• 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.
    • 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.
    • 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.
    • The 1996 Eruption Of Karymsky Volcano, Kamchatka: Detailed Petrological Study Of A Single Basalt -Triggered Eruption Cycle

      Izbekov, Pavel Edgarovich (2002)
      The current activity at Karymsky Volcano, Kamchatka, began on January 2, 1996, with simultaneous eruptions from two vents located 6 km apart: Karymsky summit vent, which erupted andesite, and a newly formed vent within Academy Nauk caldera, which erupted basalt. Detailed petrologic study of volcanic ash, bombs, and lavas of Karymsky erupted during 1996--1999 provides evidence for basaltic replenishment at the beginning of the eruptive cycle, as well as a record of compositional variations within the Karymsky magma reservoir induced by basaltic recharge. Shortly after the beginning of eruption the composition of matrix glasses of Karymsky tephra became more mafic, and then, within two months, gradually returned to its original state and remained almost constant the following three years. Further evidence for basaltic replenishment includes the presence of xenocrysts of basaltic origin in andesites erupted from Karymsky. A conspicuous portion of plagioclase phenocrysts in Karymsky andesites contain calcic cores, the composition and texture of which mimic those in Academy Nauk basalt. The earlier portions of andesite also contain rare xenocrysts of olivine, which occur as relicts in plagioclase-pyroxene aggregates. The compositions of olivine xenocrysts match those of olivines in Academy Nauk basalt. Compositional variations of glass and the presence of xenocrysts indicate that Karymsky magma reservoir was recharged by basalt at the onset of the 1996 eruptive cycle. The mixing of basalt with host andesite was both thorough and rapid, perhaps due to a modest contrast in temperature, viscosity, and density between the magmas. Academy Nauk basalt contains granophyre xenoliths, the whole-rock compositions of which are identical to that of dacites erupted twice at 40,000 yr. BP and 7,900 yr. BP, and formed the neighboring Academy Nauk and Karymsky calderas. According to hydrothermal experiments and petrologic observations both dacites last equilibrated at 3--4 km depth. At the same depth granophyre phase assemblage is reproduced by isobaric crystallization of dacites, thus implying that the granophyres represent a crystallized silicic reservoir, which produced dacites 40,000 yr. BP and formed Academy Nauk caldera. In 1996 this crystallized body was sampled by ascending basalt, which erupted in the northern part of the caldera.