USGS Professional Paper 1697 - Alaska Resources Library

Prince of Wales Island Metallogenic Belt of Continental-Margin
Arc-Related Deposits (Belt PW),
Southeastern Alaska
The Prince of Wales Island metallogenic belt occurs in
southeastern Alaska and consists mainly of a suite of porphyry
Cu-Mo, polymetallic vein, and skarn deposits (fig. 3; tables
3, 4) that primarily occur in alkalic Orodovician and Silurian
plutons in the Alexander sequence of the Wrangellia superterrane.
The deposits and metallogenic belt occur on central and
southern Prince of Wales Island, and to a much lesser extent
on Chichagof, Annette, and Gravina Islands in central- southeastern
Alaska (Nokleberg and others, 1995a). These alkalic
plutons range in age from Late Ordovician to Early Silurian.
The plutons intrude the metamorphosed Devonian(?) St. Joseph
Island Volcanics (Eberlein and others, 1983; D.A. Brew, oral
commun., 1995), Early and Middle(?) Devonian Karheen
Formation (Gehrels, 1992; D.A. Brew, oral commun., 1995),
Middle to Late Ordovician to Early Silurian Descon Formation
(Herreid and others, 1978; D.A. Brew, oral commun., 1995),
and metamorphosed Late Proterozoic and Early Cambrian(?)
Wales Group (Gehrels and Berg, 1992; D.A. Brew, oral commun.,
1995). The plutons and metasedimentary rocks form the
older part of the Alexander sequence in the region. The major
granitic-magmatism-related deposits are represented by deposits
in four areas (1) several deposits in the McLean Arm porphyry
Cu-Mo district, (2) the Klaka Inlet polymetallic skarn and vein
deposit, (3) the Kassan Peninsula Cu-Fe skarn deposit, and (4)
the major zoned mafic-ultramafic Cu-Au-PGE deposit at Salt
Chuck (table 4) (Nokleberg and others 1997a,b, 1998).
McLean Arm Porphyry Cu-Mo District
The McLean Arm porphyry Cu-Mo district (fig. 14)
contains a group of porphyry copper-molybdenum deposits,
which consist of precious metal stockworks and veins at
Poison, Ickis, Veta, Apex, and Stone Rock Bay prospects in
the central part of a northwest-trending belt of middle Paleozoic,
multiphase plutons composed of pyroxenite, syenite,
quartz monzonite, and mixed intermediate-composition igneous
rocks. The plutonic rocks intrude the clastic rocks of the
Descon Formation on the extreme tip of southern Prince of
Wales Island. The central part of the complex, which contains
the deposits, is mainly syenite. The altered and mineralized
syenite at Stone Rock Bay has a a U-Pb zircon age of 436 Ma
(Gehrels, 1992). The sulfide deposits occur mainly in stockwork
that occurs along joints and faults that strike 25° or 295°
and dip steeply. The deposits and their host joints and faults
appear to be related to a concentric alteration zone of about
5 km2 area, with a carbonate-albite center and an albite and
sericite rim. The higher grade veins and stockwork range from
0.4 to 5.6 percent Cu, 0.01 to 0.08 percent Mo, and 2.1 to 11.0
g/t Au. Anomalous Ag, Pt, Bi, Te, and base metals also occur
in the deposits (MacKevett, 1963; F.D. Forgeron and L.W.
Leroy, written commun., 1971; T.K. Bundtzen, unpub. data,
1990; Nokleberg and others, 1995a). Soil sampling, trenching,
Cambrian through Silurian Metallogenic Belts (570 to 408 Ma) 35
and limited diamond drilling done in 1972 suggests a potential
for 40 million tonnes of Cu-Mo ore at the Apex zone.
Polymetallic Vein, Skarn, and Disseminated Deposits in
Paleozoic Plutons at Klakas Inlet and Kassan Peninsula
A suite of polymetallic vein, skarn, and disseminated
deposits, which form part of the Prince of Wales Island metallogenic
belt, are associated with Silurian or older alaskite and
granodiorite in Klakas Inlet. The granodiorite, with a minimum
K-Ar isotopic age of 428 Ma (Turner and others, 1977), contains
sericite-altered veinlets of chalcopyrite, molybdenite, and galena
in a 100 m2 area. The deposit contains as much as 0.23 percent
Cu, 0.06 percent molybdenum, 0.05 percent Co, 0.05 percent Sn,
and 0.01 percent W. The high Sn and W values occur adjacent to
the main Cu and Mo deposits (Herreid and others, 1978).
A suite of polymetallic Cu vein, Cu-Fe (magnetite) skarn,
and disseminated deposits also occurs in or near altered Late
Ordovician to Early Silurian, intermediate-composition plutons
on Kasaan Peninsula. About 607,690 tonnes of Fe and Cu
ore were mined in this area prior to World War II (Warner and
Goddard, 1961). Most of the deposits consist of irregular bodies
of magnetite, chalcopyrite, and pyrite, and contain lesser
amounts of sphalerite and galena. The deposits contain minor
Au and Ag and generally occur in skarn associated with alkali
gabbro, diorite, and granodiorite. The plutonic rocks exhibit
U-Pb zircon isotopic ages ranging from the Late Ordovician to
the Early Silurian (Gehrels and Berg, 1992). The largest skarn
deposit in the area occurs at the Mount Andrew-Mamie mine,
the biggest Cu producer in the district (Bundtzen, 1978). The
deposits in this area are associated with peripheral polymetallic
veins and stockworks that contain chalcopyrite and pyrite
and Au values. Concentric magnetic anomalies in the area are
interpreted by Warner and Goddard (1961) as reflecting as a
buried porphyry Cu deposit.
Salt Chuck Zoned Mafic-Ultramafic Cu-Au-PGE Deposit
The Salt Chuck zoned mafic-ultramafic Cu-Au-PGE
(platinum group element) deposit (fig. 15) consists of irregularly
and randomly distributed veinlets of bornite and associated
minor chalcopyrite, chalcocite, covellite, native copper,
and magmatic magnetite (Donald Grybeck and David A. Brew,
written commun., 1985; Loney and others, 1987; Loney and
Himmelberg, 1992; Foley and others, 1997). The deposit produced
about 300,000 tonnes grading 0.95 percent Cu, 1.2 g/t
Au, 5.8 g/t Ag, 2.2 g/t PGE (mainly Pd and Pt), and produced
610,400 g PGE from 1907 to 1941. The sulfides and oxides
occur as disseminations and along cracks and fractures in
pipe-like late Paleozoic or Mesozoic gabbro-clinopyroxenite
stock intruding Silurian metagraywacke. Clinopyroxenite
and gabbro grade irregularly into one another. Bornite, the
principal sulfide; occurs mainly as interstitial grains in clinopyroxenite
in amounts as much as 15 percent. Extensive, late
magmatic or hydrothermal epidote veins occur in gabbro and
clinopyroxenite. Low-K, altered biotite from clinopyroxenite