Geology topics
Research about Nevada
Source-linked reports with geographic coverage including Nevada.
Studies of hydrothermal gold deposition (I). Carlin gold deposit, Nevada: The role of carbonaceous materials in gold deposition
Studies of fresh carbonate host rocks and unoxidized gold ores of the Carlin mine indicate that gold, quartz, barite, pyrite, and other sulfides were introduced into the Roberts Mountains Formation by acid hydrothermal solutions. Laboratory investigations on the carbonaceous materials in the host rocks and ores and studies of reactions between carbonaceous materials and gold-bearing solutions show that the rocks contain: (1) An activated carbon component capable of adsorbing gold chloride or gold cyanide complexes from solution; (2) A mixture of high-molecular-weight hydrocarbons usually associated with the activated carbon components; and (3) An organic acid, similar to "humic acid," containing functional groups capable of interacting with gold complexes to form gold organic compounds. Although the exact structure of the gold organic compound(s) is not known, the most attractive possibility is chelation, where ligands such as N, S, or O in organic acids would easily displace the chloride ion from aurous chloride complexes and form stable gold chelates. Subsequent oxidation of the gold organic compounds destroys the organic component and leads to the formation of metallic gold. The relative amounts and types of carbonaceous materials are of principal importance in determining the chemical state and amount of gold deposited in carbonaceous limestone, although such factors as temperature, pH, and the oxidation state of the system are also involved.Although most of the gold ores at the Carlin mine are in the Silurian Roberts Mountains Formation and are at least several hundred feet stratigraphically below the Roberts Mountains thrust fault, disseminated replacement-type gold deposits could form in the lower-plate Devonian limestones (designated the Popovich Formation at the Carlin mine) and in carbonate and shale units of the upper-plate Ordovician Vinini Formation. In a structural setting that would provide satisfactory channels for movement of gold-bearing solutions, physical characteristics and chemical and mineralogical compositions, including the presence of organic materials, indicate that all three formations are favorable for replacement and deposition of gold.
Geochronology of Tertiary igneous rocks in central Nevada
Potassium-argon dating of Tertiary igneous rocks in Lander County, central Nevada, indicates that igneous activity was episodic and can be separated into three periods. Igneous activity started abruptly about 37 m.y. ago with local extrusion of andesitic to quartz-latitic lava flows and intrusion of hypabyssal rocks of similar composition. This activity ceased about 33 m.y. ago and was followed by extrusion of rhyolite ash-flow sheets that blanketed large parts of the region. These ash-flow sheets range from about 34 to 22 m.y. in age. The final phase, represented by basalt and basaltic-andesite flows and intrusive rhyolite flow-dome complexes, took place about 16 to 10 m.y. ago. Andesitic to dacitic lava and hypabyssal rocks about 35 m.y. old are widespread east of Lander County and rhyolitic ash-flow tuffs 34 to 20 m.y. old are found south and east of Lander County. The younger (16 to 10 m.y.) basalt and basaltic-andesite flows are related to volcanism of the Snake River plain province to the north. The precision of the ages was evaluated by means of: (1) repeat analyses of the same mineral separate, (2) age determination of mineral pairs from the same hand specimen, and (3) age determinations on widely spaced samples from the same geologic body or formation. The last method seems most meaningful from a geologic point of view.
Radiometric ages and stratigraphic sequence of volcanic and plutonic rocks, southern Nye and western Lincoln Counties, Nevada
The geochronology of Tertiary igneous events at the Nevada Test Site and adjacent area is outlined by 36 recently determined K-Ar ages, together with other published K-Ar ages. The first evidence of Tertiary igneous activity is the ash-fall bedded tuffs in the Horse Spring Formation. One such tuff has been dated as 29 m.y. old (late Oligocene). Other ash-flow tuffs and lavas formed during the Miocene and Pliocene, according to radiometric age determinations. The youngest ash-flow tuff in this area is about 6 m.y. old. Great volumes of ash and lava were spewed forth 13 to 11 m.y. ago to form the Paintbrush and Timber Mountain Tuffs. Sixteen replicate age determinations on minerals from four densely welded ash-flow tuffs from these formations gave a pooled standard deviation of about ± 2 percent error, provided anomalous ages were rejected on the basis of rock alteration or analytical difficulties. In the Air Force Gunnery Range, just north of the test site, K-Ar ages suggest that the oldest ash flows, the Monotony Tuff, were emplaced 27.6 m.y. ago (late Oligocene) and were followed by outpourings of lava and ash throughout most of the Miocene. Youngest dated lava is about 13 m.y. old. In the southern Egan and northern Seaman Ranges of central Nevada, the Needles Range (?) Formation has an averaged K-Ar age of about 30 m.y., which compares closely with 29.2 m.y., the average of four earlier K-Ar ages determined by other investigators on known Needles Range Formation in eastern Nevada and western Utah. K-Ar ages given by micas from two exposed plutons in the Nevada Test Site suggest emplacement of these plutons at about 93 m.y. ago (early Late Cretaceous), although earlier emplacement in the Mesozoic would be more consistent with Pb-α ages
Fission-track and K-Ar ages of Tertiary ash-flow tuffs, north-central Nevada
Ages obtained from three Tertiary ash-flow tuffs in central Nevada by fission-track and K-Ar dating are concordant. Samples dated by these methods from the same localities give the same age within the limits of analytical uncertainty. Samples of three units from widely separated localities were dated further to confirm the concordance of the dating methods and to establish that ages can be used, in conjunction with normal geologic techniques, as a criteria for correlation. The minerals dated by K-Ar were biotite and sanidine; and sphene zircon and apatite were used to determine the fission track ages. The Bates Mountain Tuff was dated at 24.0 m.y. (F.T.) and 24.0 m.y. (K-Ar). The Fish Creek Mountains Tuff has a fission-track age of 24.4 m.y., and a K-Ar age of 23.9 m.y. The average fission-track age for the Caetano Tuff is 34.9 m.y., and it has an average K-Ar age of 32.3 m.y. The amount of analytical uncertainty is slightly greater in the fission track method. Sampling, preparation, and determination of the age by the fission-track method is competitive with the K-Ar method.
Aeromagnetic and generalized geologic map of the Austin area, Lander County, Nevada
No abstract available.
Hydraulic testing and sampling of hole Ue-1 in central Nevada
Hydraulic testing and sampling of hole Ue-1 began on January 31, 1967, and ended on February 13, 1967. Depth intervals tested and sampled were: zone 1, 1,710 feet to total depth; zone 2, 1,508 to 1,706 feet; zone 3, 1,266 to 1,464 feet; zone 4, 788 to 986 feet; and zone 5, 400 to 598 feet. Injection and recovery tests were run on each depth interval. Relative specific capacities calculated from injection data were much smaller than those calculated from recovery data. More mud and lost circulation material in the hole and in the formation during the injection test account for the different relative specific capacities. Comparative composite hydrostatic heads before and after the injection and recovery tests were 211.5 feet below lsd (land-surface datum) and 218.0 feet below lsd. The difference may be due to differences in the conditions of the measurements. Of the five zones tested, only zone 5 did not have at least partial communication around the bottom packer.
Preliminary analysis of infrared imagery, Pahute Mesa area, Nevada Test Site, NASA site 52
Infrared images from two aircraft missions, flown by HRB-Singer Inc., August 10, 1966, and by NASA, Mission 30, September 3, 1966, have been analyzed. The HRB-Singer Inc., data, obtained in the 3-5 micrometer wavelength band between 10:32 and 11:53 p.m. (MST) in Spite of smoke in the atmosphere, revealed a fault that had not been mapped in the field and that was not visible on vertical black and white aerial photography. Twenty-four days later under clear weather conditions, the NASA aircraft obtained infrared imagery in the 8-14 micrometer band on a night overflight of the same area, but the fault mentioned above is not readily identified. Nighttime infrared observations assist stratigraphic and structural studies. Rhyolitic lava flows and welded tuff layers, having relatively high thermal inertia, are well outlined as bright (warm) areas on both sets of the nighttime imagery.
Rock analyses around the Copper Canyon open pit mine, Lander County, Nevada
No abstract available.
Newly discovered bedded barite deposits in East Northumberland Canyon, Nye County, Nevada
No abstract available.
Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site
Olivine trachybasalt metamorphosed by nuclear explosion is classified into categories of progressive metamorphism: (i) Weak. Plagioclase is microfractured, and augite cotainis fine twin lamellae. (ii) Moderate. Plagioclase is converted to glass, and mafic minerals show intragranular deformation (undulatory extinction, twin lamellae, and, possibly, deformation lamellae), but rock texture is preserved. (iii) Moderately strong. Plagioclase glass shows small-scale flow, mafic minerals are fractured and show intragranular deformation, and rocks contain tension fractures. (iv) Strong. Plagioclase glass is vesicular, augite is minutely fractured, and olivine is coarsely fragmented, shows mosaic extinction, distinctive lamellar structures, and is locally recrystallized. (v) Intense. Rocks are converted to inhomogeneous basaltic glass.
A demographic analysis of continuously irradiated and nonirradiated populations of the lizard, Uta stansburiana
A natural population of the lizard Uta stansburiana occupying a fenced 20-acre area in southern Nevada has been exposed to essentially continuous gamma irradiation since February 1964. Tissue doses averaged about 2 rads/day. Nonirradiated populations occupying three adjoining 20-acre areas have also been investigated. Five years of sampling data drawn from the experimental and control populations showed no statistically significant differences in the sex ratios. Comparisons of maximal life span (44 months) and χ 2 tests of age distributions did not indicate a statistically significant difference between the experimental and untreated populations, but the tests were not sensitive to small changes in the proportions of individuals living to the age of 44 months. Both the 59% increase of the irradiated population between 1966 and 1967 and the 43% decline between 1967 and 1968 were generally matched by corresponding changes in the three untreated areas.
Areal geologic and structural interpretation map of an area in the Sheep Creek Range, Lander County, Nevada
No abstract available.