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  • Porphyry copper, copper skarn, and volcanogenic massive sulfide occurrences in the Chandalar copper district, Alaska

    Nicholson, Lisa; Keskinen, Mary (1990-05)
    Metamorphosed porphyry copper, copper skarn, and volcanogenic massive sulfide (VMS) occurrences have been found in 5 key prospects within Devonian rocks of the Chandalar copper district, Alaska. The Venus, Victor, Eva, and Evelyn Lee prospects contain "proximal" porphyry copper/copper skarn mineralization, whereas the Luna prospect contains "distal" Cu-Zn skarn and Cu-Zn VMS mineralization. Porphyry copper mineralization is recognized by granodiorite composition meta-intrusives; zoned potassic, sericitic and propylitic alteration; and del34S values of -1.5 to -0.6 per mil. Skarns consist of andraditic garnet (Ad30-100) and diopsidic pyroxene (Hd9-46), and have del34S values of -4.7 to -1.1 per mil. Alteration types in intrusive rocks and adjacent skarn are generally compatible. VMS occurrences contain chloritic and silicic alteration, and massive sulfides have del34S values of -0.8 to 6.9 per mil, consistent with values from known Devonian VMS deposits.
  • The kinetics of glucose limited growth by a marine yeast

    McNab, Allen David (1969-05)
    The kinetics of glucose limited growth by a marine yeast, shown to be a Rhodotorula species, have been studied in a continuous culture apparatus. The saturation constant, in synthetic media, has been calculated to be 0.25 mg/l, on the assumption that saturation kinetics are followed, The maximum growth rate was determined in both synthetic media, and artificial sea water. On the basis of inhibition kinetics, the kinetic behavior of this yeast in the marine environment has been predicted. The effect of temperature on the maximum growth rate has been determined and, on the assumption of a similar effect on the saturation constant, the saturation constant has been postulated to be in agreement with similar values determined for other microorganisms.
  • The changing vista of the northern Northwest Coast Indian Deer Ritual

    Austin, Kenneth Frank (1999-08)
    From time immemorial until the start of the 20th century, when disputing Tlingits decided to end a conflict, Tlingit clan leaders and elders met in council and negotiated an equitable peace settlement. After reaching a satisfactory negotiation, a peace dance took place to validate the settlement. Besides the Tlingits, the neighboring Indian groups in Southeast Alaska and British Columbia practiced this custom. When the European and Western powers assumed governance, the deer ritual--a judicial function of the Pacific Northwest Coast Indians--was modified, and new forms appeared. Presently, while elders know their regional history, many do not remember the protocol and formalities of the rite that was performed. This thesis undertakes a step into the past when the rite had an active and viable purpose in settling disputes and validating agreements
  • Modeling of Arctic stratus cloud formation and the maintenance of the cloudy Arctic boundary layer

    Zhang, Qiuqing; Stamnes, Knut; Harrington, Jerry; Sentman, Davis; Watkins, Brenton (1999)
    The formation of Arctic stratus clouds (ASCs) and the maintenance of the cloudy Arctic boundary layer are studied with two models: a one-dimensional radiative-convective model and a three-dimensional large eddy simulation (LES) model. The one-dimensional radiative-convective model consists of a comprehensive radiative module, a cloud parameterization with detailed microphysics and a convective adjustment scheme. The model is designed specifically for studying ASC formation. With this model, the roles of radiation and cloud microphysics in the formation of ASCs and multiple cloud layers are investigated. The simulations reproduce both single and multiple cloud layers that were observed with inversions of temperature and humidity occurring near the cloud top. The detailed cloud microstructure produced by the model also compares well with the observations. The physics of the formation of both single and multiple cloud layers is investigated. Radiative cooling plays a key role during the initial stage of cloud formation in a atmosphere. It leads to a continual temperature decrease promoting water vapor condensation on available cloud condensation nuclei. The vertical distribution of humidity and temperature determines the radiative cooling and eventually where and when the cloud forms. The observed temperature inversion may also be explained by radiative cooling. The three-dimensional LES model is adopted to evaluate the one-dimensional model, especially the convective adjustment scheme. The advantages and limitations of the one-dimensional model are discussed. The LES results suggest that the convective adjustment scheme is capable of capturing the main features of the vertical heat and moisture fluxes in the cloudy Arctic boundary layer. The LES model is also used to investigate the maintenance of the cloudy Arctic boundary layer. The turbulence in the cloudy Arctic boundary layer is primarily maintained by the buoyancy effect due to the cloud top cooling. It is found that weak large scale downward motion aids in cloud development and maintenance.
  • Small spatial and fast temporal ionosphere -magnetosphere coupling processes

    Zhu, Hua; Otto, Antonius (2000)
    I have developed a two-dimensional, three-fluid model (electrons, ions and neutrals) to simulate small-scale magnetosphere-ionosphere coupling processes. The code includes ionization and recombination processes, the Hall term in Ohm's law, and various heat sources in the energy equations. The electro-dynamic response and the evolution of the collision frequencies are treated self-consistently in a height resolved ionosphere. The model allows for the propagation of Alfven waves. The simulation is particularly suited for fast temporal variations and small spatial scale ionospheric structures associated with filamentary aurora and ionospheric heating experiments (e.g. HAARP). I have investigated the evolution of field-aligned currents in the magnetosphere-ionosphere system and found several notable effects---ion heating due to plasma-neutral friction, electron heating resulting from energetic particle precipitation and ohmic dissipation by strong field-aligned currents. The simulation of plasma. heating in the ionosphere is motivated by a specific auroral event that was simultaneously observed with optical and radar instruments. The results indicate that a consistent explanation of this event requires ohmic heating of electrons in a strong field-aligned electric current layer. They suggest strongly that the observed sequence of events can be explained only if spatial structure is present in the ionosphere so that it requires at least a two-dimensional model. Electron heating in strong field-aligned currents also provides a mechanism to deposit energy in the F-region of ionosphere and thus can explain the formation of tall auroral arcs. The simulation of the formation of field-aligned currents shows a strong plasma density depletion in the region of downward field-aligned current layer. The depletion is due to the divergent flow of the plasma. Similarly, the plasma density increases in the region of upward field-aligned current because of the convergent plasma motion. A modification of the ionospheric conditions by localized particle precipitation has an interesting effect. At the edge of the precipitation region, a new field-aligned current filament is formed. Finally, the simulation code is not limited by steady state assumptions commonly used for the Hall model and Pedersen conductivities.
  • 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.
  • Cloud and surface properties and the solar radiation budget derived from satellite data over the Arctic Ocean: Comparisons with surface measurements and in situ aircraft data

    Xiong, Xiaozhen; Stamnes, Knut (2000)
    Use of satellite data to study the surface and cloud properties and the solar radiation budget (SRB) is very important for improving our understanding of cloud and sea-ice albedo feedback in the Arctic. Based on an accurate and comprehensive Radiative Transfer Model (RTM), algorithms were developed for using the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data for the discrimination of cloud from snow/ice surfaces, retrieval of snow surface properties and surface albedo, and retrieval of cloud optical depth (tau) and effective droplet size ( re). Through the improved estimation of solar reflectance in AVHRR channel 3 (3.75 mum) and atmospheric anisotropic correction, a threshold function was found and used for developing an automatic cloud discrimination algorithm over snow/ice surfaces. Thin cirrus was discriminated using the brightness temperature difference between AVHRR channels 4 and 5 and brightness temperature in channel 4. Retrieval of snow grain size and mass-fraction of soot from AVHRR is difficult because of the effects of aerosol in channel 1 and the strong water vapor absorption in channel 2. Retrieval of surface albedo is more promising, but, with the melt of snow/ice, different narrow-to-broadband conversion relations should be used to derive broadband albedo. AVHRR channels 2, 3 and 4 are used to retrieve tau, r e and cloud top temperature simultaneously. Validation of these algorithms with in-situ aircraft measurements by the NCAR C-130 and the NASA ER-2 and with surface measurements obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment indicates that the retrieved re is close to the "true" value of re, but the retrieved tau tends to be overestimated. Uncertainties of cloud retrievals with regard to cloud cover fraction, vertical inhomogeneity, multi-layer stratification and cloud phase were examined. Inter comparison of different satellite data demonstrates that NOAA-14 AVHRR data for SHEBA is overestimated by 10--20% using the calibration by Rao and Chen (1996). Finally, seasonal variation of surface albedo, cloud properties and SRB over SHEBA was derived based on 1 or 2 AVHRR overpasses per day from April to August, 1998.
  • Till deformation beneath Black Rapids Glacier, Alaska, and its implication on glacier motion

    Truffer, Martin; Harrison, W. D. (1999)
    The motion of a glacier is largely determined by the nature of its bed. The basal morphology and its reaction to the overlying ice mass have been subject to much speculation, because the glacier bed is usually difficult to access, and good field data are sparse. In spring 1997 a commercial wireline drill rig was set up on Black Rapids Glacier, Alaska, to extract cores of basal ice, subglacial till, and underlying bedrock. One of the boreholes was equipped with three tiltmeters to monitor till deformation, and a piezometer to record pore water pressure. The surface velocity and ice deformation in a borehole were also measured. The drill successfully reached bedrock twice after penetrating a till layer, some 5 to 7 m in thickness, confirming an earlier seismic interpretation. The till consisted of a sandy matrix containing clasts up to boulder size. Bedrock and till lithology indicated that all the drill holes were located to the north of the Denali Fault, a major tectonic boundary along which the glacier flows. The mean annual surface velocity of the glacier was 60 ma-1 , of which 20 to 30 ma-1 were ice deformation, leaving 30 to 40 ma-1 of basal motion. The majority of this basal motion occurred at a depth of more than 2 m in the till, contradicting previously held ideas about till deformation. Basal motion could occur as sliding of till over the underlying bedrock, or on a series of shear layers within the till. This finding has implications for the interpretation of the geologic record of former ice sheets, for geomorphology, and for glacier dynamics. The effect of a thick till layer on ice flow and on quantities observable at the glacier surface was calculated. These include velocity changes on secular, seasonal, and shorter time scales. A mechanism for uplift events and dye tracing responses was suggested. An easy surface observation that could serve to clearly distinguish a glacier underlain by till from the more traditional view of a glacier underlain by bedrock could not be identified.
  • Predator-prey dynamics between mountain lions and mule deer: Effects on distribution, population regulation, habitat selection, and prey selection

    Pierce, Becky Miranda; Bowyer, R. T.; Bleich, V. C. (1999)
    Mountain lions (Puma concolor) and mule deer ( Odocoileus hemionus), which share a winter range in the Eastern Sierra Nevada in Round Valley, California, USA, were fitted with radio-telemetry collars and tracked to determine their movements and cause of mortality. The mountain lion population of Round Valley refers to a group of individuals that lived in close proximity to one another, essentially isolated from similar groups during the winter, and fed on the migratory herd of mule deer that overwinter in Round Valley. Mountain lions migrated seasonally with the deer population, and two distinct patterns for coping with variability in abundance of prey were observed. The unique migratory behavior identified for the mountain lions in this study indicates a more flexible social system for mountain lions than previously described. Tests of whether the presence of another mountain lion affected where individuals to killed deer indicated that social interactions had no effect and that social behavior was not regulating the population of mountain lions via spatial partitioning of prey. Examination of habitat selection by mule deer and mountain lions revealed that mule deer selected habitat at higher elevations (P < 0.001) with more bitterbrush ( Purshia tridentata) and less rabbitbrush (Chrysothamnus nauseosum ) than random locations. Mountain lions killed deer in relatively open areas with more desert peach (Prunus andersonii) than locations in which deer foraged. Those results indicated that deer were not confronted with a tradeoff in terms of habitat selection on the winter range because habitat with the best forage (e.g. bitterbrush), also provided the least predation risk. Comparisons of mule deer killed by mountain lions, coyotes, and automobiles indicated that mountain lions selected young (<1 year old) deer and both predators selected older age classes among adults. Furthermore, there was no selection by either predator for animals in poor condition. Among mountain lions in different social categories, female mountain lions with kittens selected more young deer than did other social categories. This study indicated that ambush predators (mountain lions) may be as selective for prey as coursing predators (coyotes) and that lactation in mountain lions may play a role in determining prey selection. ion.
  • 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.
  • Seismic investigations of subsurface volcanic structures and processes at Mount Spurr, Alaska and Soufriere Hills Volcano, Montserrat, West Indies

    Power, John A.; Wyss, Max (1998)
    Seismological techniques are used to infer the subsurface structures and volcanic processes at two recently active volcanoes: Mount Spurr, Alaska, and Soufriere Hills Volcano, Montserrat, West Indies. The three-dimensional P-wave velocity structure of Mount Spurr is determined to depths of 10 km by tomographic inversion of 3,754 P-wave arrival times from local earthquakes. Results show a prominent low-velocity zone beneath the southeast flank of Crater Peak extending from the surface to 3--4 km below sea level, spatially coincident with an active geothermal system. Beneath Crater Peak an approximately 3-km-wide zone of relatively low velocities correlates with a near vertical band of seismicity, suggestive of a magma conduit. No large low-velocity zone indicative of a magma chamber occurs within the upper 10 km of the crust. In the three years bracketing the 1992 eruptions of Mount Spurr's Crater Peak vent, approximately 2,500 located events were classified as Volcano-Tectonic (VT) earthquakes, Long-Period (LP) events, or Hybrid events. An unusual mix of VT, LP, and hybrid events at 20 to 40 km depth began coincident with the onset of unrest and peaked shortly after eruptive activity ended. The classified seismic events are combined with geophysical and geological data to develop a simplified model of the magmatic plumbing system of Mount Spurr. The major components of this model are a deep magma source zone at 20--40 km depth, a smaller storage zone at about 10 km depth, and a pipe-like conduit that extends to the surface. The frequency-magnitude distribution of earthquakes measured by the b-value is determined as a function of space beneath Soufriere Hills Volcano, from data recorded between August 1, 1995 and March 31, 1996. A volume of high b-values (b > 3.0) with a 1.5 Ian radius is imaged between 0 and 1.5 Ian beneath English's Crater and Chance's Peak. This anomaly extends southwest to Gage's Soufriere. At depths greater than 2.5 km, volumes of comparatively low b-values ( b <math> <f> ~</f> </math> 1) are found beneath St. George's Hill, Windy Hill, and below 2.5 kin to the south of English's Crater.
  • Single and multiple electromagnetic scattering by size -shape distributions of small nonspherical particles

    Schulz, Frank Michael (1998)
    A comprehensive model for light scattering by size-shape distributions of randomly oriented nonspherical particles is developed. The model uses spheroids as model particles. The vector Helmholtz equation is solved with a new separation of variables (SVM) approach that allows one to calculate the ensemble-averaged single scattering optical properties of ensembles of randomly oriented particles analytically. Since the use of the SVM in spheroidal coordinates properly accounts for the geometry of the particles, the method is applicable to a large range of shapes ranging from elongated prolate needles via spheres to flat oblate disks. The relation between geometric symmetries of particles and symmetry relations of the electromagnetic scattering solution is investigated systematically in the general framework of the theory of point groups. The results are exploited in the model for increasing the computational efficiency. A comprehensive vector radiative transfer model is in part developed in this work. This radiative transfer model takes the output of the single scattering model as input and computes the Stokes vector components in a vertically inhomogeneous, plane parallel medium as a function of polar and azimuth angle and as a function of optical depth. The single scattering model is applied to investigate the impact of particle shape on the optical properties of size-shape distributions of randomly oriented particles, such as aerosol layers or ice clouds in the atmosphere. The optical properties are found to be much more sensitive to a variation in the effective aspect ratio than to a variation in the effective variance of a shape-distribution. The results of this study are used as input to the vector radiative transfer model in order to study the shape-sensitivity of the radiation field in a macroscopic medium containing a size-shape distribution of randomly oriented particles. It is found that both the radiance, and the degree of linear polarization, and the degree of circular polarization are strongly shape-sensitive in most viewing directions.
  • Twentieth century Inupiaq Eskimo reindeer herding on northern Seward Peninsula, Alaska

    Simon, James Johnson Koffroth; Schweitzer, Peter P. (1998)
    Domesticated reindeer were introduced to Alaska from the Russian Far East at the end of the nineteenth century as a project in social engineering designed to assist in the assimilation of Alaska Natives into Euroamerican society. Most previous discussions of Alaska Native reindeer herding have focused on reindeer introduction as an agent of culture change associated with culture contact and economic modernization. This diachronic study of more than a century of Bering Strait Inupiaq reindeer herding, however, demonstrates that reindeer herding was incorporated into traditional Inupiaq culture and society to the extent that it now helps to maintain and reproduce traditional Inupiaq values and social relations. Inupiaq reindeer herding emerged as a result of the previous experience the Bering Strait Inupiat had with the intercontinental trade of Chukchi reindeer herding products prior to reindeer introduction. Bering Strait Inupiat were already aware of the economic potential of reindeer herding, such that reindeer herding was incorporated into traditional Inupiaq conceptions of property, wealth, prestige, social organization, subsistence, and land use practices. This incorporation provided the opportunity for the Bering Strait Inupiat to improve standards of life during a period of rapid social change associated with increasing Euroamerican influences. Furthermore, it also provided a means to maintain Inupiaq cultural identity through the emergence of reindeer umialiks and through the importance of reindeer herding in maintaining traditional social relations. In effect, reindeer herding became part of Bering Strait Inupiaq traditional culture through its importance to Inupiaq cultural reproduction.
  • Three-dimensional diving behavior of ringed seals

    Simpkins, Michael A.; Kelly, Brendan P. (2000)
    The three-dimensional movements of 13 freely diving ringed seals were recorded during the spring of 1990, 1991, 1992, 1996, and 1997 in the Canadian Arctic near Resolute Bay, Nunavut. These data were used to investigate the diving behavior of ringed seals more fully than was possible using previous data, which only recorded the vertical movements of diving animals (time-depth data). During a third of all dives, ringed seals focused much of their effort within a reduced volume, suggesting local search behavior within patches of prey. Local search occurred during descent, ascent, and bottom phases (time spent at depth between the end of descent and the beginning of ascent) of dives, but local search most commonly occurred during bottom phases. Location data from five seals were detailed enough to allow analysis of three-dimensional movements within individual dives. Behaviors were defined for the dives of these five seals based on the character of movements within the dives. Ringed seal dives included horizontally convoluted, travel, and exploration dives, but vertically convoluted, rest, and sit-and-wait foraging dives were not observed. Horizontally convoluted (presumed foraging), travel, and exploration dive behaviors were defined with similar frequency for V-shaped dives (dives with only descent and ascent phases) and U-shaped dives (dives with descent, bottom, and ascent phases). The lack of behavioral differences between dives with distinct time-depth profiles suggested that time-depth profiles were not a reliable means of classifying behavioral dive types for ringed seals.
  • The evolution of higher education in Zimbabwe

    Maunde, Raymund Zaranyika; Barnhardt, Ray (2000)
    This study examines the origins and development of higher education for the indigenous peoples in southern Africa as a whole, while focusing on the evolution of higher education in Zimbabwe in particular. The study examines the role that higher education plays in a developing social, economic and political context by reviewing the relevant literature on the history of higher education in southern Africa and conducting a survey of the current status of emerging higher education institutions in Zimbabwe. The government of Zimbabwe is pursuing multiple avenues of public-private co-operation in providing higher education in response to the growing demand from its citizens. The fieldwork included interviews with government officials and an extended visit to each of the four major new public and private universities in the country, during which focused interviews were conducted with university officials and relevant documents were obtained. The first generation of universities in Africa is being reassessed and new institutions are being created as a result of changes that have occurred in the world, in Africa and in the universities themselves. Internationally, the emergence of global markets has created a competitive world economic system characterized by rapid knowledge generation and technological innovation. Therefore the African universities are not evolving in isolation. They are becoming an integral part of the world university systems. This study documents the reciprocal relationship between the structure and function of educational institutions and the time and place in which they are situated. The current explosion of new higher education institutions across Zimbabwe is clearly a product of its historical and contemporary evolution as an independent country. At the same time, it is apparent that Zimbabwe's future as a player in the family of global nations is increasingly dependent on a strong and responsive system of higher education institutions focusing on the needs of the country and its citizens. Zimbabwe's future as a nation and the future of its higher education institutions are inextricably linked.
  • 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.

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