Monday, March 8, 2010

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).

A 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.

Ultramafic metakomatiite with spinifex texture from the Bradley
Peak Ultramafic Schists.

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
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!

Gossaniferous komatiite, Seminoe Mountains district.
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!



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