Preliminary geologic map of the Independence Valley NE quadrangle, Elko County, Nevada

Authors: Andrew V. Zuza, Christopher D. Henry, Michael W. Ressel, Charles H. Thorman, Seth Dee, and Jeffrey E. Blackmon
Year: 2018
Series: Open-File Report 2018-04
Format: plate: 39 x 31 inches, color; text: 12 pages, b/w
Scale: 1:24,000
View/download/purchase:

http://pubs.nbmg.unr.edu/Prel-geol-Indep-Valley-NE-p/of2018-04.htm

The Independence Valley NE 7.5-minute quadrangle encompasses the northern Pequop Mountains and adjacent Goshute Valley in eastern Elko County. Active mining in the northeast corner of the quadrangle is focused on newly recognized Carlin-type gold deposits in the east-tilted Pequop Mountains that are hosted in a geographic and geologic setting distinct from similar deposits elsewhere in Nevada. Mapping was conducted in 2017 and 2018.

The northern Pequop Mountains are comprised of east-southeast-dipping Cambrian through Permian sedimentary rocks. Cambrian and Ordovician rocks are metamorphosed and strongly foliated. Although contacts on the geologic map suggest a parallel undeformed stratigraphy, the lower and middle Paleozoic units are variably deformed with local boudinage development, shearing, thrust faulting, and folding. Upper Paleozoic rocks exhibit open folds. This deformation is strongly partitioned to the mechanically weaker horizons, with some beds completely undeformed. Well-developed lineations and asymmetric shear fabrics across the range suggest top-southeast shear. A large thrust fault, named the Independence thrust, cuts across the western and central parts of the map area, juxtaposing lower Paleozoic rocks over younger Paleozoic rocks with an apparent southeast transport direction (present-day orientation). Total offset along this thrust fault is a minimum of two kilometers, based on mapped cutoff relationships. Sparse Jurassic sills and dikes intrude the Paleozoic stratigraphy, including the Independence thrust, which requires this structure to be older.

In the northern map area, the Pequop structural plate consists of Devonian rocks thrust over Pennsylvanian-Permian strata, which are juxtaposed over Ordovician rocks along the enigmatic Pequop fault. This fault has been regarded as a thrust (Thorman, 1970) or a low-angle normal fault (Camilleri, 2010). We interpret that the Pequop plate consists of the structurally highest part of the Independence thrust system—i.e., hanging wall Devonian rocks thrust over footwall Permian strata—that was faulted over Ordovician rocks via the low-angle Pequop normal fault system during an unconstrained phase of post-Jurassic extension. Eastward tilting and exhumation of the entire range was accommodated by late Cenozoic high-angle normal fault activity on the western flank of the range.

In Goshute Valley, lacustrine gravels are deposited in beach bars, and spits recording the high-stand and recessional stages of latest Pleistocene Lake Clover (Munroe and Laabs, 2013). Lacustrine sediments are buttressed against Pleistocene fan deposits (Qfi) along a lake high-stand shoreline at an elevation of approximately 1765 m.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G17AC00212, 2018.

Read about the authors

Mike Ressel, author of two of these newly released maps, was featured in the Geological Society of Nevada “Faces of GSN” in November. You can read his story here:

http://www.gsnv.org/about/faces-of-gsn.php?face=mressel

http://www.gsnv.org/about/faces-of-gsn.php

NBMG staff pages

You can also read about the other geologic mappers and their work on the individual NBMG staff pages:

http://www.nbmg.unr.edu/Staff.html

Preliminary geologic map of the Ravens Nest quadrangle, Elko and Eureka counties, Nevada

Authors: Michael W. Ressel and Seth Dee

Year: 2018

Series: Open-File Report 2018-05

Format: plate: 37 x 30 inches, color, 2 cross sections; text: 20 pages, color

Scale: 1:24,000

View/download/purchase:

http://pubs.nbmg.unr.edu/Prel-geol-Ravens-Nest-p/of2018-05.htm

The Ravens Nest 7.5-minute quadrangle is located in the northern Piñon Range south of the town of Carlin in north-central Nevada. Mapping at Ravens Nest was undertaken to address several components of the complex geology of north-central Nevada, including the nature of sedimentation and deformation associated with the Early Mississippian Antler orogeny, the effects of post-Antler contractional deformation, development of the Eocene Elko Basin and slightly younger Robinson Mountain volcanic field, and deformation associated with Cenozoic extension in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex. Importantly, the northern Piñon Range lies within the southern segment of the Carlin trend gold belt, which is one of the premier gold mining jurisdictions in the world. Gold produced from disseminated, sedimentary rock-hosted deposits, or Carlin-type deposits, in Nevada comprised about 90.5% of Nevada’s production and about 73.6% of U.S. production in 2016 (Muntean et al., 2017). The southern Carlin trend in the vicinity of the Ravens Nest quadrangle has seen several new Carlin-type gold discoveries since 2010 in non-traditional Paleozoic host strata. The Piñon Range and flanking Pine and Huntington valleys are also important areas for conventional and unconventional hydrocarbon resources. The Blackburn and Tomera Ranch oil fields in Pine Valley produce from Cenozoic and Paleozoic rocks widely distributed in the Ravens Nest quadrangle, and the Elko Basin has seen assessment of its oil-bearing shale by the U.S. Bureau of Mines in the 1970s and in 2012-15 for its hydrofracture potential by energy companies.

Mapping shows that three principal Paleozoic structural-stratigraphic domains exist at Ravens Nest. In particular, Late Devonian to Early Mississippian strata, which straddle the leading edge of the Roberts Mountains allochthon, are interpreted to comprise: 1) the deformed base of the allochthon, 2) the incipiently deformed foreland basin, and 3) the least-deformed passive margin. The allochthonous rocks are separated from rocks of the passive margin by a west-dipping thrust or thrusts. In addition, a fourth domain, the Antler overlap sequence, consists of relatively undeformed Late Mississippian through Permian strata widespread at Ravens Nest, which overlies all three other Paleozoic domains. The Early Mississippian Chainman Shale, previously mapped as a component of all three domains, is herein restricted to an allochthonous shale-dominant Lower Mississippian facies that occurs in the western half of the Ravens Nest quadrangle. The Chainman along with underlying deformed units of the Webb and Woodruff Formations were thrust upon a coarse chert- and quartzite-grain sandstone and fine conglomerate formerly assigned to the Chainman Shale by Smith and Ketner (1978) but herein attributed to the Melandco Sandstone. Two key Early Mississippian units, the Webb of allochthon derivation, and the Tripon Pass of passive margin derivation, straddle allochthon, foreland basin, and passive margin, and thus, provide important ties between Antler tectonic domains. Late south-vergent reverse faults cut some Lower Mississippian rocks, indicative of a change from Antler east vergence that may reflect either post-Early Mississippian (i.e., post-Antler) contraction, or local variation of Antler deformation through time and space.

Rocks of the Eocene Elko Basin unconformably overlie Paleozoic rocks at Ravens Nest. The basin developed as primarily lacustrine sedimentation from ~45 Ma to 38 Ma, and only a small part of the lacustrine facies is present in the far northeast corner of the Ravens Nest quadrangle. An Eocene basal fluvial-alluvial gravel is present in many places in and near Ravens Nest quadrangle, which in previous mapping was commonly assigned a Paleozoic age (Hollingsworth et al., 2017). By ~40 Ma at Ravens Nest, significant volcanic input is evident in strata of the Elko Basin, as distally derived small-volume ash-flow tuff and tephra. By ~38.5 Ma, significant local volcanic input began, and lacustrine deposition completely ceased by ~38 Ma, during major deposition of volcanic products of the nearby Robinson Mountain volcanic field (RMVF). Activity associated with the RMVF was short-lived, with abundant rhyolite and dacite lavas and domes with lesser tuff emplaced between ~38.5 and 37.5 Ma. Coeval with calc-alkaline volcanism was the emplacement of the Bullion granodiorite to high-silica rhyolite intrusion in the Railroad district as well as other smaller stocks and swarms of silicic to intermediate sills and dikes. The intrusions of the Railroad district are responsible for polymetallic skarn and carbonate-hosted replacement deposits. In addition, widespread marble and hornfels occurs over 40 km2 in the southern half of the quadrangle, indicative of a much larger underlying pluton. Carlin-type gold deposits are coeval with Eocene intrusions and occur peripherally to them and higher-temperature polymetallic deposits (Henry et al., 2015; Hollingsworth et al., 2017).

The mid-Miocene Humboldt Basin partly overlapped the Elko Basin in the vicinity of the Ravens Nest quadrangle. However, extensive areas of lacustrine shale and siltstone, tuff, and fluvial and alluvial gravels occur over a large area in the southwest part of the quadrangle where lacustrine deposition postdated emplacement of the Palisade Rhyolite (Wallace et al., 2008). Strata of the Humboldt Basin were faulted against Paleozoic bedrock along north striking normal faults on the eastern margin of Pine Valley. Pliocene and younger gravels and extensive tuffaceous lacustrine deposits of the Hay Ranch Formation were deposited synextensionally into a hydrologically closed Pine Valley. Quaternary normal faulting along the western margin of the Piñon Range has continued at least into the middle Pleistocene. The effects of the stream capture of Pine Creek and integration of Pine Valley into the Humboldt River watershed during the middle Pleistocene include deeply incised drainages, oversteepened hillslopes and extensive landslide deposits.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G17AC00212, 2018.

Read about the authors

Mike Ressel, author of two of these newly released maps, was featured in the Geological Society of Nevada “Faces of GSN” in November. You can read his story here:

http://www.gsnv.org/about/faces-of-gsn.php?face=mressel

http://www.gsnv.org/about/faces-of-gsn.php

NBMG staff pages

You can also read about the other geologic mappers and their work on the individual NBMG staff pages:

http://www.nbmg.unr.edu/Staff.html

Preliminary geologic map of the Kelly Creek area, Humboldt, Elko, and Lander counties, Nevada

Author: Joseph A. Laravie

Year: 2018

Series: Open-File Report 2018-06

Version: supersedes Open-File Report 2005-01

Format: 2 plates and text: plate 1: 50.5 x 41 inches, color, with cross section; plate 2: 49 x 27 inches, color; text: 10 pages, color

Scale: 1:24,000

View/download/purchase:

http://pubs.nbmg.unr.edu/Prel-geol-Kelly-Creek-area-p/of2018-06.htm

This 1:24,000-scale geologic map of the Kelly Creek area in Humboldt, Elko, and Lander counties, Nevada contains descriptions of 43 geologic units and one cross section in two plates. Accompanying text includes field observations and inferences, full unit descriptions, and references. This geologic map covers the Kelly Creek (Clover) valley, part of the Dry Hills, the western edge of the Snowstorm Mountains, and the northern part of the Red House Flat area. It updates a previous open-file report (OF2005-01), and now includes the Twin Creeks Mine area.

Plate 1 covers all of these two 1:24,000-scale quadrangles: Dry Hills South and The Knolls, and parts of these quadrangles: Dry Hills North, Kenny Creek (southern portions), Red House Flat East, and Elevenmile Well (far northern portions).

Plate 2 covers most of these two 1:24,000-scale quadrangles: Red House Flat East and Elevenmile Well and the northern third of Knight and Hot Pot quadrangles.

This publication was prepared with support from the Geological Society of Nevada.

New NBMG Publication – Geologic Map of the Humboldt Peak Quadrangle, Elko County

Geologic Map of the Humboldt Peak Quadrangle, Elko County
By Allen J. McGrew, University of Dayton
Year: 2018
Series: Map 186
Format: plate: 43 x 37 inches, color; text: 23 pages, color
Scale: 1:24,000
View/purchase: http://pubs.nbmg.unr.edu/Geol-Humboldt-Peak-quad-p/m186.htm
GIS files: http://pubs.nbmg.unr.edu/CDP-Geol-Humboldt-Peak-quad-p/m186z.htm

Located in central Elko County, Nevada, the Humboldt Peak quadrangle exposes the central and highest part of the East Humboldt Range (EHR). It is flanked to the east by Clover Valley and a northerly trending line of hills. The central East Humboldt Range probably represents the structurally deepest part of the Ruby Mountains–East Humboldt Range metamorphic core complex. The high-grade core of the EHR consists of migmatitic upper amphibolite facies rocks that achieved peak P-T conditions during Late Cretaceous metamorphism up to 8 kb and 775°C. The deepest structural levels, exposed in Lizzies Basin in the west-central part of the quadrangle, form a migmatite complex with >67% leucogranitic rock. In the northwest part of the quadrangle, this migmatite complex is overlain in the cirque wall above Winchell Lake by a southward-closing, kilometer-scale recumbent fold known as the Winchell Lake fold-nappe (WLN). The WLN folds a sequence of intensely metamorphosed, migmatized and profoundly attenuated Neoproterozoic to Mississippian metasedimentary rocks. Farther north in the adjacent Welcome quadrangle the WLN is cored by Nevada’s oldest rocks—Neoarchean to early Paleoproterozoic orthogneiss and paragneiss that were thrust over the Neoproterozoic to Mississippian metasedimentary sequence before peak metamorphism and WLN-related folding. Overprinting this assemblage at higher structural levels is a WNW-directed, kilometer-scale mylonitic shear zone that diachronously exhumed this high-grade terrain during extensional tectonism bracketed between late Eocene and Miocene. Together, the mylonitic shear zone and the detachment fault that forms its roof probably accommodated much more than 15 km of extensional displacement. Cutting through the metamorphic core along the steep, eastern face of the East Humboldt Range is a younger, post-Miocene normal fault that remained active into the Quaternary. In the northern part of the quadrangle this normal fault uplifts and juxtaposes the high-grade core against moderately east-dipping Middle Miocene volcanic and hypabyssal intrusive rocks (rhyolitic quartz porphyry). A single exposure of flat-lying vitric tuff, also of Miocene age, lies to the east of the east-dipping rhyolite-bearing sequence, but it is unclear at present whether the flat-lying strata are faulted down against the rhyolitic rocks or overlie them in angular unconformity. The line of hills in the northeastern part of the quadrangle consists of Paleozoic sedimentary rock that probably represents the down-faulted “cover” of the metamorphic core complex. Finally, the late Quaternary basin fill to the east of this line of hills was faulted down against the Paleozoic sedimentary rocks along the Clover Valley fault, which strikes northward through the adjacent Welcome quadrangle toward the town of Wells where it may correlate with the fault that produced the Mw 6.0 Wells earthquake in 2008.

NMEC 2018 Great Basin Rendezvous—September 13–16

The 6th Annual Great Basin Rendezvous registration is now available!

“It is that time of year again to begin sign-up for Nevada Mineral Exploration Coalition’s 2018 Great Basin Rendezvous. This year, we have added an extra day to the front, so the GBR will be held from Thursday, September 13 at 1:00 p.m. through Sunday, September 16, 2018, at Camp Lamoille in the Ruby Mountains. The cost for an NMEC member is $20.00 and for a non-member is $100.00 (includes a 1 year NMEC membership – good through 2019). Additional family members are $10.00.

For those of you who have never been to Camp Lamoille in the Ruby Mountains, it is situated in a beautiful glacial valley located approximately 25 miles southeast of Elko, Nevada.”

On Saturday, September 15 there will be a “9:00 AM field trip led by Larry Garside, Research Geologist Emeritus, Nevada Bureau of Mines and Geology. This will be a driving field trip focusing on geology of the Ruby Mountains with added historical discussions from the town of Lamoille, up and over Secret Pass, south along the east flank of the Rubies, up and over Harrison pass…” (from NMEC website)

To sign up, please go to the NVMEC website:
https://www.nvmec.org/gbrregistration

New Open-File Report—in Elko County

Subsurface Trenching Investigation in Support of the Barrick Goldstrike N. Carlin Trend Structure Project
by Richard D. Koehler and Colin M. Chupik
Year: 2018
Series: Open-File Report 18-2
Format: text: 18 pages, color; 12 plates: 11 x 17 inches, color
Scale: 1:60
View/download/purchase

Surface geomorphic analyses and subsurface geologic trenching was conducted by the Nevada Bureau of Mines and Geology at a site in the Tuscarora Mountains in Elko County, Nevada within the northern Carlin Trend. The purpose of the investigation was to assess the presence or absence of active faults, refine previous bedrock mapping, and document the locations of stratigraphic contacts and the character of faults and other discontinuities. Surficial evidence of Quaternary faulting was not observed in the project area; however, the Southeast Sheep Creek Range fault and the Tuscarora fault zone project towards the site. Two east-west oriented trenches were excavated for a total exposure of ~3,000 feet (~915 meters). Bedrock exposed in the trenches included the Devonian Slaven Formation and the Tertiary Carlin Formation. Numerous small displacement faults, joints, and other discontinuities associated with carbonate seams and secondary mineralization were documented. A well-developed textural B soil horizon is developed across the entire exposure indicating that tectonic deformation predates the late Pleistocene. Together, the results provide information applicable to refining bedrock maps and improving regional and local structural models.

New Geologic Maps in Northern Nevada: Mount Rose NW and Herder Creek Quadrangles

 

mrnw

Preliminary Geologic Map of the South Half of the Mount Rose NW Quadrangle, Washoe County, Nevada

 Authors: Nicholas H. Hinz and Alan R. Ramelli
Year: 2016
Series: Open-File Report 16-6
Format: plate: 35 x 29 inches, color; text: 3 pages, b/w
Scale: 1:24,000
View/Download/Buy: http://pubs.nbmg.unr.edu/Geol-south-half-Mount-Rose-NW-p/of2016-06.htm

This quadrangle straddles the north end of the Carson Range directly west-southwest of Reno and abuts the Nevada-California border. The Truckee River and Interstate 80 transect the northwest quarter of this quadrangle. This quadrangle also encompasses part of the rural community along Thomas Creek in the southeast quarter, and segments of the Steamboat irrigation ditch and part of the City of Reno urban area fall within the northeast corner.

The bedrock exposures in the quadrangle consist of Mesozoic granitic basement and Tertiary volcanic and sedimentary rocks. The Tertiary section includes a complex section of lavas, intrusions, and volcanic sedimentary rocks. Many of these volcanic and sedimentary rocks were derived from a ~6-7 Ma ancestral Cascades volcanic center in the Mount Rose quadrangle, directly south of this quadrangle. Plio-Pleistocene basaltic andesite lavas and rhyolite domes locally rest on the late Miocene volcanic rocks in the middle part of the quadrangle. Principal surficial deposits include late Pliocene to modern alluvial fan and fluvial deposits, Quaternary glacial deposits, and late Quaternary mass wasting deposits. Notable deep-seated landslide complexes reside in all major drainages—including Thomas Creek, Hunter Creek, Bronco Creek, and the smaller catchments along the west edge of the quadrangle. Most of the Carson Range is west-tilted with west-dipping Cenozoic strata. However, within the Mount Rose NW quadrangle, the dip domain flips and most all the Cenozoic strata dips east with numerous west-dipping normal faults. These west-dipping normal faults are cut by younger east-dipping normal faults of the Mount Rose fault zone on the east side of the range.  East-facing Quaternary fault scarps were observed on the east side of the range and west-facing Quaternary fault scarps were observed on the west side of the range.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G15AC00240, 2016.

 herdercreek

Preliminary Geologic Map of the Herder Creek Quadrangle, Elko County, Nevada
Author: Seth Dee and Michael W. Ressel
Year: 2016
Series: Open-File Report 16-5
Format: plate: 33 x 29 inches, color; text: 5 pages, b/w
Scale: 1:24,000
View/Download/Buy: http://pubs.nbmg.unr.edu/Prel-geol-Herder-Creek-quad-p/of2016-05.htm

The map area covers part of Starr Valley, the upper reaches of the Humboldt River, and the northwest part of the East Humboldt Range.

The Ruby Mountains–East Humboldt Range metamorphic core complex is exposed in the high-relief range front in the southeast part of the quadrangle. In this area, the core complex is comprised of intensely metamorphosed and highly attenuated Neoarchean through Mississippian(?) strata, thought to be part of the platform facies of the Proterozoic through Paleozoic passive margin. Contractional structures exposed in the map area are complex and difficult to discern due to overprinted extensional deformation but are likely part of the Winchell Lake nappe (WLN), a kilometer scale, southward-closing recumbent fold-nappe mapped in adjacent quadrangles to the east. Overturned Devonian to Neoproterozoic(?) meta-sedimentary strata exposed at the highest structural levels are interpreted to be in thrust contact with an underlying, upright sequence of Cambrian to Neoproterozoic(?) paragneiss and Paleoproterozoic to Neoarchean(?) orthogneiss in the core of the fold. This structural interpretation matches those from the adjacent Welcome quadrangle (McGrew and Snoke, 2015; NBMG Map 184). Rocks in the upper part of the metamorphic core complex are pervasively overprinted by a WNW-directed mylonitic shear fabric, which records middle to late Cenozoic extensional exhumation from mid-crustal depths. Abundant sills and lenses of less deformed Oligocene to Cretaceous garnet-muscovite leucogranite and biotite monzogranite intrude all metamorphic rocks in the quadrangle.

The west side of the East Humboldt Range is bound by the active, W-dipping Ruby Mountains frontal fault zone, which extends for more than 60 km to the southwest. A west step-over in the Ruby Mountains fault south of the Herder Creek drainage results in a broad, hanging wall uplift underlain by middle-Miocene to Pliocene strata comprised of NE-dipping to flat-lying tuffaceous sandstone, shale, and conglomerate of the Humboldt Formation and younger units. A tephra in the uppermost exposed section yielded a 40Ar/39Ar age on feldspar of 5.15 ± 1.82 Ma.

Repeated late Quaternary surface-rupturing earthquakes along active traces of the frontal fault are recorded by increased uplift and dissection of Quaternary surfaces as a function of relative age. Fault scarps in Holocene deposits have up to 2.5 m of vertical separation while glacial outwash deposits from the two most recent Pleistocene glacial advances have scarp heights ranging from 6 to 32 m. The upper reaches of several drainages have well-preserved glacial moraine deposits that record the Angel Lake and Lamoille glacial advances. Adjacent to the Humboldt River, in the northwest corner of the quadrangle, 3 sets of abandoned terrace surfaces are preserved, including a broad surface comprised of gravel-rich alluvium that was likely deposited during a period of increased discharge during the latest Pleistocene.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G15AC00240, 2016.