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

98 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
the southwest edge of area 4 contained 0.07 ppm gold.
Shale, siltstone, graywacke, and lesser conglomerate of the
Kuskokwim Group are the predominant alluvium in the
two areas. Minor quartz pebbles are common in alluvium.
The quartz locally is vuggy and cuts siltstone in some of
the larger pebbles. Granite and volcanic rocks of interme-
diate composition are present in varying amounts in allu-
vium from all of the drainages. Granite glacial erratics are
present and till-mantled slopes are likely in both areas.
Conglomerate of the Kuskokwim Group is abundant
in outcrop in areas 4 and 5, in the upper reaches of drain-
ages. The conglomerates typically have round to locally
angular clasts from 1 to 10 cm in diameter. Clast compo-
sitions include siltstone, sandstone, shale, graywacke, and
clear to white, locally drusy, quartz pebbles. Vein quartz
cutting the conglomerate was observed at one site. Two 9-
kg bulk conglomerate samples were collected from each
area. These samples were ground coarsely and panned in
the laboratory. No gold was found during visual examina-
tion of the panned samples.
CONCLUSIONS
Concentrate samples from the large area in the Lyman
Hills containing cinnabar -t gold in concentrates may re-
flect small epithermal cinnabar- and stibnite-bearing vein
deposits such as those found elsewhere throughout the
Kuskokwim basin of southwestern Alaska. The White
Mountain mine, in the Lyman Hills about 30 km to the
north of the study area, is hosted in similar shale and lime-
stone of the Dillinger terrane and may provide the closest
analog.
The general abundance of gold and paucity of cinna-
bar in concentrates south of the Swift River suggest a
source different from that of the cinnabar 2 gold in con-
centrates to the north. Variations in the quantity, size, and
morphology of the gold grains may reflect multiple
sources.
The presence of such widespread generally fine-
grained gold is paradoxical. The present data preclude the
determination of sources and origins of the gold. It is un-
likely that such widespread fine gold could be the result
solely of glacial or fluvial transport from the Alaska Range
to the east. If this were the case, one would expect many
more gold occurrences and anomalies in the southeastern
part of the Lime Hills quadrangle than those identified by
Allen and Slaughter (1990). Further, the delicate mor-
phologies found in areas 2 and 3 suggest local sources for
the gold in these areas.
While visible gold was not found in the two bulk
samples of conglomerate from the Kuskokwim Group col-
lected in areas 4 and 5, this potential source of gold cannot
be conclusively ruled out. However, paleoplacer gold de-
rived from rocks of the Kuskokwim Group has not been
reported elsewhere. Yet, 76 of the 85 gold-bearing con-
centrates collected south of the Swift River are from drain-
ages containing outcrops where only sedimentary rocks of
the Kuskokwim Group are known. Three of the remaining
nine concentrates were collected in drainages underlain by
limestone of the Nixon Fork terrane, but are downslope
and within 3 km of isolated outcrops of rocks of the
Kuskokwim Group. A strong spatial association is evident
between gold in concentrates and sedimentary rocks of the
Kuskokwim Group in the area. Whether the gold is de-
rived from rocks of the Kuskokwim Group or has been
introduced by subsequent mineralization is not clear.
The western half of the Lime Hills quadrangle has not
been mapped in detail. In contrast, detailed geologic map-
ping exists for the adjacent Iditarod quadrangle (Miller and
Bundtzen, 1993). The Iditarod quadrangle is underlain
principally by rocks of the Kuskokwim Group, but detailed
ground traverses have revealed the presence of numerous
mafic to felsic dikes. Further, Late Cretaceous to early
Tertiary volcanoplutonic complexes and related igneous
bodies are the major sources of lode and placer deposits in
the Iditarod quadrangle (Bundtzen and others, 1985; Miller
and Bundtzen, 1987). Similar unrecognized intrusive bod-
ies may exist in the western Lime Hills quadrangle.
This study suggests favorability for epithermal cinna-
bar + stibnite deposits in the Lyman Hills and for gold-
bearing vein or placer deposits in rocks of the Kuskokwim
Group south of the Swift River. Precious metal and path-
finder geochemical analyses of samples collected in the
western half of the Lime Hills quadrangle will help to
clarify relations and may further delineate targets.
Acknowledgments.-I thank Elizabeth Bailey, Carter
Borden, Barrett Cieutat, and Greg Lee for their help in
sample collection and field identification. Steve Sutley
performed X-ray diffraction analyses.
REFERENCES CITED
Allen, M.S., 1990, Gold anomalies and newly identified gold oc-
currences in the Lime Hills quadrangle, Alaska, and their
association with the Hartman sequence plutons, in
Goldfarb, R.J., Nash, J.T., and Stoeser, J.W., eds.,
Geochemical studies in Alaska by the U.S. Geological Sur-
vey, 1989: U.S. Geological Survey Bulletin 1950-F, p. F1-
F16.
Allen, M.S., Malcolm, M.J., Motooka, J.M., and Slaughter,
K.M., 1990, Geologic description, chemical analyses and
sample locality map for rock samples collected from the
eastern part of the Lime Hills quadrangle, Alaska: U.S.
Geological Survey Open-File Report 90-69,49 p.
Allen, M.S., and Slaughter, K.M., 1990, Mineralogical data and
sample locality map of nonmagnetic, heavy-mineral-con-
centrate samples collected from the eastern part of the Lime
Hills quadrangle, Alaska: U.S. Geological Survey Open-
File Report 90-67, 64 p.