Friday, June 1, 2012

The Carissa Gold Mine in the South Pass greenstone belt

Gold was initially reported in the Wyoming Territory in 1842 by a fur trapper from the American Fur Company who had been working a trap line along the Sweetwater River near the Wind River Mountains. The location of this discovery may have been along Strawberry Creek, a tributary of the Sweetwater River located near the eastern edge of the South Pass greenstone belt. South Pass lies south of the town of Lander.

Greenstone belts form geological (structural) basins filled with primitive volcanic flows and sediments that were intruded by dikes, sills and stocks and deposited along the margins of ancient proto-continents. These were later deformed and metamorphosed. To many geologists and prospectors, ‘greenstone belts’ are synonymous with ‘gold belts’ since they provide excellent places to search for gold. Such basins are typically >2 billion years old: the majority of rocks in the South Pass greenstone belt were metamorphosed >2.8 billion years and were likely deposited 3+ billion years ago. Some of the more notable greenstone belts in the world are located in the Slave and Abitibi regions of Canada, the Pilbara and Yilgarn regions in Western Australia, and the Kaapvaal and Barberton regions of Africa – all are well known for significant gold deposits.

Essentially all rocks in greenstone belts have slightly higher than normal gold content. Those in the South Pass greenstone belt typically average 2 to 10 times the amount of gold as average crustal abundance, thus these provided an excellent source for gold deposits that likely formed during regional metamorphism and deformation (Hausel, 1991).

When circumstances are right, greenstone belts provide good source beds for lode gold. What is required is hot metamorphic fluids to migrate through the basins and leach enough rock and then focusing the gold-bearing hydrothermal fluids in faults, veins or similar traps to produce ore shoots (enriched pockets) of gold. This happened in several greenstone belts around the world simply because mother earth was brutal to these basins but she provided several opportunities to leach gold from the layered rocks and focus the gold into traps. Such traps include fractures, intersecting fractures and veins, shear zones, stockworks, fold closures and chemically reactive rocks. For the most part, fractures, or zones of lower pressure, were ideal for gold-bearing fluids that were squeezed from the adjacent ductile rocks under great pressure and temperature.

A tight fold in mica-rich quartzite (meta-graywacke) at South Pass.
The knife sits along a fracture. The rock above this fracture
has graded bedding – coarser sand grains that decrease in size 
towards the top similar to what is found in modern streams. This 
tells geologists that the top of this rock was towards the top of the photo 
3 billion years ago. Below this fracture we have contradictory
evidence. Cross beds, (also found in modern streams), typically are cut (terminate)
by stream action. The small cross bed at the  bottom indicates the top of this part
of the outcrop was in the opposite direction as the graded beds. The only explanation
is the fracture represents a fold axis and the outcrop below & above the fracture was
 at one time facing in the same direction until folded along a very tight (isoclinal) fold.
The nose of the fold lies either somewhere at depth in the rock outcrop perpendicular
to the fracture, or lies somewhere in space and long since been removed by erosion.

To see how this works, take your hands, place them in front of you. Now touch your finger tips together. The top of each hand would represent the top of the rock unit when it was deposited. Now let your finger tips act as a hinge and bring your palms together: the top of your right hand should be in the opposite direction as your left hand.
A 34-ounce nugget found in the Rock Creek drainage during
dredging prior to World War II. Specimen from the Los
Angeles Museum
All greenstone belts exhibit many very tight folds, many faults, fractures, and shear zones as a result of the tectonic processes that crushed and mangled these regions over eons of time. In places, greenstone belts are almost like accordions because of the many folds stacked on top of one another. Often, it is difficult to tell which rocks are right side up and which are overturned. So intense is the deformation that we can actually map rock sitting right side up adjacent to rock that is completely overturned within inches of one another. To complicate matters further, these belts are often intruded along their margins by large granite batholiths. They also inter-finger (and lie on) with much older gneissic terrains that were the basement complexes that the greenstones were deposited on. To most geologists, these belts are a nightmare to map, as the geology and geological history are very complex and never obvious. The gneissic complex for the South Pass greenstone belt was mapped along Anderson Ridge and Lewiston Lakes areas. These gneisses are undated, but may be as old as 3.5 to 3.8 billion years.

Wyoming has four greenstone belts (Elmer’s Rock, Rattlesnake Hills, Seminoe Mountains and South Pass) and one large metasedimentary belt with similarities to greenstone belts (Copper Mountain). All remained unmapped and only partially explored until the 1980s and 1990s. The author mapped all of them with the exception of Elmer’s Rock, which was mapped by Graff and others (1982). For the most part, greenstone belts are formed of dark to black rocks with some dark green rocks that are difficult to differentiate and only a handful of geologists find great joy in the challenge of unraveling these complex geological puzzles.

Dark meta-basalt folded into an isoclinal fold in which the fold is so tight that the
 fold limbs parallel one another. Below – fold in banded iron formation displaying very tight folds, almost like an accordion.

Along with complex geology, there is usually gold in many fractures, faults, veins and folds. The problem is finding where the gold has been enriched enough to produce ‘ore shoots’ that contain minable quantities of gold. Often, ore bodies in greenstone belts sit right under our noses and are consistently missed by geologists and prospectors because of the complex geology. It takes considerable detective work to figure out where ore shoots are located in lodes and how they are oriented. When the lodes are located, rich placers are almost always found immediately downstream.

In 1855, territorial records show that a group of 40 prospectors including the original fur trapper traveled back to South Pass from Georgia in search of the gold in the Sweetwater River or Strawberry Creek. The group reportedly found gold everywhere in the Sweetwater River and its tributaries. In the winter, they traveled from South Pass to a more hospitable environment at Ft Laramie to the east (a distance of 220 miles as the crow flies). For some unknown reason, they were arrested upon arrival at Ft. Laramie. Three years later in 1858, one member of this group along with 8 other prospectors ventured back to South Pass, and in 1860 commenced mining along Strawberry Creek near the site of the original discovery site.
Carissa mine shear zone show location of the
primary gold shear that averages 0.15 to 0.3 opt Au,
a geologist for scale, and the surrounding rock for
at least 1,000 feet is also highly fractures and
mineralized. Samples in this structure yield gold
values of 0.02 to 0.07 opt Au over 300 feet, the
rest of it remains to be sampled.

In 1861, another group of prospectors began mining along Willow Creek, 15 miles west of Strawberry Creek. This group of 52 prospectors was attacked by Shoshone Indians and driven out. In 1863, another group of prospectors discovered gold in dry placers near Dickie Springs adjacent to the Oregon Buttes along the Overland trail several miles south of Willow Creek and worked the area for 3 months before being attacked by Indians. Most of the prospectors were killed. At this time in history, little could be done to provide security for prospecting groups as the nation was in the grips of civil war.

Over the next several years, hostilities between prospectors and the Shoshone intensified requiring that the Overland Trail be abandoned for a more southerly route (Hausel and Love, 1993). These kinds of hostilities were common in Wyoming and were one of many reasons why Wyoming remained relatively unprospected compared to other territories in the West. Wyoming remained a wilderness that few settlers dared to journey into. Today, instead of Indian hostilities, both the State and Federal Government bureaucrats have taken on hostilities against prospectors and miners.

Another group of prospectors entered South Pass in 1866 to mine gold in Willow Creek. In the following year, this group traced gold up Willow Creek to Carissa Gulch, which was rich in gold. From the mouth of the gulch, the precious metal was traced further up slope to the Carissa lode. Claims were filed on the lode on June 8th, 1867. Within a short time, this group of prospectors was attacked by Indians and three were killed: the surviving members fled, but returned more than a month later on July 28th. During the ensuing winter, more than 400 ounces of gold were recovered using very primitive hand tools and mortars. Four tons of high grade ore was hauled to Springville, Utah and milled yielding an astounding 1,400 ounces of gold! News of the discovery reached the outside world and a rush followed.

Generalized geological map of the South Pass-Atlantic City district (from
Hausel, 1989, 1991)

The district was initially called the Shoshone district. Then it was divided into smaller districts that included an area east of the Carissa near Rock Creek which became known as the California district. Four towns erupted along the pediment at South Pass. South Pass City boasted 5,000 citizens – most living in tents (some reports suggested as many as 10,000 people were in South Pass City). Hamilton City (Miners Delight) grew to 1,500 people and Atlantic City had 500 citizens. Pacific City in the Pacific district to the south near Oregon Buttes claimed 600 people. In 1870, the US Army established Camp Stambaugh a few miles east of Atlantic City and Hamilton City to add stability in the area. The camp was abandoned eight years later.

In 1872, the district had 12 operating stamp mills. A few years later, in 1879, another town was established east of South Pass City near the original gold discovery. This town became known as Lewiston (Lewiston, Miners Delight and Pacific City are now ghost towns). In 1884, an engineering project was initiated with the construction of the Granier Ditch. The ditch was built to haul water from Lake Christina 12 miles to the west in the high peaks of the Wind River Mountains to the South Pass area for hydraulic mining. This project was finished in 1890. In the following year, 6,720 ounces of gold were recovered at the hydraulic mine.

Oregon Buttes on the horizon. Nearly
all of the land in the sunlight on the
horizon is part of a giant gold deposit
known as the Dickie Springs-Oregon
Buttes paleoplacer estimaed to
contain >28.5 million ounces of gold
according to the USGS.
When I began mapping in the South Pass greenstone belt in the 1980s, I was quite impressed by the size of the belt and amazed that such an important district had remained mostly unmapped and poorly explored in modern times. Little information was available on the historical mines and mineralized zones in the district. Thus over the course of several years, I mapped 250 square miles of the complex, identified a few hundred gold anomalies, mapped all of the old mines I could get access to, and divided the greenstone belt into two separate districts based on geology: the Lewiston district along the eastern limb of the basin and the South-Pass-Atlantic City district along the western limb (Hausel, 1991). Two other districts were recognized in Tertiary and Recent sediments by the US Geological Survey that eroded from the South Pass greenstone belt. These were McGraw Flats to the north and Oregon Buttes-Dickie Springs to the south. Both contain large dry paleoplacers (fossil placers) with minor modern placers.

The greatest gold concentrations in the greenstone belt occur in faults (shear zones) in the Lewiston district and in similar structures in the South Pass-Atlantic City district. These structures that geologists refer to as shear zones, were identified by past prospectors and miners. Even though the shear zones all have anomalous gold, only sporadic ore shoots of limited strike length contain minable quantities of gold. But these ore shoots occur primarily in fold closures (known as saddle reefs) that steeply plunge down into the earth to unknown depths (possibly a few thousand feet or more). Thus in some cases, some of these saddle reefs likely host hundreds of thousands to millions of ounces of gold. But all past exploration focused only on two dimensions at the surface (the length and width) when exploration should have emphasized the forgotten third dimension, which is potentially the greatest of the three. The third dimension is that of the plunge of the ore shoots which continues down dip into the earth to unknown depths. Past drilling at the Carissa mine suggests these may plunge to very great depths. Ore shoots at South Pass have the appearance of dipping rods that pinch and swell as they continue down into the earth.

Gold panned from Dickie Springs
(photo from J.D. Love).
The richest stream placers in the belt lie immediately downstream from these structures. The nearer the placer gravels lie to shear zones, the greater the amount of gold is found in the gravel. Immense amounts of gold were described by the US Geological Survey in giant paleoplacers near Oregon Buttes to the south and in the McGraw Flats area to the north. The amount of gold estimated for the Oregon Buttes paleoplacer by the US Geological Survey was 28.5 million ounces (Figure 5). All eroded from the South Pass greenstone belt.

Rock Creek placer with old tailings

Within the greenstone belt are many significant ore shoots. The richest discovered to date, was that of the Carissa lode near South Pass City. This mine lies a short distance southeast of State Highway 28.

During mapping, I was impressed by the immensity of this gold-bearing structure. This structure consists of a prominent, relatively narrow, shear zone that has high-grade gold values. The structure averages 0.15 to 0.3 ounces per ton gold, which is not bad considering that the Homestake mine averaged about 0.3 opt throughout its lifetime. The principal structure at the Carissa is contained in a larger fracture envelope that is essentially untested and was missed by nearly every mining company. Composite chip samples collected within this larger structure over a width of many feet ranged from 0.02 to 0.07 ounce per ton gold, considered to be potentially economic and comparable to many operating gold mines in Nevada.

Past gold production from the mine is poorly documented, but available statistics suggest 50,000 to more than 180,000 ounces of gold were produced prior to 1950. The Carissa shaft was sunk to a depth of 350 feet with more than 2,300 feet of drifts on four levels over a strike length of 750 feet. A winze was later sunk to a 5th level at a depth of 400 feet below the surface.

Steve Gyorvary standing in mine-out
portion of the Carissa shear zone. The
empty space is where the shear zone was
The mined ore contained a trace to 2.6 ounces per ton of gold (opt Au). Some specimen-grade samples were recovered that assayed as high as 260 opt Au. An assay map of the mine compiled in 1926 indicates the mine terminated in mineralized rock in every direction. Later drilling in the 1970s and 1980s proved that the ore shoot continued below the mine workings suggesting the mine to have considerable potential.

The mine was developed in a shear zone in dark rock known as metagraywacke (a metamorphosed, micaceous sandstone) of the Miners Delight Formation and a mafic dike (metamorphosed igneous rock or gabbro). Essentially, all prospecting efforts concentrated on the primary shear, as it contained ore shoots of high-grade gold. The structure averaged about 6 feet wide, is 2 to 3 feet wide at the surface, but swelled to 50 feet at depth. More importantly, this primary shear lies within a much larger shear that isn’t quite as distinct and was essentially overlooked. This larger structure forms an envelope surrounding the primary shear that is >1,000 feet wide. The envelope is expressed by numerous parallel fractures with numerous quartz veinlets.
The results of the sampling suggest that the Carissa lode is a large-tonnage gold deposit that could have been developed by open pit and underground operations. The gold ore continues below the mine workings based on drilling by Anaconda Minerals Company in 1974. Anaconda intersected 16.1 feet of gold-bearing shear beneath the mine workings that averaged 0.13 opt Au. A small section in this zone assayed 1.6 opt Au! At 700 to 970 feet, the mineralized shear was intersected in four drill holes. These assayed 0.11 to 0.36 ounce per ton gold over widths of 2.3 to 11.9 feet.

In the 1980s, Consolidated McKinney Resources intersected an 80-foot mineralized zone beneath the mine workings that assayed 0.031 to 2.54 ounces per ton gold! In addition, one intercept contained >5 opt over several feet. All of these are verifiable and provide evidence for a significant to major gold deposit at the Carissa. Based on drilling, mining and surface sampling, the Carissa ore shoot has a minimum strike length of 950 feet that is open at either end. This shoot is more than 1000 feet wide and continues to a minimum depth of 970 feet and is open at depth. This shear structure is traced on the surface to the northeast and southwest for several thousand feet and most of it remains unsampled. Such shear zones typically continue to a few thousand feet deep in similar greenstone belts worldwide.
Right- folded parallel veins in the Carissa Shear Envelop. This rock
covers >1,000 feet of width surrounding the primary Carissa Shear 
and is also highly mineralized forming an ore zone that is economic and
could easily be mined by open pit. Channel samples collected in this zone
by the author ranged from 0.02 to 0.07 opt Au.
Beeler (1908) reported the ore in the primary shear to average 0.3 opt Au. Composite chip samples in the giant low-grade shear envelope enclosing this primary structure yielded anomalous gold over a width of 300 feet: these samples yielded 0.02 to 0.05 opt (Curran, 1926; Hausel, 1989). A 97-foot composite sample in this zone assayed 0.023 opt Au and a 30-foot composite assayed 0.07 opt Au (Hausel, 1991a): at today’s gold prices these would be considered economic. The remaining envelope (700 feet) remains unsampled, but undoubtedly contains gold based on the structure and presence of secondary quartz.

Based on sampling, the Carissa has a distinct mineralized zone that is likely a few thousand feet deep, as much as 1,000 feet wide, and 1,000 feet along strike. However, the mineralized zone is open in all three directions and could be enlarged considerably with additional drilling.

This would suggest, that at an average ore grade of 0.1 opt Au, this zone potentially hosts 3.7 million ounces of unmined gold worth $4.4 billion dollars. This does not include potential resources deeper than 1,000 feet or further along the strike length of the shear. In effect, the Wyoming State legislature and Governor removed (nationalized) a major gold deposit from the public sector that would have created numerous jobs in Fremont County and provided a significant tax base to Wyoming and Fremont County. Instead, in the wisdom we have come to love in government, Wyoming now has a mining Disneyland sitting on a major ore deposit. This Disneyland may produce a few thousand dollars in revenue for the state while at the same time it sucks out a few $hundred thousand for budget and salaries to operate the Wyoming Disneyland park.

The people who operate the South Pass City historic site are very good at what they do and should be respected for their knowledge. But the legislature and governor should have their heads examined. This Disneyland appears to be spreading in a district that could provide many future jobs. Dollars set aside for Federal abandoned mine reclamation programs were used to rebuild the Carissa surface buildings and are being used to rebuild buildings on private land at the Duncan mine, another probable ore body. When will government ever be required to pay for their actions?

Table 1. Chip channel and channel sample analyses in the South Pass greenstone belt (Hausel, 1989a).

Carissa Mine (low-grade envelope) (ppm)
0 to 10 ft north of shear 0.4
10 to 20 ft north of shear 1.05
20 to 37 ft north of shear 2.5
0 to 10 ft south of shear 0.65
10 to 20 ft south of shear 0.25
20 to 30 ft south of shear 0.30
30 to 60 ft south of shear 0.35
30 ft composite north of shear 2.4

The State of Wyoming purchased the Carissa mine
(which sits on a major gold deposit) and
withdrew the property from mining to reconstruct
a monument to mining where no man can mine again. This
was done by the State Legislature without considering the
geology and ore deposits. Now the old workings attract a
few hundred people a year while they sit on top of a gold
deposit estimated to contain more than a few $billion in gold
The Carissa ore is structurally controlled and is interpreted as a saddle reef deposit where high-grade gold is localized in fold closures and rehealed fractures similar to the Homestake. The geological evidence suggests the ore-body continues to great depth. Support for the presence of a major ore deposit includes drilling by Consolidated McKinney Resources who identified a highly anomalous 80-foot wide mineralized zone. Carissa Gold Inc. made the a reserve estimate years ago without knowing there was a major shear envelope and had no data on the mineralized structure beneath the mine working. Using an extraordinary high reserve cutoff grade: 208,000 tons of ore at an average grade of 0.343 opt Au; and geological reserves of 37,000 tons of ore averaging 0.85 opt Au.

Anaconda Minerals Company drilled the property prior to Consolidated McKinney Resources and all of their drill holes interested ore grade material. They intersected a high grade ore zone over widths of 2.3 to 16.1 feet that yielded 0.11 to 0.36 opt Au to depths of 700 feet.

The Wyoming legislature & governor did what the Shoshone Indians of the past could not do. The Wyoming legislature ‘nationalized’ this mine by purchasing private property and incorporating it into the South Pass City historical site without considering geological studies or scientific testimony. This was likely one of the two best gold targets in the state. The other is located within the Rattlesnake Hills where a multi-million ounce gold deposit is currently being drilled.
Adjacent to the mine is a dry gulch locally known as Carissa gulch. This gulch drains into Willow Creek from the western extent of the lode. The eastern extent of the lode is sampled by nearby Hermit Gulch. Both gulches should provide excellent places to hunt for nuggets with metal detectors, but again, these have been withdrawn from exploration as has been Willow Creek. Willow Creek is purported to have dangerous mercury levels based solely on here say, yet it would likely produce significant gold values.

Other properties of interest at South Pass include Miners Delight and the Wolf mine along with thousands of feet of unexplored shear zones in the South Pass-Atlantic City district, the Lewiston district, and in the Crows nest. The Miners Delight mine has a very attractive shear structure that yielded a 0.68 opt Au channel sample across about 8 feet and the Wolf mine yielded a grab sample that assayed 0.5 opt Au. This shear is poorly exposed by was trenched by one company that showed a shear that was 160 feet wide (Steve Gyorvary, personal communication, 2010).

Other interesting deposits in Wyoming include the Rattlesnake Hills and Seminoe greenstone belts. The author identified a few hundred gold anomalies at South Pass, found the first several significant gold anomalies in the Rattlesnake Hills, identified a significant gold anomaly in the Seminoe Mountains and identified another gold anomaly at Copper Mountain. The Elmer’s Rock greenstone belt remains essentially unexplored for mineralization even though several indicator minerals have been found in that region that suggest the presence of several hidden diamond deposits. In addition, two world-class colored gemstone deposits were found along the flanks of the Elmer’s Rock belt that remains relatively unexplored. The author also identified excellent gold targets in the Sierra Madre, southern Laramie Range, Bearlodge Mountains and Mineral Hill (Hausel, 1987).

However, due to the lack of regard for the mining sector and prospectors, the author recommends that companies and prospectors come to Arizona, Nevada or Canada to search for gold.

Recommended Reading

Blackstone, D.L., and Hausel, W.D., 1991, Guide to the geology and mineralization of the Seminoe Mountains, Wyoming, in S. Roberts, editor, Mineral Resources of Wyoming: Wyoming Geological Association 42nd Annual Field Conference Guidebook, p. 201-210.

Graff, P.J., Sears, J.W., Holden, G.S., and Hausel, W.D., 1982, Geology of Elmer’s Rock greenstone belt, Laramie Range, Wyoming: Geological Survey of Wyoming Report of Investigations 14, 22 p.

Hausel, W.D., 1987, Structural control of Archean gold mineralization within the South Pass greenstone terrain, Wyoming (USA), in The Practical Applications of Trace Elements and Isotopes to Environmental Biogeochemistry and Mineral Resources Evaluation, Theophrastus Publications, Athens, Greece, p.199-216.

Hausel, W.D., 1989, The Geology of Wyoming's Precious Metal Lode & 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., 1993, Mining History and geology of some of Wyoming’s metal and gemstone districts: in Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 39-64.

Hausel, W.D., 1994, Economic Geology of the Seminoe Mountains Mining District, Carbon County, Wyoming: Wyoming State Geological Survey Report of Investigations 50, 31 p.

Hausel, W.D., 1996, Economic Geology of the Rattlesnake Hills Supracrustal Belt, Natrona County, Wyoming: Geological Survey of Wyoming Report of Investigations 52, 28 p.

Hausel, W.D., 1997, The Geology of Wyoming's Copper, Lead, Zinc, Molybdenum and Associated Metal Deposits: Geological Survey of Wyoming Bulletin 70, 224 p.

Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming - A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.

Hausel, W.D., and Hausel, E.J., 2011, GOLD - Field Guide for Prospectors and Geologists (Wyoming Examples). CreateSpace, 366 p.

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 & Mineralization, Wyoming Province: 28th International Geological Congress guide to field trip T-332, July 19-25, 48 p.

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