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

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.

Special Paper of the Geological Society of America

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

Geological Society of America Special Papers, 2009

Scientific Investigations Report, 2011

Scientific Drilling, 2013

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