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

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 inten-
sities 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 min-
eral 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 molyb-
denite 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.