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Research about Eureka County, Nevada

Source-linked reports with geographic coverage including Eureka County, Nevada.

9 recordsLinked to original sources
USGS2015

Early Permian conodont fauna and stratigraphy of the Garden Valley Formation, Eureka County, Nevada

The lower Part of the Garden Valley Formation yields two distinct conodont faunas. One of late Asselian age dominated by M esogondolella and Streptognathodus and one of Artinskian age dominated by Sweetognathus with Mesogondolella . The Asselian fauna contains the same species as those found in the type area of the Asselian in the southern Urals including Mesogondolella dentiseparata , described for the first time outside of the Urals. Apparatuses for Sweetognathus whitei , Diplognathodus stevensi , and I dioprioniodus sp. are described. The Garden Valley Formation represents a marine pro-delta basin and platform, and marine and shore fan delta complex deposition. The fan-delta complex was most likely deposited from late Artinskian to lateWordian. The Garden Valley Formation records tremendous swings in depositional setting from shallow-water to basin to shore.

Nevada↗
USGS2013

Intraseasonal variation in survival and probable causes of mortality in greater sage-grouse Centrocercus urophasianus

The mortality process is a key component of avian population dynamics, and understanding factors that affect mortality is central to grouse conservation. Populations of greater sage-grouse Centrocercus urophasianus have declined across their range in western North America. We studied cause-specific mortality of radio-marked sage-grouse in Eureka County, Nevada, USA, during two seasons, nesting (2008-2012) and fall (2008-2010), when survival was known to be lower compared to other times of the year. We used known-fate and cumulative incidence function models to estimate weekly survival rates and cumulative risk of cause-specific mortalities, respectively. These methods allowed us to account for temporal variation in sample size and staggered entry of marked individuals into the sample to obtain robust estimates of survival and cause-specific mortality. We monitored 376 individual sage-grouse during the course of our study, and investigated 87 deaths. Predation was the major source of mortality, and accounted for 90% of all mortalities during our study. During the nesting season (1 April - 31 May), the cumulative risk of predation by raptors (0.10; 95% CI: 0.05-0.16) and mammals (0.08; 95% CI: 0.03-013) was relatively equal. In the fall (15 August - 31 October), the cumulative risk of mammal predation was greater (M (mam) = 0.12; 95% CI: 0.04-0.19) than either predation by raptors (M (rap) = 0.05; 95% CI: 0.00-0.10) or hunting harvest (M (hunt) = 0.02; 95% CI: 0.0-0.06). During both seasons, we observed relatively few additional sources of mortality (e.g. collision) and observed no evidence of disease-related mortality (e.g. West Nile Virus). In general, we found little evidence for intraseasonal temporal variation in survival, suggesting that the nesting and fall seasons represent biologically meaningful time intervals with respect to sage-grouse survival.

Nevada↗
USGS1998

Oreshoot zoning in the Carlin-type Betze orebody, Goldstrike Mine, Eureka County, Nevada

Field and laboratory investigations of the giant Betze gold orebody, the largest Carlin-type deposit known, in the north-central Carlin trend, Nevada document that the orebody is composed of individual high-grade oreshoots that contain different geologic, mineralogic, and textural characteristics. The orebody is typical of many structurally controlled Carlin-type deposits, and is hosted in thin-bedded, impure carbonate or limy siltstone, breccia bodies, and intrusive or calc-silicate rock. Most ores in the Betze orebody are highly sheared or brecciated and show evidence of syndeformational hydrothermal deposition. The interplay between rock types and pre- and syn-structural events accounts for most of the distribution and zoning of the oreshoots. Hydrothermal alteration is scale dependent, either in broad, pervasive alteration patterns, or in areas related to various oreshoots. Alteration includes decarbonatization (~decalcification) of carbonate units, argillization (illite-clay), and silicification. Patterns of alteration zoning in and surrounding the Betze orebody define a large porous, dilated volume of rock where high fluid flow predominated. Local restriction of alteration to narrow illite- and clay-rich selvages around unaltered marble or calc-silicate rock phacoids implies that fluid flow favored permeable structures and deformed zones. Gold mainly is present as disseminated sub-micron-sized particles, commonly associated with Asñrich pyrite, although one type of oreshoot contains micron-size free gold. Oreshoots form a three-dimensional zoning pattern in the orebody within a WNW-striking structural zone of shearing and shear folding, termed the Dillon deformation zone (DDZ). Main types of oreshoots are: (1) rutile-bearing siliceous oreshoots; (2) illite-clay-pyrite oreshoots; (3) realgar- and orpiment-bearing oreshoots; (4) stibnite-bearing siliceous oreshoots; and (5) polymetallic oreshoots. Zoning patterns result from paragenetically early development of illite-clay-pyrite oreshoots during movement along the DDZ, and subsequent silicification and brecciation, associated with formation of the realgar- and orpiment-bearing, and stibnite-bearing oreshoots. Additional shear movement along the DDZ followed. Polymetallic oreshoots, which contain minerals rich in Hg, Cu, Zn, Ag, and native Au, were the last ores to form and overprint most earlier oreshoots. Ore textures, gouge, phyllonitic rock, alteration style, and previously documented isotopic and fluid-inclusion data, all indicate a weakly to moderately saline fluid that ascended and cooled during structural displacements. Changing conditions, due to water-wall rock reactions and P-T changes during deformation, are probably responsible for fluid variation that resulted in zoning of the different oreshoots during dynamic interaction of the Au-bearing fluid with the wall rock. This investigation indicates that isolated As-, Sb-, and Hg-rich ores are separate parts of a larger single gold system. This large gold system was contemporaneous with post-Jurassic brittle-ductile deformation, on the basis of deformed mineralized pods of the Jurassic Goldstrike pluton, and large-scale hydrothermal flow, and together they appear to be an integral part of the formation of some Carlin-type gold deposits in north-central Nevada.

Nevada↗
USGS1974

Thallium-bearing orpiment, Carlin gold deposit, Nevada

A variety of orpiment, As 2 S 3 , containing a significant amount of thallium has been identified in the unoxidized East ore body of the Carlin gold deposit. The mineral occurs in small veinlets with barite, calcite, quartz, and realgar.

Nevada↗
USGS1970

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.

Nevada↗