Friday, September 5, 2014

Precious Metals in the Cowboy State

Available production records for metals for Wyoming are far from accurate. In the past, few records were archived and production from the Wyoming territory was often incorporated into the totals of neighboring territories, states and mining districts. However, based on the scattered reports, it appears that sufficient quantities of metals were recovered from Wyoming during past years to warrant further exploration of the State (Table 1). 

Au (oz)
Ag (oz)
Pt (oz)
Pd (oz)
Pb (lbs)
Cu (lbs)
Fe (tons)
Table 1. Historic production of selected metals for Wyoming (from Hausel, 1989a)
A 24-ounce gold nugget from Rock Creek in the South Pass district.
Metal deposits are scattered throughout the stratigraphic record; however, the greatest concentrations are in rocks of Precambrian and Tertiary age. The very ancient Archean (>2.5 Ga = more than 2.5 years old) rocks have a number of significant metal and semi-precious stone deposits and anomalies. The Archean rocks include folded and metamorphosed sedimentary, volcanic, and plutonic rocks exposed in the core of mountain ranges. Some of these include gold in shears and veins, along with nearby banded iron formation,  massive hematite, localized veinlet and disseminated scheelite, vein nephrite (jade), ruby and sapphire schist, and podiform and veinlet chromite. Granitic rocks include copper deposits, some rare earth deposits (REE), tantalite, and beryl pegmatites.  Rare aquamarine beryl and green tourmaline have been reported in some pegmatites. In addition, some of the largest colored gemstone deposits in the world were discovered in metapelite near the Elmers Rock Greenstone belt. These deposits are mostly iolite gemstones (some gems my thousands of carats in weight) found in quartzofeldspathic gneiss adjacent to sillimanite gneiss, kyanite schist and ruby- and sapphire-vermiculite schist.
Mineralized terranes and districts in Wyoming (after Hausel, 1997).
In southeastern Wyoming, thick Proterozoic (<2.5 Ga) metasedimentary successions crop out as miogeosynclinal wedges unconformably lying on the Archean basement. These include quartzite-hosted Cu-Ag-Au deposits, Au-Ag veins, and radioactive Witwatersrand-type metaconglomerates with isolated Au anomalies. This miogeoclinal terrane is separated from a predominantly metavolcanic Proterozoic terrane to the south by a major Precambrian suture, or shear zone.  The suture includes scattered Cu-Au-Ag and some Pt- Pd-bearing mineralization in the cataclastics. 
Gold in black sands from Douglas Creek placer district, Medicine Bow
Mountains, Wyoming.
South of the suture, a Proterozoic metavolcanic terrane (1.8 to 1.6 Ga) includes volcanogenic Zn-Cu-Ag massive sulfide deposits, at least one Cu-Au porphyry, and two large layered mafic complexes (1.8 Ga). The northern edge of one of these intrusives yielded some Pt and Pd with Cu, Au, and Ag at the beginning of the 20th century. A 350 mi2 anorthosite batholith (1.4-1.5 Ga) in the Laramie Mountains has long been considered as a possible source of low-grade aluminum mineralization as well as extensive disseminated and massive Ti-Fe-V deposits. At one point along the margin of the batholith is a Cu-Au-Ag-W-Mo vein (?) deposit with anomalous nickel (Ni). And in this area, may be one the largest colored gemstone deposit found on earth, the Sherman Mountains iolite deposit that was briefly investigated by Wyoming Geological Survey geologist Dan Hausel and suggested that the deposit could potentially host more than a trillion carats of iolite based on past cordierite resource investigation in 1949. In addition to these gems, large resources of spectralite (labradorite) are found and some kimberlites intrude the anorthosite near Grant Creek.
A close look at this small nugget from Douglas Creek shows it is rounded suggesting that it traveled some district from
its source. 
Mineralization for much of the Phanerozoic was not widespread. Within the Paleozoic, Au-REE anomalies are reported in Cambrian conglomerates, some Cambrian and Devonian age kimberlite intrusives contain gem and industrial diamond, some relatively unimportant Pennsylvanian and Mississippian Cu-Ag and Mn carbonate-hosted occurrences, and Permian phosphatic and black shales contain a number of metal anomalies. Ag-Cu-Zn stratabound deposits are found in bleached Jurassic redbeds in western Wyoming, and the broad Wyoming basins include many enigmatic Au occurrences and anomalies that can only partially be explained by detrital transport in fluvial systems.  Rocks of Late Cretaceous age along the flanks of some uplifts have black titaniferous sandstone deposits. Detrital heavy minerals with anomalous Ti, Zr, REE, and in some cases Au, are concentrated in many of these paleoplacers.  
Tertiary rocks host some important ore deposits, occurrences, and anomalies. Some of the largest ore deposits and areas of mineralization in the state are the Cu-Ag porphyries in the Absaroka Mountains. This deeply incised volcanic plateau includes several Tertiary age composite stocks with zoned hydrothermal alteration and mineralization.  The porphyries contain significant Cu, Ag, Pb, Zn, Au, and Mo but all were withdrawn from exploration by the US Forest Service, likely in response to UN agenda 21.
In the Black Hills of northeastern Wyoming, Au-Th-REE mineralization and large fluorite deposits are found associated with Tertiary alkalic intrusives. In addition, some Pb-Zn-Ag replacement mineralization; and Sn pegmatites are found in this region. Significant gold and silver, in particular, we detected by Hausel (1997) at Mineral Hill. The Rattlesnake Hills in central Wyoming are formed by similar Tertiary igneous rocks that intrude an Archean greenstone belt, implying this terrain to have the highest potential for major gold deposits similar to Cripple Creek. Gold was discovered in this area in 1981 by W.D. Hausel of the Wyoming Geological Survey. 
The Tertiary of Wyoming was not only a time of volcanism, but was also an important time of paleoplacer development.  Laramide uplifts were nearly leveled by erosion which resulted in deposition of large volumes of fluvial conglomerate and fanglomerate along the flanks of the uplifts. Where these conglomerates were eroded from mineralized terranes such as  greenstone belts and other supracrustal terranes, they are often mineralized in gold. Modern placers have been relatively productive. Most commercial placers have been mined for gold, although some cassiterite, native platinum, scheelite, monazite, diamonds, chromite, native silver and tantalite have also been recovered from Recent placers in the State.
The ancient geology of Wyoming has been compared to some of the richest precious metal regions of the world including South Africa, Western Australia, and the Superior Province of Canada.  These areas are all underlain by ancient Archean cratons with some of the oldest rocks on the surface of the earth.  But unlike these other cratons, the Wyoming craton (termed the Wyoming Province) has been greatly modified by Laramide tectonics which thrust slices of the ancient craton through younger Phanerozoic sedimentary rocks.  Due to the unfortunate nature of the Laramide tectonics, vast regions of the Wyoming craton remain at the bottom of the Tertiary basins unavailable to direct observation, but the exposed slices in the mountain cores contain some of the best exposures of Archean rock in the world.  
The Wyoming Province is not confined to the State of Wyoming, but extends into northern Utah, extreme northeastern Nevada, the eastern edge of Idaho, portions of Montana, and the northwestern corner of South Dakota (Figure 1).  And according to Condie (1976), the Wyoming craton may have once been connected to the Superior Province of Canada. 
Similar to other cratons of the world, the Wyoming craton is also highly mineralized.  Within and on the margin of the craton are vast mineral resources which include the Stillwater complex (Pt, Pd, Cr), Jardine (Au), Homestake (Au), and South Pass (Au, Fe).
The Wyoming Province is formed of a vast region of high-grade gneiss and migmatite intruded by granite and granodiorite plutons that contain scattered blocks, fragments, and belts of supracrustal terranes that include greenstone belts, eugeoclinal successions, layered mafic complexes, and high-grade supracrustal belts (Figure 1).  Mineral occurrences in the granite-gneiss terrane are uncommon, but the supracrustal belts have hundreds of gold, base metal, and related mineral deposits and prospects.
Greenstone Belts. Houston (1983) noted the Province contained supracrustal successions of low-rank metamorphosed rock in the southern portion of the craton that exhibited similarities to greenstone belts in other cratons of the world.  However, in the northern region of the craton, the rank of the supracrustal belts generally increases, and the belts are lithologically distinct, in that they are formed of intercalated amphibolite and gneiss with subordinate metasedimentary successions of quartzite, metapelite, BIF (banded iron formation), amphibolite, and minor metacarbonate.  Exceptions occur, such as the Jardine belt of the Snowy Range block of the Beartooth Mountains in the northern portion of the Wyoming craton, which was reported by Thurston (1986) to exhibit similarities to the South Pass greenstone belt in the Wind River Mountains in the south.  But it was Houston's contention, that the greenstone belts represented the greatest potential for Archean mineralization compared to any other part of the Province. And these belts are concentrated along the southern margin of the craton in Wyoming. 
In general, the Wyoming greenstone belts form tripartite successions of low-rank metamorphosed (upper greenschist to middle amphibolite facies) sedimentary, volcanic, and plutonic rock, folded into a regional synclinorium.  Bedding and most structural elements (i.e., foliation, isoclinal  fold axes, auriferous shears) parallel the principal axis of the synclinorium.  Although amphibolite facies metamorphism presides in these terranes, the rank is relatively low resulting in the preservation of some primary textures.  Pillow structures, and porphyritic, amygdaloidal, spinifex, and cumulus textures are preserved in some igneous rocks.  Graded bedding, quartz pebbles, cross-bedding, and cut and fill channels occur in some metasedimentary rocks. 
 The lower portions of the belts are formed of ultramafic to mafic metavolcanic successions of hornblendic amphibolite, serpentinite, tremolite-talc-chlorite schist and metabasalt. These rocks have compositions that suggest their precursors were high-magnesian tholeiitic basalts and tuffs, and komatiitic basalts and peridotites. 
The character of the greenstone successions changed through time, and  the underlying successions were replaced by calc-alkaline metavolcanics (meta-andesite, metadacite, felsic schist), high-iron tholeiitic basalts (amphibolite, greenstone, and metabasalt), and metakomatiite (actinolite schist, hornblende-plagioclase amphibolite, tremolite-talc-chlorite schist, and minor serpentinite).  Metasedimentary rocks are prominent in the upper succession and include metagreywacke, BIF, with lesser quartzite, graphitic schist, metaconglomerate, and metapelite.
Several greenstone belts and fragments are recognized in Wyoming and include South Pass in the Wind River Mountains, Barlow Gap and Tin Cup in the Granite Mountains, the Seminoe Mountains, Casper Mountain, Sellers Mountain, Esterbrook, and Elmers Rock in the Laramie Range, as well as other fragments in the Laramie, Medicine Bow, and Sierra Madre ranges (Condie, 1967; Houston, 1983). Of these, the South Pass, Seminoe Mountains and Barlow Gap regions have recieved the greastest interest in recent years.
South Pass.  The South Pass greenstone belt in the southern Wind River Mountains crops out over a 150 to 200 mi2 surface area, but a large portion of the belt is projected to continue under a thin (<1 ft to > 2,000 ft thick) blanket of Tertiary arkosic sandstone, siltstone, and conglomerate. During the past, this region has received only minor exploration interest for lode, paleoplacer, and placer gold, yet the terrane undoubtedly offers the greatest potential of all of the greenstone belts in the Wyoming craton for the discovery of significant mineralization. Precious and base metal deposits in the belt  include auriferous shear zones and veins, associated Tertiary paleoplacers and modern placers, Ag- and Au-bearing cupriferous veins, and cupriferous stockworks. 
The South Pass greenstone belt (after Hausel, 1991).
Shear zones in the greenstone belt are narrow, foliation-parallel, cataclastic zones, that exhibit both brittle and ductile deformation and have strike lengths of dozens of feet to more than 11,000 ft (Hausel and Hull, 1990).  Widths are typically between 2 to 15 ft, although greater widths occur at several mines.  For the most part, these structures are weakly mineralized along much of their trend with localized ore shoots.  The shoots occur at pinches, swells, fold closures, attitude changes, and at intersections of structures.
Historic reports indicate the tenor of the shoots ranged from a trace to as much as 3,100 opt Au.  Average mine grades varied from 2.06 ppm (0.06 opt) to 68.6 ppm (2.0 opt) Au with minor Ag (Hausel, 1989a). The continuation of these structures downdip has not been fully tested since the deepest gold mine is only 400 ft deep, and drilling has penetrated the mineralized structures to depths of only 930 ft below the surface (deQuadros, 1989).  
Many of the shears and veins are localized in metagreywacke and hornblendic amphibolite with fewer in graphitic schist, meta-andesite, greenstone, greenschist, metatonalite, and tremolite/actinolite schist.  A large proportion of the shears are found along or adjacent to lithologic contacts of rocks with contrasting competency (Bayley, 1968; Hausel, 1987).
The source of the shear zone gold was suggested by Bow (1986) to have been rocks of komatiite affinity. But recent stable isotope and fluid inclusion studies by Spry and McGowan (1989) are redolent of a greywacke source.  Hausel (1991), however, was impressed by the ubiquitous occurrence of structurally controlled gold anomalies throughout the greenstone belt independent of rock type and proposed the gold was derived by metamorphic secretion during a 2.8 Ga regional metamorphic event, and the shears served to focus the auriferous fluids.

In addition to narrow mineralized structures,  the possibility exists for large-tonnage, low-grade, precious metal deposits in this greenstone belt.  Four deposits investigated by the Wyoming Geological Survey as possible large tonnage deposits included the Carissa, Duncan, Lone Pine, and Tabor Grand properties. 
The gold-bearing Carissa shear zone hosts a major, unmined gold deposit.
At the Carissa mine in the South Pass-Atlantic City district along the northern flank of the greenstone belt, the principal shear is a narrow 5 to 50 ft wide structure enveloped by a broad, 100 to 300 ft zone of weakly mineralized wallrock with rehealed fractures. A 1.5 ft channel sample taken across the shear assayed 5.2 ppm (0.15 opt) Au (Hausel, 1989a), and Beeler (1908) reported the average ore ran 10.29 ppm (0.3 opt) Au.  Composite chip samples collected in the adjacent wallrock also yielded anomalous gold over a 97 ft width (Hausel, 1989a) (Table 1). The mineralized structure was appears to have a with of nearly 1000 feet with considerable strike: significant gold was intercepted at depth in the deepest drill holes drilled in the area to a depth of 970 feet. Based on geology, the gold bearing ore shoot is believed to continue to much greater depth (Hausel and Hausel, 2011).
At the Duncan mine, the foliation-parallel shear is folded and splayed. The splay has a aggregate width of more than 40 feet adjacent to the shaft, and is mineralized across its entire width.  But within the fold closure, the gold values are enhanced, and the nose of the steeply plunging drag fold averages nearly ten times the amount of gold in the fold limbs (Table 1).
In the Tabor Grand mine, a 1 to 5 ft wide shear cuts hornblendic amphibolite.  Samples of the shear yielded 0.06 to 58.0 ppm Au over a 350 ft length.  During recent mapping of the mine, a second shear parallel to the first was discovered 20 ft south of the primary shear.  Two samples taken in this shear yielded 1.7 and 7.0 ppm Au.   Surface mapping extended the length of the shear another 800 ft to the east where an 8 ft channel sample assayed 3.8 ppm Au (Hausel, 1991).
At the Lone Pine mine in the Lewiston district along the southeastern margin of the greenstone belt, a hidden shear was recently discovered buried under a thin veneer of Tertiary South Pass Formation.  The discovery trench exposed a 17 ft wide shear which yielded gold values of 0.47 to 3.5 ppm (Table 1). The maximum mineralized width and strike length of this structure have not been determined.  At another mine in the same district known as the Wolf mine, representative samples yielded 23.3 ppm (0.68 opt) Au (Hausel, 1989a).  This property was later examined by U.S. Borax, and it was determined the shear was mineralized over a more than 100 ft width.
Table 1. Chip channel and channel sample analyses in the South Pass greenstone belt (Hausel,1989a).
Carissa Mine
0 to 10 ft north of shear
10 to 20 ft north of shear
20 to 37 ft north of shear
0 to 10 ft south of shear
10 to 20 ft south of shear
20 to 30 ft south of shear
30 to 60 ft south of shear
30 ft composite north of shear
Duncan Mine

0 to 2 ft west of fold closure in shear
2 ft channel across fold closure
0 to 5 ft east of closure
5  to 15 ft east of closure
15 to 25 ft east of closure
25 to 35 ft east of closure
Lone Pine Mine

0 to 4 ft (E to W) in shear
4 to 7 ft (E to W) in shear
7 to 11 ft (E to W) in shear
11 to 17 ft (E to W) in shear

In addition to auriferous shears, cupriferous veins with Au and Ag values are found at several locations in the belt.  These veins have little economic value because of the lack of tonnage.  They are seldom greater than 2 ft wide and pinch and swell over short distances along strike.  However, one Cu-Ag stockwork discovered along the northwestern flank of the belt may be of interest.  The extent of the stockwork is unknown since it lies under soil in a large aspen grove.  Grab samples of the mineralized rock yielded 3.23 % Cu, 3.2 ppm Ag, and 0.16 ppm Au (Hausel, 1991).
The South Pass greenstone belt also has extensive iron deposits in the limbs of the South Pass synclinorium.  The BIF along the northern flank of the belt was structurally thickened fourfold by internal folding and plication and by repetition along faults.  This deposit was reported by Bayley (1968) to have indicated reserves of 300 million tons of 30% Fe.   From 1962 to 1983, U.S. Steel Corp. mined more than 90 million tons of iron ore from this BIF.   Recent mapping by Hausel (1990a), indicates a sizable resource is available.
The possibility of gold in the iron formation apparently was not considered during the 20 year operating lifetime of the mine.  However, samples collected a short distance southwest in the BIF by the Wyoming Geological Survey, yielded gold anomalies (maximum of 1.3 ppm).  Other anomalies were detected in BIF along the southeastern margin of the greenstone belt.  Only a small number of sulfide-bearing oxide facies BIF were collected by the Survey in the open pit mine before it was flooded in 1983.  None of the samples contained detectable gold (minimum detection limit 0.02 opt).
The Seminoe Mountains greenstone belt includes a distinct, large
altered zone surrounding gold-bearing veins at the Penn mines.
Seminoe Mountains.  The Seminoe Mountains greenstone belt is located in south-central Wyoming (Figure 1).  This belt is ~2.7 Ga, whereas South Pass is a minimum of 2.8 Ga.  The Seminoe Mountains have similar rock types as South Pass, but in different proportions.  For instance, the relative volume of metagreywacke in the Seminoe Mountains is less than South Pass, BIF is abundant, and the belt includes a thick section of aphyric and spinifex-textured basaltic komatiite and cumulate-textured serpentinite and metaperidotite (Klein, 1981; Snyder and others, 1989).
 Gold is concentrated in a 1/4 mile diameter zone of altered  metagabbro and metabasalt near the western margin of the belt (Klein, 1981; Hausel, 1989b). Samples of quartz recently collected in this region were highly anomalous and have yielded gold values as high as 98.4 ppm (2.87 opt).  BIF is also locally anomalous in Au and Ag.  Amphibolites in the altered zone have been overprinted by chlorite, carbonate, quartz, and sulfide alteration assemblages (Klein, 1981). The zone is transected by narrow quartz-calcite veins and stockworks containing minor pyrite and chalcopyrite.  Ore shoots often yield visible gold and exhibit control by folding.        
The Seminoe Mountains greenstone belt contains ubiquitous BIF. The BIF is interlayered with metabasalt and metasediments, and is predominantly oxide facies.  Harrer (1966) outlined a 100 million ton deposit of BIF, which varied from 28% to 68.7% Fe. 
Copper deposits are sporadic and appear to be unimportant in the greenstone belt.  Some samples with anomalous Zn and Pb were detected in a shear zone near some gold mineralization (Hausel, 1990b), and follow-up studies on this shear is currently in progress.  The Seminoe Mountains were explored by Kerr McGee Corp. and Timberline Minerals in the 1980s. Hausel recovered a large number of G10 pyrope garnets from the northern flank of the greenstone belt near the Miracle Mile. The source of the diamond-stability pyrope garnets has not been identified, but this area may be of interest to gold prospectors. The Tertiary gravels sitting high and dry in the flats surrounding the North Platte river in what is known as the Kortes Placer, contain gold and likely have diamonds.

The Rattlesnake Hills supracrustal belt is located in the northern Granite Mountains. Within this belt, a 500 ft long, mineralized vein (metachert), was discovered in 1981, and named the Lost Muffler prospect.  Composite chip samples 3.6 ft in length were taken in the vein, and assayed 7.5 and 4.5 ppm Au (Hausel, 1989a).  Later exploration in this area by American Copper and Nickel intersected auriferous BIF and gold-bearing Tertiary alkalic igneous rock.  Drill intercepts included 10 ft of 10.3 ppm Au in BIF and 250 ft of 2.1 ppm in the Tertiary volcanics. Additional sampling by Hausel identified anomalous gold in nearby breccias and Tertiary alkalics and drilling to depth by recent companies on these breccias intersected some very impressive gold deposits (Hausel and Hausel, 2011).
Other Supracrustal Belts. In addition to greenstone terranes, the Wyoming Province encloses medium to high-rank metamorphosed supracrustal belts.  These belts occur in the Tobacco Root Mountains, the Ruby and Gravelly Ranges of Montana, and the Copper Mountain district of the Owl Creek Mountains, Wyoming.  Also in Wyoming is the Hartville uplift in the southeastern corner of the Province.  This terrane consists of relatively low-rank metamorphosed eugeoclinal sedimentary and volcanic rocks.
Copper Mountain. The Copper Mountain district in the eastern Owl Creek Mountains is interpreted as a high-grade supracrustal belt (Hausel and others, 1985). It is intensely metamorphosed and isoclinally folded such that all primary textures have been essentially destroyed or overprinted by foliation. Two mappable units in the belt are similar in appearance and consist of intercalated amphibolite and quartzofeldspathic gneiss. A third unit is formed of gneiss, amphibolite, BIF, metapelite, and quartzite.
Scattered mineral deposits in the Copper Mountain district include stratiform scheelite, vein Cu and Au, and REE-Ta pegmatites.  One relatively significant development (the DePass mine) lies along the eastern edge of the district in a 50 ft wide Proterozoic mafic dike emplaced in Archean granite.  This mine was driven into the dike and produced a minimum of 568,000 lbs of mill concentrates in the early 1900s with receipts for Cu, Au, and Ag.  The mine has more than 11,000 ft of workings.
Hartville uplift.  The eugeoclinal terrane in the Hartville uplift in southeastern Wyoming has yielded more than 5 million lbs of copper and 45 million tons of iron from Archean schists and overlying Phanerozoic rocks.  Mineral deposits in the region include the McCann Pass pyritiferous massive sulfide deposit, and scattered Cu-Ag-Au-U unconformity deposits.  The Silver Cliff mine in the northern portion of the uplift was initially developed for gold and silver in the 1870s and later mined for uranium in the early and mid 1900s (Hausel, 1989a). Mineralization at the Silver Cliff is localized in fault gouge and along a Precambrian-Cambrian unconformity.    
The Hartville uplift is principally known for its massive hematite.  Hematite was mined by C.F.&I. Corporation for many decades from hematite schist until 1981.  Total recorded production amounted to more than 45 million tons of ore with some by-product copper.  The hematite is secondary in origin and interpreted to have been derived by groundwater oxidation and enrichment of originally ferruginous beds (Bayley and James, 1973; Snyder and others, 1989).  These deposits often contain copper as well as anomalous gold.  For instance, the Michigan mine in the central portion of the uplift encloses two hematite pods.  The northern ore body contains 75 million tons of 25% Fe, and the southern deposit contains 41 million tons of 24% Fe (Wilson, undated).  The upper segment of these deposits are copper stained and anomalous in gold (Woodfill, 1987). 
Unconformity deposits also occur in the belt.  In the northern Hartville uplift, the Silver Cliff shaft was developed on a Precambrian-Phanerozoic unconformity and mineralized reverse fault.   Available assay reports indicate the ore contained none to 10.88 % Cu, none to 15.04 opt Ag, 0.001 to 3.39% U3O8, and anomalous gold (Wilmarth and Johnson, 1954).  In the southern portion of the uplift, Kerr McGee explored a copper-stained Precambrian-Paleozoic unconformity recovering samples with cerargyrite, unmangite, electrum, and native gold (Kerr McGee Corp., 1988).
South of the Silver Cliff mine, a contact between hanging wall dolomite and footwall schist is mineralized over a 1 to 15 ft thickness.  Lenses from the Copper Belt Group assayed 2 to 8 % Cu and the adjacent altered iron-stained schist contained 0.05 to 0.58 opt Au and 2 to 5 opt Ag (Ball, 1907).     
In the southern portion of the uplift, an extensive gossan at "gossan hill" along the McCann Pass fault was prospected by Mine Finders in the 1970s, and more recently by Exxon Minerals.  Outcrop and shallow drill holes recovered samples with elevated Cu, As, and Zn.  Deeper drilling intersected thick zones of anomalous mineralization including 10 ft of 0.8% Zn, and 2 ft of 1.2 % Zn and 0.08 opt Au (Woodfill, 1987).
 Nearly all of the Proterozoic rocks in Wyoming are restricted to the southeastern corner of the state in the Laramie, Medicine Bow, and Sierra Madre Mountains. The Sierra Madre and Medicine Bow Mountains are bisected by a prominent Precambrian suture known as the Mullen Creek-Nash Fork shear zone. This shear is projected into the Laramie Range where it is intruded by a 350 mi2 anorthosite batholith.  To the south of the suture are volcanogenic schists and metasediments of the Green Mountain terrane that represent a Proterozoic island arc accreted to the craton nearly 1.7 Ga ago. Archean basement rocks are absent from this region.  To the north, however, Proterozoic miogeosynclinal metasedimentary rocks form a thick wedge lying on the Archean basement.
Green Mountain terrane.  Precious and base metals south of the shear zone occur in quartz veins, massive sulfides, a porphyry, and in layered mafic intrusions.  In the Keystone district of the Medicine Bow Mountains, N60°W-trending shears are loci for narrow veins. These veins are gold- and copper-bearing, pyritic, quartz-carbonate veins in tensional faults subsidiary to the Mullen Creek-Nash Fork shear zone (Currey, 1965). Mineralization is accompanied by silicification in the form of small, irregular quartz veinlets and the wallrock is enriched in epidote.
At the Keystone mine, gold was found in quartz, and in pyrite and pyrrhotite masses in mylonite selvages adjacent to the vein. The vein lies in sheared diabase that intrudes quartz-biotite gneiss country rock (Currey, 1965). The shear is 2 to 6 ft wide, locally splays to 300 ft, and continues 4,500 ft to the southeast to the Florence mine.  Available information indicates the Keystone ore averaged 41 ppm Au.  When the mine ceased operations in 1893, 100,000 tons of reserves were reported in site.  Eight samples collected from the mine dump by Loucks (1976) yielded 6.5 ppm to 300 ppm (0.19 to 8.75 opt) Au and averaged 117 ppm (3.41 opt) Au. 
 At the the southeastern end of the Keystone trend, the Florence mine was developed in quartz diorite.  The ore occurred as 'kidneys' of auriferous pyrite which assayed from 257 ppm to 1,656 ppm (7.5 to 48 opt) Au (Currey, 1965).  Samples collected from the mine dump by Loucks (1976) yielded values ranging from 2.06 ppm to 799 ppm (0.06 to 23.3 opt) Au.  One sample collected by the author in 1990, consisted of limonite boxwork after pyrite with abundant visible gold.  Both the Florence and Keystone patented claims are currently for sale, and were recently examined by Homestake.
Volcanogenic massive sulfide deposits occur in the Green Mountain Formation of the southern Sierra Madre. This unit consists of relatively low-grade metamorphosed calc-alkaline metavolcanics and associated metasedimentary rocks and volcaniclastics.  The massive sulfides are copper or zinc dominated with silver and traces of gold.  Between 1979 and 1982, Conoco Minerals Company explored this region for massive sulfides (Conoco Minerals Company, 1982).  Mineralized exhalites were discovered in metarhyolite and meta-andesite, and colloform-textured, pyrite-chalcopyrite was found associated with volcaniclastics (mill rock) (Hausel, 1986).  Currently, BHP-Utah International is exploring to the south in Colorado for Ag-Zn. But true to nature, the US Forest Service followed all of the massive sulfide discoveries with massive withdrawals of land to limit any mining.
At least one deposit in the Laramie Range south of the shear zone has been classified as a Au-Cu porphyry.  The Copper King mine in the Silver Crown district west of Cheyenne (Klein, 1974), was developed in weakly foliated and hydrothermally altered quartz monzonite and granodiorite. A shaft sunk in a potassium-silicate altered zone is surrounded by propylitically altered rock. 
Geology of the Silver Crown district, Wyoming
The Copper King was initially drilled by the U.S. Bureau of Mines in the 1950s, later by Asarco, and more recently by Caledonia Resources, Ltd., Royal Gold, and Saratoga Gold.  Drilling outlined a 35-million ton open pitable, ore body averaging 0.755 ppm Au and 0.21 % Cu (Nevin, 1973). Recently, Caledonia Resources outlined a higher grade zone consisting of 4.5 million tons averaging 1.5 ppm Au. The Copper King porphyry is mineralized along a 600 to 700 ft strike length, and a 300 ft width that is open at depth (Stockwatch, 1987). Saratoga gold and Strathmore Resourced identified with additional drilling, a low-grade 2 million ounce equivalent Au-Cu deposit at the site. Consultant Dan Hausel examined the Copper King fault along the east of the ore body and found the ore deposit was offset to the east in an area that has not been drilled suggesting that the deposit is larger than has been reported. Additionally, Hausel and Terry Klein of the USGS each identified similar hydrothermal alteration zones nearby suggesting additional porphyry deposits are located in the area along I-80. Hausel also suggested that erosion of the porphyry likely produced gold placers in nearby drainages leading from the Silver Crown district to Cheyenne, Wyoming. The discovery of many nearby cryptovolcanic structures by Hausel while at DiamonEx Ltd, suggests the drainages could potentially also have placer diamonds. The area is immediately east of the State Line district where more than 130,000 diamonds were mined in the past. 
Exploration in the southern Medicine Bow Mountains was recently undertaken by American Copper & Nickel, Chevron Resources, International Platinum, and Vanderbilt Gold.  The target was two large layered mafic complexes dated at about 1.8 Ga based on field relationships. The 60 mi2 Mullen Creek mafic complex abuts against, and is sheared by, the Mullen Creek-Nash Fork shear zone.  This complex is highly deformed and metamorphosed.  Along the northeastern corner of the complex is the historic New Rambler mine.  The New Rambler shaft was collared in sheared and hydrothermally altered mafic rock and was sporadically operated from 1900 to 1918 producing 6,100 tons of copper ore with values in Au, Ag, Pt, and Pd (Hausel, 1989a). The source of the platinoids has not been determined, although one possibility suggested by McCallum and Orback (1968) is hydrothermal remobilization from a platinum reef hidden at depth.
Six miles east of the Mullen Creek mafic complex is a second layered intrusive known as the Lake Owen complex.  This intrusive is of similar size, but is essentially unmetamorphosed.   Exploration activities in the past few years have isolated some significant Pt, Au, and Pd anomalies in the complex. In this same region, Hausel discovered Cu-Au-Ag-Ni-Pd at the Puzzler Hill pyroxenite complex.
The Mullen Creek-Nash Fork shear zone is a major Precambrian suture formed of mylonite, shear cataclastics, and breccia that separates the Archean craton to the north from the Proterozoic basement to the south. Several mineral deposits in the Medicine Bow Mountains occur in this zone.  For example, the New Rambler shaft, as well as several Au-Pt mines in the Centennial Ridge district, were sunk in shear zone cataclastics.
Migeoclinal terrane.  North of the shear, precious metals occur in narrow veins and in thick quartzites with copper.  Gold has also been detected in quartz pebble conglomerate and in shear zones. Vein deposits in the Gold Hill district of the Medicine Bow Mountains, occur as narrow, rich, quartz veins with common visible gold. Specimen samples taken out of the district each year testify to the richness of the veins. Historic reports claim some specimen grade material taken from the Acme mine at Gold Hill assayed 72,000 ppm (2,100 opt) Au. Unfortunately, these veins are narrow (0.5-2 ft wide) and spotty.  One shear recently examined by the Wyoming Geological Survey and the U.S. Forest Service in the Lewis Lake area of the Medicine Bow Moutains, is greater than 100 ft wide and traceable for 2,000 ft.  The shear occurs in limonitic quartz-mica schist.  Samples of pyritized schist yielded 4.1 ppm (0.12 opt) Au (Dersch, 1990).
Mineralized quartzites occur at several localities in the Sierra Madre. Gossans in quartzites were prospected in the late 1800s and led to the development of some important copper mines. Two prominent mines were the Ferris-Haggerty and the Doane-Rambler.  The Ferris-Haggerty is interpreted as a remobilized stratabound deposit  (Hausel, 1986).  During its operation (1902 to 1908) the Ferris-Haggerty was an internationally prominent mine. The massive chalcocite minor chalcopyrite ore was found filling irregular quartzite breccias along the contact between hanging wall schist and the underlying quartzite (Spencer, 1904) of the Magnolia Formation.  Ore shoots greater than 20 ft thick were high-graded for the rich (30-40 % Cu) ore, and much of the lower grade material was left as waste. These shoots averaged 6 to 8 % Cu and carried some Au and Ag (Beeler, 1905; Spencer, 1904).  Beeler (1905) reported the ore contained 3.4 to 15 ppm (0.1-0.44 opt) Au.  The mine was mapped during World War II, and according to Ralph E. Platt (pers. comm., 1988), the steeply dipping quartzite flattens out in the lower mine workings and large blocks of "low grade" ore (6-8% Cu) remain in place.  The property was explored for stratiform Cu-Au-Ag mineralization by Exxon Minerals in the early to mid 1980s.
Proterozoic age quartz pebble conglomerate occurs in the thick miogeoclinal wedge in the northern Medicine Bow Mountains and Sierra Madre.  These conglomerates were of considerable interest in the late 1970s after they were discovered to be radioactive (Houston and Karlstrom, 1979). During the mapping of these conglomerates some samples were tested for precious metal content. Gold values as high as 10 ppm were detected from a conglomerate near Dexter Peak in the Sierra Madre. Even though similarities to the Witwatersrand conglomerates have been noted (e.g. Karlstrom and others, 1981), these rocks still have received only minor attention for gold.  Exxon explored and drilled some of the conglomerates in the late 1970s in search of U and Th.  In the early 1980s, Superior Minerals initiated a project to prospect for Au.
  Gold anomalies are scattered throughout the Phanerozoic record in the State.  Many of these anomalies are enigmatic with no definite source terrane.  In Paleozoic and Mesozoic rocks, gold deposits and anomalies are much less common than in the Cenozoic.  Whereas, in the Cenozoic, there is a significant increase in anomalies compared to the earlier Eras. The increase in gold anomalies is partially due to the Cenozoic section being better preserved, but also because the Cenozoic Era represented a period of gold redistribution from Precambrian sources and an influx from volcanic sources.
Paleozoic Mineralization The Paleozoic record is not well preserved in Wyoming. However, thick limestones and dolominte crop out along the flanks of the uplifted mountain ranges, and in southeastern Wyoming and northern Colorado are more than 100 kimberlite intrusives (Devonian), more than a dozen of which are diamondiferous. Gold-bearing Cambrian Flathead Formation (Deadwood-equivalent) conglomerates occur at a few localities in the state. The better known of these is the "Deadwood conglomerate" (basal Flathead conglomerate) at Bald Mountain west of Burgess Junction in the northern Bighorn Mountains.
During the Bald Mountain gold rush near the turn of the century, more than 1,500 miners occupied the short-lived town of Bald Mountain and mined the conglomerate and associated modern placers.  However, grades were too low to sustain commercial operations and the district was deserted.  The gold was reported as flat fine-grained flakes with jagged edges. 
   In addition to gold, the conglomerate contains black sands with ilmenite, magnetite, zircon, and monazite (Wilson, 1951; Mckinney and Horst, 1953; King and Harris, 1987). Interest in the monazite resulted in a U.S. Bureau of Mines drilling project which outlined a 20 million ton low-grade resource averaging 2.5 lbs/ton of monazite with a higher-grade resource of 675,000 tons averaging 13.2 lbs/ton (Borrowman and Rosenbaum, 1962).
During testing of the Bald Mountain conglomerate, the Bureau of Mines ran assays on material recovered from six drill holes in the better-grade monazite areas. The assays showed gold contents from 0.034 ppm to 0.172 ppm, averaging 0.103 ppm (McKinney and Horst, 1953).  However, Darton (1906) reported gold assays to run as high as 3.43 ppm (0.1 opt).  Darton's samples may have been from reworked Recent placers, rather than from the Cambrian conglomerate.
Precious metal anomalies are also reported in black shales and phosphorites of the Phosphoria Formation (Permian) in the Overthrust Belt of western Wyoming.  These rocks host anomalous Ag, Au, Cr, Zn, Cu, and V (Love, 1984). The U.S. Bureau of Mines and the U.S. Geological Survey report rocks from the Phosphoria Formation to have high silver values (as much as 2,600 ppm) and anomalous gold (0.2 ppm) (Allsman and others, 1949; Love, 1984).
Mesozoic Mineralization Several argentiferous Cu-Zn redbed deposits occur in Mesozoic rocks in the Overthrust Belt.  The majority of these are in the Nugget Sandstone (Jurassic).  Similar mineralization has been identified in the Beckwith(?) or Twin Creek Limestone(?), and in the Wells Formation Sandstone (Permo-Pennsylvanian) (Hausel, 1989a).
he better known of these deposits are located in the Lake Alice district in Wyoming near the Idaho-Wyoming border.  The Lake Alice deposits are localized in an anticline formed of bleached redbeds of the Nugget Sandstone capped by the Gypsum Spring Member of the Twin Creek Limestone.  The Lake Alice deposits were drilled by Bear Creek Exploration in the 1970s.
The Griggs mine in the northern part of the district, the mineralized sandstone is at least 300 feet thick. Samples collected by Love and Antweiler (1973) ranged from 0.02 to 6.7% Cu, a trace to 0.5% Pb, a trace to 3.2 % Zn, and a trace to 1,200 ppm (35 opt) Ag.  The average mine ore contained 3.5 % Cu and 254 ppm (7.4 opt) Ag (Allen, 1942). The mineralizing fluids are interpreted as interformational or derived from a similar low temperature source and were structurally trapped (in anticlines and faults) during thrusting and folding of the Overthrust Belt (Boberg,1986; Loose and Boberg, 1987).
Cenozoic Mineralization The Cenozoic of Wyoming includes many poorly studied and unexplored gold and silver deposits and anomalies. Only a few are discussed here and the reader is referred to Albert (1986) and Hausel (1989a) for a treatise on these and other occurrences and anomalies.
In northwestern Wyoming, paleoplacers and associated reworked modern placers cover an extensive region. These auriferous quartzitic conglomerates and sandstones range in age from Late Cretaceous to Miocene and include associated Quaternary placers. The average gold content is anomalous but low, and the gold is very fine grained (Table 3) (Antweiler and Love, 1967).  Most past prospecting activities have been confined to reworked alluvial and bench placers primarily along the Hoback and Snake Rivers.
Stratigraphic unit
Average Au (ppb)
Maximum assay (ppb)
Quaternary alluvium
Miocene(?)   conglomerate
Pass Peak Fm(Eocene)
Wind River Fm(Eocene)
Early? Eocene conglomerate
Pinyon Conglomerate (Paleocene)
Ft Union Fm (Paleocene)
Harebell Fm (Late Cretaceous)

Table 3. Reported gold content of conglomerates in northwestern Wyoming (Antweiler and Love, 1967).
The Tertiary of Wyoming was a time of intense erosion, and large volumes of fanglomerate and conglomerate were shed from the mountain ranges into the adjacent valleys.  Where the source terrane included greenstone belts and other mineralized regions, the alluvial fans and fluvial sedimentary rocks often carry detrital gold. For example, the Twin Creek paleoplacer on the northeastern margin of the South Pass greenstone belt is estimated to exceed more than one billion cubic yards of gold-bearing gravel (Antweiler and others, 1980).  The Oregon Buttes paleoplacer along the southern margin of the greenstone belt is estimated to contain more than 28.5 million ounces of gold (Love and others, 1978). Other significant paleoplacers occur in and along the margins of the greenstone belt, but for the most part remain unexplored.   However, in the late 1980s, spurred on by the recovery of auriferous core from an oil well along the southern edge of the South Pass greenstone belt, Hecla Mining drilled and flew airborne magnetic and IP surveys over the buried southern edge of the belt.
Gold-bearing conglomerates and terrace gravels in the Wind River Basin have gold over large regions.  In 1910, gold was discovered along the Wind River, Little Wind River, and Popo Agie River in the basin.  The metal was found in terrace gravels capping benches and buttes and in the nearby plains, mesas, and uplands for thousands of feet to a few miles from the present drainages.  The deposits were reported to average 12 to 14 feet thick over widths of 3 to 4 miles.
The gold is very fine tablet-like particles smaller than a pinhead. In 1913, gravels were tested and varied from none to 0.016 oz/yd3 and averaged less than 0.0025 oz/yd3 Au. Two dredges operated in the Wind River. The Neble Dredge operated in a pay zone that ranged from 0.007 to 0.016 oz/yd3 and averaged 0.014 oz/yd3.  The gold-bearing gravels averaged about 22 feet thick. The Clark Dredge, a few miles west of the Neble Dredge, treated gravels that averaged 0.038 oz/yd3 (Schrader, 1913). The demise of the district was the gold was too fine to be recovered efficiently.
The Black Hills of Wyoming
During the Tertiary, Wyoming was affected both directly and indirectly by volcanism.  In northwestern Wyoming, the Yellowstone and Absaroka regions were inundated by calc-alkaline flows, flow breccias, and ash falls generated from nearby composite volcanoes.  In the Black Hills of northeastern Wyoming, several alkalic volcanic centers erupted.  Local alkalic volcanoes erupted in the Rattlesnake Hills of central Wyoming, and basalts erupted from cinder cones in the Baggs area in southern Wyoming. In southwestern Wyoming, volcanoes ejected rare leucite- and olivine-lamproites with chemical and mineralogical similarities to the Kimberly diamondiferous lamproites in northwestern Australia. The Tertiary record also records Wyoming was buried numerous times by thick ash falls erupted from the west.
Mineral deposits are associated with the Black Hills alkalics,  the Absaroka volcanics, and with the thick ash falls in the Wyoming basins. In the Black Hills, mineralization is reported in the Tertiary alkalic complexes of the Bear Lodge Mountains, Mineral Hill district, and Black Buttes.  In recent years, this area has been explored by Hecla Mining, Molycorp, FMC Gold, and International Curator to name a few.   In the Bear Lodge Mountains, gold has been reported in fluorite veins, pegmatites, and in feldspathic breccia.  Recent exploration in the area led to the discovery of an elongate intrusive breccia (120 by 2,000 ft) containing disseminated gold in values varying from 0.34 to 1.7 ppm.  The deposit averages 0.72 ppm (International Curator Resources Ltd, 1988 Ann. Rept.). This region also includes one of the largest, low-grade rare earth and thorium deposits in the United States (Staatz, 1983).
The Mineral Hill district to the east has a history of placer gold and tin production.  Preliminary work by the Wyoming Geological Survey in cooperation with TRYCCO identified two horizontal pyritiferous quartz veins at the Treadwell open cut that yielded maximum gold values of 130 ppm (3.79 opt) and silver values as high as 330 ppm (9.62 opt).  Previous work in the district by Welch (1974) identified several gold anomalies including a jasperoid that assayed 5 ppm Au and 7 ppm Ag (Hausel and Hausel, 2011).
Black Buttes lies 8 miles west of Mineral Hill and is formed of Tertiary phonolites and trachytes intruded into Paleozoic limestone. Contact replacement mineralization (Zn, Pb, Ag, Mo, F) occurs in the Pahasapa Limestone (Mississippian), but the surface exposures are limited (Hausel, 1989a). Hausel identified both gem-quality fluorite and wulfenite at one deposit at Black Buttes (Hausel, 2009).
Location of gold-silver-copper porphyry deposits in the
Absaroka Mountains, Wyoming
The Absaroka Mountains in northwestern Wyoming include several Cu-Ag porphyry complexes, several of which are located within wilderness designated land.  However, the two largest porphyries--Kirwin and Sunlight Basin lie outside of wilderness within the National Forest.  These two porphyries contain anomalous Cu, Mo, Pb, Zn, Ag, Au, and Ti, and includes disseminated, stockwork, and vein mineralization (Hausel, 1982).  
 The Kirwin porphyry was explored by AMAX for several years, and plans to develop an open pit in 1980 died because of the poor copper market.  However, in recent years interest in Kirwin (as well as the Sunlight porphyry) has increased.  Currently, the U.S. Forest Service is negotiating with AMAX to purchase the property for $3.2 million.   If acquired, the porphyry will be considered for withdrawal from mineral entry.  Published reports indicate the porphyry hosts a minimum of 1.23 billion lbs of Cu, 13,500 lbs of Mo, 121,000 oz of Au, and 5.6 million ounces of Ag (Paydirt, 1985).  The contained metals are worth a minimum of $1.5 billion at 1989 prices.
Throughout Wyoming, many enigmatic gold anomalies have been reported. These include gold associated with coal in the Black Hills, gold anomalies in uranium roll fronts (Gordon Marlatt, pers. comm., 1989), and scattered geochem anomalies in many Wyoming basins (Albert,1986). For example, dozens of NURE geochem anomalies yielding from 0.2 ppm to 6.55 ppm Au were reported by Albert (1986) in the Wyoming basins.  Several of these anomalies have been reexamined and verified.  For example, Marlatt investigated a 200 mi2 anomaly in the Green River Basin in the vicinity of Farson (south of South Pass) and recovered one sample with visible gold. The gold occurred as a irregular-shaped microscopic sliver. Many of these anomalies are possibly due to detrital gold eroded from nearby gold districts.  But others are not easily explained by detrital transportation but instead may represent geochemically transported gold leached from Tertiary ash falls (Gordon Marlatt, pers. comm., 1988).
 Most commercial modern placers in the State have been principally mined for gold, although the Douglas Creek placers in southeastern Wyoming also possess platinum and palladium, the Clarks Camp placers in the northern Wind River Range contain anomalous monazite in addition to gold, and the Mineral Hill placers in northeastern Wyoming also have tin, tantalite, and magnetite (Figure 2).  Monazite placers also occur in the Shirley Basin in southeastern Wyoming (J.D. Love, pers. comm., 1990).  Statistics on gold nuggets are incomplete, although walnut-size nuggets have been recovered from the Mineral Hill district, the Douglas Creek district, and the South Pass greenstone belt.  The largest nugget found in Wyoming may have been a 24 ounce nugget from Rock Creek in the South Pass greenstone belt.  History also records a boulder with nearly 40 pounds of gold was found in the same area prior to 1905 (Hausel, 1989a).  This region also has several potentially rich, but unexplored placers (Hausel, 1991).
Within the State boundaries are numerous gold deposits and anomalies scattered throughout the geologic record.  Many examples occur in rocks ranging in age from Archean to Tertiary, and in Quaternary to Recent unconsolidated gravels and sands.  Yet, relatively few of these deposits and anomalies have been explored and only a handful have been drilled.
Being that much of Wyoming is underlain by an Archean craton similar to the Superior Province of Canada, the eastern and southern African craton, and to the Pilbara and Yilgarn blocks of Western Australia, one would expect Wyoming to also have significant mineralization.