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

188 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
National Forest in an area designated as Land Use Desig-
nation 111 ("management for a variety of uses") in the U.S.
Department of Agriculture Forest Service land manage-
ment plan (1991). Apparently surface rights in the vicinity
of HM were conveyed to the Kootznoowoo Native Corpo-
ration and Sealaska Native Corporation owns the subsur-
face rights in the area (written communication, Rick
Harris, Vice President, Mineral Resources, Sealaska Cor-
poration, Juneau, June 28, 1991).
This paper reports the molybdenite occurrences and
the results of subsequent preliminary laboratory studies
made on rock samples collected there. The report, based
on only a few hours of field study at the two sites and on
limited laboratory investigations, is necessarily of a rudi-
mentary, preliminary, and speculative nature; nevertheless, a
number of features of potential interest have come to light.
GEOLOGIC FRAMEWORK
Prince of Wales Island lies within the Alexander
tectonostratigraphic terrane (Brew and others, 1991). In
the vicinity of Dora Bay, the Wales Group, composed of
metamorphosed pre-Middle Ordovician sedimentary and
volcanic rocks (Eberlein and others, 1983), is intruded by
an igneous pluton (fig. 2). The highest resolution regional
mapping in the area is that of Herreid and others (1978);
however, their coverage extends eastward only to the
westernmost margin of the intrusion, which they mapped
as a part of their "granodiorite and related rocks (potas-
sium-argon age 102k3.0 m.y.)." The map of Eberlein and
others (1983) displays the complete intrusive body, which
Figure 2. Locations of Holly Moly and Cliff Moly molybdenite
occurrences at margin of Dora Bay syenite pluton (shaded area),
Prince of Wales Island.
is classified as Paleozoic and (or) Mesozoic nepheline-
eudialyte-bearing syenite. Gehrels and Berg (1984) tenta-
tively classified the body as Late Pennsylvanian and (or)
Early Permian syenite. The HM occurrence is located in
the northeast border zone of the intrusive body, which ex-
tends for about 4 km in its longest exposed dimension
from northwest to southeast; the CM occurrence is located
near the southeast extremity of the body (fig. 2).
The outline of the intrusion as shown in figure 2 is
based on the map of Eberlein and others (1983). We have
confirmed the location of the contact in the field in only a
few places. The representation of the easternmost contact
has been modified in light of the sketch-map presented by
Barker and Mardock (1990). It is not clear whether the
interfingering at the eastern margin is due to dike- or sill-
like extensions, to faulting, or to topographic intersections.
Outcrop exposure tends to be good only along the shore-
line, at higher elevations, and on cliff faces.
THE HOLLY MOLY AND CLIFF MOLY
OCCURRENCES
The HM molybdenite-bearing samples were collected
on the 5-m-high southern face of a roughly circular pit, that
is about 30 m in diameter. The rocks in the pit are cut by a
large number of joints and shears including a strong near-
vertical set and several subhorizontal sets. The moIybdenite
appears to be concentrated on shears having intermediate
dip. Because of the complexity of the fracture set and the
limited field examination, we do not as yet have a good
understanding of fracture history and its relation to the min-
eralization. A siliceous, eastward-dipping pegmatite, about
8 cm wide with a selvage of fluorite and biotite, appears to
cut, and hence postdate, the molybdenite mineralization.
Abundant visible molybdenite was noted in outcrop in a
zone extending for about 5 m in an easterly direction from
the vicinity of the pegmatite vein. Much of the pit face is
discolored by tan- and rusty-weathering products that ob-
scure the nature of the rocks present and their relationships.
The molybdenite occurs in a gray rock that lacked features
whereby it could be easily classified in the field. The west-
em margin of the pit is defined by a well-jointed rock that
readily breaks down into cubes 2 to 3 cm across with fully
weathered dark surfaces. This rock is a breccia with a
continuum of clast sizes ranging up to several centimeters
across and little, if any, apparent porosity. The most visible
of the clasts are of a distinctive fine-grained red lithology.
Clasts of melanocratic syenite and other gray clasts are less
well defined against a fine-grained gray matrix. The lack of
sharp contacts on some of these clasts suggests that the
breccia may have been heated. The eastern margin of the
pit is composed of a dark porphyritic latite(?) with mafic
phenocrysts and a bleached white albitized rock, both of
which have weathered surfaces stained by iron oxide.