Monday, August 12, 2013

Diamonds In Montana's Gold Fields

Outline of the Wyoming Craton (basement rocks of Archean [>2.5
billion years old] age) showing locations of known kimberlites,
lamproites, lamprophyres, diamonds, anomalies (Hausel, 1998).
If you decide to prospect for gold in Montana, also look for diamonds, sapphires, rubies, pyroxene, peridot and garnet in your pan and sluice concentrates. Not only have several diamonds have been found in Montana in the past, and several potential host rocks occur in the State (Hausel, 1998), but Montana is also known for high-quality sapphires.

Alnöites, monticellite peridotites, carbonate-rich mica peridotites, monchiquites, lamproites, and kimberlites are all reported in Montana (these are all rare volcanic rocks that may have diamonds - Erlich and Hausel, 2000). Many of these intrusives occur in the central Montana alkalic province in eastern Montana, which overlies part of the Wyoming Province (Hearn, 1989; Scambos, 1991). A few lamproites have also been found, off-carton, in western Montana. In eastern Montana, mantle-derived breccias and dikes are described in Missouri Breaks, Porcupine Dome, and Grassrange areas. More than 30 carbonate-rich dikes and breccia pipes are reported in the Porcupine Dome and Grassrange areas, and a group of kimberlites occur in the Missouri Breaks.

Map showing known kimberlite, lamproite, lamprophyre and
diamond occurrences in the US (after Hausel, 1998)
A few diamonds have been reported in western Montana including some relatively large stones of unknown origin. Other diamonds have been reported from the Sweet Grass Hills in northern Montana, just south of the Alberta border. Diamonds were also recovered from the Homestead kimberlite in the Yellow Water Butte area in eastern Montana. With the presence of a cratonic basement underlying much of Montana, as well as favorable host rocks, Montana would be considered high priority for diamond exploration.

Beaverhead County
A diamond was reported from Grasshopper Creek southwest of Dillon in the southwestern corner of the state (Sinkankas, 1959). No details were given.

Blaine and Hill Counties
Two diamonds were found in gravels of the Etzikom Coulee in the Milk River drainage north of the Sweet Grass Hills. The diamonds weighed 0.14 and 0.17 carats, respectively (Lopez, 1995). The placer diamond occurrence lies near a buried magnetic anomaly aligned with presumed lamproitic rocks in Alberta, Canada, where considerable diamond exploration activity has occurred (Dave A. Lopez, personal communication, 1997).

Garfield County
Location map of the Smoky Butte dike complex
(from Mitchell and Bergman, 1991).
A lamproite complex, known as the Smoky Butte lamproite (27 Ma), lies 6.5 miles west of Jordan in the Missouri Breaks region of northeastern Montana. The complex consists of several lamproite exposures along a N30°E trend. The lamproites are exposed at Radial Dike Butte, Bull Snake Knob, Half Sediment Butte, Smoky Butte, Instrument Butte, Ship Rock, and Wall Rock.

The lamproites are sanidine-diopside-richterite-phlogopite lamproites. These lie along a dike that swells to 122 feet in width. The rocks include vesicular, massive, glassy, hypabyssal breccias, with minor tuffs and pyroclastics. Smoky Butte is a vent (Mitchell and Bergman, 1991). Fipke and others (1995) reported that a few chromites from the Smoky Butte intrusive yielded geochemistries similar to diamond inclusion chromites. A small (65 kg) sample, however, yielded no diamonds.

Glacier County
In 1883, a 12-grain diamond was discovered in the placer workings near Nelson Hill in the town of Blackfoot, northwestern Montana. The stone was described as a colorless dodecahedron with triangular markings (Kunz, 1885). In 1894, a 0.22-carat flawed stone was reported from the county (Sinkankas, 1959).

Elk Butte lamproite breccia (photo by the author).
Granite County
A phlogopitic lamproite flow, known as the Ruby Slipper (Bearmouth pipe), intrudes Devonian age Jefferson Formation and Tertiary basalts and rhyolites near Bearmouth along Bear Gulch in the Garnet Range. The flow is found in sections 3, 4, and 10, T11N, R14W.

The exposed plug consists entirely of flows which buried the underlying volcaniclastics. The underlying breccias were intersected by drilling (Pete Ellsworth, personal communication, 1996). The flow was initially mapped as a 1800-foot-wide leucite basalt by Carter (1982), but later identified as lamproite. According to Ellsworth (1996), the pipe forms a prominent 1500-foot circular knob, with crater facies volcaniclastics covered by talus and colluvium from the lamproite porphyry. Four other lamproitic prospects have been located in the region (Pete Ellsworth, personal communication, 1996).

Lewis and Clark County
Ming Bar monchiquite from Montana. Possible source of diamonds? Diamonds or no diamonds, the
 discovery of gem-quality
 peridot and pyroxene at Ming Bar was overlooked by prospectors.
This intrusive sits on the edge
 of the Missouri River and likely has shed many other gemstones
(photo by the author).
In 1990, a jogger reportedly found an uncut, 14-carat diamond southwest of Great Falls near the town of Craig in west-central Montana. The stone, named the Lewis and Clark diamond, was sold for $80,000 (Anonymous, 1990a). Other diamonds have been found along the Missouri River to the south near Helena, including another relatively large diamond reported to weigh 8 carats (Anonymous, 1990b).

The only known intrusive with upper mantle material within this area lies along the eastern shoreline of Holter Lake on the Missouri River northeast of Helena. The intrusive, known as the Ming Bar monchiquite, contains abundant cognate metacrysts of olivine, pyroxene, and chromite with granulite, dunite, pyroxenite, and peridotite nodules (Meen and others, 1986). The intrusive originated in the upper mantle, but probably at too shallow a depth for diamonds.

However, some diamonds were apparently found in the vicinity of Ming Bar. Five diamonds were reportedly recovered from placer operations at Spokane Bar, and a 5-carat diamond was reported from Metropolitan Bar. Both placer localities lie upstream from Ming Bar (Tony Irving, personal communication, 1994).

Kimberlites and lamprophyres in the Grassrange field
Madison County
Diamonds have been reported from Greenhorn Gulch in the Greenhorn Range east of Dillon, southwestern Montana (Sinkankas, 1959). No details were given.

Petroleum and Fergus Counties (Grassrange Field)
Elk Creek Butte & Yellow Water
Butte lamprophyres (from Mitchell
and Bergman, 1991).
A belt of ultramafic lamprophyres form the Grassrange Field approximately 4 to 6 miles south of Winnet, near Yellow Water Reservoir in east-central Montana. According to Doden (1996, 1997), the belt consists of ultramafic lamprophyric diatremes and dikes that were originally described as lamproites by Mitchell and Bergman (1991).

Elk Creek and Yellow Water Buttes, located near the center of the field, form large ultramafic lamprophyres. The Yellow Water Butte diatreme, in particular, is carbonate rich (pour some weak 10% hydrochloric acid on the rock and it will fizz as it releases bubbles of carbon dioxide). These two buttes lie within a group of ultramafic intrusives known as the Winnet sills, which are nepheline-haüyne alnöites, a carbonate-free rock (Doden, 1996).

The breccias that form Yellow Water Butte consist of massive to brecciated, olivine-phlogopite-diopside-carbonate lamprophyre (?) and massive hypabyssal olivine lamprophyre(?). The Elk Creek vent-dike complex consists of intrusive breccia and lapilli tuffs (Mitchell and Bergman, 1991). Similar, but less extensive outcrops also occur in the area (Hausel, personal field notes, 1994). Doden (1996) suggested that the Yellow Water Butte breccias were part of a highly gas-charged magma that rapidly ascended from the mantle and was virtually unaffected by crustal contamination.

During field investigations of this area, Hausel (personal field notes, 1994) working as a consultant for a major mining company highly recommended the property to company he was working for as a diamond prospect, suggesting this area had as much potential as the Kelsey Lake Kimberlite in Colorado. The company turned down both projects even though diamonds were later recovered from both areas!

Sample of Yellow Water Butte lamproite breccia
(photo by the author).
The later discovery of diamond-bearing kimberlite near Yellow Water Butte in 2004 attracted some interest. One microdiamond was recovered from the newly discovered Homestead kimberlite (Pete Ellsworth, personal communication). It is not known if any bulk samples were processed by Delta Mining Company.

Phillips County
In Phillips County north of the Grassrange Field, a group of intrusives, known as the Williams kimberlites, occur near Landusky. The Williams kimberlites form a group of four closely spaced diatremes in the eastern part of an east-northeasterly trending swarm of ultramafic alkalic diatremes, dikes, and plugs (46 to 51 Ma) in the Missouri Breaks area of north-central Montana. These rocks preceded the intrusion of Smoky Butte (27 Ma; Ma=Millions of years ago) located 67 miles to the southeast. The rocks contain a host of xenoliths including garnet-bearing lherzolites, harzburgites, and dunites. The Williams 1 diatreme occupies a surface area of about 750 by 105 feet, and contains the typical kimberlitic indicator minerals pyrope garnet, chromian diopside, and magnesian ilmenite.

Map of the Williams kimberlites near Zortman,
central Montana (after Hearn and McGee, 1983).
The Williams 2 kimberlite has a surface area covering about 128 by 375 feet, and has a zone of kimberlite breccia with abundant Paleozoic limestone and dolomite xenoliths. The Williams 3 diatreme is about 96 by 128 feet and consists of kimberlite breccia; the Williams 4 is a dike-like diatreme 1216 feet long and up to 121 feet wide and consists of massive kimberlite with desultory zones of fragmental kimberlite  (Hearn and McGee, 1983).

These intrusives enclose xenoliths from the Precambrian basement, upper crust (schist, gneiss, amphibolite), lower crust (granulite, mafic granulite, amphibolite), and the upper mantle (spinel peridotite, dunite, garnet peridotite, garnet megacrysts), as well as kimberlitic indicator mineral xenocrysts. According to Hearn and McGee (1983) neither diamond nor eclogite has been found in these intrusives.

Sample of Williams hypabyssal facies kimberlite, Montana (photo by the author).
The available analyses of peridotitic garnets from the Williams kimberlites indicate the compositions are equivalent to G9. None of the garnets analyzed by Hearn and McGee (1983) fell within the G10 (sub-calcic pyropes) field. However, pressure-temperature estimates from co-existing ortho­pyroxene-clinopyroxene pairs in some of the peridotite nodules indicate some of the nodules may have originated from depths within the diamond stability field (Fred Barnard, written communication, 1994).

Treasure and Rosebud Counties (Porcupine Dome Field)
The Porcupine Dome Field, located in southeastern Montana, includes four intrusives known as Froze-to-Death Butte, Gold Butte, Geyser Spring, and Johnson Ranch Butte. Froze-to-Death Butte is a multiple vent complex located 8.5 miles northwest of Hysham. Gold Butte lies 25 miles northeast of Froze-to-Death Butte; Johnson Ranch and Geyser Spring lie between these two along strike, suggesting structural control. These intrusives are interpreted to be Eocene in age (Doden, 1997).
Froze-to-Death Butte lamproite breccia,
Montana (photo by the author)
The rocks which form Froze-to-Death Butte are described by Mitchell and Bergman (1991) as massive hypabyssal intrusive breccias consisting of altered olivine-phlogopite-diopside lamproite. In contrast, Doden (1996) indicated the rocks forming both buttes do not resemble lamproite, but are instead ultramafic lamprophyres (aillikites) that share some affinities with kimberlite. In particular, he reports the intrusives contain picroilmenite and garnet macrocrysts.

According to Doden (1996), these garnets are similar to some Missouri Breaks garnets derived from upper crustal sources. A second type of garnet is reddish-purple and is similar to peridotitic garnets. These garnets contain 5.7-7.0 wt% Cr2O3, 4.7-7.0 wt% CaO, 18.6-21.6 wt% MgO, and 5.9-7.9 wt% FeO. The peridotitic garnet compositions primarily fall within the G9 compositional field; however, a few have sub-calcic G10 compositions similar to garnets found as diamond inclusions (Doden and Gold, 1993).

Although the lamprophyres apparently sampled the diamond stability field, the picroilmenite com­­posi­tions are depleted in Cr2O3, indicating that oxidizing conditions prevailed in the magma. This may suggest that diamond preservation was not favorable. However, since the magmas did apparently sample the diamond stability field, the search for undiscovered (buried) pipes of olivine lam­­­­­­­prophyre and lamproite in this region may be productive.

Many diamond deposits have been found north of Montana in Canada including several commercial diamond deposits in addition to the great diamond discoveries at Ekati and Diavik diamond mines. The number of discoveries in Canada have been astounding! These deposits do not stop at the Montana border and it is likely other diamond deposits will be found in Montana, Colorado and Wyoming.

For the prospector, one needs to search for cryptovolcanic structures using aerial photography and search for diamonds and kimberlitic indicator minerals while panning for gold. Some articles written by the author and published by the ICMJ Prospecting and Mining Journal describe the use of aerial photography in searching for diamond deposits. The geology of Montana is favorable for discovery of both placer and lode diamonds.

Diamond-rich kimberlite at Victor, Ontario, Canada (photo by the author).

Snap Lake kimberlite from Snap Lake diamond mine, NWT, Canada (photo
by the author).

Highwall of Kelsey Lake diamond mine, Colorado (photo by the author)

Diamond-bearing kimberlite at Sloan Ranch, Colorado (photo by the author). 

If you are prospecting for gold in Montana, get to know the
characteristics of diamond so you don't miss the mother lode
of diamonds. This photo is taken of the surface of an octahedral
diamond surface showing the common trigons often seen on
rough diamond. These can usually be seen with a 10x
prospectors hand lens (loope).

What do you think about sapphires described in the Holy Bible?

Friday, February 26, 2010

Gold Deposits of Montana

Gold was discovered in Montana in 1852, but it wasn’t until ten years later that the precious metal was found in significant amounts at Grasshopper Creek in extreme southwestern Montana that resulted in a mining camp named Bannack (45o09’31”N;112o59’50”W) that resulted in the first of several gold rushes to the state. Other gold deposits were found in Montana resulting in considerable production – and the geology of the state indicates that there are many other deposits that remain to be discovered. Several have already been identified. Finding gold requires some geological detective work.

The US Geological Survey ranked Montana as number 7 in gold production in the US and reported that the state included 31 gold mining districts. Total gold production from the 19th century to 1968 was 17.8 million ounces, but considerable amounts of gold have been mined since (Bergendahl and Koshmann, 1968). Based on geology, it is predictable that several large undeveloped and undiscovered gold deposits will be found, identified and developed in the future.

Visible gold in milky quartz
The principal placer districts lie in southwestern Montana. 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.

Bannack district
While seaching for diamonds and gold in Montana, we came across this interesting rock
- a mochiquite filled with gem material including peridot and pyroxene.  The sample is
from Ming Bar on the Missouri River.

Following discovery of significant amounts of 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, of 1,914 per ounce, this district may have produced more than $319.6 million in gold! However, just like Wyoming, gold production statistics leave a lot to be desired and much of the production is estimated. But still, this district needs considerable more prospecting as there are gold deposits awaiting discovery - such as a a group of gossans including a very impressive gossan found less than to more than a mile south-southeast of the ghost town of Bannack (45°9'18"N; 112°58'32"W) that covers an area of more than a square-mile along the north flank of Grasshopper Creek as seen on the Google Earth image (below).

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.

'Sunrise at South Pass', colored
sketch by the author
One of the principal lode deposits, the Golden Leaf group included the Golden Leaf, Priscilla, Hillside Placer, Wadams, Wallace, French, and Excelsior mines. The underground workings were ultimately integrated through an extensive network of more than 18,000 feet of drifts, winzes and raises which connected to a series of ore shoots on different levels of the Dunn, Priscilla, Golden Leaf, and Thompson lodes. The Golden Leaf group produced $2,577,000 in metals (historical prices). Based on the type of mineralization, this area is ripe for additional discoveries. Such replacement deposits are typically erratic and follow zones of brecciation – exploration geophysics around the granodiorite plugs and a search for additional plugs could be productive.

Alder Gulch-Virginia City
The Grasshopper Creek gold rush led to other discoveries in Montana including rich placers at Alder Gulch (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 mined this gulch. In three years, $30 million in gold was recovered (historical prices) (~1.4 million ounces worth about $1.4 billion at today's price 10/5/2020). Up until the 1930s, >$100 million in gold (4 to 5 million ounces worth about a maximum of $9.6 billion at today's gold price) was recovered. As prospectors worked their way upstream, the gold became more 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. 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 lies hidden in the immediate area (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 identified a significant gold resource in 1995 at the Apex-Kearsarge property (45°13'0"N; 111°55'58"W). The company suggested 600,000 ounces ($1.15 billion at todays gold price 10/5/2020) might be recovered by open pit at a grade of 0.10 ounces/short ton, but a total resource in the area amounted to 1.6 million ounces ($3.062 billion at present gold price). 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, some cryptovolcanic structures were identified. 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).

Confederate Gulch
Nearly 90 miles to the north-northeast of Alder Gulch is Confederate Gulch (46°32'28"N; 111°28'57"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 (46°35'29"N; 111°25'34"W), Montana Gulch (46°36'45"N; 111°24'39"W), Cement Gulch (46°37'2"N; 111°23'4"W), and nearby White Gulch (46°37'58"N; 111°26'52"W). 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).

"JingleBob", pencil sketch by the author
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 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 $4.5 million at today’s gold prices 10/5/2020).

Lode operations in the Confederate Gulch were interesting in that they were poorly endowed compared to placer operations. This suggests 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!

Georgetown district
"Montana Rest Stop", pencil sketch by the author
In 1867, gold was found in the Atlantic Cable quartz vein (46°11'56"N; 113°12'60"W) near Georgetown Lake west of Anaconda (46°10'49"N; 113°15'15"W). 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 town which located near the mine, saw booms and busts, and the mine operated with varying success until about 1880, when extremely rich ore was located. A 500-foot piece of ground produced $6,500,000 in gold. One piece of ore from the mine sold for $19,000 in 1889, and it was claimed to be the largest gold nugget ever found (Wiki).

In 1902, two brothers cleaned up the mine and obtained $18,000 from the first cleanup. By 1906, three shifts a day were running in the mill. By 1940, the mine was inactive and has remained that way.

The emplacement of granite masses metamorphosed a long, narrow, belt of Meagher limestone, together with lesser Wolsey shale. The Cable deposit occurs as a contact metamorphic deposit formed by replacement of limestone adjacent to the granitic intrusives. The deposit is gold dominant with lesser copper. The replacement zone has a length of 1800 feet with a width of 80 to 360 feet; the depth is not known. High grade ore contains visible flake gold with tetradymite in calcite; the low grade ore contains pyrite and pyrrhotite associated with dolomite (without visible gold). Several magnetite ore bodies were discovered in 1906 at the mine, but never developed due to inadequate gold values (Lincoln 1911a; DeMunck 1956).

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.

Helena district
Lewis and Clark County was a source of 4 to 5 million ounces of gold through 1959, with the Helna and Marysville districts producing an excess of 1 million ounces each. Gold was discovered in the Last Chance gulch near Helena. The gulch is located south of downtown Helena near the confluence of Orofino and Grizzly Gulch. Nearly every drainage south of Helena contained gold. So, as amazing as it sounds, the town of Helena is actually built over some of these rich gold properties. The primary lode deposits in this region were emplaced in the Late Cretaceous to early Tertiary age, quartz monzonite Boulder batholith and satellite stocks.

The Helena-Last Chance district, in the southern part of Lewis and Clark County, includes the famous Last Chance Gulch placers, among the most productive in Montana. Placer gold was discovered in Last Chance Gulch in 1864. Later, other placers were found as well as lode gold at the Whitlatch-Union mine, which became the top source of lode gold in the district. Many rich pay-streaks were mined prior to 1900. After 1900, only minor, gold production occurred until an increase in the price of gold in 1934. Total production of the district was 345,000 ounces of lode gold and 940,000 ounces of placer gold!

The lode deposits lie near the contact of the batholith with sedimentary rocks. Some lodes lie within the quartz monzonite, while others are hosted by altered hornfels or tactite. Ore minerals include gold, pyrite, pyrrhotite, chalcopyrite and galena.

Marysville-Silver Creek (Ottawa) district
Placer gold was discovered in Silver Creek (46°44'46"N; 112°11'23"W) in western Montana in 1864. The pay streak was 30- to more than 100-feet wide and 15- to 20-feet deep and had a relatively high silver content. Soon gold was found in the adjacent hills in quartz veins hosted by metamorphosed Belt Series rocks. The most significant lode was the Drumlmmon (46°44'51"N; 112°17'48"W), discovered in 1876. Several other lodes were found in the district as well as adjacent to the Marysville monzonite in 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 considerable gold, and 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; and a distinct linear vein (46°45'12"N; 112°17'32"W) is visible just north of the Drumlummon mine, along a N25oE to N35oE trend that is traceable from Jennies Fork for more than 3,000 feet.

The district is 18 miles northwest of Helena. Ore production from veins included gold and lead. Total gold production for the district was about was about 1,310,000 ounces, of which about 164,500 ounces came from placers.

Centered around a small stock of Late Cretaceous- to Tertiary-age quartz diorite that intrudes Belt Series limestone and shale (Precambrian), sedimentary rocks adjacent to the stock have been metamorphosed to a hard and dense-textured slate. Numerous dikes of pegmatite, aplite, and diorite porphyry cut the stock and the sedimentary rocks.

Ore deposits are steeply-dipping gold and silver veins around the border of the quartz diorite stock. Some veins are in the marginal part of the diorite, but most are in metamorphosed sedimentary rocks. The gold is finely divided and accompanies the ore minerals tetrahedrite, chalcopyrite, pyrite, sphalerite, and galena. Gangue minerals are principally lamellar quartz and calcite.

Phillipsburg district
Silver was discovered east of Phillipsburg (46°19'10"N; 113°16'21"W) near Georgetown Lake, in 1865. The district became one of the more important silver districts in Montana. Mines include Granite Mountain (46°18'57"N; 113°14'34"W), 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 during a fall in silver prices in 1893. It wasn’t until World War I that the district again became an important mining region because of its numerous manganese replacement deposits associated with 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. Manganese was found in replacement deposits of pyrolusite and rhodochrosite. Several gossans are found in this district and could lead to additional discoveries. One prominent gossan is visible on GoogleEarth at coordinates 46o19’09’N; 113o15’25”W.

Philipsburg is south of Drummond and northwest of Anaconda along State Highway-1. In 1865, silver was discovered near Flint Creek in the hills east of the present site of Philipsburg. This discovery led to a claim-staking rush and several mines and claims were staked including Comanche, Hope, Cliff, Franklin, Trout, Gem, Poorman's Joy, San Francisco, Speckled Trout, Poorman's Joy, Kitty Clyde and Pocahontas.

A 10-stamp mill (Hope mill) was constructed in 1867. The mill incorporated chloride conversion unit, which required large amounts of salt (sodium-chloride) to be hauled from the Great Salt Lake at $120 per ton. When the price of salt rose to $320 per ton, the district was no longer viable and closed. Some years later, Congress passed the 1872 mining law and increased the size of lode claims, which stimulated interest in the Philipsburg deposits, but investments dried up following the banking Panic of 1873 (Wolle 1963; Periman and Decco 1994). 

"Laramie Cattle Guard", pencil sketch by the author.

In 1875, a second mill and mining camp was constructed a mile east of Philipsburg, that was named Tower. Mines in the area included Salmon, Gem, Little Emma and Osage. South of Tower, the East Comanche and Algonquin mines were acquired by the Algonquin Company, and rich ore was discovered in 1880, and a 20-stamp mill was built and later enlarged to 80 stamps, 0.5 mile south of Tower and north of Frost Gulch. 

The original Hope mill was used to treat ore from mines as far away as Black Pine (46°26'52"N; 113°21’59”W), 10 miles to the northeast. In 1881, the Hope intersected a rich ore shoot and recovered $361,000 in silver. For the next six years, the mine produced between $100,000 and $200,000 of silver per year until 1887, when much of the high-grade ore was exhausted. 

In 1883, the Northern Pacific railroad reached the town of Drummond (46°40'7"N; 113 9’4"W) to the north, shortening ore haulage. And with a spur extended to Philipsburg four years later, wagon haulage was eliminated. Although mine production declined after 1887, reduced transportation costs allowed some operations to function profitably until the next major silver strike in 1892, a few miles north (Wolle 1963). 

Significant interest in the district was stimulated by the development of the Granite Mountain mines (46°18'57"N; 113°14’34"W) three miles east-southeast of Philipsburg. The Granite Mountain lode was staked on July 6, 1875; and developed in 1880 after a sample of ruby-silver (pyrargyrite - Ag3SbS3) was discovered on the mine dump by the Hope Mill superintendent, Charles McLure. The specimen reportedly assayed 2,000 opt Ag! 

In November of 1882, 115-feet of barren ground was tunneled into rich ore, known as the Bonanza Shoot. Samples assayed as high as 1,700 opt Ag. During the next summer and fall, 1,400 tons of ore was mined and processed returning $274,000 (Emmons and Calkin, 1913). 

A town was built in 1884 at the Granite mine, and by 1889 the town included 18 saloons, a three-story Miners Hall, other prominent buildings, including a 4-mile bobsled run from Granite to Philipsburg. 

Between 1885 and 1888 the operation made $2.5 million. In 1888, a new 100-stamp mill was constructed on Fred Burr Creek, a mile to the south (46°17'30"N; 113°14'59"W). Ore was sent transported from the mine to the mill by a 8,900-foot long tramway. From the mill, the concentrates were hauled to Philipsburg using a 7.7 mile railway extension. In 1890, the Granite Mountain operation produced $4 million in silver and gold (Swallow 1891; Emmons and Calkins 1913; Wolle 1963). 

In 1882, McLure purchased the James Blaine lode adjacent to the Granite Mountain vein, and formed the Bimetallic Mining Company. A 50-stamp mill to support the Bimetallic mine, was constructed on Douglas Creek (46°18'39"N; 113°15'51"W), one mile south of Philipsburg, in 1888. The stamp mill and small town of Kirkville (later known as Clark) were constructed at the mining operation. In 1891, the mill was enlarged by another 50 stamps, and in 1893, a 4,307 pound bar of silver was poured (Wolle 1963). 

Silver prices declined signifying the end of all silver camps throughout the West due of the repeal of the Sherman Silver Purchase Act in 1893. Overnight, the town of Granite became a ghost town as the entire population packed up and left (Emmons and Calkins 1913; Wolle 1963). 

For three years, Granite remained a ghost town. In 1898, the Granite and Bimetallic properties were consolidated. Mining resumed and production from Granite Mountain (1883-1898) was worth $22,093,106, while that of the Bimetallic was worth $7,267,813! From 1898 to 1901, the combined property represented the largest silver mine in the world with production of a million dollars of bullion per year (Wolle 1963). 

By the end of 1913, the district had produced $39,000,000 in metals, mostly from Granite, Hope and Trout mines. When the mines deepen to the water table, lower grade primary ore was mined. Between 1907 and 1932, the district produced another $7,500,000 in ore. In 1934, the Granite and Bimetallic processed 30,000 tons of ore. In 1958, a fire destroyed most of the surface improvements (Emmons and Calkins 1913; Sahinen 1935; Gilbert 1935; Wolle 1963). 

Many silver deposits have associated manganese. Thus, during World War 1, 200,000 tons of manganese ore was mined from the district. Manganese was discovered along the road from Algonquin to Philipsburg, and the Algonquin shaft was reopened to extract ore from a small high grade manganese ore body. In total, 5 mills were eventually constructed to process ore from 40 different manganese deposits reported to have 30 to 43% Mn. The most productive mines included Headlight, Scratch Awl, Trout, West Algonquin and Bernard. After the war, the district's manganese was used in dry-cell batteries. By 1939, the district had produced 477,000 tons of high grade manganese ore (Goddard 1941; Ageton 1923; Fritzberg 1927; Lorain 1950). 

Many of the Philipsburd deposits are interpreted as skarn and replacement deposits. Prinz (1967, p.4) shows a large, Philipsburg granodiroite batholith (Tertiary) intruding older sedimentary rocks east of the Philipsburg thrust fault with the district located along the western edge of the batholith and its ore deposits confined to granodiorite at the Granite-Bimetallic mine, while other deposits occur in older sedimentary rocks to the west near the contact zone, to the east of the thrust fault. 

Lower Cambrian to Pennsylvanian formations, as well rocks of the Precambrian Spokane Formation (Missoula Group), were intruded and metamorphosed by late Cretaceous to early Tertiary granodiorite. Replacement deposits occur in many carbonate sediments adjacent to the batholith, including rocks of the Silver Hill Formation (Cambrian), Hasmark Formation (Cambrian), Red Lion Formation (Cambrian), Maywood Formation (Devonian), Jefferson Limestone (Devonian), and Madison Limestone (Mississippian). 

"Montana Bar Fly", pencil sketch by the author
The replacement deposits (skarn) are formed of secondary tremolite, garnet, epidote, diopside, quartz, hornblende and scapolite. Intrusion of granodiorite was accompanied by folding with development of E-W trending, silver-bearing fissure veins in granodiorite, silver-bearing replacement veins in sediments, and silver-bearing replacement deposits along bedding planes in calcareous rocks. Some contact magnetite is also found and the silver veins and replacement deposits are associated with manganese; some of which were mined to depths of 1,000 feet (Sahinen 1935). 

Prinz (1967) classified these deposits as: (1) steeply-dipping quartz veins, (2) quartz veins along bedding, (3) manganese-rich replacement veins, and (4) contact metasomatic magnetite deposits. Bedding plane quartz veins can occur in limestone and marble and are common in the northern part of the district and mined primarily for silver. The steeply-dipping veins were mined primarily for silver, zinc and lead. The manganese-replacement deposits are occur as favorable beds adjacent to steeply dipping veins, but typically have little quartz, and are irregular in shape. These have rhodochrosite and Mn-dolomite, altered to Mn-oxides near the surface. 

Algonquin mine (46°19'39"N; 113°15’58"W). This mine is on the north side of Frost Creek, and was one of the first lode discoveries in the district, and is an iron- and manganese-stained, silver-bearing, replacement vein with cerrusite, pyromorphite and galena. The mine and its 20-stamp mill (expanded to 80 stamps), is credited with nearly half a million dollars of production in 1882 and 1883 (Emmons and Calkins 1913; Wolle 1963). 

The Algonquin was reopened to recover manganese during WWI. The mine reported eight years of production from 1923 to 1939. From 1937 to 1939, the West Algonquin and associated Bernard mine were reopened to recover 10,000 tons of manganese ore (Goddard 1941; Ageton 1923; WPA 1941). 

Granite-Bimetallic mine (sec 32 and NW sec. 33, T7N, R13W); (46°18'55"N; 113°14’41”W). The Granite and Bimetallic lodes were incorporated into the Granite-Bimetallic mine. From 1882 to 1978, the mines reached a depth of 2,625 feet, and included miles of tunnels. The Pliocene-age (5.33 to 2.58 Ma) ore occurs as intersecting veins hosted by granodiorite with chrysocolla, tetrahedrite and chalcopyrite, in a gangue of rhodochrosite, stibnite and calcite. The Granite vein is the largest and richest, and was stoped for 4,500 feet along strike. In places, it is as much as 19-feet-wide (Wolle 1963). 

As the mine reached the underlying oxidzed ore, extremely rich, primary sulfide ore was encountered. A 3-compartment Ruby shaft was sunk to 1,200 feet and 53,529,053 tons of ore was reduced to 3,930,329.69 ounces of silver and 8,583.48 ounces of gold. From 1885 to 1892, the mine was extremely prosperous and $20 million worth of metals was recovered. But, the Silver Crash of 1893, brought all operations to a halt. When the mine reopened three years later, its operations merged with the Bimetallic (Shoemaker 1894; Emmons and Calkins 1913). Total production (prior to 1913) for the two mines is estimated to be more than $32 million in silver and gold (Byrne 1900; 1902; Walsh 1906; 1910; 1912; Emmons and Calkins 1913). The vein had been stoped to 2,600 feet below the surface while total drifts and stopes measured an aggregate 20 miles (Emmons and Calkins 1913). 

Headlight mine (NE sec 30, T7N, R13W); (46o20’8”; 113o16’). The Headlight mine collars in sedimentary rock near the Philipsburg granodiorite, where the sedimentary rocks are metamorphosed to marble and banded garnet-amphibolite along the eastern limb of the Philipsburg anticline. The deposit trends N20oE trend. Host rocks include Jefferson Limestone, Maywood Formation and marble. Some ore contained as much as 200 opt Ag, 6% Cu, 7% Pb, 38 to 48% Mn, with minor antimony. Ore minerals include tetrahedrite, sphalerite, galena, chalcopyrite and wad (The Diggings, 2020). 

The property lies along Tower Road, and first produced silver in 1878. It reopened during 1908 to 1909, and because of a need for manganese during World War I, the mine reopened in 1917 and reported intermittent production between 1919 and 1928. In 1939, the mine produced 40 to 50 tons/day (Goddard 1941; WPA 1941). 

Hobo mine (NE & NW sec. 29, T7N, R13W); (46o20”N; 113o15’40”W). The Hobo mine is at Hasmark on Camp Creek about 1.5 miles east of Philipsburg. The property was staked in 1884, and dug into rich oxide ore. In 1898, a shaft was sunk (Byrne 1898; 1899; Walsh 1906; 1912; Emmons and Calkins 1913; WPA 1941) 

Like the Granite Mountain mine, the Hobo is on a silver-bearing vein in granodiorite, with a strike of N80oE and 75oSE dip. Oxidized ore in the upper levels carried lead-carbonate with some pyromorphite and wire silver. The richest sulfide ore was said to assay 100 ounces of silver and $3 gold per ton: $100,000 in ore was taken from the mine ( Emmons and Calkins 1913). 

The ore is composed of sphalerite, pyromorphite, galena, pyrite, arsenopyrite, chalcocite, realgar and tetrahedrite with gangue of quartz, stibnite and rhodochrosite. Associated rock in this area is diorite from the Upper Cretaceous epoch (100 to 66 Ma) (Marvin and others, 1995; The Diggings, 2020). 

Hope mine. All of the orebodies found on Hope Hill (46°20'28"N; 113°16'14"W) occur within the Jefferson Limestone. The Jefferson is overlain by the Madison Limestone. According to MinDat Org, the Hope was staked in 1865 and recovered low-grade ore until a high-grade deposit was discovered at depth in 1881. After the high grade ore was exhausted, the mine resumed mining lower-grade ore. 

In 1905-06 when silver prices again dropped and the Hope mine focused on extracting high-grade gold ore. Hope Hill is honeycombed with miles of stopes, cross cuts, drifts and tunnels. Much early wealth was taken from of the Porter incline shaft which connects to the Jubilee Tunnel. The Jubilee enters the west slope of the hill and was driven to the Shapleigh shaft. In its first 40 years of production, the mine produced a total of nearly $4 million in ore (Emmons and Calkins 1913). 

The deposit is described as a Ag-Cu-Pb-Au-Mn-Barite-Sb-As-F occurrence. Mineralization occurs as a Pliocene, poly-metallic, bedding-plane deposit in Late Devonian Jefferson limestone. The Jefferson Limestone is medium-grained, shaly, magnesian limestone, and lies on the crest and west limb of the Philipsburg Anticline, which trends N20oE with a 15oN pitch. 

Montana Gems (46°14'52"N; 113°35’31”W). Located 15 miles west-southwest of Philipsburg, sapphire-bearing gravels are available to the public. 

Red Lion mine (sec. 14, T16N, R12W; 46°16'9"N; 113°11'52"W). The Red Lion explored and mined a replacement deposit this is subparallel to bedding in the Hasmark Formation. This mineralized zone was 4 feet wide and contained 0.25 to 1.0 opt Au. 

Scratch Awl (46°19'54"N; 113°16’2"W). The Scratch Awl was one of the first silver lodes discovered in the district, and recorded production in 1875 when it was worked in a small way in association with the Salmon lode. The mine was reopened in 1917, when it supplied a moderate amount of manganese for the war effort. The mine recorded production nearly every year between 1919 and 1927. Between 1928 and 1939 the mine was reported to have produced 50,000 tons of manganese ore and 60,000 tons of silver-zinc ore. The mine is developed through a 500-foot shaft with three levels with 8,000 feet of workings. On the 500-foot level the mine is connected to the True Fissure lode to the north and to the Cliff lode on the west (Gilbert 1935; WPA 1941). According to Western Mining history dot com, the Scratch Awl is south of the True Fissure vein mine (46°20'3"N; 113°15’58”W). Three veins at the Scratch Awl mine contain lead and silver with manganese being part of the vein that trends easterly. The vein is 3 to 5 feet wide, and at least 180-feet long. The scratch Awl veins was partially mined in the Sharktown tunnel and the 500-foot level tunnel connects with the true Fissure Vein to the north and the Cliff Shaft to the west. 

Trout mine (46°19'48"N; 113°15'56"W), originally staked as the Speckled Trout, lies on the ridge between Camp and Frost Creeks, between the ghost-towns of Hasmark and Tower. The North-West Company was organized to develop the Trout, Poorman's Joy, Kitty Clyde and Pocahontas mines. Other mines in the area included the Salmon, Gem, the Little Emma and the Osage (Wolle 1963; Emmons and Calkins 1913). 

The mine focused on a silver-bearing replacement vein in limestone and shale. The Phillipsburg batholith contact with sedimentary rocks is located 600 feet west of the mine. The ore was extracted from a shaft primarily from the 150 and 200-foot levels and contained galena, zinc blende, pyrite, ruby silver and gray copper ore carrying 20 to 150 ounces silver and $3 or $4 gold per ton. It was estimated that in its early days, the mine produced half a million dollars worth of ore (Walsh 1910; 1912; Emmons and Calkins 1913). 

In 1917, the Gem shaft was reopened and manganese mining commenced. From September 1917 to November 1918, 30,000 tons of ore were extracted. During this period several thousand tons of ore was also extracted out of the Pocahontas shaft. From 1924 to 1937 the Trout properties were in continuous operation and their combined production was 166,034 tons of manganese ore and 240,000 tons of silver-zinc ore (Goddard 1941; Gilbert 1935). 

Emmons and Calkins (1913) separate mines with silver-bearing veins in granite to include: (1) Puritan, Silver Chief, Mitchell, San Francisco, Granite Belle, Royal Metals Tunnel, Three Metals and Salt Hill Tunnels, Pearl mine, Annie Marony claim and Young American Claim. On silver-bearing replacement veins the Gem, Blackmail, Salmon, Headlight, Midnight, Imperial, True Fissure, Levi Burr, Terrid, Cliff, Sanders, Mystery Tunnel, and Basin mine. Mines with silver-bearing deposits in bedding planes of calcareous rocks include: (2) Sweet Home, Cadgie Taylor, Two Per Cent, New Hope and Copper Jack. Mines with magnetite deposits include: (3) Redemption Iron Mine and the Silver Lode Iron Mine. Gilbert (1935) described additional manganese mines including: (4) the Moorlight, Mont-York and Two Percent mines. 

Whitehall district
"Sunday Drive in Wyoming", pencil sketch
by the author.

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.

Montana Tunnels
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).

Zortman-Landusky district
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.

"Queen of Hearts", color pencil sketch by the author
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 intrusive complex at Zartman is one of twelve similar complexes aligned in a northeast-trending belt within the Central Montana Alkalic Province (Hastings, 1988). These deposits are structurally controlled and vary from narrow veins with restricted stockworks to oxidized low-grade stockworks, breccias and intensely fractured sheeted zones near the surface. Primary mineralization consists of native gold and silver associated with pyrite, sylvanite, calaverite and hessite. Coarse-grained gold is found in the deeper veins, but scarce to nonexistent in shallow, lower-grade zones where it is found as micron-sized particles. Gangue (non-economic) minerals are reported as kaolinite, quartz, marcasite, pyrite, calcite, arsenopyrite and fluorite.

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).

Radersburg district
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.

Kendall-North Moccasin
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.

Some high grade ore averaged 0.25 opt Au but much of the ore was lower grade. Harry Kendall began using cyanide in 1901 to dissolve the gold to extract the metal in a 500 ton-per-day mill. In the early part of the 20th century, mining was conducted without regard to the environment as there were no scientific studies on environmental issues and the government encouraged prospectors to mine without any regard to the environment. Thus the gold was leached in cyanide tanks and after the precious metal was extracted from pregnant solutions, the waste cyanide was dumped into nearby drainages. This process continued from 1901 until 1921 and approximately 450,000 ounces ($495 million at today’s prices) were recovered. Just 0.5 miles north of the Kendall mine, the Barnes-King mine was developed and recovered gold in a 100 tpd mill. Like the Kendall, this property also dumped waste cyanide into nearby drainages until the mine shut down in 1920.

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.

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", color pencil sketch by the author

The Boss Tweed lode was located in 1875. Other properties included Clipper, Belle, Garnet, Eva May, Willow Creek, Oregon, Old Joe, Ben Harrison, Lone Wolf, Mountain Cliff, White Pine, Mammoth, Bozeman, Ned and White Pine (Leeson 1885; Wolle 1963). A mill constructed at the Crevice mine. The spring of 1877 brought a second stampede of miners to the district and by the end of the year five mills were operating with a combined total of 56 stamps.

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).

"Underground at the Comstock with Jay", 
pencil sketch by the author
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.

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 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 author - searching for gold!

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!

Other districts
"Shorty", pencil sketch by the author
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.