Roberts Mountains Allochthon Research Articles

Exploring for structurally concealed Carlin-type mineralization: A case study from the northern Shoshone Range, Nevada, USA

This study integrates results from previous studies with new reconnaissance-scale mapping of rock types, structures, and hydrothermal alteration at Goat Ridge and Tenabo to provide a district-scale cross section and palinspastic restoration of an area in Nevada with high potential for Carlin-type mineralization in the footwall of a dismembered thrust fault. Reinterpretation of legacy drill data in light of recent advances in the understanding of Cenozoic normal faulting in the area with the expected stratigraphy of the Roberts Mountains autochthon allows for deciphering whether previous drill holes piercements of faults are the Roberts Mountains thrust or are tilted normal faults that down-dropped the deep-water siliciclastic rocks of Roberts Mountains allochthon onto the coeval carbonate rocks of the Roberts Mountains autochthon. A cross-sectional reconstruction shows that ∼11.1 km (120 %) of extension was accomplished along one set of faults that initiated at 60-70°and tilted 40°E. Previously underappreciated Mesozoic folds are present, which likely formed with horizontal fold hinges. Because of Cenozoic normal faulting and associated tilting, the hinge lines currently plunge at moderate angles and are offset as they step across each crosscutting fault. The anticlinal fold hinges produced the modern exposures of lower plate rocks in the range. The Eocene paleogeologic map that results from a plan-view structural restoration shows that the hinge lines of the original folds trended northwest-southeast in the southern half of the range but turned northward toward the northern end of the range.Mineralization in the range includes silver-dominated polymetallic veins/lodes, weak porphyry-type, Carlin-type, and siliciclastic-hosted gold deposits of uncertain origin. Porphyry-style and silver-dominated polymetallic vein mineralization is associated with ∼39–40 Ma granodiorites, with the silver-dominated polymetallic veins interpreted as the distal veins around weakly mineralized and/or deeply buried porphyry system(s), probably of the quartz monzonitic-granitic porphyry Mo-Cu subclass of porphyry molybdenum deposits. Crosscutting relationships demonstrate that at least one of “upper plate” gold deposits is Eocene, but a Miocene age cannot be ruled out for the other gold deposits.

The Antler orogeny reconsidered and implications for late Paleozoic tectonics of western Laurentia

Abstract Mississippian rocks of the Antler foreland basin in Nevada (western United States) record its evolution in detail, clarifying the timing and tectonic significance of the Antler orogeny. Synorogenic clastic sediments mark the initiation of tectonism; they arrived in Nevada from the north or northwest in late Early Mississippian (Osagean). An orogenic highland formed in northern Nevada in late Middle Mississippian (Meramecian), producing a regional angular unconformity and terminating deposition in the Antler foreland basin. Unconformably overlying Upper Mississippian (Chesterian) rocks provide tight age control for the redefined “Antler orogeny” sensu stricto. Palinspastic restoration of Sr isotopic data reveals a projecting step in the Laurentian margin in northern Nevada. The Antler orogeny resulted from late Middle Mississippian sinistral-oblique convergence at this step. Continuing oblique convergence produced the subsequent late Paleozoic basins, structures, and unconformities in northern Nevada. The Roberts Mountains allochthon is translated material from farther north emplaced onto the margin. Its basal bounding fault, long mapped as the “Roberts Mountains thrust,” is in fact not a single continuous fault, motion on it was not synchronous, and displacement on many fault segments mapped as the Roberts Mountains thrust postdate the late Middle Mississippian. We therefore propose that the term “Roberts Mountains thrust” be abandoned. This new paradigm not only explains anomalous characteristics of the Antler orogeny but also illustrates criteria for recognizing oblique convergence at other ancient plate margins.

Using discordant U-Pb zircon data to re-evaluate the El Paso terrane: Late Paleozoic tectonomagmatic evolution of east-central California (USA) and intense hydrothermal activity in the Jurassic Sierra Nevada arc

AbstractQuantitative modeling of discordant detrital zircon U-Pb isotope data from the northern El Paso terrane reveals metamorphosed Laurentian passive-margin strata within the Kern Plateau (southeastern Sierra Nevada), resolving a 40-year-long debate regarding this terrane’s origin. Previous studies of Kern Plateau pendants identify deep-water metasediments containing detrital zir-con populations similar to the Roberts Mountains allochthon; yet structural observations seemingly contradict proposed correlations to the Mississippian Roberts Mountains thrust, which juxtaposes exotic deep-water rocks over shallow-water, passive-margin strata in central Nevada. Here, new samples are combined with published data to identify segments of the thrust within the Kern Plateau, demonstrating that the El Paso terrane was offset ~350 km by late Paleozoic sinistral translation along the braided Kern Plateau shear zone, an abandoned model first proposed more than 20 years ago.New U-Pb-Hf isotope data from plutons intruding the Kern Plateau shear zone are virtually identical to published data from the El Paso Mountains, indicating that the Sierra Nevada–Mojave arc initiated in the late Early Permian (ca. 274 Ma) along the entire length of the El Paso terrane and was active into the Middle Triassic (ca. 240 Ma). Previously implicated Late Triassic arc activity within the Kern Plateau is not corroborated by single-crystal U-Pb data. Published structural evidence indicating reactivation of the late Paleozoic Kern Plateau shear zone is reinterpreted as indicating sinistral-oblique relative plate motion during Permian arc initiation followed by Middle Jurassic extension in the southeastern Sierra Nevada arc, which facilitated intense hydrothermal activity and zircon lead loss.

Environmental Geology of Barite Deposits in Nevada

Abstract Barite is an important industrial mineral used as a weighing agent in fluids during the drilling of oil and natural gas wells, with the majority of United States production originating from Nevada. This paper considers three barite properties at varying stages of development—Rossi, Heavy Spar, and Ann—all located within the northern Nevada barite belt. At all three sites, barite is hosted in the Roberts Mountains allochthon consisting of siliceous sedimentary rocks. Geochemical characterization of waste rock, ore, and process products has been undertaken as part of project development at each site, including 212 acid-base accounting (ABA) analysis and 19 humidity-cell tests operated for up to 114 weeks. This study demonstrates that the presence of barite greatly limits the application of using traditional Sobek-style ABA methods. Barite will undergo incomplete dissolution and extraction in the ABA tests that will result in an overprediction of acid generation for materials that contain barite (i.e., ore and jig by-products). Results indicate use of the net acid generation test to inform waste rock management represents a more reliable indicator of acid generation for these three deposits and potentially also other barite deposits. Potential barite mine sites should continue to be evaluated using the same geochemical testing tools as base metal mines but using those tools in a selective way to achieve balance between regulatory requirements and costs.

Nature and timing of Late Devonian–early Mississippian island-arc magmatism in the Northern Sierra terrane and implications for regional Paleozoic plate tectonics

Abstract The Northern Sierra terrane is one of a series of Paleozoic terranes outboard of the western Laurentian margin that contain lithotectonic elements generally considered to have originated in settings far removed from their present relative locations. The Lower to Middle Paleozoic Shoo Fly Complex makes up the oldest rocks in the terrane and consists partly of thrust-imbricated deep-marine sedimentary strata having detrital zircon age signatures consistent with derivation from the northwestern Laurentian margin. The thrust package is structurally overlain by the Sierra City mélange, which formed within a mid-Paleozoic subduction zone and contains tectonic blocks of Ediacaran tonalite and sandstone with Proterozoic to early Paleozoic detrital zircon populations having age spectra pointing to a non–western Laurentian source. Island-arc volcanic rocks of the Upper Devonian Sierra Buttes Formation unconformably overlie the Shoo Fly Complex and are spatially associated with the Bowman Lake batholith, Wolf Creek granite stock, and smaller hypabyssal felsic bodies that intrude the Shoo Fly Complex. Here, we report new results from U-Pb sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) dating of 15 samples of the volcanic and intrusive rocks, along with geochemical studies of the dated units. In addition, we report U-Pb laser ablation–inductively coupled plasma–mass spectrometry ages for 50 detrital zircons from a feldspathic sandstone block in the Sierra City mélange, which yielded abundant Ordovician to Early Devonian (ca. 480–390 Ma) ages. Ten samples from the composite Bowman Lake batholith, which cuts some of the main thrusts in the Shoo Fly Complex, yielded an age range of 371 ± 9 Ma to 353 ± 3 Ma; felsic tuff in the Sierra Buttes Formation yielded an age of 363 ± 7 Ma; and three felsic hypabyssal bodies intruded into the Sierra City mélange yielded ages of 369 ± 4 Ma to 358 ± 3 Ma. These data provide a younger age limit for assembly of the Shoo Fly Complex and indicate that arc magmatism in the Northern Sierra terrane began with a major pulse of Late Devonian (Famennian) igneous activity. The Wolf Creek stock yielded an age of 352 ± 3 Ma, showing that the felsic magmatism extended into the early Mississippian. All of these rocks have similar geochemical features with arc-type trace-element signatures, consistent with the interpretation that they constitute a petrogenetically linked volcano-plutonic system. Field evidence shows that the felsic hypabyssal intrusions in the Sierra City mélange were intruded while parts of it were still unlithified, indicating that a relatively narrow time span separated subduction-related deformation in the Shoo Fly Complex and onset of Late Devonian arc magmatism. Following recent models for Paleozoic terrane assembly in the western Cordillera, we infer that the Shoo Fly Complex together with strata in the Roberts Mountains allochthon in Nevada migrated south along a sinistral transform boundary prior to the onset of arc magmatism in the Northern Sierra terrane. We suggest that the Shoo Fly Complex arrived close to the western Laurentian margin at the same time as the Roberts Mountains allochthon was thrust over the passive margin during the Late Devonian–early Mississippian Antler orogeny. This led to a change in plate kinematics that caused development of a west-facing Late Devonian island arc on the Shoo Fly Complex. Due to slab rollback, the arc front migrated onto parts of the Sierra City mélange that had only recently been incorporated into the accretionary complex. In the mélange, blocks of Ediacaran tonalite, as well as sandstones having detrital zircon populations with non–western Laurentian sources, may have been derived from the Yreka and Trinity terranes in the eastern Klamath Mountains, where similar rock types occur. If so, this suggests that these Klamath terranes were in close proximity to the developing accretionary complex in the Northern Sierra terrane in the Late Devonian.

Three‐Dimensional Evolution of the Early Paleozoic Western Laurentian Margin: New Insights From Detrital Zircon U‐Pb Geochronology and Hf Isotope Geochemistry of the Harmony Formation of Nevada

AbstractUranium‐lead (U‐Pb) geochronology and Hafnium (Hf) isotope geochemistry of detrital zircons of the Harmony Formation of north central Nevada provide new insights into the tectonic evolution of the Late Paleozoic western Laurentian margin. Using laser‐ablation inductively coupled plasma mass spectrometry, 10 arenite samples were analyzed for U‐Pb ages, and 8 of these samples were further analyzed for Hf isotope ratios. Three of the sampled units have similar U‐Pb age peaks and Hf isotope ratios, including a 1.0–1.4 Ga peak with εHf values of +12 to −3 and a 2.5–2.7 Ga peak with εHf values of +7 to −5. The remaining seven samples differ significantly from these three, but are similar to one another; having age peaks of 1.7–1.9 Ga with εHf of +10 to −20 and age peaks of 2.3–2.7 Ga with εHf of +6 to −8. The data confirm the subdivision of the Harmony Formation into two petrofacies: quartzose (Harmony A) and feldspathic (Harmony B). The three samples with 1.0–1.4 and 2.5–2.7 Ga peaks are the Harmony A, which originated in the central Laurentian craton. The other seven samples are the Harmony B, which originated in eastern Alberta‐western Saskatchewan, north of the Harmony A source. We propose that all Harmony Formation strata were deposited near eastern Alberta and subsequently tectonically interleaved with Roberts Mountains allochthon strata. We interpret that the entire package was tectonically transported south along the western Laurentian margin and then emplaced eastward onto the craton during the Late Devonian‐Early Mississippian Antler orogeny.

The Valmy thrust sheet: A regional structure formed during the protracted assembly of the Roberts Mountains allochthon, Nevada, USA

The Valmy thrust sheet: A regional structure formed during the protracted assembly of the Roberts Mountains allochthon, Nevada, USA

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon

Research Article| November 01, 2014 Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon Joseph P. Colgan; Joseph P. Colgan † 1U.S. Geological Survey, Denver Federal Center, Bldg. 25, PO Box 25046, Lakewood, Colorado 80225 †Corresponding author: e-mail, jcolgan@usgs.gov Search for other works by this author on: GSW Google Scholar Christopher D. Henry; Christopher D. Henry 2Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557 Search for other works by this author on: GSW Google Scholar David A. John David A. John 3U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information Joseph P. Colgan † 1U.S. Geological Survey, Denver Federal Center, Bldg. 25, PO Box 25046, Lakewood, Colorado 80225 Christopher D. Henry 2Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557 David A. John 3U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025 †Corresponding author: e-mail, jcolgan@usgs.gov Publisher: Society of Economic Geologists Received: 25 Jun 2013 Accepted: 08 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1554-0774 Print ISSN: 0361-0128 © 2014 Society of Economic Geologists. Economic Geology (2014) 109 (7): 1843–1862. https://doi.org/10.2113/econgeo.109.7.1843 Article history Received: 25 Jun 2013 Accepted: 08 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph P. Colgan, Christopher D. Henry, David A. John; Evidence for Large-Magnitude, Post-Eocene Extension in the Northern Shoshone Range, Nevada, and Its Implications for the Structural Setting of Carlin-Type Gold Deposits in the Lower Plate of the Roberts Mountains Allochthon. Economic Geology 2014;; 109 (7): 1843–1862. doi: https://doi.org/10.2113/econgeo.109.7.1843 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The northern Shoshone and Toiyabe Ranges in north-central Nevada expose numerous areas of mineralized Paleozoic rock, including major Carlin-type gold deposits at Pipeline and Cortez. Paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting, but here we present new data that suggest major post-Eocene extension along west-dipping normal faults. Tertiary rocks in the northern Shoshone Range crop out in two W-NW–trending belts that locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. Tertiary exposures in the more extensive, northern belt were interpreted as subvertical breccia pipes (intrusions), but new field data indicate that these “pipes” consist of a 35.8 Ma densely welded dacitic ash flow tuff (informally named the tuff of Mount Lewis) interbedded with sandstones and coarse volcaniclastic deposits. Both tuff and sedimentary rocks strike N-S and dip 30° to 70° E; the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes. The southern belt along Mill Creek, previously mapped as undivided welded tuff, includes the tuff of Cove mine (34.4 Ma) and unit B of the Bates Mountain Tuff (30.6 Ma). These tuffs dip 30° to 50° east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks in both belts were deposited on Paleozoic basement and none appear to be breccia pipes. We infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the 34.0 Ma Caetano caldera ~40° east in the central Shoshone Range (<5 km south of Mill Creek), but further mapping is necessary to trace the faults through the highly deformed Paleozoic rocks that surround the isolated Tertiary outcrops. Significant post-Eocene extensional faulting in the northern Shoshone Range may have important implications for both the structure of the Roberts Mountains allochthon and the exposure of potentially mineralized rocks in its lower plate, both of which were likely east-tilted and repeated by west-dipping faults together with overlying Tertiary rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemical and Geochronological Constraints on Mineralization within the Hilltop, Lewis, and Bullion Mining Districts, Battle Mountain-Eureka Trend, Nevada

The Hilltop, Lewis, and Bullion mining districts, located within the northern Shoshone Range, Lander County, Nevada, are part of the greater Battle Mountain-Eureka trend. Historic production of mostly Ag with Au, Cu, Pb, and Zn from all three districts exploited ≤ 3-m-wide quartz veins hosted mainly within upper plate Ordovician-Devonian siliceous and siliciclastic rocks of the Roberts Mountains allochthon. Eocene felsic intrusive rocks, located both proximal and distal to the vein deposits, were emplaced throughout the northern Shoshone Range along a west-northwest trend; molybdenite mineralization is associated with some intrusive rocks, and some intrusive rocks host vein-type mineralization. New and previously published radiometric ages for primary igneous minerals from unaltered rocks within and/or proximal to mineralized areas are nearly coincident with published Re-Os ages for several northern Shoshone Range molybdenite occurrences (40.1 ± 0.6 Ma, avg), supporting a relationship between pluton emplacement and molybdenite mineralization. Age differences between quartz vein-hosted gold ± base metal mineralization (e.g., 38.3 ± 0.1 Ma: Gray Eagle mine, Bullion district) and nearby intrusive rocks (e.g., 39.0 ± 0.1 Ma: Granite Mountain) imply that vein mineralization probably formed during prolonged hydrothermal activity, synchronous with or subsequent to igneous rock emplacement, or from heat associated with a second, slightly younger phase of intrusive igneous activity. Primary fluid inclusion salinity (0–6.4 wt % NaCl equiv) and Th (115°–425°C) data from ore-bearing vein quartz, in conjunction with measured δ18OVSMOW data (−0.97 to +17.3‰), indicate that vein-forming fluids contained variable mixtures of magmatic and meteoric components (calculated δ18OVSMOW values of −16.0 to +13.3‰) and spanned the epithermal-porphyry temperature continuum. New and previously published stable isotope (C, O, and S) data from vein-hosted ore and gangue minerals, unaltered and altered siliceous (upper plate) host rocks, and unaltered carbonate (lower plate and upper plate) rocks indicate mixed and variable sources of vein-hosted ore and gangue components and variable interaction between some vein-forming fluids and nonsiliceous rock types (carbonates, bedded barite) that underlie the northern Shoshone Range. Large vein widths, high vein densities, and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern Shoshone Range probably formed at shallow depths (≤1 km) under lithostatic conditions. Geochronology and stable isotope data support a temporal and genetic link between Eocene magmatism, quartz vein-hosted precious and base metal mineralization, and molybdenite (porphyry) mineralization within the Hilltop, Lewis, and Bullion mining districts.

Three-dimensional geologic model of the northern Nevada rift and the Beowawe geothermal system, north-central Nevada

A three-dimensional (3D) geologic model of part of the northern Nevada rift encompassing the Beowawe geothermal system was developed from a series of two-dimensional (2D) geologic and geophysical models. The 3D model was constrained by local geophysical, geologic, and drill-hole information and integrates geologic and tectonic interpretations for the region. It places important geologic constraints on the extent and configuration of the active Beowawe geothermal system. The geologic framework represented in this model facilitates hydrologic modeling of the Beowawe geothermal system and evaluation of fluid flow in faults and adjacent rock units. Basin depths were determined using an iterative gravity-inversion technique that calculates the thickness of low-density, basin-filling deposits. The remaining subsurface structure was modeled using 2D potential-field modeling software. Crustal cross sections from the 2D models were generalized for use in the 3D model and consist of six stratigraphic layers defined as low-density basin sediments, volcanic rocks, basalt-andesite rocks of the northern Nevada rift, Jurassic and Cretaceous intrusive rocks, and Paleozoic siliceous and carbonate sedimentary rocks of the upper and lower plates of the Roberts Mountains allochthon, respectively. This simplified stratigraphy was combined with mapped surface geology and was extrapolated across the 3D model area. Features along the northern Nevada rift depicted by the model may represent preexisting crustal structures that controlled the locations and character of Tertiary tectonic and magmatic events related to Basin and Range extension and emplacement of the middle Miocene northern Nevada rift. Several of the geologic features represented are important components of the Beowawe geothermal system. Prominent ENE-trending faults (e.g., Malpais fault) that bound the southern edge of Whirlwind Valley, and older NNW-striking faults (e.g., Dunphy Pass and Muleshoe faults) that form major features of the model, are likely important pathways for geothermal fluids and groundwater flow from the Humboldt River, which may recharge the Beowawe system.

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.

Widespread effects of middle Mississippian deformation in the Great Basin of western North America

Stratigraphic analyses in central and eastern Nevada reveal the importance of a deformation event in middle Mississippian time that caused widespread deformation, uplift, and erosion. It occurred between middle Osagean and late Meramecian time and resulted in deposition of both synorogenic and postorogenic sediments. The deformation resulted in east-west shortening, expressed as east-vergent folding and east-directed thrusting; it involved sedimentary rocks of the Antler foredeep as well as strata associated with the Roberts Mountains allochthon. A latest Meramecian to early Chesterian unconformity, with correlative conformable lithofacies changes, postdates this deformation and occurs throughout Nevada. A tectonic highland—created in the middle Mississippian and lasting into the Pennsylvanian and centered in the area west and southwest of Carlin, Nevada—shed sediments eastward across the Antler foreland, burying the unconformity. Post oro genic strata are late Meramecian to early Chesterian at the base and are widespread throughout the Great Basin. The tectonism therefore occurred 20 to 30 m.y. after inception of the Late Devonian Antler orogeny, significantly extending the time span of this orogeny or representing a generally unrecognized orogenic event in the Paleozoic evolution of western North America. We propose a revised stratigraphic nomenclature for Mississippian strata in Nevada, based on detailed age control and the recognition of unconformities. This approach resolves the ambiguity of some stratigraphic names and emphasizes genetic relationships within the upper Paleozoic section. We take advantage of better stratigraphic understanding to propose two new stratigraphic units for southern and eastern Nevada: the middle Mississippian Gap Wash and Late Mississippian Captain Jack Formations.

Biostratigraphy and Structure of Paleozoic Host Rocks and Their Relationshipto Carlin-Type Gold Deposits in the Jerritt Canyon Mining District, Nevada

The Jerritt Canyon mining district in the northern Independence Range, northern Nevada, contains multiple, nearly horizontal, thrust masses of platform carbonate rocks that are exposed in a series of north- to northeast-elongated, tectonic windows through rocks of the Roberts Mountains allochthon. The Roberts Mountains allochthon was emplaced during the Late Devonian to Early Mississippian Antler orogeny. These thrust masses contain structurally and stratigraphically controlled Carlin-type gold deposits. The gold deposits are hosted in tectonically truncated units of the Silurian to Devonian Hanson Creek and Roberts Mountains Formations that lie within structural slices of an Eastern assemblage of Cambrian to Devonian carbonate rocks. In addition, these multiply thrust-faulted and folded host rocks are structurally interleaved with Mississippian siliciclastic rocks and are overlain structurally by Cambrian to Devonian siliciclastic units of the Roberts Mountains allochthon. All sedimentary rocks were involved in thrusting, high-angle faulting, and folding, and some of these events indicate substantial late Paleozoic and/or Mesozoic regional shortening. Early Pennsylvanian and late Eocene dikes also intrude the sedimentary rocks. These rocks all were uplifted into a northeast-trending range by subsequent late Cenozoic Basin and Range faulting. Eocene sedimentary and volcanic rocks flank part of the range. Pathways of hydrothermal fluid flow and locations of Carlin-type gold orebodies in the Jerritt Canyon mining district were controlled by structural and host-rock geometries within specific lithologies of the stacked thrust masses of Eastern assemblage rocks. The gold deposits are most common proximal to intersections of northeast-striking faults, northwest-striking dikes, and thrust planes that lie adjacent to permeable stratigraphic horizons. The host stratigraphic units include carbonate sequences that contained primary intercrystalline permeability, which provided initial pathways for fluid flow and later served as precipitation sites for ore minerals. Alteration, during, and perhaps prior to mineralization, enhanced primary permeability by dissolution, by removal of calcite, and by formation of dolomite. Ore-stage sulfide minerals and alteration minerals commonly precipitated in pore spaces among dolomite grains. Microveinlets and microbrecciation in zones of intense alteration also provided networks of secondary permeability that further enhanced fluid flux and produced additional sites for ore deposition.

U-Pb geochronology of detrital zircons from the Snow Lake pendant, central Sierra Nevada—Implications for Late Jurassic– Early Cretaceous dextral strike-slip faulting

The Snow Lake pendant is underlain by a thick (>1500 m) sequence of predominantly quartzite, marble, and schist that has previously been correlated with miogeoclinal strata of the western Mojave Desert. In this study, U-Pb analyses of detrital zircons from the Snow Lake pendant have been conducted to test for possible correlations with strata of the Cordilleran miogeocline, as well as nearby rocks of the Roberts Mountains allochthon and the Shoo Fly Complex. Zircons from Snow Lake strata yield dominant age groups that strongly support correlation of Snow Lake strata with miogeoclinal strata in the Mojave Desert. This correlation provides additional support for previous suggestions that the Snow Lake pendant was displaced ∼400 km northward along the Mojave–Snow Lake fault.

Recognition of fine-scale imbricate thrusts in lower Paleozoic orogenic belts—An example from the Roberts Mountains allochthon, Nevada

Dating of lower Paleozoic cherts is now possible through advancements in radiolarian biostratigraphy, permitting greater refinement of structural and stratigraphic models than has previously been possible. A detailed study of the type section of the Ordovician Vinini Formation, Vinini Creek, Roberts Mountains allochthon, Nevada, has yielded anomalously young Devonian ages. These ages were unanticipated and indicate that structural imbrication exists in a classic and previously well dated section of basinal strata. Previous ages of cherts were inferred from their stratigraphic position relative to graptolite-bearing shales. New mapping and biostratigraphy reveal considerable structural interleaving of Ordovician and Devonian strata within the allochthon, and many of the cherts appear to have structural bases. The Roberts Mountains allochthon, considered to be allochthonous but not far traveled, displays a highly complex structural history, including out of sequence thrusting and/or overprinting by later contractional deformation having a similar transport direction. This deformation was previously unrecognized because of the lack of overlap sequences in critical areas and the paucity of internal age control. Because the Roberts Mountains allochthon is not considered to be exotic, but rather has undergone only a modest amount of transport, this study shows that convergent margins with high degrees of structural imbrication do not require large amounts of transport. A biostratigraphic approach that examines all lithologies, coupled with detailed observations of field relations, is required for the construction of detailed and accurate structure sections in early Paleozoic orogenic belts.

Distal Ancestral Rocky Mountains tectonism: Evolution of the Pennsylvanian-Permian Oquirrh–Wood River basin, southern Idaho

Research Article| May 01, 1998 Distal Ancestral Rocky Mountains tectonism: Evolution of the Pennsylvanian-Permian Oquirrh–Wood River basin, southern Idaho Jeffrey K. Geslin Jeffrey K. Geslin 1Department of Geology, Idaho State University, Pocatello, Idaho 83209-8072 Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey K. Geslin 1Department of Geology, Idaho State University, Pocatello, Idaho 83209-8072 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1998) 110 (5): 644–663. https://doi.org/10.1130/0016-7606(1998)110<0644:DARMTE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey K. Geslin; Distal Ancestral Rocky Mountains tectonism: Evolution of the Pennsylvanian-Permian Oquirrh–Wood River basin, southern Idaho. GSA Bulletin 1998;; 110 (5): 644–663. doi: https://doi.org/10.1130/0016-7606(1998)110<0644:DARMTE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Strata of the Oquirrh–Wood River basin in southern Idaho and northeastern Nevada provide a record of the northern extent of basin development related to the Pennsylvanian-Permian Ancestral Rocky Mountains. Sedimentary facies distributions, sediment dispersal patterns, provenance analyses, and subsidence analysis suggest that reactivated thrusting within a segment of the Devonian-Mississippian Antler orogenic belt (Roberts Mountains allochthon) caused subsidence of the Oquirrh–Wood River basin. Reactivated thrusting within the Antler belt was apparently the primary expression of Ancestral Rocky Mountains tectonism in southern Idaho and northeastern Nevada.Stratigraphic architecture of the Oquirrh–Wood River basin was generally controlled by deformation of the Roberts Mountains allochthon to the west of the basin. The Oquirrh–Wood River basin trends north-northwest and is obliquely superposed on the eastern margin of the generally north-trending Roberts Mountains allochthon. Uplift of part of the allochthon resulted in complex sediment dispersal patterns during early basin development. Early basin fill includes Middle Pennsylvanian chert-pebble conglomerate derived from chert-rich Roberts Mountains allochthon strata to the northwest and west of the basin. These conglomerates were deposited in a shallow-marine setting on the northwestern to western basin margin and were progressively reworked to the southeast. Continued reactivation of the Antler belt in Late Pennsylvanian and Early Permian time resulted in the deposition of a thick quartzite-pebble conglomerate sequence along the western basin margin. Crustal loading during this phase of deformation created a generally north-trending foredeep, in which as much as 3000 m of mixed siliciclastic-carbonate turbidites were deposited in an anoxic marine environment. The siliciclastic turbidite fraction was derived from the craton interior to the northeast of the basin, whereas the carbonate fraction was derived from a carbonate platform on the eastern basin margin.Facies relationships, sandstone and conglomerate provenance, and basin-subsidence history constrain the tectonic setting and evolution of the Oquirrh–Wood River basin. The Oquirrh–Wood River basin probably developed as a flexural foredeep in front of a reactivated Antler thrust belt in a compressional or transpressional setting. If basin formation were driven by transpression, the basin could be explained by models of the Ancestral Rocky Mountains that relate deformation to the Ouachita-Marathon orogenic belt on the southern margin of North America. Alternatively, a recently proposed model relates the Ancestral Rocky Mountains to Pennsylvanian-Permian Andean-style convergence on the southwestern margin of North America. Although the age and location of the convergent margin are poorly constrained in this model, it is generally supported by the west-east to southwest-northeast compression needed to form the Oquirrh–Wood River basin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Roberts Mountains allochthon and the western margin of the Cordilleran miogeocline in the Northern Ritter Range pendant, eastern Sierra Nevada, California

The truncated southwestern edge of the Roberts Mountains allochthon is exposed in the Northern Ritter Range pendant in the eastern Sierra Nevada, structurally overlying parautochthonous rocks of the Cordilleran miogeocline. The Northern Ritter Range pendant exposes units that have the same stratigraphic affinities and structural relationships as rocks of the Antler orogenic belt in Nevada. Paleozoic metasedimentary rocks exposed in the pendant consist of two major units: (1) a structurally complex, disrupted chert and argillite unit interpreted to be correlative with the Roberts Mountains allochthon; and (2) a stratigraphically coherent siliceous and calcareous unit, the Rush Creek sequence, that is interpreted to be part of a transitional outer shelf and slope assemblage of the lower Paleozoic Cordilleran miogeocline. In the Northern Ritter Range pendant, the Roberts Mountains allochthon structurally overlies the Rush Creek sequence along a north-striking, steeply dipping fault zone that may be a preserved remnant of the Roberts Mountains thrust, which in north-central Nevada emplaced the allochthon over outer shelf and slope strata of the Cordilleran miogeocline during the Late Devonian–Early Mississippian Antler orogeny. These stratigraphic and structural belts are truncated on the southwest side of the pendant by the Gem Lake shear zone, a northwest-trending dextral strike-slip fault associated with the Cretaceous Sierra Nevada batholith. The Northern Ritter Range pendant thus defines both the southwestern limit of the Antler orogenic belt and the westernmost exposures of parautochthonous miogeoclinal rocks in the central Cordillera.

Kinematics of Underthrusting in the Paleozoic Antler Foreland Basin

Field mapping and structural analysis of Paleozoic rocks in the northern Roberts Mountains in central Nevada reveal that fine-grained siliciclastic strata deposited early in the history of the Antler foreland basin underthrust and underplated as two discrete fault-bound packets to the Roberts Mountains allochthon as it encroached upon the North American continental shelf. Underplating also resulted in the intercalation of North American outer shelf/ slope strata between the two foreland basin packets as the basal detachment of the Roberts Mountains allochthon stepped downward in the sedimentary section. Deformation of foreland basin and outer shelf/slope strata was in response to ESE in-layer shortening accompanied by top ESE layer-parallel shear within 10 m of fault surfaces along which underplating and underthrusting occurred. Stratal disruption, fold tightness and asymmetry, and cleavage development near the lower and upper-bounding faults are greater than in the center of underthrust packages, indicat...

Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada

Research Article| July 01, 1994 Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada MOIRA SMITH; MOIRA SMITH 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GEORGE GEHRELS GEORGE GEHRELS 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information MOIRA SMITH 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 GEORGE GEHRELS 1Department of Geosciences University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (7): 968–979. https://doi.org/10.1130/0016-7606(1994)106<0968:DZGATP>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MOIRA SMITH, GEORGE GEHRELS; Detrital zircon geochronology and the provenance of the Harmony and Valmy Formations, Roberts Mountains allochthon, Nevada. GSA Bulletin 1994;; 106 (7): 968–979. doi: https://doi.org/10.1130/0016-7606(1994)106<0968:DZGATP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A long-standing debate over the provenance of lower Paleozoic sedimentary rocks in the Roberts Mountains allochthon, Nevada, is addressed through abraded single-grain U-Pb detrital zircon analysis of the Harmony and Vahny Formations. Twenty-seven zircon grains from mature quartz arenite of the Middle Ordovician Valmy Formation yielded concordant to a few percent discordant ages of ∼1050 Ma (n = 2),1830-1845 Ma (n = 4),1910-1960 Ma (n = 5),2270-2340 Ma (n = 2), 2650-2750 Ma (n = 8), and ∼2900 Ma (n =1), and moderately discordant ages of ∼2070 Ma (n =1) and ∼3240 Ma (n =1). These ages are consistent with ultimate derivation from the north (Slave craton and Medicine Hat province and central/northern Alberta) and suggest a link between the Valmy Formation and the coeval Eureka Quartzite on the adjacent shelf and miogeoclinal prism. Twenty-seven analyses of zircon from immature quartzo-feldspathic wacke of the Upper Cambrian Harmony Formation include concordant analyses in the range of 695-710 Ma (n = 9), 1015-1225 Ma (n = 12), ∼1330 Ma (n = 2), ∼1745 Ma (n = 1), ∼1915 Ma (n = 1), and 2570 Ma (n = 1). The ∼700 Ma and ∼1100 Ma ages (which represent >80 %of the grains) have no known western North American basement source of reasonable proximity and age, suggesting derivation from an unknown western or southern source, or perhaps from rocks that presently underlie the Roberts Mountains allochthon.A growing body of U-Pb isotopic evidence indicates that 1000-1270 Ma and 670-750 Ma zircon ages are relatively common in offshelf continental margin strata. The latter set of ages is also represented by orthogneiss that intrudes or is overlain by continental margin strata, necessitating the widespread exposure of felsic plutonic rocks of these ages along the Cordilleran margin during early Paleozoic time. Subsequently, such rocks may have been overlain by younger continental margin strata, structurally overlain by allochthonous units, or tectonically removed from the region by younger rifting events. The presence of widespread mid- and late Proterozoic felsic magmatic rocks along the Cordilleran margin is not easily accommodated by popular pre-570 Ma plate reconstructions that place Nevada and the central Cordillera opposite the Transantarctic Mountains or eastern Australia. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.