Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

Alaska Range-Talkeetna Mountains Igneous Belt
The Alaska Range-Talkeetna Mountains igneous belt, which hosts the southern Alaska metallogenic belt, extends for
about 700 km in the western and central parts of southern Alaska (fig. 103). The igneous belt (unit at, fig 103) consists chiefly of
(Moll-Stalcup, 1994; Moll-Stalcup and others, 1994): (1) large and small volcanic fields which are composed of rhyolite, dacite,
and andesite flows, pyroclastic rocks, and interlayered basalt and andesite flows of 50- to 75-Ma age; and (2) numerous related
diorite, quartz diorite, tonalite, granodiorite, and granite and locally monzonite and syenite plutons. The latter constitute the
plutonic part of the Late Cretaceous to early Tertiary part of the Aleutian-Alaska Range and Talkeetna Mountains batholith. The
igneous belt occurs mostly south of the Denali Fault, and is partly coeval with the Kuskokwim Mountains igneous belt to the
northwest (Moll-Staicup, 1994). The Alaska Range-Talkeetna Mountains igneous belt overlies various terranes in southern
Alaska, including the Wrangellia superterrane, and the Kahiltna overlap assemblage (Jones and others, 1987; Nokleberg and
others, 1994c, 1997~).
In the Farewell District, the granitoid rocks and adjacent areas range in age from 52 to 60 Ma (Szumigala, 1987;
Bundtzen and others, 1988; Gilbert and others, 1990; Solie and others, 1991; Nokleberg and others, 1993). In the Chulitna
District, the granitoid rocks which are associated with deposits (Golden Zone, Nim, Nimbus, Silver King), and in the Valdez
Creek district (Zackly deposit) the associated granitoid rocks range in age from 65 to 68 Ma (Swainbank and others, 1978;
Newbeny and others, 1997). The low temperature Pb-Zn skarns at Tin Creek and Sheep Creek replace and alter granodiorite dikes
with isotopic ages of 25-30 Ma. Geochemical and isotopic data indicate that the Alaska Range-Talkeetna Mountains igneous belt
has low Ti02, moderate KzO, no Fe-enrichment, a calc-alkalic compositional trend, and low initial Sr ratios (Szumigala, 1987;
Moll-Stalcup, 1994).
Origin of and Tectonic Setting for
Southern Alaska Metallogenic Belt
The Southern Alaska metallogenic belt is hosted by the Alaska Range-Talkeetna Mountains igneous belt which is
interpreted as central part of the extensive, subduction-related Kluane igneous arc which formed along the Late 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 TiOz and moderate K20 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
subduction-zone 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-southem 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; Coldfarb and
others, 1997) consist of free gold and minor pyrite, pyrrhotite, arsenopyrite, galena, and sphalerite in sheeted quartz veins which
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 biotite-blocking 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.