Gold Placer Potential of the South Pass Greenstone Belt, Western Wyoming

The Wyoming State Geological Survey (WSGS) mapped the 450 sq. mi South Pass granite-greenstone belt and many of its historic mines during the mid-1980ies (Hausel, 1991). During mapping, the general lack of development of lode and placer gold deposit deposits (other than to the prospect stage) was apparent, even though gold values appeared to be encouraging. South Pass was Wyoming’s primary gold district. Gold production from the district is estimated at 350,000 ounces.

The abundance of significant gold anomalies, the relatively common shear zones in some areas, and the general lack of development of some placers in the district suggest that there may be several opportunities for placer gold development in the district (Hausel, 2001).

Geology

Based on geological mapping and sampling, South Pass is similar to many ancient gold belts in South Africa, Zimbabwe, Australia, Russia and Canada. Many of these ancient gold belts, known as granite-greenstone belts, have been fairly prolific sources of gold as well as some other metals such as iron ore, tungsten, and some base metals.

The term greenstone belt was applied to ancient volcanic-sedimentary containing thick, folded successions of metabasalt, metakomatiite, metagreywacke, metapelite, some banded iron formation, and other related rock units. Many of these rocks are dark in color and include greenish minerals formed during low-grade (greenschist) metamorphism. However, much of the South Pass greenstone belt has been metamorphosed to low- to moderate-grade, resulting in many amphibolite-facies (black) rocks with minor amounts of greenschist (green)-facies rocks.

Greenstone belts appear to have formed in deep marine environments with volcanic islands on one side, and an ancient continent on the other side of the belt. The result was the deposition of considerable basalt, some komatiite (magnesium-rich volcanic flows), and banded iron formation from the volcanic islands, and greywacke (dark, mica-rich sandstones) from the continental side in a deep basin. These were deposited in the narrow ocean basin, which later formed the bulk of a thick synformal belt.

Most greenstone belts are ancient. The South Pass belt is >2.8 billion years old. It sits on a very ancient gneissic terrain (old continent) that is 2.8 to 3.8 billion years old. This entire region was intruded by granite more than 2 billion years ago.

Most rocks found in greenstone belts typically have higher than normal amounts of gold, and are considered good source rocks for gold. The average gold content in these rocks is too low to mine, typically being measured only in the parts per billion. But when favorable geological events occur, such as faulting accompanied by hydrothermal alteration, the country rocks contain enough leachable gold to concentrate into the narrow shear zones, to sometimes produce an attractive gold deposit.

This is what happened at South Pass. About 2.8 billion years ago, regional metamorphism and deformation resulted in relatively high-temperature (metamorphic) fluids leaching gold from many of the country rocks. The auriferous fluids migrated into nearby shear zones, and in some places they were trapped within some fold closures in the shear zones. Thus, if one can find these folded shear zones, it may be possible to find some enriched ore shoots. In addition, placer gold deposits are always located downstream from these gold-bearing shears. Luckily, most of the shears have already been mapped, and all a prospector needs to do is consult the available geological maps to find out where both the lode and placers occur. With the maps in hand, it is recommended to highlight the most favorable drainages downstream from the shear zones.

South Pass City Area

Location and Access

South Pass City lies 30 miles south of the town of Lander and is accessed from graded roads leading from Highway 28 along the western margin of the greenstone belt. The topography consists of rolling hills on a gradual sloping pediment. Near the foothills, the terrain sits 8,500 feet above sea level, and gradually slopes to the south to 7,200 feet above sea level along the Sweetwater River. The climate and vegetation is that of a high semi-desert which supports sparse grass, sagebrush, and prickly pear cactus along south-facing slopes. Sagebrush, grass, willow, aspen, and pine are found along some north facing slopes and creek valleys. Some creeks and the Sweetwater River are perennial; however, many creeks and gulches tend to dry by August and are essentially dry except during spring runoff.

Large portions of South Pass are underlain by public mineral estate administered by the Bureau of Land Management. Some patented (private) property and mining claims are also scattered throughout the greenstone belt. So be careful not to trespass.

While mapping the greenstone belt in the 1980s, several relatively untouched placer deposits, some of which had only been partially prospected, and some which appear to have never been mined, were recognized by the WSGS. Based on the location of some of these placers in relationship to auriferous shears and ore shoots, many could host potential rich pay streaks.

The greatest number of gold-bearing shears are located near the margin of a prominent belt of mafic amphibolites (metagabbro and metabasalt) that run from South Pass City, through Atlantic City to Miners Delight, along the western flank of the South Pass synclinorium.

In this region, gold if often found by prospectors searching for nuggets each year. For example, a prospector searching mine tailings with a metal detector recently found a 7.5-ounce nugget. Another prospector recovered more than 100 nuggets during the past couple of years using similar techniques. The available historical records also record many large nuggets from this region including 36, 24, 7.5, 6.0, 5.3, 5.2, 5, 3, and 0.75 ounces. However, most nuggets have gone unreported.

Reviewing the drainages downstream from the principal lodes in the South Pass City area shows several drainages of interest. A few of these lie near South Pass City (Figure 1) and may be inaccessible due to private property, etc.

Carissa Mine

The Carissa mine near South Pass City, was the principal source of gold in the district. Past production is not well documented, but the available records along with estimates, indicate that the mine produced between 50,000 to >180,000 ounces of gold prior to 1950. Based on the extent of the mine workings, the latter figure is probably more realistic.

The Carissa mine was developed on a shear in metagreywacke, amphibolite, and actinolite schist (metakomatiite). The gold-bearing structure averaged 6 feet wide in the mine. At the surface, the primary shear is 2 to 3 feet wide, but swells to 50 feet at depth (Figure 2). This primary shear is also enclosed within a much larger shear that isn’t quite as obvious, and forms an envelope surrounding the primary shear that is more than 1,000 feet wide, much of which is untested. This envelope is expressed by numerous parallel fractures with common quartz veinlets, some brecciation, and silicification. Samples taken in this envelope over an aggregate width of 300 feet in 1926 showed low-grade gold. This was confirmed by the WSGS: a 97-foot composite chip sample collected in the structure yielded 800 parts per billion gold (Hausel, 1989, 1999). Thus the property appears to have potential for open pit and underground mining.

The ore tenor ranged from a trace to 2.6 ounces per ton of gold (opt Au) and averaged 0.3 opt Au. Drilling has shown that the structure is still significantly mineralized below the current mine workings. Drilling in the 1980s showed an 80-foot mineralized structure beneath the mine workings that assayed 0.031 to 2.54 opt Au. There are also reports of specimen-grade ore being found in the mine that contained values as high as 260 opt Au.

The data shows that this structure is an excellent source for gold, and yearly erosion must have supplied considerable amounts of gold to the nearby drainages, and Willow Creek, in particular, probably represents an excellent placer. However, only small portions of Willow Creek have been mined in the past, primarily due to it being narrow, and having a very gradual gradient that would have been considered unfavorable for historical placer mining by sluicing. Examination of the area around the Carissa shear shows some other noteworthy drainages. Carissa Gulch (number 1, Figure 1), a dry gulch draining south from the Carissa shaft into South Pass City, does not have a great volume of alluvium, but its location supports the gulch should host some specimen-grade gold including nuggets, yet it remains unmined.

The eastern extent of the Carissa lode is drained by Hermit Gulch (number 2, Figure 1). Hermit Gulch is much more extensive, and should contain considerable gold, but it shows few signs of historic mining.

Duncan Mine Area

The Duncan mine is located on a folded shear zone to the northeast of the Carissa mine (Figure 3). Other mineralized lodes are located nearby including the Tabor Grand and the Mary Ellen mines. The Tabor Grand may be on the same shear at the Duncan, and the Mary Ellen was developed in a quartz vein in a tonalite porphyry, which intruded the shear zone.

A group of drainages run from the gold-bearing structures. At the Duncan mine, a 1-foot composite chip sample collected in a steeply plunging fold, assayed 0.96 opt Au. Thus portions of this structure are well mineralized. Production records indicate that 3,800 ounces of gold were recovered from the mine. However, it is likely that 3 to 5 times as much gold was actually produced from this mine.

The shaft was sunk to 250 feet with at least 1,255 feet of drifts. The ore shoot drains directly into Little Beaver Creek to the north as does the nearby Tabor Grand, Mary Ellen, and other mines and prospects (number 3, Figure 1). During sampling and mapping on the Tabor Grand 120-foot level, the shear zone was shown to contain anomalous gold along the entire length of the shear. The samples ranged from a trace to 58 ppm (1.8 opt) Au. The mine was estimated to have produced 2,400 ounces of gold.

The nearby Mary Ellen mine (to the south) reportedly produced 6,050 ounces, and the ore averaged 0.4 opt. Historical reports indicated that the tenor ranged from 0.25 to 5.25 opt Au. These early reports indicate that pockets of ore were intersected that assayed as high as 50 opt Au.

Based on the presence of these and other well-mineralized ore shoots in this area, Little Beaver Creek, located north of the mines, should contain anomalous auriferous gravels. The creek appears to have received only minimal mining interest in the past, but may be difficult to work due to the lack of water in the upper reaches, beaver ponds near the mouth of the creek, and private land.

There are other drainages in this area of possible interest. For example, Deep Gulch, a tributary of Willow Creek to the south (number 4, Figure 1), cuts the same rock units as Rock Creek, yet it remains relatively unprospected.

Rock Creek Area

Pre-1911 operations on Rock Creek appear to have been largely confined to localized hydraulic operations. Most other pre-1911 placer mining is assumed to have been relatively ineffective due to the gradual gradient of Rock Creek (2°). Some early hydraulic mining on Mill Hill (number 1, Figure 4) along the south edge of Atlantic City (the south bank of Rock Creek) was estimated to have recovered 10,500 ounces of gold. This would be considered an extraordinary amount of gold for this site, as shear zones and veins are not well developed on Mill Hill. If this could be verified, it might suggest the presence of hidden shears, some mineralized wall rock, or simply that the hydraulic operations incorporated a much larger area.

The historical reports indicate that rich quartz specimens were found in Rock Creek. For instance, one fist-size piece of quartz was reported to contain 24 ounces of gold. A boulder found nearby in 1905 contained an estimated 630 ounces of gold!

Rock Creek is one of the principal drainages in the district. The creek cuts across 12 to 14 miles of the greenstone terrain before draining into the Sweetwater River. Along the way to the Sweetwater River, it receives material from several tributaries including Little Beaver Creek.

The creek cuts across many gold-bearing structures. The channel is 100 to 250 feet wide and narrows to several feet wide, locally. The average gravel depth is 10 feet. The upper 3 feet of the deposit consists of barren loam, and much of the gold recovered from the placer, was reportedly found within 1 to 3 feet of bedrock. The gold was found as rounded nuggets and flakes, that typically had a fineness of 0.84 to 0.90 (84 to 90% pure).

Between 1933 and 1941, the E.T. Fisher Company constructed a concentrating plant and dredged about 6 miles of Rock Creek from Atlantic City downstream to the Mormon Cemetery with a dragline. Approximately 3 million yds3 of gravel were processed that averaged 0.012 oz/yd3. Production was reported to total 11,500 ounces. However, based on the volume of gravel and average grade, production may have been 3 times the reported production, and should have been more on the order of 36,000 ounces. Nuggets up to 3.4 ounces were recovered, but operations terminated in 1941 due to the outbreak of war.

Based on the available information, the operation may also have been fairly inefficient. Some old timers who worked at the mine indicated security was a problem, as nuggets were high-graded from the concentrator. In addition, considerable gold may have been lost to the tailings. A few modern operations, which reprocessed small portions of the tailings, successfully recovered several nuggets (Figure 5).

In addition to several unmined tributaries, Rock Creek also has 3 miles of unmined gravel downstream from the Mormon cemetery. Some unmined gravel also lies upstream from Atlantic City (number 2, Figure 4). In particular, unmined gravel running from Atlantic City to the Rose shear, nearly 3,500 feet distant, may be relatively rich, as this gravel was derived from nearby shear zones. The 6 miles of tailings downstream from Atlantic City should also be of interest, as the concentrator appears to have been poorly designed and possibly rejected considerable gold.

Big Atlantic Gulch

Prior to 1911, 750 ounces of gold were mined from Atlantic Gulch. Big Atlantic Gulch was later dredged, possibly by the Fisher dragline. Near the mouth of the gulch, the stream narrows considerably. Further upstream, the tailings (number 3, Figure 4) may also be a source for rejected gold, similar to Rock Creek. The upper reaches of Big Atlantic Gulch, from near the Snowbird mine running north into Cole and Placerita gulches, should provide enough gravel for a small operator for several years.

Little Atlantic Gulch, a tributary of Big Atlantic gulch, lies west and parallel to Big Atlantic Gulch, and cuts across the same rocks and shear zones. Past mining activity in this gulch has been limited, and this gulch should offer potential for placer gold. Further west, are some a group of small gulches that are dry much of the year. These include Basket Gulch, Beer Garden Gulch, and others.

East and parallel to Big Atlantic Gulch, are Smith and Promise gulches (number 4, Figure 4). Prior to 1911, 1,500 ounces were recovered from Smith Gulch and about 1,500 from Promise Gulch, yet portions of gravel remain unmined.

In the 1980s, a two-man operation tested some gravel on Smith Gulch with a trommel and backhoe (Figure 6). The operation recovered about 20 ounces of gold per week, and periodically struck pay streaks yielding 20 ounces in a day (Figure 7). The gravels were 6 to 10 feet deep, and much occurred as flatten nuggets and flakes near bedrock and in sandy layers overlying thin clay-rich zones. The gravel averaged about 0.1 ounce/yd3. Irish Gulch lies about a mile east of Smith Gulch, south of Miners Delight. There are no reports of gold from this gulch.

Miners Delight

The Miners Delight mine (number 5, Figure 4) (one of the two principal lode mines in the district) intersected a rich pocket of gold in the 1800s. The mine was developed in a 3 to 16 foot wide, 2,500 foot long, folded shear zone hosted by metavolcanic rocks. Little modern information is available on this mine, and estimated production was 60,000 ounces prior to 1911.

The mine apparently pumped sufficient water from the shaft for placer mining of Spring Gulch below the mine. Spring Gulch is a dry gulch, even so, 1,500 ounces were recovered from the drainage in the 1800s. Several 1 and 2 ounce nuggets were found in the gulch including one 6-ounce nugget. One piece of specimen-grade quartz was also recovered in 1873 that was described to be as large as a water bucket. According to one witness, it looked as if it could contain a pound of gold.

Yankee Gulch, to the northeast of the Miners Delight mine, was also auriferous, and the Miners Delight shear appears to continue north of the mine under alluvial cover. One operator in the 1800s, recovered 8 to 15 ounces of gold per day in this gulch including one nugget that weighed 5 ounces.

Lewiston District

The Lewiston district lies near the eastern flank of South Pass. Rock types are similar to those in the South Pass Atlantic City area. The shear zones in this district are typically narrower than those in the South Pass-Atlantic City area, but some are quite rich in gold.

Rich pockets of gold were found at Wilson Bar at the mouth of Burr Gulch within the Sweetwater River in 1878 (number 1, Figure 8). In the 1890s, a 500-foot strip of gravel was mined at Wilson Bar, which yielded 370 ounces of gold. The gold was traced upstream to the Burr lode.

In 1893, a pocket of ore intersected in the Burr mine reportedly yielded 3,000 ounces of gold. Some specimen-grade ore was claimed to have contained as much as 1,690 opt Au. The lode varied from 7 to 10 feet wide averaging 2 to 3.5 opt Au (in all probability, the ore grade was exaggerated, or the ore was very selectively mined). To the northeast, at the Hidden Hand mine, miners intersected a rich shoot in the 1930s that produced several sacks of specimen-grade ore reportedly assaying 75 to 3,100 opt Au! Relatively recent samples collected northeast of the Hidden Hand at the Mint-Gold Leaf mine, included two 2.5 foot composite chip samples that assayed 1.29 and 3.05 opt Au (Hausel, 1991). A narrow and presumably shallow drainage next to this locality (number 2, Figure 8), might provide some interesting specimens.

The gold in these ore shoots is probably pockety, since none of the mines were developed to any great depth (<150 feet deep). But because of the fabulously-rich pockets, placers in the district may be worth prospecting. But unfortunately, very few streams cut the area, and the few that do are small with limited gravel.

Strawberry Creek appears to be one of the better placers (number 3, Figure 8). The creek cuts several of the principal shears, and is probably enriched in gold. Historical reports indicate that some nuggets (3 and 4.5 ounces) were recovered nearby at Two Johns Gulch in 1905. In 1944, five 'good-size' nuggets were found in the Big Nugget placer. Possibly, these two placers are the same as Giblin Gulch where in 1932, several nuggets were found including nuggets weighing 5.2 and 5.3 ounces (number 4, Figure 8). Another gulch of potential interest is Deep Creek (number 5, Figure 8), as it also cuts across gold-bearing shear structures.

Strawberry Creek is also of interest. This drainage cuts across some known shear zones. Historic estimates suggest that 21,000 ounces were mined from the Bullion mine, located on Strawberry Creek at the historic site of Lewiston. In addition, about 10 years ago, a prospector working in Strawberry Creek, near the mouth of the creek, recovered about 25 ounces of gold including nuggets up to 1/2 inch in length.

Summary

South Pass may be one of the more under-prospected regions in the western US. Geological evidence supports that the district is not only a good place to search for specimen-grade gold samples, but also the area has good potential for placer gold prospecting and mining. The WSGS periodically leads field trips to the South Pass area and some other gold districts in the State (see http://wsgsweb.uwyo.edu/calendar.asp.). If you would be interested in attending one of these, please feel free to contact Dan Hausel at dhause@wsgs.uwyo.edu. If you have any questions about the South Pass area, or other similar mineral resources in Wyoming (metals or precious stones) feel free to contact the above email address, or call 307-766-2286.

References Cited.

Hausel, W.D., 1989, The Geology of Wyoming’s Precious Metal Lode and Placer Deposits: Geological Survey of Wyoming Bulletin 68, 289 p.

Hausel, W.D., 1991, Economic geology of the South Pass granite-greenstone belt, Wind River Mountains, western Wyoming: Geological Survey of Wyoming Report of Investigations 44, 129 p.

Hausel, W.D., 1999, The Carissa Gold Mine, South Pass, Wyoming – A sleeper? International California Mining Journal, v.68, no. 11, p. 14-16.

Hausel, W.D., 2001, The South Pass gold placers, western Wyoming: International California Mining Journal, v.70, no. 8, p. 29-35 & 41-42.

List of Figures

Figure 1. Highlighted areas on the South Pass 1:48,000 scale geological map (from Hausel, 1991) show some placer deposits of possible interest. Note that these all lie immediately downstream from many productive lodes.
Figure 2. View of the gold-bearing shear exposed in the glory hole at the Carissa mine (photo by W.D Hausel).
Figure 3. Duncan mine headframe (photo by W.D. Hausel).
Figure 4. Some placers near the Duncan mine-Rock Creek area are highlighted showing potential placer ground.
Figure 5. Nuggets recovered from the Stout placer along Rock Creek (photo by W.D. Hausel).
Figure 6. Buddy Presgrove and Hank Hudspeth, Jr. prospecting the Smith Gulch placer in 1987 (photo by W.D. Hausel).
Figure 7. Some gold recovered from Smith Gulch in 1987 (photo by W.D. Hausel).
Figure 8. Placers in the Lewiston district (from Hausel, 1991).

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