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

88 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
area is underlain by sedimentary rocks of the Kuskokwim
Group. The heavy-mineral-concentrate sample collected
from this site contains several grains of microscopically
visible gold, but gold was not detected analytically in ei-
ther the stream-sediment or concentrate sample. The sig-
nificance of this single-site placer gold anomaly is poorly
defined.
AREA 12
Samples collected from unnamed tributaries of the
Kuskokwim River delineate area 12. Sedimentary rocks of
the Kuskokwim Group are the only bedrock in the area.
In this area, two stream-sediment samples contain 0.018
and 0.012 ppm Au; heavy-mineral concentrates contain up
to 1,000 ppm Sn, 10,000 ppm Ba, 200 ppm Pb, and micro-
scopically visible gold, scheelite, and barite. The
geochemical anomalies of Au, Sn, Ba, and Pb, as well as
the presence of gold, scheelite, and barite in heavy-min-
eral-concentrates, are perhaps most similar to those in the
Horn Mountains (areas 1 and 2). This anomaly is enig-
matic because the geology is inconsistent with the
anomaly. The data suggest that poorly exposed or con-
cealed intrusive rocks may be present in area 12, some of
which may be mineralized. Another possibility is that the
anomaly is of glacial derivation from the nearby Horn
Mountains.
CONCLUSIONS
The reconnaissance geochemical study of the Horn
Mountains area identified 12 geochemically anomalous ar-
eas. Two of these areas are considered favorable for
polymetallic vein deposits, eight areas are interpreted as
being favorable for epithermal mercury-rich vein deposits,
and two areas are unclassified. The two areas favorable
for polymetallic vein deposits are found within or around
the Horn Mountains where intermediate to felsic intrusions
cut mafic to felsic volcanic rocks and sedimentary rocks of
the Kuskokwim Group. Stream-sediment samples col-
lected in these areas are anomalous in Au, Ag, As, Cu, Pb,
Hg, Sb, W, Cd, Bi, B, or Te; heavy-mineral concentrates
are anomalous in Au, Ag, Pb, Sb, W, Bi, Ba, or B, and
contain microscopically visible gold, scheelite, or barite.
The Horn Mountains are similar geologically and
geochemically to other areas in southwestern Alaska such
as the Russian and Beaver Mountains, where base-metal-
sulfide-, sulfosalt-, and gold-bearing polymetaIlic vein de-
posits are known. Polymetallic vein lodes are known in
the Horn Mountains, and the geochemical anomalies sur-
round these occurrences.
Eight areas are designated as favorable for epithermal
mercury-rich vein deposits. Cretaceous sedimentary rocks
of the Kuskokwim Group are most common in these areas,
but Late Cretaceous and Tertiary mafic to felsic volcanic
rocks and intrusions are found locally. Anomalous concen-
trations of Hg in stream-sediment samples and cinnabar in
heavy-mineral-concentrate samples are most consistent in
these areas. Stream-sediment samples containing anoma-
lous Sb, As, Au, W, or Te, and heavy-mineral-concentrate
samples containing anomalous Au, Ag, Ba, Zn, or Sn, and
microscopically visible gold, scheelite, or barite are less
consistently found in these areas. Numerous epithermal
mercury-rich vein deposits are found throughout south-
western Alaska in areas with similar geology and
geochemistry, suggesting that undiscovered deposits of this
type are possible in the areas delineated.
Two additional areas are delineated in the study. One
area is a single-site placer gold occurrence on a tributary
of the Iditarod River. In the final area, stream-sediment
samples are anomalous in Au, whereas heavy-mineral-con-
centrate samples contain anomalous Pb, Sn, Ba, and mi-
croscopically visible gold, scheelite, and barite. Data are
presently insufficient to classify these two deposits.
Acknowledgments.-The authors would like to thank
Phil Hageman, Bruce Roushey, Craig Motooka, and Jerry
Motooka for chemical analyses; Danny Abrams for sample
preparation; and Jimmy Carter Borden for assistance with
sample collection.
REFERENCES CITED
Bennett, G.J., Gray, J.E., and Taylor, C.D., 1988, Mineralogy
and sample locality map of the nonmagnetic, heavy-min-
eral-concentrate samples, Iditarod quadrangle, Alaska: U.S.
Geological Survey Open-File Report 88-32, 37 p., 1 plate,
scale 1 :250,000.
Box, S.E., Moll-Stalcup, E.J., Frost, T.P., and Murphy, J.R., in
press, Preliminary geologic map of the Bethel and southern
part of the Russian Mission 1 :250,000 quadrangles, Alaska:
U.S. Geological Survey Miscellaneous Field Studies Map
2226-A, scale 1:250,000.
Bull, K.F., and Bundtzen, T.K., 1987, Greisen and vein Au-W
mineralization of the Black Creek stock, the Flat area, west-
central Alaska: Geological Society of America Abstracts
with Programs, v. 19, p. 362.
Bundtzen, T.K., Cox, B.C., and Veach, N.C., 1988a, Heavy min-
eral provenance studies in the Iditarod and Innoko districts,
western Alaska, in Process Mineralogy VII: Metallurgical
Society, SMEIAIME joint meeting, Denver, Colorado, pro-
ceedings, p. 221-245.
Bundtzen, T.K., and Gilbert, W.G., 1983, Outline of geology
and mineral resources of upper Kuskokwim region, Alaska:
Journal of the Alaska Geological Society, v. 3, p. 101-119.
Bundtzen, T.K., Laird, G.M., Harris, E.E., Kline, J.T., and
Miller, M.L., 1993, Geologic map of Sleetmute C-7, D-7,
C-8, and D-8 quadrangles, Horn Mountains area, southwest-
ern Alaska: Alaska Division of Geological and Geophysical
Surveys Public-Data File 93-47, 1 plate, scale 1:63,360.
Bundtzen, T.K., and Laird, G.M., 1982, Geological map of the