Age Relationships of Ore Deposits of Southwestern Montana (A Microscopic Study)

Although considerable work has been undertaken by some prominent geologists, the best known of which is that of Paul Billingsley and J. A. Grimes', in investigating the ore deposits of the Boulder Batholith and surrounding area, there has not been any complete microscopic investigation of these deposits, as a whole, published in the literature. With this in mind it was suggested to the writer by Professor Paul A. Schafer, of the Montana School of Mines, that a microscopic study of the ores of this region would be a worthwhile geologic problem. It was thought that the mineral association and the mode of mineral occurrence might afford methods of classify­ing these deposits so that they could be correlated with the age relationships worked out by Billingsley and Grimes.

Pegmatites in Southwestern Montana

Little has been written on pegmatites in Montana except in conjunction with reports on mining districts. This subject was chosen as a senior thesis to see what facts could be ascertained re­garding pegmatites alone, and an attempt has been made by the writer to assemble all written material on pegmatites in southwestern Montana, and to study specimens from those that could be reached or from which specimens were available.

Mineral Occurences in Certain Pegmatite Dikes in Southwestern Montana

Pegmatite dikes are rather common in occurrence in parts of southwestern Montana, particularly in a region to the east of the Tobacco Root range 50 to 75 miles southeast of Butte.

Auriferous Tertiary Gravels Near Rocker, in Silver Bow County, Montana

Between the villages of Rocker and Silver Bow, in south­western Montana, are found an interesting group of placers. Gold occurs in Tertiary gravel beds that are interstratified with beds of rhyolitic volcanic ash. With the aid of a plane table and open-sight alidade, a small portion of the lake-bed area near Rocker was mapped; all distances were paced, but numerous checks assure a fairly accurate map.

Geology of Upper Bear Gulch and Dry Boulder Creek Area Madison County, Montana

Potential gold mines lie high among the rugged peaks of the Tobacco Root Mountains of southwestern Montana. This is a region where little geologic work has been done, though extensive mine operations have been carried on, and valuable ore has been shipped.

Karst Kamp Asbestos Deposits Gallatin County, Montana

Karst Kamp, a southwestern Montana recreation re­sort, is 32 road miles south of Bozeman on the east bank of the Gallatin River in a narrow V-shaped valley flanked on the west by the rugged Madison mountain range and on the east by the equally rough Gallatin range. The asbestos deposit itself lies approximate­ly one-half mile northwest of the ranch on a heavily timbered "Al­pine-like" slope nearly 1200 feet above the floor of the valley.

Exploration diamond drilling in the Boulder batholith, Jefferson and Silver Bow Counties, Montana /

Radiometric logging of the drill holes was accomplished by means of a Geiger counter-type probe. Samples for chemical assay were selected after checking the core with a Geiger counter and correlation with the radiometric logging record. The exploration project did not disclose significant amounts of ore-grade material although encouraging indications were found at two different properties.

The Application of Laser-Ablation Split-Stream ICP Petrochronology to Metamorphic Studies

Discussion of a new, innovative method for dating rocks, called laser ablation split stream (LASS) petrochronology, which is an in situ method that couples geochronological and geochemical data of minerals that remain in the rock matrix. The talk focuses on the application of this technique with U-Th-Pb dating of the phosphate minerals monazite and xenotine in metamorphic rocks. Examples from the Ruby Range in southwestern Montana and metamorphic core complexes in the northern Idaho panhandle will be explored.

An Isopach Map and a Discussion of the Cambrian Strata in the Central Cordilleran Area

In the past few years intensive studies have been going on concerning Cambrian formations as a locale for orebodies. Many of the dolomites and limestones of southwestern Montana are susceptible to replacement by mineralizing solutions, especially when overlain by a rather impervious layer of shale, as is the Pilgrim limestone of Upper Cambrian time.

Remembering Lee Metcalf

Abstract Montana's Lee Metcalf was an extraordinary Montana leader with an unbelievable record of accomplishment fighting for the little people against the forces of economic and political power. The public memory is so short that this film will serve to help reacquaint Lee & Donna Metcalf to most of those who were around during their time. But it will also provide an opportunity for new generations to receive a perspective of an important leader from an important time. (Language from YouTube version of the film, written and provided by Executive Producer Evan Barrett) Lee Warren Metcalf (January 28, 1911 – January 12, 1978) was an American lawyer, judge, and politician. A member of the Democratic Party, he served as a U.S. Representative (1953–1961) and a U.S. Senator (1961–1978) from Montana. He was permanent acting President pro tempore of the Senate, the only person to hold that position, from 1963 until his death in 1978. U.S. House of Representatives During his tenure in the House, Metcalf served on the Education and Labor Committee (1953–1959), Interior and Insular Affairs Committee (1955–1959), Select Astronautics and Space Exploration Committee (1958), and Ways and Means Committee (1959–1960). He became known as one of Congress's "Young Turks" who promoted liberal domestic social legislation and reform of congressional procedures. He introduced legislation to provide health care to the elderly ten years before the creation of Medicare. He earned the nickname "Mr. Education" after sponsoring a comprehensive bill providing for federal aid to education. He also voted against legislation that would have raised grazing permits on federal lands, and led the opposition to a bill that would have swapped forested public lands for cutover private lands. He was elected chairman of the Democratic Study Group in 1959. U. S. Senate Regarded as "a pioneer of the conservation movement", Metcalf worked to protect the natural environment and regulate utilities. He helped pass the Wilderness Act of 1964, and supported the creation of the Great Bear Wilderness and the Absaroka-Beartooth Wilderness. In 1962, he introduced a "Save Our Streams" bill to preserve natural recreation facilities and protect fish and wildlife from being destroyed by highway construction. He was a longtime member of the Migratory Bird Conservation Commission. He was also active on the issue of education. He was a leading supporter of the Elementary and Secondary Education Act, the effort to extend the G.I. Bill's educational benefits to a new generation of veterans, and the development of legislation to improve federally-aided vocational education.[1] The Peace Corps was established under leadership of Metcalf and Senator Mansfield. In 1983, by act of Congress, the Lee Metcalf Wilderness area was created in southwestern Montana in honor of the late Congressman. The Great Bear Wilderness and Absaroka-Beartooth Wilderness areas were also created as a result of Metcalf's efforts in Congress, in addition to the Lee Metcalf National Wildlife Refuge in Montana. Metcalf was ranked number 15 on a list of the 100 Most Influential Montanans of the Century by the Missoulian newspaper. This text is courtesy of Wikipedia®, a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization, and is available under the Creative Commons Attribution-ShareAlike License.

Debris flow magnitudes: a global catalog

The episodic occurrence of debris flow events in response to stochastic precipitation and wildfire events makes hazard prediction challenging. Previous work has shown that frequency-magnitude distributions of non-fire-related debris flows follow a power law, but less is known about the distribution of post-fire debris flows. As a first step in parameterizing hazard models, we use frequency-magnitude distributions and cumulative distribution functions to compare volumes of post-fire debris flows to non-fire-related debris flows. Due to the large number of events required to parameterize frequency-magnitude distributions, and the relatively small number of post-fire event magnitudes recorded in the literature, we collected data on 73 recent post-fire events in the field. The resulting catalog of 988 debris flow events is presented as an appendix to this article. We found that the empirical cumulative distribution function of post-fire debris flow volumes is composed of smaller events than that of non-fire-related debris flows. In addition, the slope of the frequency-magnitude distribution of post-fire debris flows is steeper than that of non-fire-related debris flows, evidence that differences in the post-fire environment tend to produce a higher proportion of small events. We propose two possible explanations: 1) post-fire events occur on shorter return intervals than debris flows in similar basins that do not experience fire, causing their distribution to shift toward smaller events due to limitations in sediment supply, or 2) fire causes changes in resisting and driving forces on a package of sediment, such that a smaller perturbation of the system is required in order for a debris flow to occur, resulting in smaller event volumes.

General Geology and Mines of Northwestern Montana

The five counties discussed in this paper compose the northernmost and westernmost counties in Montana. On the eastern boundary are Glacier National Park and the Continental Divide; on the southern boundary are Missoula and Powell counties; Idaho lies on the southwestern and western side; and the Canadian border lies along the northern edge. The region is on the Pacific Ocean side of the Rocky Mountains. Three major rivers, the Clark Fork, the Flathead, and the Kootenai drain this area into the Columbia River.

Late Oligocene to middle Miocene rifting and syn-extensional magmatism in the southwestern Sierra Madre occidental, Mexico: the beginning of the Gulf of California rift

Although Basin and Range style extension affected several areas of western Mexico since the Late Eocene, extension in the Gulf of California region (the Gulf Extensional Province GEP) is thought to have started as subduction waned and ended at ~14 12.5 Ma. A general consensus also exists in considering the mid Miocene Comondú group as a suprasubduction volcanic arc. Our new integration of the geology of the south east Gulf region, backed by 43 new Ar Ar and U Pb mineral ages and geochemical studies, document a widespread phase of extension in the southern GEP between latest Oligocene and Early Miocene that subsequently focused in the region of the future Gulf in the Middle Miocene. Upper Oligocene to Lower Miocene rocks across the southern Sierra Madre Occidental (SMO)(northern Nayarit and southern Sinaloa) were affected by major ~N S to NNW striking normal faults prior to ~21 Ma. Then, between ~21 and 11 Ma, a system of NNW-SSE high angle extensional faults continued extending the southwestern side of the SMO. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20-17 Ma. In northern Sinaloa, large grabens were floored by huge dome complexes at ~21-17 Ma and filled by continental sediments with interlayered basalts dated at 15-14 Ma, a setting and timing very similar to Sonora. Early to Middle Miocene volcanism, including the largely volcaniclastic Comondú strata in Baja California Sur, was thus emplaced in rift basins and was likely associated to decompression melting of upper mantle (inducing crustal partial melting) rather than to fluxing by fluids from the young subducting plate. Along the Nayarit and Sinaloa coast, flatlying basaltic lava flows dated at 11-10 Ma are exposed just above the present sea level. Here, crustal thickness is almost half that in the unextended core of the SMO, implying significant lithosphere stretching before ~11 Ma. Our study shows that rifting began much earlier than Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed at ~20-18 Ma to be narrower and likely more rapid, and again at ~12.5 Ma, when the kinematics of rifting became more oblique.

Late Oligocene to Middle Miocene rifting and synextensional magmatism in the southwestern Sierra Madre Occidental, Mexico : the beginning of the Gulf of California rift

Although Basin and Range–style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended ca. 14–12.5 Ma. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comondú Group as subduction related, whereas volcanism after ca. 12.5 Ma is extension related. Here we present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension. This extension across the southern and central Gulf Extensional Province began between Late Oligocene and Early Miocene time, but was focused in the region of the future Gulf of California in the Middle Miocene. Late Oligocene to Early Miocene rocks across northern Nayarit and southern Sinaloa were affected by major approximately north-south– to north-northwest– striking normal faults prior to ca. 21 Ma. Between ca. 21 and 11 Ma, a system of north-northwest–south-southeast high angle extensional faults continued extending the southwestern side of the Sierra Madre Occidental. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20–17 Ma. Rhyolitic rocks, in particular the domes and lavas, often show strong antecrystic inheritance but only a few Mesozoic or older xenocrysts, suggesting silicic magma generation in the mid-upper crust triggered by an extension induced basaltic infl ux. In northern Sinaloa, large grabens were occupied by huge volcanic dome complexes ca. 21–17 Ma and filled by continental sediments with interlayered basalts dated as 15–14 Ma, a stratigraphy and timing very similar to those found in central Sonora (northeastern Gulf of California margin). Early to Middle Miocene volcanism occurred thus in rift basins, and was likely associated with decompression melting of upper mantle (inducing crustal partial melting) rather than with fluxing by fluids from the young and slow subducting microplates. Along the eastern side of the Gulf of California coast, from Farallón de San Ignacio island offshore Los Mochis, Sinaloa, to San Blas, Nayarit, a strike distance of >700 km, flat lying basaltic lavas dated as ca. 11.5–10 Ma are exposed just above the present sea level. Here crustal thickness is almost half that in the unextended core of the adjacent Sierra Madre Occidental, implying signifi cant lithosphere stretching before ca. 11 Ma. This mafic pulse, with subdued Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing an upward asthenospheric flow into an upper mantle previously modified by fluid fluxes related to past subduction. Widespread eruption of very uniform oceanic island basalt–like lavas occurred by the late Pliocene and Pleistocene, only 20 m.y. after the onset of rifting and ~9 m.y. after the end of subduction, implying that preexisting subduction-modified mantle had now become isolated from melt source regions. Our study shows that rifting across the southern-central Gulf Extensional Province began much earlier than the Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed ca. 20–18 Ma to be narrower, and again after ca. 12.5 Ma, when the kinematics of rifting became more oblique.