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Sapphire Deposits of the Missouri River Near Helena, Montana part of MT Field Guides
Sapphires occur with gold, both in terrace deposits along the Missouri River and in the active river channel. Both of these minerals have been recovered from these deposits beginning with recovery of gold in the latter part of the 1800's. More recently, up until the early 1940's, sapphires were also recovered from these operations for industrial uses. In recent years recovery of sapphires by recreationists for faceting into gem stones has become important. Five deposits open for fee digging will be visited during this field excursion. They are situated on the French, Eldorado, Spokane and Gruell's bars. Also, exposures of a sapphire-bearing andesite sill close to the Missouri River and considered a source for some of the sapphires found in the gravels will be examined.

Topics: Resources, Structures, Sedimentary rocks, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

The Middle Yellowstone Valley from Livingston to Gardiner, Montana: A Microcosm of Northern Rocky Mountain Geology part of MT Field Guides
The middle Yellowstone valley, between the Great Plains at Livingston and the edge of the Yellowstone volcanic plateau near Gardiner, is a complex palimpsest of lithology, structure, and surficial processes. It shares basement rocks with the continental interior, largely to the north and east; Paleozoic lithologies with the western interior, compressive tectonics with the Fold and Thrust Belt to the west; extension with the Basin and Range to the west and south; and Cenozoic volcanism and elements of its geomorphic evolution with much of the surrounding region. The geological exploration of this region serves as a microcosm of the evolution of the geological understanding of the American West.

Topics: Hydrology, Resources, Fossils, Environmental Geology, Hazards, Surficial geology, Sedimentary rocks, Igneous rocks, Structures, Metamorphic rocks
Geographic Location: Yellowstone National Park, Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Road Log from Bozeman to Specimen Creek via Gallatin Canyon and U.S. 191 part of MT Field Guides
This trip leads southward through the Gallatin Range to Specimen Creek in Yellowstone National Park. It affords an excellent worm's-eye view of the structure and stratigraphy of this range as revealed both laterally and vertically through the quietly beautiful Gallatin Canyon. The route also borders the east margin of the Madison Range and its spectacular Spanish Peaks uplift. The Gallatin and Madison Ranges are geologically and topographically similar, an essential difference being the thick cap of andesitic lava and breccia that covers the high parts of the Gallatin Range.

Topics: Surficial geology, Hazards, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Helena and the North Border Zone of the Boulder Batholith, Montana part of MT Field Guides
The geologic descriptions for this road log roughly follow the scalloped north edge of the Boulder batholith. The will be ample opportunity to examine the intrusive rocks, and to the north, the broad border zone of contact-metamorphosed Paleozoic and Mesozoic sedimentary rocks. The route will follow the highway eastward, with a stop in granodiorite of the batholith. The log then detours to the south into the mountains along Colorado Gulch to view skarn and hornfels developed in the Madison Group and Three Forks Formation in a "septum" (or "screen") of country rock caught between two lobes of the batholith. The log returns to Highway 12 and continues east to Helena's Reeder's Alley and Grizzly Gulch. After a visit to a talc vein and teh unconformity between Precambrian (Belt) and Cambrian formations, the trip leaves Helena southbound on Interstate 15. Near Montana City are stops at an outcrop of hornfelsed, stromatolitic Helena Formation (Precambrian Belt Supergroup) and at the contact with the Boulder batholith. The final stop is to examine the complex of intrusions, wall- or roof-rock inclusions and structures in the batholith's border zone. In this area, Precambrian, Paleozoic and Mesozoic sedimentary rocks dip southwest and south toward the intrusion. Within about one to two miles from the batholith, the sedimentary sequence is contact-metamorphosed, yet in most places sedimentary structures are remarkably well preserved.

Topics: Structures, Resources, Metamorphic rocks, Sedimentary rocks, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Early Proterozoic Geology of the Highland Mountains, Southwestern Montana, and Field Guide to the Basement Rocks that Compose the Highland Mountain Gneiss Dome part of MT Field Guides
The Highland Mountains are underlain by the largest of the northwesternmost exposures of basement crystalline rocks in southwestern Montana....Metasedimentary rocks in the Highland Mountains are in part lithologically similar to the Late Archean multilithologic sequence (in the Tobacco Root, Ruby, and the northern Madison and Gravelly Ranges), but are considerably thinner. In the Highland Mountains the individual beds of aluminous schist, marble, quartzite, and iron-formation extend for only short distances, but the assemblage as a whole is mappable. It nowhere exceeds 300 ft (100 m) in thickness and appears to pinch out to the north....these rocks in the Highlands, unlike those to the southeast, are overlain by more than 10,000 ft (3000 m) of aluminous biotite gneiss that may have been deposited as muds basinward from the Late Archean shelf edge.

Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Hydrologic Aspects of Metal Mine Impacts on Upper Clark Fork Superfund Sites, Butte - Warm Springs, MT part of MT Field Guides
This road log concentrates on environmental impacts of historic mining activities in the Butte Mining District, particularly with regard to water quality. The trip decribes the general geology between Bozeman and Butte and provides a detailed overview of the geology, hydrogeology, mineralization, and mining history in the Butte district. Specific topics included in this field guide are: water quality changes in underground mines and the Berkeley Pit, impacts and remediation of mine tailings and processing plants (mills, smelters, and reduction plants), and groundwater contamination and remediation. Detailed information from geotechnical and hydrogeological studies related to these topics is included.

Topics: Hydrology, Environmental Geology
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Volcanism and Plutonism at Shallow Crustal Levels: The Elkhorn Mountains Volcanics and the Boulder Batholith, Southwestern Montana part of MT Field Guides
The Upper Cretaceous Elkhorn Mountains Volcanics (EMV) and Boulder batholith of southwestern Montana provide an example of a large-volume, epizonal, volcanic-plutonic complex whose deep level of erosion has exposed the cogenetic intrusive rocks while preserving sizeable portions of the volcanic field. Such a volcanic-plutonic association provides a unique opportunity for evaluation of many aspects of the evolution of a shallow-crustal magmatic system, such as geochemical relations of both the volcanic and plutonic rocks and the nature of intrusive-extrusive relationships at the present level of exposure.

Topics: Metamorphic rocks, Structures, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Crazy Mountains, Montana part of MT Field Guides
This trip examines a variety of mid-Eocene alkalic (feldspathoidal) stocks, laccoliths, sills, and dikes emplaced into Cretaceous and Paleozoic sedimentary strata. These rocks include mafic and felsic varieties, and are both texturally and compositionally variable. Most of the mafic alkalic rocks are unique in the Montana Alkalic Province in having Na2O > K2O. They are unusual even among feldspathoidal rocks because they are stongly enriched in incompatible elements, and have Sr, Nd, and Pb isotopic compositions which reflect an ancient source having low Rb/Sr, Sm/Nd, and U/Pb.

Topics: Structures, Metamorphic rocks, Igneous rocks, Sedimentary rocks
Geographic Location: South-Central Montana
Geologic Province: Central Rocky Mountains Foreland Province

Plutonism at Deep Crustal Levels: The Idaho Batholith, Montana and Idaho part of MT Field Guides
The Idaho batholith field trip traverses a well exposed cross-section of the northern Idaho batholith, briefly examines the broad aspects of this deep-seated granitoid batholith and its regionally metamorphosed country rocks, and considers the role of the synplutonic mafic magmas from the mantle in providing heat for melting of continental crustal rocks to form the more felsic main-phase units of the batholith.

Topics: Igneous rocks, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Geology of the Highwood Mountains, Montana: A Survey of Magma Types and Sources part of MT Field Guides
The Highwood Mountains represent a dissected Eocene volcanic and intrusive complex composed of latites and highly potassic, mafic to felsic igneous rocks. Along with the Bearpaw Mountains and Eagle Buttes, they form a northeast-trending belt within the Montana Alkalic Province, and were all emplaced between 54 and 50 Ma. In the Highwoods an older suite of quartz-normative latites is overlain and intruded by a younger suite composed mostly of mafic phonolite flows and dikes, minette and trachyte dikes, and shonkinite-mafic syenite stocks. This trip examines a variety of dikes, sills, and stocks composed of these unusual rock types.

Topics: Metamorphic rocks, Igneous rocks, Sedimentary rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains