USGS Professional Paper 1697 - Alaska Resources Library

242 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
margin of Jurassic quartz diorite and younger Cretaceous and
early Tertiary granitoid rocks of the Talkeetna Mountains
batholith, and also locally in mica schist. The Willow Creek
district consists of several mines and many prospects, most in
an area about 12.8 km long and 6.2 km wide along southern
portion of the Talkeetna Mountains batholith. The mine at
the deposit contains several thousand meters of underground
workings and produced about 18,400 kg Au from 1909 to
1950. Ore grade ranged from about 17 to 69 g/t Au.
Origin of and Tectonic Controls for Talkeetna Mountains
Metallogenic Belt
Hydrothermal micas from the Gold Bullion and Lucky
Shot mines in the Willow Creek district exhibit K-Ar isotopic
ages of 66 and 57 Ma, respectively (Madden-McGuire
and others, 1989). These ages are interpreted to indicate that
gold mineralization in the Talkeetna Mountains metallogenic
belt occurred in the Late Cretaceous or early Tertiary. Three
different tectonic events might be the cause of mineralization—(1)
Late Cretaceous and early Tertiary underthrusting of
the Chugach and Prince William accretionary-wedge terranes
along the Contact fault in southern Alaska, (2) as favored
herein, early Tertiary underthrusting of the spreading Kula
Ridge under the southern margin of coastal Alaska (Plafker
and others, 1989), and (or) (3) early Tertiary dextral-slip faulting
along on the Castle Mountain Fault system.
Chugach Mountains Metallogenic Belt of Au
Quartz Vein Deposits (Belt CM) Southern Alaska
The Chugach Mountains metallogenic belt of Au quartz
vein deposits (fig. 103; tables 3, 4) occurs on Kodiak Island,
the southeastern Kenai Peninsula, and in the central and eastern
Chugach Mountains in southern Alaska. The Au quartz vein
deposits occur mainly in the Late Cretaceous flysch of the Valdez
Group and Kodiak Formation where these units were metamorphosed
to greenschist facies (Goldfarb and others, 1986,
1997, 1998; Nokleberg and others, 1994c, 1997c, 1989b). To
a lesser extent, the belt also occurs (1) in the northern margin
of the early Tertiary Orca Group of the Prince William terrane
where locally metamorphosed to lower greenschist facies,
(2) in metasedimentary rock of the Orca Group within a few
kilometers of granitoid plutons, (3) along the southern margin
of the Late Triassic through mid-Cretaceous McHugh Complex
on the Kenai Peninsula, and (4) in Eocene and Oligocene
granitoid plutons intruding the Valdez and Orca Groups. The
deposits in this belt are of small tonnage but locally high grade.
For this widespread group of mines and deposits, a unique
mineral deposit model for Chugach-type low-sulfide Au quartz
veins has been developed by Bliss (1992). The significant
deposits are at Cliff, Alaska Oracle, Chalet Mountain, Crown
Point, Kilpatrick, Gold King, Granite, Jewel, Kenai-Alaska,
Lucky Strike (Palmer Creek), Mineral King (Herman and
Eaton), Monarch, and Ramsey-Rutherford (table 4) (Nokleberg
and others 1997a,b, 1998). Substantial Au production occurred
in this district until the early 1940’s; recent exploratory work
has been conducted at the Cliff Mine near Valdez, the largest of
the many known deposits.
Cliff Au Quartz Vein Deposit
The Cliff Au quartz vein deposit (Johnson, 1915; Pickthorn,
1982; Jansons and others, 1984) consists of quartz
veins, as much as 3 m thick and 515 m long, containing gold,
pyrite, galena, sphalerite, arsenopyrite, and stibnite. The veins
are hosted in metagraywacke and minor phyllite of the Late
Cretaceous Valdez Group. The veins occur in a complicated
system of intersecting faults. Sulfide minerals are about 3 to 5
percent of the ore. The mine contains a few thousand meters
of underground workings. Production occurred mainly from
1906 to 1940. The average grade ranged from 34 to 69 g/t Au,
and the mine at the deposit produced about 1,610 kg Au from
about 25,000 tonnes of ore.
Origin of and Tectonic Controls for Chugach Mountains
Metallogenic Belt
The Au quartz veins generally occur in the younger
of two generations of quartz fissure veins (Richter, 1963;
Goldfarb and others, 1986, 1997, 1998; Nokleberg and others,
1989b). The older and mostly barren veins are approximately
parallel to the regional schistosity and to axial planes of minor
and major folds. Their strike varies from northwest in the east
to northeast in the west. The younger Au veins generally occur
in a set of tensional cross joints or fractures and are normal to
the older, barren quartz veins. Both sets of quartz veins generally
dip steeply to vertically. Hydrothermal muscovite from
Au-bearing veins has been dated at 53 Ma in the Port Valdez
district (Winkler and others, 1981b), at 52 Ma in the Hope-
Sunrise district (Mitchell and others, 1981), and at 57 Ma in
the Nuka Bay district (Borden and others, 1992).
These field relations and isotopic ages are interpreted
as indicating that the deposits formed during early Tertiary
regional metamorphism and anatectic granite plutonism.
Regional tectonic and isotopic studies suggest that the Au veins
formed in the Orca and Valdez Groups in response to subduction
of the spreading Kula-Farallon Ridge beneath the southern
Alaska continental margin (Plafker and others, 1989; Bradley
and others, 1993; Haeussler and Nelson, 1993; Haeussler and
others, 1995; Goldfarb and others, 1995; 1997; Goldfarb, 1997).
Baranof Metallogenic Belt of Au Quartz Vein
Deposits Southeastern Alaska (Belt BN)
The Baranof metallogenic belt of Au quartz vein deposits
occurs in southeastern Alaska and consists of a 250-km-long
belt hosted in the Chugach subduction-zone terrane (fig. 103;
tables 3, 4) (Brew, 1993; Monger and Nokleberg, 1996; Goldfarb
and others, 1997, 1998). The significant deposit in the belt
are Apex and El Nido, Chichagoff, Cobol, Hirst-Chichagof, and
Reid Inlet (Nokleberg and others 1997a,b, 1998). The principal