Anaconda Range area, Montana.
The Anaconda Range, in the northwest part of the map area, is composed mainly of Tertiary and Cretaceous monzogranite, granodiorite, and quartz monzonite stocks and batholiths. The igneous rocks intrude thrust-faulted Middle Proterozoic age quartzite, argillite, and limestone. A series of high-angle normal faults bound the range on the east. Mineral deposits present include two prospects on veins and a stockwork molybdenum deposit, all of which have had little development.
Argenta (Montana) district.
Montana In the south-central part of the quadrangle; folded and complexly faulted sedimentary strata of Middle Proterozoic and Paleozoic age are intruded by a stock of monzogranite and by sills and dikes of andesite and dacite of Late Cretaceous and Tertiary age. Mineral deposits include replacements and veins in carbonate rock, veins in siliceous sedimentary rocks, and disseminations in carbonate and intrusive rocks. The principal deposits, mainly lead and silver, are replacements and veins in carbonate rocks adjacent to the Argenta stock. The district was discovered in 1864 and production continued for more than 100 years; the Hand group of mines was the largest producer.
Baldy Mountain (Bald Mountain) district, Montana.
Middle Proterozoic quartzite and Paleozoic limestone, shale, and quartzite have been intruded by Cretaceous granodiorite of the Pioneer batholith. This district is in the south-central part of the map area. Gold, silver, and base metals occur in vein and replacement deposits in Cambrian and Devonian carbonate rocks. The amount of ore mined is not known, but it was probably small. Carbonate rocks also host scheelite deposits in skarn, although no production is known.
Bannack (Bannock, Wbit:e’s Bar) district:, Montana.
The Bannack district, famous for gold placer deposits along Grasshopper Creek, is underlain by folded and faulted sedimentary rocks of Mississippian to Cretaceous age that follow a north-trending structural grain. They are intruded by several small masses of granodiorite, two of which are exposed in the valley of Grasshopper Creek. Limestones of the Madison Group are the most widespread sedimentary rocks and the most important hosts of ore. The eastern part of the district is covered by Cretaceous and Tertiary volcanic rocks. Lode ores are chiefly in carbonate rocks adjacent to granodiorite. Garnet-rich skarn is common at the contact, and the best ore occurs as replacement deposits on the limestone side of the skarn. Fractures surrounding apophyses of granodiorite localized the ore bodies. Ores consisted of native gold in quartz veins, auriferous pyrite, and minor amounts of chalcopyrite, galena, and sphalerite. The Grasshopper Creek placers were discovered in 1862, and the first significant mining in Montana ensued. Approximately 500,000 ounces of gold has been produced from this district, about’ 80 percent from placers.
Beaverhead Mountains area (Mulky, Big Swamp Creek, South Fork Big Hole), Montana .
The east slope of the Beaverhead Mountains, in the southwest part of the map area, is composed of Middle Proterozoic quartzite and argillite of the Missoula and Lemhi Groups; they are intruded by mafic sills and dikes, probably also Proterozoic. The mountains were deeply incised by glaciers. The Beaverhead Mountains are an uplifted block broken by thrust and steep faults. Lode ores are in vein and replacement deposits in quartzite and basic intrusives situated in zones of complex faulting and fracturing along the crest of the range. Galena-rich ores containing silver and gold are most common. Small amounts of ore have been mine.
Big Hole Divide area (North Bloody Dick), Montana .
This area, in the southwest part of the map area, is composed of quartzites of the Missoula and Lemhi Groups (Middle Proterozoic) overlain locally by tuffaceous Tertiary sedimentary rocks. Two Cretaceous-Tertiary stocks intrude the Missoula Group quartzites near the center of the area, but neither of these are known to be associated with mineralization. The Saginaw mine is the only significant mine in the area. Small amounts of copper-silver ore have been shipped since its discovery in 1894.
Birch Creek (Utopia, Farlin) district, Montana.
This district, in the central part of the map area, is at the edge of a large southeastward-trending protrusion of the Pioneer batholith, along its contact with Paleozoic carbonate rocks. Skarn, developed locally at the contact, contains varying quantities of scheelite, magnetite, chalcopyrite, and molybdenite. Scheelite and molybdenite are most common in the Amsden Formation, chalcopyrite in the Madison Group, and magnetite in the Hasmark Formation (or Pilgrim Dolomite). The only mine having sizable production is the Indian Queen, which produced 1,729,204 lbs of copper and 42,219 oz of silver (Geach, 1972) from an ore body consisting of garnet-epidote skarn in Mission Canyon Limestone. Most of the metals were mined in 1903 and 1904. Magnetite ore from the Jumbo Group was probably mined for use as flux in nearby smelters.
Blacktail Mountains area, Montana.
This range, along the southern edge of the map area, is a block uplift composed of thrust-faulted Paleozoic sedimentary rocks overlying Archean gneiss. These rocks are covered in the western half of the range by volcanic rocks.
Blue Wing district, Montana.
Paleozoic strata here are apparently thrust over volcanic rocks of Cretaceous age. This district is located in the south-central part of the map area. Fine-grained Upper Cretaceous granodiorite and andesite were intruded concordantly along the thrust and have bleached and recrystallized the limestone. The Mississippian Mission Canyon Limestone (lowest formation of Madison Group (Mm)), the oldest of the sedimentary rocks explored, as shown by drilling, lies above the volcanic rocks. Ore bodies are veins and replacement deposits in the bleached Mission Canyon and veins in granodiorite.
Butte (Su.ait Valley) district, Montana.
Only the southern fringe of the district, south of Butte, is within the Dillon quadrangle. The Upper Cretaceous Butte Quartz ~ Monzonite forms the principal bedrock. It is intruded by dikes and sheets of aplite, quartz porphyry, dacite, and rhyolite and overlain by tuffaceous deposits of Tertiary age. The concentric alteration zoning of the Butte district extends slightly into the Dillon quadrangle, and is probably responsible for the base-and precious-metal mines containing manganese listed here. These mines, which date from the late 1800’s, are not well known, and their locations are probably accurate only to within a mile.
Calvert Bill district, Montana.
Near the northern end of the Pioneer batholith, in the north-central part of the map area, Cretaceous or Tertiary quartz monzonite intrudes sedimentary rocks ranging in age from Middle Proterozoic to Late Cretaceous. Ore deposits are tungsten-bearing skarns and base-and precious-metal veins in upper Paleozoic carbonates near quartz monzonite contacts. In 1956 and 1957 about 102,800 tons of ore averaging 1.13 percent wo3 was mined from the Calvert mine; at least 10,000 tons has been mined since (Geach, 1972).
Divide Creek (Fleecer) district, Montana .
This district, in the north-central part of the map area, is underlain by intrusive rocks of the Boulder batholith and by two associated bodies, the Humbug and Mt. Fleecer stocks. The batholith and the stocks intrude sedimentary rocks ranging in age from Mississippian to Late Cretaceous. The district is bisected by a north-trending graben filled with Tertiary sedimentary deposits. Ore deposits include fissure fillings closely associated with quartz porphyry or aplite dikes cutting quartz monzonite, and vein and replacement deposits in Paleozoic rocks near contacts with the stocks or the batholith. Production of metals is believed to be small. Before 1900, travertine in Tertiary sediments and Madison limestone were mined for use as flux at the Glendale smelter (near the Hecla district).
Elkhorn district, Montana.
Hornblende-biotite granodiorite of the Pioneer batholith is mineralized along the north-trending Comet fault in this district, located near the center of the map area. The batholith in this vicinity is cut locally by small dikes and irregular masses of quartz porphyry and alaskite. The ores are in quartz fissure veins mostly west of the Comet fault. Veins are generally several feet thick and continuous, but ore shoots seem to be small and scattered. Sulfide minerals are pyrite, tennantite, galena, sphalerite, chalcopyrite, wolframite, and molybdenite. Although the initial discoveries were made and mining was done in the 1870’s, the most active period was from 1913 to about 1930. Metals, chiefly silver, valued at about $500,000 have been produced from the district. Exploration in recent years has focused on tungsten and molybdenum prospects.
French Gulch district. Montana
Bedrock in this district, in the center of the north part of the map area, is Missoula Group quartzite, a Tertiary or Cretaceous pluton, and Tertiary volcanic rock. Quaternary alluvium in the headwaters of French Gulch and its tributaries have been mined for placer gold intermittently from 1864 to the present. The lode source of the gold was probably narrow quartz veins in the pluton and quartzite. The total placer gold has not been ascertained, and estimates of early production vary considerably. Estimates for the period 1864-1870 range from $1 million to $5 million (Lyden, 1948).
informazioni tratte da https://pubs.usgs.gov/imap/1803c/report.pdf
