Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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192 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 estimated several percent of iron indicated for this phase would be most unusual for albite, especially a presumed low-temperature albite, inasmuch as pegmatitic albites typically have less than 0.01 percent iron (Smith, 1983). Possibly the phase is a quenched iron-bearing glass, but then its lack of potassium might seem puzzling. In fact, a potassic mineral is present on the smooth areas on the natural fracture surface of CM rock sample 92DB-05C where it occurs as (light gray) hexagonal plates (fig. 9A). It is even more apparent in the gray area at the lower left of figure 9A. The area, shown enlarged in the upper right of figure 9B, can be seen to consist of two phases. The darker areas showing between the hexagons in figure 9B are of (iron-free) albite composition (ab). The lighter phase (kim) contains abundant potassium (fig. 11). Over a range of a factor of five in potassium peak inten- sity, FeK is acceptably invariant (3 to 3.6) for 9 of 12 analyses. Similarly, eight analyses gave MoK peak intensities from 2.1 to 3.1. In contrast, SiIK intensities ranged from 1 to 27 and Si/Al intensities are close to those for albite. We conclude that the spectrum shown in figure 11 likely represents a mixture of albite and a non-silicate mineral containing potassium, iron, and molybdenum. Be- cause the readily observed X-ray peaks for molybdenum and sulfur have overlapping energies that cannot be easily distinguished, the mineral could be a sulfide. It also could be an oxide or hydroxide among other possibilities. Ferrimolybdite is a known weathering product, and there are several potassic molybdates. This raises the question of the origin of thls hexagonal phase. Because we are ana- lyzing a natural, unpolished surface, the phase almost cer- tainly represents some kind of surficial deposit or reaction. The hexagons appear to be oriented parallel to the surface and nucleation appears to have dominated crystal growth kinetics (that is, there are numerous small crystals). If they are weathering products, a source of potassium would be required, and this could be the potassium feldspar con- tained in the bulk rock. Another possibility is that this potassium-iron-molybdenum phase may have crystallized (perhaps subsolidus) as part of a system quenched in a manner similar to that ascribed above to the conjectured iron-albite glass. DISCUSSION The Dora Bay molybdenite occurrences are only showings, but molybdenite occurs here in similar settings at two localities that are 3 km apart; readily visible molybdenite occurs over a width of about 5 m in the HM pit face; and analyses of two bulk HM samples yielded 0.5 and 1.2 percent molybdenum, respectively. These discov- eries offer at least some prospect of a molybdenite deposit in the vicinity. At our current level of understanding, the Figure 6. Scanning electron microscope backscattered-electron Figure 7. Scanning electron microscope backscattered-electron image of HM rock sample CDP-92-29. A grain of scapolite (sc) is image of HM rock sample CDP-92-29. Molybdenite (mo) veined by calcite (ca) and potassium feldspar (or). Molybdenite lenses and veins occur in potassium feldspar (or) and albite (ab). (mo) occurs within a calcite vein in the upper left comer of the Mottled mineral at the top of the photograph is scapolite (sc), scapolite grain. Accessory minerals incl~ide sphene (sph), apatite which is adjacent to a grain of biotite (bt). Field of view is (ap), and zircon (zr). Field of view is approximately 2 mm. approximately 8 mm.
NEWLY DISCOVERED MOLYBDENITE OCCURRENCES AT DORA BAY, PRINCE OF WALES ISLAND Figure 8. Scanning electron microscope backscattered-electron images of CM rock sample 92DB-05C. A, Albite (ab) is the dominant mineral. Grains at bottom of image are partially oxidized pyrite (py). At the top of the image a discontinuous vein of potassium feldspar (or) lies above a discontinuous vein of scattered molybdenite (mo) which intersects a vein of iron oxide or carbonate (v2). Field of view is approximately 4 mm. B, Enlargement of outlined area of A. Molybdenite vein (mo) cuts both the albite matrix and vein v2. Field of view is approximately 0.4 mm. 193 Figure 9. Scanning electron microscope backscattered-electron images of a natural fracture surface of the CM rock sample 92DB- 05C. A, Smooth albitic patches (fab) occur in the upper left and the lower right of the image adjacent to pitted albitic areas (ab) and plates of molybdenite (mo). Field of view is approximately 1 mm. B, Enlargement of outlined area in A. Molybdenite plates (mo) surround an area in which a phase of albitic composition (ab) alternates with a mineral (kim) containing potassium, iron, and molybdenum. Some grains of the latter mineral have hexagonal form. Field of view is approximately 0.1 rnm.
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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GOLD AND CINNABAR IN HEAVY-MINERAL
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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134 GEOLOGIC STUDIES IN ALASKA BY '
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Page 180 and 181: Table 1. Isotopic ages of intrusive
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Page 212 and 213: Table 3. Summary of geochemical sig
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Page 219 and 220: GEOCHEMICAL CHARACTER OF UPPER PALE
Page 225 and 226: RECONNAISSANCE GEOCHEMISTRY OF BASA
Page 235 and 236: OSTRACODE ASSEMBLAGES FROM MODERN B
Page 243 and 244: RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
Page 245 and 246: RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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