 |
manganese carbonate, in Mineral Museum, Butte |
Manganese is indispensable in the production of steel and is thus essential to our Nation's economy. The chief value of manganese is as a desulfurizer, and more than 13 pounds are consumed in the production of each ton of steel. Some is also used as an alloying metal in high-strength steels. More than 95 percent of the manganese consumed in the United States is for metallurgical purposes; the remainder is used, generally as oxide ore or concentrate, as the depolarizer in dry-cell batteries, in the manufacture of manganese chemicals, as a drying agent in paints and varnishes, as a pigment or to neutralize the effects of iron in glass making and ceramics, and in the leaching of uranium ores.
Manganese occurs in a variety of minerals in the earth's crust, but insofar as ore deposits are concerned only two types are important: (1) the manganese oxide minerals, which are too numerous to mention individually here, and (2) the manganese carbonate, rhodochrosite. Some manganese deposits are sedimentary in origin; others are in veins or replacement deposits that formed from hydrothermal solutions. Later action by ground water modified many deposits of both types by; converting rhodochrosite and manganese silicates to manganese oxide minerals changing original oxide minerals to different oxide minerals, and enriching or concentrating manganese.
Most of the manganese reserves of the United
States are in low-grade sedimentary deposits which, except for the manganiferous
iron ores of the Cuyuna Range, Minn., are not economical to mine at the present
time. Domestic reserves of high-grade manganese ore are limited, and the United
States therefore relies almost entirely on imported manganese. In 1958 production
of ores and concentrates (+35 percent manganese) by domestic mines reached 22
percent of consumption (DeHuff and Fratta, 1959, p. 724); this was accomplished
however, only because of Government purchases at premium prices for stockpiling.
After Government purchasing ceased, domestic production declined sharply and
in 1961 amounted to less than 3 percent of consumption. By the end of 1961,
Montana was the only State producing high-grade manganese ores or concentrates.
Montana is the leading State in total production of this type of material, most
of which has come from two districts--Philipsburg and Butte (fig. 25).
In 1999, the U.S. was wholly dependent on imports for its manganese needs; Gabon
was the leading source of imports.
The Philipsburg district (fig. 25, locality No. 1), which started as a silver camp in 1864, first produced manganese in 1900 (Pardee, 1922, p. 146), and became the country's leading producer of high-grade manganese ore during World War I. This ore, all oxide, was used mainly for metallurgical purposes. After the war, the district was unable to compete in the metallurgical market because of its distance from steel-producing centers; however, it was found that manganese oxide concentrates from the district were well suited for the manufacture of dry-cell batteries. Since then, Philipsburg has been the leading, and for the most part sole, domestic source of natural battery-grade manganese dioxide. Oxide ore is upgraded in local mills to battery-grade concentrates containing 65 to 70 percent manganese dioxide. Middlings containing 10 to 35 percent manganese are also marketed. Through 1961, production from the Philipsburg district, including both oxide and carbonate ores, has accounted for approximately half of Montana's total production of more than 3,230,000 tons of manganese ore.
The Philipsburg district straddles part of the western border of an early Tertiary granodiorite batholith that was intruded into folded and faulted Precambrian and Paleozoic sedimentary rocks (Emmons and Calkins, 1913). Silver- and zinc-bearing quartz veins cut both the granodiorite and the sedimentary rocks. Although the veins have yielded a small amount of manganese ore, most has come from irregular manganese-rich replacement deposits which are distributed erratically in favorable limestone and marble beds adjacent to the quartz veins (Goddard, 1940, pp. 157-202). The primary manganese minerals in these deposits are the carbonates, rhodochrosite, and manganoan dolomite, which were deposited from hydrothermal solutions. Near the surface and to depths varying from 100 to more than 850 feet, the manganese carbonates were oxidized by ground water to manganese oxide minerals. The bulk of the ore produced from the district has been oxide; some carbonate was mined during World War II and for Government stockpiles in the early and mid-1950's, but little, if any, carbonate has been sold commercially. Reserves of manganese oxide and carbonate ore at Philipsburg have been estimated at 710,000 long tons averaging 22.5 percent manganese (Hewett and others, 1956, p. 214).
The veins around the periphery of the famous copper, silver, and zinc district of Butte (No. 2) contain abundant rhodochrosite, and the area west and south of the heart of the district has yielded considerable manganese ore. The veins are in an early Tertiary quartz monzonite batholith, and in the manganese-rich areas they contain rhodochrosite, silver, lead, and zinc sulfides in a quartz gangue. The depth of oxidation at Butte is relatively shallow; thus manganese production from the district has been principally carbonate ore with only a minor amount of oxide.
Manganese was first mined in Butte during World War I, but, as with Philipsburg, production declined after the armistice. In the late 1920's and the 1930's, carbonate ore containing 38 percent manganese was roasted in revolving kilns to produce oxide nodules containing 56 percent manganese. However, reserves of these high-grade ores were limited and manganese production was small until 1941 when the Anaconda Co. built a manganese concentrating plant and its own kilns. Ores averaging 18 percent manganese were then treated by flotation to recover the associated zinc, lead, and silver sulfides and to produce rhodochrosite concentrates containing 38 percent manganese. The rhodochrosite concentrates were dried and then roasted to drive off CO2 and produce nodules of manganese oxide grading 57 to 60 percent manganese (U.S. Congress, House Committee on Public Lands, Subcommittee on Mines and Mining, 1948, pp. 279-298). The high manganese and low impurity content of these nodules made them well suited for the manufacture of ferromanganese, the most common form in which manganese is added to steel, and in 1946 the Anaconda Co. began production of ferromanganese from its own nodules in an electric furnace at Great Palls. Furnaces were also built at Anaconda, and in the late 1940's and early 1950's Anaconda consumed a large quantity of its own nodules to make ferromanganese. However, high freight rates to the steel-producing centers in the East and competition from low-cost imported manganese ore forced Anaconda to reduce production in the late 1950's. The company's last remaining manganese mine, the Emma, was closed during the strike in 1959 and has not been reopened. The ferromanganese furnaces have been operated intermittently since then on stockpiled ore.
Many mines in the west Butte area other than those operated by the Anaconda Co. have also produced manganese ore, chiefly during times when the Government was paying premium prices. Reserves of the Butte district were reported in 1956 to be 4,460,000 short tons of carbonate ore averaging 14 percent manganese (Hewett and others, 1956, p. 214). Reserves of oxide ore are nil.
Manganese also occurs at many other places in southwestern Montana (Pardee, 1919; 1922, pp. 177-179; Sahinen and Crowley, 1959, p. 21), and some areas have yielded a little ore. Hydrothermal deposits similar to those at Butte or Philipsburg occur in the Wickes (Fig. 25, No. 3), and Cataract Creek (No. 4), districts between Butte and Helena, in the Neihart district in the southeastern part of Cascade County (No. 5), in the Castle Mountains 10 miles southeast of White Sulphur Springs (No. 6), in the Cramer Creek district east of Missoula (No. 7), in west-central Gallatin County (No. 8), and at several places in Beaverhead and Madison Counties (Nos. 9, 10, 11, and 12). Irregular lenses or pods of manganese that follow bedding in sedimentary rocks occur in central and northwestern Meagher County (Nos. 13, 14, and 15), southeastern Lewis and Clark County (No. 16), and in a few places in Beaverhead and Madison Counties (Nos. 17, 18, 19, and 20). It is not known whether the manganese was concentrated by ground water from that occurring in surrounding or overlying beds or whether it was introduced by hydrothermal solutions. A manganiferous bed of iron ore has been reported near Renova in northern Madison County (No. 21) (Pardee, 1919, p. 132).
Molybdenum is a white metal that is commonly used as an alloy to increase hardness, strength, and resistance to corrosion of steel, particularly at elevated temperatures. It is also used in electrical equipment and in aircraft and missile parts.
The most important ore mineral is molybdenite (MoS2). More than half of the world supply comes from molybdenite ores mined at Climax, Colo. Molybdenite is also an important byproduct of porphyry copper deposits. Less important are contact metamorphic tungsten deposits that yield molybdenum as a byproduct.
Montana has not produced molybdenum, although
several low-grade deposits are known. The Big Ben deposit near Neihart, Cascade
County, has been prospected and drilled, and it appears to contain between 3
and 4 million tons of material that averages 0.2 to 0.3 percent molybdenum (fig.
26, northeasternmost locality) (Creasey and Scholz, 1945). Known tonnage and
grade of the material do not favor production at present prices and mining costs,
but further exploration may increase the estimates of reserves. (In
2001, the price of molybdenum was $5.20 per kilogram. Byproduct molybdenum was
produced in Montana, as well as Arizona, New Mexico, and Utah; primary mine
production came from Colorado, New Mexico, and Idaho. The U.S. in 1999 was a
net exporter of molybdenum.)
Molybdenite occurs with pyrite and chalcopyrite in quartz veins at the Bismarck mine, Madison County. The amount of ore is not known, and the grade appears to be low (Tansley and others, 1933, p. 31).
The copper deposits at Butte contain traces of molybdenite, and in a few local areas it is reported to be abundant (Weed, 1912, p. 79). None has been recovered as a byproduct, however.
Molybdenum is known to occur in aplitic quartz monzonite in NE 1/4 sec. 9, T. 1 N., R. 7 W., at the head of Blacktail Deer Creek about 10 miles south of Butte, and in the Emigrant Gulch area, Park County (Sahinen, U. M., written communication, 1962). The potential of these occurrences is unknown, but is probably not great. Neither occurrence is shown on the map.
An additional potential source of molybdenum in Montana, still essentially untested, is the occurrence of powellite, the calcium molybdate, in tactite bodies associated with Mesozoic intrusives. (See the chapter on tungsten for a more comprehensive discussion of these deposits.)
(By S. L. Groff, Montana Bureau of Mines and Geology, Butte, Mont.)
Columbite, the commonest ore mineral of niobium (columbium) is a niobate-tantalate of iron and manganese. It is an end member of an isomorphous mineral series (Fe,Mn)(Nb,Ta)2O8, of which tantalite, the principal ore of tantalum, is the other. Columbite is usually recognized by its iron-black, commonly iridescent color.
The minerals of the columbite-tantalite series
are commonly found in pegmatites. The world's main source of supply comes from
Nigerian alluvial deposits formed by the weathering of pegmatite dikes. Niobium
is a steel-gray, lustrous metallic element used principally as an alloying agent
in metals for jet aircraft engines. Niobium alloys are stable and retain their
strength at temperatures up to 1,550ºF. Minor uses of the metal include applications
in the vacuum-tube industry, in tungsten-carbide cutting tools, and as a shielding
material in some types of nuclear reactors.
Purchasers of columbite are the Electro-Metallurgica1 Division, Union Carbide & Carbon Co., Niagara Falls, N.Y.; and the Fansteel Metallurgical Corp., North Chicago, Ill. Columbite ore (65 percent pentoxide) was being purchased in October 1962, at the following prices:
Ratio of Nb2O5 to Ta2O5:
Per pound
10:1____________________________________ $1.10-$1.25
8.5:1___________________________________ 1.00- 1.10
(In 2000, the price of columbite was $6.25 per pound.)
There is no record of columbite production from Montana, but columbite and other niobium-bearing minerals are found in several localities in Montana (fig. 27). Minor amounts of columbite were reported in carbonate rocks in the Rocky Boy stock, Bearpaw Mountains, Hill County. Fergusonite--a complex mineral containing rare earths, niobium, and uranium--has been found in the Sappington
pegmatites in northern Madison County, and in pegmatites near Hamilton, Ravalli County, and in placers at the head of California Gulch in Madison County (Heinrich, 1949, pp. 31-32).
The largest deposit in Montana is on Sheep Creek, a tributary of the West Fork, Bitterroot River, in southern Ravalli County (Crowley, 1960). In this deposit columbite is associated with euxenite and fersmite, along with the rare-earth minerals ancylite, allanite, and monazite. There has been extensive development, but the deposit is apparently subcommercial.
(By F. C. Armstrong, U.S. Geological Survey, Spokane, Wash.)
Optical calcite is the transparent variety of calcite (CaCO3). It is used in polarizing microscopes, polariscopes, colorimeters, saccharimeters, and other optical devices. First-grade material, often referred to as Iceland spar, must be colorless, water clear, and free from cracks, twinning, inclusions, and other flaws.
Both before and during World War II a small but unknown amount of first-grade material was mined in Montana. For a short time during World War II optical calcite was mined in Montana for use by the Navy in a special gun sight. Calcite for this purpose did not have to meet several of the specifications required of Iceland spar. During the period 1942-44 about 7,400
 |
Veins containing optical calcite occur in Park and Sweet Grass Counties, Mont. (fig. 28), and have been described by Stoll and Armstrong (1958). Most of the optical calcite has been obtained from veins cutting the sedimentary rocks of the Livingston group of Late Cretaceous and Tertiary age. The veins occupy faults. The productive veins are large; many of them are conspicuously banded parallel to their walls, and the individual bands exhibit comb structure. Vugs created by incomplete filling of fissures have been the source of all optical calcite mined; most of the usable material came from the clear crystal tips that project into the vugs. The calcite veins were deposited in open fractures by rising hydrothermal solutions of low temperature and pressure. The veins appear genetically related to the local igneous activity, and may have been deposited during an earlier hypogene period of carbonate mineralization by ancient hot springs similar to the ones in the area today.
The demand for optical calcite has never exceeded several hundred pounds annually, and sales are usually individually negotiated. A factor adversely effecting the market is the increasing use of synthetic Polaroid in many instruments that formerly required calcite. If, however, uses demanding the unique qualities of optical calcite develop to the extent that cost is not a major factor, a limited amount of optical calcite could be recovered from the veins in Montana.
(By S. L. Groff, Montana Bureau of Mines and Geology, Butte, Mont.)
Pegmatite minerals that may attain commercial
importance include potash feldspar, quartz, muscovite, and lithium silicates,
with smaller amounts of niobium-tantalum, rare-earth minerals, beryl, cassiterite,
and wolframite. These minerals occur in pegmatite dikes, which are coarsely
crystalline igneous rocks commonly associated with larger intrusive bodies of
finer-grained rocks. Pegmatite bodies are believed to have formed during a late
stage in, the normal sequence of crystallization of a magma, when residual fluids
were sufficiently enriched in volatile materials to permit the formation of
coarse-grained rocks more or less equivalent in composition to the parent rock.
Montana has, thus far, produced little material from pegmatites and of the pegmatite
minerals only mica, feldspar, and beryl are discussed here.
MICA
"Mica" is the name given to a group of hydrous aluminum silicates containing varying amounts of iron and magnesium. The more important members of this group are muscovite (white mica), phlogopite (amber mica), biotite (black mica), vermiculite (expanding mica), and lepidolite (lithia mica). Of these only vermiculite and muscovite have been mined in Montana. Vermiculite is not a pegmatite mica, and is treated elsewhere in this publication.
Muscovite mica has the general formula K2Al4(0H)4[(Al2Si6)O20], but minor amounts of other elements affect its color and cause slight differences in its properties. Muscovite is valuable for its very low heat and electrical conductivity and the perfect basal cleavage which permits it to be split into flexible sheets as little as 1/1000 inch in thickness. The mineral occurs as rough crystals, called books, in pegmatites. Books with a high proportion of structural flaws, or too small for minimum sheet size, fire classified as "scrap" or grinding mica.
Sheet mica is mostly used for electrical insulation and has many applications in electrical and electronic instruments and utensils. Mica board is made up of thin splittings bonded together and is widely used as forms for mounting electrical equipment. Ground mica is used in molded insulators, or it may be used in roof coating, plaster, paints, lubricants, or as a binder and filler.
Prices (North Carolina district) as of October 18, 1962, are quoted here as a general guide for prospective producers. Prices for punch mica (material that will yield trimmed, unflawed books of less than 1.5 x 2 inches) and sheet mica vary considerably, depending on the degree of stain. Clear punch runs 7¢ to 12¢ per pound; clear sheet mica prices depend on the size of the trimmed sheets.
Size
Per pound
1.5 by 2 inches__________ $0.07-$1.10
3 by 4 inches____________ $2.00-$2.60
2 by 2 inches____________ $1.10-$1.60
3 by 5 inches____________ $2.60-$3.00
2 by 3 inches____________ $1.60-$2.00
4 by 6 inches____________ $2.75-$4.00
3 by 3 inches____________ $1.80-$2.30
6 by 8 inches____________ $4.00-$8.00
Scrap or grinding-quality mica is valued at $20 to $30 per short ton, depending on relative amounts of other mineral impurities. The nearest scrap mica buyer is in Colorado. Buyers of sheet mica are all east of the Mississippi River. (In 2001, the price of scrap mica was $140 per ton.)
Mica-bearing pegmatites are shown on figure 29. Some 10 tons of hand cobbed books were marketed from the Sappington deposit (fig. 1, locality No. 1) in northeast Madison County, and a small amount was mined from the Dulea and Montana deposit (No. 2) near Virginia City (Heinrich, 1949, p. 23; Stoll, 1950, p. 57). Beginning in 1958 small quantities of hand sorted mica were produced from the Thumper Lode (No. 4), Gallatin County, and from a deposit 15 miles south of Ennis (No. 3), Madison County. Small amounts have also been produced from the San Miguel district (No. 5), Judith Basin County (Robertson and Roby, 1952, p. 46).
FELDSPAR
The feldspar minerals are potassium, sodium, or calcium aluminum silicates and are the most abundant of the rock-forming minerals. The relative concentration of potash, soda, and lime determine the properties of the minerals. Silicic pegmatites may contain a concentration of large crystals of potash feldspar together with soda feldspar.
The chief use for feldspar is in ground form as a constituent of enamels, glasses, and pottery. It is the chief source of aluminum in glass and is also the chief flux. Additional uses are as fillers, bases for scouring powder, ceramic binders, poultry grit, and roofing granules. In the ceramic industry potash feldspar, chiefly the variety microcline, is used almost exclusively but soda feldspar can be used in enamels and glazes. Both types have been found suitable for glass manufacturing.
Most feldspar produced commercially has come from pegmatites containing concentrations of very large crystals. Hand sorting is necessary to separate the feldspar from the other minerals. Depletion of rich deposits throughout the Nation brought about a search for methods of beneficiation; as a result the flotation method of feldspar separation and concentration is now a common process. Feldspar is sold in ground form. Maximum price is $17 per short ton. (In 2001, the price of feldspar was $54 per ton.)
No feldspar has been produced in Montana, mainly because of distance and shipping costs to areas of industrial use. Feldspar-bearing pegmatites are common in west and southwest Montana; most of those that have produced mica could also produce feldspar. Many more exist than are shown on the map (fig. 29).
BERYL
Beryl, found mainly in silicic pegmatites, is at present virtually the only ore mineral of the important strategic metal beryllium. Because of the need for beryllium, low-grade deposits of other beryllium minerals such as phenacite and bertrandite are being investigated in Nevada, Utah, Colorado, and elsewhere. Deposits of these minerals are not yet known in Montana.
Beryl, Be3Al2(SiO3)6, occurs as prismatic hexagonal crystals that may sometimes be confused with quartz. Beryl is brittle and hard (7.5-8), and, when pure, contains 13.9 percent BeO, though the usual range is 9-11 percent. It is commonly green, but occurs in several varieties with colors of emerald green, yellow, blue, white, and even pale-rose red. The emerald-green clear beryl (emerald) is one of the most valuable of all precious stones. Other gem varieties are the bluish-green "aquamarine," the yellowish "golden beryl" or "heliodor," and the rose-colored "morganite." Helvite, a beryllium mineral not associated with pegmatites, has been found at Butte, Mont.
Beryllium is the lightest of all metals except lithium. The largest single use is in copper alloys where beryllium imparts properties to copper somewhat analogous to those which carbon imparts to steel. The major uses of the pure metal are in X-ray tubes and in nuclear reactors. Beryllium oxide has a melting point of 2,400ºC., low thermal expansion, high electrical resistance, and resists abrupt temperature changes, properties that make it useful for various purposes in rockets and missiles.
Beryllium is priced on the basis of BeO content. This is presently $30 to $32 per unit for ore containing 10 to 12 percent BeO, or $300 to $320 per ton for essentially pure beryl. (In 2001, the price of beryllium oxide was $100 per ton.)
No beryl has been produced in Montana, but there are three known occurrences. It occurs in pegmatites of the Tobacco Root Mountains (No. 7), Madison County, and near Monarch (No. 6) 46 miles south of Great Falls. A pegmatite containing scattered green prisms of beryl up to 2 or 3 inches long in a clear quartz-feldspar matrix is located near Sula (No. 8) in southern Ravalli County. The pegmatites of the Sula area are perhaps the most promising in Montana.
Phosphorus, meaning light bringer, burns spontaneously in air. Because of its great affinity for oxygen, it is never found uncombined in nature but almost always as a phosphate. Having many valence states, it combines with many metals and nonmetals to form a great many compounds for industry. Phosphorus is so widely used that phosphorus compounds affect the everyday life of most Americans and much of American industry. It is an essential constituent of all life, hence its principal use is in fertilizers, yet the most commonly produced white variety of the element is highly toxic and must be used with appropriate caution. A large part of the elemental phosphorus produced in furnace treatment is used in the manufacture of detergents.
Most phosphorus in nature occurs as one of the apatite series minerals. Common apatite or fluorapatite, (CaF)Ca4(P04)3, is present in almost all igneous rocks, and this is the ultimate source of most phosphate on earth. It contains about 42 percent P205 (or 18 percent P), 50 percent CaO, and 8 percent CaF2.
The largest, richest, and most important occurrences of phosphate, including those of Montana, are the sedimentary apatite deposits (McKelvey and others, 1953a), composed of the mineral carbonate, fluorapatite. These deposits are marine in origin, deposition having occurred from cold phosphate-rich waters that upwelled from the ocean reservoir onto a Continental Shelf environment (McKelvey and others, 1953b).
Very little raw phosphate rock (phosphorite) is used directly. For most of the fertilizer industry sulfuric acid is added, freeing the phosphate of its combined fluorine and forming synthetic gypsum, which is usually filtered off. In the elemental phosphorus industry, phosphorite is smelted in an electric furnace, vaporizing the phosphorus which is then condensed and collected under water (see Ruhlman, 1958, and Waggaman and Ruhlman, 1960, for summary of industry methods). Most of the net value of phosphate stems directly from such treatment.
The United States ranks first in world production with 42 percent of the nearly 44 million tons of phosphate rock produced in 1961 (Lewis and Tucker, 1962). Montana ranks fourth among the States after Florida (74 percent), Tennessee (12 percent), and Idaho (9 percent). Montana and Wyoming production in 1961 was more than 1 million tons, having a value of more than $8 million, and the larger part of this came from Montana (data for the two States are combined to avoid disclosure of information from individual companies). In 1961, 55 percent of U.S. production was used in agriculture, 23 percent in industry, and 22 percent was exported (most of that was also used in agriculture). In 1999, only 4 states produced phosphate: Florida and North Carolina accounted for 85%, with Idaho and Utah providing the remainder. 41 million tons of marketable material was valued at $1.21 billion; the U.S. went from being a net exporter in 1995 to a 7% reliance on imports in 1999.
It should be noted that some apatite is present in virtually all rock and therefore all soils. This helps make the soil of any area a valuable resource. If the content is insufficient for agricultural needs (this may be due to cropping), as is true for some Montana soils, phosphate must be added for successful agriculture. It is partly in satisfaction of this type of need and partly to satisfy an ever-growing chemical industry that the minable phosphate deposits of southwest Montana attain their importance as one of the State's valuable mineral resources.
The western phosphate field of the United States, extending from western Montana through southeastern Idaho and western Wyoming to northern Utah, contains one of the world's largest occurrences of sedimentary apatite. These have been investigated extensively since World War II by the U.S. Geological Survey (Swanson and others, 1953); these investigations included the collection of more than 1,500 channel samples from more than 60 localities in Montana, and this report is based on these studies.
Most of the phosphate in the western phosphate field occurs in the Meade Peak (lower) and the Retort (upper) phosphatic shale members of the Phosphoria formation, which are part of a complex sequence of sedimentary rocks of Permian age (McKelvey and others, 1959). The Permian rocks near Lima, at the southwest corner, are more than 800 feet thick. They thin northward and eastward to near zero northeast of Three Forks and northwest of Drummond, partly by unconformable relations to underlying and overlying formations. The Meade Peak member pinches out between Dillon and Butte, but the Retort member has been identified in every area of southwest Montana in which Permian rocks have been found.
The phosphatic shale members are composed of
interbedded shaly siltstone, phosphorite, and some chert, dolomite, and sandstone.
Their dark-brownish-gray to black color is due chiefly to included organic matter.
The phosphate is mostly pelletal to oolitic, with grains in the fine- to medium-sand-size
range (1/8-1/2 mm), but some is nodular and some is composed of phosphatic shell
and bone fragments. The grains occur scattered through other rock types, in
thin layers interbedded with other rocks, and in thicker layers of mostly apatite.
Beds of pure carbonate-fluorapatite (39.1 percent P2O5)
do not occur. Minable thicknesses almost everywhere include thin layers rich
in other rock materials that dilute the ore, and the quality of an ore is chiefly
a function of the amount, and partly of the identity, of such dilution. Thus
carbonate is undesirable for both acid and furnace treatment. Silica, on the
other hand, is relatively inert in the acid treatment, but in the furnace if
acts as a flux to help smelt the ore so it is a valuable ingredient and must
be present in furnace feed in a fairly definite ratio to the phosphate.
Sedimentary rocks of Paleozoic and Mesozoic age, including the Permian strata, accumulated over all of southwest Montana, covering the older rocks like a blanket 10,000 to 15,000 feet thick. Later these rocks were folded and faulted and invaded locally by granite bodies. The region was uplifted, and much of the material from the higher areas was eroded, including the Permian rocks from some fairly large parts of the region. As a result the Permian rocks crop out in a narrow band that winds back and forth across southwest Montana, interrupted by faults, igneous intrusions, or later cover (fig. 30). In some areas these rocks occur high on the mountains, in others they lie deeply buried. In downfolded areas they are continuous from one outcrop band to the next, in places beneath many thousands of feet of younger rocks.
above entry level | 1st 160 ft below entry level | tonnage | |
| Rock with >31% P2O5 | |||
| Retort member | 40 | 16 | 450 |
| Meade Peak member | 35 | 4 | 150 |
| TOTAL | 75 | 20 | 600 |
| Rock with >24% P2O5 (1) | |||
| Retort member | 200 | 53 | 4,000 |
| Meade Peak member | 150 | 32 | 3,500 |
| TOTAL | 350 | 85 | 7,500 |
| Rock with >18% P2O5 (1) | |||
| Retort member | 820 | 250 | 17,000 |
| Meade Peak member | 230 | 50 | 7,250 |
| TOTAL | 1,050 | 300 | 24,250 |
| (1) includes tonnages in rock of higher grade |
A summary of the phosphate industry of Montana (Crowley, 1962) shows the rapid expansion of this industry since 1940. Phosphorite is being mined from the Retort member near Melrose for treatment in the electric furnaces of the Stauffer Chemical Co.'s Victor Chemical Works at Silver Bow southwest of Butte. It is mined at several places north of Garrison by the Montana Phosphate Products Co. for treatment in Canada at the fertilizer plants of the Consolidated Mining & Smelting Co. of Canada, Ltd. Another mine is being developed near Maxville to produce rock for shipment to Canada. The Relyea mine north of Garrison supplies phosphate reek to other producers. Phosphorite has been mined from the Meade Peak member in the Centennial Range west of Yellowstone Park by the J.R. Simplot Co. Many other small mines have produced limited tonnages of phosphate.
Many other elements of economic interest occur with the western phosphate. Fluorine (see chapter on fluorite) is part of the phosphate mineral and is present in the approximate ratio of 1 part F for every 10 parts P2O5. There is thus 1 ton of P for every 32 tons of rock containing 31 percent P2O5, for every 40 tons at 25 percent grade, or for every 50 tons at 20 percent grade. The reserves of fluorine therefore, are very large. Fluorine is being recovered from some of the Florida phosphorite, but none has been recovered on a commercial basis from Montana phosphorite.
Uranium, vanadium, chromium, nickel, molybdenum, and rare earths are present in small amounts in most western phosphorite. Vanadium has been recovered from phosphorite mined in Idaho (Care, 1949). Uranium (see chapter on uranium) is being recovered from some Florida phosphorite, in which it occurs in comparable amounts to western phosphorite. Vanadium concentrates in ferrophosphorus, and it is being recovered from ferrophosphorus produced by electric furnace in Idaho (Fulkerson and others, 1962). Chromium and nickel also concentrate in the ferrophosphorus and can be recovered therefrom (Banning and Rasmussen, 1951). Data on the content of these metals in ferrophosphorus produced in Montana are not available.
Approximately 1.25 tons of synthetic gypsum is produced in the sulfuric acid treatment of phosphorite to make 1 ton of triple superphosphate (high quality) fertilizer. This is removed by filtration and is discarded, but it could be recovered and used in a variety of building materials and for agricultural purposes.
Oil shale occurs in beds associated with much of the phosphorite in the Retort member in southwest Montana (Condit, 1920) (see chapter on oil shale). Beds 18 feet thick contain more than 18 gallons per ton at one locality and beds 24 feet thick contain nearly 15 gallons at another.
As the phosphate in the western phosphate field is a bedded deposit, like coal or oil shale, application of neither the lode nor the placer laws proved satisfactory for mining purposes. Western phosphorite lands were therefore withdrawn from entry (Gale and Richards, 1910), but they are available under lease from the Government or the State for mining of phosphate, for which a small royalty is charged (see Crowley, 1962, p. 4). Most of western Montana underlain by Permian rocks is public land (Willey and others, 1954) and is subject to the leasing regulations.
(By A. F. Bateman, Jr., U.S. Geological Survey, Great Falls, Mont.)
Halite (NaCl) or rock salt is present in the subsurface throughout most of the Williston Basin. In the Montana portion of the basin there are 10 separate salt beds ranging in age from Devonian to Triassic (Pierce and Rich, 1962, p. 51-61; Anderson and Hansen, 1957).
The Prairie formation (Prairie evaporite formation of Canada) of Middle Devonian
age, consists of a rather thin lower member that is mostly anhydrite and dolomite
interbedded with shale and thin beds of halite and a thicker upper member that
is mostly halite with some interbedded anhydrite (Baillie, 1955, p. 590-597;
Sandberg and Hammond, 1958, p. 2304, 2307; Sandberg, 1961, p. 112-114). On the
flanks of the Williston Basin an anhydrite bed overlies the salt. 
The salt is colorless to moderate reddish-orange and greyish-red, and varies
from fine- to very coarse-grained, with the coarse-grained material chiefly
in the upper part of the unit. The salt member is more restricted than the lower
member and in Montana underlies about 3,800 square miles in Sheridan, Daniels,
Roosevelt, and Richland Counties, as is shown on figure 31. The formation attains
a thickness of about 600 feet where best developed in south-central Saskatchewan,
but in Montana it has a maximum thickness of about 250 feet in the northeast
corner of the State, from which it thins southward and westward. West of this
area, the salt has been removed by solution as a result of movement of ground
water across this part of Montana in a general northeast direction (Milner,
1956, p. 111). Depth below ground surface ranges from 5,000 to 11,600 feet.
In Saskatchewan, the potash minerals sylvite (KCl) and carnallite (KMgCl3.6H2O) occur with the halite in at least three zones in the upper 200 feet of the salt member (Goudie, 1957, unpublished report). These zones are continuous and recognizable over large areas. To date potash minerals have not been reported in Montana, but no special search for potash seems to have been made. Tracing of the potash-bearing zones into Montana is difficult because of wide well-spacing, but two zones have been identified tentatively in a test for oil and gas in Sheridan County. In the last 3 years, potash has been produced commercially in Saskatchewan, where reserves of recoverable ore containing 25 percent or more potash, and lying at depths of less than 3,500 feet, are estimated at 6.4 billion tons (Pearson, 1960, p. 2).
During deposition of the Charles formation of Early Mississippian age, environmental conditions in the Williston Basin fluctuated between penesaline and evaporitic, resulting in a thick alternating sequence of carbonates (limestone and dolomite) and evaporites (anhydrite and salt) (Anderson, 1958; Andrichuck, 1955, p. 2175-2182, 2199-2206; Nordquist, 1953, p. 68). In Montana there are six salt beds designated "A" through "F" (Pierce and Rich, 1962, p. 56-57, Anderson and Hansen, 1957, pls. 1-2, figs. 4-9). The "A" bed is the thickest and underlies the greatest area. Data on the salt beds are as follows:
thickness, ft | thickness, ft | sq mi |
cover, ft | |
| A | 30 to 60 | 110 | 12,400 | 5,600 to 8,500 |
| B | 20 | 50 | 6,900 | 6,000 to 8,600 |
| C | 20 | 30 | 4,850 | 6,700 to 8,650 |
| D | 30 to 40 | 70 | 11,500 | 5,800 to 8,850 |
| E | 20 | 30 | 1,800 | 7,650 to 8,950 |
| F | 30 | 75 | 6,700 | 6,100 to 9,050 |
PERMIAN SALT
A small part of Montana along the North Dakota border is underlain by a salt bed 30 to 80 feet thick in the Opeche formation of Permian age (Pierce and Rich, 1962, p. 58; Anderson and Hansen 1957, pls. 1-2, fig. 3). The area covers about 1,300 square miles. Beneath a smaller, separate area of about 500 square miles, the salt occurs either in very thin layers or disseminated in the orange-red, anhydritic and dolomitic siltstones, shales, and sandstones. Cover ranges from 7,000 to 7,300 feet in the larger area and from 5,950 to 7,600 in the smaller area.
The Pine salt, one of the most extensive salt beds in the Williston Basin, underlies the extreme eastern edge of Montana from Carter County northward through Roosevelt County (Pierce and Rich 1962, p. 59, fig. 24; Anderson and Hansen, 1957, pls. 1-2, fig. 2). Thickness ranges from a featheredge to more than 300 feet just east of the Montana-North Dakota line in Slope County, N. Dak. The Pine salt is predominantly halite, but contains thin interbeds of reddish-brown mudstone and anhydrite and is capped by a layer of anhydrite. It underlies approximately 6,200 square miles in Montana and has from 6,650 to 7,100 feet of overburden. A stratigraphically higher bed, the Dunham salt was restricted in deposition to the deeper part of the Williston Basin (Pierce and Rich, 1962, p. 59, fig. 26; Anderson and Hansen, 1957, pls. 1-2, fig. 1). It covers an area of about 1,150 square miles in Montana, mostly in Richland County; has a maximum thickness of nearly 50 feet; and has from 5,450 to 7,050 feet of overburden.
To date there has been no production of salt in Montana. Near Williston, N. Dak., however, the Dakota Salt & Chemical Co., a subsidiary of General Carbon & Chemical Corp., is mining salt by solution methods from the A bed of the Charles formation at a depth of about 8,200 feet, and propane gas is being stored in the cavities produced.
U.S. salt production averages about 41 million metric tons per year, valued at about $965,000,000. Half of salt sales go to the chemical industry.
(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)
Sand and gravel, so abundant that few people think of them as mineral deposits, actually comprise the largest mineral industry in the United States in terms of tonnage, and the fourth largest in terms of dollar value. Only crushed stone, petroleum, and portland cement exceed sand and gravel in value. U.S. production in 1961 amounted to 751,784,000 tons, valued at $751,301,000. (In 1999, the U.S. produced 1,080,000,000 tons of sand and gravel for construction, valued at more than five billion dollars.) During the period 1862-1961 Montana produced more than 137 million tons (written communication, R.D. Geach, Montana Bureau of Mines and Geology, 1961); in 1961 it produced 14,702,000 tons valued at $13,506,000 (D'Amico, Kathleen J., 1962, p. 138).
Major uses for sand and gravel are in aggregates such as concrete, mortar, plaster, and asphalt, and as ballast on railroads and highways. Construction of dams and highways consume the greatest tonnages, but hardly any construction of any type is undertaken without using sand or gravel in one form or another.
Sand and gravel constitute a low-cost commodity. Prices are generally less than a dollar per ton at the source for washed and graded products meeting various specifications. Transportation is a major cost item, and every attempt is made to locate large deposits as close as possible to major users; as a result there are thousands of sand and gravel producers, each supplying the needs of local markets. Where local supplies of sand and gravel are unavailable, it is a common practice to substitute locally manufactured crushed rock.
Sand and gravel deposits may be glacial, fluvial, marine or lake, or residual in origin. Because most operations require a substantial quantity of raw material with the highest possible proportion of grains within a preferred size range, selection of suitable sand and gravel deposits requires an understanding of the geologic processes responsible for their formation. As an example, glaciation can form a wide variety of unconsolidated deposits, ranging from unsorted till containing a high proportion of clay- and silt-size material mixed with boulders of almost unmanageable size, to deposits of outwash sands and gravels that are clean and well sorted, and suitable for immediate use. The success or failure of any enterprise dealing with such a low-cost commodity depends to a large degree on adequate geologic information.
Sand and gravel deposits in Montana are principally glacial or fluvial in origin. Major glacial deposits are therefore generally north of a line approximately halfway between the Missouri an Yellowstone Rivers (the area of continental glaciation) and in valleys in many of the mountain ranges where alpine glaciation occurred. Fluvial deposits are found in stream valleys, except where streams drain areas of soft bedrock, or where the valleys are too short or narrow to permit significant accumulation. Only major areas of sand and gravel deposits are shown on figure 32.
(By R. D. Geach, Montana Bureau of Mines and Geology)
Silica (SiO2) is used in a wide variety of industrial applications. Quartz, its commonest mineral form, is the principal mineral in the silica of commerce. It is a common gangue mineral in veins of hydrothermal origin, where it is generally associated with other minerals, although veins in which quartz is essentially the only constituent are not uncommon. Quartz is also found as large masses in the cores of some pegmatites. Quartz in many rocks has been set free by weathering, and segregated during transportation and deposition into silica sand. Some ancient deposits of sand have been lithified into sandstone or quartzite. Some of these in turn have been weathered and reconcentrated into silica sands of exceptional purity.
Most of the silica of commerce is obtained from silica sand deposits. High-quality sands going into industrial uses are known as "industrial sands"; 17,128,000 tons of industrial sands valued at $50,929,000 was produced in the United States in 1961 (Cotter and Mallory, 1962, p. 1062). (U.S. industrial sand production in 1999 was 28,300,000 tons, with a value of more than $500,000,000.) A much lesser quantity is quarried from deposits of massive silica. Whether sand or crushed rock from massive deposits is utilized depends in part on the type of material that is available but to a much greater extent on the use to which it is to be put. For some purposes physical properties are of paramount importance, for others the chemical composition is the chief concern, for still others both chemical and physical properties are important.
Silica is used chiefly as an essential constituent in the manufacture of glass. It is also used in large amounts for metallurgical flux, for ferroalloys and for refractories, as well as for molding sands, engine sands, hydrafrac sands, and for a variety of other specialized uses. In much smaller quantities, silica is the source of metallic silicon, an element used for semiconductors which serve a variety of specialized electronic purposes. Silicon is also used for the manufacture of silicones a group of plastics with many unusual properties.
Specifications vary greatly according to use. For many purposes, washing and sizing are the only treatments necessary. Other products have particular requirements as to purity; glass sand must contain less than 0.06 percent iron, as greater amounts impart color to the final product. Raw material for the production of silicon metal must contain at least 99 percent SiO2, and only traces of certain impurities. For hydrafrac send, which is packed into artificial openings in the producing rocks of oil wells to promote recovery, physical properties are of greatest importance. The sand must be made up of clean, tough, well-rounded grains. Material used for sandblasting must be tough and closely sized, but need not be especially pure.
The entire silica production in Montana through 1961 was as crushed rock. Little is known of specific sources of high-quality silica sand in the State, but it seems probable that several exist. The Tensleep Sandstone, of Pennsylvanian age, crops out in many places in eastern Montana (fig. 33), and where surface weathering has separated the rock into individual grains, or cheap, mechanical treatment could do so, the formation maybe of potential value as industrial sand. Other formations, such as the Quadrant formation (the correlative of the Tensleep in southwestern Montana), may also in places yield sands of value. The Quadrant formation of Pennsylvanian age is a persistent widespread stratigraphic unit in southwestern Montana. It has been quarried for silica rock in at least three localities. Should the demand develop, it is probable that numerous suitable sources of silica sand could be developed in Montana.
At the Dalys Spur deposit near Dillon, Mont. (fig. 33, locality No. 1), the Quadrant formation is more than 150 feet thick. It is a friable rock composed principally of small well-rounded clear quartz grains, with some dark minerals present in minor amounts. A quarry at this site has produced about 100,000 tons of metallurgical material and the remaining reserves are apparently very large. The Oregon Short Line (Union Pacific) Railroad is near the base of the quarry, and loading facilities and a connecting spur to the quarry have been constructed. Duplicate chemical analyses (Carter and others, 1962, p. 23) of a sample taken at the quarry face are (table below right):
| Fe2O3 | 0.05% | 0.049% |
| SiO2 | 98.9% | 97.5% |
| Al2O3 | 0.41% | 0.45% |
| CaO | <.05% | <.05% |
| MgO | <.05% | <.05% |
The quartz cores of certain pegmatites are sources of silica in Washington. A few similar deposits are known to occur in southwestern Montana (Heinrich, 1949), though undoubtedly many more remain to be discovered. The quartz is generally pure and white, though some is clear or gray with minor iron staining along joints and fracture surfaces.
The white quartz mass between Basin and Boulder (No. 9) is evidently of pegmatitic origin. The deposit is estimated to contain 200,000 tons of high-purity silica and has in the past produced silica for metallurgical purposes. A spur from the main line of the Great Northern Railroad runs to the base of the deposit. Duplicate chemical analyses (Carter and others, 1962, p. 24) of a sample taken across the outcrop are (below, left):
| Fe2O3 | 0.029% | 0.036% |
| SiO2 | 98.9% | 99.2% |
| Al2O3 | 0.29% | 0.31% |
| CaO | .05% | .05% |
| MgO | .05% | .05% |
The Sappington pegmatite deposit in Madison County (No. 6), contains a quartz core estimated to be 75 feet long, 30 feet wide, and about 70 feet deep (Heinrich, 1949, p. 24). The northern edge of the deposit is covered by surface debris and the extent of the quartz core northward is unknown. Other quartz-core pegmatites are the Rim Rock (No. 4) and Montana (No. 5) deposits in Madison County, and the Pohndorf Amethyst mine (No. 7) in Jefferson County, but their potential as silica sources is uncertain owing to limited exposures.
The Corral Creek (No. 11) and Brown (No. 10) deposits in Jefferson County, and the Crystal Butte deposit (No. 2) in Madison County are examples of quartz-filled fissure veins in granitic rock. The two Jefferson County deposits are estimated to contain in excess of 200,000 tons each of high-grade silica (Roby and others, 1960, p. 86). The reserves of the Crystal Butte deposit are not accurately known but are estimated to be more than 50,000 tons. The Corral Creek deposit is lenticular in shape and is reported to be about 150 feet wide and more than 160 feet long. The vein on the Brown property is 60 feet wide and 1,200 feet long. Both veins contain massive white quartz with minor iron staining along joint and fracture surfaces. Quartz from the Crystal Butte deposit is of the same general appearance, but slightly pinkish in color.
Chemical analyses (Carter and others, 1962, pp. 25-27) of samples from the Brown, Corral Creek, and Crystal Butte deposits, respectively, are:
| Brown Deposit | ||
| Fe2O3 | 0.02% | 0.07% |
| SiO2 | 99.59% | 99.63% |
| Al2O3 | 0.36% | 0.14% |
| CaO | .10% | - |
| MgO | .007% | - |
| Corral Creek Deposit | ||
| Fe2O3 | 0.02% | - |
| SiO2 | 99.46% | - |
| Al2O3 | 0.45% | - |
| CaO | .09% | - |
| MgO | .011% | - |
| Crystal Butte Deposit | ||
| Fe2O3 | 0.032% | 0.036% |
| SiO2 | 93.6% | 98.7% |
| Al2O3 | 0.26% | 0.44% |
| CaO | .05% | .05% |
| MgO | .05% | .05% |
The Brown and Corral Creek analyses "B" were furnished by the owner; for the Crystal Butte deposit, analyses A and B are duplicate analyses.
Both the Corral Creek and Brown deposits contain extremely pure silica, and are thus potential sources of premium-grade material. The Crystal Butte deposit is of lower grade, but may be suitable for ferrosilicon production. See also Some high-purity quartz deposits in Montana, by J.M. Chelini, MBMG Bulletin 54, 1966.
(By S. L. Groff, Montana Bureau of Mines and Geology, Butte, Mont.)
A wide variety of refractory materials are required by industry, for many different uses. Among them are four minerals with identical applications: sillimanite, kyanite, andalusite, and dumortierite (Bateman, 1950, p. 297-299). The first three have identical composition (Al2O3·Si02); dumortierite is a basic aluminum borosilicate. At high temperatures (1,100°-1,650° C.) all change over to mullite (3A1203·2Si03) and vitreous silica. This material remains stable up to 1,810° C., and is heat resistant, a good high-temperature insulator, and is particularly resistant to thermal shock. Although costly it is much used for spark plugs, electrical and laboratory porcelains, and in the high-temperature ceramic industry.
Kyanite is a common mineral of metamorphic rocks, and also occurs in some pegmatites and locally in quartz veins.
Andalusite occurs in metamorphic rocks, alkaline crystalline rocks, and in pegmatites.
Sillimanite usually occurs in highly metamorphosed alumina-rich rocks.
Dumortierite is commonly associated with pegmatites or quartz veins that cut aluminous rocks.
Prices for ground bagged kyanite in South Carolina (October 18, 1962) are as follows: 35 mesh, $47 per short ton; 200 mesh, $53 to $55 per short ton. (In 2001, the price of raw USA kyanite was $165 per metric ton.)
Apparently the kyanite price includes the other minerals of the sillimanite group (Eng. Mining Jour., Metal and Minerals Markets, October 18, 1962).
There has been no production of the sillimanite minerals in Montana to date, although deposits of these minerals of potential commercial importance exist within the State (fig. 34). Industrial purchasers, however, are too far removed to permit the profitable working of such deposits.
Montana's major deposits of kyanite, andalusite,
and sillimanite are found 13 miles southwest of Ennis (fig. 34, locality No.
1) in Madison County, where they are associated with pegmatite dikes and Precambrian
gneiss (Heinrich, 1948), These deposits are irregular and pockety, and accurate
estimates of tonnage and grade cannot be made. However there are vast tonnages
of low-grade material with local concentrations of from 50 to 60 percent aluminum
silicates (Sahinen and Crowley, 1959, p. 18). Other occurrences are the Bozeman
deposit, 12 miles southwest of Bozeman (No. 2), with corundum in syenite; the
Gallatin deposit, 17 miles southwest of Bozeman (No. 3), similar to the Bozeman
deposit, of variable grade, developed for corundum during World War II; Bear
Trap deposit, 10 miles southeast of Norris (No. 4), Madison County, has some
high-grade sillimanite and corundum. Still other deposits about which little
is known are found in the vicinity of Dillon (Nos. 5 and 6), Beaverhead County;
23 miles southeast of Dillon (No. 7) (Heinrich, 1950); in the Jardine district
(No. 8) Park County; and in the Philipsburg area (No. 9), Granite County.
Only two deposits of dumortierite are known in Montana, one in secs. 3 and 4, T. 7 S., R. 6 W., about 14 miles east of Dillon (No. 10), Beaverhead County; the second, 7 miles north of Basin on Jack Creek, in sec. 7, T. 7 N., R. 6 W. (No. 11), in Jefferson County (Graham and Robertson, 1951, p. 916). No production has been recorded from either deposit, nor are tonnage estimates available.
(By A. E. Weissenborn, U.S. Geological Survey, Spokane, Wash.)
Montana has 56 districts, scattered throughout 19 different counties in which the sum of the recorded production plus the estimated reserves of metal exceeds 100,000 ounces of silver, 1,000 tons of lead, or 1,000 tons of zinc. Fifty-five or all but one of these, have produced significant (i.e., more than the minimum stated above) amounts of silver. Of the 35 lead districts, 34 are silver districts as well; and of the 23 zinc districts, all are also lead and silver districts. Because of the intimate association of these metals, which in many instances are mined from the same deposits, all three are discussed together.
Gold is found in many of the same districts and even in the same deposits as silver, lead, or zinc, but gold occurrences have been described separately because gold was of great importance in Montana's early history; much of the gold was derived from placers rather than lode deposits; and 14 of Montana's 52 gold-producing districts did not produce significant amounts of silver, lead, or zinc.
The following discussion of the geology of silver, zinc, and lead deposits has been condensed from "Zinc in the United States" and "Lead in the United States" (McKnight, Newman, and Heyl, 1962 a and b) and from "Silver in the United States" (McKnight, Newman, Klemic, and Heyl, 1962). The tabulation of Montana's silver-zinc-lead districts which is at the end of this chapter has also been compiled with a few modifications from these same sources.
 |
| Explanation: Numbers refer to districts discussed in text. Production + reserves more than 50 million oz Ag, or more than 1 million tons Zn or Pb: #12, 30, 36. Production + reserves from 5 million to 50 million oz Ag, or 50,000 to 1 million tons Zn or Pb: #3, 4, 16, 35, 37, 45, 50, 51. All others, production + reserves = 100,000 to 5 million oz Ag, or 1,000 to 50,000 tons Zn or Pb. |
The igneous bodies with which the deposits are affiliated are typically small bodies of porphyritic textures whose apices are truncated by erosion. Where deposits are associated with larger batholiths such association is with special parts, usually the smaller satellitic protuberances or cupolas.
Although the host rocks of the deposits range from Precambrian to Tertiary in age, most of the deposits appear to be related to igneous rocks that were intruded from about the end of the Jurassic Period to near the end of the Miocene.
For convenience, Montana's silver-zinc-lead districts have been grouped into the following categories:
First magnitude: Districts in which production plus estimated reserves totals more than 50 million ounces of silver, 1 million tons of lead, or 1 million tons of zinc.
Second magnitude: Districts in which production plus reserves totals from 5 to 50 million ounces of silver, 50,000 to 1 million tons of lead, or 50,000 to 1 million tons of zinc.
Third magnitude: Districts in which production plus reserves totals from 100,000 to 5 million ounces of silver, 1,000 to 50,000 tons of lead, or 1,000 to 50,000 tons of zinc.
The locations of these districts are shown on figure 35; the numbers on the figure refer to districts mentioned in the text. Only districts on which production plus reserves is equal to or greater than magnitude 3 are considered.
Montana has three districts which by this definition are of the first magnitude silver districts--Philipsburg (No. 12), Butte (No. 30), and Colorado-Wickes (No. 36). Butte is also a first-magnitude zinc district and a second-magnitude lead district, Colorado-Wickes is a second-magnitude lead district. Three others--Bryant (No. 16), New World (No. 51), and Barker (No. 51)--are second-magnitude districts for both silver and lead. Four--Hog Heaven (No. 3), Elkhorn (No. 37), Marysville (No. 45), and Neihart (No. 50)--are second-magnitude districts for silver only. One--Eagle district (No. 4)--is a second-magnitude district for lead only. Of the 11 first- or second-magnitude districts, all but the Hog Heaven and Eagle districts have produced substantial amounts of gold and in some of them silver, lead, and zinc have been subordinate to gold. Butte's major importance is as a copper producer.
Since the inception of mining, Montana has been an important silver producer. The peak of production was reached in 1918 when Montana was the country's leading silver State (fig. 36). In that year Montana's mines produced 16,797,477 fine ounces of silver or nearly a fourth of the domestic mine production. Since 1957 Montana has ranked third or fourth as a silver-producing Seats but the amount produced has been steadily declining. In 1961 Montana's silver production was only 3,490,350 fine ounces or about 10 percent of domestic mine production.
Although silver is, or has been, produced in many localities within the State, by far the greater part of the silver has been obtained as a byproduct of base-metal mining, especially at Butte. Through 1961 this district alone has produced 627,753,711 fine ounces of silver (The Anaconda Co.--Montana operations), a figure that has been exceeded only by the Coeur d'Alene district in Idaho. In 1961 Silver Bow County (essentially the Butte district) accounted for 79 percent of the State's output of silver. Granite County (mostly the Philipsburg district) accounted for an additional 13 percent; the remaining 8 percent came from 16 other counties (Fulkerson and others 1962, p. 624). The rise in the price of silver which resulted from the suspension in November 1961 of Treasury sales of silver will undoubtedly tend to stimulate mining of other silver or of other argentiferous lead-zinc deposits. However, the great bulk of the silver will be obtained in the future as in the past, from the Butte area. Because of the size of the Butte reserves Montana should remain an important silver-producing area for a long time to come. (In 2001, the price of silver was $5 per troy ounce.)
 |
zinc sulfide |
Montana
was the Nation's leading zinc producer. Zinc production has declined sharply since
then; in 1961 Montana ranked only 13th in the listing of domestic producers. The
10,262 short tons credited to the State accounted for only 2.2 percent of domestic
mine production. Preliminary U.S. Bureau of Mines production figures for 1962
show a substantial rise to 38,830 short tons of zinc.
Zinc has been produced in quantity from 23 different districts in Montana but, as with silver, the Butte district has been by far the chief source of the State's zinc. Through 1961 over 2,290,000 short tons has been mined from this single district. In 1956, a typical year of fairly high zinc production, 90 percent of Montana's 70,520 tons of zinc was obtained from the Buffs mines. Nevertheless, unlike silver, zinc production is nearly independent of copper mining. Some zinc is recovered as a byproduct of copper mining, but most of the zinc at Butte is obtained from zinc-lead-bearing veins peripheral to the copper-rich central part of the district. (See section on copper.) These deposits normally are worked only when market conditions make it economic to do so. Periods of high demand for zinc may or may not correspond with periods of demand for copper. Thus in 1956 Montana produced nearly seven times as much zinc as in 1961, although more copper was produced in 1961 than in 1956. The increased output of zinc in 1962 over 1961 reflects the opening of the Anaconda's Elm Orlu-Black Rock project at Butte.
The largest known reserves of zinc in Montana are in the Butte district but potential reserves both within and without the district are large. Wide fluctuation in the production rate can be expected from year to year but, under suitable market conditions, Montana will continue to be an important source of zinc.
Montana has been a substantial but not a major producer of lead. Lead output has fluctuated but this fluctuation has not been as pronounced as in the case of zinc (fig. 37). Peaks of production average about 20,000 tons of lead a year. From 1946 through 1956 Montana production has been maintained at an average rate of about 18,000 tons a year, or about 5 percent of U.S. mine production. Since 1956 there had been a progressive decline until in 1961 Montana produced only 2,643 short tons of lead or about 1 percent of domestic mine production. With the opening of Anaconda's Elm Orlu-Black Rock project this was increased to 6,556 short tons in 1962.
Lead is found with zinc in the veins in the peripheral part of the Butte district and the Butte veins have produced a total of 402,000 short tons of lead or about a fifth of the amount of zinc credited to the State. However, other districts in the State such as Colorado-Wickes, Eagle, and Bryant have been substantial producers in the past and in 35 different districts production plus reserves exceeds 1,000 tons of lead. In 1956, a year of relatively high lead production, 80 percent of the State's output of 18,642 short tons was derived from the Butte district. In 1961, a year of relatively low lead production 23 percent of the 2,643 short tons produced came from mines in Lewis and Clark County, 16 percent from Granite County--mostly the Philipsburg district--and an equal amount from the Butte district. Eight other counties yielded the remainder (Fulkerson, Kauffman, and Knostman 1962, p. 623).
Reserves of lead in the Butte district are large and potential reserves outside the Butte district are substantial, but market conditions in the lead industry do not at present encourage their extensive development. The rise in the price of silver resulting from the suspension in November 1961 of Treasury sales of that metal should tend to stimulate development outlying deposits of silver-rich lead ores which are marginal at present lead prices. However, any really significant increase in the current rate of lead production probably must await higher price levels for lead--or for zinc with which much of the lead is associated. (In 2001, the price of North American lead was 44¢ per pound.)
Pertinent statistics on Montana's silver, zinc, and lead districts are summarized in table 7. Note: Table 7 is very large and resides on its own page in this digital edition; click the link in the previous sentence to see it.
(By U. M. Sahinen, Montana Bureau of Mines and Geology, Butte, Mont.)
The sodium sulfate of commerce is of two kinds: (1) natural sodium sulfate and (2) byproduct sodium sulfate. Until about 1925 the supply of byproduct sodium sulfate, or "salt cake," from the manufacture of hydrochloric and nitric acids tended to exceed the demand; but due to the changes in the processes for acid manufacture, the supply of salt cake was diminishing at about the same time that its demand in the manufacture of kraft paper was increasing. The increasing demand was partly offset by utilizing natural sodium sulfate, deposits of which are widespread throughout Western United States. Most of the demand, however, was met by imports. Prior to the last war, Germany was perhaps the main source of imported salt cake, although some was recovered from deposits in Belgium, Netherlands, and Canada. In later years Chile and Russia also contributed to United States imports.
Sodium sulfate is principally used by the pulp and paper industry in the manufacture of kraft paper, and the demand for this purpose is steadily rising. Each ton of pulp produced consumes 120 pounds of sodium sulfate. Sodium sulfate is also used in container and plate glass manufacture, in curing hides, in the dye and coal far industry, in stock feeds, in the medical and chemical industries, in the manufacture of rayon and textiles, and in the metallurgical industry. The newest use is in soapless detergents.
The specifications of commercial sodium sulfate vary with industrial uses. The paper pulp industry acquires material that is from 94 to 98 percent anhydrous sodium sulfate, but different giants have different limits for different impurities. Dye industries prefer natural sodium sulfate to salt cake because the latter may contain nitrates or nitrites that oxidize the dyes.
The United States produces annually about 300,000 tons of sodium sulfate, about 40 percent of which is from natural sources. About 60 percent is produced as byproduct salt cake from various manufacturing industries. Price per ton ranges from $28 to $54 depending on grade. (In 2001, the price of sodium sulfate averaged $114 per short ton.) Imports exceed exports by about 130,000 tons. Montana has not produced sodium sulfate (production reported from Montana in 1951 was actually from North Dakota), but deposits in the State could readily supply the demands of the western paper pulp industry.
Sodium sulfate is a white salt that occurs
in nature chiefly as the anhydrous mineral thenardite with the decahydrate mirabilite.
It is found in different degrees of purity from pure mirabilite crystals to
massive deposits containing mixed salts or minerals of a wide variety of composition
together with insoluble impurities. In Montana sodium sulfate occurs as crusts,
as crystals intermixed with mud, and as massive beds in certain intermittent
lakes. Deposits in Chouteau and Sheridan Counties have long been known to exist
and have been described by Sahinen (1956).
In Chouteau County the deposits occur along Shonkin Sag, a topographically low area which was formerly the course of the Missouri River (fig. 38). The lakes, which are 2 to 24 miles southeast of Fort Benton, have no outlets and dry up during the summer. The C.M. St.P. & P. Railway traverses the Sag skirting both White and Big Lakes. The nearest rail shipping point is Geraldine, about 10 miles easterly from Big and Kingsburt Lakes. Four of the lakes, listed below, show thick crusts and concentrations of crystals in mud which suggest the possible existence of permanent crystal beds of economic significance; however, as yet no drilling has bean done to verify this.
| White Lake | Teal Lake | 900 acres | Secs. 2, 3, 10, and 11, T 22 N, R 10 E |
| Lost Lake | 160 acres | Sec. 29, T 22 N, R 10 E | |
| Big Lake | Crane Lake | 2 square miles | Secs. 25, 26, 35, and 26, T 22 N, R 10 E |
| Kingsburt Lake | Mallard Lake | 1.8 square miles | Secs. 17, 18, 19, and 20, T 22 N, R 10 E |
The reserves of these deposits cannot be estimated from the present state of knowledge.
In Sheridan County intermittent soda lakes form part of a chain extending from Saskatchewan through northeastern Montana into North Dakota. These lakes occupy shallow undrained depressions in channelways in glacial drift. Sulfate salts are deposited as the water evaporates during the hot summer months. Similar deposits in Canada and North Dakota have been thoroughly examined and described (Cole, 1926; Binyon, 1952; Witkind, 1959). There are some 66 lakes in Sheridan County, most of which are intermittent. Some contain deposits of sodium sulfate that warrant further investigations (Sahinen, 1956), but only 2 have been described in detail (Binyon, 1952; Witkind, 1959). Binyon (1952, p. 34) indicates a reserve of 2,824,000 tons for S.E. Brush Lake and 3,813,000 tons for Westby B. Lake, the latter partly in Montana and partly in North Dakota.
S.E. Brush Lake in secs. 26, 27, 34, and 35, T. 33 N., R. 58 E. (313 acres) has a permanent crystal bed 5.65 feet thick with 77.41 percent glauber's salt (commercial term for sodium sulfate). This bed contains 2,710,000 tons of salt; another 113,000 tons is contained in a mixture of salt and mud a few inches above and below the permanent bed.
Westby B. Lake in sec. 12, T. 36 N., R. 58 E. has an area of 386 acres, one-third of which is in Montana and two-thirds in North Dakota. The lake has a permanent crystal bed which is at least 9 feet thick.
Reserves for the entire lake are estimated (Binyon, 1952, p. 24) at 3,677,000 tons for the drilled portion of the permanent bed and 136,000 tons in about 1 foot of mixed mud and crystals overlying the permanent bed. About a third of the reserves, or 1,270,000 tons, should underlie the Montana portion of the lake.
Other lakes in this area that might be possible sources of sodium sulfate lie in sec. 13, T. 36 N., R. 58 E., just west of Westby B. Lake and the lake in secs. 24 and 25, T. 36 N., R. 58 E., south of the town of Westby.
Lakes northwest of these, near Sybouts, Saskatchewan, have been worked for sodium sulfate and there is no apparent reason why the Montana deposits could not also be worked to supply the paper pulp industry to the west. Lignite coal for power and processing the salt is plentiful in the area.
