MINERAL AND WATER RESOURCES
OF MONTANA

Mineral Resources: Antimony through Limestone


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METALLIC AND INDUSTRIAL MINERAL RESOURCES

INTRODUCTION

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

The early development of the mineral resources of Montana was almost entirely directed toward the metallic minerals. As in so many parts of the West, gold was the initial goal of the early miners and prospectors. Exploitation of other metallic deposits soon followed, but it was not until a larger and more diversified population arrived that other mineral resources drew attention. By the early 1900's, Montana had a significant population engaged in ranching, lumbering, and commerce, and the need for some of the less glamorous mineral commodities began to be felt. Cement, stone, sand and gravel were used in increasing quantities for roads and building construction. The rapid changes and advances in technology created demands for many mineral products formerly considered of little or no value. Vermiculite is an outstanding example; it was not until the initiative and vision of a few people developed some of the many present-day uses for vermiculite that it was considered a mineral commodity at all. There are many other similar examples. The net result of Montana's growing population and industrial variety is the long list of mineral commodities now produced in the State. Although the metallic minerals are as important as ever, and although Montana remains a leading producer of them, the importance of nonmetallic minerals continues to grow. As will be seen in the text that follows, not only the quantity, but the variety, of Montana mineral production is great.

ANTIMONY, ARSENIC, BISMUTH, CADMIUM, GERMANIUM

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

These elements, chiefly recovered as byproducts, are present in different ores and in varying degrees of abundance; all except arsenic are in relatively short supply.

Antimony and bismuth are soft, silver-gray metals with low melting points. Both are rare in the United States. Antimony is recovered as a, byproduct from silver refining (almost all U.S. production comes from the Sunshine mine in Idaho) and bismuth is a byproduct from certain lead ores including those from Butte. The Anaconda Co. produces the only bismuth recovered from Montana ores. (In 1999, there was no bismuth recovery from domestic mines; the U.S. was 100% dependent on imports.) Antimony ores (stibnite) are mined in some parts of the world but, although no economic deposits are presently known in the United States, some deposits in Sanders County once produced a little more than 200 tons of ore.

Both metals are used in alloys such as type metal, babbitt metal, ammunition, battery lead alloys (antimony), aluminum alloys, malleable iron and steel, pharmaceuticals, and laboratory chemicals (bismuth). Antimony oxide is used in the plastics industry in flame retardants. Most of the U.S. supply of both metals is imported.

Butte produces almost the entire domestic supply of arsenic, and could increase its production by many times if required (John R. Cooper, U.S. Geological Survey written communication, 1960). The arsenic is contained mostly in the mineral enargite, an important copper ore mineral at Butte, and is recovered from the smelter at Anaconda principally to prevent contamination of the surrounding area with poisonous arsenical fumes.

Chief use of arsenic is as the oxide As2O3 (white arsenic of commerce) and calcium arsenate, for insecticides, pesticides, and cattle and sheep dips. Arsenic compounds are also used in wood preservatives (90% of trioxide consumption in 1999), dyestuffs, weed killers, glass manufacture, fireproofing, and hide-tanning compounds.

Present supply far exceeds demand, and prospecting for mineral deposits to produce arsenic for the market at present does not offer much promise. In 1999, all arsenic and arsenic compounds consumed in the U.S. (1,000 tons metal, 29,000 tons compounds) were imported, principally from China.

Cadmium is a soft, silver-white metal that is markedly rust resistant. Chief uses for cadmium are in batteries (72% of consumption), electroplating automobile and aircraft engine parts, radio and television parts, nuts and bolts, and as an alloy in bearing metals, low fusion-point metals, photography, paint pigments, and dyes.

Cadmium is recovered almost exclusively as a byproduct from zinc smelting. The Anaconda Co. recovers it from Butte ores, and in 1960 was the only producer in Montana. Current U.S. needs are adequately supplied from this and other domestic zinc producers.

Germanium is an extremely rare element. When pure it is a white, brittle, crystalline metal. It is recovered principally from certain zinc ores (Fisher, 1960, pp. 1252-1253). Chief uses are for fiber optic systems, polymerizing catalysts, infrared optics, and transistors, diodes, and rectifiers in the electronics industry. It is also used sparingly as a color modifier in fluorescent lights. Germanium is known to occur with enargite and some sphalerites (Fleischer, 1961), but no production has been reported from Montana. U.S. germanium consumption in 1999 was 28,000 kilograms.

ASBESTOS

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

Asbestos is the term given a group of silicate minerals with fibrous habit, which can be separated into very thin, more-or-less flexible fibers. Asbestos minerals are of two types: chrysotile, which forms from serpentine, yields highly flexible, tough fibers, which, if long enough, can be spun and woven; and amphibole asbestos, which produces brittle fibers that cannot be woven but can be mixed with binders such as cement, plaster of paris, or resins to make useful products (Jenkins, 1960, p. 49).

Chief uses for asbestos are as thermal and electrical insulation. Fireproof clothes, gloves, and curtains are made from chrysotile as are clutch facings, brake linings, and gaskets for the automotive industry. Amphibole asbestos is used in fireboard, shingles, furnace coverings, and similar uses not requiring flexibility.

Asbestos has been reported at four places in Montana (fig. 12) (Chidester and Shride, 1962). Chrysotile occurs at Cliff Lake, Madison County, and at the Anderson deposit southwest of Armstead, Beaverhead County (Perry, 1948, p. 35). Several attempts have been made to produce asbestos from the Cliff Lake deposit but, as of 1959, none were successful. Activity at the Anderson deposit has been confined to prospecting (Sahinen and Crowley, 1959, p. 4-5).

Although no other deposits of chrysotile asbestos are known in Montana, metamorphosed dolomites in the Precambrian rocks of southwestern Montana constitute a favorable environment for their occurrence, and the potential exists for discovery of minable deposits.

Amphibole asbestos was mined at the Karst deposit, 35 miles south of Bozeman. The deposit was operated by Interstate Products Co., Bozeman. Production prior to 1959 was approximately 1,800 tons; the mine is now inactive and remaining reserves appear small.

Large quantities of amphibole asbestos (tremolite) are present at the Rainy Creek vermiculite deposit near Libby. No attempts have been made to market the material as yet, but commercial exploitation may be possible.

BARITE

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

Barite is a white, heavy (specific gravity about 4.5), relatively soft crystalline mineral with the composition BaS04. It occurs in veins and replacement deposits, either alone, with quartz, calcite and similar minerals, or commonly as a gangue mineral in metallic sulfide deposits (Brobst, 1958, p. 83). Commercial deposits generally contain few impurities. Market price for crude barite in 1961 was $18 per ton. (In 2001, the price of barite at the mine was $25 per ton.)

A major use for barite is in drilling muds, for which it is normally ground to minus 325 mesh and mixed with bentonite to keep it in suspension. The resulting muds are used in oil well drilling, where their high specific gravity assists in controlling oil and gas pressures.

A second important use is in lithopone, a mixture of barite and zinc sulfate used as a white pigment in paint. Other uses are in glass manufacture; filler in paper, asbestos products, linoleum, textiles, and rubber goods; and for a variety of products in the chemical and drug industries. Annual domestic consumption of barite and its derivative materials ranges from about 1.5 to 2 million tons, of which about one-third is imported (Brobst, 1960, p. 59). In 1999, domestic consumption of crude barite was 1.14 million metric tons, plus 1.5 million metric tons of ground and crushed barite. Two-thirds was imported.

There are three productive deposits in Montana; two are near Greenough, about 25 miles east of Missoula. One of these, the Elk Creek deposit (fig. 13, locality No. 1) was discovered in 1950, and mining began shortly thereafter (DeMunck and Ackerman, 1958, p. 12). The other, the Coloma vein (No. 2), about 2 miles to the south, was discovered in 1955, and mining began in 1956. The barite has been used mostly for drilling mud. The deposits are fracture fillings of relatively pure crystalline barite; principal impurity is a mixture of wall rock (principally Belt series) that has been faulted into the vein.

Some shipments have been made from the Kennelty deposit, about 35 miles southeast of Libby, Lincoln County (No. 3), where barite occurs in a massive vein cutting Belt argillites. Similar barite veins are known elsewhere in western Montana, and several may be large enough to justify exploitation. Among them are a deposit about 4 miles south of Missoula (No. 4); the Copper Mountain veins near Troy, Lincoln County (No. 5); the Whaley deposit northeast of Florence, Missoula County (No, 6); the Shook vein, between Woods Creek and Deer Creek south of Darby, Ravalli County (No. 7); and the Fletcher deposit north of York, Lewis and Clark County (No. 8) (DeMunck and Ackerman, 1958, p. 17-26). A number of other deposits are known; some are probably sufficiently large and high grade to constitute potentially productive deposits, provided a market can be found for the product. See also Barite in Montana, by R.B. Berg, MBMG Memoir 61, 1988.

BENTONITE

(By U. M. Sahinen, Montana Bureau of Mines and Geology, Butte, Mont.)

Bentonite is the commercial name given to a type of clay consisting essentially of the mineral montmorillonite. Bentonite exhibits certain unusual physical properties that lead to its utilization in industry. When wet, it swells greatly, and thus finds use as a watertight seal in irrigation ditches, soils, dam foundations, and drill holes. When mixed with water and allowed to stand if forms a gel which becomes liquid again upon agitation. This property, known as thixotropy, leads to its use in drilling muds to keep heavy minerals in suspension. Bentonite is also used as a bonding agent in foundry sands, as a filler and carrier in insecticides and fungicides, and as a filler in paper. A variety found in southeastern United States swells less than western bentonite, and is used as an adsorbent and decolorizer. Some Montana bentonite has been used as a binding agent in pelletizing iron ore.

Bentonite is formed by devitrification of volcanic glass. Tuff beds of Cretaceous and Tertiary age are widespread in Montana, Wyoming, the Dakotas, and elsewhere in western United States. Many, particularly in Cretaceous rocks, have been altered to bentonite, and some of the larger and more accessible deposits are mined. The main use is for foundry sands and in drilling muds; therefore, most of the bentonite produced has come from deposits near oilfields or iron and steel centers.

Bentonite occurs in many parts of Montana (fig. 14). In the southwestern part of the State it occurs as beds of highly altered volcanic ash interbedded with soft clay-shale, sand, and gravel in the Tertiary sediments of the intermontane valleys. The bentonite beds vary in thickness from a few inches to beds measurable in tens of feet; quality also is variable. The clay has been mined on a small scale near Gregson and near Melrose, both in Silver Bow County, but the deposits, as a whole, have not been surveyed, and estimates of reserves cannot be made. Some of the bentonitic clays and soils of the larger valleys of southwestern Montana have been tested and found suitable for compacted irrigation canal-lining (Green and Agey, 1960, Everett and Agey, 1960 and 1961).

In central Montana, bentonite of good quality is found at several places, but the best known are the deposits in the Hardin area described by Knechtel and Patterson (1956) where some 24 beds ranging from 3 to 45 feet thick are interbedded in a shaly sequence 4,800 feet thick. Regarding these deposits Knechtel and Patterson (1956, p. 48-49) state:

* * * beds of minable thickness lying in belts more than 50 feet wide under less than 30 feet of overburden are estimated to contain about 110 million short tons of bentonite * * *. Apparently, all the beds contain some bentonite that is suitable for use as foundry clay. * * * Some of the bentonite of this district also could be used as an ingredient of drilling mud, but most of it is not comparable in quality to the best grades of drilling clay mined in the Black Hills district. ***

A small bentonite-processing plant has been operated by the Wyotana Mining Co. at Aberdeen siding, 6 miles south of Wyola, but the clay came from the Wyoming side of the State line. Bentonite from a deposit near Geraldine, in Chouteau County, in central Montana has been used for lining irrigation ditches.

Bentonite of good quality has been produced from Carter County in the Montana section of the Black Hills district in southeastern Montana. Here nine beds of bentonite with average thicknesses ranging from 1 to 5 feet occur in about 3,000 feet of shaly sediments of Cretaceous age. The quality ranges from very poor to excellent. Only one bed, the Clay Spur bed in the Mowry Shale, has been mined, but material from this bed ranks from good to excellent in quality. Bentonite from the Black Hills district is usable as an ingredient in drilling muds, for preparation of metallurgical molding sand of superior strength, and for bonding material in pelletizing taconite iron ore of the Lake Superior district (Knechtel and Patterson, 1962, p. 1). See also Bentonite in Montana, by R.B. Berg, MBMG Bulletin 74, 1969.

CHROMIUM

(By Everett D. Jackson, U.S. Geological Survey, Washington, D.C.)

Chrome ore has been one of the critical commodities of the United States for many years. At present, no domestic chrome deposit can compete with foreign ore, either by reason of low grade, small size, or remote location. However, over 80 percent of the potential chromite resources of the Nation are in Montana.
U.S. consumption of chromium ferroalloys and metal in 1999 was predominantly for the production of stainless and heat-resisting steel and superalloys, respectively. The value of 522,000 tons of chromium material consumed was about $421 million. The U.S. in 1999 had no primary mine chromium production; imports came primarily from South Africa (44%), Russia (14%), Turkey (10%), and Zimbabwe (9%).
In the past, these deposits have been mined under contracts with the U.S. Government, but in the future they may become economic in their own right. They are, therefore, important in any resource evaluation of the State. Consumption of chrome ore in the United States averages about 1.3 million short tons per year, for three principal uses: (1) metallurgical, (2) refractory, and (3) chemical. Metallurgical use constitutes about 55 percent of the total; chromium is added to improve the strength and corrosion resistance of steel, and in the production of other ferroalloys and chromium metal. Refractory use consumes about 30 percent of the total chrome ore in the manufacture of bricks for lining open hearth steel and other high-temperature furnaces. Chemical use amounts to about 15 percent of the total for the manufacture of sodium dichromate for tanning, pigments, and electroplating. Historically, the metallurgical, refractory, and chemical industries have specified different types of chrome ore for these uses.

The mineral chromite is the only commercial source of chromium. However, pure chromite contains four major metallic constituents-chromium, aluminum, magnesium, and iron--and its composition varies widely. For example, the chromic oxide (Cr2O3) content of natural pure chromite ranges from more than 60 weight percent to less than 30 and its Cr:Fe ratio ranges from more than 4:1 to less than 1:1. In addition to chromite, commercial chrome ores contain small amounts of gangue minerals of which silica (SiO2) is an important constituent. The "standard" specifications for metallurgical-grade chrome ore call for 48 percent Cr2O3, a Cr:Fe ratio of 3:1 and an SiO2 content of less than 5 percent. Refractory-grade ores should have Cr2O3 plus Al203 in amounts greater than 57 percent and Fe and SiO2 must be low. Specifications for chemical-grade ore are more variable, but in general it should be high in Cr203 and low in Al2O3 and SiO2. These specifications interest metallurgical and chemical users only insofar as they affect the cost of extracting chromium, and are subject to change with economic factors. Thus, in 1961, about 15 percent of the chrome ore used in the metallurgical industry was chemical-grade (Prokopovitch and Heidrich, 1962, p. 432). Users of refractory-grade chrome ores, however, are concerned with the physical properties of the chromite itself, and until recently high-iron chromites were unsuitable for chromite refractories. However, successful service trials of refractories made from chemical-grade chromite have recently been reported (Heiligman, Mikami, and Samuel, 1961).

Chromite deposits may be classified into three principal types (1) stratiform, (2) podiform, and (3) placer. Stratiform chromite deposits occur in the lower parts of great tabular gabbroic intrusions as uniform layers that commonly can be traced for miles. Podiform chromite deposits occur as irregular lenses in dunite or troctolite of alpine mafic intrusions. Placer chromite deposits are derived from weathering of peridotite and chromite deposits, and occur either as black sand, or as chromiferous lateritic iron ore. Most stratiform chromite deposits are chemical grade, whereas podiform chromite deposits are typically either metallurgical- or refractory-grade (Thayer, 1946; 1960).

The total measured, indicated, and inferred reserves of chrome ore in the United States were estimated to be equivalent to about 3.5 million long tons of Cr203 in the ground as of 1956; of this total more than 80 percent was estimated to be in Montana (Estimate by T. P. Thayer and E. D. Jackson, Department of the Interior Information Service, press release, June 5, 1957). Although both stratiform and podiform deposits occur in the State, nearly all of the reserves are contained in the great stratiform intrusion known as the Stillwater Complex, in Stillwater and Sweet Grass Counties.

The chromite deposits of the Stillwater Complex occur in a belt or ribbon across the northern front of the Beartooth Range in south-central Montana. The belt is about a mile wide and 30 miles long, is gently arcuate in shape, and trends from east-west to northwest-southwest.

The deposits are stratiform in type and chemical in grade. The chromite is concentrated in layers that are parallel to those of the enclosing rocks, which are olivine- and pyroxene-rich harzburgites and bronzitites. Chromite-rich layers, however, occur only within the olivine-rich layers. About 13 zones of chromite enrichment, called chromitite zones, are known, and these vary in thickness from less than an inch to more than 12 feet. Although the chromitite zones vary in thickness from layer to layer, and along the strike, they are remarkably continuous. Several of them can be traced laterally for more than 15 miles. Dips of the chromitite zones--and of the harzburgites and bronzitites that enclose them--are steep, ranging mostly from vertical to about 50. The deposits extend downward to great depths, except where terminated by major faults.

The pure chromite varies considerably in composition: the Cr2O3 content ranges between 36 and 49 percent, and the Cr:Fe ratio ranges between 0.8:1 and 2.3:1. Average values are about 43 percent Cr2O3 and 1.5:1 Cr:Fe. These compositions were obtained on laboratory purified material; commercial concentrates made from these chromites would contain somewhat less Cr2O3. The composition of the chromite in the chromitite zones tends to change in a regular way--between different zones, along the same zone, and from bottom to top of the same zone (Jackson, Dinnin, and Bastron, 1960; Jackson, in press).

Interest in the chromite deposits of the Stillwater Complex was first stimulated by the war demand of 1917 and 1918 (Westgate, 1921). Several properties were developed, but no ore was shipped. Development continued sporadically during the 1920's and 1930's and a number of claims in the eastern part of the area were patented in 1933. Schafer (1937, p. 7-20) summarized the knowledge of ore deposits at that time. In the period 1939-45 the U.S. Geological Survey, in cooperation with the U.S. Bureau of Mines, began a detailed study of the geology and grade of the chromite deposits of the Stillwater Complex.

Results of this work, including geologic maps and drill hole data, have been published by Peoples and Howland (1940); Wimmler (1948); Howland, Garrels and Jones (1949); Jackson, Howland, Peoples, and Jones (1954); Peoples, Howland, Jones, and Flint (1954); and Howland (1955).

In June 1941, the Anaconda Copper Mining Co., as agent of the Defense Plant Corporation of the U.S. Government started underground development for chromite in three favorable areas in the Stillwater Complex: (1) The Benbow area near the head of Little Rocky Creek; (2) the Mouat or Mountain View area just west of the Stillwater River; and (3) the Gish area on the Boulder River. Large camps and mills were constructed in the Benbow and Mouat areas, and a camp was built at the Gish property. At Benbow, the Anaconda Copper Mining Co. mined 200,625 long tons of ore and milled 166,400 long tons to produce 64,791 long tons of concentrates averaging 41.5 percent Cr2O3 and 1.61:1 Cr:Fe. At Mouat, they mined 163,571 long-tons and milled 69,371 long tons to produce 26,373 long tons of concentrates averaging 38.8 percent Cr2O3 and 1.44:1 Cr:Fe. Some ore was broken at the Gish mine, but none was milled. All operations were stopped by Government order in September 1943, when higher grade foreign chrome ore again became available.

In 1952 the American Chrome Co. entered into a contract with the U.S. Government to produce 900,000 short tons of Stillwater chromite concentrates averaging 38.5 percent Cr2O3. The Mouat mine was reopened in 1953, the mill was rebuilt, and the concentrates were delivered to the Government stockpile at Nye, Mont. In 1958 American Chrome Co. installed a pilot plant to investigate production of charge-grade ferrochromium by electrical reduction. The Government contract was completed in 1961, and the mine was closed the following year.

Estimated chromite reserves of the Stillwater complex as of 1962, corrected from Thayer and Jackson's earlier estimates (Department of the Interior Information service press release, June 5, 1967), are equivalent to 2,520,000 long tons of Cr2O3 content in the ground.

Podiform chromite deposits occur in two areas of Montana: near Red Lodge in Carbon County, and near Sheridan in Madison County. Both areas contain a number of small deposits, and both are unusual in that they contain predominantly chemical-grade chromite.

The Red Lodge district is a 45-mile-square area near the southeastern end of the Beartooth Range just north of the Montana-Wyoming boundary. In contrast to the stratiform deposits of the Stillwater complex, the Red Lodge chromite deposits consist of irregular pods and lenses in remnants of sill-like masses of serpentine. The serpentine, an alteration product of original olivine and pyroxene rocks, occurs as roof pendants in gneissoid Precambrian granite. Chromite deposits range in size from bodies a few feet square to a lens averaging 40 feet wide by 150 feet long; they contain from a few pounds to as much as 35,000 tons of chromite. The size of the deposits is not related to the size of the enclosing serpentine bodies, and neither chromite nor serpentine extends to any great depth. The pure chromite mineral varies considerably in composition; Cr2O3 contents range from 36 to 52 percent, and Cr:Fe ratios range between 0.7:1 and 2.1:1.

Chromite was first discovered in the Red Lodge area in 1916; in 1933 Montana Chrome, Inc., was organized to develop the deposits (Schafer 1937, pp. 21-34). In the period 1941-43 the U.S. Geological Survey, in cooperation with the U.S. Bureau of Mines, mapped and evaluated the chromite deposits (James, 1946; Herdlick, 1948). In 1941 the U.S. Vanadium Corp., as agent for the Government, began development work in the area and built a mill in Red Lodge. U.S. Vanadium Corp. mined a total of 67,943 long tons of ore, from which were obtained 21,958 long tons of lump ore averaging about 32 percent Cr2O3, and 11,689 long tons of concentrates averaging about 40 percent Cr203. Operations ceased in 1943. Estimated chromite reserves based on data by James (1946, p. 178) are equivalent to about 4,600 long tons of Cr203 content in the ground.

The Sheridan deposits occur in an area in the northern part of Madison County bounded by the towns of Sheridan, Silver Star, and Pony. The deposits are irregular pods and lenses in sill-like masses of altered ultramafic rocks, and are very similar to those of the Red Lodge area. The largest chromite deposit reported was between 5 to 25 feet wide and 250 to 300 feet long. The chromite of the Sheridan deposits is somewhat lower in grade than those of the Red Lodge area; reported Cr2O3 content of clean chromite ranges from 38 to 45 percent, and Cr:Fe ratios range between 0.8:1 and 1.3:1.

Chromite in the area was first described by Jones (1931), and later by James (1943). The Silver Star deposit was developed by the Silver Star Chrome Corp. in the period 1941-44, and several thousand tons of crude lump and concentrates were produced. Small amounts of chromite were also mined from nearby properties. All operations stopped in 1944, and remaining chromite reserves are equivalent to about 1,000 long tons of Cr2O3 in the ground. See also The Stillwater Complex, Montana: Geology and Guide, by G. Czamanske and M. Zientek, MBMG Special Pub. 92, 1985.

CLAYS

(By U. M. Sahinen, Montana Bureau of Mines and Geology)

Clays have been a subject of interest and commercial significance in Montana for many years. Since World War II, with the growth of industry in the State, increasing interest has been shown in clays for ceramic use and as raw material for the manufacture of expanded shale lightweight concrete aggregate; and since the establishment of the Anaconda aluminum plant at Columbia Falls in 1955, interest in Montana clays as possible sources of alumina for the manufacture of metallic aluminum has rapidly increased.

The term "clay" is applied to a large variety of minerals, but the common concept is that of an earthy material which is plastic when wet; while in the plastic state it can be molded into many shapes and subsequently fired (baked) in kilns to a highly indurated product in an infinite number of forms ranging from common bricks to the most intricately designed pottery.

Clays can be classified into three broad categories based on composition and crystal structure: the kaolin group, composed essentially of hydrous aluminum silicates, that include the highest grade ceramic clays and most fire clays; the illite group, composed of complex micaceous silicates; and the bentonite group, composed essentially of montmorillonite. The bentonite group is described separately in another section of this report.

The U.S. clay industry in 1999 produced 42.2 million metric tons, valued at $1.71 billion. Ball clay was used primarily for floor and wall tile and sanitaryware; bentonite was used for foundry sand binder, pet waste absorbent, drilling mud, and iron ore pelletizing; common clay was used for brick, cement, and lightweight aggregate; fire clay was used primarily in refractories; fuller's earth was used in absorbents; and kaolin's primary use was in the paper industry.
Since 1894 Montana, has produced 1,017,389 tons of clay valued at $1,730,428. This includes miscellaneous clays used for heavy construction and fire clay (kaolin type mostly). The value per ton of fire clay in Montana is about four times that of other clays, excluding bentonite. Production of clay as a source of expanded shale lightweight aggregate was begun in 1959 by the Montana Lightweight Aggregate Co. in Billings. The Treasurelite lightweight aggregate plant at Great Falls began operations in 1960.

The uses of clay are innumerable and depend on quality. High quality white-firing dickite (kaolin) clay occurs in the South Moccasin Mountains (Dougan, 1947) but is not mined for lack of sufficient markets within economic freight-rate distances for the high-grade ceramic products that could be made from it. It is a high quality whiteware clay. Similar material, but in thin beds of doubtful economic importance, occurs on the south side of the Little Rocky Mountains.

Figure 15 shows the locations of clay pits and clay products plants in Montana. High-grade kaolin-type fire clay is mined at Armington, Mont. (fig. 15, locality No. 1), where it is associated with a workable coal bed. Siliceous fire clay is mined at a pit on Lost Creek just north of Anaconda (No. 2). A red kaolin clay has been mined from a deposit on Dyce Creek (No. 3), west of Dillon, for use as refractory patching in an electric furnace. Armington and Lost Creek fire clays are used as refractories in the Anaconda Co.'s smelters.

Common brick clay and clay suitable for the manufacture of tile and similar grade ceramic products has been mined from pits at Butte (No. 4), Deer Lodge (No. 5), Missoula (No. 6), Thompson Falls (No. 7), Whitehall (No. 8), Blossburg (No. 9), Great Falls (No. 10), Havre (No. 11), Lewistown (No. 12), Billings (No. 13), and Fromberg (No. 14). At present, brick plants are in operation at Helena, Great Falls, Lewistown, and Billings. A plant at Havre is presently inactive.

The newest use of clay in Montana is in the manufacture of lightweight aggregate for concrete. For this use a clay is needed that will expand to a firm cellular product when heated suddenly to the temperature of incipient fusion. For economic practice this temperature should not exceed 2,200F. Many of Montana's bentonite clays and shales, which are useless for the manufacture of ceramic products, are admirably suited for this purpose. At present, the Treasurelite Co. of Great Falls utilizes bentonitic clay from the Blackleaf formation of early Cretaceous age. In the Billings area, the bentonitic Claggett shale of Late Cretaceous age is expandable. At Three Forks the Builder's Products Co. will soon be expanding shale for lightweight aggregate from a pit in Colorado shale of Cretaceous age, a few miles north of Logan (fig. 15, No. 15). Expandable clay shales have also been found near most of the principal towns and cities of Montana. In 1960 nearly 40,000 tons of clay were used for expansion into lightweight aggregate-and the industry is due to expand.

The Montana Bureau of Mines and Geology is conducting a survey of Montana's clay resources, and a great many deposits have been sampled. The sampling sites and the analyses of these samples have been published in progress reports of the Montana Bureau of Mines and Geology (Sahinen and others, 1958, 1960, 1962).

Although some high-alumina clays have been found in Montana, no published data is available on their usefulness as a source of alumina for the manufacture of metallic aluminum. See also Progress report on clays and shales of Montana, by J.M. Chelini, R.I. Smith, and D.C. Lawson, MBMG Bulletin 45, 1965.

COPPER

Malachite and azurite,
copper carbonates,
in Mineral Museum, Butte
(By A. E. Weissenborn, U.S. Geological Survey, Spokane, Wash.)

Copper is one of the metals vital to the needs of an industrial economy. Prior to 1927 the United States supplied more than 50 percent of the world's production of copper. In 1961 domestic mine production of 1,165,155 tons of copper was the highest of record (U.S. Bureau of Mines Minerals Yearbook 1961, vol. 1, p. 497) but this was only 23 percent of world production. Since about 1938 consumption has exceeded domestic production, and the United States became dependent on foreign sources to make up the deficit. However, in 1960 and 1961, domestic production and consumption were nearly in balance. In 1999, 27% of U.S. copper needs were supplied by imports.

Montana, has long been one of the three important copper-producing States. In 1961 it produced 104,000 short tons of copper, or approximately 8 percent of that produced from domestic ores. Of about 47,176,425 short tons of copper produced from domestic ores from earliest records through 1961, Montana has produced 7,683,960 short tons or approximately 16.2 percent. This is a little less than half the total production of Arizona (17,782,444 short tons), and a little less than that of Utah (8,392,059 short tons) (U.S. Bureau of Mines Minerals Yearbook 1961, vol. 1; p. 499).

In contrast to Arizona where copper is, or has been, produced from at least a dozen major mining districts, more than 99 percent of the entire Montana production has come from a single area, the Summit Valley district-better known as the Butte district. Essentially all of the copper of the Butte district--and most of the zinc, lead, manganese, and silver--has been produced from an area 2.5 miles wide and 5 miles long (Hart, 1935, p. 289). The copper deposits form a series of steeply dipping veins in a fissure system of great complexity, cutting quartz monzonite as well as dikes of aplite and quartz porphyry. The ore, found mainly as replacements along fissures, contains pyrite, chalcocite, enargite, tennantite, bornite, sphalerite, chalcopyrite, and covellite in a scant gangue of quartz (Lindgren, 1933, p. 615-616). A primary zoning of the ore has been recognized with a central copper zone, an intermediate zone with copper and zinc, and a peripheral zone with zinc, lead, and manganese. Silver enrichment is common in the peripheral zone (Sales, 1914, p. 58; Hart, 1935, p. 297).

In the eastern part of the Butte district (No. 5 on fig. 16), the veins tend to become discontinuous, lose their identity, and break up into a series of narrow, closely spaced transverse fractures, known locally as horsetail structure. Ore has been mined from the "horsetail" area for many years, but with the successful introduction in the Butte district a few years ago of lower cost block-caving and open pit methods of mining, the "horsetail" area has become of increased economic significance.

Copper is found in many localities outside the Butte district. The more important of these are the Hellgate (No. 1 on Fig. 16) and Radersburg (No. 4) districts in Broadwater County; the Basin-Boulder (No. 2) and Wickes (No. 3) districts in Jefferson County; the Hecla (No. 6) and Utopia (No. 7) districts in Beaverhead County; the Heddleston (No. 8) district in Lewis and Clark County; the Neihart (No. 9) district in Cascade County; the Philipsburg (No. 10) district in Granite County; and the New World (No. 11) district in Carbon County (fig. 16; in all of the deposits shown on the map, copper may be subordinate to other metals and may not be economically recoverable. Tonnage refers to produced metal plus estimated remaining reserves. Adapted from Kinkle, Jr., and Peterson, 1962.). With the exception of the ankerite-quartz-chalcopyrite deposits of the Hellgate district (Pardee and Schrader, 1933, pp. 165-166) and the contact metamorphic deposits of the Utopia district (Winchell, 1914, pp. 63-64; Myers, 1952, p. 36) nearly all of the copper was mined from deposits chiefly valuable for other metals. Total production of copper from these districts is probably of the order of 150 million pounds, but this represents only about 1 percent of the total production of the State. Numerous other occurrences of copper are known in the State but few of these have produced more than a few tons of ore. Their locations are indicated on figure 16 by purple dots.

Copper ore was known in Butte as early as 1865 but early attempts to smelt the ore were unsuccessful. Copper mining did not begin in earnest until 1882 when the construction of a successful smelter and the coming of a railroad made copper mining at Butte feasible (Weed, 1912, pp. 18-19). Development was rapid, and by 1896 yearly copper production from Butte reached 100,000 tons (fig. 17) and consistently exceeded this figure until the 1930's with the exception of a short period following World War I. Since the depression years, the value of Montana's copper production has followed an upward trend, and in 1961 totaled $62,400,000, a figure exceeded only in 1956 and by the World War I peak of 1916-18. However, since 1944, only in 1961 has the annual tonnage of copper--as contrasted with the dollar value of the production-exceeded the 100,000-ton figure which was so consistently maintained from 1896 through 1918.

Although it is by no means inconceivable that significant copper discoveries may be made in Montana outside of the Butte area, copper mining in Montana for the foreseeable future is tied to the Butte district. Despite its 80 years of continuous production the outlook for Butte seems brighter than it has in recent years. Development of low-cost methods of extraction through block caving through the Kelly shaft, and open-pit mining at the Berkeley pit, have added large tonnage of low-grade ore to the reserves. Recent development of the Butte veins in depth is reported to have resulted in a significant addition to the reserves of high-grade ore. E. P. Shea, chief geologist, Montana Division, Anaconda Co. stated recently ("The Glittering Hill 8O years later, geology"; paper given at the Seventh Annual Rocky Mountain Minerals Conference, Butte, Mont., September 1962) that Butte ore reserves are at an all-time high. It seems, therefore, that if market conditions permit, Butte can continue to produce copper at an annual rate of 100,000 tons for an indefinite period.

FLUORSPAR

Fluorite, calcium fluoride,
in Mineral Museum, Butte
(By R. D. Geach, Montana Bureau of Mines and Geology, Butte, Mont.)

Fluorspar is the commercial term for the mineral fluorite (CaF2). It displays a wide array of colors, ranging from colorless, yellow, green, and blue, to purple. Fluorite is harder than calcite and softer than quartz, but heavier than either one. Use is made of this latter property as both calcite and quartz are commonly intermixed with fluorite in nature, and an upgraded fluorspar product can sometimes be made by using mineral dressing techniques based on specific gravity.

Major use of fluorspar is in the iron and steel and aluminum industries. In the aluminum industry, high-grade fluorspar is used to produce hydrofluoric acid, a primary ingredient in the manufacture of synthetic cryolite and aluminum fluoride for electrolytic reduction of alumina. The iron and steel industry utilizes fluorspar principally in the basic open-hearth steel process, where it reduces the viscosity of the slag and assists materially in dissolving lumps of lime which may have resisted fusion. Amounts as high as 10 pounds per ton of steel may be used; and in 1961, out of a total fluorspar consumption by industry of 681,833 tons, 155,938 tons were used for this purpose (Kuster and Schreck, 1962, p. 571). 1999 U.S. fluorspar consumption, all from imports (mostly from China and South Africa), was 552,000 tons; 72,000 tons were metallurgical grade.

Ceramic consumption (35,589 tons in 1961) of fluorspar is less than that consumed in the aluminum and the iron and steel industries, but nevertheless its use is vital in the production of enamels for coating steel and cast iron, and in the manufacture of opal and flint container glass.

Hydrofluoric acid, derived from fluorspar, is a starting material for a diversity of chemical compounds. Refrigerants, fluorocarbon elastomers, plastics, and drugs are but a few items requiring hydrofluoric acid for their manufacture. The catalytic action of hydrofluoric acid is used in the manufacture of high-octane aviation fuel. Fluorspar is marketed under three grades, acid, ceramic, and metallurgical. Acid-grade fluorspar is the source material for production of hydrofluoric acid (1961 production, 412,155 tons) and must contain at least 97 percent CaF2 with specific limitations on quartz, calcite, and sulfur content. Specifications for ceramic-grade fluorspar, however, are not as high, and depend somewhat on qualifications set forth by the buyer; in general, acceptable raw material must contain at least 95 percent CaF2, and the amount of quartz, calcite, and iron oxide is limited. Metallurgical-grade fluorspar (1961 production, 228,181 tons) must contain at least 60 percent effective CaF2-effective CaF2 is that percentage of CaF2 remaining after subtraction of 2.5 percent CaF2 for each percent of SiO2 in the raw material.

Fluorspar occurrences in Montana are many and varied. Sahinen (1962, p. 5) has grouped the deposits into the following three geologic provinces:

1. Deposits related to the Idaho batholith.

2. Deposits related to the Boulder batholith.

3. Deposits related to the Potassic province of central Montana. Each will be considered separately in following paragraphs. The discussion is largely drawn from Sahinen's report (1962).

The fluorspar deposits related to the Idaho batholith are, and have been, the primary producers of commercial ore in Montana. Most deposits are in the Belt series of Precambrian age, or their metamorphosed equivalents, and are within the contact zone of the Idaho batholith. They are similar mineralogically in that they all contain masses of quartz and calcite as well as fluorite.

The Crystal Mountain deposit in Ravalli County is the State's largest individual producer, having been brought into production in 1952 (fig. 18). The host rock is granite and fine-grained gneiss; the trend of the ore bodies is east-west. The fluorite ranges in color from white or pale green to deep purple. Gangue minerals are altered feldspar, sericite, quartz, and biotite. Massive quartz overlies the west outcrop. Reserves are not known, but the ore is reported to contain more than 96 percent CaF2. Presently (1962), the deposit is mined by the Roberts Co. of California. Their product is used for metallurgical purposes.

The Snowbird property in southwestern Mineral County and the Spar property in the central part of the same county are in some respects similar types of occurrences. However, they contrast with the Crystal Mountain deposit in that they are nowhere In contact with igneous rock. Massive quartz and calcite are present in large amounts. Both deposits were productive in the late forties to 1950, and though they are considered exhausted, it is conceivable that similar fluorspar deposits might be disclosed through exploration and development work.

The more promising fluorspar deposits related to the Boulder batholith occur near the boundary of the granitic and surrounding rocks. Examples are the Albion mine in eastern Granite County, the Silver Bow deposit in Silver Bow County, the Bald Butte and Boeing prospects in Lewis and Clark County, and the Boulder Mountain and Normany prospects in Broadwater County. Others are the Jetty and Weathervane prospects in Deer Lodge County. No commercial ore bodies of fluorspar are known in any of these properties. However, fluorite appears to be associated with the metalliferous veins, and minable fluorspar bodies may yet be found.

Sweet Grass Hills, Montana, U.S.A. May 1991. The snow-covered buttes of the Sweet Grass Hills, near the border of Montana and Alberta, Canada, are visible in this southeast-looking, low-oblique photograph. Each of the hills, formed approximately 50 million years ago, is a miniature mountain range composed of igneous intrusions and older sediment rocks. During the Bull Lake Ice Age, the great glacier flowed around the three big buttes of the Sweet Grass Hills, leaving them to stand as islands above a sea of ice. Visible is the narrow valley of the Milk River in southern Alberta, Canada (lower left). (NASA photo)
The Potassic Province is an area in central Montana characterized by alkali intrusives, particularly syenitic rocks. It includes the Belt, Highwood, Bearpaw, Little Rockies, North and South Moccasin, and Judith Mountains, and the Sweet Grass Hills. In this province fluorite occurs in veins and disseminations within the syenite, and as veins and replacement deposits in the surrounding limestones. The more promising seem to be those localized in zones in Madison Limestone of Mississippian age. The best known are those in the Sweet Grass Hills in Liberty County, particularly those in the Tootsie Creek area (Ross, 1950, p. 195).

Other fluorspar occurrences within the Potassic Province are in the South Moccasin Mountains, the Judith Mountains and the Little Rocky Mountains. Here its general occurrence is as a gangue mineral in gold and lead-silver veins, narrow stringers in or near fault zones, disseminations and replacements in the Madison limestone, and fine seams or disseminations in rocks of syenitic composition. Even the best deposits probably are too small to be considered of commercial significance.

An important potential source of fluorine, though not of the mineral fluorite, is also present in Montana. The Phosphoria formation of Permian age contains a large reserve of phosphate in the State and is being mined at many places (see chapter on phosphate). The phosphate occurs as the mineral carbonate-fluorapatite, which contains approximately 1 percent fluorine for each 10 percent P2O5 (Altschuler, Clarke, and Young, 1958, p. 49).

No phosphate processing plant in Montana is currently recovering fluorine, but under certain conditions it is technically feasible to do so. The tonnage of rock treated is great enough to make Montana phosphate rock an important potential source of fluorine.

GEMS AND GEM MATERIALS

 

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

Gents and gem materials are naturally occurring substances that combine the properties of beauty, rarity, and durability in sufficient degree to make them prized for personal adornment.

A wide variety of minerals have been used as gems. Many of them are found in placers, where their durability and weight permit their concentration. A few are recovered from lode deposits.

Montana agate
in Mineral Museum, Butte
Prices of gemstones depend on their scarcity, their beauty, and their popularity. Prices range from thousands of dollars per caret for best-quality rubies, sapphires and emeralds to a few cents per caret for some of the varieties of quartz (Jahns, 1960, p. 435). Certain gem material is also used in industry. Diamond and corundum, because of their great hardness, are used in dies, bearings, abrasives, and as cutting agents. Garnet is a widely used abrasive. Quartz, calcite, and fluorite are used as piezoelectric elements in strain gages.

Montana contains several deposits and occurrences of gemstones (fig. 19) (Note: Fig. 19 is provided as an oversized image of the whole state map, here (Fig. 19), and as an enlargement of the area of southwest Montana with numerous localities, below.) The most valuable are the sapphires, which have been found in several places. In fact, the Montana sapphire deposits comprise the most valuable gemstone deposits in the United States. Sapphire is the term given to the colorless, yellow, or blue varieties of the mineral corundum (Al2O3). The red variety is called ruby. Although corundum found in Montana has a variety of colors, very few rubies have been found. However the highly prized "cornflower blue" sapphire occurs in several places, and is especially characteristic of the well-known deposits at Yogo Gulch, Judith Basin County (Clabaugh, 1952, p. 21-22).

The importance of Montana sapphire deposits is indicated by past production figures; $3 to $5 million worth of stones have been produced. Material from Yogo Gulch alone, prior to 1929, was worth about $2.5 million in the rough; cut stones from that deposit have a present value of more than $25 million (Clabaugh, 1952, p. 2) (fig. 19, locality No. 1). Production from this deposit has been intermittent since 1929, but some 20,000 carats were recovered in 1958 (Sinkankas, 1959, p. 60). About 20 percent of this material was of gem quality.

Photo by Will Heierman

Yogo Gulch sapphires occur in a nearly vertical pyroxene-biotite dike that intrudes Madison limestone. The dike is almost 5 miles long and is from 8 to 20 feet in width. Sapphires are distributed uniformly throughout the dike, and the average yield has been estimated to range from 20 to 50 carets per ton of rock (Clabaugh, 1952, pp. 11-18). Sapphires were first found in gold placers in Yogo Gulch some time after 1878, but the dike was not recognized as the source of the gems until later. The deposit has been extensively worked, both at the surface and at depth by underground methods, since the 1880's. The dike is altered to a soft, easily worked material in its upper part, and thus far all production has come from this material. Remaining reserves of altered material are about twice as great as the amount already mined; reserves below the present workings are undoubtedly many times larger (Clabaugh 1952, p. 34).

The first Montana sapphires were found in gravels along the Missouri River northeast of Helena. These placer deposits are chiefly in gravel beds or terraces as much as 200 feet above river level. Sapphires were found at Magpie Gulch (No. 2), about 15 miles due east of Helena, and at American Bar (No. 3). Early attempts to mine the gravels were not encouraging, and operations were mostly small and sporadic until about 1940, when construction of dams and submergence of some of the deposits made large-scale gold dredging possible. Sapphires have since been recovered as a byproduct of gold dredging, but most of these stones have been sold for industrial purposes.

The Rock Creek sapphire deposits on the north side of the West Fork of Rock Creek, about 16 miles southwest of Philipsburg, Granite County (No. 4), are in gravels. Fragments of igneous rock similar to the sapphire-bearing dike near Canyon Ferry have been found, but neither the dike nor the gems have been found in place. Sapphires from this locality generally show a wide range of deep colors.

The upper 4 miles of the South Pork of Dry Cottonwood Creek, Deer Lodge County (No. 5), have been worked for sapphires, but the source rock has not been identified; recovery has apparently been entirely from alluvium and residuum. Most of the stones are of poor quality, and only a few thousand carets is believed to have been produced (Clabaugh, 1952, p. 54).

Sapphires have been reported from other localities in Montana, but the deposits listed above appear to exceed all others in productivity and reserves. It is of interest to note that all of the gem-quality sapphires in Montana have come from such rocks. Deposits of corundum are also known in metamorphic rocks in Gallatin and Madison Counties, but no gemstones occur in these deposits (Clabaugh, 1942, p. 58) (Nos. 6 and 7). In terms of dollar value, the sapphire production of Montana far exceeds that of other gemstones.

A number of other gem materials are known to occur in Montana, and some may be better known than the sapphires. Moss agate, a translucent gray variety of chalcedony with black or brown dendritic inclusions, is a well-known semiprecious stone in Montana. Large quantities of this material are found in gravels along the Yellowstone River between Billings and Glendive. No commercial workings are known; collectors and gem cutters pick the rough stones from gravel bars along the river. The polished slabs cut from them are distinctive and are widely distributed in gift, souvenir, and rock shops. The quantity of moss agate collected each year in Montana is unknown but it is undoubtedly large enough to be of considerable value.

The Pohndorf amethyst mine, about 2 miles northeast of the Toll Mountain picnic grounds in southwestern Jefferson County (No. 8), was worked for many years. Clear, smoky, and amethystine quartz were recovered from cavities in pegmatite (Sinkankas, 1959, p. 356). This deposit is now believed to be worked out, but similar occurrences are known in the area.

Almandine garnet, occurring as small but good quality grains and fragments, is abundant in the gravels of the Ruby River upstream from Alder, Madison County (No. 9). Rhodochrosite, a pink manganese carbonate ore, has been mined from deposits in Butte (No. 11) and Philipsburg (No. 12) and some specimens have been collected for gem material. Rhodonite, a pink manganese silicate, has also been mined at Butte. Gem quality quartz is known in a number of places, as is silicified wood. Clear calcite, which is sometimes used as a novelty gem, has been mined from several veins in Park (No. 13) and Sweet Grass (No. 14) Counties for use in optical instruments (Stoll and Armstrong, 1958). (See also chapter on optical calcite.) See also Directory of Montana mining enterprises for 1986 with a section on the Sapphire deposits of Montana, by D.C. Lawson, MBMG Bulletin 126, 1987.

GOLD

(By A. E. Weissenborn, U.S. Geological Survey)

Most of the data on the individual gold-producing districts of Montana have been abstracted from an unpublished treatise on the occurrence of gold in the United States by A. H. Koschmann and M. H. Bergendahl of the U.S. Geological Survey. Because of the untimely death of Mr. Koschmann, publication of this volume has been delayed. Mr. Bergendahl has kindly allowed the writer to make use of the chapter on Montana in preparing this report. Without access to the great mass of information so painstakingly compiled from many sources by these men, the following summary could have been prepared only in a most superficial form.

Gold has been prized since earliest times and has been mined in almost every part of the world. Because of its wide distribution, its relative scarcity, and its indestructibility, it has been used for monetary purposes since the beginning of history. Because of its beauty, its resistance to tarnish and corrosion, its malleability, and its high value, it is much used for jewelry and in the decorative arts. It has limited but important uses in industry.

Montana gold nugget,
in Mineral Museum, Butte
Although Montana currently ranks only about ninth among the gold-producing States, the discovery of gold and the resultant influx of population had much to do with the early development of the State. Total gold production is not accurately determinable, but best estimates (U.S. Bureau Mines Minerals Yearbooks 1960 and 1961) credit Montana with a production from 1862 through 1961 of 17,657,400 ounces valued at $402,475,000, including both placer and lode gold, or about 6 percent of the total U.S. production. The bulk of the placer gold was produced before 1875 and probably most of it in the 1860's. From 1904 to 1961, as compiled from Mineral Resources and Minerals Yearbooks, the total gold production has been 6,219,865 ounces, of which 5,225,462 ounces has been derived from lode mining and 994,403 ounces from placer mining. Montana has 53 districts, in 17 counties, that have produced in excess of 10,000 ounces of gold each. Four districts--Butte, Helena, Marysville, and Virginia City--have produced more than 1 million ounces, and 27 districts have produced between 100,000 and 1 million ounces.

Many of the gold deposits of Montana and their associated placers are found near the margins of granitic rocks or in roof pendants in these rocks. As shown by Pardee and Schrader (1933), most of the principal gold districts are centered around the Boulder batholith and its satellite stocks (fig. 20 and fig. 4). All of the Montana gold deposits except those at Jardine are late Cretaceous or Tertiary in age; the Jardine deposits are regarded as Precambrian.

In Montana, as elsewhere in the West, placer gold deposits were the first ore deposits to be discovered. The placers along Gold Creek in Powell County, which were found in 1852, were the first discoveries of placer gold in Montana (Lyden, 1948, p. 118), but the discovery in 1862 of the placers along Grasshopper Creek near Bannack in Beaverhead County started the influx of prospectors into Montana (Winchell, 1914, p. 18). Other discoveries followed in rapid succession including the very rich deposits along Alder Gulch near Virginia City, which became the most extensive and productive in Montana. The Last Chance placers on the present site of Helena were discovered in 1864 (Knopf, 1913, p. 15) as were the placers in the Butte district (Weed, 1912, p. 18; Lyden, 1948, p. 144-145). Placer mining flourished during the late 1860's. Some of the deposits were quickly exhausted, but others were worked on a substantial scale until World War II. Since then very little placer mining has been done.

The first lode mine in Montana is said to have been discovered in 1862 in the Bannack district (Shenon, 1931, p. 27). Among the early rich lode discoveries were the Whitlatch-Union in the Helena district in 1864--the first patented claim in Montana--(Knopf, 1913, p. 15), several lodes in the Sheridan district (Winchell, 1914, p. 133), in the Argenta district (Winchell, 1914, p. 69), all in 1864, and several lodes in the Silver Star district in 1867 (Winchell, 1914, p. 139-140), including the Greene Campbell--the second claim patented in Montana. Lode production first became significant in the 1870's but until rail transportation became available in 1882 and 1883 only the richest ores could be mined.

Gold mining in Montana has followed a more or less regular pattern in most districts. Most of the bonanza placers and the richest and more easily mined lode deposits were exhausted in an initial period of feverish activity. Mining declined temporarily, but a second and longer period of activity ensued during which dredging and other mechanized methods replaced hand
Gold microcrystals
operations of the placer mines, and mills were constructed to treat lower grade ore at the lode mines. Production declined sharply as costs increased after the First World War. Mining reached a low ebb in the early 1930's but underwent a dramatic revival when the United States went off the gold standard in September 1933 and the price of gold was officially raised in January 1934 to $35 an ounce (fig. 21). Gold mining declined drastically after October 1942 when a governmental order (L-208) prevented acquisition of vital supplies. Many gold mines were reopened after the end of the Second World War but few were able to continue for more than a few years. In the face of rising costs, gold mining as such declined drastically, and since about 1950 most of Montana's production of gold has been derived as a byproduct from base metal mining--chiefly at Butte. Lode gold mining in Montana is now essentially confined to small operations and placer gold mining has nearly ceased to exist; production from this source was only 132 ounces in 1961, and 135 ounces in 1960. Figure 21 illustrates this decline very clearly. The story of gold mining in Montana thus is largely a recital of past glories. Montana will continue to produce substantial quantities of gold from the operation of its base-metal mines but only a drastic change in the price of gold relative to that of other commodities would induce a revival of gold mining such as occurred in the 1930's. (In 1997-2002, the price of gold ranged from $280 to $330 per ounce.)

The occurrence and production of gold by counties is summarized in the following pages and the salient features of all of the Montana districts which have produced 10,000 or more ounces of gold are tabulated on table 5. The locality numbers in the text and the tabulation correspond to the numbered locations on figure 20, which has been taken from Koschmann and Bergendahl's report (1962). NOTE: Table 5 is provided as a link to the top of the full table, and for each county in the list below, a link is provided directly to the appropriate part of the full table. Use your browser's "back" function to return to this page. In Internet Explorer, Fig. 20 may display as a small, low-resolution version. If this happens, move the cursor over the image, and click the expander button that appears in the lower right corner.


BEAVERHEAD COUNTY

Districts (Table 5)   Beaverhead County has produced at least 370,000 ounces of gold, but early records are incomplete. Before 1900 production from placers was probably considerably larger than production from lodes. From 1904 through 1958 the county has a recorded production of 116,350 ounces of lode gold and 14,800 ounces of placer gold.

The gold deposits are found chiefly in the northern half of the county where the Mount Torrey granitic stock and several smaller satellite lobes have intruded Precambrian, Paleozoic, and Mesozoic sedimentary rocks (Corry, 1933, fig. 6). Most of the gold came from the Bannack, Argenta, and Bryant (Hecla) districts (localities 1, 2, and 3). The placer deposits in the Bannack district were the first significant gold discoveries in Montana and were responsible for the first rush to the territory. The Bannack district was also the site of the first lode discovery in Montana (Shenon, 1931, p. 21).


BROADWATER COUNTY

Districts (Table 5)   The placers of Broadwater County were among the most productive in the State. Records prior to 1903 are not available but from 1904 through 1959 the county has a recorded production of 318,000 ounces from lodes and 34,000 ounces from placers. Lode gold production prior to 1903 probably was small. The total gold production from the county from the beginning of mining through 1960 is probably between 900,000 and 1,225,000 ounces. Production has come mainly from two rich placer districts (Confederate Gulch and White Creek) (Nos. 4 and 5) on the west flank of the Big Belt Mountains, and the Winston, Park, and Radersburg districts (Nos. 6, 7, and 8) on the east side of the Elkhorn Mountains.


CASCADE COUNTY

Districts (Table 5)   Gold production in Cascade County came almost entirely from lode deposits in the Neihart (Montana) district (No. 9). Gold is chiefly a byproduct from the mining of silver-rich, base-metal ores. The total recorded gold output of the district is about 67,000 ounces. The more important ore deposits occur as veins in Precambrian gneisses, schists, and diorite; along contacts of these rocks with Tertiary intrusives and, in a few instances, as low-grade disseminated deposits in Tertiary intrusive bodies. Gold is dominant only in the Snow Creek area where the gold-silver ratio of the ores is high.


DEER LODGE COUNTY

Districts (Table 5)   Deer Lodge County has produced- both placer and lode gold. The Georgetown district (No. 11) has yielded most of the lode gold; the French Creek district (No. 10) most of the placer gold. Records of early production, particularly of placer gold, were not kept but the county is believed to have yielded at least 470,000 ounces of gold, about 425,000 ounces of which was from lode mining. The most productive mines were the Cable and the Southern Cross in the Georgetown district.


FERGUS COUNTY

Districts (Table 5)   Although it is far east of the main mining area in the State, Fergus County has had a respectable production of gold, almost all of it from lodes. Most of the output has come from the Warm Springs district in the Judith Mountains (No. 12) and from the North Moccasin district in the North Moccasin Mountains (No. 13). Total production from 1886 through 1950 was about 653,000 ounces; periods of greatest activity were from 1901 through 1922 and from 1936 through 1942. Production of other metals has been insignificant.


GRANITE COUNTY

Districts (Table 5)   Early records are inaccurate or nonexistent but Granite County is estimated to have produced a minimum of about 710,000 ounces of gold from the beginning of mining through 1962. Of this, 332,000 ounces is thought to be placer gold and 376,000 ounces is lode gold. Most of the lode gold was a byproduct of ores that were mined for small amounts of copper, lead, and zinc. Chief districts are the First Chance (No. 14) in the Garnet Range, Henderson Gulch on the east flank of the John Long Mountains (No. 15), and Boulder Creek and Flint Creek (Philipsburg) in the Flint Creek Range (Nos. 16 and 17).


JEFFERSON COUNTY

Districts (Table 5)   Mining began in Jefferson County about 1864 with the discovery of silver, lead, and gold at Wickes (Pardee and Schrader, 1933, pp. 232-234) and has continued at a fluctuating rate to the present. Gold mining declined during the 1920's but activity increased after the price of gold was raised in 1934. Since 1950, gold mining again has been reduced sharply. Through 1960 Jefferson County has produced a minimum of about 735,000 ounces of gold--615,000 ounces from lodes and 125,000 from placers. Except for the Whitehall district (No. 21) all the gold-producing areas are in the northern part of the county. They include the Clancy-Wickes-Colorado, Basin and Boulder, and Elkhorn districts (Nos. 18, 19, and 20). The lode gold deposits are found in granitic rocks of the Boulder batholith and in the invaded sedimentary and volcanic rocks near the contact. More than 2,000,000 ounces of gold have been produced from the Golden Sunlight Mine, at the south end of Bull Mountain (Whitehall District). This open pit mine operated from 1982-2003.


LEWIS AND CLARK COUNTY

Districts (Table 5)   Lewis and Clark County has produced well over 4 million ounces of gold, about equally divided between placer and lode gold. Two districts within the county--Helena and Marysville--have each produced in excess of 1 million ounces and six others have produced in excess of 100,000 ounces.

Mining began in 1863 or 1864 with discovery of placer gold in the Sevenmile-Scratchgravel district (No. 25) 4 miles northwest of the present site of Helena. Other rich lode and placer deposits were discovered soon afterward in the Helena region. Most of the placers and some of the lodes were soon exhausted, and by 1900, mining had dwindled to small-scale operations. After the price of gold was raised in 1934, gold mining, both lode and placer, again became a major industry, but declined again after 1942.

The Helena (Last Chance) placers (No. 23) have been the most productive in the county; the Marysville district (No. 26) has been the largest producer of lode gold. Other gold districts are Missouri-York (No. 24) east of the Missouri River, and the Rimini-Tenmile, Stemple-Virginia Creek, McClellan, and Lincoln districts (Nos. 22, 27, 28, and 29).


LINCOLN COUNTY

Districts (Table 5)   The mining of gold has been a relatively minor activity in Lincoln County but the county has produced since 1901 a minimum of about 29,000 ounces from lodes and 2,500 ounces from placers. There is no record of earlier production although both lode and placer mines were worked prior to 1901. Most of the lode production has come from the Libby and Sylvanite districts (Nos. 30 and 3l); the placer production has come chiefly from the former.


MADISON COUNTY

Tobacco Root Mountains, Beaverhead River, and Ennis Lake, Montana, U.S.A. March 1992. This near-vertical photograph shows the valleys of the Beaverhead, Madison, and Jefferson Rivers. The confluence of three rivers near the barely discernible city of Three Forks forms the headwaters of the Missouri River (bottom right of photograph). The snow-covered, rugged Tobacco Root Range, from which were mined gold and silver until the early 1900s, consists mostly of Precambrian basement rocks with a core of granite emplaced about 70 million years ago. East of the Tobacco Root Range lies Ennis Lake. The Highland Range and the small, circular McCartney Mountain are visible in the upper portion of the photograph. (NASA photo)
Districts (Table 5)   Madison County ranks third in gold production in Montana, following Silver Bow and Lewis and Clark Counties, and is one of the three counties that has produced more than 1 million ounces. The greater part of the Madison County production has come from placer deposits, and most of it came from Alder Gulch during the first few years following discovery. Small amounts have been produced from at least 40 other gulches, but none of these has been a consistent producer. Unlike other gold-producing areas, placer mining in Madison County was but little affected by the rise in the price of gold in 1934.

Gold lodes are numerous and are found chiefly near the contacts of Precambrian and Paleozoic rocks with the Tobacco Root batholith and smaller intrusives and satellitic stocks. Minimum total gold output of the county is 3,707,600 ounces--2,507,250 from placer and 1,200,000 from lode mining.

The chief gold-producing districts are Virginia City-Alder Gulch (No. 32), Norris (No. 33), Pony (No. 34), Renova (No. 35), Silver Star-Rochester (No. 36), Tidal Wave (No. 37), and Sheridan (No. 38).


MINERAL COUNTY

Districts (Table 5)   Practically the entire gold output of Mineral County has been derived from placer deposits along creeks that drain the east slope of the Bitterroot Mountains; the known lode deposits have produced only minor amounts of byproduct gold. The two most productive areas are the Cedar Creek and Trout Creek districts (Nos. 39 and 40). Output is estimated at 120,000 ounces, most of which was recovered before 1908.


MISSOULA COUNTY

Districts (Table 5)   Gold production in Missoula County has come mainly from placer deposits in the Ninemile and Elk Creek-Coloma districts (Nos. 41 and 42) which have yielded 152,000 to 225,000 ounces of gold. Lode mining in the Elk Creek-Coloma district in the Garnet Range north of the better known Garnet district (No. 14) accounts for an additional 17,000 ounces.


PARK COUNTY

Districts (Table 5)   Placer gold was discovered in 1862 near Gardiner, but the bulk of the gold has come from the lodes of the Jardine district (No. 44) with a smaller amount produced as a byproduct of silver-lead deposits in the New World district (No. 45). A still smaller amount has been contributed by the Emigrant Creek district (No. 43). Lodes and placers have produced a total of about 286,000 ounces of gold, of which placers have accounted for 16,000 ounces.


PHILLIPS COUNTY

Districts (Table 5)   The entire metal production of the county has come front two closely-spaced areas in the Little Rocky Mountains, the Zortman-Landusky or Little Rocky Mountains district (No. 46), the easternmost gold-producing area in Montana. Placers were discovered in 1884 and lode deposits in 1893. The deposits are chiefly gold- and silver-bearing veins in a porphyritic laccolith which has intruded Precambrian schist and Paleozoic sedimentary rocks. Of lesser importance are disseminated deposits in porphyry and replacement bodies in limestone. Early activity was sporadic but the district flourished between 1905 and about 1912 and, except for the war years, again from the early 1930'9 through 1950. It has been nearly dormant since.

Total lode gold production is estimated at about 380,000 ounces. Recorded placer production dating back only to 1928 has been trivial.


POWELL COUNTY

Districts (Table 5)   Most of the gold produced in Powell Counts has come from placer deposits in the southern part of the county. The discovery in 1852 of gold-bearing gravels along Gold Creek is said to be the first discovery of gold in Montana (Lyden, 1948, pp. 118-120) although the placers were not worked until 1862. Placer gold has come chiefly from the Finn, Ophir, and Pioneer districts (Nos. 47, 48, and 49); lode production from the Ophir and Zosell districts (Nos. 48 and 50). Estimated production totals 539,000 ounces of placer gold and 50,000 ounces of lode gold.


RAVALLI COUNTY

Districts (Table 5)   Most of the 9,000 ounces of gold produced in Ravalli Counts since 1903 has come from placer deposits in the Hughes Creek district (No. 51). These were discovered in the early days of mining. Since 1946 production has been small and sporadic. Total production is probably in excess of 10,000 ounces.


SILVER BOW COUNTY

Districts (Table 5)   Silver Bow County is the leading mining county in Montana. Most of the gold has come as a byproduct of mining copper and other base metal in the Summit Valley (Butte) district (No. 52), but the Highland district (No. 53) has also produced both lode and placer gold. Early records are lacking but the total amount of gold recovered from Silver Bow County may be in excess of 4 million ounces; 2,406,000 ounces is the recorded production of the Butte veins--the remainder has come from placer mining and the Highland district lodes.

GRAPHITE

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

Graphite is pure crystalline carbon. It is characterized by its black color, extreme softness, complete opacity, high electrical and heat conductivity, and great resistance to chemical decomposition under all conditions. It is insoluble in all common chemical reagents, but decomposes slowly in the presence of oxygen if heated to 600-700C.

100% of U.S. graphite needs were met in 1999 by imports, mostly from Mexico, Canada, and China. The average price ranged from $220 to $1100 per ton.
Graphite is a common and widespread mineral in metamorphic rocks, although minable concentrations are rare. It also occurs in veins and in thermally metamorphosed coal seams. The three geologic environments in which it occurs produce three distinctive commercial types: lump and chip, flake, and "amorphous." Vein graphite deposits, which produce lump and chip graphite, are rare; flake graphite is widespread in metamorphic rocks in many parts of the world. "Amorphous" graphite (actually extremely fine-grained crystalline graphite) comes only from metamorphosed coal seams.

Important uses are as carbon brushes for electric motors (lump); crucible and refractory wares, especially brake linings, and lubricant (flake); foundry facings, paints, rubber, dry-cell batteries, and pencil leads (amorphous).

Montana has one known commercial graphite deposit, located about 10 miles southeast of Dillon in Beaverhead County (Perry, 1948, p. 13). This deposit, the Crystal Graphite mine, has been worked intermittently since about 1902 (Cameron and Weis, 1960, p. 252), and total production is on the order of 2,200 tons. Most of the material was mined during World War I; no production is reported after 1948.

The deposit contains lump graphite in veins that cut the Precambrian Cherry Creek Group. Wall rocks are principally gneiss and schist, with local pegmatites and some quartzite and dolomitic marble (Armstrong and Full, 1950). Veins appear to be fracture fillings around the nose of a plunging isoclinal fold. The graphite resembles the material mined in Ceylon, which, for many years, has furnished, essentially, the entire world supply of this type. Although high-grade graphite is present in the deposit, tonnage does not appear to be great, and distribution is erratic and difficult to predict.

Crystalline graphite is also reported from an occurrence on Kate Creek, about 15 miles southwest of Armstead. Reserves and quality are not known (Sahinen and Crowley, 1959, p. 15).

GYPSUM AND ANHYDRITE

(By P. L. Weis, U.S. Geological Survey, Spokane, Wash.)

Gypsum, the hydrated calcium sulfate, and anhydrite, the anhydrous form, are salts formed by the evaporation of sea or lake water of appropriate composition. The country's reserves of both minerals are very large, although some of the largest and purest deposits are too far from major markets to be worked profitably under present conditions (Withington, 1962, p. 1). Of the two minerals, gypsum is the most widely used.

Gypsum is used extensively in the building industry. It is calcined to drive off some of the water of hydration, then ground for use in plaster, wallboard, lath, sheathing, tile, and related interior construction materials (Havard, 1960, p. 485). Many other uses exist, but major uses in 1961 were as follows: building industry, about 9.1 million tons; calcined gypsum for industrial uses, 258,000 tons uncalcined gypsum used in cement, agricultural gypsum, etc., about 3.9 million tons. Of somewhat more than 13 million tons used in the United States, about 9.3 million tons were from domestic deposits (Kuster and Jensen, 1962, p. 629-642). In 1999, more than 19 million tons of crude gypsum were produced domestically and were valued at $134 million. This filled 71% of our needs; the remaining 29% was imported primarily from Canada and Mexico.

Montana has large reserves of gypsum, but in 1961 only one deposit was being worked. Production in Montana for the period 1926-59 amounted to 2,827,726 short tons, valued at more than $12,768,000 (R. D. Geach, Montana Bureau of Mines and Geology, written communication, 1962). Deposits of commercially important gyps are exposed in outcrops of four formations: the Ellis formation near Lewistown and in parts of Cascade and Meagher Counties (fig. 22, localities Nos. 1 and 2); the Chugwater formation near Bridger, Lodge Grass Creek (No. 3); the Otter formation in central Montana near Riceville (No. 4); the Kibbey sandstone in central Montana near Kibbey and Lingshire (No. 5); and in southwest Montana near Lima (No. 6) (Sahinen and Crowley, 1959, p. 16; Perry, 1949).

In the Gravelly Range a large tonnage of gypsum formed from hot springs can be observed at irregular intervals along the north slope of the valley of Cottonwood Creek from the south end of Monument Ridge to stream level 2 miles west. Along the Gravelly Range Ridge road east of Monument Ridge there are several extinct geysers with mounds of white gypsum revealing their presence. The gypsum is in the Ellis group of Jurassic age (Mann, 1954).

In addition to the surface exposures of gypsum-bearing formations listed above, a large volume of gypsum-bearing material is present in buried sedimentary rocks in the Big Horn, Powder River, and Williston Basins (Withington, 1962). Montana's reserves of gypsum are therefore undoubtedly great enough to last for many decades; activity of the industry in the State depends on price of the commodity an demand for it, rather than presence of raw material.

IRON

(By R. D. Geach, Montana Bureau of Mines and Geology)

Iron and steel are the foundation of the industrial economy of the United States. No other metal is, or is likely to be, used in such large quantities for so many purposes.

U.S. iron ore production in 1999 totaled about 70 million tons. Iron also was produced using an additional 70 million tons from iron and steel scrap, and 20 million tons more from iron and steel slag.
Pure iron is too soft for most uses, and pig iron, the initial product of the blast furnace, is too brittle. Ultimate use is therefore mostly in the form of cast iron, wrought iron, steel, or one of the hundreds of alloys with iron and other metals. Much smaller quantities of iron are used for paint pigments, cements, and a host of other uses (Tucker, 1960). The United States consumes more than 100 million tons of iron ore each year.

Principal iron ore minerals are the oxides hematite (70 percent iron) and magnetite (72.4 percent iron). Smaller tonnages of iron have been produced from limonite, a mixture of hydrous oxides, and siderite, the iron carbonate, but neither has been a significant source of ore in this country. The largest iron deposits of the world are in Precambrian sedimentary iron formations or their metamorphosed equivalents, and ores of this type have provided by far the largest tonnage for world production.

Until the end of World War II almost all of the iron ore mined in this country was shipped directly to the smelters without further beneficiation other than washing. These crude and washed ores generally contained at least 50 percent iron; most shipping-grade ore contains from about 51 percent to about 60 percent iron, and a few deposits produced ore with as much as 68 percent iron. Since World War II, however, processes have been developed for concentrating and pelletizing lower grade ores, and the proportion of U.S. production that is upgraded has risen from less than 20 percent at the end of the war to 55 percent in 1960. As a result, low-grade deposits (25-45 percent iron) that were formerly considered to be of no value are now worth serious consideration as sources of iron ore.

Montana deposits are generally believed to contain no more than about 30 percent iron, but some appear to be large, and most of the iron-bearing mineral is magnetite which, if not too fine grained, can be easily and cheaply upgraded. Of importance to Montana deposits and to western ores in general is the accelerated growth of population and industry west of the Mississippi, which is creating a potential market in which western iron and steel may enjoy a competitive advantage over distant eastern sources.

The Carter Creek deposit (fig. 23, No. 1), unknown until recently, is believed to contain reserves in excess of 60 million tons of iron ore. The ore body, consisting mostly of magnetite with minor amounts of hematite, is in the Precambrian Cherry Creek group, which is made up of hornblende-biotite-garnet gneisses and metamorphosed limestones. The iron-rich zone is 12,000 feet long and 700 to 900 feet wide. The grade of the deposit is not precisely known, but a limited number of samples show a range of iron content from 30 to 32 percent. Calculations made by the author of the results of a series of tests made by the Bureau of Mines (Holmes and others, 1962, p. 10) show that wet grinding the material to minus 200-mesh followed by wet-magnetic separation will yield a concentrate averaging 59.3 percent iron and recover 85 percent of the iron.

Montana contains several other deposits in the same geologic environment, specifically the Kelly, Ramshorn (Copper Mountain), Johnny Gulch, and Dry Boulder Creek deposits. Although the exact amount of ore reserves is unknown, it is believed that for each deposit quantities estimated at tens of millions of tons are realistic. It is also noteworthy that all of these deposits are within a few score miles of each other, and a thorough investigation of the entire area might well result in finds of great value and importance. There is little question that deposits of this type make up by far the most important Iron ore reserves in Montana.

Index to localities on map (fig. 23)
Number, name
Type
Potential
1 Carter Creek Sedimentary Precambrian significant
2 Kelly Sedimentary Precambrian significant
3 Ramshorn (Copper Mtn.) Sedimentary Precambrian significant
4 Johnny Gulch Sedimentary Precambrian unevaluated
5 Dry Boulder Creek Sedimentary Precambrian unevaluated
6 Blackfoot Indian Res. beach sand,
titaniferous magnetite
unevaluated
7 Radersburg beach sand,
titaniferous magnetite
significant
8 Choteau beach sand,
titaniferous magnetite
significant
9 Elkhorn Mountains magmatic affiliation significant
10 Sheep Creek magmatic affiliation significant
11 Running Wolf magmatic affiliation significant
12 Southern Cross magmatic affiliation significant
13 Cable magmatic affiliation significant
14 Sweet Grass Hills beach sand,
titaniferous magnetite
significant
15 Thunder Mountain magmatic affiliation unevaluated
16 Iron Mountain magmatic affiliation unevaluated
17 Yogo Peak magmatic affiliation unevaluated
18 Great Falls Slag smelter slag unevaluated
19 Anaconda Slag smelter slag unevaluated
20 Helena Slag smelter slag unevaluated

Concentrations of titaniferous magnetite of detrital origin are present along a belt of ancient beach deposits that extends intermittently from the Blackfoot Indian Reservation on the Canadian border southward to Radersburg, Mont., a distance of about 190 miles (Wimmler, 1946b). The ores have been examined superficially at outcrops and it is believed that they are not contiguous from place to place. Iron-rich zones occur in two horizons, the Horsethief sandstone and the Virgelle sandstone, both of Late Cretaceous age. The deposits are lenticular in shape and their thickness may taper from a few inches at the ends to 20 feet at the widest portion. The ore minerals are magnetite and ilmenite in a gangue of quartz and feldspar. The grade of these deposits is not precisely known, but a chemical analysis of a composite sample taken by the U.S. Bureau of Mines at the Choteau deposit showed 43.7 percent iron and 7.2 percent titania (Wimmler, 1946b, p. 7), whereas chemical analyses of channel samples from the other deposits give 10.5 to 55.6 percent iron and 2.01 to 12.7 percent titania (Hubbard and Hencks, written communication, 1962). The exposure hear Radersburg, Mont., contains magnetite, hematite, and minor amounts of copper oxides and calcite veinlets. It is presently operated by Harris Brothers and John Ralls, and their product is shipped directly to the cement plant of the Ideal Cement Co. at Trident, Mont., where it is used in the manufacture of types II and V cements.

The more numerous, though smaller, iron occurrences known in Montana have a magmatic affiliation. The deposits are in general associated with limestones or shales at or near a contact with intrusive igneous rocks. They may, however, be entirely enclosed within the intrusive rock, as for example, the Elkhorn Mountain deposits near Boulder. Their shape is tabular or lenticular, and they swell and pinch horizontally and down dip. Reserves are not accurately known, but some deposits may contain more than several million tons. The iron minerals are predominantly magnetite, hematite, and limonite. Minor amounts of recoverable copper, lead, silver, and gold are also present. The ore mined was high grade, and many of these ores were especially sought in the early days not for the manufacture of iron and steer, but as a fluxing material needed by the copper and lead industry.

At the Sheep Creek iron deposit in Meagher County two major steeply dipping veins are known, and each has been traced for about 1,300 feet along the strike. The width is variable but 38 feet may be an average. Bedded ores also occur nearly at right angles to the dip of the veins. The iron mineral in the veins is chiefly hematite; whereas the bedded ores contain more limonite than hematite. The difference in mineralogy is further emphasized because the iron content in the veins is higher, probably 48 percent iron and 4 percent silica as opposed to 39 to 46 percent iron with 4 to 21 percent silica in the bedded ores (Reed, 1949).

Notable other occurrences are the Running Wolf (Willow Creek) deposit in Judith Basin County (Roby, 1949), the Southern Cross and the Cable deposits in Deer Lodge County (Wimmler, 1946a), the Sweet Grass Hills deposit in Liberty County, Thunder Mountain deposit in Cascade County, and the Iron Mountain deposit in Meagher County.

The Running Wolf deposit is the only property in the State that has produced commercial iron ore for the steelmaking industry within the last 5 years. The ores were mined and shipped to the Great Lakes region and elsewhere by the Young-Montana Co.

The area south of Yogo Peak in Judith Basin County contains many small outcrops of high-grade magnetite deposits. It has been suggested that a magnetometer survey of this area might reveal larger hidden deposits (DeMunck, 1956, p. 49).

Numerous deposits derived from oxidation of preexisting pyritic veins are known in Montana, as well as limonite deposits attributed to hot spring deposition. Their value and potential as a source of metallic iron is probably doubtful due to the small tonnages of ore available and because of their isolation from transportation facilities.

The use of smelter slags as a source of iron has aroused considerable interest in Montana. In 1959, Webb & Knapp, Inc., announced that construction of an integrated steel plant utilizing natural ore and smelter slag derived from the Anaconda Co.'s reduction works at Anaconda, Mont., was being considered. It was explained that the Strategic-Udy process employing electric smelting techniques would be applied to reduce the raw material and that the final products would consist of finished shapes. It was also stated that initiation of the project would depend on obtaining investment capital, estimated at $40 million, and solving marketing problems. As of 1962, Gulf, State Lands & Industries, Inc., a subsidiary of Webb & Knapp, Inc. has applied to the Federal Area Redevelopment Administration for financial assistance. No decision is believed to have been reached (1962).

The slag dump at Anaconda is reported to contain 40 million tons of material. Other slag dumps comparable in size are at the plant of the American Smelting & Refining Co. at East Helena and at the plant of the Anaconda Co. at Great Falls.

Forty-five percent seems to be a realistic estimate of the iron content in the slags, although the exact grade is not known. Most of the iron evidently occurs as a synthetic silicate. Small amounts of copper, lead, and zinc are present, since nonferrous metal recovery is not complete.

If appears likely that Montana contains enough iron ore to provide for at least a moderate-size iron and steel industry. Chief obstacles at present are the distance from existing major markets, and the limited population and consumer demand within the area where Montana iron could realize an advantage in transportation costs.

LIMESTONE (including lime and cement)

(By J. M. Chelini, Montana Bureau of Mines and Geology, Butte, Mont.)

Limestone including dolomite, is the most widely used of all rocks. Over 450 million tons are consumed annually in the United States.

It occurs in some form in every State, is produced in thousands of localities, and is sold as a low-cost mineral commodity to many different industries, which utilize it either in raw crushed form or calcined to lime.

In Montana, limestone is used for concrete aggregate, roadstone, flux, agriculture, railroad ballast, riprap, fill material, filler, sugar refining, portland cement, and lime. Important uses in other areas include coal mine dusting, filtration, limestone whiting, mineral food, and alkali, calcium carbide, glass, and paper manufacture.

Lime production in the US in 1999 totaled more than 20 million tons valued at $1.24 billion. This does not include limestone used as stone or for cement manufacture, but only that used for lime. Major markets for lime are in steel making, flue-gas desulfurization, mining and construction, pulp and paper, water treatment, and precipitated calcium carbonate.
In terms of total domestic production, concrete aggregate and roadstone consume the greatest volume of the raw product. However, cement and lime, the major products manufactured from limestone, exceed in dollar value that of all limestone sold or used in the United States in any one year. Lime is an essential material for more than 7,000 uses, involving many different industries (Patterson, 1960). By far the greatest number of uses are chemical and industrial. In 1961 the value of cement produced in the United States was $1,105,537,000 (West and Lindquist, 1962, p. 391), the value of lime was $210,141,000 (Patterson and Schreck, 1962, p. 799), and the combined value of crushed limestone and limestone dimension stone was $608,139,000 (Cotter and Jensen, 1962, p. 1145, 1156).

Commercially, the word "limestone" is a general term for that class of rocks which contains at least 80 percent of the carbonates of calcium and magnesium. The marketed products are further defined depending upon composition and use. When calcium carbonate is present in excess of 95 percent, the rock is called a high-calcium limestone. High-calcium limestone, the variety used by lime and cement industries, should contain less than 2 percent magnesium carbonate and less than 3 percent of other impurities (commonly silica, alumina, and other insolubles). If 10 percent or more of magnesium carbonate is present, the rock is called magnesian or dolomitic limestone. This decreases its usefulness in the manufacture of lime and makes it unfit for the manufacture of cement; cement manufacturers do not like to use a limestone containing more than 5 percent magnesium carbonate and prefer even less. It does, however, find use for agricultural processes. Portland cement-is commonly made from "natural cement" rock, an argillaceous limestone containing clay and silica in the correct proportions to make cement. However, some cement is made from high-purity limestone by adding the proper amount of clay and silica.

When the content of magnesium carbonate approaches 45 percent in a carbonate rock, it is known as a dolomite. Dolomite is used in the manufacture of high-magnesium lime; its predominant use is in the production of magnesium compounds and as a refractory.

Marine limestones are formed on the sea bottom by any one or combinations of the following processes: slow accretion of organic remains, such as shells; accumulation of carbonate detritus in the same manner as with other elastic sediments; and chemical precipitation.

The above types may be altered to dolomites or dolomitized limestones by replacement of part of the calcium carbonate by magnesium carbonate. Limestones may also be deposited in lakes or streams or from springs. Impurities such as sand, clay, iron oxide, or other detritus may be mixed or interbedded with the calcareous material. Limestones are named, in part, on the basis of these impurities. Thus, siliceous or cherty limestones contain considerable quantities of silica; ferruginous limestones contain iron oxides; and argillaceous limestones contain clay or shale.

Limestones are also classified according to their physical character. Lithographic stone is a very fine-grained crystalline variety, deriving its name from one of its early uses for making lithographs. Travertine is a banded variety of carbonate rock that has been deposited from ground or surface water or from hot springs. Its pleasantly variegated coloring and ease of polishing makes the travertine generally more valuable as building stone than for its calcium carbonate content. Waste material from travertine quarries is marketed as chicken feed, agricultural limestone, and for the manufacture of lime.

In Montana, limestone is found in strata of nearly every geologic age. However, three units of Paleozoic age are the major sources of limestone. These are the Mission Canyon and Lodgepole limestones of Mississippian age, and the Meagher limestone of Cambrian age. Exposures of these units are confined to the central and western part of the State; younger rocks form a thick cover to the east (fig. 24). NOTE: Figure 24 is an oversized image which Internet Explorer may display in a low-resolution mode. To view the full image, move the cursor over the image and click the enlarger button which appears in the lower right corner.

Ridge of Mission Canyon limestone in
the northern Tobacco Root Mountains.
The Mission Canyon limestone is the most important source of high-purity limestone in Montana and crops out in many places in the western part of the State. It characteristically contains more than 95 percent CaC03. At Limespur (fig. 24, locality No. 7) where the limestone was quarried for flux for the Butte smelters and for sugar refining, the calcium carbonate content exceeds 99 percent (Perry, 1949, p. 40). At Sappington (No. 8), where the rock was quarried for use in sugar refining, and at Elliston (No. 5) where rock is currently produced for the manufacture of lime, the calcium carbonate content exceeds 98 percent. Good quality limestone is also found in northwest Montana, at the Stahl quarry near Roosville (No. 1).

The Lodgepole limestone contains thin-bedded limestone, interbedded with chert and shale. Its composition is suitable for cement manufacture, and the Ideal Cement Co., is quarrying it for that purpose at Trident (No. 10).

Cambrian carbonate formations in Montana are commonly dolomitic in composition, but the Meagher limestone is in places a very pure limestone. In the Helena-Townsend region the magnesium carbonate content of the Meagher is less than 3 percent (Perry, 1949, p. 39). Hanson (1952, p. 15) states, "East of a line connecting Ennis and Whitehall, the Meagher formation is entirely limestone, whereas west of a line connecting Butte and the Upper Ruby Valley it is entirely dolomitic." Between these two areas there is a transition zone of intermediate composition.

The upper part of the Meagher is characteristically mottled in dark gray or black and buff and is sometimes termed "black and gold marble." A quarry in the Limestone Hills north of Radersburg (No. 9) was operated for a time by the Vermont Marble Co. Polished slabs for facing work were marketed under the name "Egyptian Limestone." The quarry has not been operated in recent years.

The Meagher limestone is quarried at two localities east and south of Helena. At Maronick (No. 14) it is quarried for flux for the American Smelting Refining Co. plant at East Helena. A few miles to the west it will be quarried for cement rock in the new plant of the Permanente Cement Co. (No. 13).

Other potentially important Paleozoic carbonate formations include the Pilgrim dolomite, the Big Horn dolomite, the Jefferson dolomite, and the Ellis formation (Perry, 1949, p. 32). These limestones are suitable for lime manufacture, but they are not likely to be sought because adequate limestone is generally available in more accessible localities. In addition, beds of impure limestone are found in Precambrian (Belt series), Mesozoic (Cretaceous), and Cenozoic (Tertiary-Quaternary) sedimentary rocks. In the northwestern and extreme western parts of Montana and in the Belt Mountains in the central part of the State, great thicknesses of rocks of the Belt series are found. The calcareous formations of this series are the Helena, the Wallace or Newland, the Siyeh, and the Altyn formations. Assay reports by Johns (1960 and 1961) show that the calcareous rocks of the Belt series in northwestern Montana generally average about 44 percent silica, 10 percent alumina, between 40 and 50 percent calcium carbonate, and 4 percent magnesium carbonate.

Also of potential value among Precambrian calcareous deposits are the massive marbles of the Cherry Creek group exposed in the low foothills of Ruby Range southeast of Dillon, in the vicinity of Virginia City, in the low foothills of the Gravelly Range 15 miles south of Ennis, and in the Bridger Range 5 miles north of Bozeman. They range in composition from nearly pure calcite to nearly pure dolomite. Thicknesses range up to 800 feet. South of Ennis there is an area of magnesium-bearing marble about 2 miles wide and 5 miles long in which the strata stand nearly vertical (Ferry, 1949, p. 33).

The only important Mesozoic limestone is the gastropod limestone member of the Cretaceous Kootenai formation. This member is present in southwestern Montana and ranges in thickness from 10 to 75 feet (Perry, 1949, p. 22); it has been quarried for lime burning near Drummond (No. 3). Recent assays of a grab sample from the quarry site show a calcium carbonate content of 87 percent; magnesium carbonate, 2 percent; insolubles, 9.38 percent; and alumina, 1.13 percent.

Cenozoic travertines of hot spring origin in Montana are believed to be of late Tertiary or early Quaternary age (Ferry, 1949, p. 32). Two large, unusually pure deposits have been developed, and several smaller occurrences are known. The more important of these deposits is near Gardiner (No. 16) (Mansfield, 1933, p. 7). It is being quarries by the Montana Travertine Co. for use decorative building stone.

Rock from the Gardiner quarry is used for building and ornamental stone, though the rock is chemically suitable for the manufacture of lime and chemical compounds. A typical analysis of the stone is: calcium carbonate, 95 percent; magnesium carbonate, 0.9 percent; silica, 0.9 percent; and iron oxide, 0.2 percent.

Another deposit of travertine is on the south flanks of the North Moccasin Mountains of central Montana, north and west of Lewistown (No. 19). The deposit is contained in an area of about 6 square miles and has a maximum thickness of 250 feet (Calvert, 1909, p. 36) and is presently being quarried for building stone, but it is also suitable for many uses requiring pure high-calcium carbonate rock. Other undeveloped travertine deposits occur in the general area north and east of Lewistown (Perry, 1949, p. 42). See also Limestone, dolomite, and travertine in Montana, by J.M. Chelini, MBMG Bulletin 44, 1965.