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Chad Johnson

Publications and source records attributed to Chad Johnson.

7 recordsLinked to original sources
USGS2010

Hydrothermal zebra dolomite in the Great Basin, Nevada--attributes and relation to Paleozoic stratigraphy, tectonics, and ore deposits

In other parts of the world, previous workers have shown that sparry dolomite in carbonate rocks may be produced by the generation and movement of hot basinal brines in response to arid paleoclimates and tectonism, and that some of these brines served as the transport medium for metals fixed in Mississippi Valley-type (MVT) and sedimentary exhalative (Sedex) deposits of Zn, Pb, Ag, Au, or barite. Numerous occurrences of hydrothermal zebra dolomite (HZD), comprised of alternating layers of dark replacement and light void-filling sparry or saddle dolomite, are present in Paleozoic platform and slope carbonate rocks on the eastern side of the Great Basin physiographic province. Locally, it is associated with mineral deposits of barite, Ag-Pb-Zn, and Au. In this paper the spatial distribution of HZD occurrences, their stratigraphic position, morphological characteristics, textures and zoning, and chemical and stable isotopic compositions were determined to improve understanding of their age, origin, and relation to dolostone, ore deposits, and the tectonic evolution of the Great Basin. In northern and central Nevada, HZD is coeval and cogenetic with Late Devonian and Early Mississippian Sedex Au, Zn, and barite deposits and may be related to Late Ordovician Sedex barite deposits. In southern Nevada and southwest California, it is cogenetic with small MVT Ag-Pb-Zn deposits in rocks as young as Early Mississippian. Over Paleozoic time, the Great Basin was at equatorial paleolatitudes with episodes of arid paleoclimates. Several occurrences of HZD are crosscut by Mesozoic or Cenozoic intrusions, and some host younger pluton-related polymetallic replacement and Carlin-type gold deposits. The distribution of HZD in space (carbonate platform, margin, and slope) and stratigraphy (Late Neoproterozoic Ediacaran–Mississippian) roughly parallels that of dolostone and both are prevalent in Devonian strata. Stratabound HZD is best developed in Ediacaran and Cambrian units, whereas discordant HZD is proximal to high-angle structures at the carbonate platform margin, such as strike-slip and growth faults and dilational jogs. Fabric-selective replacement and dissolution features (e.g., collapse breccias, voids with geopetal textures) are common, with remaining void space lined with light-colored dolomite crystals that exhibit zoning under cathodoluminescence. Zoned crystals usually contain tiny (<1–3 μm) fluid inclusions with vapor bubbles, requiring Th > ∼70 °C. The oxygen isotopic compositions of HZD are consistent with formation temperatures of 50–150 °C requiring brine circulation to depths of 2–5 km, or more. The few HZD occurrences with the highest concentrations of metals (especially Fe, Mn, and Zn) and the largest isotopic shifts are closely associated with Sedex or MVT deposits known to have formed from hotter brines (e.g., Th > 150–250 °C). These relationships permit that HZD formed at about the same time as dolostone, from brines produced by the evaporation of seawater during arid paleoclimates at equatorial paleolatitudes. Both dolostone and HZD may have formed as basinal brines, which migrated seaward from evaporative pans on the platform, with dolostone forming at low temperatures along shallow migration pathways through permeable limestones, and HZD forming at high temperatures along deeper migration pathways through basal aquifers and dilatant high-angle faults. The small MVT deposits were chemical traps where hot brines encountered rocks or fluids containing reduced sulfur. The abundant Sedex deposits mark sites where hot brine discharged at the seafloor in adjacent basins. Thus the distribution of HZD may map deep migration pathways and upflow zones between eastern shallow marine facies, where evaporative brine could have been generated, and western Sedex deposits, where heated brines discharged along faults into platform margin, slope, and basin facies. The small size and scarcity of Pb-Zn deposits and the abundance of barite deposits in the Great Basin suggests the brines were generally reduced, possibly due to reactions with carbonaceous rocks along deep migration pathways. While this scenario may have occurred at several times, the age and abundance of Sedex deposits suggest that such a hydrology was best developed in the Late Ordovician, Late Devonian, and Early Mississippian, possibly in response to episodes of extension and forebulge faults associated with the Antler orogeny. The improved understanding of HZD may aid future exploration for ore deposits in the Great Basin.

Nevada↗
USGS2009

Surveillance for transmissible spongiform encephalopathy in scavengers of white-tailed deer carcasses in the chronic wasting disease area of wisconsin

Chronic wasting disease (CWD), a class of neurodegenerative transmissible spongiform encephalopathies (TSE) occurring in cervids, is found in a number of states and provinces across North America. Misfolded prions, the infectious agents of CWD, are deposited in the environment via carcass remains and excreta, and pose a threat of cross-species transmission. In this study tissues were tested from 812 representative mammalian scavengers, collected in the CWD-affected area of Wisconsin, for TSE infection using the IDEXX HerdChek enzyme-linked immunosorbent assay (ELISA). Only four of the collected mammals tested positive using the ELISA, but these were negative when tested by Western blot. While our sample sizes permitted high probabilities of detecting TSE assuming 1% population prevalence in several common scavengers (93%, 87%, and 87% for raccoons, opossums, and coyotes, respectively), insufficient sample sizes for other species precluded similar conclusions. One cannot rule out successful cross-species TSE transmission to scavengers, but the results suggest that such transmission is not frequent in the CWD-affected area of Wisconsin. The need for further surveillance of scavenger species, especially those known to be susceptible to TSE (e.g., cat, American mink, raccoon), is highlighted in both a field and laboratory setting.

Journal of Toxicology and Environmental Health, Pa↗
USGS2009

Genetic susceptibility to chronic wasting disease in free-ranging white-tailed deer: complement component C1q and Prnp polymorphisms

The genetic basis of susceptibility to chronic wasting disease (CWD) in free-ranging cervids is of great interest. Association studies of disease susceptibility in free-ranging populations, however, face considerable challenges including: the need for large sample sizes when disease is rare, animals of unknown pedigree create a risk of spurious results due to population admixture, and the inability to control disease exposure or dose. We used an innovative matched case&ndash;control design and conditional logistic regression to evaluate associations between polymorphisms of complement C1q and prion protein (Prnp) genes and CWD infection in white-tailed deer from the CWD endemic area in south-central Wisconsin. To reduce problems due to admixture or disease-risk confounding, we used neutral genetic (microsatellite) data to identify closely related CWD-positive ( n = 68) and CWD-negative ( n = 91) female deer to serve as matched cases and controls. Cases and controls were also matched on factors (sex, location, age) previously demonstrated to affect CWD infection risk. For Prnp, deer with at least one Serine (S) at amino acid 96 were significantly less likely to be CWD-positive relative to deer homozygous for Glycine (G). This is the first characterization of genes associated with the complement system in white-tailed deer. No tests for association between any C1q polymorphism and CWD infection were significant at p < 0.05. After controlling for Prnp, we found weak support for an elevated risk of CWD infection in deer with at least one Glycine (G) at amino acid 56 of the C1qC gene. While we documented numerous amino acid polymorphisms in C1q genes none appear to be strongly associated with CWD susceptibility.

Wisconsin↗
USGS2004

The late cretaceous Donlin Creek gold deposit, Southwestern Alaska: Controls on epizonal ore formation

The Donlin Creek gold deposit, southwestern Alaska, has an indicated and inferred resource of approximately 25 million ounces (Moz) Au at a cutoff grade of 1.5 g/t. The ca. 70 Ma deposit is hosted in the Late Cretaceous Kuskokwim flysch basin, which developed in the back part of the arc region of an active continental margin, on previously accreted oceanic terranes and continental fragments. A hypabyssal, mainly rhyolitic to rhyodacitic, and commonly porphyritic, 8- × 3-km dike complex, part of a regional ca. 77 to 58 Ma magmatic arc, formed a structurally competent host for the mineralization. This deposit is subdivided into about one dozen distinct prospects, most of which consist of dense quartz ± carbonate veinlet networks that fill north-northeast–striking extensional fractures in the northeast-trending igneous rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, and, typically younger, stibnite; gold is refractory within the arsenopyrite. Sericitization, carbonatization, and sulfidation were the main alteration processes. Fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent CO 2 ± CH 4 and a few tenths to a few mole percent NaCl + KCl. The gold-bearing fluids were mainly homogeneously trapped at approximately 275° to 300°C and at depths of 1 to 2 km. Some of the younger stibnite may have been deposited by late-stage aqueous fluids at lower temperature. Measured δ 18 O values for the gold-bearing quartz range between 11 and 25 per mil; the estimated δ 18 O fluid values range from 7 to12 per mil, suggesting a mainly crustally derived fluid. A broad range of measured δ D values for hydrothermal micas, between –150 and –80 per mil, is suggestive of a contribution from devolatilization of organic matter and/or minor amounts of mixing with meteoric fluids. Gold-associated hydrothermal sulfide minerals are characterized by δ 34 S values mainly between –16 and –10 per mil, with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin. A smaller group of δ 34 S measurements, which shows values as depleted as –27 per mil, suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite. Initial ϵ Nd of –8.7 to –3.1 and 87 Sr/ 86 Sr measurements of 0.706 to 0.709 for the ore-hosting dikes also indicate a crustal reservoir for some of the Late Cretaceous magmatism. Overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both. Copper- and gold-bearing stockwork veinlets in hornfels occur at Dome, a prospect located at the northern end of the Donlin Creek deposit. These stockworks are cut by the younger auriferous gold veins that define the main Donlin Creek gold mineralization. Highly saline, gas-rich, heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100°C hotter than those of the main gold-forming event at Donlin Creek. The genetic relationship of the Dome prospect to the main Donlin Creek gold resource is equivocal. The epizonal Donlin Creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related; it shows important differences from typical epithermal and Carlin-like deposits. The ore-forming fluids were derived by either broad-scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component.

Economic Geology↗
USGS2000

Origin of rainwater acidity near the Los Azufres geothermal field, Mexico

The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal field (88 MWe) and its surroundings during May - September 1995, which is the rainy season. Samples were collected from eight sites: three within the field, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl-, SO42- and NO3- were measured in about 350 samples and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, which suggests a nearby industrial source for the sulfate and nitrate. A mixing model for Cl-, SO42- and ??34S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sufficient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H2CO3(*). Values of alkalinity were found to range from 10-4 to 10-6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO42- and NO3- are in general not in acidic form (i.e. balanced by Na+, Ca2+, etc. rather than H+). Sulfate ??34S values were about -1.5??? in Los Azufres and its surroundings, and in Morelia, but differed from the value of -0.2??? for Guadalajara. The ??34S values for H2S from the Los Azufres geothermal wells are in the range -3.4 to 0.0???. The ??34S ranges for the natural and anthropogenic sources for environmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of ??34S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres. (C) 2000 Published by Elsevier Science Ltd on behalf of CNR.The chemical compositions of rainwater were analyzed at Los Azufres geothermal field in Spain from May-September 1995. The concentrations of Cl-, SO42- and NO3- were measured and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, suggesting a nearby industrial source. A mixing model for Cl-, SO42-, and ??34S also suggested an industrial source for the rainwater sulfur.

Geothermics↗
USGS1984

Ferromanganese crust resources in the Pacific and Atlantic Oceans

Ferromanganese crusts on raised areas of the ocean floor have joined abyssal manganese nodules and hydrothermal sulfides as potential marine resources. Significant volumes of cobalt-rich (about 1% Co) crusts have been identified to date within the US Exclusive Economic Zone (EEZ) in the Central Pacific: in the NW Hawaiian Ridge and Seamount region and in the seamounts in the Johnston Island and Palmyra Island regions. Large volumes of lower grade crusts, slabs, and nodules are also present in shallow ( greater than 1000 m) waters on the Blake plateau, off Florida-South Carolina in the Atlantic Ocean. Data on ferromanganese crusts have been increased by recent German and USGS cruises, but are still sparse, and other regions having crust potential are under current investigation. The authors discuss economic potentials for cobalt-rich crusts in the Central Pacific and Western North Atlantic oceans, with special reference to US EEZ areas. Additional research is needed before more quantitative resource estimates can be made.

Conference Paper↗