The principal placer districts are in the southwestern part of the State. The most productive placer deposit was Alder Gulch near Virginia City in Madison County. Other important localities are on the Missouri River in the Helena mining district. The famous Last Chance Gulch is the site of the city of Helena. There are many districts farther south on the headwaters and tributaries of the Missouri River, especially in Madison County which ranks third in total gold production in the State. Gold was produced at many places on the headwaters of the Clark Fork of the Columbia River, particularly in the vicinity of Butte. Placer production from the Butte district, however, has been over-shadowed by the total output of byproduct gold recovered from the mining of lode deposits of copper, lead, and zinc.
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|
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.
At the historic Bartlett mine, a 15- to 20-foot wide vein was discovered between dolomite and gneiss that yielded ore averaging 0.4 opt Au and 1.4 opt Ag. This property was mined from 1935-41. A vein extension in the bottom of this mine found in 1939 was 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 principal rocks underlying Confederate Gulch are sedimentary, including shales of the Spokane and Greyson formations and limestones of the Newland Formation. These are cut by diorite and quartz diorite dikes, stocks and sills. Narrow quartz veins along fractures in the diorite and along bedding planes in the shale contain high grade gold. Ore values decreased at depth in the lode mines, and few were developed deeper than 150 feet. In addition to the quartz veins in shales, the diorite contains low-grade mineralized shears (Sahinen 1935; Pardee and Schrader 1933; Reed 1951). The decrease in gold at shallow depth is puzzling and likely due to supergene enrichment. It is very likely that these lodes contain gold mineralization continuing to greater depth. Gold distribution suggests that the source of most of the placer gold in Confederate Gulch and nearby White Creek was from a series of quartz lodes on Miller Mountain on the divide between the two drainages (Pardee and Schrader 1933; Reed 1951).
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|
Northwest of Confederate Gulch is the White Creek District. Gold was found in drainages at White Creek, Avalanche Creek and in the upper part of White Creek and its tributary Johnny Gulch. Magpie Gulch also had rich placer ground. Hellgate Creek and its tributaries also produced gold.
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, multidirectional 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.
|Keeping it clean in Montana|
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.|
According to Wolle (1963) placer gold was discovered at the mouth of Bear Gulch during 1865-66. Quartz veins were uncovered not developed until 1877. Large parts of the placer were mined hydraulic methods. In addition to gold, scheelite (a tungsten ore) was discovered.
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, 2011 in press - GOLD - A prospector's Guide to Finding Precious Metals).
The district extends 2 miles south into Yellowstone and continuesnorth 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).
The Republic deposit, the northernmost of the two mines, was discovered in 1869 by a group of fur trappers. The mine consists of 3,000 feet of open cuts, 1,500 feet of tunnels with a 225-foot-deep shaft developed in fissures heavily stained by psilomelane (manganese oxide). Manganese staining is characteristic of the complex lead-silver-zinc ores of the district. Oxidized ore in some stopes assayed as high as 1,000 opt Ag (Lovering, 1929a).
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 aquiring mining property would be an excellent start. If adopted by the states, one would see a tremendous wealth produced in the West.