USGS Professional Paper 1697 - Alaska Resources Library

244 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
tain minor gold and considerable pyrite, chalcopyrite, galena,
minor sphalerite, and secondary copper minerals (Bundtzen
and others, 1990; Goldfarb and others, 1991, 1997; Brew
and others, 1991; Swainbank and others, 1991). Pyrite is the
dominant sulfide. Gold associated with pyrite occurs as minute
blebs in goethite rims and fracture fillings in corroded crystals.
The gangue consists of quartz and lesser ankerite, chlorite,
and sericite. The mine produced about 1.5 million g Au, and
reserves are estimated at 907,000 tonnes of ore grading 10.5
g/t Au. The age of mineralization is interpreted as 55 Ma. The
deposit contains more than 5,500 m of horizontal workings.
The deposit is hosted in Cretaceous quartz diorite that exhibits
proximal ankerite, quartz, and sericite alteration adjacent to
veins and more widespread propylitic alteration. The quartz
diorite intrudes Late Triassic greenstone, graywacke, and
argillite of Alexander sequence of the Wrangellia superterrane.
Riverside Au Quartz Vein Deposit
The Riverside Au quartz vein deposit consists primarily
of disseminated galena, pyrite, tetrahedrite, pyrrhotite, chalcopyrite,
sphalerite, gold, and scheelite in two large quartz veins,
but in the Lindeberg lode, part of the deposit is a combined
quartz vein and epigenetic replacement deposit (Byers and
Sainsbury, 1956; Smith, 1977). The mine at the deposit produced
about 27,200 tonnes, yielding 93,300 g Au, 3.1 million
g Ag, 45,400 kg Cu, 113,500 kg Pb, 9,080 kg Zn, and 3,500
units (318,000 kg) WO3. The veins occur in a shear zone,
schist inclusion, and mylonitic gneiss derived from the Triassic
Texas Creek Granodiorite of the informally named Coast
plutonic-metamorphic complex of Brew and Ford (1984).
Sumdum Chief Au Quartz Vein Deposit
The Sumdum Chief Au quartz vein deposit consists of
two quartz-calcite fissure veins that contain gold, auriferous
pyrite, galena, sphalerite, chalcopyrite, and arsenopyrite (Brew
and Grybeck, 1984; Kimball and others, 1984; Goldfarb and
others, 1988, 1991, 1997). Gold is unevenly distributed and
occurs mainly in pockets where small veins intersect the
main veins. Mining produced about 750,000 g each of Ag and
Au from ore that averaged about 13.7 g/t Au. The veins, as
much as 6 m thick, occur in upper Paleozoic(?) or Mesozoic
graphitic slate and marble of the informally named Coast plutonic-metamorphic
complex of Brew and Ford (1984).
Treadwell Au Quartz Vein Deposit
The large Treadwell Au quartz vein deposit consists of
disseminated sulfides in quartz and quartz-calcite vein systems
in shattered albite sills and dikes. The veins contain sparse
gold, pyrite, magnetite, molybdenite, chalcopyrite, galena,
sphalerite, and tetrahedrite. The best ore grade is associated
with abundant quartz and calcite veinlets (Light and others,
1989; Goldfarb and others, 1991, 1997). Mining produced
about 90.1 million gm Au from 25 million tonnes of ore. The
albite sills and dikes are Eocene and intrude Jurassic(?) and
Early Cretaceous(?) slate and greenstone derived from basaltic
tuff or agglomerate (part of the Treadwell Slate in the Gravina-
Nutzotin belt). Some ore forms a zone at least 1,100 m long
in slate inclusions and in adjacent wall rock. The deposit was
mined from above sea level to 790 m beneath sea level in
Gastineau Channel from 1885 to 1922, when most workings
flooded during a catastrophic influx of sea water.
Origin of and Tectonic Controls for Juneau Metallogenic Belt
The Juneau metallogenic belt is hosted in a wide variety
of metamorphosed sedimentary, volcanic, and plutonic rocks
that occur adjacent to a complex series of faults between
the Alexander and Wrangellia sequences of the Wrangellia
superterrane (Nokleberg and others, 2000), the Gravina
sequence (Gravina-Nutzotin-Gambier overlap assemblage),
and in syn- to postaccretionary granitoid plutonic rocks,
including the foliated tonalite sill of southeastern Alaska
(Brew and Ford, 1984). Late Paleozoic and early Mesozoic
metasedimentary and metavolcanic rocks host the Jualin
(Berg, 1984) and Kensington (Bundtzen and others, 1988)
deposits in the Berner Bay district and the Sumdum Chief
(Kimball and others, 1984) and Sea Level deposits. Flysch of
the Gravina-Gambier overlap assemblage hosts the Treadwell
group of quartz-sulfide-gold deposits, across the Gastineau
Channel from the Alaska-Juneau mine (Spencer, 1905), and
the Gold Standard and Goldstream deposits. Hydrothermal
muscovite from Au-bearing veins from the Alaska-Juneau
mine has been dated at about 54 to 57 Ma (Haeussler and
others, 1995; Goldfarb and others, 1997). As described
above for the origin of the Chugach Mountains, Baranof,
and Maclaren metallogenic belts, the Au quartz veins of
the Juneau metallogenic belt are interpreted as forming 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).
Metallogenic Belts Formed During Early Tertiary
Spreading Along Kula-Farallon Oceanic Ridge,
Southern and Southeastern Alaska
Prince William Sound Metallogenic Belt of
Besshi and Cyprus Massive Sulfide Deposits
(Belt PW), Eastern-Southern Alaska
The Prince William Sound metallogenic belt of Besshi
and Cyprus massive sulfide deposits (fig. 103; tables 3,
4) occurs in the Prince William Sound district along the
eastern and northern margins in eastern-southern Alaska.
The metallogenic belt is hosted in the southern margin of
the Chugach and the Prince William accretionary-wedgeturbidite
terranes (Jones and others, 1987; Nokleberg and