By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Lewis and Clark County produced between 4 and 5 million ounces of gold through 1959 and is one of three counties in Montana to exceed an output of 1 million ounces. Within the county two districts - Helena and Marysville - have each produced in excess of 1 million ounces, and six others have produced in excess of 100,000 ounces. The gold yield is roughly divided equally between placer and lode deposits.
The first mineral deposits discovered and mined in the county, either in 1863 or 1864, were the gold-bearing gravels along Iowa Gulch in the northern part of the Scratchgravel Hills, about 4 miles northwest of Helena (Pardee and Schrader, 1933, p. 36). The famous Last Chance Gulch placers on the present site of Helena were discovered shortly afterwards in the summer of 1864, and rich gold lodes were discovered a few miles south of Helena in September of the same year (Knopf, 1913, p. 15).
Most placer deposits and some lode deposits were quickly exhausted, and mining languished by 1900. However, after the price of gold was raised to $35 per ounce in 1934, both placer and lode districts were reactivated and gold mining again became a major industry.
The placers of the Helena-Last Chance district have been the most productive in the county, but other highly productive placer deposits occur chiefly in the western part of the county and, from south to north, include the following (Lyden, 1948, p. 54-74) : Tenmile Creek and its tributaries in the Rimini district, Sevenmile Creek in the Scratch-gravel district, Silver Creek in the Marysville district, Virginia Creek in the Stemple district, McClellan Creek west of Stemple, and Lincoln Gulch in the Lincoln district.
Rich placers were also exploited in the southeastern part of the county east of Helena in the York district on the western slope of the Belt Mountains along the Missouri River, York Gulch, and Oregon Gulch (Pardee and Schrader, 1933, p. 120-122, 176-182; Lyden, 1948, p. 68-74).
The chief lode deposits are in and near the Boulder batholith and its satellite stocks and in the roof rocks (Pardee and Schrader, 1933, pi. 1). The Boulder batholith, of Late Cretaceous or early Tertiary age, is 60 miles long from north to south and averages 18 miles in width. The Marysville district has been the largest producer of lode gold in the county, but sizable production has also come from the Helena, Rimini, Stemple, Dry Gulch, and Scratchgravel districts.
HELENA-LAST CHANCE DISTRICT
The Helena-Last Chance district, in the southern part of Lewis and Clark County, in and around Helena, the capital of Montana, includes the famous Last Chance Gulch placer deposits, among the richest and most productive in Montana. Placer gold was discovered in Last Chance Gulch in 1864; other placer deposits in neighboring gulches were discovered soon after, and in the fall of the same year lode gold was discovered at the Whitlatch-Union mine, the most productive lode in the district. Gold accounts for more than 99 percent of the total value of the mine production from the district.
Most of the rich placers and lodes were mined out before 1900, and mining operations after that time were intermittent and on a small scale until 1934, when the gold price increase reawakened activity. During 1935-50 a successful dredging operation yielded considerable gold. Lode mines were also reactivated and were productive to 1940 but declined thereafter (Lyden, 1948, p. 56-57). There was no recorded production from lodes or placers during 1954-59.
The estimated value of the early placer gold production, most of which was taken out before 1868, ranges from $10 to $35 million (Knopf, 1913, p. 15, 86). Pardee and Schrader (1933, p. 186) credited the district with a placer production of $17,079,000 (826,275 ounces) and with lode production of $6,304,000 (305,000 ounces) from 1864 through 1928.
From 1929 through 1959 the district produced about 40,120 ounces of lode gold and about 110,600 ounces of placer gold. Thus the total production of the district through 1959 was at least 345,000 ounces of lode gold and about 940,000 ounces of placer gold.
The Helena-Last Chance district lies along the north edge of the Boulder batholith, a mass of quartz monzonite and related rocks of Late Cretaceous or early Tertiary age which has intruded a thick sequence of sedimentary rocks of late Precambrian, Paleozoic, and Mesozoic age and volcanic rocks of Late Cretaceous age. Other igneous rocks in the district are porphyry dikes and sheets of pre-Tertiary age and small intrusive masses, lava flows and tuffs of rhyolite, all of Miocene age.
The sedimentary and volcanic rocks were folded into a large dome about 25 miles in diameter which extends beyond the district. The folds were ruptured by the intrusion of the Boulder batholith, and additional faulting occurred as the intrusive forces relaxed.
The ore deposits are mainly near the contact of the batholith with the sedimentary rocks; some are in the granitic rocks, and others are in the adjacent hornfels or tactite. The ore minerals in the contact deposits are pyrite, pyrrhotite, and gold, and local chalcopyrite and galena; they occur in aggregates of lime-silicate minerals, tourmaline, quartz, ankerite, and chlorite.
The Whitlatch-Union lode, the most productive in the district, lies partly in the granite and partly in hornfels. The vein ranges in width from a thin seam to 15 feet and averages about 4 feet. The ore taken out in the early years averaged from $20 to $25 per ton in gold (Knopf, 1913, p. 99).
The Lincoln district includes Lincoln Gulch and several other tributaries of the Blackfoot River near the town of Lincoln in the western part of Lewis and Clark County. Most of the gold mined in the district was from placer deposits; a small amount was from lodes. The gold placers, which were discovered about 1865, were rich and hastily worked, and by about the middle 1870's the camp was virtually abandoned (Pardee and Schrader, 1933, p. 115-117).
Pardee and Schrader (1933) estimated that during these early years a stretch of the gulch 7,400 feet long yielded about $7 million (338,653 ounces) in gold. The placers were worked intermittently from 1904 through about 1955 and yielded at least 2,700 ounces of gold. The total placer production through 1959 was about 342,000 ounces.
Lode production, which probably totaled less than 200 ounces, was mined sporadically, mostly during the 1930's.
Low-grade gold ore, which averages $2.20 to $3 per ton, is found in a diorite dike that has intruded calcic argillite of the Belt Series (Pardee and Schrader, 1933, p. 116-117). The lode follows a shear zone and is as much as 30 feet wide. The diorite in the shear zone is largely replaced by quartz, siderite, and pyrite.
All production of the McClellan district, which is in the western part of Lewis and Clark County about 8 miles south of Lincoln, has been placer gold. Placer mining in McClellan Gulch dates back to 1864, and by 1875 these deposits yielded an estimated $7 million (338,653 ounces) in gold (Pardee and Schrader, 1933, p. 117). The gravels were very rich and have been reworked in places as many as two or three times since 1875.
The amount of gold recovered since 1875 is not known. The total minimum production of these placers through 1959 was about 340,000 ounces. About 10 ounces of lode gold was produced in the late 1940's.
The source of the gold probably was the low-grade gold-quartz lodes that crop out on the slopes at the head of the gulch (Pardee and Schrader, 1933, p. 118).
MARYSVILLE-SILVER CREEK DISTRICT
The Marysville-Silver Creek (Ottawa) district, near the headwaters of Silver Creek about 18 miles northwest of Helena, also includes the Bald Butte area. The district has been one of the most productive precious-metal mining districts in Montana. Most of the gold has come from veins, although a smaller amount has come from placer deposits. Some mines have also produced substantial amounts of lead.
The first placer mining in the district was along Silver Creek in 1864, and these placers accounted for at least 75 percent of the placer production of the district (Lyden, 1948, p. 60). The placers were rich and were mined out in the early years; in fact, no placer activity was reported in the district from 1904 to 1933 (Lyden, 1948, p. 60). From 1938 to 1941, dredging and dragline shovel operations were undertaken.
The placer production during the early period was estimated at about $3,200,000 (154,813 ounces) (Lyden, 1948, p. 60), and the total through 1959 was about 164,500 ounces.
Lode mining dates back to 1876 and the discovery of the rich Drumlummon lode, the most productive and most steadily mined lode in the district (Knopf, 1913, p. 61-62). In the early 1890's, the Drumlummon property became involved in protracted litigation and the mine was worked only intermittently. In 1911 the mine was sold, and the new owners rehabilitated both the milling plant and mine and began exploration for new ore bodies. In later years tailings from the Drumlummon mill were also reworked (Pardee and Schrader, 1933, p. 63). The last significant lode gold production was reported in 1951.
The lode production of the district before 1903 was valued at about $30 million in gold and silver (Knopf, 1913, p. 62), of which possibly 60 percent was in gold. About half of the early production was from the Drumlummon mine. Total lode gold production through 1959 was about 1,145,800 ounces. If placer production is included, the district had a total yield through 1959 of about 1,310,000 ounces.
The Marysville-Silver Creek district is centered around a small stock of quartz diorite of Late Cretaceous or Tertiary age that has intruded limestone and shale of the Belt Series of Precambrian age (Barrell, 1907, p. 7-19). The sedimentary rocks adjacent to the stock have been metamorphosed to a hard and dense-textured hornstone locally called slate, in a zone ranging from 1/2 to 2 miles in width. Numerous dikes of pegmatite, aplite, and diorite porphyry cut the stock and the sedimentary rocks.
The ore deposits are steeply dipping gold and silver veins around the border of the quartz diorite stock. Some veins are in the marginal part of the diorite, but most are in metamorphosed sedimentary rocks. The gold is finely divided and accompanies the ore minerals tetrahedrite, chalcopyrite, pyrite, sphalerite, and galena. The gangue minerals are chiefly lamellar quartz and calcite (Knopf, 1913, p. 64-66); the calcite contains some iron and manganese.
MISSOURI RIVER-YORK DISTRICT
Located in the southeast corner of Lewis and Clark County, on the west side of the Belt Mountains, the Missouri River-York district includes Trout Creek, York, Clark, Oregon, Cave, and Magpie Gulches - all tributaries of the Missouri River. Most of the production of this district came from placer deposits, but a significant amount came from lodes.
Placer gold was discovered in this area in 1864 about half a mile above the mouth of York Gulch (Pardee and Schrader, 1933, p. 176) ; placers along its tributaries and other streams in the district were discovered about the same time or a year or two later. Most of the placers were rich, and within a few years after discovery they were either mined out or the richest parts had been depleted and they were then abandoned. Years later some were worked by dredges, and again substantial amounts of gold were produced, especially during the periods 1909-13 and 1934-44. No placer production was reported from 1950 through 1959.
Authentic records of the early placer production of the Missouri River-York district have not been found. On the basis of the size of deposits and reported grades, the production to about 1928 of the individual deposits was estimated (Pardee and Schrader, 1933, p. 177-182) as follows:
The figures given probably should be accepted as the minimum production of these placers. Some estimates of the output of the York-Trout Creek placers alone were as high as $5 million (Pardee and Schrader, 1933, p. 176). The production of the district from 1928 through 1950 was about 41,200 ounces. The minimum placer production of the district was therefore about 265,000 ounces.
The lode deposits occur chiefly in Dry Gulch and other tributaries of Trout Creek above the old town of York. Discovery of the first lode in the district, the gold-quartz lode of the Old Amber mine, was probably made soon after mining had begun (Pardee and Schrader, 1933, p. 120), and a mill to work the ore was built before 1870. Outcrops of gold-bearing quartz, occurring mainly in shale along a dike, attracted early attention. Numerous veins along the dike were developed, and the ore was hauled to Trout Creek or other streams where it was worked in small mills and arrastres.
Several mines were active between 1895 and 1900 (Pardee and Schrader, 1933, p. 121), but after 1900 all were abandoned except the Golden Messenger mine in Dry Gulch, the most productive in the district. Mining of the Golden Messenger began in 1899 (Pardee and Schrader, 1933, p. 121, 146) and continued through 1942. The lode mines were virtually idle from 1942 through 1959.
Incomplete records credit the lode mines with production worth $450,000 to $600,000, chiefly in gold, before 1932 (Pardee and Schrader, 1933, p. 121-122). Lode production from 1933 through 1943 was about 51,440 ounces, most of which came from the Golden Messenger mine. The minimum lode production of the Missouri River-York district was about 70,000 ounces. The district can thus be credited with a total gold output through 1959 of about 335,000 ounces from lodes and placers.
The area is underlain by shaly, slaty, and calcareous rocks of the Belt Series, which are folded into a large northwest-trending anticline whose northeast limb is cut by an overthrust fault. The sedimentary rocks are cut by quartz diorite dikes and stocks of Late Cretaceous or Tertiary age (Pardee and Schrader, 1933, p. 123-134).
The lodes and the placer deposits of the Missouri River-York district are closely associated with the quartz diorite dikes. Most of the lodes are small quartz veins in fractures in diorite and along the bedding planes in the adjacent shale. The veins are valuable chiefly for gold (Pardee and Schrader, 1933, p. 139-144; 147-160); silver and lead are minor constituents. The veins consist almost entirely of quartz, a little pyrite, and scattered grains of galena.
The ore shoots range from a few inches to several feet in width and from a few feet to several hundred feet in length. The ore bodies in the Golden Messenger mine are irregular replacement deposits along fractures in the quartz diorite which has been altered and more or less replaced by quartz, ankerite, and small amounts of sulfides. Pyrite is the most abundant sulfide, but galena, sphalerite, and small amounts of chalcopyrite are also present. Most of the ore mined, both in the replacement shoots and in the veins, was oxidized.
The Rimini-Tenmile (Vaughn) district in the valley of Tenmile Creek in the southern tip of Lewis and Clark County, about 14 miles southwest of Helena, has produced chiefly gold, silver, and lead. The first location was probably made on the Lee Mountain lode in 1864, and the first mine was a tunnel driven on the Eureka vein in 1865 (Pardee and Schrader, 1933, p. 246).
Through 1957 the district had an output of about 194,000 ounces of lode gold and about 4,275 ounces of placer gold. The most active and productive period of the district was before 1907, when about 169,500 ounces of gold was mined. Thereafter the deposits were worked on a small scale until 1957, and the district was idle from 1957 through 1959.
The prevailing country rock is coarse-grained quartz monzonite and aplite of the Boulder batholith, which is intrusive into Upper Cretaceous andesitic and quartz latitic volcanic rocks. Rhyolite of Tertiary age caps the older rocks (Knopf, 1913, p. 80-85, and pi. 1).
In the Remini-Tenmile district, two periods of mineralization are recognizable: one of Late Cretaceous and one of late Tertiary age (Knopf, 1913, p. 81). The older and more productive ore bodies are auriferous silver-lead veins in tourmalinized and sericitized quartz monzonite in the upper part of the batholith. The principal ore mineral is galena accompanied by sphalerite, pyrite, arsenopyrite, and a little chalcopyrite and tetrahedrite; most of the lodes are notably tourmaline-bearing.
The Tertiary deposits are low grade and consist of altered rhyolite impregnated with gold along small fractures (Knopf, 1913, p. 82). The rhyolite rests un-conformably on the eroded surface of the quartz monzonite.
The Sevenmile-Scratchgravel district includes the Scratchgravel Hills which are about 4 miles northwest of Helena and immediately north of Sevenmile Creek. The ore deposits include gold placers, rich silver-lead veins, and gold veins (Pardee and Schra-der, 1933, p. 35-62).
Placer deposits were discovered in Iowa Gulch, in the northern part of the Scratchgravel Hills, shortly before gold was discovered in Last Chance Gulch at Helena in 1864. These deposits were not large, and there has been little activity on them since the early days. The amount of gold produced is not known (Pardee and Schrader, 1933, p. 36). Other placer deposits along Sevenmile Creek and its tributaries, including Greenhorn Creek, were considerably more productive.
These creeks were mined for an aggregate length of 12 miles or more, and by 1930 yielded an estimated $1,200,000 (58,055 ounces) worth of gold (Pardee and Schrader, 1933, p. 59). From 1930 through 1959 the yield probably was less than 750 ounces (Lyden, 1948, p. 58-59).
The lode deposits were discovered before 1872. Some rich silver-lead ore was mined in the early years, and in 1914 rich gold ore was found in the Franklin and Scratchgravel mines. The period 1916-18 was one of great prosperity in which the lode mines, primarily the Franklin and Scratchgravel mines, produced at least $550,000 (26,600 ounces) in gold. By 1919, however, costs of supplies and labor forced these mines to close, and lode production was then limited to desultory output of several smaller mines.
Total lode gold production of the district through 1959 was about 48,700 ounces; total lode and placer production through 1959 was at least 108,000 ounces.
The country rock consists of shale, sandstone, and limestone of the Belt Series of late Precambrian age and quartzite, shale, and limestone of Cambrian, Devonian, and Mississippian age. The bedded rocks are folded and are intruded by diorite and quartz monzonite, probably offshoots of the Boulder batholith, of Cretaceous or Tertiary age (Pardee and Schrader, 1933, p. 36-37; 59).
The ore deposits in the Scratchgravel Hills include contact-metamorphic deposits, gold veins, and lead-silver veins. Most of the gold deposits are gold-quartz veins in quartz monzonite. Pyrite is the most common ore mineral in the unoxidized ores, and in some veins there are scattered grains and bunches of galena and a little sphalerite. Gold can be panned from the oxidized ore (Pardee and Schrader, 1933, p. 37-58).
In the Sevenmile Creek area the lodes yielded chiefly silver, lead, copper, and gold. The lodes are irregular pockets or pipelike bodies in limestone near the quartz monzonite contact. Most of the ore was oxidized and the chief constituents were iron oxides, gold, silver-bearing galena, and copper carbonates.
STEMPLE-VIRGINIA CREEK DISTRICT
Located about 28 to 35 miles northwest of Helena in the drainage basin of Virginia Creek, the Stemple (Gould)-Virginia Creek district contained both placer and lode ore deposits. The lode ore was valued chiefly for gold; only about 5 percent of its value was silver (Lyden, 1948, p. 63). Mining began at least as early as 1878 when the Homestake lode in the Stemple area was located; in 1884 the Jay Gould, the principal mine in the district, was discovered (Pardee and Schrader, 1933, p. 77, 86). These and other mines in the district were worked intermittently.
Beginning in 1922 the Jay Gould mine operated almost continuously to 1942 when all minework was suspended. A small production from the district was reported for the period 1943-51.
The gravels along Virginia Creek have been mined from Stemple to its mouth, a distance of about 8 miles. Their date of discovery has not been ascertained. The gravels were moderately rich but not very deep; prior to 1927 they yielded at least $600,000 (29,028 ounces) in gold (Pardee and Schrader, 1933, p. 86). From 1927 to 1942 small intermittent production was reported (Lyden, 1948, p. 63), but it probably totaled less than 200 ounces.
The Stemple-Virginia Creek district was most productive during the early years, but the amount cannot be definitely ascertained. The production of the Jay Gould mine to 1914 has been estimated to be worth $2.5 million, more than 95 percent of which was the value of gold and the remainder, silver. The lode production of the Gould area, including some silver, through about 1927 was about $3 million (Pardee and Schrader, 1933, p. 77, 81) or about 135,000 ounces of gold.
In the Stemple area the output before 1927 was about $420,000 (20,319 ounces) in gold (Pardee and Schrader, 1933, p. 86). Total production of lode gold through 1959 was about 216,000 ounces, most of which was from the Jay Gould mine. Placers yielded about 29,200 ounces, which made a district total of about 245,000 ounces.
The country rock of the district comprises shale and argillite of the Belt Series, a stock of quartz diorite, and a sill and dikes of diorite, all of Cretaceous or Tertiary age. Near the contact with the stock, the sedimentary rocks are altered to hornstone. The rocks are tilted and cut by small faults (Pardee and Schrader, 1933, p. 78).
The ore bodies are in veins in sedimentary rocks and in the quartz diorite. The Jay Gould vein cuts the metamorphosed sedimentary rocks that adjoin the stock of quartz diorite. The vein is banded, has probably filled an open fissure, and consists largely of lamellar calcite, quartz, and small amounts of chalcopyrite, argentite, and native gold distributed along streaks and bands.
Small amounts of iron and manganese oxides and malachite accompany the streaks of ore minerals. The argentite and gold are closely associated. The veins in the stock consist of coarse granular quartz, with pay streaks of gold-bearing iron oxide and copper carbonate (Pardee and Schrader, 1933, p. 79-84).