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

skarn bodies occur in limestone, chert, and argillite of the Late Triassic Kamishak Formation and in overlying metavolcanic rocks
of the Late Triassic(?) to Early Jurassic Talkeetna Formation (Nokleberg and others, 1994d). The largest skarn body at Sargent
Creek contains epidote, garnet, actinolite, quartz, pyrite, and chalcopyrite. Lenses up to 1 m wide and 10 m long average 7% Cu.
Numerous airborne magnetic anomalies occur in the area surrounding the granodiofite stock. The Crevice Creek deposit produced
11 tonnes of ore from high-grade zones, with an average grade of 4.5 g/t Au, 5 14 git Ag, and 17.5% Cu.
Origin of and Tectonic Controls for
Alaska Peninsula Metallogenic Belt
The Cu-Au and Cu-Zn skarn deposits of the Alaska Peninsula metallogenic belt occur in areas where Jurassic(?) quartz
diorite and tonalite intrude calcareous sedimentary rock, and generally consist of epidote-garnet skarn in limestone or marble,
containing disseminations and layers of chalcopyrite, sphalerite, and pyrrhotite. The Fe skarn deposits occur in dolomite or marble
and generally consist of magnetite skarn containing lesser garnet, amphibole, and rare chalcopyrite. The Fe skarns occur in areas
where Jurassic(?) quartz diorite and tonalite intrude calcareous sedimentary rocks. These skam deposits occur in marine
sedimentary rocks of the Late Triassic Kamishak Formation, in Early Triassic marble, and in volcanic and volcaniclastic rocks of
the Late Triassic(?) to Early Jurassic Talkeetna Formation.
The Alaska Peninsula metallogenic belt occurs in, or adjacent to, the Late Triassic(?) and Early to Middle Jurassic,
Talkeetna part of the Talkeetna-Bonanza island arc which extends for several hundred km along the strike length of the Alaska
Peninsular part of the Wrangellia superterrane (Burns, 1985; Plafker and others, 1989; Nokleberg and others, 1994c, 1997c;
DeBari and Coleman, 1989). Abundant field, chemical, and isotopic data indicate that the Talkeetna arc is mainly gabbro, diorite,
and tonalite, and rarely granodiorite, has calc-alkaline composition and lower initial Sr ratios, and is interpreted as having formed
in an island arc above a subduction zone (Reed and others, 1983; Burns, 1985; Plafker and others, 1985). The Jurassic(?) plutonic
rocks, which host the Alaska Peninsula metallogenic belt, form the older part of the Alaska-Aleutian Range batholith, which along
with the Late Triassic(?) and Early Jurassic Talkeetna Formation and Border Ranges ultramafic-mafic complex, collectively define
the Talkeetna arc which is a key comlonent of the Peninsular sequence (Nokleberg and others, 1994a).
Talkeetna Mountains-Alaska Range
Metallogenic Belt of Kuroko Massive
Sulfide Deposits (Belt TM) Northern Part of
Southern Alaska
The Talkeetna Mountains-Alaska Range metallogenic belt of kuroko massive sulfide deposits (fig. 42; tables 3,4) occurs
in the northern part of southern Alaska. The metallogenic belt is hosted in submarine tuff, andesite, and dacite of the Late
Triassic(?) and Early Jurassic Talkeetna Formation, which is a major unit in the Peninsular sequence and Talkeetna-Bonanza
islanc arc of the Wrangellia superterrane (Nokleberg and others, 1994c, 1997~). The one significant deposit is the Johnson River
prospect (table 4) (Nokleberg and others 1997a, b, 1998).
JOhnSbn River Massive Sulfide(?) Deposit
The Johnston River kuroko massive sulfide(?) deposit (R. L. Detterman, oral cornmun., 1984; Steefel, 1987; Madelyn
Mollholyn, written comrnun., 1988; J. Proffett, written cornmun., 1991) consists of quartz-sulfide veins and massive sulfide lenses
containing chalcopyrite, pyrite, sphalerite, galena, and gold which occur in discordant pipe-like bodies of silicified volcanic rock.
Veins of chlorite, sericite, and anhydrite, and a cap of barite occur proximal to the four ore horizons. The deposit occurs in
pyroclastic and volcaniclastic rocks of Portage Creek Agglomerate in the Talkeetna Formation; s~milar mineralized horizons have
been found along strike to the northeast. Local stockworks, which cut the metavolcanic rock, suggest either mobilization or
additional deposition. The deposit is interpreted as forming from deposition of sulfides directly over a capped submarine vent
system during Jurassic volcanism. The deposit contains an estimated 997,540 tonnes grading 10.35 g/t Au, 7.84 glt Ag, 8.3% Zn,
1.1% Pb, and 0.76% Cu (Bundtzen and others, 1994). In the same region, in the Oshetna River drainage of the Nelchina district,
northeast of Anchorage, tuff in the Talkeetna Formation contains disseminated chalcopyrite and barite. Also in this region, Au-
enriched massive sulfide deposits in the Eskay Creek district contain many similar morphological features to those described at
Johnson River.
Oflgin of and Tectonic Controls for
Talkeetna Mountains-Alaska Range Metallogenic Belt
The Late Triassic(?) and Early Jurassic Talkeetna Formation (fig. 42) which hosts the Talkeetna Mountains-Alaska
Range metallogenic belt consists mainly of bedded andesitic volcaniclastic sandstone and tuff, ignimbrite, breccia, and
agglomerate; andesite and lesser rhyolite and basalt flows; and shale (Plafker and others, 1989; Nokleberg and others. 1994a).The
Talkeetna Formation is linked to Middle Jurassic plutonic rocks which form the older part of the Alaska-Aleutian Range batholith,
which along with the Border Ranges ultramafic-mafic complex, define the Talkeetna arc (Nokleberg and others, 1994a). The
Peninsular sequence forms a major part of the Talkeetna-Bonanza island arc, and is one of three major sequences in the
Wrangellia superterrane. The Talkeetna arc is tectonically linked to a discontinuous series of Early Triassic to Jurassic(?)