|Large crustal xenolith exposed in the rib of the|
Sloan 2 kimberlite adit, Colorado
Magmatic deposits in Wyoming include platinum, palladium, chromite, magnetite, and ilmenite in layered mafic complexes; disseminated chromium in serpentinites and in ultramafic schists; massive and disseminated titaniferous magnetite and labradorite gems in anorthosite; and diamonds in kimberlite and lamprophyre as well as associated detrital placers. Disseminated and cumulate mineralization provides direct textural evidence of crystallization from host magmas. This intimate association links mineralization to the host igneous rock implying the mineralization and host rock have a common heritage.
Other than nickeliferous schists and diamondiferous lamproite, most types of magmatic deposits are recognized in Wyoming in one form or another. And since a major lamproite field occurs in southwestern Wyoming and high-magnesian schists with peridotitic komatiite composition occur in more than one mountain range in the state (i.e., South Pass in the Wind River Mountains the Seminoe Mountains), it is likely these latter deposits may be found.
Platinum Group Mineralization
Worldwide, platinum-group metals (PGE) show a strong affinity for large, intracontinental, layered, mafic complexes of tholeiitic affinity. This affinity is demonstrated in southeastern Wyoming where PGE mineralization is intimately associated with two ~1.8 Ga (age from Houston and others, 1968) Precambrian layered mafic intrusives (Lake Owen and Mullen Creek) in the Medicine Bow Mountains. These layered complexes in the Medicine Bow Mountains intrude Proterozoic schist and gneiss of the Green Mountain terrain. This terrain is interpreted as part of a Precambrian island arc which was accreted to the Wyoming craton about 1,770 Ma. The terrain was intruded by the Mullen Creek mafic complex at about the time of accretion (Loucks and others, 1988).
In mineralized layered complexes elsewhere in the world, the PGE have been described to occur in sulfides in stratiform layers spatially associated with cyclic cumulus pyroxenite, dunite, anorthosite, and troctolite. Platinum-bearing sulfides include cooperite, sperrylite, braggite, and laurite (Edwards and Atkinson, 1986).
Typically, PGE metals are found in cyclic cumulus layers of undisturbed layered complexes. In the Lake Owen intrusive, anomalous platinum is syngenetic and found in labradorite-bearing gabbroic norite (Loucks, 1991). In the Mullen Creek mafic complex, however, significant platinum mineralization is epigenetic and in hydrothermally altered igneous rock. The effects of deformation on the Lake Owen and Mullen Creek complexes differ greatly.
Figure 2. Exposed layered complex with anomalous mineralization between
the two red flags in trench behind author.
Mullen Creek mafic complex. The northeastern edge of the 60 mi2 Mullen Creek mafic complex is intensely sheared and truncated by the Mullen Creek-Nash Fork shear zone which contributed greatly to the deformation of the complex. Platinum and palladium occur in shear zones in hydrothermally altered metadiorite, metagabbro, metapyroxenite, and metaperidotite (McCallum and others, 1975). McCallum and others (1975) recognized two hydrothermal alteration assemblages overprinted by supergene assemblages at the New Rambler mine. Hydrothermal propylitic mineral assemblages include chlorite, epidote, clinozoisite, albite, magnetite, and pyrite. Phyllic alteration assemblages consist of sericite, quartz, and pyrite.
|Map of the Medicine Bow Mountains |
and Sierra Madre showing Lake Owen,
Mullen Creek, Centennial Ridge and Puzzler Hill
|Copper sulfide and carbonate mineralization associated |
with platinum-group metals in gabbro in Mullen Creek
Loucks and others (1988) recognized more than 21 cyclic units in the Mullen Creek complex. It is not known if syngenetic platinum is associated with these units, but the association of platinum and palladium with shear zones in the layered complex suggests remobilization of the metals from the complex.
Lake Owen mafic complex. In contrast to the Mullen Creek complex, the Lake Owen complex is virtually unaffected by deformation and metamorphism. It forms a 20- to 25-mi2 funnel-shaped intrusion tilted 75° on its side exposing a cross-section of at least 16 cyclic units. Vanadiferous titanomagnetite cumulates are persistant in gabbronorite near the tops of some cyclic units (Loucks, 1991).
Cumulus sulfides occur in at least 12 stratigraphic horizons in the complex, with some zones containing elevated gold and platinum ± palladium. Four of the horizons have laterally persistant precious metal anomalies of a few hundred to a few thousand ppb and contain Au-Ag alloys, Pt-arsenides, Pt-Pd tellurides and sulfides associated with disseminated chalcopyrite, pentlandite, pyrrhotite, pyrite, gersdorffite, bornite, millerite, PGE-bearing carrollite (Loucks, 1991). The mineralized zones are generally lensy and spotty and include zones up to 15-feet thick with strike lengths of more than 1 mile.
Centennial Ridge district. North of the Lake Owen complex in the Centennial Ridge district, late 19th Century mine developers cut mafic metaigneous rock in search of gold associated with platinum-group metals. The mineralization is spotty and found in shears and veins. The richest ores were found in sulfide-rich zones in mafic mylonites, graphitic fault gouge, and in strongly chloritized zones (McCallum, 1968).
|The author speaks to a group of University of Wyoming geologists and|
company geologists from the Rocky Mountain region about the Centennial
Ridge mining district along the flank of the Medicine Bow Mountains
It is apparent some Wyoming platinum is magmatic as well as hydrothermal. Platinum in the Lake Owen complex is clearly magmatic and associated with cumulus layers. In the New Rambler district in the Mullen Creek complex, the platinum is in hydrothermally altered mafic cataclastics. Possibly, the New Rambler ore was leached from discrete mafic rock units by hydrothermal solutions, or remobilized from the deformed layered complex (McCallum and Orback, 1968). Platinum in the Centennial Ridge district is restricted to narrow zones of altered, mafic, metaigneous schist and gneiss and appears to have been remobilized from the mafic country rock (McCallum, 1968). In addition to these metals, some pyrope garnet (diamond indicator minerals) were recovered in the Middle Fork of the Little Laramie River indicating a potential to find diamond deposits in this region.
Titaniferous-magnetite is found within a 350 mi2 anorthosite batholith of the Laramie Mountains, and in stratiform layers of the Lake Owen mafic complex of the Medicine Bow Mountains (refer to the Platinum group mineralization section above for information on Lake Owen). In addition to titanium, these deposits also contain significant iron and anomalous vanadium and chromium. Gemstones include considerable labradorite as well as some of the largest iolite gem deposits on earth.
|Overlooking the Platinum City mine dump along the flank of the Medicine|
Bow Mountains, Wyoming.
The deposits consist of lenses of ilmenite, magnetite, and magnetite-ilmenite intergrowths containing minor to accessory olivine, apatite, spinel, mica, and sulfides (pyrrhotite and pyrite). The magnetite and ilmenite occur as discrete grains and as intergrowths and overgrowths. The intergrowths consist of fine interpenetrating networks of ilmenite lamallae along octahedral partings in magnetite. In some samples the titaniferous magnetite partially replaces feldspar and pyroxene indicating the metals to be paragenetically late (Hagner, 1968). Early workers interpreted these deposits as magmatic segregations or injections (Diemer, 1941). Later work by Hagner (1968) considered the titaniferous magnetite to have formed by replacement of the anorthosite along a zone of en echelon fractures. More recently, the titaniferous magnetite has been considered as either a crystal cumulate or the result of magma unmixing (Frost and Simons, 1991) .
Dow (1961) described to types of ore: (1) massive ore and (2) disseminated ore. Chemical analyses of the massive ore show 16 to 23% TiO2. The ore is enriched in vanadium (<1.0% V2O5) and locally enriched in chromium (0.03% to 2.45% Cr2O3) (Diemer, 1941; Hagner, 1968). In addition to massive deposits, disseminated titanomagnetite forms relatively large, low-grade, deposits in the complex (Dow, 1961; Frost and Simons, 1991).
Available resource estimates based on drilling and magnetic surveys indicate reserves of massive titaniferous-magnetite ore in the Laramie anorthosite complex at 30 million tons averaging 45% Fe, 20% TiO2, and 0.64% V2O5 with little or no sulfur, and disseminated ore at 148 million tons averaging 20% Fe, 9.7% TiO2, 0.17% V2O5, and 0.17% S (Dow, 1961). The amount of disseminated ore in the district could be as much as 300 million tons (John Simons, personal communication, 1990).
In the 1950s, 1,091,452 tons of titaniferous-magnetite were mined from the anorthosite complex. The ore was used as a heavy mineral concrete in submerged petroleum pipelines in the Gulf Coast.
The principal chromite deposits are magmatic and associated with >2.0 Ga layered complexes (Edwards and Atkinson, 1986). With the exception of the Lake Owen layered complex, most of the known Wyoming chromium deposits are associated with serpentinites and ultramafic schists in greenstone belts and related supracrustal successions. Typically, these deposits occur as weakly anomalous zones to low-grade mineralized zones in the high-MgO rocks. Although no nickel anomalies have been identified in Wyoming, some high MgO and Cr2O3 serpentinites have whole-rock compositions similar to nickeliferous peridotitic komatiites in Western Australia.
The known Wyoming chromite deposits are too low grade or too small to be considered economic (Hausel, 1987a). However, some chromite schist mined from the Deer Creek deposit in 1908 and during the first world war yielded grades of 35 to 45% Cr203 (Spencer, 1916; Beckwith, 1955). The chromite-schist on Casper Mountain averages only 2% Cr203, but includes bands of high grade chromitite that vary from 5 to 25% Cr203. Drilling by the Bureau of Mines on Casper Mountain identified relatively large low-grade resources (Julihn and Moon, 1945). These chromitites are stratified and associated with serpentinized cumulate peridotite and magnetite-talc-chlorite schist.
The bulk of the world's nickel deposits occur in peridotitic komatiites with >36% MgO. These komatiites are aluminum undepleted with CaO/Al203 ratios of about 1, Al2O3/TiO2 ratios of nearly 20, and flat chondrite normalized heavy rare earth element (HREE) patterns. They are depleted in light rare earth elements (LREE) and TiO2 (Marston and others, 1981).
Rocks with compositions similar to the Western Australian nickeliferous rocks occur in some Wyoming greenstone belts. In the South Pass greenstone belt, serpentinites and talc-tremolite-chlorite-serpentine schists of the lowermost unit of the greenstone belt vary from 21.15% to 38.1% MgO, 1,700 ppm to 10,100 ppm Cr, and 289 ppm to 2,570 ppm Ni. The CaO/Al2O3 ratios average about 0.6, and the Al2O3/TiO2 ratios average about 22, These rocks yield some weak Cr2O3 anomalies, but nickel systematically increases with increasing MgO and is not anomalous in any of the samples collected to date (Hausel, 1991). The available REE contents for these rocks are incomplete. Two ultramafic samples partially analyzed for REE chemistry, possess flat HREE patterns similar to the Australian rocks. But the LREE data are lacking.
Serpentinites from the Seminoe Mountains have compositions consistent with peridotitic komatiite (Klein, 1981). Chemically, they have 25.7 to 36.7% MgO, 1,900 ppm to 6,200 ppm Cr, 810 ppm to 1,600 ppm Ni. CaO/Al2O3 ratios average 0.41, Al2O3/TiO2 ratios average 27. Tremolite schists with spinifex texture have 7.01 to 28.4% MgO, 150 to 6,000 ppm Cr, 60 to 1,400 ppm Ni. CaO/Al2O3 ratios average 0.86 and Al2O3/TiO2 ratios average 23.
In the Elmers rock greenstone belt, similar ultramafic schists have 11.2 to 28.4% MgO and 713 to 4,900 ppm Cr. CaO/Al2O3 ratios average 1.25, Al2O3/TiO2 ratios average 21 (Smaglik, 1987). Five samples were analyzed for REE content and only one sample showed a REE pattern consistent with the Western Australian nickeliferous komatiites. The remaining samples showed LREE enrichment inconsistant with the Western Australian rocks.
Worldwide, commercial diamond deposits are confined to kimberlites and lamproites in stable shield terranes, and to placer deposits presumably derived from these and related mantle rocks. Kimberlite and lamproite intrusives have unique nodules, mineral assemblages, and geochemistry indicative of mantle origin. Pressure-temperature estimates based on the chemistry of mineral associations of some ultramafic xenoliths and diamond xenocrysts place the source terrane of the kimberlite intrusives at minimum depths of 120 miles. Phanerozoic mobile belts tend to lack penecontemporaneous diamondiferous kimberlite or lamproite.
|Colorado-Wyoming diamond province|
Geochemically, kimberlite is a potassic ultrabasic igneous rock. Whole-rock analyses of kimberlite from the Colorado-Wyoming region show SiO2 contents of 24.8 to 34.2 %; K20 contents of 0.16 to 1.4%; and Mg0 contents in the range of 12.6 to 31.2 % (Smith and others, 1979). The intrusives are Early Devonian dikes, blows, and diatremes that range from inches wide to the largest known pipe in the region (Sloan 1) with dimensions of 1,800 by 500 feet (McCallum and others, 1977). Kimberlite eruption in Wyoming was enhanced by deep north-northwest fractures developed during the Early Devonian. These intrusives exhibit a variety of textures including porphyries and breccias (McCallum and Mabarak, 1976). Many of the kimberlites, particularly the diamondiferous intrusives, host abundant mantle and lower crustal xenoliths.
diamonds were recovered in the adjacent stream placers. In total, much more than 100,000 diamonds have been recovered from the State Line district (McCallum and Waldman, 1991). One diamond fragment mined at Kelsey Lake likely was fragmented from a parental diamond of around 90 carats.
The potential for the discovery of additional kimberlite intrusives is high. To date, only modern drainages have been sampled, and these have yielded over a hundred kimberlitic heavy mineral anomalies along with several detrital diamonds. Additionlly, several prominent magnetic and conductivity anomalies in the State Line district have not been tested. Along with diamonds being recovered from essentially every kimberlite in the State Line district that has been bulk tested, diamonds were also recovered from some kimberlites in the Iron Mountain kimberlite district to the north, and diamonds were also recovered from lamprophyres at Cedar Mountain in the Green River Basin.
|Flawless, 14.2 carat octahedral diamond recovered from the|
Kelsey Lake mine in Colorado.
MAGMATIC HYDROTHERMAL DEPOSITS
Hydrothermal alteration accompanies many magmatic metalliferous deposits. Temperature gradients associated with the deposits results in zoned alteration and mineralization. The classic magmatic hydrothermal deposits are the porphyry copper deposits and their associated vein systems. Spatially associated with some porphyries with high average Au/Ag ratios are large-tonnage, disseminated gold deposits. Porphyries, veins, and disseminated gold mineralization have all been recognized in Wyoming. Volcanogenic massive sulfides, another type of magmatic hydrothermal deposit, have also been recognized in Wyoming.
Porphyry deposits and disseminated gold
Several large copper-silver porphyries (with high Ag/Au ratios) occur in the Absaroka Mountains of northwestern Wyoming (Fisher, 1981; Hausel, 1982). This region includes one of North America's great copper districts, but limited accessibility has precluded development and extensive exploration of these deposits. Total copper, molybdenum, lead, zinc, silver, titanium, and gold resources are unknown, but the available drilling records indicate ore tonnages exceed a hundred million tons. Although, no mineralized breccia pipes are reported in the region, similarities of these porphyry deposits to those found in the Basin and Range in Arizona, suggest breccia pipes likely exist and remain undiscovered.
The Absaroka Mountains form a deeply dissected Tertiary volcanic plateau of calc-alkaline flows and flow breccias. Some eruptive centers possess classical hydrothermal alteration mineral assemblages and zonation typically seen in many porphyry copper deposits in the southwestern United States. Several mineralized porphyries have been recognized, but only the Kirwin and Sunlight districts are accessible.
In the Kirwin district in the southern Absaroka Mountains, at least three intrusive centers have been recognized (Wilson, 1964). But only the Bald Mountain porphyry has been extensively drilled. This porphyry is surrounded by deuterically altered andesite containing secondary calcite, chlorite, and clay. The andesite gives way to hydrothermally propylitized (quartz-epidote-montmorillonite-calcite-chalcopyrite with chalcopyrite-calcite-quartz veinlets) andesite within 1,500 feet of the intrusive center. Near the intrusive center, phyllically altered assemblages are overprinted by argillic assemblages (quartz-sercite-pyrite-biotite-kaolinite-chlorite- illite/montmorillonite). This phyllic-argillic altered zone encloses a poorly defined potassic zone represented by secondary orthoclase, quartz, and veinlet sulfides (Wilson, 1964; Nowell, 1971).
Zoned mineralization is characteristic of these deposits. Copper-molybdenum-trace gold mineralization surrounds the stocks and gives way to zinc-lead-silver mineralization laterally. Drill hole data show a pyrite-chalcopyrite-molybdenite stockwork at Kirwin with a secondary enriched blanket of chalcocite, digenite, and covellite overlying a portion of the stockworks (Wilson, 1964). Veins in the altered area are chalcopyrite-pyrite-molybdenite-quartz veins (Wilson, 1960). Wilson (1964) reported vein and mine dump samples to assay a trace to 8.58 ppm Au and a trace to 3,835 ppm Ag (111.8 opt). A portion of the ore body was drilled outlining a minimum resource of 70 million tons of 0.75% Cu (Rosenkranz and others, 1979). Estimated contained metals in the porphyry include 1.23 billion lbs of Cu, 121,000 oz of Au, 5.6 million oz of Ag with significant Pb, Zn, Mo, and anomalous Ti (Pay Dirt, 1985) worth more than $1.5 billion (1989 prices).
|Mineralized districts and regions of Wyoming|
Possibly another similar deposit of Proterozoic age was recently examined by the author in the southern Sierra Madre. This property was developed as the Kurtz-Chatterton copper mine somewhere near the turn of the last century. The mine is surrounded by a well-developed mineralized zone with a 3,500-foot strike length and a minimum width of 600-feet. The mineralized zone is confined to the Sierra Madre granite and contains secondary (?) K-spar, biotite, muscovite, and propylitic mineral assemblages as well as a stockwork. Historic reports indicate the mined ore contained 10 to 20% Cu with some gold and silver (Hausel, 1989, p. 156). Hand specimens contain chalcopyrite, cuprite, malachite, and minor chrysocolla.
The Bear Lodge Mountains in the northwestern Black Hills of Wyoming (Figure 1), form a large multiple intrusive complex of alkalic igneous rock ranging in age from 38.0 to 50.0 Ma (Staatz, 1983; Lisenbee, 1985). Staatz (1983) described the complex as a porphyry-type intrusive containing one of the largest, low-grade, disseminated and vein-type REE and thorium deposits in the United States. Disseminated gold mineralization is also associated with feldspathic breccia in the complex (Jenner, 1984). One mineralized zone discovered in an elongate intrusive breccia (2,000 by 120 ft) was recently drilled yielding gold values of 0.343 to 1.72 ppm (Anonymous, 1988). Current geologic resource estimates for the intrusive breccia are 8.2 million tons averaging 0.686 ppm gold (Anonymous, 1991).
Twelve to 15 miles southeast of the Bear Lodge Mountains, another Tertiary alkalic intrusive at Mineral Hill shows similar mineralization. Anomalous gold is reported in feldspathic breccia, quartz veins, and in jasperoid (Welch, 1976). Welch (1976) reported breccias with 6 ppm Au and 115 ppm Ag, and jasperoids with 5 ppm Au and 7 ppm Ag. Recently, the author collected quartz vein samples at Mineral Hill that assayed 130 ppm Au and 330 ppm Ag. The possibility for similar mineralization at Black Buttes, 6 miles to the southwest, is indicated by the presence of epithermal replacement galena, wulfenite, fluorite, and hemimorphite in altered Pahasapa Limestone along a contact with Tertiary alkalic igneous rock (Hausel, 1989). Disseminated gold was also recently discovered by in Tertiary alkalics in the (Figure 1).
Some potential for epithermal disseminated gold may also exist in the Rattlesnake Hills of central Wyoming in the vicinity of UT Creek where American Copper and Nickel Company identified several gold anomalies between 1983 and 1987. This area is underlain by Archean supracrustals intruded by Tertiary alkalics and includes some untested jasperoids (Hank Hudspeth, personal communication, 1988). Aspen (Quaking Asp) Mountain along the Rock Springs uplift south of Rock Springs, is another anomalous area with highly silicified sandstones and siltstones covering several square miles. Alunite, kaolinite, localized jasperoid(?), and minor travertine are anomalous. Some weak gold anomalies were recently detected in the silicified zone (Hausel and others, 1992).
Volcanogenic Massive Sulfides
|Colloform volcanogenic massive sulfide sample from the Itmay mine in the|
Encampment district of Wyoming.
Wallrock alteration associated with the massive sulfide mineralization consists of localized sericite-pyrite with broad zones of sausseritization (epidote ± chlorite ± garnet ± calcite ± actinolite) (Conoco Minerals Company, 1982). The geological setting and physical characteristics of these deposits suggests formation by sulfide precipitation in mounds near vents on a Proterozoic seafloor, similar to volcanogenic massive sulfide deposits in Arizona. Other massive sulfides in the Encampment district include those found at the Ferris-Haggarty mine.
The Ferris-Haggerty mine in the Sierra Madre was Wyoming's premier copper mine. The Ferris-Haggerty ore body is a stratabound massive sulfide, hosted by mill rock breccia e formed between a hanging-wall schist and the footwall quartzite of the Magnolia Formation (Proterozoic). The massive sulfide is as much as 20 feet thick and grades into laminated disseminated sulfides in the nonbrecciated quartzite. The ore deposit was reported to average 6 to 8% Cu with some high grade shoots containing 30 to 40% Cu with some Ag and 3.43 to 12.7 ppm Au (Beeler, 1905).
The mine operated from 1902 until 1908. Operations terminated following a series of disasters. First the mill at Riverside was partially destroyed by fire in 1906, followed by the destruction of the Riverside smelter by fire in 1907, and a 35% drop in the price of copper in 1908. The ore body was not exhausted and large blocks of 'low-grade' ore (averaging about 5% Cu) remain unmined (Ralph Platt, personal communication, 1988). In addition to the Ferris-Haggarty deposit, similar quartzite-hosted deposits are described at several other locations in the Sierra Madre (Hausel, 1986).
Quartz veins are common in both volcanic and metamorphic terranes. In Tertiary volcanics, veins are clearly associated with hydrothermal activity and often show classical ore zonation. Some Proterozoic veins in southeastern Wyoming show similar characteristics. In the Archean craton, the association is often not clear, and many veins are undoubtedly related to metamorphic secretion during regional metamorphism and deformation, rather than magmatic hydrothermal processes.
Quartz veins associated with the porphyry stocks of the Absaroka Mountains are zoned. In the Sunlight district, copper-rich veins with trace gold occur near the porphyry center and grade into lead-silver and barren veins away from the stock. Some Proterozic veins are also zoned. Spencer (1904) described precious metal zonation in the Bridger vein of the Sierra Madre, where gold decreased and silver increased with depth.
Several factors may cause ore shoots in hydrothermal veins whether it be host rock chemistry or structure. For example, Schoen (1953) noted a close association of host rock lithology and the type of metals found in the Albion mine. Copper dominated the ore assemblage in metalimestone, and lead and silver in quartzite. In the Mineral Hill district, the strongly mineralized, near-horizontal, pyritiferous veins of the Treadwell mine are reported to form ore shoots at intersections with a series of vertical fractures.
During regional metamorphism, fluids released at elevated temperatures and pressures may leach metals from the surrounding rocks and transport them to dilational zones, forming shear zone and vein deposits. Contact metamorphic deposits may result by the intrusion of igneous rock into country rock leading to their recrystallization and replacement at elevated temperatures. These types of deposits are closely related to metamorphic processes.
Shear zone gold
During the initial stages of deformation (D1) of Wyoming's Archean supracrustal belts, regional shortening produced isoclinal folding (F1), regional foliation (S1), and shear zones parallel to S1 and to the F1 fold hinges. Regional metamorphism during D1 liberated fluids from the supracrustal pile which tended to focus in the shear structures. Precipitation of silica synchronous with this early stage of deformation produced quartz veins which were stretched, boudinaged, and sheared parallel to S1. Wallrock alteration associated with gold mineralization is chlorite-carbonate-quartz dominated with minor sericite, microcline, and tourmaline. Most zones are stained by hematite.
The shears and veins contain trace gold with sporadic ore shoots enriched in gold (Hausel, 1987). Where recognized, fold closures, shear-fault, and shear-shear intersections appear to localize some ore shoots. The Hidden Hand shaft in the Lewiston district was sunk at the intersection of coalescing shears in metagreywacke. The chloritizied-hematized shear intersection is many feet wide with a gold tenor of a trace to 3,100 opt (ounces per ton) (Pfaff, 1978). At the Bullion mine on Strawberry Creek, a shoot was mined at the intersection of a Archean shear and a Laramide(?) tear fault. The localization of this latter shoot suggests gold may have been mobilized synchronous or subsequent to Laramide deformation. My impression is the intersection produced a zone of high permeability that was supergene enriched. Thus this shoot may be surficial and not extend below ground-water.
Tight to open fold closures control shoots at several mines including the Carissa, Alpine, Diana, Duncan, and Miners Delight. The Duncan shaft was sunk on a steeply plunging drag fold in hornblendic amphibolite. Compared to the adjacent shear splay, the fold closure is more than 10 times enriched in gold. A 2-foot channel sample from the fold nose assayed 33 ppm Au compared to a 37-foot composite chip sample taken in the shear splay that averaged 2.5 ppm Au.
In general, the shears occur as relatively narrow, foliation-parallel zones with brittle and ductile deformation. They are traceable for hundreds of feet to more than 11,000 feet along strike (Hausel, 1991) and are continuous to minimum depths of at least 900 feet based on drilling (deQuadros, 1989).
Later mineralizing episodes occurred after the development of the auriferous shear zones. This is clearly seen at South Pass where a swarm of copper-gold-silver quartz veins cut the earlier auriferous shears. These veins occur in greater frequency near the margins of the greenstone belt, and have also been identified in the adjacent granodiorite plutons, implying a possible relationship to the cratonization event that produced the major 2.6 Ga batholiths along the margin of the greenstone belt.
Veins related to metamorphic processes are characteristically not zoned. The Mary Ellen vein (Archean) in the South Pass-Atlantic City district along the northwestern flank of the South Pass greenstone belt is a crosscutting vein hosted by metatonalite porphyry. This milky quartz vein is dominated by gold with uncommon pyrite. Gold values are found to increase where the vein pinches, and no mineralogical zonation has been noted. Veins along the Sweetwater River in the Lewiston district to the southeast parallel foliation and contain argentiferous arsenopyrite and minor gold. Again, no mineralogical zonation has been noted.
In the Seminoe Mountains greenstone belt, gold-chalcopyrite-pyrite-quartz veins occur in a 1/4-mile diameter chlorite-calcite-sulfide alteration haloe in metagabbro and metabasalt (Klein, 1981). The quartz veins develop shoots at vein intersections and in fold closures.
The intrusion of rock by magma and its associated hydrothermal fluids often leads to replacement and recrystallization. When such hot mineralizing fluids contact carbonates, the resulting replacements (skarns) can be profound. These contact metamorphic deposits, until very recently, have been essentially unknown in Wyoming.
Some localized replacement lead-zinc-silver mineralization has been described in the Black Buttes area of the Black Hills. Recent mapping in the Cooper Hill district of the Medicine Bow mountains led to the discovery of several skarns in metalimestone of Proterozoic age associated with gabbrioic and basaltic intrusives (Hausel and others, 1992). These include (1) garnet (hydrogrossular), epidote, actinolite, chlorite, idocrase(?), calcite, limonite, (±) magnetite hornfels, (2) epidote, pyrite, calcite, quartz hornfels, (3) magnetite hornfels, (4) calcite, epidote, actinolite, pyrite, magnetite marble, (5) actinolite, calcite, quartz, chlorite, (±) chalcopyrite hornfels, (6) tremolite, calcite, quartz marble, and (7) uvarovite-magnetite-calcite hornfels.
STRATIFORM AND STRATABOUND DEPOSITS OF SEDIMENTARY AFFILIATION
Stratiform and stratbound deposits of sedimentary affiliation include some of the largest known metalliferous deposits in Wyoming. The source of the metals of these deposits is not always clear. Typically, the statiform deposits contain mineralization that is concordant to stratification, and the stratabound deposits are confined stratigraphically, and mineralization can be either concondant or disconcordant to stratification.
Copper-silver-zinc redbed mineralizaton is widespread in the thrust belt of western Wyoming (Hausel and Harris, 1983). Many of these deposits and occurrences lie along the contact of the Nugget Sandstone (Triassic-Jurassic) and the overlying Gypsum Spring Member (Jurassic) of the Twin Creek Limestone (Boberg, 1986) Some deposits show evidence of both structural and stratigraphic control. The redbeds are bleached indicating the mineralizing fluids were reducing.
The best exposure of this type is at the Griggs mine in the Lake Alice district (Figure 1), where several adits were driven into mineralized sandstone. Fluid inclusion studies indicate the mineralizing fluids were deposited at less than 100°C (Loose and Boberg, 1987). The source of these fluids may have been interformational fluids generated during deformation of the thrust belt (Boberg, 1986), or they may have originated from metalliferous hydrocarbons (Love and Antweiler, 1973). The ore fluids migrated into anticlinal traps along permeable fault and breccia zones (Loose and Boberg, 1987; Loose, 1988), and produced a mineralized zone 300 feet thick (Love and Antweiler, 1973).
Ore shipped from the district between 1914 and 1920, and ore recovered from the mine in 1942 averaged 3.5% Cu and 254 ppm Ag (Allen, 1942). Samples collected by Love and Antweiler (1973) contained 180 ppm 6.7% Cu, a trace to 0.5% Pb, 26 ppm 3.2% Zn, and a trace to 1,200 ppm Ag.
Banded iron formation
Significant resources of Archean banded iron formation (BIF) occur in the Copper Mountain, South Pass, and Seminoe Mountains supracrustal belts, and additional BIF is found in the Barlow Gap, Sellers Mountain, and Elmers Rock belts. In the Hartville uplift of southeastern Wyoming, giant resources of hematite schist occur in a eugeoclinal belt of Archean (?) age.
Banded iron formation in the South Pass greenstone belt occurs in a metasedimentary-metaigneous unit containing quartzite, metapelite, and amphibolite. The BIF typically shows well-developed banding expressed by alternating magnetite-rich and metachert-rich layers with subordinate amphibole (principally hornblende and grunerite), chlorite, and local sulfides (pyrite and chalcopyrite). The rock averages 33.0% iron (Bayley, 1963). Bayley (1968) reported indicated resources in the range of 300 million tons. Ninety million tons of taconite were mined at the Atlantic City mine between 1962 to 1983, suggesting a substantial resource remains in place (Hausel, 1991).
Sulfides are uncommon in the BIF, but locally may form up to 5% of the rock. The sulfides (pyrite with subordinate chalcopyrite) are principally stratiform with some crosscutting veinlets. The BIF has been structurally thicken by internal folding and plication and by repetition by slippage along faults.
Gold distribution has been incompletely examined in the BIF. Available records indicate one mine was developed in a crosscutting quartz vein adjacent to BIF at the Atlantic City iron mine. The ore averaged 2.06 ppm Au (Bayley, 1963). Elsewhere, samples of BIF have assayed as high as 1.1 ppm Au with some quartz stingers containing 0.4 ppm Au (Hausel, 1991).
|Wyoming banded iron formation|
BIF in the Seminoe Mountains greenstone belt forms a large resource in the Bradley Peak thrust sheet (Blackstone, 1965). The iron formation is intercalated in metasediments, follows schistosity in metabasalt and metagabbro, and occurs as interflow sediments between basaltic komatiite flows. Locally, the BIF is structurally thickened producing a giant iron ore resource. Harrer (1966) indicated the north slope of Bradley Peak contains a resource of 100 million tons. The iron deposits continue beyond the north slope, suggesting the total resource to be much greater. Chemical analyses of the Seminoe BIF give iron contents of 28.7% to 68.7% Fe (Harrer, 1966). Localized gold and silver anomalies have been detected (Hausel, 1989).
BIF in the Barlow Gap supracrustal belt of the Granite Mountains is found in a metavolcanic-metasedimentary sequence intruded by Tertiary alkalics. Gold anomalies have been identified in a variety of rock types in this area (John T. Ray, personal communication, 1991) including iron formation (Bob Kellie, personal communication), and metachert (Hausel, 1989).
Iron deposits in the Hartville uplift were commercially mined until 1981. About 45 million tons of ore were mined from the Sunrise deposit, where the ore occurs as hematite schist in the Silver Springs Schist (Snyder and others, 1989). Gold anomalies have also been detected in some hematite schists in the Hartville uplift (Woodfill, 1987).
One of the greatest untapped resources in the State are extensive paleoplacers and placers that cover thousands of square miles of surface area. Paleoplacers have been recognized in rocks of Precambrian, Cambrian, Jurassic, Cretaceous, and Tertiary age. Precambrian conglomerates of Archean and Proterozoic age in the State exhibit possible equivalents to the Blind River, Canada and Witwatersrand, South Africa uranium and gold deposits. Cambrian conglomerates exhibit possible equivalents to the Deadwood, South Dakota gold deposits, and the vast auriferous Cretaceous and Tertiary conglomerates may someday yield commercial gold deposits. But incredibly, much of these remain essentially unexplored.
Modern placers have yielded some gold and other valuable heavy minerals in the historic past. These placers ranged from relatively restricted occurrences to widespread deposits.
Paleoplacers are widespread in the State. To date, production from paleoplacers and their associated reworked placers has been minimal, although the shear volume of paleoplacer material implies that these deposits should become important sources of gold and other heavy minerals in the future.
|Gold recovered from the Dickie Springs area of the Oregon Buttes paleo paleoplacer.|
Cambrian paleoplacers are described at several locations in the state. At South Pass, Flathead Sandstone conglomerates were explored for gold, although the paleocurrent directions (into the greenstone belt) indicate limited gold content. In the Bald Mountain district of the Bighorn Mountains, low-grade gold and monazite paleoplacers form relatively large resources. Several other localities in the State, the Flathead Sandstone has yielded anomalous monazite, gold, or other heavy minerals. One similar unexplored conglomerate is the Fountain Formation(?) conglomerate of Pennsylvanian age. In southeastern Wyoming near the State Line district, native copper and copper carbonate were discovered in this conglomerate prior to 1906. The conglomerate remains unexplored for other heavy minerals.
Tertiary paleoplacers are abundant in the State. These consist of fanglomerate and fluvial conglomerates that locally include giant boulders eroded from the nearby uplifts. The more highly mineralized conglomerates lie adjacent to greenstone belts. For example, the South Pass greenstone belt is flanked by two giant paleoplacers known as the Twin Creek (Antweiler and others, 1980) and Oregon Buttes paleoplacers (Love and others, 1978; Hausel and Love, 1991), and large areas of the greenstone belt are also overlain by additional paleoplacers. One of these was recently explored by magnetic surveys and trenching revealing a complex braided paleo-channel with gold values (Fred Groth, personal communication, 1989).
Titaniferous Black Sandstone
Titaniferous black sandstone (Late Cretaceous) is relatively widespread. These paleobeach sands occur primarily in the Mesaverde Formation. The black sandstones are enriched in heavy mineral suites which include anatase, sphene, rutile, ilmenite, titanomagnetite, magnetite, monazite, zircon, and gold. Minerals of potential economic value include the titanium-bearing assemblage of sphene, rutile, anatase, ilmenite, and titanomagnetite; zircon for zirconium and hafnium; monazite for rare earth metals; a niobium-bearing opaque; and gold (Houston and Murphy, 1962, 1970).
The deposits differ greatly in grades and size. The Grass Creek deposit in the Bighorn Basin is the largest high-grade deposit in Wyoming (Houston and Murphy, 1962) and includes significant resources of titanium (averages 16% TiO2), and zircon (3 million tons averaging 4.8% ZrSi04) (William Graves, personal communication, 1990). Some younger deposits (i.e., Sheep Mountain in southeastern Wyoming and deposits in northeastern Wyoming) contain a similar suite of heavy minerals and are also anomalous in gold (Madsen, 1978). Values as high as 1.3 ppm Au have been reported by Houston and Murphy (1970) and 1.7 ppm by William Graves (personal communication, 1990).
Gold tenors of modern placers in the state range from a trace to more than an 1.0 oz/yd3. Generally, the commercial placers average about 0.01 oz/yd3, although exceptional placers have averaged 0.1 oz/yd3.
Some extensive placers extend from their river beds into the adjacent terraces covering hundreds of square miles. These widespead placers typically contain fine gold flakes and colors difficult to recover by mechanical concentration. The placers of the Wind River were mined in 1910, but recovery was difficult even though gravels encountered by two dredges averaged 0.014 oz/yd3 and 0.038 oz/yd3 (Hausel, 1989). This placer was found along much of the Wind River and reported as 12 to 14 feet thick with widths as great as 3 to 4 miles (Schrader, 1913).
Nuggets recovered from Wyoming placers include walnut-size nuggets from Mineral Hill, Douglas Creek, and the South Pass greenstone belt. The largest nugget found in Wyoming may have been a fist-size specimen of mixed rock with 24 ounces of gold reported to have been found on Rock Creek in the South Pass area prior to 1905. Another interesting specimen found in the same area consisted of country rock with an estimated 630 ounces of gold. Several other nuggets ranging in weight up to 5 ounces have also been described (Hausel, 1991). Most nuggets in the State are rounded typical of detrital transport, although Day and others (1988) report slivers and hairs of gold from gravels in the Lewiston district. In addition to placer gold, other heavy minerals of potential value have been identified in Wyoming placers (Table 1).
Table 1. Predominant heavy minerals reported in some modern placers in Wyoming.
Gold placer or district Heavy Minerals
Lewiston district, South Pass Gold, scheelite, cassiterite
South Pass-Atlantic City district Gold, scheelite, cassiterite, chromite
Crows Nest, South Pass Gold, scheelite.
Mineral Hill district, Black Hills Gold, cassiterite, tantalite
Douglas Creek, Medicine Bow Mtns Gold, platinum, palladium, diamond, pyrope
Cortez Creek, Medicine Bow Mtns Gold, diamond
Clarks Camp, Wind River Mountains Gold, monazite
Bald Mountain, Bighorn Mountains Gold, monazite
Nugget Creek, Sierra Madre Silver, gold
Muddy Creek, Shirley Basin Monazite
Big Creek, Medicine Bow Mountains Monazite, rare earth metals
Big Creek, Medicine Bow Mountains Monazite, rare earth metals
A study of sand and gravel deposits for gold occurrences resulted in the identification of gold colors at many of the locations sampled (Hausel and others, 1992). This widespead occurrence of gold could lead to the recovery of by-product gold from some sand and gravel operations.
Gemstones are found in a variety of geologic settings and may be the result of crystal growth and/or replacement during regional metamorphism producing jade, sapphire, or ruby; magma cooling producing peridote or aquamarine phenocrysts or megacrysts; partial melting at great depths where diamond xenocrysts may be captured and brought to the earth's surface; or just simply low temperature silica replacement of fossils and wood at the earth's surface. In the past, the State has been noted for its abundant and high-quality jade and varieties of chalcedony, although a variety of gemstones and semi-precious and lapidary stones have been found in Wyoming.
Wyoming jade (nephrite) is a monomineralic rock composed of calcium- and magnesium-rich amphibole. Nephrite has been found at a number of localities in central Wyoming including the Prospect Mountains of the southern Wind River Mountains, the Granite Mountains, the Seminoe Mountains, the northern Laramie Mountains, and has also been found in fanglomerates derived from these areas. The jade recovered in past years has included material ranging from the poorest quality black jade to some of the highest quality apple green jade ever found in the world.
In the Granite Mountains, nephrite is associated with amphibolite inclusions in quartzofeldspathic gneiss. The Granite Mountains have been the primary source area of jade in the State (Sutherland, 1990). Other areas include the Laramie Mountains, where nephrite has been identified in orthoamphibolite dikes containing quartz veins. In the Seminoe Mountains, nephrite is reported with amphibolite inclusions and dikes (Sherer, 1969).
Field, chemical, and petrographic data suggest nephrite was formed by metasomatic alteration of amphibolite. Sherer (1969) suggested the following reaction in the presence of water: hornblende » prismatic actinolite » fibrous actinolite (nephrite) » chlorite + talc » serpentine.
Based on a small sampling of 78 diamonds recovered from the Colorado-Wyoming district, McCallum and others (1979) reported the majority of the diamonds were aggregates, octahedra, and transitional octahedra-dodecahedra crystals. Subordinate morphologies include macles, irregulars, flattened dodecahedra, and dodecahedra. Octahedra and macles were the principal growth forms. Dodecahedra forms evolved from octahedra.
Diamonds recovered from Wyoming kimberlites include both gem and industrial quality diamonds. A few of the gemstones are brown to tinted but most are white or better with GIA (Gemological Institute of America) color grades of H-I or better. Some gems have exceptional white colors and GIA color grades as high as D-E-F (grading system ranges from D to X with diamonds of grades D to I being most desirable; see Hurlbut and Switzer, 1979, p.130-132). The clarity of the gems varies from VVS (very very slightly included) to I (imperfect), thus some gems are lightly included, although near inclusion-free diamonds also occur.
Up until 2006, the author discovered dozens of gemstone deposits that were previously unrecognized in Wyoming (Hausel, 2014). These deposits include rubies and sapphire from the Granite Mountains of central Wyoming that occur in soft, light-green, aphanitic nodular masses enclosed by a darker green to grey mica schist. Diffraction patterns of the nodules encasing the rubies show a homogeneous mass of sericite. Gem quality rubies have been recovered from at least two localities in the Granite Mountains, and float schist with high quality rubies have been found on Green Mountain. Also in the Granite Mountains, pale-blue to colorless sapphires occur in rounded nodules in biotite-chlorite schist. The schist forms an enclave in granite.
|Iolite gemstones found in schist in the Laramie Mountains, Wyoming.|
Pyrope garnet and chromian diopside are found in ant hills in the Green River Basin. These minerals produce attractive faceted stones. The source of the stones is unknown.
Beautiful high-quality amethyst and smoky quartz has been recovered from open-space fractures in granite south of the Battle Lake area in the Sierra Madre (Ralph E. Platt, personal communication, 1989). Amethyst and drusy lavendar chalcedony was also recently found at the Artic mine in the Mineral Hill district (Sutherland, 1990).
Some other attractive gemstones found in Wyoming include labradorite in the Buttes area of the Laramie Range anorthosite complex which produces a 'fire' similar to opal. Small amounts of poor quality opal and some gem quality amber are found in the Absaroka Mountains. Peridote crystals, up to 1 to 2 cm are found in some of the olivine lamproites in the Leucite Hills. Other gemstones of note include giant iolite deposits along with many other gemstones. Information on these and other gemstones can be found in Hausel (2014).
This paper was initially prepared for a geology conference at the University of Wyoming. That original paper was reviews by W. W. Boberg and Dr. Robert S. Houston. I very much appreciate their comments and suggestions and I am indebted to both Bill and Bob for helping me clarify some of my ideas. Dr. Sheila Robert's editorial review greatly improved the organization of the paper. Some updates were recently added to this paper. This paper is dedicated to three geologists of the Wyoming Geological Survey who died at the institute. May Ray E. Harris, Robert, and Richard Jones RIP and one day receive accountability. Even though the institute only had about 20 to 25 people, there was never any investigation to their deaths, the heart related problems of six other staff members, nor to the turnaround of staff 50% of the staff and advisory board.
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