and pyrope garnets and chromian diopsides were recovered from nearby Tertiary paleoplacers. All of the kimberlitic indicator minerals tested to date have yielded diamond-stability geochemistry. These minerals are found along with detrital gold in the paleoplacer. The paleoplacer remains unexplored.

Targets of Intererst

Gossaniferous komatiite, Seminoe Mountains district.

The Bradley Peak gold deposits occur in propylitically altered metatholeiites that are altered over an area of 0.25 to 0.5 square miles. Everywhere in this altered zone, one can find gold anomalies in the quartz veins, banded iron formation and the metatholeiites themselves. This area is so underexplored that possibilities for significant gold discoveries are high. In addition, the presence of mafic and ultramafic komatiites suggest that some exploration for gold and nickel in these rocks might identify additional anomalies.

Furthermore, no one knows the extent of the Miracle Mile paleoplacer along the northern flank of the Seminoe Mountains, other than it is a large paleoplacer and one can pan gold from the concentrates along with pyrope garnets. ALL pyrope garnets tested to date have been G10-diamond stability garnets suggesting that somewhere in this region is one or more major diamondiferous kimberlite pipes!

Summary

In 1981, while conducting reconnaissance to this area the author recovered more than a dozen samples of quartz with visible gold and one assay of quartz without visible gold assayed 2.87 opt Au, and a sample of banded iron formation assayed 1.15 opt Au. This zone of mineralization occurs in a larger propylitically altered zone that likely hosts a large tonnage, low grade gold deposit with high grade quartz veins.

As incredible as it seems - this deposit remains essentially unexplored to date (2009), yet it has excellent potential for diamonds and gold!

Some Publications

HOW TO PROSPECT for GOLD