USGS Professional Paper 1697 - Alaska Resources Library

tonnes grading 0.53 percent Ni, 0.33 percent Cu, 0.03 percent
Co, and minor PGE. Grab samples contain from 0.18 to 1.30
g/t PGE. The deposit occurs almost entirely beneath Brady
Glacier, but exposed in small nunataks. Considerable drilling
and exploration occurred in the 1970’s. Exploration stopped
short of development. The deposit is now part of the Wrangell-
St. Elias National Park.
Origin of and Tectonic Controls for Yakobi
Metallogenic Belt
The gabbroic Ni-Cu deposits and host mafic and ultramafic
rocks in the Yakobi metallogenic belt formed during
intrusion into the Oligocene La Perouse Plutonic Suite into
highly-deformed and low-grade, regionally metamorphosed
rocks of the Chugach accretionary-wedge and subduction-zone
terrane (Woodsworth and others, 1991; Brew, 1993, 1994;
Moll-Stalcup and others, 1994). Intrusion occurred in the early
Tertiary after cessation of accretion and subduction of the
Chugach terrane in the earliest Tertiary. The early Tertiary La
Perouse Plutonic Suite is interpreted as forming during final
spreading along the Kula-Farallon oceanic ridge during ridge
1,000 m
800
600
400
200
0
SW NE
NUC-3
NUC-5
Glacial ice and snow
Plagioclase-augite cumulate
Plagioclase-olivine-augite cumulate
Plagioclase-olivine cumulate
Plagioclase-olivine-chromite cumulate
Olivine-bronzite-chromite cumulate
Olivine-chromite cumulate
Biotite schist
Zone with Ni+Cu greater than
0.5 weight percent
Drill hole
Contact
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 247
NUC-2
NUC-3
subduction at about 50 to 60 Ma along the margin of southern
and southeastern Alaska (Bradley and others, 1993; Kusky and
others, 1997). The early Tertiary subduction of the spreading
oceanic ridge, locally partly preserved in ophiolites in the
Prince William terrane (Lytwyn and others, 1997; Kusky and
Young, 1999), is interpreted as causing (1) a regional metamorphic
welt and formation of anatectic granites (Plafker
and others, 1989b; 1994), (2) rapid change in component of
strike-slip movements along the subduction zone bordering the
early Tertiary continental margin (Bradley and others, 1993),
and (3) intrusion of the early Tertiary granitic and maficultramafic
plutonic rock of the Sanak-Baranof plutonic belt
(Hudson, 1979; Moll-Stalcup and others, 1994) in Southern
and southeastern Alaska. The intrusions are interpreted as
forming in a near-trench environment during subduction of the
Kula-Farallon oceanic ridge (Bradley and others, 1993; Kusky
and others, 1997). Because of temporal and spatial proximity
to subduction of the Kula-Farallon oceanic ridge along the
margin of southeastern Alaska from about 50 to 60 Ma, the
Oligocene La Perouse Plutonic Suite and associated gabbroic
Ni-Cu deposits of the Yakobi metallogenic belt are interpreted
as forming either (1) during the last gasp of oceanic ridge
NUC-19 NUC-24
0 200 m
Early
Tertiary
Figure 117. Brady Glacier gabbroic Ni-Cu
deposit, Yakobi metallogenic belt, southeastern
Alaska. Schematic southwestnortheast
cross section. Adapted from
Himmelberg and Loney (1981). See figure
103 and table 4 for location.