• A Detailed Structural Analysis Across A Regional Unconformity, Forks Of The Canning River, Franklin Mountains, Northeastern Brooks Range, Alaska

      Ziegler, Jennifer Ann (1989)
      Structural analysis on the northern flank of the 'Franklin Mountains anticlinorium,' northeastern Brooks Range, Alaska, addressed the geometry and sequence of structures, and the deformational mechanics of the Franklinian and Ellesmerian sequences, which are separated by a sub-Mississippian unconformity. The anticlinorium is comprised of two horses of Franklinian sequence rocks in a Cenozoic north-vergent duplex thrust system. South-dipping pre-Mississippian slaty cleavage may have been a plane of preferred failure during ramp formation. Above the unconformity, the Kekiktuk Conglomerate remained attached to pre-Mississippian rocks, deforming with them beneath a roof thrust in the Mississippian Kayak Shale. Increased shear stress and overburden pressure beneath overthrust Franklinian sequence rocks may have led to local detachment near the unconformity surface. Above the Kayak Shale, progressive detachment folding and thrust faulting occurred in the Lisburne and Sadlerochit Groups as a result of emplacement of the two underlying horses. <p>
    • Application Of Argon-40/Argon-39 Chronostratigraphy To Geologic Problems In Yellowstone Caldera And Cook Inlet Basin

      Dallegge, Todd A.; Layer, Paul (2002)
      The 40Ar/39Ar dating method is a useful tool in addressing problems concerning stratigraphic distribution of rock units. This tool is used where previously K-Ar-dated units are problematic, and where further chronostratigraphic control is needed for lavas of the Central Plateau Member (CPM) of the Plateau Rhyolite in Yellowstone National Park and volcanic ash dispersed in the sedimentary units of the Kenai Group in Cook Inlet Basin. Sanidine 40Ar/39Ar ages for CPM rhyolite lavas indicate that xenocrysts, excess argon, and incomplete degassing affected the K-Ar age interpretations. With the 40Ar/39Ar dating method these effects are removed, revealing four apparent eruptive episodes at approximately 135, 122, 107, and 90 ka. The 40Ar/ 39Ar eruption ages are 10's ky younger than U-Th ages from zircon indicating significant residence time of the CPM magma chamber prior to eruption. 40Ar/39Ar dating using multiple analytical runs and statistical assessment of the data produced satisfactory results from the potassium-poor, plagioclase-bearing, tephras of the Kenai Group. The results indicate that excess argon, argon loss, and xenocrystic contamination exist. The chronostratigraphic framework indicates that faulting offsets strata of widely different ages causing repetition of the stratigraphy. Age data indicate that the Sterling and Beluga Formations are time-equivalent strata representing lateral facies variations rather than distinct time-stratigraphic formations. Based on crosscutting relations and structural folding, the dated horizons suggest that structural deformation is no older than early Pliocene in age. Based on the structural complexities affecting the Kenai Group strata, the previously measured coal resource estimates for the Kenai Peninsula are overestimated by 23% in the Clam Gulch area and 50% in the Diamond Creek area. Desorbed gas contents, ranging between 66 and 123 scf/t (daf) from cuttings collected from the Beaver Creek and Kenai Gas Fields, are applied to coal volumes to estimate coalbed methane gas-in-place. CBM in-place estimates suggest values of 93 Tcf for the Kenai Peninsula and 272 Tcf for Cook Inlet Basin. Hypothetical recovery rates suggest 4.7 and 13.6 Tcf for the Kenai Peninsula and Cook Inlet Basin, respectively. The hypothetical CBM recovery rates could continue natural gas supplies to the Anchorage area for an additional 60 years beyond the estimated shortened supply date of 2011.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirement: Adobe Acrobat.
    • Beach ridge geomorphology of Kotzebue Sound: Implications for paleoclimatology and archaeology

      Mason, Owen Kenneth (1990)
      Beach ridges occur on all continents and record the horizontal addition of shoreface beyond the reach of storms. Improved cartographic methods in the nineteenth century allowed British historians to link shoreline changes with abandoned villages. This scientific trajectory was paralleled in the Bering Strait region from the 1880's to the 1930's. In the 1950's J. L. Giddings formalized "beach ridge archaeology" as a survey strategem using relative position to infer relative cultural chronology in northwest Alaska. Modern researchers use archaeological dates and data to document past climates or environments. At Cape Espenberg, on Seward Peninsula, my use of archaeological, stratigraphic, pedological, granulometric and photogrammetric data allows the delineation of 4000 years of coastal evolution. Four chronostratigraphic units are distinguished, using archaeological dates as minimum age assignments. Dune ridges formed in discrete intervals: 3300 to 2000 BP and from 1200 BP to the present; while low, berm ridges are predominant 4000-3300 and from 2000-1200 BP. The two different types of ridges correspond to variable climatic conditions: dune ridges formed after higher storm surges and winter winds while the lower berm ridges are related to less intense storm surges. Coastal dunes at Cape Espenberg are soon altered by plant succession processes with distance from the beach. As primary dunes are eroded, a complex blowout topography results. Erosional processes in blowouts were monitored during 1987-1989, revealing substantial vertical changes, up to 10 cm of erosion per yr. These rapid changes have considerable influence on archaeological site stability. Studies of the gravel ridge systems confirm the proxy storm record apparent in the coastal dunes atop the beach ridges on the Seward Peninsula. The geoarchaeological methodology allows correlations between depositional units within nine of the principal beach ridge and chenier complexes of northwest Alaska. The onset of deposition was at 4000-3500 BP. The complexes at Cape Espenberg and Choris Peninsula contain elevated, broader transgressive ridge sets 3300-2000 BP and from 1100-200 BP, connected with increased storm activity in the North Pacific. Erosional disconformities between successive sets of beach ridges occur at Cape Krusenstern at ca. 3000 BP and before 2000 BP. Between 2000-1000 BP extensive progradation occurred at nearly all complexes, indicating that less stormy conditions predominated.
    • Depositional Environments Of The Lower Triassic Ivishak Formation In The Northeastern Brooks Range, Alaska

      Harun, Nina (1994)
      The Lower Triassic Ivishak Formation in the northeastern Brooks Range represents a progradational wave-dominated delta system. Eight distinct lithofacies were recognized based upon 27 measured sections in the Sadlerochit and Shublik Mountains and at Leffingwell and Bathtub ridges. These lithofacies record a series of progradational episodes beginning in the Kavik Shale, and continuing upward into the Ledge Sandstone Member, and the overlying Fire Creek Siltstone Member. Isopach and correlation diagrams indicate that the paleoshoreline was located in the Sadlerochit and northern Shublik Mountains area, roughly parallel to the present trend of the mountains front. The major dispersal centers were located in the Katakturuk River area of the western Sadlerockit Mountains and in the Marsh Creek area in the eastern Sadlerochit Mountains to Fire Creek area in the eastern Shublik Mountains. The Ivishak Formation contains texturally mature, very fine- to coarse-grained, light gray to light gray-blue, sublitharenite to chert arenite with an average framework: matrix:cement:porosity ratio of $\rm F\sb{75}:M\sb{<1}:C\sb{24}:P\sb1$. The Ledge Sandstone Member was derived from a recycled sedimentary provenance, with an ultimate metamorphic provenance. Compaction and late silica cementation resulted in major porosity destruction. <p>
    • Eruptive Stratigraphy And The Transport And Deposition Of Pyroclastic Material From The Caldera-Forming Eruption Of Volcan Ceboruco, Nayarit, Mexico

      Browne, Brandon Lanquist; Gardner, James (2001)
      The ~1000 A.D. eruption of Volcan Ceboruco produced 3--4 km<super>3</super> of rhyodacitic to dacitic magma erupted as the Jala Pumice, forming a 3.7-km-wide caldera. The tephra sequence consists of alternating Plinian fall and pyroclastic surge deposits, and a series of lithic-rich pyroclastic flow deposits. The latter are known as the Marquesado and North-Flank Pyroclastic Flow deposits. Stratigraphic evidence and results of a comprehensive analysis of accidental lithic and pumice populations constrain the timing of caldera collapse, and the emplacement of the pyroclastic flow deposits to the end of the Jala Pumice. Mass accumulation calculations of particles from fall layers display sedimentation patterns consistent with deposition from dilute and turbulent currents with low particle concentration. Pyroclastic flow and surge deposits likely resulted from density-stratified currents, with a basal region of relatively high particle concentration, and an upper dilute layer that transports particles in turbulent suspension. <p>
    • Evolution of late Cretaceous-early Tertiary depositional sequences in the Beaufort-Mackenzie Basin, Canada

      Myers, Mark D. (1994)
      The Maastrichtian to Lower Eocene rocks of the Fish River and Aklak sequences of Arctic Canada's Beaufort-Mackenzie basin were deposited during the northward migration of a fold and thrust belt across a north-facing passive margin. Detailed outcrop analysis performed in this study has resulted in delineation of the depositional processes, environments and history of systems tracts contained within the Fish River sequence. The Fish River sequence contains Maastrichtian to Lower Paleocene rocks of the Tent Island and Moose Channel Formations of the Fish River Group. This 1900 m thick terrigenous clastic succession consists of channelized conglomerate, sandstone and siltstone overlain by mudstone and capped by a large scale coarsening-upward package of interbedded mudstone, sandstone and conglomerate. Regional unconformities separate the Fish River Group from both the underlying shale of the Cenomanian to Turonian Boundary Creek Formation and the overlying conglomerate, sandstone, mudstone, and coal of the Paleocene to Eocene Aklak Member of the Reindeer Formation. The Fish River Group records a systematic vertical succession of depositional environments including submarine canyon, slope to outer shelf, prodelta, delta front and lower and upper delta plain. The succession of paleoenvironments, regional stratigraphic correlations and relationships, thickness and age control, and bounding unconformities suggest that the Fish River Group records the evolution of a complete type-1, second order, depositional sequence. The Cuesta Creek Member at the base of the Fish River sequence consists of stacked fining-upward channel fills consisting of conglomerate, sandstone and siltstone. The channel fills contain a complex assemblage of high and low density turbidites, current deposits, debris flows, slumps, and slide blocks. These deposits record the complex fill history of a coarse-grained lowstand submarine canyon system. The internal stratigraphic organization of the Fish River sequence records alternating periods of uplift and subsidence in the Beaufort-Mackenzie basin. The scale and timing of these episodes, combined with the environments of deposition and lithologic character of the rocks, suggests that the driving mechanism was alternating episodes of subsidence due to thrusting followed by flexural rebound.
    • Experimental And Petrologic Constraints On Magma Movement, Storage, And Interactions At Two Volcanoes In Katmai National Park, Alaska

      Coombs, Michelle Lynn; Eichelberger, John C. (2001)
      Between 1953 and 1974, ~0.5 km3 of lava and tephra erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest Trident. The eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. The dacite lava flows contain andesitic enclaves as well as compositional banding. Dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ~3.5 wt % H2O, equivalent to 3 km depth. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of the earliest lava by approximately one month. The enclaves in the dacite had experienced a complex history by the time they were erupted. Quantitative analysis of groundmass microphenocrysts in enclaves from the lava shows that the enclaves underwent a textural maturation. I have run experiments that replicate the path taken by andesite during magma mixing in which the andesite was annealed at 1000�C, cooled at various rates to 890�C, held for residence time t, and then quenched. The andesite experimentally cooled at the slower rates (2�C/h and 10�C/h) most resembles enclave groundmass. This is consistent with cooling of the andesite below an andesite-dacite interface, suggesting that pre-enclave formation crystallization caused vapor exsolution and enclave flotation. Decompression experiments on the dacite suggest an average ascent time for the eruption of 30 hours. The high silica rhyolite erupted during the June 1912 eruption of Katmai is notable both for its large volume and evolved composition. Hydrothermal, water-saturated experiments constrain the magma's pre-eruptive storage condition to a region in P-T space between 800�C and 100 MPa and 850�C and 40 MPa. Amphibole is only present in the rhyolite of Novarupta dome, the last product of the eruption. Novarupta dome rhyolite probably was stored under the same conditions but underwent magma mixing with andesite and dacite prior to effusion.
    • Extent, timing, and paleogeographic significance of multiple Pleistocene glaciations in the Bering Strait region

      Heiser, Patricia Anne; Hopkins, David M. (1997)
      This study utilizes a multidisciplinary approach to the investigation of the extent, timing, and potential effects of repeated Pleistocene glaciation in Bering Strait region. A major focus of this study was directed toward testing the hypothesis that a continental-scale ice sheet existed in Beringia during the Late Wisconsin glacial period. Satellite synthetic aperture radar (SAR) imagery was used to compile a map of glacial moraines in Chukotka, Russia, and to attempt preliminary correlations with the glacial record in Alaska. Geophysical modelling of the solid-earth response to postulated glacial loading, and the reconstruction of regional snowline were combined with the results of the SAR investigation to test the ice sheet hypothesis. Finally, a detailed study of the Quaternary stratigraphy and surficial geology of St. Lawrence Island was used to correlate the glacial and sea level histories of western Alaska and Chukotka, Russia. The sequences of moraines in Chukotka, mapped from SAR imagery, are similar in morphology and position to moraine sequences described in Alaska, recording a succession of glacial events that most likely began in the middle Pleistocene and ended with the Late Wisconsin. The record of repeated mountain glaciation, characterized by radial flow out of high topographic areas provides strong evidence against the existence of a southward-flowing, continental-scale ice sheet in Beringia at any time in the latter part of the Pleistocene. Geophysical modelling of the solid-earth response to glacial loading predicted relative sea level changes on the scale of meters to tens of meters (rising or falling depending on forebulge effect) around the shores of present-day Bering Strait if a large ice sheet had, indeed, occupied the Beringia during Late Wisconsin time. There is no evidence of these predicted sea level changes anywhere in the region. The reconstruction of Late Wisconsin snowlines in Russian and Alaska show that the paleoclimatic conditions needed to 'grow' the hypothesized ice sheet did not exist. Field mapping and stratigraphic work on St. Lawrence Island revealed that ice advanced onto the island twice in the late Pleistocene, once in the Middle Pleistocene and once after the Last Interglacial, probably during the Early Wisconsin. The record of glaciers advancing from Chukotka onto the island provides an important 'Rosetta Stone' for correlating the glacial histories of northeast Siberia and Alaska.
    • Fluvial and hillslope geomorphology of Hoseanna Creek Watershed, central Alaska

      Wilbur, Stephen Crawford (1995)
      Hoseanna Creek Watershed is rapidly eroding and provides excellent opportunities to describe and quantify hillslope and fluvial processes in the subarctic and in discontinuous permafrost terrain. High landslide and badland densities occur due to asymmetric geologic structure and weakly consolidated lithologies. Late Quaternary regional glaciofluvial processes and tectonism have changed local base level at least 100 m, inducing headward incision through weak lithologies and yielding high rates of sediment production. Earthflows, translational blocks, rotational blocks, lateral spreads or complex landslide types form in coal-bearing formations in response to lateral corrasion of toes by avulsing streams or to undermining of foot areas by headward incising streams. Slides undergo episodic resurgent activity when new lithostatic or hydrostatic thresholds are reached. Average horizontal displacement rates of seven slides monitored between 8/85 and 9/88 ranged from 0.2 m/yr in rotational blocks to 48 m/yr in the earthflows. Although unique sliding mechanisms are not apparent, permafrost and subarctic climate generate delays or catalysts for failure atypical of warmer climates. Freezing/thawing fronts affect soil strength and permeability; break-up/freeze-up processes affect the timing of water supply to the slide mass and affect development of aufeis-related ground-water pore pressures. Aspectual and lithologic variations combine to yield three geohydrologic subbasin types which govern discharge ranges. Regressions were performed on multiple sets of sediment-discharge (Ts-Q) data. Regression variance (r$\sp2$) was found to have a maximum natural threshold indicative of intrinsic variability. Wide ranges in Q (0.001 to 2350 cfs) and Ts (0.005 to 1600 g/l) necessitated log-log scales and power functions. Each geohydrologic subbasin has a unique Ts-Q relationship termed here the mean sediment concentration potential Cp. Systematic differences in regression parameters indicate that variations in spatial conditions define Cp, while systematic changes in Ts-Q regression residuals R (termed here the maintenance rate R$\sp*$) describe the temporal variability of Ts through time with respect to Cp. 50-95% of the annual sediment load is transferred during less than 3% of the year. Erosion rate indexes were established from peak load estimates; Nenana Gravel basins are eroding 260 times faster than schistose basins and ten times coal-bearing basins.
    • Geology and timing of zinc-lead-silver mineralization, northern Brooks Range, Alaska

      Werdon, Melanie Beth; Newberry, Rainer J. (1999)
      The north-central and northwestern Brooks Range of Alaska hosts widespread Carboniferous Zn-Pb-Ag +/- Ba shale-hosted massive sulfide (Sedex) deposits, and Zn-Pb-Ag +/- Cu vein-breccia and disseminated sulfide occurrences. The Sedex deposits are hosted by black carbonaceous shale and siliceous mudstone of the Mississippian to Pennsylvanian Kuna Formation and are spatially associated with minor (e.g. Red Dog) to locally abundant (e.g. Drenchwater) volcanic and hypabyssal intrusive rocks. The vein-breccia and disseminated sulfide occurrences show no obvious igneous association and are hosted by a deformed but only weakly metamorphosed package of Upper Devonian to Lower Mississippian mixed continental and marine elastic rocks (the Endicott Group). Textural, mineralogical, isotopic, chemical, and fluid inclusion data indicate that sulfides, quartz, and lesser carbonates in the Kady vein-breccia and disseminated sulfide prospect were deposited from slightly acidic, low salinity, carbon-destructive, relatively oxidized, low temperature (<250�C) hydrothermal fluids, under evolving chemical conditions (i.e. decreasing temperature and pressure, and increasing pH, fo2, fs2). The lack of known Sedex mineralization in the north-central Brooks Range and the presence of sulfide mineralization within the Endicott Group suggests that Kady represents the hydrothermal fluid pathway below a failed or non-existent Sedex system. Trace element analyses of volcanic rocks and 40Ar/ 39Ar laser step-heating ages indicate the following geologic history for the north-central and northwestern Brooks Range: within-plate alkaline volcanic rocks at Red Dog and Drenchwater were emplaced from approximately 344 Ma to 336 Ma in a continental extensional environment. This presumably set up an elevated geothermal gradient, which heated basinal fluids. Sedex mineralization is estimated to have formed between 337 and ~314 Ma by basinal dewatering. 40Ar/39Ar ages of recrystallized white mica in Upper Devonian sandstone adjacent to large sulfide-bearing vein-breccia zones fall within the independently estimated time frame for Sedex mineralization. Tholeiitic gabbro magmatic activity occurred around 276 +/- 15 Ma. The transition with time from within plate alkaline to tholeiitic magmatism suggests progressive episodic extension in a continental basin.
    • Hydrothermal history of the Long Valley Caldera, California: Life after collapse

      Mcconnell, Vicki Sue (1995)
      Drilling of the Long Valley Exploratory Well (LVEW) on the resurgent dome in the 760 ka Long Valley Caldera opened a window to view the geologic history of the central caldera. Stratigraphic relationships indicate piston/cylinder (Valles-type) collapse for this caldera, and a resurgent structure intimately linked to post-caldera-collapse rhyolitic intrusions. Samples recovered from this and other wells proximal to the resurgent dome were characterized through isotope microanalytical techniques, petrographic and microprobe study, and analysis of fluid inclusions within alteration minerals. This work revealed the complexity of primary magmatic and secondary hydrothermal activity involved in the formation of a resurgent dome. Measurements of the $\rm\delta\sp{18}O$ composition of silicate components forming the intracaldera lithologies display disequilibrium within samples as a result of variable exchange with hydrothermal fluids. A maximum calculated temperature of $350\sp\circ\rm C$ at 1800 m depth in LVEW indicates paleohydrothermal temperatures exceeded the known present-day hydrothermal conditions by more than $100\sp\circ\rm C.$ Contouring of $\rm\delta\sp{18}O$ values from wells on a line crossing the caldera define a pattern of convective flow with upwelling beneath the resurgent dome. Although surface volcanism at the LVEW site ended about 650 ka, laser probe $\rm\sp{40}Ar/\sp{39}Ar$ microanalysis of samples from sill-like intrusions into the intracaldera ignimbrite reveals intrusive events at ${\sim}650$ ka, ${\sim}450$ ka, and ${\sim}350$ ka. Sanidine phenocrysts from the Bishop Tuff at 1772 and 1792 m depths and whole rock samples of the Mesozoic metavolcanic basement rocks at 1957 m depth record times of disturbance by hydrothermal pulses at ${\sim}530$ ka and ${\sim}350$ ka. Repeated emplacement of intrusions into the centrally located caldera ignimbrite was a primary process of resurgence. In turn, the feeders for the intrusions and the intrusions themselves supplied heat for resurgent-dome-centered hydrothermal flow. After approximately 300 ka, all activity shut off in the central caldera only to resume at ${\sim}40$ ka in response to renewed Holocene volcanic activity in the West Moat. Geophysical evidence of recent intrusive activity beneath the resurgent dome indicates this shallow magma emplacement mechanism is not totally extinct in the central caldera. Most likely a new cycle of volcanism and hydrothermal circulation is underway as the caldera matures.
    • Ice -wedge networks and "whale-hole" ponds in frozen ground

      Plug, Lawrence J.; Hopkins, David M. (2000)
      The patterns of ice-wedge networks and of whale-hole ponds in frozen ground self-organize by strong interactions between pattern elements. Mechanisms for the consistent spacing (15--25 m) and orientation between ice wedges are examined in a model encapsulating the opening of fractures under a combination of thermally-induced tensile stress, stress reduction near open fractures, and heterogeneity of frozen ground and insulating snow. Modeled networks are similar to ice-wedge networks on the Espenberg coastal plain, Bering Land-Bridge National Park, Alaska, at the level of variation among Espenberg networks, as indicated by: (i) comparisons of distributions of relative orientation and spacing between wedges; and (ii) application of nonlinear spatial forecasting to modeled and Espenberg network patterns. Spacing in modeled networks is sensitive to fracture depth and weakly sensitive to thermally-induced tensile stress and substrate strength, consistent with the narrow range of spacing between natural ice wedges in different regions. In an extended model that includes recurring fractures over thousands of winters, networks similar to natural ice-wedge networks form. The annual pattern of fractures diverges from the ice-wedge pattern, with only &frac12;--&frac34; of wedges fracturing in a single year at a steady-state reached after approximately 103 y. Short-lived sequences of extreme stress from cooling can permanently alter the spacing between and the fracture frequency of modeled ice wedges, suggesting that the existence and characteristics of existing and relic natural ice-wedge networks reflect extreme, not mean, climate conditions. Ponds on the Espenberg beach-ridge plain, approximately 2 m across and 1 m deep and surrounded by raised rings of ice-rich permafrost 2 m across and 0.5 m high, form through an interplay between localized bacterial decomposition of peat, thawing of frozen ground and frost heaving of peat in rings. Groups of hundreds of ponds at Espenberg assemble through time because new ponds are favored to form adjacent to raised rings around existing ponds. The nonlinear behavior that results from strong interactions in patterns of ice-wedge networks and in ponds suggests general limitations in the application of linear approaches to inferring the response of geomorphic systems to changes in forcing, such as climate change.
    • Mid -Cretaceous plutonic -related gold deposits of interior Alaska: Characteristics, metallogenesis, gold-associative mineralogy and geochronology

      Mccoy, Daniel Thomas; Newberry, Rainer J. (2000)
      Mid-Cretaccous gold deposits in interior Alaska are hosted in or near apices of low magnetite plutons that formed in a broad continental arc. Ore is hosted in (1) anastomosing quartz veins with potassic or albitic envelopes, (2) planar veins and shear zones with sericitic alteration, and (3) pyroxene-rich skarn deposits. This study was undertaken to constrain the fluid and metal source and composition, formation conditions, gold associative mineralogy, age relationships, and areal extent of this mineralizing event Techniques included reflected light petrographic, 40Ar/39Ar step-heating, stable isotope, fire assay, Mossbauer spectroscopy, electron microprobe, and scanning ion mass spectroscopy analysis. Results suggest ages between 85 Ma and 107 Ma with a 0 to 2 million-year differential between magmatic biotite and hydrothermal veins in the same deposits. Deposits are 10 to 20 million years younger than local metamorphism. Fluid calculated stable isotopic ratios (delta13C = -9 to -10 per mil; delta18O = 5--10 per mil; deltaD = -47 to -100; delta34S = -5 to +5 per mil) suggest gold precipitated from magmatic fluids. Fluid inclusions in ore-bearing quartz contain high CO2 with trapping temperatures and pressures of 270� to 570�C and 0.5 to 2 kb respectively. The Fort Knox and Pogo deposits have a strong Au-Bi association and high relative amounts of potassic; and albitic alteration with mineralogical evidence for the original existence of maldonite (Au2Bi) or Au-Bi melt subsequently overprinted by native gold + bismuthinite. The True North deposit has a strong Au-As association and no Au-Bi association. It lacks potassic or albitic alteration and contains only sub-micron gold, approximately half chemically bound to arsenopyrite or arsenian pyrite. The Dolphin and Ryan Lode deposits are intermediate in Au-Bi association, gold-associative mineralogy and alteration features. Arsenopyrite geothermometry yield temperatures between 300� and 630�C for albitic and potassic alteration and between 250� and 420�C for sericitic alteration. 40Ar/39Ar dating and metal ratios suggest that gold mineralization is (1) solely mid-Cretaceous in the Fairbanks mining district, (2) mid-Cretaceous and late Cretaceous in the Kantishna mining district, and (3) mid-Cretaceous and early Tertiary in the Livengood, district.
    • Origin, character, application and correlation of tephra partings in tertiary coal beds of the Kenai Peninsula, Alaska

      Reinink-Smith, Linda Margareta; Hopkins, David M. (1989)
      Volcanic and non-volcanic partings occur in coal beds of the Neogene Beluga and Sterling Formations along the shores of the Kenai lowland, Alaska. The partings were systematically characterized to determine their potential geological applications: Two-thirds of the partings originated as air-fall tephra. Of these, partly altered, Pliocene tephra typically contain volcanic glass + feldspar $\pm$ montmorillonite $\pm$ quartz $\pm$ kaolinite $\pm$ opal-CT. Highly altered Miocene partings are characterized by feldspar $\pm$ kaolinite $\pm$ montmorillonite $\pm$ quartz $\pm$ crandallite $\pm$ altered volcanic glass, where crandallite appears to have formed by replacement of volcanic glass prior to clay formation. About one-third of the partings are of detrital origin and contain detrital chlorite + illite + smectite + quartz $\pm$ feldspar $\pm$ siderite $\pm$ kaolinite. A Pliocene pumice parting near the top of the Sterling Formation was correlated from the northwestern to the southeastern Kenai lowland on the basis of similar glass morphologies, an absence of opaque minerals, and geochemical similarities. A crystal-tuff near the middle of the section could be traced across the Kenai lowland as one or two ash-falls, based on inertinite contents of adjacent coal, mineralogy, and geochemistry. Some other prominent tephras could not be correlated. The tephra partings are time-equivalent to DSDP cores from the Gulf of Alaska and along the Aleutian Island chain. Tephras occur every 125-500 yr in the lower part of the Beluga Formation, and their deposition probably coincides with a volcanic pulse 10.5 m.y. ago. This pulse is not well recorded in nearby DSDP cores. In the upper part of the Beluga Formation, during volcanic quiescence, tephras are recorded at an average rate of one every 9,000 yr. Time equivalent DSDP cores show a near absence of tephras. A volcanic pulse occurred during the deposition of the lower Sterling Formation, about 7.5 m.y. ago, with intervals between volcanism which averages 11,000 yr or longer. Volcanic sources appear to have been distant, which is consistent with an absence of tephra layers in a Gulf of Alaska core. About 5 m.y. ago, concurrent with the deposition of the upper Sterling Formation, the thicknesses of the tephra layers dramatically increase and the frequency increases to an average of one tephra every 2,000 years. This increase is recorded in DSDP cores as well.