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Robert L. Earhart

Publications and source records attributed to Robert L. Earhart.

At least 19 recordsLinked to original sources
USGS1998

Geochemical studies of rare earth elements in the Portuguese pyrite belt, and geologic and geochemical controls on gold distribution

This report describes geochemical and geological studies which were conducted by the U.S. Geological Survey (USGS) and the Servicos Geologicos de Portugal (SPG) in the Portuguese pyrite belt (PPB) in southern Portugal. The studies included rare earth element (REE) distributions and geological and geochemical controls on the distribution of gold. Rare earth element distributions were determined in representative samples of the volcanic rocks from five west-trending sub-belts of the PPB in order to test the usefulness of REE as a tool for the correlation of volcanic events, and to determine their mobility and application as hydrothermal tracers. REE distributions in felsic volcanic rocks show increases in the relative abundances of heavy REE and a decrease in La/Yb ratios from north to south in the Portuguese pyrite belt. Anomalous amounts of gold are distributed in and near massive and disseminated sulfide deposits in the PPB. Gold is closely associated with copper in the middle and lower parts of the deposits. Weakly anomalous concentrations of gold were noted in exhalative sedimentary rocks that are stratigraphically above massive sulfide deposits in a distal manganiferous facies, whereas anomalously low concentrations were detected in the barite-rich, proximal-facies exhalites. Altered and pyritic felsic volcanic rocks locally contain highly anomalous concentrations of gold, suggesting that disseminated sulfide deposits and the non-ore parts of massive sulfide deposits should be evaluated for their gold potential.

Professional Paper↗
USGS1982

Geologic and structure map of the Choteau 1° x 2° quadrangle, western Montana: A digital database

The geologic and structure map of Choteau 1 x 2 degree quadrangle (Mudge and others, 1982) was originally converted to a digital format by Jeff Silkwood (U.S. Forest Service and completed by the U.S. Geological Survey staff and contractor at the Spokane Field Office (WA) in 2000 for input into a geographic information system (GIS). The resulting digital geologic map (GIS) database can be queried in many ways to produce a variey of geologic maps. Digital base map data files (topography, roads, towns, rivers and lakes, etc.) are not included: they may be obtained from a variety of commercial and government sources. This database is not meant to be used or displayed at any scale larger than 1:250,000 (e.g. 1:100,000 or 1:24,000. The digital geologic map graphics and plot files (chot250k.gra/.hp/.eps and chot-map.pdf) that are provided in the digital package are representations of the digital database. They are not designed to be cartographic products.

Montana↗
USGS1981

The Conterminous United States Mineral Appraisal Program; background information to accompany folio of geologic, geochemical, geophysical, and mineral resource maps of the Choteau 1 degree x 2 degrees Quadrangle, Montana

The Choteau l? x 2? quadrangle in northwest Montana was studied by an interdisciplinary research team in order to appraise its mineral resource and hydrocarbon potential The appraisal is based on field and laboratory investigations of the geology, geochemistry, and geophysics. The results of the investigations are published as a folio of maps, figures, tables, and accompanying discussions. This circular provides background information on the investigations and integrates the published components of the resource appraisal. A comprehensive bibliography cites both specific and general references to the geology, geochemistry, geophysics, and mineral deposits of the Choteau l? x 2? quadrangle.

Circular↗
USGS1980

The Lewis thrust fault and related structures in the Disturbed Belt, northwestern Montana

The classical Lewis thrust fault in Glacier National Park has now been mapped 125 km south of the park to Steamboat Mountain, where the trace dies out in folded middle Paleozoic rocks. The known length of the fault is 452 km, extending northward from Steamboat Mountain to a point 225 km into Canada, where the fault also dies out in Paleozoic rocks. At the south end, the surface expression of the Lewis thrust begins in a shear zone in folded Mississippian rocks. To the north, the thrust progressively cuts downsection into Proterozoic Y (Belt) rocks near Glacier National Park. Displacement on the Lewis plate increases northward from approximately 3 km on an easterly trending hinge line at the West Fork of the Sun River to a postulated 65 km at the southern edge of the park, where the stratigraphic throw is about 6,500 m. Present data indicate the thrust formed during very late Paleocene to very early Eocene time. The Lewis thrust and related structures, the Hoadley thrust and the Continental Divide syncline, probably formed concurrently under the same stress field. The northern limit of the trace of the Hoadley thrust is within the lower portion of the Lewis plate, about 28 km north of where the Lewis thrust develops, and the Hoadley extends for at least 125 km to the south. Displacement of the Hoadley increases southward from about 1 km at the hinge line to an inferred 70 km near its known southern extent. If our inference is correct, the Hoadley is nearly the southern mirror image of the Lewis to the north. The Continental Divide syncline, a doubly plunging, broad, northerly trending open fold that is about 120 km long, is a major fold within the Lewis plate.

Professional Paper↗
USGS1977

A summary of the mineral resources of the proposed Great Bear wilderness, Flathead, Teton, and Pondera counties, Montana

The proposed Great Bear wilderness has a good potential for oil and gas (mainly gas), and a moderate potential for submarginal resources of copper and silver in a belt 25 miles (40 km) long and about 1 mile (1.6 km) wide that is partly within the study area along the western boundary. The eastern part of the area may contain small submarginal resources of coal. Sand and gravel, dimension stone, and high calcium limestone are in the area, but they occur in abundance in more accessible areas elsewhere in northwest Montana. The potential for geothermal resources is nil. A mineral survey of the proposed Great Bear wilderness was made in 1974 and 1976. The work by the U.S. Geological Survey consisted of geologic mapping, and geochemical and geophysical surveys. The U.S. Bureau of Mines made a courthouse search for mining claims, and sampled and mapped prospects.

Montana↗