Battle Mountain Research Articles

Indigenous art as decolonising truth-telling: Battle Mountain Memorial

Indigenous art as decolonising truth-telling: <i>Battle Mountain Memorial</i>

Hyperspectral mapping of magmatic-hydrothermal sericite, Battle Mountain mining district, Nevada

We demonstrate the utility of multi-scale imaging spectroscopy data (hyperspectral data) to differentiate magmatic-hydrothermal sericite from other white micas based on grain size, in a case study of the Battle Mountain mining district in Humboldt and Lander Counties, Nevada. The Battle Mountain mining district contains porphyry, skarn, and distal disseminated deposits that formed in magmatic-hydrothermal systems, as well as sedimentary-rock hosted gold deposits of less certain origin. We generated spectral-based mineral maps of the Battle Mountain mining district to examine the spatial distribution of spectrally dominant minerals such as kaolinite, montmorillonite, and white micas (muscovite, sericite, and illite). We then used spectral-based mineral mapping to characterize the grain size and composition of white micas, enabling us to differentiate igneous muscovite from magmatic-hydrothermal sericite. By integrating our spectral-based maps with geologic mapping, airborne aeromagnetic data, and prior studies on mines and deposits in Battle Mountain, we determined that finer grained, higher Al sericite spatially correlated with the footprints of known magmatic-hydrothermal systems and is interpreted as forming from magmatic-hydrothermal alteration. Larger grained, lower Al muscovites are distal from known magmatic-hydrothermal systems and are interpreted as being igneous derived siliciclastic sediments that became constituents of the host sedimentary rocks. Techniques described in this contribution can be applied to any appropriate hyperspectral data, regardless of sensor type used to collect the data.

New constraints on fluid chemistry and elemental mobility during hydrothermal alteration from thallium isotope systematics of the battle mountain district, North-central Nevada

The Battle Mountain area is a major porphyry style Au (–Cu) district in northern Nevada, forming one end of the Battle Mountain-Eureka mineral trend. This trend, along with several others in the north-central Nevada region, hosts some of the world's most prolific gold mineralization, which is oftentimes associated with a suite of trace elements including arsenic, antimony, tungsten, and thallium and can be related to or overprinted by hydrothermal alteration. However, determining the extent of control hydrothermal fluids exert on some of the deposits in the region has been difficult to ascertain. One of the associated trace metals in the region, thallium (Tl), is a highly incompatible element and, as such, dominantly resides in the continental crust, where it can be easily remobilized during hydrothermal alteration. While previous work has demonstrated the effects of hydrothermal alteration on Tl distribution and fractionation, the controls responsible for the observed Tl fractionation during hydrothermal alteration are still poorly characterized but may provide insight into fluid behavior. Here, we present new Tl isotope composition and concentration data for a suite of 54 mineral separates obtained from 43 samples from the Battle Mountain area, which range from unaltered intrusive igneous rocks through varying types and degrees of hydrothermally altered rocks.Measured Tl concentrations vary by more than an order of magnitude, from below detection limit (0.2 ppm in this study) to 2.0 ppm, while ε205Tl ranges between −5.0 and +2.2 (ε205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). Thallium concentrations correlate positively with whole-rock potassium (K), thus show strong increases during K alteration, and demonstrate significantly lower ε205Tl values within K-altered samples. Conversely, during later, overprinting Na–Ca alteration, both K and Tl are removed, resulting in a noted decrease in Tl concentrations coupled with a shift to significantly higher ε205Tl values. It appears that during hydrothermal alteration, 203Tl is more easily (re)mobilized and (re)distributed, which reflects: 1) a first-order hydrothermal alteration control that relates to the transport of Tl during the formation of a new, metasomatic mineral assemblage (particularly the breakdown and/or formation of K-bearing minerals) and 2) a second-order mineralogical control, relating to inter-mineral equilibrium, which also results in a small fractionation effect.

The Deadliest Shortcut by Lee Bergthold

Reviewed by: The Deadliest Shortcut by Lee Bergthold Thomas J. Lyon, emeritus Lee Bergthold, The Deadliest Shortcut. Palmdale, CA: Center for Wilderness Studies, 2021. 453 pp. Paper, $21.95. It seems likely that Lee Bergthold is not a familiar figure to readers of Western American Literature, but he has written four books describing immense stretches of the western landscape, from 282 feet below sea level at Badwater to the summit of Mount Whitney; from Battle Mountain, Nevada, to Death Valley, California; from Death Valley southward to the Salton Sea; and finally, in the present book, from the Utah–Nevada border back to Battle Mountain. All of this is done on foot, with a large backpack, alongside a few select companions male and female, similarly equipped. The motivation for some of these treks was to point out and hike alternative routes for the ill-fated Donner party of 1846–47, who themselves took the “deadliest shortcut” infamously sponsored by Lansford Hastings. As is well known, the Donner party thus came late in the year to the eastern flank of the Sierra Nevadas, attempted to cross that range, and became hopelessly stuck by several big snowstorms. Bergthold’s four hikes were potentially life-threatening. Chief among the many dangers was the lack of good water sources. Bergthold arranged matters with friends who had four-wheel-drive trucks, and who knew, generally, where to meet the hikers and supply them with fresh water. In the enormous valleys of Nevada, it was often difficult for Bergthold to locate, say, a couple of plastic one-gallon bottles. The hikes were from two weeks to thirty-five days long. The author meets the difficulties with intelligence but doesn’t present himself as a specimen of heroism. A veteran of the Korean War, he is today somewhere in his eighties and seems well acquainted with his own lacks. He is a leader, however, and his companions gravitate to his integrity. I believe that upon finishing The Deadliest Shortcut, you will seek out the other three books: There Must Have Been an Angel, The Night of the Triangle, and To Walk Away from Battle Mountain. [End Page 437] Thomas J. Lyon, emeritus Utah State University Copyright © 2023 Western Literature Association ...

Estimates of pit-lake evaporation and its potential effects on groundwater interactions with the Humboldt River

The Humboldt River, in northern Nevada supplies water mostly for irrigation of nearby croplands. Groundwater pumping both near and distant from the river has increased over the past 50 years. The primary use of groundwater is for irrigation of croplands but it is also an important source for municipalities, mining and other industries. Large gold deposits that require dewatering were discovered in the mountains of the central part of the drainage basin in the 1970’s and 1980’s. Much of the water pumped to dewater the mines is recharged back to groundwater in the valleys or is released to the Humboldt River or one of its tributaries. Of concern is the long-term loss of groundwater from pit-lake evaporation and how those losses could affect groundwater interactions with the Humboldt River once mine operations cease. Flows in the Humboldt River are mostly dependent on snowmelt runoff from the mountains, particularly from mountains in the drainage area upstream of the Palisade gaging station. The mean flow increases with drainage area between the gaging stations upstream of Elko and at Palisade, whereas the mean flow decreases downstream of the Palisade gaging station. Much of the decrease in mean flow downstream of Palisade is caused by: (1) minimal additional contributions from runoff and groundwater flow; (2) spreading of water to native pastures on the floodplain; and (3) infiltration of river water into its associated alluvium. During low-flow periods, particularly during periods of drought, the Humboldt River between Battle Mountain and Comus gaging stations often has flows less than 1 cubic foot per second (cfs) even before there was any mine dewatering. Estimated groundwater loss to evaporation for the five largest pit lakes in the drainage area between the Palisade and Comus gaging stations is about 5,400 acre-feet per year (afy) (7.6 cfs or five times less than the estimated net annual evaporation loss from the Rye Patch and the Pitt-Taylor Reservoirs upstream of Lovelock, Nevada. The groundwater loss to three of the pit lakes (Goldstrike Mine, Cortez operations, and Twin Creeks Mine) is unlikely to affect river flow because even prior to mine dewatering, groundwater flow from the low-lying mountains was lost to evapotranspiration near the base of the alluvial fans and did not contribute flow to the river. The Lone Tree Mine ceased dewatering in December 2006 and since then a lake has formed in the pit. The floodplain of the Humboldt River in the reach between Battle Mountain and Comus is underlain by a layer of blue clay at shallow depth. Between 2007 and 2019, the net mean annual streamflow loss between Palisade and Comus is nearly the same as during a period of little groundwater pumping from 1946 to 1969 suggesting that even the relatively large amount of water pumped from the mine has had a little effect on Humboldt River flows. Groundwater evaporation from the eventual pit-lake at the Gold Quarry Mine is estimated at 740 afy (about 1 cfs) and could perhaps decrease the gain in the Humboldt River upstream of Palisade. How much of a decrease is uncertain because some of it could be lost to evapotranspiration at the edge of the floodplain prior to reaching the river. Even if the groundwater loss from the pit lake at Gold Quarry Mine were to reduce the gain of river flow between Carlin and Palisade, the loss would be too little to measure accurately.

Fe-oxides in jasperoids from two gold districts in Nevada: Characterization, geochemistry, and (U-Th)/He dating

This study examines the whole-rock geochemistry, Fe-oxide texture and crystal morphology, and Fe-oxide (U-Th)/He date variation of jasperoids, as well as the relationships between these characteristics, in order to evaluate jasperoids as an indicator for gold mineralization. Jasperoid samples with a range of textures, appearances, and expected gold concentrations were collected from two gold districts in Nevada: the northern Carlin trend (Gold Quarry Mine), and the Battle Mountain district (Marigold Mine; Battle Mountain prospect). In our jasperoid samples, whole-rock concentrations of Au are positively correlated with As, Ag, Pb, Sb, U, W, and Tl. The following Fe-oxide textures are associated with anomalous gold concentrations: disseminated, submicron crystals; feathery and acicular crystal morphologies; concentric zonation from hematite to goethite; and botryoidal textures. Calculated closure temperatures for the (U-Th)/He system in observed jasperoid Fe-oxide morphologies within our dataset range from 26 to 136 °C, indicating variable susceptibility to He diffusion at near-surface conditions. Fe-oxide (U-Th)/He dates range from 29.4 ± 0.5 Ma to 0.11 ± 0.01 Ma, displaying intra- and intersample variation. We attribute the date variation to multiple factors including He implantation, parent isotope exchange, and He loss through closure temperature sensitivity and alpha-ejection. The intra- and intersample variation within the Fe-oxide (U-Th)/He dates limits our ability to make geologic interpretations. Our case study identifies important limitations of the (U-Th)/He method in jasperoids and provides insight on how He date variation might be avoided in future studies.

Magmatic-Hydrothermal Gold Mineralization at the Lone Tree Mine, Battle Mountain District, Nevada

Abstract The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry. The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma. Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

Trace elements in fluid inclusions of sediment-hosted gold deposits indicate a magmatic-hydrothermal origin of the Carlin ore trend

Abstract The Carlin-type deposits in Nevada (western USA) constitute the world’s second-largest gold ore province. These structurally and stratigraphically controlled, sediment-hosted ore bodies are characterized by carbonate dissolution attending hydrothermal precipitation of gold-rich arsenian pyrite. The origin of the mineralizing fluids and the source of the gold remain debated. Conceptual models, favoring either sedimentary, metamorphic, or magmatic fluid sources, are based on isotopic tracers, giving ambiguous results. Here we use the trace element compositions of fluid inclusions to separate geochemical signals of the large-scale fluid source from effects of deposit-scale fluid interaction with the sedimentary host rocks. Specifically, we compare the ratios of Rb, K, B, As, Sr, and Ba between clearly magmatic-hydrothermal Cu-Au ores at Copper Canyon in the Battle Mountain–Eureka trend with the Gold Quarry and Chukar Footwall deposits on the Carlin trend that contain high-grade gold in similar sedimentary host rocks. Results indicate that both ore districts can be related to upper crustal hydrous magmatic intrusions, but are now exposed at different levels of erosion and formed at different distances from their magmatic fluid source. Fluid compositions are best explained by separation of a deep magmatic fluid into Rb-K–enriched brine and B-As-Au–enriched vapor, followed by cooling and contraction of the magmatic vapor phase to an epithermal liquid, which reacted with Sr-Ba–bearing sedimentary rocks during ascent and eventual precipitation of Au-rich arsenian pyrite.

A Gallium Anomaly Utilized in Palaeogeographic Reconstruction of Battle Mountain, Rappahannock County, Virginia

This paper explores the geostatistical relationship between volcanic and plutonic rock on Battle Mountain, one of the northernmost units of the Robertson River Igneous Suite in central Virginia (U.S.A.), in an effort to reconstruct the original appearance of the formation after rift volcanism at 704 Ma. This Neoproterozoic anorogenic formation dates from the initial rifting of the supercontinent Rodinia and may be one of the oldest intact visible volcanic formations in Virginia. This paper explores the use of geostatistical trace element analysis in geochemical data related to A-type granitoid rock, as well as mathematical and feed-forward neural network palaeogeographic reconstructions including the use of Markov chains. The paper concludes with evidence and analysis that indicates gallium may be used as a reliable indicator trace element for volcanic A-type rhyolitic rock. Furthermore, the paper indicates a lateral rhyolitic stratovolcano explosive event at approximately 704 Ma with a corresponding rhyo...

Erratum

The article by J. W. Rector, et al., “Tomographic imaging of surface waves: A case study from the Phoenix Mine, Battle Mountain, Nevada” (November 2015 TLE) incorrectly listed the authors.

Tomographic imaging of surface waves: A case study from the Phoenix Mine, Battle Mountain, Nevada

Conventional surface-wave analyses for estimating shallow-seismic properties typically use 2D geometries. A new geometry uses tomographic imaging of surface waves to reconstruct surface-wave velocity and attenuation profiles in three dimensions. Sources and receivers are deployed around a border encompassing an area on the earth's surface. The surface wave is assumed to propagate along a simple ray between source and receiver that attenuates with depth as a function of frequency. Using conventional raypath-tomography techniques, surface-wave velocity and attenuation tomograms can be generated as a function of frequency and then used to estimate shear-wave velocity and attenuation versus depth using standard surface-wave inversion techniques. The methodology is described in a field study designed to image old mine workings. Velocity and attenuation tomograms exhibit anomalous areas that correspond to known voids.

The Meteoritical Bulletin, No. 101

AbstractMeteoritical Bulletin 101 contains 2639 meteorites accepted by the Nomenclature Committee in 2012, including 1 fall (Battle Mountain), with 2308 ordinary chondrites, 156 carbonaceous chondrites, 63 HED achondrites, 17 relict meteorites, 16 Rumuruti chondrites, 15 enstatite chondrites, 15 ureilites, 10 iron meteorites, 9 lunar meteorites, 9 primitive achondrites, 8 ungrouped achondrites, 7 mesosiderites, 4 Martian meteorites, and 2 Pallasites, and with 1812 from Antarctica, 437 from Asia, 301 from Africa, 43 from South America, 21 from Europe (including Russia), 21 from North America, 3 from Oceania, and 1 from unknown. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available on line at http://www.lpi.usra.edu/meteor/.

The Pliocene–Quaternary Buffalo Valley volcanic field, Nevada: Post-extension, intraplate magmatism in the north-central Great Basin, USA

The Buffalo Valley volcanic field consists of Pliocene through Quaternary lava flows and spatter cones located south of Battle Mountain and adjacent to the Fish Creek Mountains, north-central Nevada. The volcanic rocks are split into two groups by age and geochemistry. The Pliocene sequence (4.02 to 2.75Ma) consists of olivine- and plagioclase-bearing alkali basaltic lava flows with minor pyroclastic deposits, found primarily along the south flank of Battle Mountain and also at the north end of the Fish Creek Mountains and within the Fish Creek Mountains caldera. The Quaternary series (1.99 to 1.14Ma) includes nearly a dozen trachybasaltic spatter cones with short lava flows erupted along the northwest flank of the Fish Creek Mountains. Normalized rare earth element and incompatible element plots for both groups are light rare earth and Nb–Ta enriched, resembling alkali basalts from ocean islands, but the Quaternary lavas are more light rare earth element-enriched and cross the Pliocene basalt patterns at Eu. Radiogenic and stable isotope ratios are consistent with an asthenospheric mantle source, and the rare earth element patterns indicate a shift from melting in the spinel to garnet peridotite field with time. Basaltic rocks from other intraplate fields in the Great Basin, including the Lunar Crater and Cima fields, only include lavas that originated at depth in the garnet peridotite field. Buffalo Valley is located at the margin of a proposed lithospheric drip (delamination) and within a zone of lithospheric thinning that extends across northern Nevada, both of which may control where melting in the asthenosphere may occur. The proximity to the edge of Precambrian–Phanerozoic lithosphere boundary may also be a factor in melt generation.

Distinct mantle sources for Pliocene–Quaternary volcanism beneath the modern Sierra Nevada and adjacent Great Basin, northern California and western Nevada, USA

Latest Pliocene to Quaternary, mildly alkaline, mafic to intermediate volcanic activity extends in a swath from the Lake Tahoe region in the eastern Sierra Nevada across the western Great Basin to the Battle Mountain area, Nevada. From west to east, the volcanic centers exhibit a dramatic gradient in chemical and isotopic composition. Centers situated in or adjacent to the Sierra Nevada have incompatible element and isotopic compositions consistent with an old, subduction-modified lithospheric mantle source (87Sr/86Sr > 0.7045; 143Nd/144Nd < 0.5127; δ18O > +6.5‰). Mafic volcanic centers east of the Sierra Nevada, in the Carson Sink and in the Buffalo Valley region, have an intraplate incompatible element and isotopic signature (87Sr/86Sr < 0.7045; 143Nd/144Nd > 0.5127; δ18O < +6.5‰) consistent with an asthenospheric mantle source. Earlier 20–3 Ma arc volcanism in the Sierra Nevada also tapped the old lithospheric mantle source along with the mantle wedge, indicating that the lithospheric mantle source existed well prior to the onset of Tertiary arc volcanism and probably prior to Mesozoic igneous activity of the Sierra Nevada. Thus, the lithospheric mantle beneath the Sierra Nevada has remained a geochemically consistent, fertile, fusible source for at least the past 20 m.y. Old lithospheric mantle likely still exists east of the Sierra Nevada, but lithospheric thinning and/or exhaustion of fusible components inhibit its melting, such that during the Quaternary, melting could only occur in the asthenosphere.

Rapid middle Miocene collapse of the Mesozoic orogenic plateau in north-central Nevada

The modern Sierra Nevada and Great Basin were likely the site of a high-elevation orogenic plateau well into Cenozoic time, supported by crust thickened during Mesozoic shortening. Although crustal thickening at this scale can lead to extension, the relationship between Mesozoic shortening and subsequent formation of the Basin and Range is difficult to unravel because it is unclear which of the many documented or interpreted extensional episodes was the most significant for net widening and crustal thinning. To address this problem, we integrate geologic and geochronologic data that bear on the timing and magnitude of Cenozoic extension along an ∼200 km east–west transect south of Winnemucca, Battle Mountain, and Elko, Nevada. Pre-Cenozoic rocks in this region record east–west Palaeozoic and Mesozoic compression that continued into the Cretaceous. Little to no tectonism and no deposition followed until intense magmatism began in the Eocene. Eocene and Oligocene ash-flow tuffs flowed as much as 200 km down palaeovalleys cut as deeply as 1.5 km into underlying Palaeozoic and Mesozoic rocks in a low-relief landscape. Eocene sedimentation was otherwise limited to shallow lacustrine basins in the Elko area; extensive, thick clastic deposits are absent. Minor surface extension related to magmatism locally accompanied intense Eocene magmatism, but external drainage and little or no surface deformation apparently persisted regionally until about 16–17 Ma. Major upper crustal extension began across the region ca. 16–17 Ma, as determined by cross-cutting relationships, low-temperature thermochronology, and widespread deposition of clastic basin fill. Middle Miocene extension was partitioned into high-strain (50–100%) domains separated by largely unextended crustal blocks, and ended by 10–12 Ma. Bimodal volcanic rocks that erupted during middle Miocene extension are present across most of the study area, but are volumetrically minor outside the northern Nevada rift. The modern physiographic basins and ranges formed during a distinctly different episode of extension that began after about 10 Ma and has continued to the present. Late Miocene and younger faulting is characterized by widely spaced, high-angle normal faults that cut both older extended and unextended domains. Major widening of the Basin and Range at this latitude thus took place during a relatively brief interval in the middle Miocene, and the lack of major shortening west of the Sierra Nevada at this time suggests that the change in the plate margin from microplate subduction to lengthy transtensional strike-slip played an important role in allowing extension to occur when it did, as rapidly as it did. The onset of extension ca. 16–17 Ma was coeval with both Columbia River flood-basalt volcanism and the hypothesized final delamination of the shallow Farallon slab that lay beneath the western USA in the early Tertiary. However, it is unclear if these events were necessary prerequisites for extension, simply coincidental, or themselves consequences of rapid extension and/or reorganization of the plate boundary.

A prominent geophysical feature along the northern Nevada rift and its geologic implications, north-central Nevada

We consider the origin and character of a prominent large-scale geophysical feature in north-central Nevada that is coincident with the western margin of the northern Nevada rift—a mid-Miocene rift that includes mafi c dike swarms and associated volcanic rocks expressed by a NNW-striking magnetic anomaly. The geophysical feature also correlates with mid-Miocene epithermal gold deposits and is coincident with the central part of the Battle Mountain–Eureka mineral trend. The Reese River Valley, a 2-km-deep Cenozoic basin, is located along the western margin of this feature and is inferred from the inversion of gravity data to be infl uenced by, and perhaps in part structurally controlled by, the geophysical feature. Geophysical modeling indicates that the source of the geophysical anomaly must extend to mid-crustal depths, perhaps refl ecting a transition from Paleozoic crust in the southwest to Precambrian crust in the northeast, the presence of felsic intrusive rocks in the middle crust, or the edge of mid- to subcrustal mafi c intrusions related to late Tertiary magmatic underplating associated with hotspot magmatism. These cases offer very different possibilities for the age, depth, and origin of the source of the geophysical anomaly, and they present distinct implications for crustal evolution in the northern Great Basin. For example, if the anomaly is due to a pre-Cenozoic basement structure, then its coincidence with the midMiocene northern Nevada rift suggests that the trend of the rift was guided by the preexisting crustal structure. On the other hand, if the anomaly is related to Tertiary mafi c intrusions, then the western limit of this magmatism may have been infl uenced by hotspot fracturing of the crust.

Anatomy of the Early Cambrian ‘La Sentinella’ reef complex, Serra Scoris, SW Sardinia, Italy

Abstract All bioherms from the Early Cambrian (Botoman) Matoppa Formation of the Nebida Group in SW Sardinia were previously thought to be dominated by Epiphyton . However, at ‘La Sentinella’ (Serra Scoris Hill), they are composed of Girvanella, Razumovskia, Botomaella and Renalcis , with Epiphyton and archaeocyaths as accessory components. This association forms two unusual types of crust boundstone, consisting of stacked flat or curved crusts and saucer-like archaeocyaths delimiting shelter cavities. Dendrolitic Renalcis archaeocyath-cement boundstone caps the bioherm. Analysis of the La Sentinella reef complex and comparison with similar constructions from Mongolia (Zuune Arts, Salaany Gol), Nevada (Stewart's Mill, Battle Mountain), Mexico (Sonora) and China (Tianheban Formation) suggest that episodic deposition of fine-grained siliciclastic or carbonate sediment followed by periods of non-deposition enabled the calcimicrobial rafts and crusts to colonize the substrate and then provide synoptic relief for the development of a dendrolitic Renalcis -cement framework. ‘La Sentinella’ is one of the rare examples of Cambrian reef complex displaying community replacement, from an initial stage of thrombolitic and/or flat-stacked microbial crusts on a muddy substrate to an arched microbial crust system, to a more resistant Renalcis -cement boundstone. Such bioherms reflect an open-shelf, shallow-marine environment of increasing energy.

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.

Can Shade Structures Help Riparian Areas?: A look at using constructed shades to pull cattle off riparian areas in northeastern Nevada

T he damage caused by unmanaged cattle grazing on riparian habitats is well documented.1 At the same time, ample evidence exists that well-managed cattle grazing is compatible with maintaining, and even improving, riparian habitats.2 The key to creating stream-friendly grazing systems is to reduce the amount of time cattle spend in these sensitive riparian habitats.3 We faced this challenge when we began our study on Antelope Creek during the spring of 1996. Antelope Creek is located on private land approximately 40 miles north and east of Battle Mountain, Nevada. The active stream channel had dropped to approximately 10–15 feet below the surrounding valley bottom, and bank erosion still occurs during high spring runoff events. Although many positive improvements had already been begun, we were interested in methods to foster continued improvements along this stream. Antelope Creek has already formed a small floodplain, and herbaceous, riparian plant communities are reestablishing along most of the banks. Intermittent willow clumps are also growing along the length of the stream. Established methods to exclude cattle from riparian areas along sensitive stream banks include fencing and the development of off-site water and mineral sources. Some researchers have even tried genetic selection of cattle herds and negative conditioning with electrical shock.4,5 In the spring of 1996, we began a project to determine whether constructed shade structures could reduce the amount of time cattle spend in the Antelope Creek riparian areas and, in turn, the impacts on riparian plant communities from grazing. The shades were intended to provide a more desirable location for cattle to loaf and ruminate than the adjacent riparian habitats that had little or no shade. Previous research that showed benefits in cattle production were primarily confined to feedlots and dairies,6,7 and studies that concerned themselves with shade in range and pastures systems had conflicting results.8–11 Our main goals were to determine whether 1) artificially constructed, shaded areas would be used preferentially by significant numbers of cattle in place of adjacent riparian bottoms; and 2) riparian-vegetation use classes, as estimated by stubble height and currentyear shrub use, were lower adjacent to the shade structures when compared with the nonshaded areas, and if so, how far out from the structures. Our project was conducted for 3 years, and changes in water quality (chemical analysis), riparian vegetation, and the size and shape of the stream bank were also investigated. We will not report these results here. We are focusing on the effects of constructed shade areas on Antelope Creek because of the recent interest and recommendations for use of shade to control cattle movements.

Seed weight variation of wyoming sagebrush in Northern Nevada

Seed size is a crucial plant trait that may potentially affect not only immediate seedling success but also the subsequent generation. We examined variation in seed weight of Wyoming sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young), an excellent candidate species for rangeland restoration. The working hypothesis was that a major fraction of spatial and temporal variability in seed size (weight) of Wyoming sagebrush could be explained by variations in mean monthly temperatures and precipitation. Seed collection was conducted at Battle Mountain and Eden Valley sites in northern Nevada, USA, during November of 2002 and 2003. Frequency distributions of seed weight varied from leptokurtic to platykurtic, and from symmetry to skewness to the right for both sites and years. Mean seed weight varied by a factor of 1.4 between locations and years. Mean seed weight was greater (P < 0.05) in 2003 than in 2002 at both sites. This can partially be attributed to 55% greater precipitation in 2003 than 2002, since mean monthly temperatures were similar (P > 0.05) in all study situations. Simple linear regression showed that monthly precipitation (March to November) explained 85% of the total variation in mean seed weight (P = 0.079). Since the relationship between mean monthly temperature (June-November) and mean seed weight was not significant (r2 = 0.00, P = 0.431), this emphasizes the importance of precipitation as an important determinant of mean seed weight. Our results suggest that the precipitation regime to which the mother plant is exposed can have a significant effect on sizes of seeds produced. Hence, seasonal changes in water availability would tend to alter size distributions of produced offspring.