By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Located in north-central Montana, Phillips County is the easternmost gold-producing area in the State. Prior to 1915 Phillips County was part of Blaine County and before 1912 both were part of Chouteau County. Almost the entire metal production of the county has come from gold-silver lodes in the Little Rocky Mountains.
Gold placers were discovered in 1884 and were worked intermittently for a number of years, but they yielded rather insignificant returns. Gold lodes were found in 1893 at the site of the August mine, and small amounts of gold were mined, even though the area at that time was part of the Belknap Indian Reservation. The mineral-rich land was thereupon withdrawn from the reservation; soon additional deposits were found and developed, mills were built, and by about 1906, the mines produced $950,000 worth of ore (Emmons, 1908, p. 97).
Except for the periods 1919-21, 1925-29, and 1943-45, the lode mines were fairly active until 1951. The Ruby Gulch and August mines were the major gold-producing properties. From 1951 through 1959, only 6 ounces of gold was reported from Phillips County. Total gold production through 1959 was about 380,000 ounces; virtually all production was from lodes.
LITTLE ROCKY MOUNTAINS DISTRICT
The Little Rocky Mountains (Zortman-Landusky) district is in southwestern Phillips County. Its history and production are synonymous with that of Phillips County and need not be repeated here. A laccolithic dome forms the Little Rocky Mountains. Its core of Precambrian gneiss is exposed in the deeper gullies in the central part of the mountains.
Overlying the Precambrian rocks are, in ascending order, the Flathead Sandstone of Cambrian age, the Emerson Formation of Cambrian and probable Ordovician age, the Bighorn Dolomite of Ordovician age, the Maywood Formation and Jefferson Limestone of Devonian age, the Three Forks Shale (?) of Devonian and Mississippian age, and the Madison Group of Mississippian age. Unconformably overlying rocks of the Madison Group is the Rierdon Formation of Jurassic age, and this is succeeded upward by the Swift and Morrison Formations of Jurassic age, the Kootenai Formation, the First Cat Creek sand (of drillers), the Thermopolis Shale, Mowry Shale, Warm Creek Shale, and the Montana Group of Cretaceous age (Knechtel, 1959, p. 726-745). These Mesozoic sedimentary rocks have an aggregate thickness of 4,000 feet.
The sedimentary formations were arched and invaded by magma which spread along bedding and formed laccolithic masses. These igneous rocks range in composition from syenite porphyry to tinguaite porphyry (Corry, 1933, p. 32). The rocks are offset by a complex system of thrusts and circular faults.
The major ore deposits are in shear zones in porphyry; other less important deposits are replacement bodies in limestone. The mineralized shear zones contain auriferous pyrite and sylvanite in a gangue of quartz and fluorite. Native gold and limonite and manganese oxides are the principal components of the oxidized deposits (Corry, 1933, p. 33-35).