• Deformation microstructures, mechanisms, and history of a shear zone within the Chugach accretionary complex in the Nelchina area, South-Central Alaska

      Yakimova, Veselina T.; Nadin, Elisabeth; Mezger, Jochen; Regan, Sean (2020-05)
      Ductile-to-brittle fault zones reveal mineralogical processes that are thought to be responsible for the mechanical behavior of faults. I examined a pervasively deformed zone within the Jurassic to Cretaceous accretionary complex of southern Alaska that preserves hydrothermal alteration, dissolution precipitation, carbonaceous material (CM), clay minerals, and intracrystalline plasticity, all of which influence the strength of a fault. I characterized microstructures by SEM and EBSD, determined compositions by XRD, XRF, and Raman spectroscopy for one carbon-rich sample, and dated whole rock, rotated K-feldspar, and metamorphic muscovite by ⁴⁰Ar/³⁹Ar thermochronology to constrain the timing and conditions of accretion, uplift, and deformation recorded by this fault zone. I interpret the specific mineralogy and complex network of deformation microstructures as a result of multiple deformation events. Highest-temperature deformation recorded within the shear zone is lower greenschist facies (400-450°C). Quartz-rich clasts preserve deformation lamellae, grain bulges, sweeping undulose extinction, pressure solution, and brittle fractures characteristic of low grade (300-400°C) at the brittle-ductile transition. Brittle overprint is expressed by fractures cross-cutting the stretched quartz phacoids, and black fault rock that has entrained stretched quartz grains. Raman spectroscopy places precipitation of the CM at ~300˚C. I therefore associate the fault-rock fabrics with progressive down-temperature deformation as the fault was exhumed. I suggest that pressure solution and mineral alteration in all fault-zone samples, as well as quartz and phyllosilicate preferred orientation in a subset of the samples, indicate aseismic slip. Growth of clay and precipitation of CM reduced the friction coefficient, lowering the frictional strength and influencing the dynamic behavior of this fault zone. Constraining the relative timing of the different slip behaviors is hard to determine. It is possible they were active at the same time, especially with the increase of width and complexity at the deeper part of the fault. What is preferentially preserved in the rock record is the latest stage of slip. Pseudotachylite structures generated during earthquakes, however, are rarely preserved due to their susceptibility to alteration. In my field area, consequent exhumation and cooling lead to progressive down-temperature brittle deformation and strong hydrothermal alteration, which could have eradicated any evidence for frictional melting. Using ⁴⁰Ar/³⁹Ar thermochronometry alongside regional and local age constraints, I was able to provide some insight on timing of fault-zone and local tectonic activity. The fault lies between the McHugh Complex and Valdez Group, the two main components of the Jurassic to Cretaceous Chugach accretionary prism whose development and disruption is still debated. I interpret that fault activity lasted from ca. 120 Ma to ca. 60 Ma., and was followed by two stages of accelerated uplift and cooling during ca. 40 Ma and ca. 20 Ma. The cease of major fault activity after ca. 60 Ma, the lack of pervasive strike-slip motion indicators, and the presence of undeformed Eocene dikes as well as Eocene sediments deposited on top of both the McHugh Complex and Valdez Group, suggest they were deposited in proximity and were in place in Southern Alaska at the start of the Eocene epoch.
    • Insights into deep structure and evolution of Alaska based on a decade of observations of shear wave splitting and mantle flow

      Bellesiles, Anna K. (2011-05)
      This thesis covers shear wave splitting results from a decade of temporary networks deployed throughout Alaska. The analysis and interpretation of data from the MOOS (Multidiscipline Observations Of Subduction) and ARCTIC (Alaska Receiving Cross Transect for the Inner Core) PASSCAL (Program for Array Seismic Studies of the Continental Lithosphere) deployments, combined with the previously published BEAAR (Broadband Experiment Across the Alaska Range) results provide anisotropy and flow observations across the state. In south central Alaska, a region dominated by the subduction of the Pacific plate under the North American plate, fast directions are dominantly in the direction of convergence (NNW-SSE), or trench-normal. This is either due to entrained flow below the subducting portion of the Yakutat block, or anisotropy within the block itself. Farther north above the mantle wedge the shear wave splitting results are dominated by fast directions along the strike of the subducting slab (NE-SW), due to along strike flow within the mantle wedge. North of the mantle wedge, fast directions transition into a more NNE to SSW orientation which is the Brooks Range and North slope are in the direction of absolute plate motion.
    • Metal and mineral zoning and ore paragenesis at the Kensington Au-Te deposit, SE Alaska

      Heinchon, Sarah H.; Newberry, Rainer; Severin, Kenneth; Keskinen, Mary (2019-05)
      The Kensington gold mine is a structurally controlled mesothermal gold deposit at the northern end of the Juneau Gold Belt. The Kensington is the only known gold deposit where over 90% of the gold occurs as calaverite (AuTe₂) rather than native gold (Au, Ag). Calaverite and native gold occur as microscopic inclusions in pyrite. Very little geochemical work had been performed on the ore at Kensington. This project generated a large geochemical data set of metals in sulfide concentrations of the ore. To better understand this unique gold deposit I collected and analyzed 214 sulfide concentrates by X-ray fluorescence. The concentrates were collected from various locations, vein types, and sulfide textures to examine possible correlations between the different sample types and between trace metals associated with sulfides and tellurides. Kensington sulfide concentrates have a consistent and strong correlation between Au and Te that indicates 90% of the gold occurs as calaverite. I surveyed 26 polished sulfide grain mounts. There are varying amounts of native gold and calaverite, depending on the presence of non-Au telluride minerals (petzite, hessite, coloradoite, altaite, tellurobismuthite, and melonite) and Cu sulfide minerals (chalcopyrite, tetrahedrite, and bornite). The corresponding sulfide concentrates still had a consistent Au to Te ratio despite the increased observed ratio of native gold to calaverite independent of Cu or non-Au telluride associated elements. The consistent Au to Te ratio indicates a single Au-Te event, however the variable native gold to calaverite ratios and variable inclusion textures in pyrite and Cu-sulfides suggest two Au-Te events: a pre/syn-pyrite and a separate post-pyrite Au-Te event. I used an electron microprobe to generate Co maps of pyrite with varying gold and/or calaverite inclusion textures. There are multiple pyrite growth phases with two distinct pyrites: Co-rich pyrite generations (> 2 weight % Co) and Co-poor pyrite generations (< 10 ppm). Co maps distinctly show multiple generations of native gold and calaverite inclusions relative to pyrite growth. The majority of non-Au tellurides and Cu-sulfide are post-pyrite. There are three main stages of metal mineralization in the Kensington ore: Early barren pyrite; Calaverite, minor native gold, and more pyrite; Cu-sulfides and non-Au tellurides.
    • Thirty Summers and a Winter: U.S. Geological Survey Illustrations

      ; Mertie, Evelyn (Mineral Industry Research Laboratory, School of Mineral Engineering, University of Alaska, 1982)
    • Trace element copper distribution and areal geology in a portion of the Clearwater Mountains, Alaska

      Glavinovich, P.S. (University of Alaska Mineral Industry Research Laboratory, 1967)
      The study concerns that portion of the Clearwater Mountains defined by north latitudes 63' 03' and 63' 08' and west longitudes 147' 09' and 147' 30'. Outcrop within the area consists predominantly of a sequence of intercalated andesitic and basaltic flows. Sedimentary rocks are present but comprise a very small percentage of the total section. Dikes and a small pluton are also present. The prevailing attitude of the volcanic and sedimentary rocks is east-northeast with a consistent north dip. A Triassic age is accepted for the volcanic and sedimentary rocks. Areal and local sampling indicates that all rock types are abnormally high in trace copper content, and average background is 1000 ppm. Copper distribution suggests a syngenetic origin. Frequent small copper deposits crop out along the north side of the area. The deposits are epigcnctic and are structurally controlled. The origin of these deposits may have potential exploration significance.