USGS Professional Paper 1697 - Alaska Resources Library

and southeastern Alaska (Moll and Patton, 1982; Bundtzen
and Gilbert; 1983; Swanson and others, 1987; Plafker and
others, 1989; Szumigala, 1993; Miller and Bundtzen, 1994;
Moll-Stalcup, 1994; Nokleberg and others, 1995a; Bundtzen
and Miller, 1997). Supporting data for this interpretation
include (1) the alkali-calcic nature of the igneous rocks in the
Kuskokwim Mountains igneous belt that are more alkalic than
the coeval Alaska Range-Talkeetna Mountains igneous belt,
(2) trans-tensional tectonism associated with the Kuskokwim
Mountains igneous belt, and (3) the existence of peraluminous
igneous rocks and the deposits in the Kuskokwim Mountains
igneous belt. In a similar setting, alkalic porphyry Cu-Au and
polymetallic Sn deposits occur in back-arc extensional environments
in South America (Hollister, 1978), and early Tertiary
peraluminous intrusions in Yukon Territory are interpreted by
Sinclair (1986) as forming during extensional, wrench fault tectonism
related to strike-slip faulting. This interpretation is also
advocated by Miller and Bundtzen (1994) for the mineralized
plutons of the Kuskokwim Mountains igneous belt.
In Alaska, the other major parts of the Kluane arc are the
Alaska Range-Talkeetna volcanic-plutonic belt to the southeast,
the Yukon-Kanuti igneous belt to the northwest, and the Late
Cretaceous and early Tertiary granitoid rocks of the Yukon-
Tanana igneous belt to the northeast (Moll-Stalcup, 1994).
The Kluane arc is interpreted as forming immediately after the
accretion of Wrangellia superterrane in the mid-Cretaceous and
was consequently substantially dismembered by major dextralslip
faulting in the Cenozoic (Plafker and others, 1989; Nokleberg
and others, 1994d, 2000). These belts and the Kuskokwim
Mountains sedimentary and volcanic belt are interpreted as a
subduction-related arc that was tectonically linked to the Late
Cretaceous part of the Chugach accretionary-wedge terrane
(Valdez Group and equivalent units) and to the early Tertiary
part of the Prince William accretionary-wedge terrane (Orca
Group and equivalent units) (Nokleberg and others, 2000).
Felsic Porphyry Mo Deposit, Kuskokwim Mountains
Metallogenic Belt, Southwestern Alaska
A north-south-trending, linear belt of quartz monzonite to
granite porphyry stocks and plutons, which host porphyry Mo
deposits and prospects, intrude the Late Cretaceous flysch of the
Kuskokwim Group and older metamorphic rocks of the Ruby
terrane in the west-central Kuskokwim Mountains. From south to
north along a linear distance of about 125 km, these deposits and
prospects include the McLeod deposit in the eastern Kaiyuh Hills
along the lower Innoko River Drainage and the Molybdenum
Mountain deposit on the Owhat River. These two felsic porphyry
Mo deposits are generally similar to the model of Lowell and
Guilbert (1970). All three deposits and prospects contain concentric
phyllic and argillic alteration zones, plot in the granite field of
a normative QAPF diagram, and are peraluminous.
McLeod Porphyry Molybdenum Prospect
The McLeod porphyry Mo deposit strikes east-northeast
and consists of a molybdenite-bearing quartz stockwork in
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 229
a quartz porphyry stock about 3 km 2 in area (Mertie, 1937a;
Chapman, 1945; West, 1954; Nokleberg and others, 1995a).
The stock intrudes undifferentiated Cretaceous greenstone of
the Kuskokwim Group and is interpreted as associated with
quartz latite dikes that occur near the eastern boundary of
the pluton. In addition to quartz and molybdenite, the stockwork
contains pyrite, pyrrhotite, and chlorite. The stockwork
locally constitutes as much as 10 percent of the intrusion.
Veinlets also occur in adjacent greenstone. The quartz-feldspar
porphyry stock and latite dikes exhibit a phyllic alteration core
flanked by silicic and argillic zones in a 300 by 1,100 m area.
The stockwork averages 0.09 percent MoS 2 over a 30 by 350
m surface area. The quartz porphyry stock has a K-Ar biotite
age of 69.3 Ma (T.K. Bundtzen, unpub. data, 1987).
Molybdenum Mountain Porphyry Molybdenum Prospect
The Molybdenum Mountain porphyry Mo prospect (T.K.
Bundtzen, unpub. data, 1987; Nokleberg and others, 1995a)
consists of a stockwork of vein quartz with massive and disseminated
molybdenite, galena, and pyrite in the Molybdenum
Mountain stock, a small 2 km2 hypabyssal felsic intrusion
that occurs about 45 km northeast of Aniak. Alterations are
mainly silicic and sericitic. A large, elongate contact metamorphic
aureole surrounds the Molybdenum Mountain stock
and several smaller intrusions that occur about 4 to 6 km to
the northeast. Selected rock samples from the Molybdenum
Mountain stock contain as much as 5.0 percent MoS2; however,
average estimates of grade are not available. The stock
intrudes contact-metamorphosed, Late Cretaceous flysch of
the Kuskokwim Group along a large shear zone that is a splay
of the Iditarod-Nixon-Fork Fault, a major dextral-slip Cenozoic
fault in west-central Alaska. A K-Ar white mica age of
60.9 Ma is obtained from the stock.
Alkalic-Calcic Porphyry Cu-Au Prospects, Kuskokwim
Mountains Metallogenic Belt
Alkalic-calcic, porphyry Cu-Au deposits and prospects
have only been recently identified in the central Kuskokwim
Mountains. The geologic structure and alteration features
of the porphyry deposits in the southwestern Kuskokwim
Mountains metallogenic belt are generally those of the alkalic
porphyry Cu-Au model of Lowell and Guilbert (1970) or the
porphyry Cu-Au deposit model of Cox (1986b), except that
the classic alteration patterns are lacking. The significant
deposits are at Chicken Mountain and Von Frank Mountain.
These deposits display similar characteristics, including
tourmaline-bearing breccia pipes, stockworks, metal zoning,
and petrology. The host plutons for the deposits range in age
from 66 to 71 Ma and intrude Late Cretaceous flysch of the
Kuskokwim Group.
Chicken Mountain Cu-Au Deposit
The Chicken Mountain porphyry Au and polymetallic
vein deposit (fig. 108) (Bull, 1988; Bundtzen and others, 1992;