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

GEOCHEMICAL EVALUATION OF STREAM-SEDIMENT DATA
GEOLOGY AND KNOWN MINERAL
OCCURRENCES
The study area (fig. 1) is underlain by rocks of the
Alexander, Wrangellia, Chugach, Prince William, and
Yakutat terranes (Jones and others, 1987). Reconnaissance
geologic mapping of the easternmost Chugach Mountains
was conducted by Hudson and Plafker (1982), Gardner
and others (1988), and Plafker and others (1989). Detailed
geologic mapping of the entire Bering Glacier and Icy Bay
quadrangles (at a scale of 1:250,000 or greater) has not yet
been carried out.
The northeastern comer of the Bering Glacier quad-
rangle is underlain by a thick sequence of early Paleozoic
through Early Jurassic rocks informally named the
"Kaskawulsh Group" by Gardner and others (1988). This
complex sequence of rocks, comprising the basement rocks
of Alexander terrane, includes marble, limestone, schist,
phyllite, siltstone, argillite, basic volcanic rocks, and felsic
to intermediate intrusive bodies. Late Paleozoic basaltic to
andesitic flow rocks, flow breccia, volcanic graywacke,
tuff, and limestone of the Wrangellia terrane crop out
south and west of the Alexander terrane. Prior to the
Early to mid-Cretaceous docking of the amalgamated
Alexander and Wrangellia terranes onto North America
(Saleeby, 1983), Late Jurassic quartz monzodiorite, with
lesser quartz diorite, granodiorite, and tonalite of the
Chitina Valley batholith, intruded Triassic and older rocks
of the Wrangellia terrane.
The Border Ranges fault system separates the
Wrangellia terrane and the Chugach terrane to the south.
The Chugach terrane underlies most of the northern half of
the Bering Glacier quadrangle and was underthrust beneath
the Wrangellia terrane in Late Cretaceous or early Tertiary
time (Plafker and others, 1977). Within the study area, the
Chugach terrane consists of highly deformed marine clas-
tic rocks and subordinate interbedded basaltic units of the
Cretaceous Valdez Group. The northern third of the
Valdez Group is metamorphosed to greenschist facies,
whereas the remainder consists of higher grade schist and
gneiss of the informally named, high-temperature, low-
pressure "Chugach Metamorphic Complex" of Hudson and
Plafker (1982) (see also, Sisson and others, 1989). The
Contact fault system separates the Chugach terrane from
the early Eocene Prince William terrane, a similar turbid-
ite-rich sequence of rocks (the Orca Group) that was tec-
tonically underthrust by about 51 Ma (Plafker, 1987).
Small, Eocene stocks and dikes of felsic to intermediate
composition intrude rocks of both the Chugach and Prince
William terranes.
The southern third of the Bering Glacier quadrangle
and land areas on the Icy Bay quadrangle are underlain by
bedded sedimentary rocks of the Yakutat terrane described
by Winkler and Plafker (1981) and Plafker (1987). The
Chugach-St. Elias fault system separates the Yakutat
179
terrane from the Prince William terrane. Units in the
Yakutat terrane consist of the Eocene sandstone, siltstone,
and shale of the alluvial-plain Kulthieth Formation, the
deltaic Tokun Formation, and the pro-delta Stillwater For-
mation. A late Eocene to early Miocene marine transgres-
sion is recorded by the organic-shale-dominated Poul
Creek Formation. This unit contains basaltic flows and
related mafic intrusive rocks. Oligocene(?) and younger
clastic strata of the Redwood and Yakataga Formations,
reflecting glacially derived material from uplift of the
Chugach and St. Elias Mountains, were deposited on top of
the Poul Creek Formation in a continental-shelf environment.
The only known mineral deposits within the Bering
Glacier and Icy Bay quadrangles are the river, beach, and
marine terrace placer gold deposits of the Yakataga mining
district. These placers stretch for approximately 50 km,
from Cape Yakataga to eastward of longitude 141" (fig. 1).
Fine-grained gold has been mined by small-scale placer
operations from 1897 until the present, with an estimated
15,709 oz of gold recovered through 1959 (Koschmann
and Bergendahl, 1968). Wave action has concentrated
much of the gold within heavy-mineral layers in the beach
sand and in raised terraces. Reimnitz and Plafker (1976)
noted beach sand about 8 km east of the mouth of the
White River that contained 1.87 ppm Au; a sample of sand
just west of the mouth of the White River analyzed by
Eyles (1990) contained 37 ppm Au. Gold in the placers
has been speculated to have been reworked most recently
from rocks of the Yakataga Formation, but ultimately the
gold is interpreted to have been derived from bedrock
sources in the Chugach and St. Elias Mountains (Maddren,
1914; Reimnitz and Plafker, 1976; Eyles, 1990). The
gold-bearing heavy mineral layers are largely composed of
garnet, with some chromite, native copper, magnetite, il-
menite, xenotime, rutile, sphene, ilmenite, zircon, plati-
num, and U- and Th-bearing minerals (Moxham and
Nelson, 1952; Thomas and Benyhill, 1962).
A few isolated metallic lode occurrences are scattered
throughout the Bering Glacier quadrangle. None of these
are described in great detail, and it is impossible to relate
these occurrences to specific mineral deposit models. Hy-
drothermal quartz-feldspar veins containing up to 20 per-
cent graphite and pyrite cut bleached metamorphic rocks
of the Wrangellia terrane about 1.5 km north of Slender
Lake along the northern edge of the Bering Glacier quad-
rangle (Brabb and Miller, 1962). Along the southern shore
of the lake, Brabb and Miller (1962) reported argillite con-
taining hematite both disseminated and in fracture zones.
In rocks of the Valdez Group within the Granite Range,
minor gold was reported in what is probably a silicified
felsic dike (60" 51' N., 142" 26' W.; Brabb and Miller,
1962). A few kilometers to the northeast of this occur-
rence, gold-bearing placers are recognized along the
Kiagna River. Moffit (1918) described numerous quartz
veins cutting metasedimentary and granitic rocks of the