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

190 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
analysis after the other samples in order to minimize con-
tamination. Of the other rock samples from the HM pit
itself, bulk analyses of all but the albitized sample show
measurable molybdenum, although all have less than 10
ppm; no molybdenite was noted in preliminary study of
any of these samples. Only two of our rock samples from
the Dora Bay area, other than the high-grade samples, had
molybdenum contents exceeding 10 ppm, and then only
barely (1 1 ppm). One of these was a sample of country
rock with reddish coloration. The other was the albitized
rock sample from CM in which molybdenite had been ten-
tatively identified by visual examination. Interestingly,
Barker and Mardock (1990), in a study of rare-earth
enrichments in the Dora Bay area, reported their highest mo-
lybdenum value of 51 ppm from a sample collected at a site
located at the nmws of Dora Lake close to the CM locality.
SCANNING ELECTRON MICROSCOPY
Many of the features noted in hand specimen receive
corroboration and amplification when viewed under a
scanning electron microscope (SEM). Samples were hand
polished on fresh abrasive paper and then carbon coated.
The SEM used is an Etec Autoscan with a Kevex Delta
Microanalysis Energy-Dispersive System (EDS) and
Quantex software. Operating conditions of the SEM were
accelerating voltage of 30 kV, filament current of about
125 PA, and working distance of 14-18 mm. Output was
in the form of backscattered-electron images, in which
grains of higher average atomic number appear brighter,
and X-ray spectra plots of relative intensity versus energy.
Spectral assignments were made using the system's inte-
gral software. Because thick slabs were used in this SEM
study in the interest of rapid and inexpensive sample
preparation, there is very little a priori control or knowl-
edge of what is being analyzed at depth below the surface
of the slab. Some of the EDS spectra are no doubt reflect-
ing compositional data from more than one mineral; how-
ever, consistency in ratios of peak intensities for analyses
at neighboring spots can often be used as a criterion for
evaluating whether a spectrum represents a single phase or
a mixture of phases. Because of the lack of sampling con-
trol, we have not attempted to make the corrections to raw
peak intensities necessary to convert these into quantitative
element abundances. Because of the geometric constraint,
the lack of corrections to the intensity data, and other
problems inherent to SEM-EDS analysis (especially for el-
ements of lower atomic number), any intimations of exact
compositions must be considered tentative. It follows
from this, and from the general lack of crystallographic in-
formation, that our SEM mineral "identifications" are rea-
sonable but not definitive.
An abundant mineral in the HM molybdenite-bearing
rock sample CDP-92-29 is a sodium-calcium-aluminum-
silicate containing some potassium and major amounts of
chlorine. An EDS spectrum for this mineral is shown in
figure 5. The mineral was tentatively identified as scapolite
[3NaA1Si3081*NaC1,3CaA12Si208*CaC0~. Subsequent
X-ray diffraction on a powdered whole-rock sample
produced a complex spectrum, which indicated that scapolite
was a likely major component. The scapolite appears
to be partially altered and replaced by other phases. Under
the SEM, the scapolite has a birds-eye appearance and is
cut by numerous intersecting veins of both calcite and potassium
feldspar. Figure 6 is a backscattered-electron image
of altered and veined scapolite (dark gray). The area
above and to the left of the scapolite consists largely of
patches of potassium feldspar, biotite, and residual areas of
scapolite. A number of accessory minerals can be seen in
the figure, including sphene, apatite, and zircon; sphalerite
and iron sulfide occur elsewhere in the sample. In the upper
left comer of the scapolite grain in figure 6 is a small
group of molybdenite grains (white) set within a calcite vein
(gray). This indicates that some, at least, of the molybdenite is
later in the paragenetic sequence than the scapolite.
Molybdenite also occurs as lenses and sheets, tens of
micrometers thick and millimeters in length, composed of
polycrystalline molybdenite in a matrix of potassium feldspar
(fig. 7); the molybdenite is presumed to be filling
small fractures. The molybdenite appears to be quite pure.
The EDS spectrum reveals only molybdenum and sulfur
peaks and trace peaks for silicon and aluminum. Although
the latter are very small, they do appear consistently, and it
may be that a fine-grained aluminum silicate (feldspar?)
was crystallizing intermixed with the molybdenite. Potassium
feldspar, in sizable grains, is a common associate of
the molybdenite. It is usually quite pure, without any detectable
sodium or calcium. Small amounts of pure albite
also occur, but this mineral is at least an order of magnitude
less abundant. The apparent lack of measurable solid
solution in these feldspars suggests low-temperature crystallization.
The accessory phases sphene, pyrrhotite, and
sphalerite appear to be more abundant in potassium feldspar-rich
regions. Pyrrhotite was not found immediately
adjacent to molybdenite. Biotite is common, but irregularly
distributed; EDS spectra show that the biotite contains
minor titanium, manganese, and fluorine.
Figure 8A is a low-magnification backscattered-electron
image of a polished slab of a CM rock sample. In the
lower part of the figure, a patchwork iron sulfide (pyrite?)
vein crosses a largely albitic (dark gray) matrix. Within
this vein, many of the grains are rimmed by or composed
entirely of iron oxide (or hydroxide or carbonate, all anions
that cannot be readily distinguished in our SEM
work). Oxidation of pyrite is a probable cause for the rust
stains on this rock. The fine vein (v2) toward the top of
the figure is also composed of iron oxide or carbonate.
Figure 8B, a closeup of the central portion of this vein,
shows a molybdenite stringer (white) cutting both the al-