USGS Professional Paper 1697 - Alaska Resources Library

240 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
The stockwork varies from weak to intense, consists mainly
of quartz and sulfide minerals, and is developed both in the
dacite porphyry and in overlying volcanic rocks. Extensive
propylitic and silicic alteration occurs in the dacite porphyry.
The deposit contains a possible resource of 90 million tonnes
and some samples containing as much as 0.25 percent Cu and
0.17 percent Mo (T.K. Bundtzen, written commun., 1984).
Golden Zone Deposit
The Golden Zone deposit exhibits features common to
polymetallic vein, Au-Ag breccia pipe, and porphyry Cu deposits.
The sulfide mineralogy consists of auriferous arsenopyrite
and minor chalcopyrite, sphalerite, and pyrite in a quartz gangue
that fills open spaces in a breccia pipe (Hawley and Clark, 1974;
Swainbank and others, 1978; C.C. Hawley, written commun.,
1985, 1990). The breccia pipe occurs in the highly fractured
central part of an Late Cretaceous early Tertiary quartz diorite
porphyry. The ore zone is about 125 m in diameter. At the
surface, high-grade mineralization occurs in a breccia pipe that
is approximately 75 m wide. Abundant polymetallic veins occur
adjacent to the porphyry. The breccia pipe contains an estimated
8.9 million tonnes grading 3.2 g/t Au and minor Cu and Ag. The
deposit contains an inferred reserve of 1.6 million tonnes grading
5.2 g/t Au and 0.5 percent Cu. An old mine at the deposit
produced 50 kg Au, 267 kg Ag, and 19 tonnes Cu. The diorite
porphyry has been dated at 68 Ma (Swainbank and others,
1978) and intrudes Permian to Jurassic sedimentary rocks of the
Chulitna (ophiolite) terrane, a fault-bounded fragment within
the highly deformed Kahiltna overlap assemblage.
Nabesna Glacier Polymetallic Vein(?) Deposit
The Nabesna Glacier polymetallic vein(?) deposit (Richter,
1975) occurs in three contiguous areas that contain (1) quartz
veins and veinlets containing pyrite and minor chalcopyrite and
sphalerite, (2) a zone of disseminated malachite and azurite, and
(3) a zone of intense alteration and breccia cemented by quartz,
pyrite, chalcopyrite, and galena. The deposit occurs in late
Paleozoic metavolcanic porphyry and metabasalt flows of the
Tetelna Volcanics and may be related to nearby Late Cretaceous
and early Tertiary granitoid plutons and dikes.
Sn and Mo Lode Deposits Hosted by Granitoid Plutons of
McKinley Sequence
A distinctive group of Sn-greisen, polymetallic vein, and
porphyry Mo deposits occur in part of the southern Alaska metallogenic
belt at Boulder Creek, Ohio Creek, Coal Creek, and Treasure
Creek (Nokleberg and others, 1987, 1988, 1993). These and
other lesser deposits occur in or near the early Tertiary McKinley
sequence of granite and granodiorite plutons that exhibit a narrow
age range of 55 to 60 Ma (Reed and Lanphere, 1973; Lanphere
and Reed, 1985). Many of the McKinley sequence plutons are
peraluminous granites having high initial Sr ratios, suggesting
incorporation of large amounts of crustal material (Reed and
Lanphere, 1973; Lanphere and Reed, 1985). The plutons of the
McKinley sequence intrude the the Wrangellia sequence of the
Wrangellia supterterrane and the Kahiltna overlap assemblage
(Jones and others, 1987). Lanphere and Reed (1985) interpreted
these granitoid rocks as forming during early Tertiary collision
of the Wrangellia composite terrane with various parts of the
passive continental-margin Central composite terrane to the north.
However, because more recent tectonic analyses suggest this
collision occurred in the mid-Cretaceous (Nokleberg and others,
1985, 1994d; Plafker and others, 1989b; Plafker and Berg, 1994),
the McKinley sequence granitoid plutons are herein interpreted as
forming during the crustal contamination of magmas from early
Tertiary subduction along the southern margin of Alaska.
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 that are composed of rhyolite,
dacite, and andesite flows, pyroclastic rocks, and interlayered
basalt and andesite flows of 50 to 75 Ma in 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-Stalcup, 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, 1997c).
The granitoid rocks in the Farewell District 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 that 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;
Newberry 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 to 30 Ma. Geochemical
and isotopic data indicate that the Alaska Range-Talkeetna
Mountains igneous belt has low TiO2, moderate K2O, no Feenrichment,
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 the central part of the extensive, subduction-related
Kluane igneous arc that formed along the Late