By A. H. KOSCHMANN and M. H. BERGENDAHL – USGS 1968
In 1866, prospectors from the Elk Creek area found gold on Napias Creek in the unexplored north-central part of Lemhi County. The town of Leesburg was soon founded and became a base from which discoveries were made in the northern part of the county at Gibbonsville, Moose Creek, Bohannon Bar, and Yellow Jacket. Gold placers and then lode deposits were discovered at Yellow Jacket in 1868. Nearly all the gold discoveries in Lemhi County were made before 1900, and most of the production, which was largely from placers, occurred in the early days. In recent years byproduct gold from copper-cobalt ores in the Blackbird district caused a significant increase in gold output.
Umpleby estimated that the total gold production of Lemhi County to 1911 was worth $13,702,256 (about 665,060 ounces), of which $6 million represented the output of placers before 1881.
Staley listed annual gold production data for Lemhi County from 1874 through 1942. Total output for this period was 514,430 ounces. From 1942 through 1958, the county was credited with 56,295 ounces of gold. Total production through 1959 was 570,725 ounces, according to Staley’s data, or 720,000 ounces, according to Umpleby’s estimate.
The oldest rock is Archean granitic gneiss, exposed around Shoup, in the northwestern part of Lemhi County. This is unconformably overlain by argillites, phyllites, and quartzites of the Belt Series of Precambrian age. In the southern part, the Precambrian rocks are overlain by Cambrian quartzite and Ordovician, Silurian, Devonian, and Mississippian limestones and dolomites. Farther north in the Leesburg quadrangle, Ordovician quartzite lies unconformably on the Belt Series. Lake beds of Miocene age, occupying an area 8 miles wide and 90 miles long, occur in the valley of the Lemhi River near the east boundary of the county. At several localities in the northern part, granitic rocks probably related to the Idaho batholith cut the pre-Tertiary rocks. Numerous dikes, ranging from granite porphyry to lamprophyre, are associated with the larger intrusives. The dominant igneous rocks of Tertiary age are lava flows and welded tuffs that cover much of the central part of the county. These are known regionally as the Challis Volcanics of Oligocene age.
The gold lodes are fissure veins and replacement deposits along shear zones. There are two recognized periods of mineralization: Late Cretaceous or early Eocene, and late Miocene or early Pliocene.
The Blackbird district, about 35 miles southwest of Salmon, was discovered in 1893. Ores were first worked for gold, with discouraging results, but copper was found in 1896 and cobalt in 1901. Before 1957 the district produced only 1,205 ounces of gold. However, increased activity at the Calera cobalt-copper mine yielded 3,683 ounces of gold in 1957, a total of 9,506 ounces in 1958, arid an undisclosed amount in 1957; all production was a byproduct of cobalt-copper ore.
Rocks of the Blackbird district are metamorphosed sedimentary rocks of the Belt Series of Precambrian age, intruded by granitic rocks of the Idaho batholith. At the Calera mine, the only significant producer in the district, quartz-biotite and garnet-chloritoid schists are the favorable host rocks for copper-cobalt deposits. These rocks are contained in a structural unit, the Blackbird block, which is 5 miles long, 2 miles wide, and is elongate northward. Copper-cobalt mineralization is controlled by north-trending shear zones and north-plunging folds; deposits are in veins and lenses formed by replacement. The ore minerals are chalcopyrite, cobaltite, safflorite, and gold; the gold is present in trace amounts to a few hundredths of an ounce per ton.
CARMEN CREEK-ELJDORADO-PRATT CREEK-SANDY CREEK DISTRICT
Carmen Creek, Eldorado, Pratt Creek, and Sandy Creek are contiguous small camps in the northeastern part of Lemhi County along the flanks of the Beaverhead Mountains. The Kirtley Creek camp is also in this area but it will be considered separately.
Mining first began in the 1870’s in the Eldorado area, where Chinese were mining the gravels on Bohannon Bar. From 1895 through 1911, the placers produced $350,000 in gold. Lode mines were also developed but were not very successful.
In the Carmen Creek camp, only small amounts of bullion were produced. The only property of significance was the Oro Cache mine, opened about 1897. At Pratt Creek, gold lodes were found in the early 1890’s at the Goldstone mine. According to Ross, total production from all camps was about $1 million, including $500,000 in gold credited to the Kirtley Creek camp. Considering that about $350,000 came from the Eldorado camp, the Pratt Creek, Carmen Creek, and Sandy Creek camps produced about $150,000 in gold. In the 1930’s and 1940’s this area was active, but only small amounts of gold were mined. Total gold production through 1959 was about 24,500 ounces.
The rocks underlying this district are quartzites, quartzitic slates, and schists of the Precambrian Belt Series which are covered in the valleys by Miocene lake beds. The Precambrian rocks are cut locally by diorite dikes and granitic intrusions. Most of the lode deposits are found in the metasedimentary rocks and are fissure fillings of quartz with variable amounts of pyrite, chalcopyrite, galena, and sphalerite. Gold occurs with the sulfides.
The Gibbonsville district, at the northern apex of Lemhi County, was discovered in 1877. Gold placers were worked extensively and several lode properties, particularly the A. D. and M. mine, were, very productive . From the time of a disastrous fire in 1907 through 1959 mining was sporadic. Total production, according to Ross, was about $2 million in gold, about half of which came from the A. D. and M. mine.
Bedrock in the district consists of thick beds of quartzite, quartzitic slate, and micaceous slate, which strike northwest and dip steeply to the east. Ross assigned these rocks to the Belt Series of Precambrian age. Diorite dikes which cut the Precambrian rocks were believed to be Precambrian in age by Umpleby and Mesozoic by Ross.
The gold lodes occur in the quartzite and slate as narrow east-trending veins that are broken by numerous faults. The primary vein minerals are auriferous pyrite and chalcopyrite in a gangue of quartz and local calcite.
KIRTLEY CREEK DISTRICT
The Kirtley Creek district, in Tps. 21 and 22 N., R. 23 E., about 6 miles east of Salmon, was for a short time the most productive placer area in the State. Gold-bearing gravels were found along Kirtley Creek some time before 1890, and extensive hydraulic operations were conducted in them between 1890 and 1894. Production in the district declined in the late 1890’s but was revived in 1910 when a dredge was brought in which successfully mined the gravels until the deposits were depleted in 1918. From 1932 through 1959 the district was active for short periods on a small scale.
During 1910-18 about $500,000 (about 24,300 ounces) in gold was dredged from the placers. No data were found on the production before 1910. Production from 1932 to 1947 was 2,146 ounces; none was reported from 1947 through 1959.
Some gold-quartz veins at the head of Kirtley Creek were worked. At the White House mine, $40,500 in gold was produced. The veins contain pyrite, chalcopyrite, galena, and free gold in a quartz gangue.
Total minimum production from the district was about 27,500 ounces, mostly from placers.
The Mackinaw (Leesburg) district is about 10 miles west of Salmon.
In 1866 gold placers were discovered at Wards Gulch along Napias Creek, and for the next 14 years these placers were worked on a large scale. The population of the town of Leesburg in the district was 7,000 during this period, but by 1911 it had dwindled to 25 . Lodes were productive in this district, though on a much smaller scale than the placers. The vein deposits were first developed in 1870 but were worked for only a few years; however, the placers were worked intermittently to about 1954. The district was inactive from that date through 1959.
The estimated the value of placer production of the district at not more than $5 million and the lode output at about $250,000. Shockey listed annual production from 1902 through 1954, which totaled 7,635 ounces from placers and 1,146 ounces from lodes. Total gold production through 1959 from all sources, including the estimate of early production, was about 271,200 ounces.
The oldest rocks in the area are phyllite and quartzite of the upper Belt Series of Precambrian age. These are overlain by the Kinnikinic Quartzite of Ordovician age. Intrusive into these metasedimentary rocks are quartz monzonite apophyses of the Idaho batholith. After a long period of erosion, the rocks were buried by early Tertiary conglomerate and later by the Challis Volcanics. Parts of the Leesburg basin are underlain by Tertiary lake beds, and these sediments are overlain by late Tertiary volcanic ash deposits. Prior to Ordovician sedimentation, the Precambrian rocks were deformed, and afterward the Precambrian and Ordovician rocks were faulted and compressed into a northwest-trending synclinorium some time before the monzonitic intrusion. During Tertiary time minor warping occurred.
The lode deposits are of five types: (1) quartz veins and stockworks in quartz monzonite, containing pyrite, sphalerite, specularite, and a little galena; (2) replacement veins along fault planes in metasedimentary rocks and quartz monzonite, containing quartz and pyrite as the chief constituents; (3) replacement deposits along a shear zone in schist, containing garnet, epidote, and magnetite; (4) mineralized lenses in schist and probably monzonitic rock, possibly the source of much of the placer gold; and (5) quartz veins along biotite monzonite dikes, containing chalcopyrite, sphalerite, and subordinate pyrite and galena. All five types are of equal economic importance as lode-gold producers.
The placers were distributed along Moose Creek, Beaver Creek, and Napias Creek and its tributaries; most of the production was from the Leesburg basin, an area drained by Napias Creek. Umpleby regarded some of the placers as of Miocene and Pliocene age, but Ross was of the opinion that much of the auriferous gravel was Pleistocene or younger.
MINERAL HILL AND INDIAN CREEK DISTRICT
The Mineral Hill and Indian Creek district is in Tps. 23 and 24 N., Rs. 17, 18, and 19 E., in northwestern Lemhi County, near the town of Shoup.
Most of the gold mined in the district was from lodes, but small amounts came from placers along Boulder Creek. In the Mineral Hill area most of the lode properties were discovered and developed in the 1880’s. These include the Kentuck and Grunter mines which were discovered in 1882. In the Indian Greek area there was some activity in 1895, but no extensive development occurred until 1901 when the Kittie Burton and Ulysses lode mines became operative. Most of the production of the district was before 1910. The early production which in addition to gold included an undisclosed amount of silver and small amounts of base metals was estimated at $1,350,000 by Umpleby and at $1,400,000 by Ross. From 1932 through 1959 the district produced only 21,937 ounces of gold.
The rocks of this area, as summarized by Ross, consist of schist and quartzite of the Belt Series and intrusive gneissic granite that Umpleby believed to be Precambrian in age but Ross believed to be related to the Idaho batholith. Much of the southern part of the district is underlain by granitic rocks of the Idaho batholith. Mineral deposits occur as fissure fillings and also as replacement deposits in both the schistose and granitic rocks. Quartz, pyrite, arsenopyrite, galena, sphalerite, and some calcite, magnetite, and muscovite are the predominant vein minerals. Gold occurs with the pyrite.
The Texas district is in parts of Tps. 13 and 14 N., Rs. 26 and 27 E., near Gilmore.
In the early 1880’s prospectors swarmed over the east slope of the Lemhi Range in search of lead-silver deposits similar to those discovered at Nich-olia to the southeast. Some promising deposits were found in the area known as the Texas district, but after about 10 years of occasional activity, the district became dormant for 10 to 12 years. After the discovery of large ore bodies at the Pittsburg-Idaho mine in 1902, the future looked bright for the Texas district; however, transportation difficulties hampered any major activity. In 1910, a railroad was constructed that linked the mines to a smelter; large-scale mining then began and continued through 1929. After a decline for a few years, the district again became active, and infrequent small-scale operations continued through 1956.
Production which was mainly in silver and lead from the Pittsburg-Idaho mine, was small during 1903-10; data before 1903 were not found. Total gold production from 1903 to 1959 was 21,745 ounces.
Most of the district is underlain by eastward-dipping sedimentary rocks of Cambrian, Ordovician, Silurian , Devonian , and Mississippian age. With the exception of the Cambrian rocks, which are quartzite, all the Paleozoic strata are limestone or dolomitic limestone. Unconformably overlying the Paleozoic rocks in the eastern part of the district are Miocene lake beds.
The ore deposits occur in the belt of calcareous rocks, an area bounded by the Cambrian quartzite on the west and the lake beds on the east. They are in flat and steeply dipping veins, parallel with the strike of the country rocks; some of the larger and more productive deposits occur at the intersection of the steep and flat veins. Most of the ore is valued for its lead and silver, but one deposit, the Martha vein, was mined for gold alone. Almost all the workings were in oxidized ore containing cerussite, anglesite, smithsonite, calamine, cerargyrite and iron, and manganese oxides.
YELLOW JACKET DISTRICT
The Yellow Jacket district is at about lat 44Â°58′ N. and long 114Â°31′ W., near the settlement of Yellow Jacket, in west-central Lemhi County.
Lode deposits were discovered in the district in 1868, and they were worked in the early 1870’s, but their greatest production was during 1893-97. Activity throughout the district declined with the closing of the Yellow Jacket mine in 1897, and except for intermittent activity during 1911-14, in the late 1920’s, and in the 1930’s, the district never approached its pre-1900 production.
The estimated production from the Yellow Jacket mine at $450,000 (about 21,840 ounces) in gold. Ross listed a total of $121,761.56 in bullion from the Yellow Jacket mine for 1893-97. From 1902 through 1949 the district produced 3,855 ounces of gold, and from 1949 through 1959, only 8 ounces. Gold has been the chief commodity, but small amounts of silver, copper, and lead have also been recovered. Total gold production, according to Umpleby’s estimate, was about 25,000 ounces.
The Yellow Jacket district is underlain by sedimentary, metasedimentary, and igneous rocks that were folded and faulted into a complex pattern and then mineralized. The oldest sedimentary rocks consist of two subdivisions of the Belt Series of the Yellowjacket Formation, which is composed of argillite and calcareous quartzite, and the Hoodoo Quartzite. In the western part of the district these are overlain by Challis Volcanics of Oligocene age. Intrusive rocks of three ages are in the district: dikelike and stocklike bodies of gabbro of Precambrian age; a large body of hornblende-biotite diorite and smaller bodies of syenite, diabase, diorite, and gabbro, all of Late Cretaceous or early Tertiary age; and dikes of granophyre, granite porphyry, and vitrophyre, all of Miocene age. The older rocks were deformed and fractured by several diastrophic events.
Most of the lodes are either fissure fillings or replacement deposits in breccia zones. The Yellow-jacket Formation, which has been deformed most extensively, contains most of the ore deposits. Primary minerals in the deposits are quartz, calcite, siderite, and barite in the gangue, and the ore minerals are pyrite, specularite, chalcopyrite, tetrahedrite, galena, and gold. The ores are somewhat oxidized near the surface: much of the early production was from these free-milling oxidized ores.