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

82 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
1986; Reifenstuhl and others, 1984; Miller and Bundtzen,
1993); ages for intermediate to mafic dikes are poorly con-
strained because they are commonly altered, but one dike
is 71 Ma (Miller and Bundtzen, 1993).
Rocks of the Cretaceous Kuskokwim Group surround
the Horn Mountains. The Kuskokwim Group consists pre-
dominantly of sedimentary rocks, but also contains minor
interbedded volcanic tuffs and flows in the Iditarod quad-
rangle (Miller and Bundtzen, 1993). The Kuskokwim
Group is a unit of flysch representing turbidite fan,
foreslope, shallow-marine, and shelf facies deposited in a
Late Cretaceous basin (Bundtzen and Gilbert, 1983; Miller
and Bundtzen, 1992). Cady and others (1955) first de-
scribed and named rocks of the Kuskokwim Group, sug-
gesting that graywacke and lesser siltstone compose almost
all of the sequence; conglomerates are found locally.
Rocks of the Kuskokwim Group are deformed into broad
open folds, and locally into tight chevron folds, but they are
not regionally metamorphosed (Miller and others, 1989).
MINERAL OCCURRENCES
Few mineral occurrences have been recognized in the
Horn Mountains study area. The Rhyolite prospect on
Juninggulra Mountain is the only epithermal mercury-rich
vein deposit known in the study area (fig. 2). Cinnabar-
rich quartz-carbonate veins cut granite porphyry at this lo-
cality. The Rhyolite prospect was part of a larger study of
epithermal cinnabar-stibnite-bearing deposits in the region
(Gray and others, 1991). Other deposits of this type near
the study area include the stibnite-rich Snow Gulch lode
and the cinnabar-rich DeCourcy mercury mine, both about
7 km north of the Sleetmute quadrangle in the Iditarod
quadrangle (fig. 1). Numerous epithermal mercury-rich
vein lodes are scattered over several hundred thousand
square kilometers in southwestern Alaska, in which cinna-
bar and stibnite are the dominant ore minerals, with lesser
amounts of realgar, orpiment, native mercury, pyrite, gold,
limonite, and hematite (Sainsbury and MacKevett, 1965;
Gray and others, 1990). Scheelite, arsenopyrite, and base-
metal-sulfide minerals are rare, but are found with cinna-
bar in some occurrences such as those near Flat (Bundtzen
and others, 1992). Gangue is typically quartz, carbonate,
and dickite. The deposits are found in all sedimentary and
igneous rock types in the region, but typically show a
close spatial association with Late Cretaceous and Tertiary
igneous rocks. Many of these deposits are small, consist-
ing of veins a few centimeters wide and extending for
strike lengths of only a few meters, but in the Red Devil
deposit veins exceeded a meter in width and 100 m in
length (MacKevett and Berg, 1963). Red Devil produced
approximately 35,000 flasks of mercury (1 flask = 76 lb or
34.5 kg) and total production from southwest Alaska is
about 41,000 flasks of mercury (Miller and others, 1989).
Mercury, Sb, and As in minus-80-mesh stream-sediment
samples, and cinnabar in nonmagnetic heavy-mineral-con-
centrate samples are the most consistently anomalous data
in samples collected around epithermal cinnabar-stibnite-
bearing deposits in southwestern Alaska (Gray and others,
1991).
Polymetallic veins were recently discovered in the
Horn Mountains (Bundtzen and others, 1993) near the
headwaters of Jungjuk Creek, Getmuna Creek, and
Whitewing Valley (fig. 3), and are base-metal-sulfide-bear-
ing with anomalous gold or silver. The Whitewing Valley
occurrence cuts rocks of the Horn Mountain pluton, and
the Jungjuk and Getmuna Creek occurrences are found in
volcanic rock hornfels. Similar polymetallic vein lodes are
found in other volcanoplutonic complexes in southwestern
Alaska including the Russian Mountains (Bundtzen and
Laird, 1991), Beaver Mountains (Bundtzen and Laird,
1982), the Flat area (Mertie, 1936; Bull and Bundtzen,
1987; Bundtzen and others, 1988c), Camelback Mountain
(Bundtzen and others, 1988b), and the Moore Creek pluton
(Bundtzen and others, 1988b). The mineralized veins are
found in faults, shear zones, and breccias within cupolas of
monzonite stocks or in surrounding sedimentary- and vol-
canic-rock hornfels aureoles (Bundtzen and Gilbert, 1983;
Bundtzen and others, 1988a). The lodes contain arsenopy-
rite, pyrite, chalcopyrite, bornite, sphalerite, galena, marca-
site, tetrahedrite, gold, bismuthanite, stephanite, cassiterite,
scheelite, barite, or wolframite. Quartz, tourmaline,
axinite, and fluorite comprise most of the gangue. Stream-
sediment samples collected around volcanoplutonic com-
plexes with known polymetallic vein deposits in the
Iditarod quadrangle contain anomalous concentrations of
Au, Ag, As, Cu, Pb, Zn, Mo, Sn, Hg, B, Bi, or Be; heavy-
mineral-concentrate samples contain anomalous Au, Ag,
Cu, Pb, Zn, Mo, Sn, Sb, As, W, Bi, Ba, B, or Nb, and may
contain gold and scheelite, but rarely base-metal sulfides
or cinnabar (Gray and others, 1988; Bennett and others,
1988). However, anomalous concentrations of base metals
in drainage basin samples collected around some of the
volcanoplutonic complexes are inconsistent with the min-
eralogy of polymetallic veins known in the areas. For ex-
ample, in the Beaver Mountains, chalcopyrite- and
galena-bearing veins are known (Bundtzen and Laird,
1982), but heavy-mineral concentrates collected from the
area show no anomalous Cu and Zn (Gray and others,
1988). This probably reflects the small-scale nature of the
polymetallic veins, sampling distance from the vein depos-
its, and mechanical breakdown of the sulfides in the
weathering environment.
Placer gold mines are not known in the study area, but
approximately 1,500 oz of gold was recovered from a
placer mine on New York Creek, about 5 km south of the
study area (Maddren, 1915; Cady and others, 1955; Miller
and others, 1989). In addition, approximately 25,000 oz of
gold has been produced from placer mines in the Donlin-