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Robert C. Jachens

Publications and source records attributed to Robert C. Jachens.

At least 55 records · Page 3Linked to original sources
USGS1991

Regional study of mineral resources in Nevada: Insights from three-dimensional analysis of gravity and magnetic anomalies

A three-dimensional interpretation of the basins of Nevada was developed based on statewide data bases of digital-gravity, magnetic, geologic, well, and topographic information. An iterative technique was applied to isostatic residual gravity anomalies in Nevada in order to define the location and shape of pre-Tertiary basement and to produce a gravity map that reflects variations of density within the pre-Tertiary basement. The dominant feature of the basement gravity of Nevada is an enormous area of low gravity that spans the entire state between latitudes 37°N and 40.5°N. This regional low strongly correlates with the distribution of middle and late Tertiary volcanic rocks and may reflect silicic intrusions within the mid-crust and upper crust that are the counterparts of volcanic rocks at the surface. Although 80% of Nevada is covered by Cenozoic deposits, these deposits are thicker than 1 km over only about 20% of the state. The remaining 60% of Nevada may have pre-Tertiary basement rocks within reach of current mineral-exploration techniques. Aeromagnetic profiles from the National Uranium Resource Evaluation (NURE) were analyzed in order to produce a map showing the location of shallow magnetic sources in Nevada. This analysis shows that 46% of the state has magnetic sources, generally Mesozoic and Cenozoic igneous rocks, within 1 km of the surface. A linear magnetic anomaly in north-central Nevada has been interpreted by others as a rift zone active during middle Miocene time. The rift also is evident in NURE magnetic profiles, but our interpretation suggests that the magnetic expression of the rift continues south-southeast with similar strike to at least 38°N and perhaps to the amagmatic zone (lat. 37°N). The survival since the middle Miocene of this narrow crustal feature, essentially linear over a distance of 500 km, is difficult to interpret in light of later Basin and Range deformation. Our analysis of gravity anomalies shows that many deep Cenozoic basins are located near the rift, yet only two basins cut across it, and at least five others change strike near the rift, as if to avoid it. The rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation.

Nevada↗
USGS1990

Mineral Resources of the Black Mountains North and Burns Spring Wilderness Study Areas, Mohave County, Arizona

At the request of the U.S. Bureau of Land Management, approximately 19,300 acres of the Black Mountains North Wilderness Study Area (AZ-020-009) and 23,310 acres of the Burns Spring Wilderness Study Area (AZ-02D-010) were evaluated for mineral resources and mineral resource potential. In this report, the area studied is referred to, collectively or individually, as the 'wilderness study area' or simply 'the study area'; any reference to the Black Mountains North or Burns Spring Wilderness Study Areas refers only to that part of the wilderness study area for which a mineral survey was requested by the U.S. Bureau of Land Management. The study area is located in western Arizona, about 30 mi northwest of Kingman. There are no identified resources in the study area. An area surrounding the Portland mine and including the southern part of the Black Mountains North Wilderness Study Area and the extreme northwestern part of the Burns Spring Wilderness Study Area has high resource potential for gold and moderate resource potential for silver, lead, and mercury. The area surrounding this and including much of the northern part of the Burns Spring Wilderness Study Area has moderate potential for gold, silver, and lead. The northeastern corner of the Black Mountains North Wilderness Study Area has moderate potential for gold and low potential for silver, copper, and molybdenum resources. The central part, including the narrow strip of land just west of the central part, of the Black Mountains North Wilderness Study Area and the southern and extreme eastern parts of the Burns Spring Wilderness Study Area have low resource potential for gold. The central and southern parts of the Black Mountains North Wilderness Study Area and all but the southwestern part of the Burns Spring Wilderness Study Area have moderate resource potential for perlite. Moderate resource potential for zeolites is assigned to a large area around the Portland mine that includes parts of both study areas, to a narrow strip of land just west of the central part of the Black Mountains North Wilderness Study Area, and to all but the southwest corner of the Burns Spring Wilderness Study Area. There is no potential for oil and gas in either study area. Sand and gravel are present in both study areas, but abundant quantities of these resources are available closer to existing markets.

Bulletin↗
USGS1990

Mineral Resources of the Mount Nutt Wilderness Study Area, Mohave County, Arizona

The Mount Nutt Wilderness Study Area (AZ-020-024) is located in the Black Mountains about 15 mi west of Kingman, Arizona. At the request of the U.S. Bureau of Land Management, approximately 27,210 acres of the wilderness study area was evaluated for mineral resources (known) and mineral resource potential (undiscovered). In this report, the area studied is referred to as the 'wilderness study area' or simply 'the study area'; any reference to the Mount Nutt Wilderness Study Area refers only to that part of the wilderness study area (27,210 acres) for which a mineral survey was requested. The U.S. Geological Survey and the U.S. Bureau of Mines conducted geological, geochemical, and geophysical surveys to assess the identified mineral resources and mineral resource potential of the study area. Fieldwork for this report was carried out in 1987 and 1988. A gold resource totaling at least 56,000 troy oz has been identified at two sites in Secret Pass Canyon, less than 0.5 mi north of the study area. No other metallic mineral resources were identified inside the study area. An area near the center of the study area contains fire agate, a gem stone. On the basis of tonnage, site accessibility, and current production methods, this area is considered an indicated subeconomic fire-agate resource for the foreseeable future. Sand and gravel are present in the study area. An area surrounding the Tincup mine and including a small portion of the extreme north-central part of the study area has high potential for gold and low potential for silver, lead, and mercury. Three areas in the extreme northwestern, north-central, and southwestern parts of the study area have moderate potential for gold and low potential for silver, lead, and mercury. A small area near the known fire-agate resource in the south-central part of the study area has low potential for fire agate. Large areas in the eastern and central parts of the study area have low potential for perlite and zeolite resources. The entire study area has no potential for oil and gas and no potential for geothermal resources.

Bulletin↗
USGS1990

Mineral Resources of the Wabayuma Peak Wilderness Study Area, Mohave County, Arizona

The Wabayuma Peak Wilderness Study Area (AZ-020-037/043), for which a mineral survey was requested by the U.S. Bureau of land Management, encompasses 40,118 acres in northwestern Arizona. Fieldwork was carried out in 1986-88 by the U.S. Bureau of Mines and the U.S. Geological Survey to appraise the identified (known) resources and assess the mineral resource potential (undiscovered) of the wilderness study area. Within the Wabayuma Peak Wilderness Study Area are 14 private parcels of land totaling 1,315 acres. The Wabayuma Peak Wilderness Study Area, including the 14 private parcels of land, is herein referred to as the 'wilderness study area' or the 'study area'. The Boriana, Antler, and Copper World mines lie near the east boundary of the study area. The Boriana mine was a major tungsten-producing mine of the United States during World War II. The Antler and Copper World mines produced relatively small amounts of copper and zinc prior to 1970. Copper and zinc were mined within 100 ft of the study area at the Antler mine. The Antler mine contains subeconomic resources of 350,000 to 400,000 short tons of copper-zinc ore; a minimum of 2,000 short tons, at grades of 1 to 4 percent copper and 1 to 2 percent zinc, lie within the study area. No other mineral resources were identified within the study area. Four small tracts in the eastern part and one in the central part of the study area have high resource potential for copper, zinc, and minor lead, silver, and gold in massive sulfide deposits. A large central tract and two eastern tracts have moderate resource potential for the same metals. An eastern and a western tract within the wilderness study area have high resource potential for tungsten, copper, and combinations of beryllium, gold, silver, arsenic, bismuth, molybdenum, tin, indium, thorium, niobium, yttrium, lanthanum, scandium, tantalum, rhenium, lead, zinc, and iron in granite-related tungsten-polymetallic vein deposits. Most of the rest of the study area has moderate resource potential for these metals. A northern tract in the study area has moderate resource potential for gold, copper, and combinations of silver, zinc, lead, tungsten, and molybdenum in polymetallic vein deposits of several types.

Bulletin↗
USGS1989

Isostatic residual gravity and crustal geology of the United States

A new isostatic residual gravity map of the conterminous United States presents continent-wide gravity data in a form that can be readily used, with geologic information and other geophysical data, in studies of the composition and structure of the continental crust. This map was produced from the gridded gravity data on which the recently released Gravity Anomaly Map of the United States is based. About 1 million onland and 0.8 million offshore gravity observations interpolated to a 4- by 4-km grid serve as the basis for both maps. The Airy-Heiskanen model of isostatic compensation of topography applied to topographic and bathymetric data averaged over 5- by 5-min compartments was used to remove, to first order, the large, long-wavelength Bouguer gravity anomalies caused by deep density distributions that support topographic loads. The parameters used in the Airy-Heiskanen model were topographic density, 2.67 g/cm 3 ; sea-level crustal thickness, 30 km; and density contrast across the base of the model crust, 0.35 g/cm 3 . Many of the conspicuous short-wavelength anomalies (widths less than several hundred kilometers) on the isostatic residual gravity map correlate with mapped or near-surface geologic features, and primarily reflect shallow-density distributions rather than any departures from isostatic equilibrium. In general, gravity highs occur over (1) mafic igneous bodies emplaced in rift or magmatic arc settings or as isolated intrusions controlled by structures; (2) accreted slices of mafic oceanic, island-arc, or transitional crust; and (3) uplifted crystalline basement. Gravity lows are found over (1) thick bodies of felsic intrusive or extrusive rocks; (2) sedimentary deposits in extensional, convergent, or transform settings; and (3) depressed crystalline basement. Anomalies with widths as much as 1,000 km or more also appear to reflect crustal properties in many cases—several broad gravity highs are associated with crust having a high average seismic wave velocity, and comparable broad gravity lows occur over areas of low average seismic velocity. Alternative ways of viewing the isostatic residual gravity data provide additional information about density distributions in the crust. The first-vertical derivative map accentuates gravity anomalies over shallow, abrupt density changes at the expense of those resulting from deeper or more gradual density transitions. The maximum horizontal gradient map contains information about the locations of pronounced density boundaries. Two-dimensional spectral analysis of the gravity data provides a quantitative means for identifying dominant fabrics in the gravity field and for distinguishing various terranes from each other. Neither Bouguer nor isostatic residual gravity anomalies are particularly well suited for practical modeling of deep structure in conjunction with deep seismic information. However, a scheme in which the entire Earth outside the area of interest is approximated by laterally homogeneous layers and isostatically compensated topography, and in which the area of interest is modeled using the seismic constraints applied in a two-and-one-half-dimensional geometry, holds promise for exploiting useful features of both the Bouguer and isostatic residual gravity anomalies.

conterminous United States↗
USGS1989

Mineral resources of the Mount Tipton Wilderness Study Area, Mohave County, Arizona

The Mount Tipton Wilderness Study Area (AZ-020-012/ 042) comprises 33,950 acres in Mohave County, Ariz. At the request of the U.S. Bureau of Land Management, this area was evaluated for identified mineral resources (known) and mineral resource potential (undiscovered). This work was carried out by the U.S. Bureau of Mines and the U.S. Geological Survey in 1984-87. In this report, the area studied is referred to as the "wilderness study area" or simply "the study area." There are no identified mineral resources in the study area. The southernmost part of the study area is adjacent to the Wallapai (Chloride) mining district and has low mineral resource potential for gold, silver, copper, lead, zinc, and molybdenum in hydrothermal veins. This area also has a low mineral resource potential for tungsten in vein deposits and for uranium in vein deposits or pegmatites. In the central part of the wilderness study area, one small area has low mineral resource potential for uranium in vein deposits or pegmatites and another small area has low resource potential for thorium in vein deposits. The entire study area has low resource potential for geothermal energy but no potential for oil or gas resources.

Arizona↗