The GOLD HUNTER

The Search for Gold, Silver, Platinum Groups & Gemstones

South Pass Gold

 About the author's work: No one knows South Pass better than W. Dan Hausel. Beginning in the early 1980s, Hausel took on this 250 square mile+ greenstone belt with is many gold districts and mapped in from one end to the other finding hundreds of gold anomalies, enormous regions of unexplored terrain, identified several signficant gold deposits, and more importantly, determined the source of the gold and ore controls. During this project, eight 7.5 minute quadrangles were completed (five had no previous mapping in the quadrangles), more than 3 dozen gold mines were mapped, and the rock precursors were determined. This work provided assistance to several companies that later explored this area including Hecla Mining. At the end of this project, dozens of professional papers were published including a 129-page book on South Pass which contains a 1:48,000 scale map of the entire greenstone belt.

 

Historically, South Pass was Wyoming’s principal gold district, but was never explored to depths greater than 137 m (450 ft). The district hosts several minor to significant gold deposits that include classical Archean (greater than 2.5 billion years old) shear zone deposits, quartz vein deposits, isoclinally to open folded, steeply plunging ore shoots, Tertiary paleoplacers and modern placers. Detailed geological maps of the district were produced by Hausel on the scale of 1:24,000. A compilation map of the greenstone belt was published at a scale of 1:48,000.

The land is a mixture of BLM (public land), State, and patented land. Possibly the best deposit, the Carissa, was nationalized by the State and incorporated into the South Pass City historic site. The Carissa appears to be a major gold deposit that is very likely a multi-million ounce deposit.

The greenstone belt is located at the south tip of the Wind River Mountains 50 km (30 mi) south of Lander at 2,380 m (7,800 ft) elevation.  The district encloses two villages: Atlantic City and South Pass City.

Geology

South Pass is a classical Archean greenstone belt and was initially explored, sampled and mapped by Hausel. Research on this district began in 1983 and culminated in 1991 with the publication on a treatise of the greenstone belt. Suring this time, more than 250 square miles were mapped resulting in the completion of eight 1:24,000 scale geological maps, more than 3 dozen underground maps, and hundreds of samples collected for analysis. A few hundred gold anomalies were identified, several that were significant (Hausel, 1991). More than a hundred gold anomalies were recognized including significant a major gold deposit and several potentially significant gold deposits. A distinct belt of mineralized gold-bearing shears trend through South Pass City, through Atlantic City and further northeast to Miners Delight. At both ends of this structure are significant paleoplacers. Several placers lie downstream from this structure.

Schematic geological map of the South Pass-Atlantic City district in the South Pass greenstone belt.

The Carissa shear zone exposed in mine workings at the southwestern edge of the greenstone belt. This shear yielded >180,000 ounces of gold based on incomplete production records. Missing production records over a period of several years suggests production could have been more.

Mapping by Hausel and drilling by various companies indicates that a significant gold anomaly exists beneath the historic mine workings.

The primary shear at the Carissa mine contains high-grade gold in a 0.45 to 24 m (1.5-80- ft) –wide, steeply-dipping and plunging cataclastic zone. This high-grade shoot is enclosed in larger (previously unrecognized) major shear that is as much 300 m (1000 ft) wide! The shear envelope is untested!  A 30 m (97-ft) composite chip sample taken within this structure on the south side of the high grade shear yielded 0.023 opt Au. Another 30-ft sample taken on the north side of the high-grade shear yielded 0.07 opt Au!  The remainder of this envelope remains untested!

Resources

The Carissa is structurally controlled and represents a steeply-plunging saddle reef deposit with high-grade gold localized in fold closures and rehealed fractures similar to the Homestake. Drilling by Consolidated McKinney Resources identified a highly anomalous 80 ft wide zone at depth. Drill core assays from this zone ranged from 0.03 to 2.54 opt Au (the shear envelop was not tested). This mineralized structure was intersected at depths of 930 ft. Earlier, Carissa Gold Inc. made the following reserve estimates using an extraordinary high reserve cutoff grade. They reported 208,000 tons of ore at an average grade of 0.343 opt, and geological reserves of 37,000 tons of ore averaging 0.85 opt! Anaconda Minerals Company drilled the property and all holes interested ore grade material and included a high-grade ore zone over widths of 2.3-16.1 ft that yielded 0.11 to 0.36 opt Au at depths up to 700 ft.

The Miners Delight mine at the northeastern edge of the shear trend was developed on a 3-16-ft -wide shear that has been traced over 2500 ft on the surface. Gravels in Spring Gulch draining this shear produced 1500 ounces of gold including several 1 to 2-ounce nuggets, a 6 oz nugget, and a sample of auriferous quartz described as “the size of a water bucket that was filled with gold”.  Samples collected across the shear by me ranged from 0.01 to 0.36 opt Au.  A historic prospectus reported there were 2,400 ft of drifts accessed from a 250 ft deep shaft. The mine produced 60,000 ounces. The gold tenor ranged from 0.3 to 110 opt Au. There has been no production (and very little exploration) since the late 1800s.

The Duncan mine northeast of the Carissa mine, has a significant gold anomaly in a fold.  A 2.5-ft channel sample yielded 0.96 opt Au in the steeply plunging fold adjacent to the shaft!

Several historical mines along the Carissa-Duncan-Miners Delight belt yield gold anomalies. These include the Mary Ellen, Tabor Grand, St. Louis, Diana and Caribou mines.  At several locations, one can find specimens with visible gold.

All drainages downstream of the shear trend have yielded anomalous placer gold.

Another parallel belt occurs in the Lewiston district to the east. During mapping, I was noted secondary rock alteration in the Crows Nest area between these two mineralized belts that is characteristic of gold mineralization and probably represents hidden shear zones in that area. The Crows Nest area remains unevaluated.

The greenstone belt continues under Tertiary sedimentary rock of the South Pass and Wasatch Formations to the northeast and to the south. It is notable that both of these regions are overlain by giant unconsolidated gold paleoplacers (McGraw Flats-Twin Creek to the north, and Oregon Buttes-Dickie Springs to the south) suggesting the presence of at least two major hidden gold deposits.

Visible gold in sample from Carissa mine.

Geophysical exploration and drilling by Hecla mining identified greenstone belt rocks to continue under Tertiary sedimentary rock for at least 9.6 km (6 mi) to the south of South Pass City.

Gold Geochemistry.

Gold geochemistry reported by the US Geological Survey show Au/Ag ratios are high & Au/Cu ratios are low. Trace metal contents (Bi, Pb, As, Sb, V, Mo, W, B, Nb, Zn, Cr, Co, Ni) are typical of hypothermal veins in other greenstones worldwide. Stable isotopes and fluid inclusion studies support that the South Pass gold is similar to that of a hypothermal vein system. The carbon and oxygen isotopes in shear zones along with hydrogen isotopes from fluid inclusions support that the much of the gold solutions were derived from the dewatering of the Mineral Delight Formation during compaction. 

 Structurally, some gold systems at South Pass are located adjacent to a distinct group of metagabbos, metatholeiites, and actinolite schists (metakomatiites) that trend from South Pass City to Miners Delight. The localization of gold in this region is believed to be due to competency contrasts between the metagabbros and adjacent Miners Delight metagraywackes. During folding, it is thought that these were favorable for the development of numerous fractures and faults. Much of the ore is found in these shear structures contain enriched ore shoots developed in folds suggestive of a reef-type structural control. Placers downstream from the shear structures are highly enriched in gold.

Placers

Some placers contain coarse gold.  XL Dredging mined portions of Big Atlantic Gulch south of Miners Delight in 1910 and recovered nuggets weighing 0.07 to 1 ounce. The ET Fisher Company dredged Rock Creek near the Duncan mine from 1933 to 1941 and produced 11,000 to 30,000 ounces: 75% of the gold was found within 3 ft of bedrock. Some of the gold was coarse and several nuggets were recovered. Nuggets included many small specimens and some large nuggets. A boulder with as much as 630 ounces of gold was also reported.

A 7.5-ounce nugget found in recent years in dredge tailings on Big Atlantic Gulch at South Pass. Other placer gold recovered from Rock Creek. 

Paleoplacers

Gold paleoplacers cover large areas of South Pass. The paleoplacers were explored by the US Geological Survey. These are found in and on the edge of the greenstone belt.

At Oregon Buttes south of South Pass City, Wasatch Formation conglomerates are estimated to be 1300 ft thick and cover 8 mi2 and host a gold resource estimated to be as much as 28.5 million ounces.  Some gold-bearing oil well cuttings were recovered from depths of 6500-7000 ft about0.5 mi north of the Continental Fault adjacent to Oregon Buttes indicating a possible buried auriferous shear zone. Geophysical exploration by Hecla Mining Company identified a distinct magnetic linear trend with IP anomalies in what was interpreted by Hecla as an expression a sulfide-bearing iron formation (at depth) in this area. Portions of the paleoplacers at Oregon Buttes have been reworked yielding some enriched Modern dry placers.

The source of the gold for Oregon Buttes has never been identified but in all probability lies at depth beneath the sedimentary cover near the buttes.  A similar paleoplacer (McGraw Flats-Twin Creek) along the northern edge of the belt has a projected source near the historical Miners Delight mine and the abandoned Atlantic City iron ore mine.

Recommendation.

Investigate several properties in the greenstone belt and conduct a regional sampling program in the Carissa-Duncan-Miners Delight shear structure to search for saddle reefs. Several gold deposits of note lie along this trend that include the Carissa, Miners Delight, Duncan, Caribou, St. Louis, Tabor Grand, Mary Ellen, Diana, Caribou and more.  A parallel structure to the east includes several interesting deposits including the Wolf, Hidden Hand, Bullion, Mint and Two-Johns gulch.

References

Hausel, W.D., 1989, The geology of Wyoming's precious metal lode and placer deposits: Geological Survey of Wyoming Bulletin 68, 248 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., and Hull, J., 1990, Guide to gold mineralization and Archean geology of the South Pass greenstone belt, Wind River Range, Wyoming, in Roberts, S., Geologic field trips to western Wyoming: Geological Survey of Wyoming Public Information 29, p. 178-191.

Hausel, W.D., and Love, J.D., 1991, Guide to the geology and mineralization of the South Pass area, in S. Roberts, editor, Mineral Resources of Wyoming: Wyoming Geological Association 42nd Annual Field Conference Guidebook, p. 181-200.

Snyder, G.L., Hausel, W.D., Klein, T.L., Houston, R.S., and Graff, P.J., 1989, Precambrian rocks and mineralization, Wyoming Province: 28th International Geological Congress guide to field trip T-332, July 19-25, 48 p.