Lewis and Clark County Montana Gold Production
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.
SEVENMILE-SCRATCHGRAVEL DISTRICT
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.