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Jeffrey Phillips

Publications and source records attributed to Jeffrey Phillips.

14 recordsLinked to original sources
USGS2021

Integrated geophysical imaging of rare-earth-element-bearing iron oxide-apatite deposits in the eastern Adirondack Highlands, New York

The eastern Adirondack Highlands of northern New York host dozens of iron oxide-apatite (IOA) deposits containing magnetite and rare earth element (REE)-bearing apatite. We use new aeromagnetic, aeroradiometric, ground gravity, and sample petrophysical and geochemical data to image and understand these deposits and their geologic framework. Aeromagnetic total field data reflect highly magnetic leucogranite host rock and major structures that likely served as fluid conduits for the hydrothermal system. Bandpass filtering of the aeromagnetic data reveals individual deposits that were verified in the field or from historical records. A three-dimensional inversion for magnetic susceptibility images these deposits at depth, allowing inference of plunge directions and relative size. Radiometric data highlight variations in the surface geology and several large tailings piles that contain REE-bearing apatite. Within the host rock, eTh (equivalent Th), K and the eTh/K ratio are variable with high eTh/K near several of the IOA deposits. Areas with elevated K or low eTh/K representing potassic alteration appear to be rare; instead elevated eTh/K ratios likely reflect widespread sodic alteration overprinting potassic alteration. Bouguer gravity anomalies show limited correspondence to the surface geology, radiometric data, or magnetic data, but do exhibit ~10-km wide highs in areas where deposits are observed. Two-dimensional forward models of the gravity and magnetic data show that deeper dense material beneath the leucogranite is quantitatively feasible. If these dense rocks represent intrusions that were emplaced or still cooling at the time of mineralization, they may have served as a heat source that helped to drive the hydrothermal system. Combining datasets, we find that deposits occur towards the distal ends of major structures within the host leucogranite and mostly above gravity highs. The geophysical modeling thus suggests that IOA deposits formed in structural, thermal, and chemical traps near the distal ends of the hydrothermal system.

New York↗
USGS2020

Three-dimensional shape and structure of the Susitna basin, south-central Alaska, from geophysical data

We use gravity, magnetic, seismic reflection, well, and outcrop data to determine the three-dimensional shape and structural features of south-central Alaska’s Susitna basin. This basin is located within the Aleutian-Alaskan convergent margin region and is expected to show effects of regional subduction zone processes. Aeromagnetic data, when filtered to highlight anomalies associated with sources within the upper few kilometers, show numerous linear northeast-trending highs and some linear north-trending highs. Comparisons to seismic reflection and well data show that these highs correspond to areas where late Paleocene to early Eocene volcanic layers have been locally uplifted due to folding and/or faulting. The combined magnetic and seismic reflection data suggest that the linear highs represent northeast-trending folds and north-striking faults. Several lines of evidence suggest that the northeast-trending folds formed during the middle Eocene to early Miocene and may have continued to be active in the Pliocene. The north-striking faults, which in some areas appear to cut the northeast-trending folds, show evidence of Neogene and probable modern movement. Gravity data facilitate estimates of the shape and depth of the basin. This was accomplished by separating the observed gravity anomaly into two components—one representing low-density sedimentary fill within the basin and one representing density heterogeneities within the underlying crystalline basement. We then used the basin anomaly, seismic reflection data, and well data to estimate the depth of the basin. Together, the magnetic, gravity, and reflection seismic analyses reveal an asymmetric basin comprising sedimentary rock over 4 km thick with steep, fault-bounded sides to the southwest, west, and north and a mostly gentle rise toward the east. Relations to the broader tectonic regime are suggested by fold axis orientations within the Susitna basin and neighboring Cook Inlet basin, which are roughly parallel to the easternmost part of the Alaska-Aleutian trench and associated Wadati-Benioff zone as it trends from northeast to north-northeast to northeast. An alignment between forearc basin folds and the subduction zone trench has been observed at other convergent margins, attributed to strain partitioning generated by regional rheologic variations that are associated with the subducting plate and arc magmatism. The asymmetric shape of the basin, especially its gentle rise to the east, may reflect uplift associated with flat-slab subduction of the Yakutat microplate, consistent with previous work that suggested Yakutat influence on the nearby Talkeetna Mountains and western Alaska Range. Yakutat subduction may also have contributed to Neogene and later reverse slip along north-striking faults within the Susitna basin.

Alaska↗
USGS2018

The use of curvature in potential-field interpretation

Potential-field anomalies can be transformed into special functions that form peaks and ridges over isolated sources. All special functions have a common mathematical form over an isolated source, which leads to a common equation for estimating the source depth from the peak value and the curvature at the peak. Model-specific special functions, usually calculated from a transformed version of a potential field, are used to estimate the locations of very specific source types. Model-independent special functions calculated from an observed or transformed potential field can be used to estimate the locations of a variety of source types. Vertical integration is a particularly useful transformation for reducing the effects of noise and increasing the coherency of solutions from model-independent special functions. For gridded data, the eigenvalues and eigenvectors of the curvature matrix associated with a quadratic surface that is fitted to a special function within 3 × 3 windows can be used to locate the sources and estimate their depths and strikes. Discrete source locations estimated in this manner can be connected into lines that follow contacts, faults, and other mappable features based on distance and azimuth criteria. These concepts are demonstrated on aeromagnetic data from the Albuquerque basin of New Mexico, USA.

New Mexico↗
USGS2017

Crustal structure interpreted from magnetic anomalies

In this review, we discuss publications during the last quadrennium (1987–1990) that used aeromagnetic data, marine magnetic data, satellite magnetic data, and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies. Abstracts and papers by non‐U.S. authors are not described. The publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies, particularly in deep crustal magnetization, exploration for hydrocarbons, and inversion of marine magnetic anomalies. Interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard. The magnetic anomaly map of North America appeared, the first such map of an entire continent. MAGSAT data contributed to the development of a new model for the magnetization of the oceanic crust. Forward modeling of sources of magnetic anomalies contributed to a wide range of geologic investigations on land, and inverse magnetic modeling techniques became popular for marine geologic investigations.

Reviews of Geophysics↗
USGS2017

Undiscovered porphyry copper resources in the Urals—A probabilistic mineral resource assessment

A probabilistic mineral resource assessment of metal resources in undiscovered porphyry copper deposits of the Ural Mountains in Russia and Kazakhstan was done using a quantitative form of mineral resource assessment. Permissive tracts were delineated on the basis of mapped and inferred subsurface distributions of igneous rocks assigned to tectonic zones that include magmatic arcs where the occurrence of porphyry copper deposits within 1 km of the Earth's surface are possible. These permissive tracts outline four north-south trending volcano-plutonic belts in major structural zones of the Urals. From west to east, these include permissive lithologies for porphyry copper deposits associated with Paleozoic subduction-related island-arc complexes preserved in the Tagil and Magnitogorsk arcs, Paleozoic island-arc fragments and associated tonalite-granodiorite intrusions in the East Uralian zone, and Carboniferous continental-margin arcs developed on the Kazakh craton in the Transuralian zone. The tracts range from about 50,000 to 130,000 km 2 in area. The Urals host 8 known porphyry copper deposits with total identified resources of about 6.4 million metric tons of copper, at least 20 additional porphyry copper prospect areas, and numerous copper-bearing skarns and copper occurrences. Probabilistic estimates predict a mean of 22 undiscovered porphyry copper deposits within the four permissive tracts delineated in the Urals. Combining estimates with established grade and tonnage models predicts a mean of 82 million metric tons of undiscovered copper. Application of an economic filter suggests that about half of that amount could be economically recoverable based on assumed depth distributions, availability of infrastructure, recovery rates, current metals prices, and investment environment.

Urals↗
USGS2016

Magnetic and gravity gradiometry framework for Mesoproterozoic iron oxide-apatite and iron oxide-copper-gold deposits, southeast Missouri, USA

High-resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in Mesoproterozoic basement rocks of southeast Missouri. The data are used to calculate mineral prospectivity for iron oxide-apatite (IOA) ± rare earth element (REE) and iron oxide-copper-gold (IOCG) deposits. Results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas. The airborne data are also inverted to three-dimensional density and magnetic susceptibility models over four concealed deposits at Pea Ridge (IOA ± REE), Boss (IOCG), Kratz Spring (IOA), and Bourbon (IOCG). The Pea Ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km. A smaller density source, located within the shallow Precambrian basement, is partly coincident with the magnetic source at Pea Ridge. In contrast, the Boss models show a large (625-m-wide), vertically extensive, and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2,600 m across the shallow Precambrian paleosurface. The Kratz Spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits. A prospective area identified south of the Kratz Spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface, and is interpreted as prospective for a hematite-dominant lithology that extends from the top of the Precambrian to depths exceeding 2 km. The Bourbon deposit displays a large bowl-shaped volume of coincident high density and high-magnetic susceptibility rock, and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the Precambrian basement. In order to underpin the evaluation of the prospectivity and three-dimensional models, an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits, altered wall rocks, and unaltered country rocks.

Missouri↗
USGS2015

Sharpening the boundaries - 3D terracing applied to physical property inversions

Physical property inversions typically result in smooth property distributions, without the sharp boundaries expected of true geologic sources. Here we explore the use of 3D terracing as a way to force the inverted physical properties to have sharp boundaries. Terracing during inversion is preferable to post-inversion terracing because it results in a better fit to the observed data.

Conference Paper↗
USGS2015

11.12 – Tools and techniques: gravitational method

The gravitational method is used to investigate density variations within the subsurface at depths of several meters to tens of meters, as in depth-to-bedrock investigations, or at depths of several kilometers, as in sedimentary basin thickness investigations. This chapter covers fundamental relations, densities of Earth materials, instruments, field procedures, data reduction, filtering, forward modeling, inversion, and field examples. The focus is on near-surface investigations as distinct from the solid Earth studies found elsewhere in this treatise. The gravitational method is often used in conjunction with other geophysical methods, such as the magnetic method or the seismic method, which target similar physical properties at similar depths.

Book chapter↗
USGS2014

Using vertical Fourier transforms to invert potential-field data to magnetization or density models in the presence of topography

A physical property inversion approach based on the use of 3D (or 2D) Fourier transforms to calculate the potential-field within a 3D (or 2D) volume from a known physical property distribution within the volume is described. Topographic surfaces and observations at arbitrary locations are easily accommodated. The limitations of the approach and applications to real data are considered.

Conference Paper↗
USGS2014

Can we estimate total magnetization directions from aeromagnetic data using Helbig's integrals?

An algorithm that implements Helbig’s (1963) integrals for estimating the vector components ( m x , m y , m z ) of the magnetic dipole moment from the first order moments of the vector magnetic field components (Δ X , Δ Y , Δ Z ) is tested on real and synthetic data. After a grid of total field aeromagnetic data is converted to vector component grids using Fourier filtering, Helbig’s infinite integrals are evaluated as finite integrals in small moving windows using a quadrature algorithm based on the 2-D trapezoidal rule. Prior to integration, best-fit planar surfaces must be removed from the component data within the data windows in order to make the results independent of the coordinate system origin. Two different approaches are described for interpreting the results of the integration. In the “direct” method, results from pairs of different window sizes are compared to identify grid nodes where the angular difference between solutions is small. These solutions provide valid estimates of total magnetization directions for compact sources such as spheres or dipoles, but not for horizontally elongated or 2-D sources. In the “indirect” method, which is more forgiving of source geometry, results of the quadrature analysis are scanned for solutions that are parallel to a specified total magnetization direction.

Earth, Planets and Space↗
USGS2010

Estimating structural dip from gravity and magnetic profile data

The anomalies of ideal sources, dipping magnetic contacts and dipping density sheets, provide the theoretical basis for estimating structural dip from gravity and magnetic profile data. The dip is always related to the local phase angle of a complex analytic signal evaluated directly over the source. For magnetic sheets, the complex analytic signal is constructed from the anomaly and its Hilbert transform. For magnetic contacts or density sheets, it is constructed from the first‐order horizontal and vertical derivatives of the anomaly. Dip estimation is implemented as an integral part of source‐location algorithms. The analytic signal and local wavenumber methods require interpolation of the local phase from sampled anomaly or derivative values. In the case of extended Euler deconvolution and multiple‐source Werner deconvolution the dip is derived as part of the least squares solution.

Conference Paper↗