USGS Professional Paper 1697 - Alaska Resources Library

Cretaceous and early Tertiary continental margin of southern
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; Bundtzen and Miller, 1997). Supporting
data for this interpretation include (Moll Stalcup, 1994;
Moll-Stalcup and others, 1994) (1) the calcic nature of igneous
rocks, (2) low initial Sr ratios, low TiO 2, and moderate K 2O
values, and (3) the occurrence of the Alaska Range-Talkeetna
Mountains igneous belt along the southern margin of southern
Alaska, a few kilometers north of major, coeval subductionzone
and accretionary-wedge complexes to the south, which
are thrust northward, under the Alaska Range-Talkeetna
Mountains igneous belt to the north.
Metallogenic Belts Formed During Early
Tertiary Oblique Subduction of Kula-Farallon
Oceanic Ridge Under Margin of Southern and
Southeastern Alaska
Maclaren Metallogenic Belt of Au Quartz Vein
Deposits (Belt MC), Northern Part of Eastern-
Southern Alaska
The Maclaren metallogenic belt of Au quartz vein
deposits occurs in the Maclaren continental-margin arc terrane
in the central Alaska Range in the northern part of eastern-southern
Alaska (fig. 103; tables 3, 4) (Nokleberg and
others, 1997b, 1998; Goldfarb and others, 1997, 1998). The
metallogenic belt is hosted in the Maclaren Glacier metamorphic
belt, which is probably derived from Late Jurassic(?)
flysch composed chiefly of argillite and metagraywacke
(Nokleberg and others, 1985, 1994d). The significant deposits
are at Timberline Creek and Lucky Hill.
Lucky Hill and Timberline Creek Au Quartz Vein Deposits
The Lucky Hill and Timberline Creek Au quartz vein
deposits (Smith, 1981; Adams and others, 1992; Goldfarb
and others, 1997) consist of free gold and minor pyrite,
pyrrhotite, arsenopyrite, galena, and sphalerite in sheeted
quartz veins that strike east-northeast and dip steeply to the
northwest. The veins occur in semischist of the Maclaren
Glacier metamorphic belt at Lucky Hill and in granodiorite
at Timberline Creek. A distinctive, yellowish, ankerite-carbonate
assemblage also occurs in some veins. Incremental Ar
isotopic ages on primary micas indicate an emplacement age
of 90 to 100 Ma, whereas the age of vein formation is 57 to
63 Ma. The latter ages are the same as determined for biotiteblocking
temperature in the Maclaren Glacier metamorphic
belt. The deposits contain an estimated 348,000 tonnes, averaging
7.1 g/t Au. One of Alaska’s largest placer Au mines at
Valdez Creek occurs downstream from the Lucky Hill and
Timberline Creek deposits.
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 241
Origin of and Tectonic Controls for Maclaren
Metallogenic Belt
The Maclaren Glacier metamorphic belt, which hosts
the Maclaren metallogenic belt of Au quartz vein deposits,
and part of the Maclaren terrane, displays classic prograde,
Barrovian-type metamorphism in which the lower, greenschist
facies portions are judged as highly favorable for Au-quartz
vein deposits. 40Ar-39Ar ages of metamorphic minerals in auriferous
quartz veins that occur in the lower greenschist facies
and have isotopic ages rangeing from about 58 to 62 Ma (early
Tertiary) (Adams and others, 1992). These ages are interpreted
to indicate that Au mineralization occurred in the early Tertiary
during oblique subduction of Kula-Farallon oceanic ridge
under margin of southern Alaska (Bradley and others, 1993;
Haeussler and Nelson, 1993; Haeussler and others, 1995;
Goldfarb and others, 1995; 1997; Goldfarb, 1997)..
The Maclaren terrane is interpreted as a displaced continental-margin
arc fragment that was tectonically separated
from the Kluane schist and the Ruby Range batholith that both
occur on the northeast side of the Denali Fault some 400 km
to the southeast in the Yukon Territory (Nokleberg and others,
1985, 1994d, 2000; Plafker and others, 1989). Similar major
Au quartz vein deposits occur in the Juneau metallogenic belt
of southeastern Alaska (described herein) (Nokleberg and others,
1985, 1993, 1994d).
Talkeetna Mountains Metallogenic Belt of Au
Quartz Vein Deposits (Belt TM), Northern Part of
Southern Alaska
The Talkeetna Mountains metallogenic belt of Au quartz
vein deposits occurs in the Talkeetna Mountains in the northern
part of southern Alaska (fig. 103; tables 3, 4) (Nokleberg
and others, 1997b, 1998). The metallogenic belt is hosted in
the Late Triassic(?) and Early Jurassic Talkeetna Formation,
where intruded by Jurassic and Cretaceous granitoid rocks that
were deformed and metamorphosed to lower greenschist facies
during the early Tertiary (Nokleberg and others, 1994c, 1997c).
The significant deposit is the Independence mine in the Willow
Creek District. Other deposits in the district include those at
Gold Bullion, Gold Cord, Lucky Shot, and Thope
Independence Au Quartz Vein Deposit
The Independence Au quartz vein deposit, which occurs
in the Willow Creek district in the Talkeetna Mountains (Ray,
1954; Madden-McGuire and others, 1989), consists of quartz
veins containing a few percent or less pyrite, chalcopyrite,
magnetite, and gold and minor arsenopyrite, sphalerite, tetrahedrite,
Au-tellurides, and galena. The veins average 0.3 to 1
m thick, but some are as much as 2 m thick. The veins occupy
east-northeast and north-south-striking shear zones as much as
7 m wide. Considerable alteration of wall rocks occurs marked
by formation of sericite, pyrite, carbonate, and chlorite in parallel
bands. The veins occur in zones in and along the southern