Tuesday, September 27, 2011
Friday, February 26, 2010
Following discovery of significant gold in the streams within the Bannack district in 1862, the precious metal was mined along a 10-mile stretch of Grasshopper Creek and several tributaries at the southern end of the Pioneer Range. Mining included placer, hydraulic and some lode activity during the period of 1862 to 1875, and dredging occurred on Grasshopper Creek from 1895 to 1902. More than 132,000 ounces of gold were recovered from placers while the lodes only produced 35,000 ounces of gold. At today’s gold price, this district produced more than $184,000 million in gold!
The geology of Bannack was described by Winchell (1914) as consisting primarily of Paleozoic Madison Limestone. Immediately above the Madison are remnants of the Quadrant Quartzite. These sedimentary rocks were intruded by granodiorite that is nearly circular in plan on the south side of Grasshopper Creek. A minor granodiorite intrusion is also found on the north side of the creek to west of Bannack. The eastern portion of the district is overlain by Tertiary volcanics.
Lode deposits are interpreted contact replacement mineralization with some fissure veins. The ores are dominantly gold with some silver, lead and copper and generally found along the contact between the limestone and granodiorite, or in stringers or small fissures. Large amounts of garnet, with some epidote, are also along the contact.
|Artwork by the author|
Alder Gulch-Virginia City
The Grasshopper Creek gold rush led to other discoveries in Montana including rich placers at Alder Gulch (GoogleEarth coordinates 45o19’25’N; 112o05’38”W) near Nevada and Virginia cities about 40 miles east of Bannack in 1863.
Placers at Alder Gulch were mined upstream for 8 to 10 miles. This was the richest placer operation in the world at its time of discovery. At one point, 4 dredges operated in this gulch. In three years, $30 million in gold was recovered (historical prices) (~1.4 million ounces). Up until the 1930s, >$100 million in gold (4 to 5 million ounces) was recovered. As prospectors worked their way upstream, the gold became coarse and lodes were found near Summit Mountain to the southeast. Gold was later mined at Oro Cache, Kearsarge, Keystone, Atlas and the Bartlett mines.
Sizable nuggets were recovered from the district. In 1902, the New York Times reported a 42-pound (672-ounce) nugget was found in California Gulch – this followed the recovery of an 84.48 ounce nugget in the same gulch. These nuggets support that a major undiscovered lode or ore shoot is found in the immediate area (potentially under the gulch).
The district is underlain by Archean gneiss interbedded with iron formation, marble, graphitic schist and amphibolite intruded by pegmatite and ultramafic dikes. Gold was found in place in northeast-trending shear zones associated with pyritiferous veins and breccias that exhibited both sericitic and potassic alteration. Exploration by Kennecott resulted in discovery of a 1.6 million ounce deposit in 1995 at the Apex-Kearsarge property. At the nearby Atlas property, Anaconda identified 3- to 5-foot wide vein with a 2,000-foot strike length. This vein averaged 0.2 opt Au and 0.3 opt Ag with a minimum resource of 52,000 ounces.
mineralized over a width of 19 feet and averaged 0.7 opt. The nearby Oro Cache mine produced gold from 1864 to 1872 and 1889 to 1892. This vein contains gold in quartz-ankerite-pyrite breccia and stockworks in potassically-altered gneiss. Surface samples yielded an average grade of 0.7 opt Au.
Alder Gulch along the Ruby River has headwaters a short distance upstream from Virginia City. Based on the amount of gold found in lodes in this district, it is likely that other deposits remain to be discovered. The identified lodes do not account for the amount of gold found in the placers. And the presence of >2 million ounces in resources identified by Kennecott and Anaconda in this district should make this area very attractive for future exploration. Based on current gold prices, this district produced gold worth about $5.5 billion and holds a minimum of $2.2 billion. With detailed geological mapping, other deposits will be found in this district. During regional examination of this district, I was able to identify some cryptovolcanic structures in this region. Can you find these? Are they diamondiferous kimberlites? Diamondiferous kimberlites, numerous lamproites and lamprophyres and some detrital diamonds have been found in this craton (Hausel, 1998).
Nearly 90 miles to the north-northeast of Alder Gulch is Confederate Gulch (coordinates 46o35’59’N; 111o24’37”W). This gulch contained some of the richest placer ground in Montana and lies on the western slope of the Big Belt Mountains between Helena and Townsend east of the Missouri River. A rush followed the discovery of gold in 1864. During the boom, miners worked placer claims on the upper stretch of Confederate Gulch and its tributaries, especially at Boulder Creek, Montana Gulch and Cement Gulch. The district continued to produce gold from both lode and dredge operations, at least into the late 1940s (Malone et al. 1991; Wolle 1963; Lyden 1948).
The richest placer ground was found at Montana Bar at the foot of Gold Hill. Roughly two acres produced more gold per acre than any other placer in Montana. While one pan reportedly contained a high of $1,400 in gold (historical gold price), it was not uncommon to wash out $1000 from a pan full of Montana Bar gravels. A single shipment in the fall of 1866 weighed two tons and was valued at $900,000 (historical price). The total from Montana Bar estimated at $1 million to $1.5 million (Pardee and Schrader 1933), or about $55 to 83 million at today’s gold prices.
Hydraulic operations continued in Confederate Gulch and its tributaries for many years which were followed by dredging operations in the 1930s. The best returns were from 1939 when two operations recovered 2,357 ounces of gold (worth $2.6 million at today’s gold prices).
Lode operations in the Confederate Gulch were interesting in that they were poorly endowed compared to placer operations. This suggests that some hidden lodes occur in the district that might be discovered with detailed geological mapping. Lodes in the district include Hummingbird, Slim Jim, Schabert, Baker Group and Three Sisters. These only yielded $100,000 in gold, while placers at Confederate Gulch yielded 120 times as much gold ($12 million)! This dramatic difference in placer and lode contribution indicates that nearby, hidden lodes await discovery!
|Don's coffee break|
In 1867, gold was found in the Atlantic Cable quartz vein near Georgetown Lake west of Anaconda. In this lode, rich shoots were intersected including a mass of gold that weighed 550 to 900 ounces. Some gold was also produced at the nearby Southern Cross mine. The total production from the district was about 460,000 ounces of gold from lode and placers worth more than $500 million at today’s gold price.
The district is underlain by faulted and folded Paleozoic sedimentary rocks intruded by small granitic masses. The deposits represent contact metamorphic gold-copper replacement deposits and veins in granite. The ore occurs as pyrite, pyrrhotite, chalcopyrite, gold and magnetite in a gangue of quartz, calcite and garnet (Bergman and Koschmann, 1968). Based on the type of geology and mineralization, exploration geophysics would likely reveal additional targets.
Near Georgetown Lake, silver was discovered east of Phillipsburg in 1864. The district became one of the more important silver districts in Montana. Mines included Granite Mountain, Bi-Metallic and Hope. In 1887, the district produced 2.2 million ounces of silver, making it the largest silver producer in the US at the time. The district suffered from the fall in silver prices in 1893. It wasn’t until World War I that it became active again due to manganese associated with the silver. Manganese was needed for steel additives – being that manganese is often a carrier for silver and gold, it is hoped that the manganese operations recovered by-product silver and gold.
Silver at Phillipsburg is found in veins as fracture fillings in limestone. Ore minerals included polybasite, pyrargyrite, prustite, sphalerite, galena and tennanite (See Hausel, 2009 for information on mineralogy). Manganese was found as replacement deposits containing pyrolusite and rhodochrosite. Several gossans are found in this district and could lead to additional discoveries. One prominent gossan occurs at: GoogleEarth coordinates 46o19’09’N; 113o15’25”W.
Gold was found at Bull Mountain north of Whitehall and mined at the Golden Sunlight open pit mine (45o54’13”N; 112o01’17”W) east of Butte. Mining began in 1975 and in 2008, about 120,000 ounces were recovered. Reserves were estimated at 540,000 ounces of gold from a Late Cretaceous rhyolitic breccia pipe in Late Precambrian Belt Supergroup sedimentary rocks. The breccia pipe is 300 to 700 feet in diameter and includes disseminated sulfides (pyrite and minor tellurides). Stockworks extend >100 feet into the wallrock. The pipe is thought to grade downward into an alkalic porphyry molybdenum system (DeWitt and others, 1996). As you examine this open pit on Google Earth, see if you can spot the impressive gossan just north of the open pit within 1 to 2 miles.
Thirty miles north of the Golden Sunlight open pit, is another breccia pipe. Montana Tunnels mine was a bulk-minable, gold-silver-lead zinc deposit in southwestern Montana (GoogleEarth coordinates 46o22’02"N; 112o07’42”W). This mine was developed on a diatreme (breccia) emplaced along the faulted contact between andesitic volcaniclastic rocks (Late Cretaceous Elkhorn Mountains Volcanics), and a sequence of quartz latitic ignimbrites (Lowland Creek Volcanics) of middle Eocene age. A north-northeast-striking swarm of quartz latite porphyry dikes was emplaced during the waning stages of Lowland Creek volcanism.
Two large dikes (middle Eocene) were intruded into the diatreme prior to the cessation of brecciation and mineralization. The diatreme extends downward for at least 1000 feet and the principal host rock is a matrix-rich breccia characterized by a sand-size tuffaceous matrix of quartz latitic composition with fragments of contiguous volcanic wall and intrusive rocks derived from the Late Cretaceous Boulder batholith. Down-dropped blocks of volcanic wall rock lie at various attitudes against the walls of the diatreme and within it. Those composed of Elkhorn Mountains Volcanics underwent hydraulic brecciation after they sank into the diatreme. The diatreme is separated into two unequal parts by a west-northwest-striking oblique slip fault. Sulfide minerals are disseminated in the breccia matrix and as widely spaced, multi-directional veinlets. Mineralization consists of pyrite, sphalerite, galena, minor chalcopyrite, and rare electrum accompanied by a gangue of calcite, siderite, and minor quartz. Ore was accompanied by sericitic alteration with weak kaolinization and silicification (note - knowing and recognizing the characteristics of hydrothermal alteration, can lead to additional discoveries during mapping and exploration). Sericitization grades outward beyond the ore zone to chlorite-montmorillonite-carbonate alteration, an assemblage that also characterizes the interiors of the late mineral dikes in the diatreme. Gold occurs in electrum and as inclusions in pyrite and sphalerite and silver is present in solid solution in galena. The Montana Tunnels is characterized by anomalously high concentrations of zinc, lead, and manganese (Sillitoe and others, 1985).
Gold was discovered in the Little Rocky Mountains in 1884 (GoogleEarth coordinates 47o54’58’N; 108o36’50”W). The mineralization was found in breccia deposits in magmatic-hydrothermal pipe-like breccias associated with stocks, plugs sills and dikes that intruded a variety of Precambrian to Tertiary age rocks. Gold is in small quartz veins in fissure zones of shattered and altered rock. Some low-grade gold is present as auriferous pyrite disseminated locally in syenite and other intrusive rocks. In a few areas gold occurs in high-grade replacement deposits in limestone.
|Montana bath time|
Development of veins in the early 1900's required use of cyanide to dissolve and extract gold. This continued into the 1920's when the operation became marginal. Pegasus Mining developed shallow, low grade stockwork mineralization in 1979 and initiated heap leaching in 1980 yielded yielding 1.06 tonnes of gold and 1.65 tonnes of silver. In 1994, 13.46 million tonnes of ore were treated to produce 3.35 tonnes of gold.
The major veins from both the Zortman and Landusky open pits are continuous along strike for 3000 feet and were stoped to vertical depths of more than 650 feet. The veins vary from sheeted zones up to 40 feet wide to well defined faults 1 to 6 feet wide. The ore averaged 0.1 opt Au, although values of up to 30 opt Au were reported. Drusy quartz, comb structures and chalcedony were characteristic of the veins (Hastings, 1988).
A low-grade stockwork overlies the veins and occurs as large shear zones up to 1,900 feet wide extending up to 5,700 feet along strike. The Zortman and Landusky trends, although similar in overall aspect, exhibit some differing characteristics. At Zortman the host rocks are syenite and quartz-latite/rhyodacitic porphyries and locally gneiss. The overall shear trend is north-northwest, with associated shears and veins striking north-northeast. Where latter veins cut major shears, the intensity of mineralization increases: a major factor in localization of ore.
At Landusky the host rocks are undifferentiated Tertiary intrusives including porphyritic rhyodacite/quartz-monzonite/quartz-latite and syenite, as well as Proterozoic quartz-feldspar gneiss and shale. The major structural trend is north-easterly, with intersecting northerly striking shears and veins. As at Zortman, this structural pattern localized ore. Trachyte dikes are common along major shear zones, and considered an indication of structural preparation (Hastings, 1988).
Ten miles west of Toston, on the east side of the Elkhorn Mountains is the Radersburg district. The Keating Mine was one of a group of productive operations. Along Crow creek and its tributaries were some rich placers. Below the town, along Eagle and Sam Creeks are other rich placer gravels as well as some placer gold found in Johnny Gulch.
Radersburg is located in western Montana about halfway between the Golden Sunlight mine and Confederate Gulch (GoogleEarth coordinates 46o11’20”N; 111o39’15”W). The district includes Backer, Canyon and Diamond City, along with several gulches that are tributaries of the Missouri river. Historically, the district produced 600,000 ounces of gold worth more than $650 million at today’s price. As you look at this district, you should be able to spot several gossans, particularly around the Radersburg lode mine.
Gold was discovered in central Montana on the east flank of the North Moccasin Mountains as well as in the Judith Mountains in the 1890s. Gold was identified in argillaceous layers in the Madison Limestone and the precious metal was not free milling, thus it was recovered by fine grinding and dissolving the metal with cyanide. Gold was initially recovered at the Kendall mine (GoogleEarth coordinates 47o16’56”N; 109o28’03”W) by underground and open pit operations at the beginning of the 20th century.
|The author stands on a classical gossan.|
Years later, in 1989, a large, low-grade ore deposit was identified in the area of the historic mining operations. The deposit averaged 0.05 opt Au and like the earlier operations, was amenable to cyanide leach. From 1989 to 1996, 300,000 ounces of gold and 135,000 ounces of silver were recovered from the open pit operations by scientific and environmentally approved cyanide leach processes. The presence of cyanide later identified in adjacent drainages appears to have been the result of misinformation from environmental groups and local ranchers. These anomalies were from historical cyanide dumping in the early part of the 20th century. The drop in the water table was due to years of drought, but the modern mining company was blamed for the effects of drought and for the historical environmental damage.
Twelve miles east of the Kendall mine, one should be able to spot some interesting gossans that are 5 miles north of the Gilt Edge gold mine.
Marysville (Ottawa) district
Gold-bearing gravel was discovered in Silver Creek in western Montana in 1864. The pay streak was 30 to 50 feet wide and 15 to 20 feet deep and had a relatively high silver content. Soon gold was found in the adjacent hills in gold-silver quartz veins in metamorphosed Belt Series rocks. The most significant lode was the Drumlmmon lode, discovered in 1876. Several other lodes were developed in the district and found adjacent to the Marysville monzonite within metamorphosed slate and limestone. The veins exhibit three trends: (1) northeast (North Star mine), (2) north-south (Drumlummon mine) and (3) northwest. The ore occurred in quartz fissure veins containing gold and sulfides. The Drumlummon vein was developed 3000 feet horizontally and 1600 feet deep.
The district produced $31,000,000 - essentially half was recovered from the Drumlummon mine. The district is attributed to 1,146,000 ounces of gold. The Marysville district exhibits a very impressive gossan (GoogleEarth coordinates 46o45’20”N; 112o17’36”W) visible on aerial photos. This gossan likely hosts additional undiscovered gold deposits and is so distinct and large, that it is mappable on Google Earth.
Mineral Hill (Pony)
Placers were discovered in this region of the Tobacco Root Mountains in southwestern Montana in the early 1870s. Gold was found in 1874 at what became known as the Ned mine (GoogleEarth coordinates 45o38’52”N; 108o58’56”W). The Willow Creek vein was found and the Keystone lode was staked. About two miles above the present town of Pony, is the Crevice and Strawberry lodes. At a depth of 14 feet, a 10-foot vein was discovered with free gold that assayed 1 to 5 opt Au. Several other mines were located and a mill erected at the Crevice mine (Parker 1934; Wolle 1963).The district became known as Mineral Hill because of the abundance of ore in the slopes above town.
|Night Crossing in Montana - one of my favorite color-pencil|
The town received rail service in 1890 and some mines began to ship low-grade ores to smelters in Butte (GoogleEarth coordinates 46o08’12”N; 112o30’36”W), Anaconda (GoogleEarth coordinates 46o08’13”N; 112o50’00”W) and East Helena (GoogleEarth coordinates 46o35’04”N; 111o55’04”W) (Fiege 1985). The district remained active until the 1930s and together with Potosi and Bismark districts a short distance south and west, yielded a total of $6,430,2336 (historical prices), mainly in gold but also in appreciable amounts of silver, copper, and lead (Sahinen 1935; Wolle 1963).
The district is on the east side of the Tobacco Root range. While the mines on the western slope were silver-lead producers those on the eastern slopes were gold producers. The northern part of the district is underlain by metamorphic rocks of the Pony series (pre-Cherry Creek). The Pony series are characterized by light and dark colored gneiss and schist cut by aplite, pegmatite and mafic dikes, and Tobacco Root quartz monzonite batholith which occupies the southern part of the district (Cope 1888; Sahinen 1935).
Ore occurs as veins. In general, the veins strike east parallel to schistosity. About 40 distinct vein systems were identified at the Keystone, Proctor Knott, Bozeman, Clipper-Tweed, Fourth of July, Ned and Willow Creek deposits (Cope 1888; Winchell 1914; Parker 1934; Sahinen 1935). The most important mines were the Boss Tweed-Clipper, Atlantic-Pacific, Strawberry-Keystone and Garnet.
The Boss Tweed was a large gold deposit in gneiss associated with faults. This ore body was developed by 15,000 feet tunnels extending along strike. The mine produced $500,000 by 1914 (Winchell 1914; Parker 1934). The Clipper mine is on the south side of the Boss Tweed and has ore similar to that of the Boss Tweed; one ore shoot of 6 to 30 feet in width was worked from surface to the 600 foot level. The mine is credited with $1,500,000 in ore in the 1890s (Winchell 1914; Parker 1934). From 1904 to 1935 the Boss Tweed properties yielded 44,223 ounces of gold; 49,728 ounces of silver; and 57,090 pounds of copper. Total value for the ore from 1874 to 1930 was calculated to be over $6,400,000 (Tansley et al 1933; Lorain 1937).
The Strawberry vein was 4 to 7 feet wide. The Keystone vein was more productive. The Keystone changed from pegmatite to quartz within the walls of single fissure (Winchell 1914). Ore values averaged 1.5 opt Ag and 1 opt Au. The ore also contained 0.4% Cu (Walsh and Orem 1910; 1912; Winchell 1914).
Although the Jardine District, located in the vicinity of the town of Jardine in the upper Yellowstone River valley, was initially known for placer gold, the success of the lode mining outweighed placers. This district is located just north of Yellowstone Park.
Mining of gold and low-grade scheelite ores continued until 1921. Ore rich in arsenopyrite was discovered shipped to Tacoma in 1922. A arsenic plant was built in 1923 to produce both crude and refined arsenic trioxide from arsenical gold concentrates. The plant operated almost continuously from 1923 to 1926 and again from 1932 to 1936. The plant closed in 1942 due to war-time restrictions. During its years of operation, the mill treated 33,416 tons of gold ore, much of it from the mine immediately outside Jardine. From 1902 to 1947 the district yielded 171,815 ounces of gold; 33,262 ounces of silver; 4,368 pounds of copper; and 1,292 pounds of lead (Reed 1950; Wolle 1963).
Archean gneiss and schist are folded, faulted, and intruded and locally covered by volcanic rocks of Cretaceous or early Tertiary age. The ore, consisted of gold, arsenopyrite, and scheelite, in lenses in shear zones in biotitic quartz schist. Ore shoots range from three to 20 feet wide, 50 to 200 feet high, and 300 to 800 feet long (Sahinen 1935; Duykers 1938).
New World (Cooke City)
This district (GoogleEarth coordinates 45o03’27”N; 109o57’00”W) is located along the Montana-Wyoming border adjacent to Cooke City, Montana and extends to the Wyoming border. I highly recommend that you explore this district with Google Earth and/or Virtual Earth – there are dozens and dozens of gossans (a gossan is a tawny to reddish-brown colored weathered and oxidized zone produced from the oxidation of mineral sulfides and may sit over the top of an ore deposit). Anyone of the gossans seen in this area could sit on top of a major, world-class gold deposit. In fact, I would bet that several do sit over gold deposits that contain more than 1 million ounces each (Hausel and Hausel, 2011).
The district extends 2 miles south into Yellowstone and continues north of Cooke City for at least 16 miles. It lies primarily in Montana. The area has many gossans including two giant gossans: one northeast of town and the other to the northwest of Cooke City. The gossan to the northwest can be traced several miles! The gossan to the northeast (45o01’60”N; 109o55’40”W) is very distinct and traceable for miles to the north (look for the tawny to yellow color on Google Earth). The combined surface area of these gossans is 400 to 500 mi2 suggesting major gold, silver and base metal deposits are likely to be found. North of Cook City, Montana (25 miles to the north) is the Stillwater mine and complex. The Stillwater mine is a primary palladium mine with associated values in platinum, chromium, gold and silver.
Dozens of gossans are found in this area. These continue along a major NW-SE trend known as the Crooke City structural zone that lines up with Republic mining area, Sheep Creek prospects, Independence, Horseshoe Mountain, Emigrant district, and Sunlight mining district. One of the gossans along this trend is traceable from Cooke City to a few miles south of Stillwater. While looking at aerial photography, it should become apparent that this area is highly fractured and one could literally map hundreds of fractures, some dikes and old mines. More than 2 million ounces of gold and 9 million ounces of silver have been identified in the region and based on the extensive gossans these numbers would dramatically increase with exploration. We would not be surprised to see this area host more than 10 times these amounts.
The stocks north of Cooke City are principal centers of mineralization. With few exceptions, the known ore bodies are localized adjacent to the stocks. For example, the Como, Fisher Mountain, McLaren, Miller Creek and Homestake deposits are largely replacement deposits in Cambrian limestone adjacent to the Tertiary intrusive complexes. The district also encloses breccia pipes including the prominent Homestake pipe at Henderson Mountain. The Homestake pipe has identified drilled resources amounting to 1.48 million ounces of gold! The combined reserves of the replacement deposits in the area total more than 12 million tons of ore with an average grade of 0.22 opt Au, 0.87 opt Ag, and 0.75% Cu (Kirk and others, 1993). Based on the size of gossans, these could be increased substantially. Other mines in this area include the Little Daisy, Glengarry and Gold Dust.
Precious metal reserves (New World District) (after Van Gosen):
DEPOSIT TONS Au (opt) Ag (opt) Ounces (Au) Ounces (Ag)Como 707,318 0.11 0.546 77,805 386,195
Fisher Mtn 334,200 0.189 1.13 63,164 377,646
McLaren 2,171,035 0.091 0.381 197,564 827,164
Miller Creek 2,218,268 0.387 1.54 858,505 3,416,286
Homestake 6,600,696 0.224 0.83 1,478,616 5,478,801
Total 12,031,887 2,675,657 10,486,092
These ore deposits are open at depth and laterally; thus reserves can be substantially increased. The values of the contained metals are $178 million in silver, $3 billion in gold and $500 million in copper (a total of 179 million pounds of copper have been identified in these gold deposits).
Mineralization extends outward from Henderson Mountain in irregular metallogenic zones. Contact metamorphic gold-copper deposits lie adjacent to the stock and grade into copper-lead dominant mineralization away from the stock. This mineralization is further zoned forming a halo of copper-lead-zinc. Farther from the stock, mineralization takes on characteristics of complex lead-silver-zinc deposits. These grade into an aureole of silver-bearing sideritic calcite veins and finally into barren carbonate veins greater distances from the stocks. The better deposits are developed where the veins cut limestone beds and form replacement deposits in the Pilgrim Limestone (Gallatin Formation equivalent) of Late Cambrian age (Lovering, 1929a; Reed, 1950; Butler, 1965).
Reported production in the district has been 62,000 ounces of gold, 700,000 ounces of silver, 2 million pounds of copper, 3 million pounds of lead, and 900,000 pounds of zinc. Nearly all the copper and gold came from the McLaren gold mine, and a large proportion of the lead and silver was produced from the Irma-Republic mines (Kirk and others, 1993).
The Cooke City structural zone is a zone of weakness that provided access for calc-alkalic magmas during the Eocene along with mineralization. One cannot emphasize enough the size of these deposits and potential for discoveries. The mappable gossans likely host dozens of major ‘blind’ ore deposits similar to those found at Homestake and Miller Creek. As a prospector, you might examine this region for nuggets and also check the BLM records for mining claims and look for areas to stake a claim.
Ore deposits are related to a deeply dissected intrusive-volcanic complex. Due to uplift, this complex has been more deeply eroded other mineralized center in the Absaroka Mountains (Hausel, 1986). Precambrian gneiss, Paleozoic sedimentary rocks and Tertiary intrusives (diorite to syenite) are exposed. On the western and southern edges of the district, andesitic flows are still preserved and unconformably rests on Paleozoic sedimentary rocks (Nelson and others, 1980).
The Goose Lake and Henderson Mountain stocks, located north of Cooke City, are the principal centers of mineralization (Hausel, 1982). With few exceptions, the major ore bodies are localized adjacent to these stocks. For example, the Como, Fisher Mountain, McLaren, Miller Creek and Homestake deposits are largely replacement deposits in Cambrian limestone adjacent to the Tertiary intrusive complexes. One breccia pipe is also reported in the district. The combined reserves of the replacement deposits total more than 12,000,000 tons of ore with an average grade of 0.22 opt Au, 0.87 opt Ag, and 0.75% Cu (Kirk and others, 1993).
Mineralization at Henderson Mountain extends outward in irregular metallogenic zones. Contact metamorphic gold-copper deposits occur adjacent to the stock grade outward into copper-lead deposits. The copper-lead mineralization is further zoned to copper-lead-zinc. Farther from the stock, the mineralization takes on the characteristics of complex lead-silver-zinc deposits. These grade into an aureole of silver-bearing sideritic calcite veins and finally into barren carbonate veins at greater distances from the stock. The better deposits are developed where the veins cut limestone beds and form replacement deposits in the Pilgrim Limestone (Gallatin Formation equivalent) of Late Cambrian age (Lovering, 1929a; Reed, 1950; Butler, 1965).
Reported production from the district totals 62,311 ounces of gold, 692,386 ounces of silver, 1,963,800 pounds of copper, 3,242,615 pounds of lead, and 920,200 pounds of zinc. Nearly all the copper and gold production came from the McLaren gold mine, and a large proportion of the lead and silver was produced from the Irma-Republic mines (Kirk and others, 1993).
The Irma-Republic deposits are part of a mesothermal vein system of the Henderson Mountain complex, adjacent to the Montana-Wyoming border. The vein is near vertical and strikes N30°W to N40°W, and was formed by fracture filling and replacement of the host oolitic beds of the Pilgrim Limestone. The oolitic beds are overlain by a hanging-wall shaly member that is interpreted to have acted as an impermeable barrier to uprising hydrothermal solutions. Butler (1965) pointed out that the same conditions occur in the underlying Gros Ventre Formation (Middle to Upper Cambrian), which should offer an attractive exploration target.
Ore mineralogy consists of galena, sphalerite, pyrargyrite, chalcopyrite, polybasite(?), anglesite, cerussite, proustite, native silver, freibergite, argentite and rhodochrosite. Gangue minerals include quartz, jasperoid, calcite, dolomite, manganian ankerite, arsenopyrite, pyrite, marcasite, pyrolusite, psilomelane and iron oxides. Wallrock alteration consists of dolomitization and silicification (Butler, 1965).
In 1920, exploration led to the discovery of a southern extension of the Republic vein, which became known as the Irma mine. The Irma shaft lies on the Snowslide claim, and most of the workings lie on the Blackrock claim in Montana. The shaft reached a depth of 250 feet with several hundred feet of workings, including a 740-foot-long drainage adit that emptied into Republic Creek. In total, the mine workings in the Irma-Republic lode amounted to more than 2,700 feet (Nelson and others, 1980).
From 1922 to 1959, the Irma mine was a small but consistent producer of lead, zinc and silver (Butler, 1965; Nelson and others, 1980). Several carloads, totaling more than 200 tons of ore averaging 12% Pb, 13% Zn and 34.5 opt Ag, were shipped from the mine. The ore was mined from the lower Pilgrim beds in the same horizon as in the Republic mine. The ore consisted of coarse- to fine-grained argentiferous galena and sphalerite with small amounts of pyrite and native silver (Lovering, 1929).
Recorded production from the Irma-Republic properties included 18,400 tons of concentrates that contained gold, silver, lead, copper and zinc (Nelson and others, 1980). Direct shipping grade ore averaged about 40 opt Ag, 20% Pb and 6% Zn. Mill-grade ore contained about 12 opt Ag, 4% Pb and 5% Zn (Butler, 1965).
More than 2.4 million ounces of gold were recovered from the porphyry system at Butte, Montana (GoogleEarth coordinates 46o08’12”N; 112o30’36”W). Butte was a world-class gold, copper, silver, zinc and manganese deposit and started as a gold placer district. This district has numerous veins and deposits in the surrounding region. A sizable gold nugget was found in the Highland mountains south of Butte. The nugget weighed 27.5 ounces!
Several gold districts produced significant amounts of gold and likely contain many hidden and undiscovered gold deposits. These include Gilt Edge in the Judith Mountains where gold was found as replacement deposits in limestones adjacent to intrusive contacts associated with sulfides, chalcedony and fluorite, at the Last Chance placers in the Helena district, produced about 350,000 ounces of gold were recovered in the 19th century and early part of the 20th century. Gold was also found in lodes to the southwest along the northern edge of the Boulder batholith at the Whitlatch-Union and Spring Hill properties.
Montana, like all of the Western States, has considerable mineral wealth in gold, silver, copper, as well as gemstones (sapphire and diamonds). The US could easily climb out of its current depression by changing mining laws, eliminating most permits and shorting the permitting process for exploration and mining. All the states need to do is to tax the mineral wealth, rather than its citizens and encourage exploration and mining. Using Canadian's province guidelines for exploration and acquiring mining property would be an excellent start. If adopted by the states, one would see a tremendous wealth produced in the West.
Websites on GOLD
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more information on gold, see our sites at:
World-Class Gold Deposit discovered at Donlin Creek, Alaska
South Pass Gold, Wyoming
New Gold District discovered at Rattlesnake Hills, Wyoming