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Gold-quartz veins south of Libby, Montana
The area described in this paper is one of rugged mountains, composed chiefly of folded and faulted sedimentary rocks of the Belt series. In the nearby region the beds are intruded by diorite sills and granitic stocks. Glacial drift, including silt deposited in a lake, covers much of the lower slopes of the mountains and partly fills the valleys. Incomplete records show a production of $200,000 in gold, mostly from the oxidized free-milling parts of quartz veins. Except a few in the Wallace formation the veins are found in sandstone and shale of the Prichard formation. They partly replace the country rock along fractures that commonly follow bedding planes but also cut across them at moderate angles. The veins average between 1 and 2 feet in thickness and reach a maximum of 6 feet. They are cut by faults of small displacement, with which the ore bodies are commonly associated. Quartz forms 95 percent or more of the veins. Of the remainder, one or more of the minerals pyrite, galena, sphalerite, and pyrrhotite are relatively abundant, and chalcopyrite, arsenopyrite, tetrahedrite, and scheelite are present sparingly. A little silver is present, and in some veins it exceeds the gold considerably. In many veins an original quartz filling has been slightly brecciated or sheared by movement parallel to the walls. Sulphides and gold have been introduced along the shear planes, partly replacing the quartz and giving the vein a "ribbon" structure. The gold is closely associated with the sphalerite, and silver is more plentiful in the veins that carry galena. As a rule oxidation has not extended far below the surface, and its chief product is limonite. Cerusite and anglesite are present in some veins, and a little malachite, azurite, pyromorphite, or manganese oxide occurs in a few. Native gold is present in both the oxidized and unoxidized ores. Although mining of these veins in the past may have been unprofitable, that result was due partly, at least, to difficulties of transportation and milling. With improvement in roads and methods, not to mention appreciation in the value of gold, the deposits are believed to have a promising future.
Metalliferous deposits of the greater Helena mining region, Montana
The ore deposits described in this bulletin are distributed through a region of about 3,000 square miles surrounding the city of Helena, Mont. In general the surface of this region is mountainous, but it includes several large intermontane valleys. Large areas in the northern and eastern parts of the region sire underlain by sedimentary rocks of the Algonkian Belt series, and on the northeast and southwest the Belt rocks are overlain without any noticeable angular unconformity by Paleozoic and Mesozoic beds. Oligocene, Miocene, and possibly Pliocene sediments, composed chiefly of volcanic ash and land waste of local origin, occupy large areas in the intermontane valleys and lie unconformably upon Cretaceous and older rocks. A thin veneer of Pleistocene and Recent alluvium generally overspreads the Tertiary. In the extreme northern part of the region are large deposits of glacial drift that represent two stages of the Pleistocene. The principal igneous body of the region is the northern part of the early Tertiary or late Cretaceous Boulder batholitb of quartz monzonite. The main exposure of this body occupies an area of nearly 1,200 square miles and extends southward beyond the limits of the particular region considered. Smaller areas of similar rocks are clustered around this exposure. Most of the exposures probably represent bodies that are connected in depth to form a single mass. The late Cretaceous and older sedimentary rocks are involved in a series of northwestward-trending folds. Along the east side of the region overthrust faults related to the great Lewis overthrust of Glacier National Park cause Belt rocks to overlie rocks of Paleozoic and Mesozoic ages. Large normal faults occur near Marysville and faults of moderate displacement near Helena. The Tertiary beds are slightly deformed by folds and faults that are unrelated to the structure of the older rock. The geologic history of the region includes two contrasting periods, the earlier of which was characterized by the accumulation of marine sediments and the later by mountain building and erosion. The later period began with folding and elevation in late Cretaceous or early Eocene time, followed by overthrust faulting and the intrusion of the Boulder batholith. Next, there ensued a period of crustal stability, during which erosion reduced the region to a surface of low relief and cut away at least 10,000 feet of strata in the area north of Helena. In Oligocene, Miocene, and Pliocene (?) time sediments composed of land waste and volcanic ash were deposited, and this event was followed by warping and faulting that elevated the present mountains. During Pliocene and Pleistocene time the mountains were maturely dissected, and in middle and late Pleistocene time local glaciers formed in the higher mountains and large valley glaciers invaded the extreme northern part of the region. The ore deposits include lodes and placers that have yielded gold, silver, lead, copper, and zinc to a value of at least $176,860,000. The placers were formed mostly during interglacial stages of the Pleistocene. They have been almost entirely exhausted. Most of the lodes are classified as regards age in two groups, an older and a younger. The older lodes are related in origin to the Boulder batholith or some of the neighboring intrusive granitic bodies and were probably formed during early Eocene time. The younger lodes were formed after dacite of probable Miocene age was erupted. They are possibly related in origin to some unexposed intrusive granitic rock. For convenience in description the region is divided into three parts districts north of Helena, districts in the Belt Mountains, and districts south of Helena. The districts north of Helena include a 50-mile stretch of the Continental Divide that forms a broad ridge surmounted with considerable areas of flat or gently sloping surface at a general altitude of 7,000 feet. Narrow valleys 1,000 to 2,000 feet deep are cut into this surface and lead out to the neighboring wide intermontane valleys. The area is underlain mostly by shale, sandstone, and limestone of the upper part of the Belt series. Beds of Paleozoic and Mesozoic age occur south of the Belt area and extend from Helena west and northwest. The igneous rocks of the area include diorite and gabbro sills and dikes of probable Cretaceous age, extrusive andesite that is probably Oligocene or Miocene, and stocks of quartz monzonite, granodiorite, and quartz diorite, probably of Oligocene or Miocene age. The ore deposits of the northern districts are chiefly lodes that are valuable for gold and silver but contain some lead and copper. In the Ophir district bodies of gold and silver ore occur mainly in limestone near a body of quartz monzonite. In the Scratchgravel Hills and Grass Valley districts veins of gold quartz and veins containing lead-silver ore occur in quartz monzonite and in the adjoining metamorphic rocks. In the Austin district lodes containing gold; silver, lead, and copper are found in limestone near intrusive quartz monzonite. An unusual mineral in one of these lodes is corkite, a hydrous sulphate of lead containing arsenic. A small stock of quartz diorite in the Marysville district has invaded and domed Belt rocks. Marginal and radial fractures formed during the cooling and contraction of the igneous body became the receptacles of gold and silver veins, one of which, the Drumlummon, has produced $16,000,000. The veins filled open fractures and are characterized by a gangue of platy calcite and quartz. Lodes in Towsley Gulch in the western part of the district contain lead in addition to gold. In the Gould district a small stock of the granodiorite has invaded the Belt rocks and caused the deposition of veins similar to those near Marysville. In the Heddleston district lodes valuable for gold, silver, lead, and copper occur in Belt sedimentary rocks and diorite, some of them associated with porphyry dikes. In the Wolf Creek district veins in Belt rocks have produced copper ore composed mainly of chalcopyrite or chalcopyrite and tennantite accompanied by pyrite and a gangue of quartz and barite. Placer deposits along the western slope of the Belt Mountains have produced $17,500,000 in gold. Sapphires were formerly obtained from some of these deposits. The central part of the Belt Mountains is a plateaulike area considered to be the remnant of a surface produced by erosion during Tertiary time. This surface was elevated and has been deeply trenched by narrow, transverse valleys that are bordered with remnants of low terraces in which most of the placer deposits occur. Most of the western slope of the mountains is underlain by sedimentary rocks of the Belt series. At the foot of the mountains these give place to Paleozoic rocks, and these in turn are overlain unconformably in Townsend Valley by Tertiary and later deposits. The main structural feature is a great arch called the York anticline, which occupies most of the west side of the mountains. At the west foot of the mountains this fold is bordered by a series of small synclines that are tightly squeezed, faulted, and overturned as a result of pressure exerted from the west or southwest along a fracture described as the El Dorado overthrust On another fracture called1 the Scout Camp overthrust the Belt rocks composing the western slope of the mountains are thrust eastward over Paleozoic beds. Both faults are regarded as branches of the Lewis overthrust of Glacier Park. Igneous rocks that probably range in age from early Eocene to Pleistocene are widely but sparingly distributed.' They include sills and dikes of quartz dibrite, porphyry dikes, small stocks of quartz monzonite and quartz diorite, and surface flows of andesite and basalt. The deposits in the Belt Mountains that are of most interest at the present time are lodes that are chiefly valuable for gold. Most of them are found in the vicinity of York and Confederate Gulch. Nearly all are small quartz veins formed along fractures in diorite dikes and stocks or on bedding planes in the adjoining Belt sedimentary rocks. An exception is the Golden Messenger, a replacement deposit of large size but low grade, formed along fractures in a quartz diorite dike. Other veins in the same dike belong to the rather uncommon class called ladder veins. Many of the small veins contain shoots and bunches of rich ore in their upper parts. Downward- enrichment in gold is indicated to have occurred in some of the veins near York that lie below an old erosion surface. Elsewhere the origin of the placer deposits from erosion o'f the lodes during interglacial stages of the Pleistocene is indicated. Lodes containing chalcopyrite occupy tension fractures in the Belt shales that were produced by lateral movements of the mass composing the mountain front. In the districts south of Helena mining began with the discovery, on July 14, 1864, of rich placer deposits at the present site of the city of Helena, on Last Chance Creek. Since then the placer and lode deposits of these districts have produced metals worth $130,000,000 or more, of which about one-third was gold, the remainder chiefly lead and zinc. Sedimentary rocks ranging in age from Algonkian to Cretaceous underlie parts of the region, and other parts are underlain by a bedded series of andesite and latite tuffs, breccias, and flows. These rocks have been intruded and severely metamorphosed by the quartz monzonite of the Boulder batholith, the exposures of which occupy a large area. Rocks later than the intrusion of the batholith are chiefly a series of late Tertiary dacites and rhyolites. The placers of the southern districts have been almost entirely worked out. The lodes have yielded metals worth $111,600,000, but many of them are still productive. They include veins and contact-metamorphic deposits. Some of the contact deposits contain copper ore, and others contain iron ore valuable for fluxing. The veins are of two ages. The older veins have yielded most of the metallic production of the region. Their ores in general are heavy sulphide aggregates composed mainly of galena, sphalerite, and pyrite. Arsenopyrite is generally present; tetrahedrite and chalcopyrite are less common. Many of the veins are distinguished from the usual type of ore body by the occurrence of abundant tourmaline. The, metals produced are chiefly silver, lead, gold, and zinc, with some copper. The younger veins are essentially precious-metal deposits. They are mainly fissure veins but include some disseminated deposits of low grade. They are widely distributed and include several productive bodies. A distinguishing feature is the occurrence in the gangue of cryptocrystalline quartz and lamellar calcite. A dominant eastward trend of the vein fractures of the older group indicates them to be tension cracks in the crust block lying west of the Lewis overthrust that were produced by stretching in a direction at right angles to the thrust.
Geology and mineral resources of parts of Carbon, Big Horn, Yellowstone and Stillwater Counties, Montana
No abstract available.
The Forsyth coal field, Rosebud, Treasure, and Big Horn Counties, Montana
No abstract available.
The Kevin-Sunburst oil field and other possibilities of oil and gas in the Sweetgrass arch, Montana
No abstract available.
Deposits of vermiculite and other minerals in the Rainy Creek district, near Libby, Mont.
No abstract available.
The northward extension of the Sheridan coal field, Big Horn and Rosebud Counties, Montana
No abstract available.
Thrust faulting and oil possibilities in the plains adjacent to the Highwood Mountains, Montana
No abstract available.
Geology of the Cat Creek and Devils Basin oil fields and adjacent areas in Montana
No abstract available.
Phosphate rock in the Three Forks-Yellowstone Park region, Montana
The region described is about 7,000 square miles in area. It includes most of Gallatin and Madison Counties, in Montana, and some contiguous parts of Idaho and Wyoming. Within it are several broad fertile valleys and a number of prominent mountain ranges. A considerable part is underlain by a bed of phosphate rock which occurs in the Phosphoria formation, of Permian age, and is elevated to the surface of the mountainous area. In most of the middle and southwestern parts of the region this deposit is about 2 feet thick; locally it is 3 feet or more. Outcrops of beds in which the deposit is more than 6 inches thick aggregate more than 200 miles in length. Toward the northeast the deposit thins out, and its limit in that direction is represented fairly accurately by a line drawn from Yellowstone Park northwestward through Salesville to Willow Creek, on Jefferson Kiver. Its thicker parts contain 45 to 65 per cent of tricalcium phosphate. In this region the phosphate deposit is thinner and of poorer quality than in the adjoining parts of Montana on the west and northwest, where it is commonly 4 feet thick and contains 65 to 70 per cent of tricalcium phosphate. In southeastern Idaho the deposit is still thicker and richer.
The Montana earthquake of June 27, 1925
The earthquake of June 27, 1925, in Montana caused considerable damage within an area of 600 square miles or more, the center of which is in latitude 46° 5' N. and longitude 111° 20' W., a short distance southeast of Lombard. It was a seismic disturbance of the first order of magnitude, but, owing to the hour at which it occurred and to other fortunate circumstances, no lives were lost and no fires broke out. The shock was startling throughout an area extending 75 miles or more in all directions from the epicenter and was sensible to persons within an area of 310,000 square miles. Within the epicentral area brick buildings suffered severely, rocks fell from cliffs, cracks opened in the ground, and the inhabitants experienced the usual symptoms of illness and emotions of alarm. Isoseismals drawn according to the Rossi-Forel scale show a wide indentation at the south due to a rapid decline of intensity in the volcanic area of Snake River Plain and Yellowstone Park.
Geology and possible oil and gas resources of the faulted area south of the Bearpaw Mountains, Montana
No abstract available.
The Scobey lignite field, Valley, Daniels, and Sheridan counties, Montana
No abstract available.