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

80 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
typical of those found in southwestern Alaska (fig. 1).
Volcanic rocks of these complexes generally overlie or are
in fault contact with coeval plutons that intrude and partially
assimilate the volcanic rocks (Bundtzen and Gilbert,
1983). The plutonic rocks in the Horn Mountains (fig. 2)
are part of a stock of mostly quartz monzonite that intrudes
Late Cretaceous and Tertiary mafic to felsic tuff,
lava, and breccia and sedimentary rocks of the Cretaceous
Kuskokwim Group (Cady and others, 1955). In contactfi
ALASKA
metamorphic aureoles adjacent to the plutons, sedimentary
and volcanic rocks are converted to hornfels. The rocks of
the Horn Mountains are proposed to be part of a caldera
with a surrounding ring fracture zone and densely welded
ash-flow tuffs (Doherty and Bergman, 1987). Igneous
rocks in the Horn Mountains have not been dated, but K-
Ar studies of similar volcanoplutonic complexes in south-
western Alaska yield ages of 73 to 59 Ma for the plutons
and 77 to 58 Ma for the associated volcanic rocks (Decker
EXPLANATION
A Volcanoplutonic complexes (Tertiary and Late Cretaceous)
HM - Horn Mountains
RM - Russian Mountains
BM - Beaver Mountains
CM - Camelback Mountain
TM - Takotna Mountain
MP - Moore Creek pluton
C - Candle Hills
X Mines
RD - Red Devil
CC - Cinnabar Creek
DC - DeCourcy
X Prospects
R - Rhyolite
S - Snow Gulch
Figure 1. Volcanoplutonic complexes and selected mines and prospects in southwestern Alaska (compiled from Cady and
others, 1955; Bundtzen and Gilbert, 1983; Miller and others, 1989; Miller and Bundtzen, 1992; Box and others, in press).