Pat Shanks | U.S. Geological Survey (original) (raw)

Papers by Pat Shanks

Stratabound base-metal sulfi de deposits and occurrences are present in metasedi-mentary rocks of... more Stratabound base-metal sulfi de deposits and occurrences are present in metasedi-mentary rocks of the Neoproterozoic and Paleozoic Nome Complex on south-central Seward Peninsula, Alaska. Stratabound and locally stratiform deposits including Aurora Creek (Zn-Au-Ba-F), Wheeler North (Pb-Zn-Ag-Au-F), and Nelson (Zn-Pb-Cu-Ag), consist of lenses typically 0.5–2.0 m thick containing disseminated to semi-massive sulfi des. Host strata of the Aurora Creek and Wheeler North deposits are variably calcareous and graphitic siliciclastic metasedimentary rocks of Middle Devonian or younger age based on detrital zircon geochronology; the Nelson deposit is within Ordovician–Devonian marble (Till et al., this volume, Chapter 4). Deformed veins such as Quarry (Zn-Pb-Ag-Ba-F) and Galena (Pb-Zn-Ag-F) occur in a unit composed mainly of marble and schist; fossil and detrital zircon data indicate that this unit contains rocks of Ordovician, Silurian, and Devonian age. None of these Zn-and Pb-rich deposits or occurrences has spatially associated metavolcanic or intrusive rocks. All were deformed and metamorphosed to blueschist facies and then retrograded to greenschist facies during the Jurassic and Early Cretaceous Brookian orogeny. Disseminated Cu-rich deposits including Copper King (Cu-Bi-Sb-Pb-Ag-Au) and Wheeler South (Cu-Ag-Au) occur in silicifi ed carbonate rocks and have textures that indicate a pre-to syn-metamorphic origin. The Zn-and Pb-rich sulfi de deposits and occurrences consist mainly of pyrite, sphalerite, and/or galena in a gangue of quartz and carbonate. Minor minerals include arsenopyrite, chalcopyrite, magnetite, pyrrhotite, tetrahedrite, barite, fl uorite, and

Geological Society of America Special Papers, Jul 1, 2014

Treatise on Geochemistry, 2014

alteration hydrothermal silica spires stable isotopes trace elements vents 54 GEOTHERMAL BIOLOGY ... more alteration hydrothermal silica spires stable isotopes trace elements vents 54 GEOTHERMAL BIOLOGY AND GEOCHEMISTRY IN YELLOWSTONE NATIONAL PARK ABSTRACT

Results of sulfur and oxygen isotopic studies of sedimentary-exhalative (SEDEX) Zn-Pb(-Ag-Au-Ba-F... more Results of sulfur and oxygen isotopic studies of sedimentary-exhalative (SEDEX) Zn-Pb(-Ag-Au-Ba-F) deposits hosted in metamorphosed Paleozoic clastic and carbonate rocks of the Nome Complex, Seward Peninsula, Alaska, are consistent with data for similar deposits worldwide. Stable isotopic studies of the Nome Complex are challenging because the rocks have undergone Mesozoic blueschist-and greenschist-facies metamorphism and deformation at temperatures estimated from 390 to 490 °C. Studies of sulfur and oxygen isotopes in other areas suggest that, in the absence of chemical and mineralogical evidence for metasomatism, the principal effect of meta-morphism is re-equilibration between individual minerals at the temperature of metamorphism, which commonly leads to a narrowing of the overall range of isotope values for a suite of rocks but generally does not signifi cantly modify the average whole-rock value for that suite. Sulfur isotopic studies of the stratabound and locally stratiform sulfi de lenses at the Aurora Creek–Christophosen deposit, which is of possible Late Devonian–Early Carboniferous age, show a large range of δ 34 S sulfi de values from –9.7‰ to 39.4‰, suggesting multiple sulfur sources and possibly complex processes of sulfi de formation 235 *Current address: 7309 W.

$Research paper thumbnail of Multiple sulfur isotope fractionation and mass transfer processes during pyrite precipitation and recrystallization: An experimental study at 300 and 350 °C$

Equilibrium multiple sulfur isotope fractionation factors (33 S/ 32 S and 34 S/ 32 S) between aqu... more Equilibrium multiple sulfur isotope fractionation factors (33 S/ 32 S and 34 S/ 32 S) between aqueous SO 4 , H 2 S, and coexisting pyrite under hydrothermal conditions were determined experimentally at 300–350 °C and 500 bars. Two different experimental techniques were used to determine the fractionation factors and the rate of S isotope exchange between pyrite and constituent aqueous species, H 2 S and SO 4 ; (1) closed system gold capsule pyriteÀH 2 S exchange experiments and (2) complimentary time-series experiments at 300 and 350 °C, 500 bars using flexible gold cell hydrothermal equipment, which allowed monitoring the multiple S isotope composition of dissolved S species during pyrite precipitation and subsequent recrystallization. The three isotope technique was applied to the multiple S isotope data to demonstrate equilibrium S isotope fractionation between pyrite and H 2 S. Results at 350 °C indicate ln 34 a Pyrite=H 2 S = À1.9& and ln 33 a Pyrite=H 2 S = À1.0&. The ln 34 a Pyrite=H 2 S is not only different in magnitude but also in sign from the commonly used value of 1& from Ohmoto and Rye (1979). This experimental study also demonstrated initial S isotope disequilibrium amongst the aqueous S-species and pyrite during rapid precipitation, despite aqueous speciation indicating pyrite saturation at all stages. Textural, crystallographic , and S isotope interpretations suggest that pyrite formed by means of the FeS pathway. The initial S isotope disequi-librium between formed pyrite and dissolved S-species was effectively erased and approached isotopic equilibrium upon recrystallization during the course of 4297 h. Interpretation of seafloor hydrothermal vent sulfides using the revised equilibrium 34 S/ 32 S fractionation between pyrite and H 2 S suggests that pyrite is close to S isotope equilibrium with vent H 2 S, contrary to previous conclusions. The experimental data reported here broaden the range of pyrite formation mechanisms at seafloor hydrothermal vents, in that mineral formation pathway and equilibration rates need to be considered to account for the well-recognized S isotope variability that often characterizes these systems. Published by Elsevier Ltd.

. Values of 6'" ant1 6D are calibrated relative to the VSMOW standard and have reproducibility of... more . Values of 6'" ant1 6D are calibrated relative to the VSMOW standard and have reproducibility of approximately 0.1 and I .(1?60, respectively. 6D values of water samples determined by H2 equilibration methods, which give isotope activity ratios, may he significantly different fro111 those determined by Zn-or C[-reduction methods, which gibe isotopc concentration ratios, if the sanlplcs contain dissolveti salts. The effect of salts on the isotopic activity ratio of seawatcr-may be expected to cause 6D values determined by the H, equilibration method at 30°C to be approximately 1 . 1 -1.2760 higher than those dctermineti by a Zn-or U-reduction method (Horita et al. 1903). Results of d hydrogen and oxygcn isotope values uf vent fluids are presented as A"o,,,,,,,, and ADc,,ncunlrat,,,,, relative to local bottom waters (see Shanks et al. 1095). Oxygen isotopc activlty values and hydrogen isotopc concentration values are chosen because most data in the literature and all of the baslc isotope fractionation data fiom experimental st~idies are presented on thcse scales. 'I'he A approach removes global ocean 6D and S1"'O variations arid allows i b c~~s on processes that affect the isotopes during hydrothermal processes. VSMOW values for 6D rind 6'" of bottom waters in V~I~~O L~S areas arc Shanks r. F F, F, r., ,-,r ,

Special Paper of the Geological Society of America

Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boilin... more Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boiling water, steam, mud, and rock fragments from source craters that range from a few meters up to more than 2 km in diameter; associated breccia can be emplaced as much as 3 to 4 km from the largest craters. Hydrothermal explosions occur where shallow interconnected reservoirs of steam-and liquid-saturated fl uids with temperatures at or near the boiling curve underlie thermal fi elds. Sudden reduction in confi ning pressure causes fl uids to fl ash to steam, resulting in signifi cant expansion, rock fragmentation, and debris ejection.

GSA Field Guide 10: Roaming the Rocky Mountains and Environs: Geological Field Trips, 2008

This fi eld trip highlights various stages in the evolution of the Snake River Plain-Yellowstone ... more This fi eld trip highlights various stages in the evolution of the Snake River Plain-Yellowstone Plateau bimodal volcanic province, and associated faulting and uplift, also known as the track of the Yellowstone hotspot. The 16 Ma Yellowstone hotspot track is one of the few places on Earth where time-transgressive processes on continental crust can be observed in the volcanic and tectonic (faulting and uplift) record at the rate and direction predicted by plate motion. Recent interest in young and possible renewed volcanism at Yellowstone along with new discoveries and synthesis of previous studies, i.e., tomographic, deformation, bathymetric, and seismic surveys, provide a framework of evidence of plate motion over a mantle plume.

Geological Society of America Special Papers, 2009

Scientific Investigations Report, 2011

Scientific Drilling, 2013

HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake ... more HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic history of the SRP, which represents the surface expression of this hotspot, and to understand how it affected the evolution of continental crust and mantle. An additional goal is to evaluate the geothermal potential of southern Idaho.

Geology, 2007

We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large calde... more We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large caldera-forming ignimbrites and postcaldera lavas of the Heise volcanic fi eld, a nested caldera complex in the Snake River Plain, that preceded volcanism in Yellowstone. Early eruption of three normal δ 18 O voluminous ignimbrites with δ 18 O quartz = 6.4‰ and δ 18 O zircon = 4.8‰ started at Heise at 6.6 Ma, and was followed by a 2‰-3‰ 18 O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km 3 Kilgore ignimbrite, and post-Kilgore intracaldera lavas with δ 18 O quartz = 4.3‰ and δ 18 O zircon = 1.5‰. The Kilgore ignimbrite represents the largest known low-δ 18 O magma in the Snake River Plain and worldwide. The post-Kilgore low δ 18 O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal δ 18 O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low δ 18 O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in δ 18 O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low δ 18 O signature from mantle or crustal sources. Instead, δ 18 O age trends indicate progressive remelting of low δ 18 O hydrothermally altered intracaldera rocks of previous eruptions. This trend may be generally applicable to older caldera complexes in the Snake River Plain that are poorly exposed.

Stable isotopic (δD and δ,C and then mix with pore waters prior to venting on the lake bottom. De... more Stable isotopic (δD and δ,C and then mix with pore waters prior to venting on the lake bottom. Depositional