The Centennial Ridge Gold-Platinum District, Wyoming

Placer gold was discovered in the Middle Fork of the Little Laramie River south of the present site of Centennial village in 1874. The source of the gold was traced up-slope to a nearby ridge in the following year. The initial sample taken from the discovery site assayed 47 opt Au! The discovery was staked as the Centennial Lode, and a rush followed. The Centennial Ridge district was later organized in 1876.

Since 1876, the region surrounding the Centennial Ridge district has been subjected to other rushes and prospecting activity. Most notable were rushes for uranium in Witwatersrand-type Proterozoic quartz-pebble metaconglomerates in 1978, diamonds and gold in the 1980s, and more recently for platinum-group metals.

Location & Access

The Centennial Ridge district lies along the eastern edge of the Medicine Bow Mountains of southeastern Wyoming (Figure 1). Dozens of old prospect pits (Figure 2), a few shafts, several mine adits (tunnels), old miners' cabins, some good placer ground, very scenic country, and many old mine dumps and artifacts (Figure 3). The area is accessible, but a 4-wheel drive vehicle is recommended due to some steep jeep trails in the interior of the district. Much of the district lies on public land within the Medicine Bow National Forest, with a few, scattered, patented mining claims.

Because of the district's location on a mountain front at 9,000 to 9,600 feet above sea level, it is best to be prepared for inclement weather. To get to the district, drive either west from Laramie, or east from Saratoga along Highway 130 (Snowy Range Scenic Byway). This is a very scenic highway, due to the efforts of the local Forest Service district office in Laramie. At the Forest Service information center a few miles west of Centennial village, turn south on Forest Service Road 338. At 2.2 miles from the turnoff, turn left, and the jeep trail branches into several other trails leading to different mines and placers. You will need 4-wheel drive.

History

The district gained international attention following the discovery of the Centennial Lode at the base of Centennial Ridge in 1875 (Figure 4). The lode was described as an N45øE-trending quartz vein in a shear zone (fault) hosted by iron-stained, mafic (dark, magnesium-rich), hornblende schist.

The Centennial discovery was rich! The ore was reported to average about 1.5 ounces per ton in gold. Sporadic ore shoots were intersected that yielded some highly prized ore specimens. One of these won first prize as an ore specimen at the Paris Mining Exposition in 1878. Another (a sample of gold-bearing garnet schist) was placed in the Smithsonian collection, and yet another was sent to the Denver Mint in 1876. This latter sample assayed an incredible 2,263 ounces per ton gold and 210 ounces per ton silver!

Mining continued on the lode and the operator ordered a 10 stamp mill. Since the mine adit was located on the steep hillside 260 feet above the valley floor, a 425-foot long tramway was constructed to lower the ore from the mine portal, down-slope to where the ore was received at the stamp mill. At the mill, the ore was crushed and the gold was recovered on amalgamation (mercury-coated) plates. However, since a large portion of the gold occurred in pyrite; significant gold values were undoubtedly lost during milling.

Because of the many rich gold specimens produced from the mine, it was the talk of the mining community. Everything seemed to be very favorable until the summer of 1877, when the worst fears of the miners were realized. The mine face was drilled and blasted. When the dust cleared, instead of the rich vein; gray, barren, hornblende schist was exposed at the face. The vein had terminated in fault gouge! Several more blasts into the face revealed no clues to where the vein went. The offset vein was never found.

In addition to the Centennial mine, several other veins and shear zones were prospected in the district. Three other relatively extensive mines - the Platinum City, Utopia, and the Cliff gold mines were also developed (Figure 5). A considerable amount of tonnage was mined from each property. But like many historical mines in Wyoming, very little information is available on these mines.

Geology

The geology of the Centennial district is dominated by hornblende schist with a strong northeasterly foliation. Two types of lode deposits were recognized: (1) gold-bearing quartz veins that parallel the foliation of the country rock; and (2) gold-platinum veins that occur as fracture fillings and replacements in shear zones (Figure 6) cutting the gneisses and schists (Figure 7).

The placer deposits in the district are associated with gravels along the Middle Fork of the Little Laramie River, Fall Creek, and also the Queen Mill Run drainage along Centennial Ridge. This latter placer is unusual in that it was mined all the way to the top of the ridge.

Mines in the district

Some other mines in the district include:

Cliff mine

The Cliff mine was developed by a 775-foot tunnel that followed the schistosity of the country rock. Near the end of the main drift, a 325-foot crosscut tunnel intersected four quartz-sulfide (pyrite-rich) fracture zones. The first zone yielded samples that assayed 0.58 ounce per ton gold over a 15-foot width. The number 3 vein located 140 feet from the main drift, was reported to carry some platinum.

Presently, the mine workings are inaccessible as the mine portal has collapsed. An iron-stained mineralized zone is visible adjacent to the portal, and samples with pyrite can be found on the mine dump. Some miners' cabins and the remains of an old mill lie adjacent to the portal.

Empire mine

Two adits were driven on iron- and copper-stained shears parallel to schistosity. Samples collected from the sulfide-bearing zones in 1926, were reported to range from a trace to 0.99 ounce per ton silver, a trace to 0.06 ounce per ton gold, a trace to 1.04 ounce per ton platinum, a trace to 63.72 ounces per ton palladium, and a trace to 2.84 ounces per ton iridium. The mine adits are currently inaccessible, as they were sealed and reclaimed a few years ago. The platinum-group assays are suspect.

Free Gold claims

Located near the top of Centennial ridge running along the same trend as the mineralized veins in the Cliff gold mine. These prospects were probably developed to search for the Cliff vein. Dozens of prospect pits, trenches, and a few shafts lie on the ridge above the mine. In places, the Free Gold vein was reported to contain visible gold in boxworks. Boxworks are pitted to spongy masses of limonite (a rusty iron oxide), which represent the remains of pyrite or some similar sulfide. Boxworks often provide excellent samples to search for visible gold, particularly if the former pyrite contained any hidden gold. Samples collected from various prospects on the Free gold claims were reported to have yielded from 0.08 to 2.84 ounces per ton gold.

Independence mine

Located on the west bank of the Middle Fork of the Little Laramie River. The tunnel was driven N35øW crosscutting five shear zones. At 30 feet from the portal, a tunnel was driven to the south parallel to an N68øE shear zone. Reported assays from the mine yielded 0.27 ounce per ton platinum, 0.46 ounce per ton iridium, 0.03 ounce per ton rhodium, and 0.10 ounce per ton osmium. Samples collected in the during mapping of the mine by the Wyoming State Geological Survey, contained no detectable platinum, and none to 0.82 ounce per ton gold.

Middle Fork Placers

A large flat area located a short distance downstream from the Independence and Cliff gold mines, occurs along the Middle Fork of the Little Laramie River. The drainage cuts across the country rock foliation, as well as several veins and shear zones. Because it lies immediately downstream from several mineralized veins, this placer should contain some precious metals. But there is little evidence of much past mining activity. This might be a good placer for someone to prospect.

Mother Lode

The Mother Lode lies along the western margin of the district. This is a pyrite-rich eluvial quartz deposit along the bank of the Middle Fork of the Little Laramie River. Several years ago, the Wyoming State Geological Survey collected some samples of the pyrite-rich quartz. The sulfide-rich quartz contained no detectable gold. However, in August 2000, some attendees on the 2000 Snowy Range General Public field trip led by members of the Wyoming State Geological Survey's geology staff, panned several gold colors from the river, and several pyrope garnets were also recovered.

The presence of pyrope is significant. Pyrope garnet is a common kimberlitic indicator mineral used as a tracer mineral to find diamond deposits. The pyropes, which erode from kimberlite (or similar rock), are swept downstream for short distances (typically less than 2 miles). Several years ago, the Wyoming State Geological Survey recovered several pyrope garnets west and northeast of this locality. The source of the garnets has never been found, but suggests potential for discovery of diamond deposits in the Medicine Bow Mountains.

Queen Mill Run Placer

This placer lies high on Centennial Ridge between 9,400 feet to 9,600 feet above sea level. Portions of the placer were probably mined in the 1930s. The position of the placer is interesting. It lies at the top of a ridge, and parallels the country rock foliation. Historic reports indicate that the Queen Mill Run placer produced several nuggets weighing about 0.1 ounce with others up to 0.25 ounce. Since the placer was mined to the top of the ridge and parallels the country rock foliation, this may suggest that it lies on top of a hidden gold vein. Some unmined ground can be found between the mined areas.

Summary

The Centennial Ridge district has not been heavily prospected and awaits new discoveries (Figure 8). With the current rise in platinum-group metal prices, one might anticipate this area will soon receive considerable interest. Already, large portions of the Medicine Bow Mountains to the south and west of Centennial Ridge, have been staked for platinum-group metals.

Other minerals of interest are the kimberlitic 'diamond' indicator minerals. The source of these minerals has not been found, and there has been practically no follow-up work on the anomalies. This means a prospector or rock hound would have a very good opportunity of making a new diamond discovery by tracing these minerals to their source. This area is considered to have very high potential for the discovery of diamonds, as diamonds have been found only 14 miles to the northwest at Cortez Creek within the Medicine Bow Mountains, 40 miles to the southeast in the State Line district, and reported 50 miles to the northeast in the Iron Mountain district.

The offset vein of the Centennial mine is also of historic, if not economic, interest. One might be tempted to search for the offset in the Queen Mill Run placer, or on the Free Gold Claims further to the south.

Figures

Figure 1: General geological map of the Medicine Bow Mountains and Sierra Madre showing location of the Centennial Ridge district.

Figure 2: Topographic map of the Centennial Ridge district showing location of mines, prospects, etc.

Figure 3: Mining artifact (3 mill stamps) found near the old Utopia mine dump in the Centennial Ridge district (photo by Hausel).

Figure 4: View of Centennial Ridge showing the approximate location of the Centennial Mine, hidden in the trees (photo by Richard W. Jones).

Figure 5: Looking down on the Platinum City mine dump from Centennial Ridge (photo by Hausel).

Figure 6: A mine adit driven in a 3 foot wide shear zone on the Middle Fork of the Little Laramie River. The shear zone, a zone of intense fracturing, is located above the left half of the mine entrance (photo by Hausel).

Figure 7: Geological Map of the Centennial Ridge district (modified from McCallum, 1968).

Figure 8: The Queen mine head-frame at the top of Centennial Ridge. A sample from the mine in 1926 assayed 0.03 ounce per ton platinum, 0.05 ounce per ton iridium, less than 1 ounce per ton silver, and a trace of gold (photo by Hausel).

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