USGS Professional Paper 1697 - Alaska Resources Library

32; tables 3, 4) occurs along the northern margin of Kodiak
Island, on the Kenai Peninsula, and along the northern flank
of the Chugach Mountains in southern coastal Alaska (Foley,
1985; Foley and others, 1997). This belt occurs discontinously
along a strike distance of several hundred kilometers
from Kodiak Island in the southwest to the eastern Chugach
Mountains on the east. The metallogenic belt is hosted in the
Border Ranges ultramafic-mafic assemblage, which forms the
southern part of the Talkeetna part of the Talkeetna-Bonzana
island arc in Wrangellia superterrane (Burns, 1985; Plafker
and others, 1989; Nokleberg and others, 1994c, 1997c, 2000).
The significant deposits are at Halibut Bay, Claim Point, Red
Mountain, and Bernard and Dust Mountains; a possibly related
gabbroic Ni-Cu deposit is at Spirit Mountain (table 4) (Nokleberg
and others 1997a,b, 1998).
Red Mountain Podiform Cr Deposit
The Red Mountain podiform Cr deposit (fig. 35) (Guild,
1942; Bundtzen, 1983b; Burns, 1985; Foley and Barker, 1985;
Foley and others, 1985, 1997) consists of layers and lenses of
chromite in dunite tectonite; the layers and lenses are as much
as several hundred meters long and 60 m wide. The largest
chromite layer is about 190 m long and as much as 1.5 m
Tom Stringer Zone
Tram Stringer Zone
Turner Stringer Zone
A
5 4 3 2
Star Stringer Zone
wide, and more than 10 smaller ore bodies exist. The host Late
Triassic to Early Jurassic dunite tectonite is interlayered with
subordinate pyroxenite in zones about 60 m thick. Serpentinite
is locally abundant along contacts of bodies. Exploration and
development occurred sporadically from about 1919 to the
1980’s. Several hundred meters of underground workings and
trenches were constructed. An estimated 26,000 tonnes of ore,
ranging from 38 to 42 percent Cr 2O 3, was produced from 1943
to 1957. The two largest remaining deposits are estimated to
contain 87,000 tonnes grading about 25 to 43 percent Cr 2O 3.
An additional, low-grade deposit contains an estimated 1.13
million tonnes Cr 2O 3. The nearby Windy River chromite
placer deposit, which occurs downstream from Red Mountain
deposit, in hosted in glaciofluvial sand and gravel deposits
and is estimated to contain 15.6 million m 3 grading about 1.33
percent Cr 2O 3.
Origin of and Tectonic Controls for Kodiak Island and
Border Ranges Metallogenic Belt
The Kodiak Island and Border Ranges metallogenic belt
of podiform Cr and associated deposits occurs in the Late
Triassic to Early Jurassic Border Ranges ultramafic and mafic
assemblage (Burns, 1985; Plafker and others, 1989; DeBari
B
Late Triassic Metallogenic Belts (230 to 208 Ma; fig. 32) 85
Star #4
2
Quaternary alluvium
containing placer chromite
Dunite
0 800 m
Pyroxenite
Graywacke, argillite,
chert, and greenstone
Chromitite stringers
Chromitite stringer
zone
Drill hole
Contact
Mesozoic
Figure 35. Red Mountain podiform Cr deposit, Kodiak Island and Border Ranges metallogenic belt, southern Alaska. Schematic
geologic map showing locations of the larger chromite deposits. Modified from Guild (1942), and Foley and others (1997). See
figure 32 and table 4 for location.