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Bruce Smith

Publications and source records attributed to Bruce Smith.

6 recordsLinked to original sources
USGS2024

Deep resistivity geophysics of the San Juan–Silverton caldera complex, San Juan County, Colorado (USA)

Magnetotelluric (MT) and audiomagnetotelluric (AMT) data are used to better understand the subsurface geology and mineral resources in the San Juan–Silverton caldera complex located near Silverton, Colorado, western United States, as part of the extensive southern Rocky Mountains volcanic field that covers much of southwestern Colorado and northern New Mexico. Seven MT and AMT profiles of varying lengths image resistivity structure to depths of ~5 km. The AMT inversion models characterize geophysical responses of near-surface lithologies, structures, and mineralized systems and also help corroborate airborne electromagnetic data at shallow levels. The MT inversion models extend our depth of investigation from near the surface to great depths (~5 km) and help to form hypotheses about roots of the hydrothermal plumbing that fed shallower mineralized systems. Subsurface high resistivities occur beneath intermediate-composition lava flows and Proterozoic units. Subsurface moderate- to low-resistivity values may reflect hydrothermal plumbing that served as flow paths for mineralizing fluids and metallic ore formation. The model interpreta­tions presented in this study could be utilized in remediation planning or mineral resource applications. The methods used could be applied to other watersheds with similar volcanic environments containing acid-generating historical mines or hydrothermally altered and mineralized source rocks.

Colorado↗
USGS2023

Geophysical data provide three dimensional insights into porphyry copper systems in the Silverton caldera, Colorado, USA

The Silverton caldera in southwest Colorado, USA hosts polymetallic veins and pervasively altered rocks indicative of porphyry copper systems. Nearly a kilometer of erosion has exposed multiple levels of the hydrothermal systems from shallow lithocaps down to quartz-sericite-pyrite veins. New airborne electromagnetic and magnetic survey data are integrated with previous alteration mapping and porphyry models to show the subsurface geophysical response of shallow to deep levels of the porphyry system. Qualitative map views show lateral changes in the magnetization and resistivity of the hydrothermally altered rocks. The volcanic terrain exhibits high magnetization and high amplitude anomalies map near-surface plutonic rocks associated with porphyry systems. Magnetic susceptibility measurements on outcrops of hydrothermally altered rocks indicate magnetite content decreases upward and outward from the source intrusions where magnetic anomaly lows are observed over the lithocaps. The resistivity maps highlight hydrothermal alteration as resistivity lows with exception being rocks having propylitic alteration. Quantitative resistivity models show low resistivity zones with an apparent thickness around 50–150 m beneath quartz-sericite-pyrite veins interpreted to be the result of supergene processes that may continue today, and the calculated magnetic source depths occur near the top of this zone. The resistivity models also show rocks having propylitic, silicic, and quartz-alunite-pyrophyllite assemblages exhibit high resistivity with depth, and argillic alteration assemblages had high resistivity due to high quartz content. This integrated approach presented in a three-dimensional environment provides guidance when exploring for porphyry copper systems in less exposed terrains.

Colorado↗
USGS2019

Monitoring brine contamination using time-lapse airborne electromagnetic surveys, East Poplar Oil Field, Montana

Integrated geophysical and water-quality studies have been used to delineate areas of saline groundwater in shallow unconfined aquifers underlying the East Poplar oil field in northeastern Montana. In 2004, a RESOLVE survey was conducted over the oil field to identify high conductivity areas potentially associated with brine contamination and to map the shale unit comprising the base of aquifer. In 2014, a SkyTEM 301 survey was conducted over the same flight paths to examine possible changes in groundwater conductivity and to complete the base-of-aquifer mapping where the depth of investigation from the 2004 survey was inadequate. We present a preliminary comparison between the 2004 and 2014 surveys.

Montana↗
USGS1983

Time-domain electromagnetic tests in the Wadi Bidah District, Kingdom of Saudi Arabia

A time-domain electromagnetic (TDEM) method was tested in two areas of mineralization in Precambrian rocks in the Wadi Bidah district, Kingdom of Saudi Arabia. Transient-decay voltages in profile mode were measured across the Sha'ab at Tare and Rabathan prospects by use of three transmitterreceiver loop configurations. At the Sha'ab at Tare prospect all of the loop configurations indicated the mineralized zone. Analysis of the coincident loop data at Sha'ab at Tare reveals that gossanous and altered rock of i0 ohm-m resistivity extends to a depth of 35 m, where there is an unweathered, dry mineralized zone of about 1 ohm-m resistivity. The model further suggests that the rocks at a depth of 55 m and below the water table are even less resistive (0. 1 ohm-m). The TDEM method successfully discriminated conductors within from those below the weathered zone at the Rabathan prospect. Conductors below the weathered zone are identified by a lack of transient response in the early part of the transient decay curve, followed by an increasing response in the middle to late parts of the transient decay curve. Results of these limited tests suggest the potential value of integrating TDEM with other geophysical tools in the Kingdom. Recommendations are made to expand these tests into a more comprehensive program that will evaluate the TDEM potential in various geologic environments that are host to mineral deposits of diverse origin.

Open-File Report↗
USGS1976

Induced-polarization surveys applied to exploration for roll-front uranium deposits

This open-file report summarizes the oral presentation given at the 45th Annual Meeting of the Society of Exploration Geophysicists. The data analysis given here is brief because of time limitations imposed at the meeting. A more detailed analysis of the data (both geological and geophysical) will be given in subsequent publications. Our oral presentation was preceded by one given by Jeffrey Daniels and James Scott entitled "Borehole Investigations of South Texas Uranium Deposits." Statements made in their presentation that are pertinent to this report are as follows: (1) a Tertiary sandstone is the host for the South Texas uranium occurrence, (2) the roll-front geometry is similar to those found in Wyoming, and (3) the roll-front does not occur at a visually obvious change from reduced to oxidized environments. A careful comparison of the IP (induced polarization) borehole logs and lithology revealed the following general points: (1) the largest IP response logged in the boreholes occurred at the uranium roll-front, and (2) the IP response can be correlated with percentage pyrite and with variations in the percentage and type of clay present. The IP measurements reported here were made with a time-domain Huntec IP receiver. The transmitted wave-form had a two-second on-and-off period. Four windows were measured during the off-time. The second window, M2, which is given here, has a window duration of 130 milliseconds from 50 to 130 milliseconds. IP coupling is inferred not to be a serious problem in the survey data reported here, because the variations in M measurements are the same as those seen for the2 window of longest delay and length. The data presented herein are part of a broad data set which the U.S. Geological Survey has gathered on surface geophysical methods. Other people who have been involved in the data collection, interpretation, and presentation are John Cady, David Campbell, Ralph Corken, Richard Meyer, Louis O'Connor, Tommas Nash, and James Scott.

Open-File Report↗