South Pass greenstone belt
Encyclopedia
The South Pass greenstone belt (2.8 Ga) is located within the Wyoming craton
in the US. The region was the site of Wyoming's initial gold discovery in 1842.
was discovered in the Lewiston district of the South Pass granite-greenstone belt in 1842 along the eastern margin of the complex. Other discoveries followed including the discovery of high-grade gold mineralization at the Carissa Mine along the western flank of the belt in 1867. Some high-grade gold was recovered from the Lewiston and the South Pass-Atlantic City districts. One large 34-ounce nugget resides in the Los Angeles Museum of Natural History that was recovered from Rock Creek along the western flank of the greenstone belt.
In addition to gold, significant iron ore deposits were found along the northern edge of the South Pass greenstone belt. Some minor copper
, silver
, tungsten
, asbestos
, beryl
(aquamarine), ruby
including a diamond have also been reported within the region. Development of the large taconite
(iron ore) deposits in the 1950s led to the opening of the Atlantic City iron mine, which produced more than 90 million tons of iron ore between 1962 and 1983 from banded iron formation
in the Goldman Meadows Formation. Extensive gold development took place across the entire Precambrian greenstone belt in the late 1860s to early 1870s, and again in the 1930s until World War II. Lode deposits occur primarily in strike-trending shear zones within metagreywacke and hornblendic orthoamphibolite
s, and placer
operations have developed at various times along most streams downstream from these shear structures.
Hausel reported cursory investigations of the historic Carissa Mine during the late 1980s indicated this mine hosted an identifiable reserve of more than 109,000 ounces averaging 0.343 to 0.850 oz/ton with a cutoff grade of 0.299 oz/ton. But due to scattered drilling and lack of deep drilling on this property, this high-grade zone most likely is much more extensive along strike and depth. The high cutoff grade combined with the essentially untested large tonnage low-grade mineralized zone associated with the property (>1000 feet wide and >1000 feet long) suggests actual gold resources are much greater. In addition to the Carissa property, a distinct belt of prospects and historic mines lie between the Carissa mine and Miners Delight mine along a 4 to 5 mile northeasterly trend in the belt. This provides a structurally favorable zone for a major gold resource. Mapping by Hausel identified numerous shear structures along with steeply plunging ore shoots associated with isoclinal and open folding
that may be favorable for saddle reef-type gold mineralization. There is little evidence that any of the historic operations recognized the steeply plunging structures as being significant.
The margins of the greenstone belt contain major gold anomalies within Tertiary sediments eroded from the greenstone belt. The Cathedral Bluffs Tongue of the Eocene Wasatch Formation, is estimated to host more than 28.5 million ounces of placer gold in the Dickie Springs-Oregon Gulch area.
Some aquamarine beryl is known in pegmatite in the South Pass Granite in the Anderson Ridge area in the northwestern corner of the belt. Non-gem beryl is reported from several coarse-grained tourmaline-beryl granite pegmatites in the area.
This elongate greenstone belt is characterized by Hausel as a synclinorium of metamorphosed
volcanic, sedimentary and plutonic rocks that have been regionally metamorphosed to greenschist
and amphibolite facies, and locally overprinted by a later retrogressive greenschist facies event. The axis of the belt is paralleled by foliation, shear zones, and lower order fold axes, and has been intruded along its flanks by granite and granodiorite
.
and granite pegmatite
(2545 ± 30 Ma) intrude metasedimentary rock of the Miners Delight Formation in the western part of the greenstone belt. The South Pass pluton is a pegmatitic granite west of South Pass City, and the Sweetwater granite is a fine-to medium-grained leucocratic granite that occurs to the west of the South Pass pluton along the Sweetwater River and Lander Creek.
The first granitic event in the South Pass area includes tonalite dikes and plugs intruded into shear zones in the South Pass supracrustal rocks along with light-colored, metamorphosed leucodacite porphyry and quartz diorite. This unit is exposed in small areas near the Mary Ellen mine, just west of the Duncan mine, and in a 6-mile long north-trending intrusion east of Lewiston. A quartz vein included within the Mary Ellen stock carries visible gold, and a quartz stringer within sheared metaleucodacite porphyry dike contained disseminated arsenopyrite, and showed 0.01 oz/ton gold along with 0.01 oz/ton silver.
of the Miners Delight Formation, ranging in thickness from 5000 to 20,000 feet, is dominated by gray to dark brown feldspathic and biotitic metagreywacke
, and mica schist. An Rb-Sr whole-rock isochron of about 2.8 Ga reported for the greywacke by Stuckless and others (1985). The contact between the Miners Delight formation and the Roundtop Mountain Greenstone is the Roundtop fault, which is locally marked by mylonitized
, breccia
ted, and strongly folded rocks. The Miners Delight formation which hosts several epigenetic shear zone and vein gold deposits is subdivided into several lithologic units, the relative ages of which are not known.
The Roundtop Mountain Greenstone is predominantly greenstone, greenschist, and amphibolite, but also includes mica schist, hornblende-mica schist, and metabasalt, accompanied by minor metagreywacke, metatuff, chlorite schist, tremolite-actinolite schist and rare grunerite schist. The unit is exposed on both limbs of the South Pass synclinorium, and in most places conformably, but locally unconformably, overlies the Goldman Meadows Formation. Much of the unit is composed of metamorphosed pillow basalt
s and cusp-shaped pillow structures are preserved on Roundtop Mountain. A broad zone of carbonated breccias and intensely folded schists, representing a major break in the geologic record, is found at the top of the formation.
The Goldman Meadows Formation overlies the Diamond Springs Formation and contains two distinct lithologies: a schist member that includes pelitic schists, quartzites and massive to schistose amphibolites; and iron formation members composed of banded quartz-magnetite-amphibolite iron formation. The iron formation consists of laminated dark gray to black, fine-grained, hard, dense alternating 0.1 to 2 in (2.5 to 50.8 mm) layers of magnetite and metachert and varying amounts of amphibole. The average iron content in the Atlantic City area is about 33.5% and ranges as high as 56.23%.
The Diamond Springs Formation is conformable with the overlying rocks and consists predominantly of serpentinite
, tremolite
-talc
-chlorite
schist, and amphibolite. Compositionally, these rocks are typical of the basal volcanic members in other Archean terranes, and represent high-magnesian, sub-marine flows and sills
.
gneiss
and granite migmatite
, interlayered with supracrustal rocks and intruded by granodiorite
, is intercalated in places with the supracrustal rocks of the greenstone belt. It is suggested that the likely origin for this unit is an ancient basement or earlier supracrustal succession tectonically interleaved as thrust splinters into the South Pass supracrustals.
Wyoming craton
The Wyoming craton is a craton located in the west-central United States and western Canada – more specifically, in Montana, Wyoming, southern Alberta, southern Saskatchewan, and parts of northern Utah...
in the US. The region was the site of Wyoming's initial gold discovery in 1842.
Economic geology
GoldGold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
was discovered in the Lewiston district of the South Pass granite-greenstone belt in 1842 along the eastern margin of the complex. Other discoveries followed including the discovery of high-grade gold mineralization at the Carissa Mine along the western flank of the belt in 1867. Some high-grade gold was recovered from the Lewiston and the South Pass-Atlantic City districts. One large 34-ounce nugget resides in the Los Angeles Museum of Natural History that was recovered from Rock Creek along the western flank of the greenstone belt.
In addition to gold, significant iron ore deposits were found along the northern edge of the South Pass greenstone belt. Some minor copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, asbestos
Asbestos
Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
, beryl
Beryl
The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al26. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare...
(aquamarine), ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
including a diamond have also been reported within the region. Development of the large taconite
Taconite
Taconite is a variety of iron formation, an iron-bearing sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate...
(iron ore) deposits in the 1950s led to the opening of the Atlantic City iron mine, which produced more than 90 million tons of iron ore between 1962 and 1983 from banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
in the Goldman Meadows Formation. Extensive gold development took place across the entire Precambrian greenstone belt in the late 1860s to early 1870s, and again in the 1930s until World War II. Lode deposits occur primarily in strike-trending shear zones within metagreywacke and hornblendic orthoamphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s, and placer
Placer
Placer may refer to one of the following:*Placer deposit*Placer sheep*Placer mining*Placer, rugby league football role.Geographical names:* Placer, Masbate, Philippines* Placer, Surigao del Norte, Philippines...
operations have developed at various times along most streams downstream from these shear structures.
Hausel reported cursory investigations of the historic Carissa Mine during the late 1980s indicated this mine hosted an identifiable reserve of more than 109,000 ounces averaging 0.343 to 0.850 oz/ton with a cutoff grade of 0.299 oz/ton. But due to scattered drilling and lack of deep drilling on this property, this high-grade zone most likely is much more extensive along strike and depth. The high cutoff grade combined with the essentially untested large tonnage low-grade mineralized zone associated with the property (>1000 feet wide and >1000 feet long) suggests actual gold resources are much greater. In addition to the Carissa property, a distinct belt of prospects and historic mines lie between the Carissa mine and Miners Delight mine along a 4 to 5 mile northeasterly trend in the belt. This provides a structurally favorable zone for a major gold resource. Mapping by Hausel identified numerous shear structures along with steeply plunging ore shoots associated with isoclinal and open folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
that may be favorable for saddle reef-type gold mineralization. There is little evidence that any of the historic operations recognized the steeply plunging structures as being significant.
The margins of the greenstone belt contain major gold anomalies within Tertiary sediments eroded from the greenstone belt. The Cathedral Bluffs Tongue of the Eocene Wasatch Formation, is estimated to host more than 28.5 million ounces of placer gold in the Dickie Springs-Oregon Gulch area.
Some aquamarine beryl is known in pegmatite in the South Pass Granite in the Anderson Ridge area in the northwestern corner of the belt. Non-gem beryl is reported from several coarse-grained tourmaline-beryl granite pegmatites in the area.
Geologic units
The geology for the South Pass belt was investigated by Hausel.This elongate greenstone belt is characterized by Hausel as a synclinorium of metamorphosed
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
volcanic, sedimentary and plutonic rocks that have been regionally metamorphosed to greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
and amphibolite facies, and locally overprinted by a later retrogressive greenschist facies event. The axis of the belt is paralleled by foliation, shear zones, and lower order fold axes, and has been intruded along its flanks by granite and granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
.
Archean
GraniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
and granite pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
(2545 ± 30 Ma) intrude metasedimentary rock of the Miners Delight Formation in the western part of the greenstone belt. The South Pass pluton is a pegmatitic granite west of South Pass City, and the Sweetwater granite is a fine-to medium-grained leucocratic granite that occurs to the west of the South Pass pluton along the Sweetwater River and Lander Creek.
The first granitic event in the South Pass area includes tonalite dikes and plugs intruded into shear zones in the South Pass supracrustal rocks along with light-colored, metamorphosed leucodacite porphyry and quartz diorite. This unit is exposed in small areas near the Mary Ellen mine, just west of the Duncan mine, and in a 6-mile long north-trending intrusion east of Lewiston. A quartz vein included within the Mary Ellen stock carries visible gold, and a quartz stringer within sheared metaleucodacite porphyry dike contained disseminated arsenopyrite, and showed 0.01 oz/ton gold along with 0.01 oz/ton silver.
South Pass greenstone belt metasedimentary and metaigneous rocks
The Miners Delight Formation is a metagreywacke (2.8 Ga). The diverse lithologyLithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
of the Miners Delight Formation, ranging in thickness from 5000 to 20,000 feet, is dominated by gray to dark brown feldspathic and biotitic metagreywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
, and mica schist. An Rb-Sr whole-rock isochron of about 2.8 Ga reported for the greywacke by Stuckless and others (1985). The contact between the Miners Delight formation and the Roundtop Mountain Greenstone is the Roundtop fault, which is locally marked by mylonitized
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
, breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
ted, and strongly folded rocks. The Miners Delight formation which hosts several epigenetic shear zone and vein gold deposits is subdivided into several lithologic units, the relative ages of which are not known.
The Roundtop Mountain Greenstone is predominantly greenstone, greenschist, and amphibolite, but also includes mica schist, hornblende-mica schist, and metabasalt, accompanied by minor metagreywacke, metatuff, chlorite schist, tremolite-actinolite schist and rare grunerite schist. The unit is exposed on both limbs of the South Pass synclinorium, and in most places conformably, but locally unconformably, overlies the Goldman Meadows Formation. Much of the unit is composed of metamorphosed pillow basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s and cusp-shaped pillow structures are preserved on Roundtop Mountain. A broad zone of carbonated breccias and intensely folded schists, representing a major break in the geologic record, is found at the top of the formation.
The Goldman Meadows Formation overlies the Diamond Springs Formation and contains two distinct lithologies: a schist member that includes pelitic schists, quartzites and massive to schistose amphibolites; and iron formation members composed of banded quartz-magnetite-amphibolite iron formation. The iron formation consists of laminated dark gray to black, fine-grained, hard, dense alternating 0.1 to 2 in (2.5 to 50.8 mm) layers of magnetite and metachert and varying amounts of amphibole. The average iron content in the Atlantic City area is about 33.5% and ranges as high as 56.23%.
The Diamond Springs Formation is conformable with the overlying rocks and consists predominantly of serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
, tremolite
Tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
-talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
-chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
schist, and amphibolite. Compositionally, these rocks are typical of the basal volcanic members in other Archean terranes, and represent high-magnesian, sub-marine flows and sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
.
Gneiss complex
FelsicFelsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
and granite migmatite
Migmatite
Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...
, interlayered with supracrustal rocks and intruded by granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
, is intercalated in places with the supracrustal rocks of the greenstone belt. It is suggested that the likely origin for this unit is an ancient basement or earlier supracrustal succession tectonically interleaved as thrust splinters into the South Pass supracrustals.