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

156 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
OTHEROCCURRENCES
Disseminated pyrrhotite, pentlandite, and chalcopyrite
occur within quartz diorite of the Miners Bay pluton
(Jansons and others, 1984). This syngenetic occurrence is
distinct from the epigenetic Blackjack and Columbia Red
Metals prospects, which are in mineralized zones that
strike approximately north-south, have little or no shearing
along the mineralized zone, and have dendritic quartz vein-
Iets that locally brecciate the wall rock. The epigenetic oc-
currences are either within -2 km of, or within, an
intrusion.
There is one quartz-fluorite occurrence in the study
area (fig. 2)-the only known occurrence of this type
within the Chugach-Prince William terrane. The main vein
consists of a 1- to 2-m-wide yellow-weathering zone that
contains -70 percent quartz and -30 percent fluorite.
Within the vein there is abundant vuggy quartz and cres-
cent and phacoid-shaped blobs of fluorite up to 8 cm by 3-
4 cm. The fluorite blobs are oriented parallel to the
margins of the vein. The vein strikes NNW-SSE (fig. 14),
is steeply dipping, and its exposed length is 300 m. There
is a network of dendritic quartz veinlets adjacent to the
main vein, and the margins of all veins are sharp with no
-
0 27
N = 224
- -
Figure 13. Rose diagram of lineament
trends in study area south of lat 61' N.,
and north of Glacier Island, from Condon
and Cass (1958). N, number of samples.
evidence of faulting or shearing. These veinlets are remi-
niscent of those at the Columbia Red Metals prospect, but
they are more planar and continuous and generally
subparallel the main quartz-fluorite vein.
DISCUSSION OF STRUCTURAL AND
TECTONIC HISTORY
Ductile deformation in the study area is characterized
by kilometer-scale tight folds with steep bedding dips and
an absence of cleavage. Bol and Gibbons (1992) studied
the structural geology of a correlative belt of rocks south-
west of our study area (fig. 1) (Bol and Gibbons' eastern
belt) and found that bedding generally has moderate dips,
in contrast to the steep dips in our study area, and a similar
absence of cleavage. They favored the interpretation that
these rocks are an incorporated slope basin, in order to ex-
plain its slightly deformed character. However, they stated
that vitrinite reflectance and mineral assemblage data (Gib-
bons, 1988) indicate burial depths of 10 to 15 km, which
might argue that the rocks were underplated, but escaped
intense deformation. The steeper dips of bedding in our
study area suggest that there are along-strike variations in
the intensity of deformation and possibly that this belt of
rocks is more deformed than Bol and Gibbons (1992) ap-
preciated. Perhaps this greater degree of deformation indi-
cates that these rocks were underplated. However, the
roughly symmetric distribution of lithofacies in our study
N
Figure 14. Equal-area stereogram of poles to quartz vein
orientations (dots) and quartz-fluorite vein orientations (boxes).