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At least 361 records · Page 20Linked to original sources
USGS1975

Application of ERTS images and image processing to regional geologic problems and geologic mapping in northern Arizona

The purpose of this study was to apply the techniques of computer image processing to ERTS images as an aid to the solution of some regional geologic problems of significant interest. ERTS-1 images were applied to studies in the Shivwits Plateau, Coconino Plateau, and north-central Arizona regions. Unprocessed ERTS images revealed a wealth of new structural information and enabled a broad regional study to be made of the tectonic history of the southwestern Colorado Plateau. Spectral information from ERTS-1 was shown to be compatible with ground spectral reflectance measurements made with a portable field instrument developed during this investigation, provided that allowance was made for atmospheric effects. Computer image processing yielded lithologic boundary information within the Coconino Plateau region not obtainable from unprocessed images. Various enhancement techniques were compared in the three areas of study, and some simple rules were developed to guide the processing of images in unknown areas. A new hypothesis was developed for the history of the Colorado River. An ancestral and relatively old Upper Colorado Drainage followed approximately its present course as far as the western side of the Kaibab Uplift, where it diverged from the present course by following a strike valley trending north-northwest. This drainage was captured relatively recently by a much younger Lower Colorado Drainage, which developed by headward erosion after the opening of the Gulf of California. A byproduct of the regional studies in each of the three areas was the identification of areas favorable for the localization of shallow and deep ground waters. On the Shivwits Plateau water trapped in the axis of the old strike valley underlying the Shivwits lavas is a potential source. On the Coconino Plateau sandstone lenses, underlain by clays, within the otherwise permeable Kaibab Limestone are potential perched aquifers. These can be identified on computer-enhanced images. Lineaments detected first on ERTS-1 frames and later verified on aerial photographs were used south of Flagstaff, Arizona, to identify maximum fracturing along the Oak Creek fault in the Woody Mountain well field; a site for a new deep well has been selected and is being drilled by the city of Flagstaff.

Arizona↗
USGS2023

Effect of thermal and mechanical processes on hydraulic transmissivity evolution

Fracture healing is a critical component of enhanced geothermal systems, the earthquake cycle, and induced seismicity. Accordingly, there is significant interest in understanding the process of healing and its effects on fluid transport. The creation, reactivation, and sustainability of fracture networks depend on complex coupling among thermal, hydraulic, mechanical, and chemical processes. We use laboratory slide-hold-slide experiments, at temperatures from 22 to 200 ˚C, to examine effects of fracture reactivation and quasi-static loading on the evolution of fluid transport properties of simulated fractures in Westerly granite. At all temperatures, the in-plane hydraulic transmissivity consistently decays during hold periods resulting in an overall reduction in transmissivity. During the first three to fifteen hours of an experiment, transmissivity decreases rapidly due to the generation of wear products, development of a sliding surface, and compaction of the resulting gouge. Once the sliding surface has developed, the long-term transmissivity decay rate at 22 and 100 ˚C is significantly lower than the transmissivity decay rate during the initial 3 to 15 hours of the experiment. However, at 200 °C, the decay of hydraulic transmissivity remains high throughout the experiment. The long-term decay of hydraulic transmissivity can be fitted with a power law model with more rapid reduction of hydraulic transmissivity at higher temperature. Periods of sliding on the fracture surface result in transient increases in the transmissivity, due to shear dilation, as is expected for Coulomb materials. These transients are superimposed on the long-term decay. When sliding ceases and a new hold period commences, there is a rapid reduction in transmissivity and return to the long-term rate of transmissivity decay. The rate of decay of the transmissivity transients is inversely proportional to temperature, in contrast to the long-term decay and the expected behavior for processes like subcritical crack growth and indentation creep. The higher decay rates that are observed during the initial 3-15 hours of the tests and following sliding, are associated with times that the porosity of the gouge is expected to be high. The difference in decay rates suggests that when the gouge is driven far from equilibrium by active shearing, densification may be dominated by a different mechanism from long-term compaction.

Conference Paper↗
USGS2009

Geological processes and sedimentation rates of wind-tidal flats, Laguna Madre, Texas

Coastal flats worldwide that are periodically exposed to arid climates and periodically flooded by marine waters are unique depositional environments because they receive sediments surficially and interstitially from both land and sea. The wind-tidal flats bordering Laguna Madre, Texas, which fit this unique category, are modified by eolian processes when subaerially exposed, and by wave and current processes when submerged. Floodwater is derived from the lagoon and driven onto the flats by strong and persistent winds during the passage of cold fronts and tropical cyclones. Low surface gradients of the flats prevent rapid drainage and promote seawater evaporation. The depositional products of these processes are interbedded and interlaminated sand, mud, marine shells, algal mats, and evaporites. This assemblage of sediments is geologically diagnostic evidence for intertidal marine deposition and the same assemblage of sediments have been reported for modern marginal-marine flats in the Middle East. The wind-tidal flat surface at Laguna Madre is constantly changing. However, the net effect of natural changes during the past century is either negligible or the changes occur at such a slow rate that they are almost imperceptible. Sediments are repeatedly added to and removed from the surface of the flats in minor increments and in different areas at different times. Preservation potential is enhanced at a particular site by the development of thick mats of blue-green algae. The 14 C ages of buried algal mats yield average long-term (centuries to millennia) sedimentation rates for the wind-tidal flats that range from 0.13 to 0.96 mm/yr and average 0.57 mm/yr. The 210 Pb profiles yield average short-term (150 yr) sedimentation rates that are an order of magnitude higher, ranging from 0.7 to 8.3 mm/yr and averaging 2.9 mm/yr. The minimum present rate of relative sea-level rise in Laguna Madre (3.4 mm/yr) exceeds the historical sedimentation rates for most of the flats. If future sea-level rise is faster than the rates of aggradation, then the wind-tidal flats will progressively become more frequently flooded and will eventually become permanently submerged.

Texas↗
USGS1999

Component flow processes at four streams in the Catskill Mountains, New York, analysed using episodic concentration/discharge relationship

Plots of solute concentration against discharge have been used to relate stream hydrochemical variations to processes of flow generation, using data collected at four streams in the Catskill Mountains, New York, during the Episodic Response Project of the US Environmental Protection Agency. Results suggest that a two-component system of shallow and deep saturated subsurface flow, in which the two components respond simultaneously during hydrologic events, may be applicable to the study basins. Using a large natural sea-salt sodium input as a tracer for precipitation, it is argued that an additional distinction can be made between pre-event and event water travelling along the shallow subsurface flow path. Pre-event water is thought to be displaced by infiltrating event water, which becomes dominant on the falling limb of the hydrograph. Where, as appears to be the case for sulfate, a solute equilibrates rapidly within the soil, the pre-event-event water distinction is unimportant. However, for some solutes there are clear and consistent compositional differences between water from the two sources, evident as a hysteresis loop in concentration-discharge plots. Nitrate and acidity, in particular, appear to be elevated in event water following percolation through the organic horizon. Consequently, the most acidic, high nitrate conditions during an episode generally occur after peak discharge. A simple conceptual model of episode runoff generation is presented on the basis of these results.Plots of solute concentration against discharge have been used to relate stream hydrochemical variations to processes of flow generation, using data collected at four streams in the Catskill Mountains, New York, during the Episodic Response Project of the US Environmental Protection Agency. Results suggest that a two-component system of shallow and deep saturated subsurface flow, in which the two components respond simultaneously during hydrologic events, may be applicable to the study basins. Using a large natural sea-salt sodium input as a tracer for precipitation, it is argued that an additional distinction can be made between pre-event and event water travelling along the shallow subsurface flow path. Pre-event water is thought to be displaced by infiltrating event water, which becomes dominant on the falling limb of the hydrograph. Where, as appears to be the case for sulfate, a solute equilibrates rapidly within the soil, the pre-event - event water distinction is unimportant. However, for some solutes there are clear and consistent compositional differences between water from the two sources, evident as a hysteresis loop in concentration-discharge plots. Nitrate and acidity, in particular, appear to be elevated in event water following percolation through the organic horizon. Consequently, the most acidic, high nitrate conditions during an episode generally occur after peak discharge. A simple conceptual model of episode runoff generation is presented on the basis of these results.

Hydrological Processes↗
USGS2014

Advances in interpretation of subsurface processes with time-lapse electrical imaging

Electrical geophysical methods, including electrical resistivity, time-domain induced polarization, and complex resistivity, have become commonly used to image the near subsurface. Here, we outline their utility for time-lapse imaging of hydrological, geochemical, and biogeochemical processes, focusing on new instrumentation, processing, and analysis techniques specific to monitoring. We review data collection procedures, parameters measured, and petrophysical relationships and then outline the state of the science with respect to inversion methodologies, including coupled inversion. We conclude by highlighting recent research focused on innovative applications of time-lapse imaging in hydrology, biology, ecology, and geochemistry, among other areas of interest.

Hydrological Processes↗
USGS2022

Can machine learning accelerate process understanding and decision-relevant predictions of river water quality?

The global decline of water quality in rivers and streams has resulted in a pressing need to design new watershed management strategies. Water quality can be affected by multiple stressors including population growth, land use change, global warming, and extreme events, with repercussions on human and ecosystem health. A scientific understanding of factors affecting riverine water quality and predictions at local to regional scales, and at sub-daily to decadal timescales are needed for optimal management of watersheds and river basins. Here, we discuss how machine learning (ML) can enable development of more accurate, computationally tractable, and scalable models for analysis and predictions of river water quality. We review relevant state-of-the art applications of ML for water quality models and discuss opportunities to improve the use of ML for emerging computational and mathematical methods for model selection, hyperparameter optimization, incorporating process knowledge into ML models, improving explainablity, uncertainty quantification, and model-data integration. We then present considerations for using ML to address water quality problems given their scale and complexity, available data and computational resources, and stakeholder needs. When combined with decades of process understanding, interdisciplinary advances in knowledge-guided ML, information theory, data integration, and analytics can help address fundamental science questions and enable decision-relevant predictions of riverine water quality.

Hydrological Processes↗
USGS2008

Advancing process‐based watershed hydrological research using near‐surface geophysics: A vision for, and review of, electrical and magnetic geophysical methods

We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3–5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three‐dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a ‘top down’ approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation.

Hydrological Processes↗
USGS2016

Insights into shallow magmatic processes at Kīlauea Volcano, Hawaiʻi, from a multiyear continuous gravity time series

Continuous gravity data collected near the summit eruptive vent at Kīlauea Volcano, Hawaiʻi, during 2011–2015 show a strong correlation with summit-area surface deformation and the level of the lava lake within the vent over periods of days to weeks, suggesting that changes in gravity reflect variations in volcanic activity. Joint analysis of gravity and lava level time series data indicates that over the entire time period studied, the average density of the lava within the upper tens to hundreds of meters of the summit eruptive vent remained low—approximately 1000–1500 kg/m 3 . The ratio of gravity change (adjusted for Earth tides and instrumental drift) to lava level change measured over 15 day windows rose gradually over the course of 2011–2015, probably reflecting either (1) a small increase in the density of lava within the eruptive vent or (2) an increase in the volume of lava within the vent due to gradual vent enlargement. Superimposed on the overall time series were transient spikes of mass change associated with inflation and deflation of Kīlauea's summit and coincident changes in lava level. The unexpectedly strong mass variations during these episodes suggest magma flux to and from the shallow magmatic system without commensurate deformation, perhaps indicating magma accumulation within, and withdrawal from, void space—a process that might not otherwise be apparent from lava level and deformation data alone. Continuous gravity data thus provide unique insights into magmatic processes, arguing for continued application of the method at other frequently active volcanoes.

Hawaii↗
USGS2013

Fish biodiversity sampling in stream ecosystems: a process for evaluating the appropriate types and amount of gear

Because human impacts and climate change threaten aquatic ecosystems, a need exists to quantify catchment-scale biodiversity patterns and identify conservation actions that can mitigate adverse human impacts on aquatic biota. Whereas many traditional aquatic resource questions can be answered by repeatedly sampling a few target species with limited types of gear in the same habitats, sampling fish biodiversity patterns at larger scales requires a different approach. Researchers and managers need to determine the types of sampling gear and amount of effort that provide a representative estimate of biodiversity in a range of habitats across a catchment. Using a randomized block design within a 90-m stream reach that contained the same habitats as the scientific study area, fish assemblages were compared using three different types of gear (minnow traps, backpack electrofishing, and hoop nets) at three levels of effort (one, two, and three mixed-gear units) over four replicate days. A mixture of gear types best quantified fish assemblages. A combination of 10 minnow traps, 20-m of backpack electrofishing, and two hoop nets caught the most species. Additional gear added few new species. Resampling confirmed these results. When researchers and managers initiate sampling on a new stream or river system, they do not know how effective each gear type is and whether their sampling effort is adequate. Although the types and amount of gear may be different for other studies, systems, and research questions, the five-step process described here for making sampling decisions and evaluating sampling efficiency can be applied widely to any system to restore, manage, and conserve aquatic ecosystems. It is believed that incorporating this gear-evaluation process into a wide variety of studies and ecosystems will increase rigour within and across aquatic biodiversity studies.

Massachusetts↗
USGS2023

Assessment of recovery potential for the American horseshoe crab (Limulus polyphemus): An application of the IUCN green status process

According to an International Union for Conservation of Nature (IUCN) Red List assessment (RLA), the American horseshoe crab ( Limulus polyphemus ), an iconic coastal species, is at risk of extirpation in some regions within its range where small and vulnerable populations occur. However, the RLA does not consider future status beyond viability and does not attempt to identify the conservation necessary to effectively mitigate threats and recover the species to full ecological functionality. To aid in conservation planning for vulnerable species, the IUCN developed the Green Status of Species assessment (GSA) process to complement the RLA. This paper describes the application of the GSA process to assess the recovery potential of the American horseshoe crab. First, specific Limulus populations within spatial units for conservation were delineated, and their statuses were defined based on viability and ecological functionality. Then conservation actions were identified that would promote recovery and affect their near- and long-term population status under different conservation scenarios. Horseshoe crab conservation has relied on, and will continue to depend on, effective harvest regulation. However, as currently conceived, conservation is not expected to mitigate habitat loss at the scale required to restore range-wide ecological functionality, primarily because habitat loss is widespread and affected by climate change. Thus, the GSA results, while indicating that there is potential for near-term recovery gains, reveal that long-term recovery is in doubt owing to expected loss of habitat. To conserve critical habitats for spawning and early life stages and achieve ecological functionality, it is imperative to identify and develop conservation plans at appropriate spatial scales. Unfortunately, such plans do not currently exist and need to be established. The GSA Green Score can then serve as a metric for monitoring recovery and gauging the effectiveness of conservation implementation.

Aquatic Conservation: Marine and Freshwater Ecosys↗
USGS2021

Spatial Gaussian processes improve multi-species occupancy models when range boundaries are uncertain and nonoverlapping

Species distribution models enable practitioners to analyze large datasets of encounter records and make predictions about species occurrence at unsurveyed locations. In omnibus surveys that record data on multiple species simultaneously, species ranges are often nonoverlapping and misaligned with the administrative unit defining the spatial domain of interest (e.g., a state or province). Consequently, some species display differentially restricted extents within a study area. Assuming hard boundaries based on expert opinion or published range maps to restrict species occurrence predictions implies a false sense of certainty in model-based inferences. We propose a multi-species occupancy model with a spatial Gaussian process on site-specific effects for each species as a model-based solution. Specifying informative Bayesian hyperpriors on the spatial hyperparameters encapsulates broad-scale correlation among site occupancy probabilities for each species. We fit this model to acoustic detection/nondetection data collected with autonomous recording units during summer of 2016–2019 throughout Oregon and Washington, USA, on 15 bat species. We found vast improvements in spatial predictions of spotted bat ( Euderma maculatum ), canyon bat ( Parastrellus hesperus ), and Brazilian free-tailed bat ( Tadarida brasiliensis ) when the available environmental predictors were insufficient for characterizing their restricted ranges within the region. In contrast, widespread species ( Lasionycteris noctivagans , Myotis californicus , Myotis evotis , Myotis volans ) were appropriately modeled using only environmental predictors, such as percentage forest cover and cliff and canyon cover. Utilizing spatial Gaussian processes within a community or multi-species model incorporates uncertainty in range boundaries and allows for simultaneous predictions for the entire faunal assemblage even if species have nonoverlapping or restricted ranges within a spatial domain of interest. Such modeling improvements are essential if species distribution models are to accurately inform monitoring, species recovery plans, and other conservation efforts.

Ecology and Evolution↗
USGS2022

Woody plant encroachment of grassland and the reversibility of shrub dominance: Erosion, fire, and feedback processes

Many grass-dominated ecosystems in dryland regions have experienced increasing woody plant density and abundance during the past century. In many cases, this process has led to land degradation and declines in ecosystem functions. An example is the Chihuahuan Desert in the southwestern United States, which experienced different stages of shrub encroachment in the past 150 years. Among a wide variety of mechanisms to explain the grass–shrub transitions in this dryland system, soil erosion (both wind and water) and fire are particularly well studied. Here, we synthesize recent developments on the drivers and feedback in the process of shrub encroachment in the Chihuahuan Desert through the intercomparison of two Long Term Ecological Research (LTER) sites, namely Jornada and Sevilleta. Experimental and modeling studies support a conceptual framework, which underscores the important roles of erosion and fire in woody plant encroachment. Collectively, research at the Jornada LTER provided complementary, quantitative support to the well-known fertile-islands framework. Studies at the Sevilleta LTER expanded the framework, adding fire as a major disturbance to woody plants. Conceptual models derived from the synthesis represent the general understanding of shrub encroachment that emerged from research at these two sites, and can guide management interventions aimed at reducing or mitigating undesirable ecosystem state change in many other drylands worldwide.

New Mexico↗
USGS2019

Monitoring the Riverine Pulse: Applying high-frequency nitrate data to advance integrative understanding of biogeochemical and hydrological processes

Widespread deployment of sensors that measure river nitrate (NO3-) concentrations has led to many recent publications in water resources journals including review papers focused on data quality assurance, improved load calculations, and better nutrient management. The principal objective of this paper is to review and synthesize studies of high-frequency NO3- data that have aimed to improve understanding of the hydrologic and biogeochemical processes underlying episodic, diel, and long-term stream NO3- dynamics. Investigations have provided unprecedented detail on hysteresis and flushing patterns during high flow, seasonal variation during baseflow, and responses to multi-year climate variation. Analyses of high-frequency data have led to notable advances in understanding how climate variation affects spatial and temporal NO3- patterns, especially dry-wet cycles and antecedent moisture. Further advances have been limited by few investigations that include high-frequency measurements outside the channel and the short duration of many records. High-frequency data for multiple constituents have provided new insight to the relative roles of hydrology and biogeochemistry as highlighted by studies of the roles of autotrophic uptake, denitrification, riparian evapotranspiration, and temperature-driven changes in viscosity as drivers of diel patterns. Comparisons of short-duration high-frequency data with long-duration low frequency data have described similarities and differences in concentration – discharge patterns and highlighted the role of legacy stores. Investigators have applied innovative analysis approaches not previously possible with low-frequency or temporally-irregular data. Future availability of long-duration high-frequency data will provide new insight to processes, resulting in improved conceptual models and a deeper understanding of the role of climate variation.

WIREs Water↗
USGS2013

Ecosystem services: developing sustainable management paradigms based on wetland functions and processes

In the late nineteenth century and twentieth century, there was considerable interest and activity to develop the United States for agricultural, mining, and many other purposes to improve the quality of human life standards and prosperity. Most of the work to support this development was focused along disciplinary lines with little attention focused on ecosystem service trade-offs or synergisms, especially those that transcended boundaries of scientific disciplines and specific interest groups. Concurrently, human population size has increased substantially and its use of ecosystem services has increased more than five-fold over just the past century. Consequently, the contemporary landscape has been highly modified for human use, leaving behind a fragmented landscape where basic ecosystem functions and processes have been broadly altered. Over this period, climate change also interacted with other anthropogenic effects, resulting in modern environmental problems having a complexity that is without historical precedent. The challenge before the scientific community is to develop new science paradigms that integrate relevant scientific disciplines to properly frame and evaluate modern environmental problems in a systems-type approach to better inform the decision-making process. Wetland science is a relatively new discipline that grew out of the conservation movement of the early twentieth century. In the United States, most of the conservation attention in the earlier days was on wildlife, but a growing human awareness of the importance of the environment led to the passage of the National Environmental Policy Act in 1969. Concurrently, there was a broadening interest in conservation science, and the scientific study of wetlands gradually gained acceptance as a scientific discipline. Pioneering wetland scientists became formally organized when they formed The Society of Wetland Scientists in 1980 and established a publication outlet to share wetland research findings. In comparison to older and more traditional scientific disciplines, the wetland sciences may be better equipped to tackle today’s complex problems. Since its emergence as a scientific discipline, the study of wetlands has frequently required interdisciplinary and integrated approaches. This interdisciplinary/integrated approach is largely the result of the fact that wetlands cannot be studied in isolation of upland areas that contribute surface and subsurface water, solutes, sediments, and nutrients into wetland basins. However, challenges still remain in thoroughly integrating the wetland sciences with scientific disciplines involved in upland studies, especially those involved with agriculture, development, and other land-conversion activities that influence wetland hydrology, chemistry, and sedimentation. One way to facilitate this integration is to develop an understanding of how human activities affect wetland ecosystem services, especially the trade-offs and synergisms that occur when land-use changes are made. Used in this context, an understanding of the real costs of managing for a particular ecosystem service or groups of services can be determined and quantified in terms of reduced delivery of other services and in overall sustainability of the wetland and the landscapes that support them. In this chapter, we discuss some of the more salient aspects of a few common wetland types to give the reader some background on the diversity of functions that wetlands perform and the specific ecosystem services they provide to society. Wetlands are among the most complex ecosystems on the planet, and it is often difficult to communicate to a diverse public all of the positive services wetlands provide to mankind. Our goal is to help the reader develop an understanding that management options can be approached as societal choices where decisions can be made within a spatial and temporal context to identify trade-offs, synergies, and effects on long-term sustainability of wetland ecosystems. This will be especially relevant as we move into alternate climate futures where our portfolio of management options for mitigating damage to ecosystem function or detrimental cascading effects must be diverse and effective.

Book chapter↗
USGS1998

Processes of fluvial island formation, with examples from Plum Creek, Colorado and Snake River, Idaho

A fluvial island is a landform, elevated above and surrounded by stream-channel branches or waterways, that persists sufficiently long to establish permanent vegetation. Natural fluvial islands occur in any part of a drainage network but most commonly in montane, piedmont-valley, and coastal flood-plain environments. Processes, often interactive, by which islands form include avulsion (the sudden separation of land by a flood or by an abrupt change in the course of a stream), rapid and gradual channel incision, channel migration, dissection of both rapidly and slowly deposited bed sediment, and deposition of bed sediment on a vegetated surface or behind a channel obstruction. Products of high-energy conditions, fluvial islands typically lack stability over decades to millennia. Fluvial islands in Plum Creek, Colorado, USA, results of sorting processes following a recent high-magnitude flood, and in the Snake River, Idaho, USA, partly results of the Pleistocene Bonneville Flood, illustrate how islands form, develop, and disappear. The examples consider differing conditions of island shape, size, height, sediment, and vegetation.

Colorado, Idaho↗
USGS2025

Noble and base metal distribution and processes affecting ore tenors in the disrupted lower stratigraphy of the Stillwater Complex, USA

Exploration continues for contact-style Ni-Cu sulfide and chromitite-associated PGE mineralization in ultramafic rocks of the Stillwater Complex. At the Iron and Chrome Mountain areas, massive sulfides occur along the complex’s footwall contact and anomalous concentrations of PGE+Au are associated with the three lowermost chromitite seams. Southeast of Chrome Mountain, magmatic layering is highly disrupted by the presence of faults, magmatic breccias, serpentinized discordant dunites, pyroxenite pegmatoids, and disaggregated chromitite seams. The bulk rock chemistry, sulfide chemistry, and noble metal mineralogy of samples from this area were examined to determine the deportment of PGE and processes that led to enrichments in PGE, Au, Cu, Co and Ni. Results show that a sulfide liquid was the principal collector of PGE. If sulfide liquid was initially deposited with chromite, it was disaggregated or redistributed by subsequent melt or fluid infiltration, which may have resulted in the offset of peak PGE(+Cu, Ni) from peak Cr 2 O 3 concentrations, and upgraded PGE tenors. Upon cooling, PGE exsolved from sulfides to form discrete bismuth tellurides, arsenides, arsenic sulfides, antimonides, and alloys, commonly along the margins of sulfide globules. Calculated metal tenors are highest in the disseminated sulfides southwest of Chrome Mountain, whereas massive and net-textured sulfides near the Iron Mountain-Camp zone represent monosulfide solid solution cumulates. At progressively shallower levels, higher metal tenors combined with lower S/Se ratios are consistent with increasing R-factors from 100 to 100,000. Serpentinization and talc-tremolite alteration resulted in S loss through partial replacement of sulfides by secondary silicate+carbonate+magnetite+sulfide assemblages, further upgrading Ni-Cu-PGE tenors. The present work shows that processes responsible for the disruption of magmatic layering and post-magmatic fluid alteration along the intrusion’s lower contact led to noble and base metal enrichments.

Montana↗
USGS1997

Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Glass Mountain consists of a 1 km 3 , compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H 2 O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km 3 ) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H 2 O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H 2 O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust.

California↗