USGS Professional Paper 1697 - Alaska Resources Library

88 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
sulfide stringer stockworks. The deposit contains reserves
of 265 million tonnes grading 1.44 percent Cu, 0.07 percent
Co, and 0.20 g/t Au. Five additional stratiform Cu occurrences
were discovered in the area in 1992. The deposit age is
interpreted as Late Triassic on the basis of Norian conodonts
in limestone from the deposit (M. Orchard, written commun.,
1983). The host rocks are intruded by calc-alkaline diorite sills
and dikes and overlain by calc-alkaline pillow basalt at least
1,500 m thick. The large, Cu-bearing pyrite-pyrrhotite massive
sulfide bodies are folded, faulted, and sheared. The deposit is
transitional between Cyprus and Besshi volcanogenic massive
sulfide deposit types.
Greens Creek Kuroko Zn-Pb-Cu Massive Sulfide Deposit
The Greens Creek Kuroko Zn-Pb-Cu massive sulfide
deposit (fig. 38) occurs on Admiralty Island and consists of
sphalerite, galena, chalcopyrite, and tetrahedrite in a pyriterich
matrix. The sulfides occur in massive pods, bands,
laminations, and disseminations and are associated with
pyrite-carbonate-chert exhalite (Berg, 1984; Wells and others,
1986; Brew and others, 1991; Newberry and others, 1997).
The structural hanging wall contains chlorite- and sericite-bearing
metasedimentary rocks. The structural footwall
contains black graphitic argillite. Black ore forms an extensive
blanket deposit and is composed of fine-grained pyrite,
sphalerite, galena, and Ag-rich sulfosalts in laminations in
black carbonaceous exhalite and argillite. “White ore” occurs
along edges of massive sulfide pods and is composed of minor
tetrahedrite, pyrite, galena, and sphalerite in laminations,
stringers, or disseminations in massive chert, carbonate rocks,
or sulfate-rich exhalite. Local veins occur below the massive
sulfides and contain bornite, chalcopyrite, and gold. The veins
may constitute brine conduits. The sulfides and host rocks are
underlain by serpentinized mafic volcanic flows and tuffs. An
1,600 m
Adit
1,200 m
Argillite
Volcanic rocks
Contact
North Zone
0 100 m
Sulphide stringer zone
Massive sulphide - pyrite
Massive sulphide - pyrrhotite
Figure 37. Windy Craggy Cyprus massive sulfide deposit, Alexander
metallogenic belt, northern British Columbia, Canadian Cordillera.
Schematic cross section through North Zone. Adapted from
Downing and others (1990). See figure 32 and table 4 for location.
incremental Ar age of 211 Ma was obtained for hydrothermal
mariposite from a small massive sulfide occurrence near
Greens Creek (Taylor and others, 1995). The host rocks are
part of a Triassic suite of metasedimentary and metavolcanic
rocks in the Wrangellia sequence that overlies the early to
middle Paleozoic rocks of Alexander sequence. Both are part
of the Wrangellia superterrane (Nokleberg and others, 1994c,
1997c). The host rocks are tightly folded into a southeastplunging,
overturned antiform. The deposit is interpreted as
forming during marine exhalation in a Triassic back-arc or
wrench-fault extensional basin during deposition of the arc.
From 1989-1999, the Greens Creek mine produced 299,480
tonnes zinc, 122,400 tonnes lead, 1,896 tonnes silver, and
10,617 kg gold from 2,924,294 tonnes of ore (Swainbank and
Szumigala, 2000).
Castle Island Bedded Barite Deposit
The Castle Island bedded barite deposit consists of lenses
of massive barite interlayered with metamorphosed limestone
of probable Triassic age and with metamorphosed calcareous
and tuffaceous clastic rock (Berg and Grybeck, 1980; Berg,
1984; Grybeck and others, 1984; Brew and others, 1991).
Sulfide-rich interbeds contain sphalerite, galena, pyrite, pyrrhotite,
bornite, tetrahedrite, and chalcopyrite. The deposit
produced 680,000 tonnes of ore grading 90 percent barite.
Sulfide-rich layers contain as much as 5 percent galena and
sphalerite, and 100 g/t Ag.
Origin of and Tectonic Controls for Alexander
Metallogenic Belt of Massive Sulfide Deposits
The deposits in the Alexander metallogenic belt of massive
sulfide and associated deposits (tables 3, 4) are hosted in
a variety of rocks. At the Windy Craggy deposit, the basalt
flows hosting the massive sulfide deposit consist of a thick
unit of dominantly alkaline to subalkaline composition and
abundant interleaved, craton-derived clastic sedimentary
rocks that are more characteristic of a Besshi-type deposit.
Hosting the Greens Creek and other kuroko massive sulfide
deposits are suites of Triassic metasedimentary rocks, argillite,
and siliceous metavolcanic rocks. Hosting the Castle Island
bedded barite deposit is Triassic(?) limestone and calcareous
and tuffaceous clastic rocks. The Triassic units constitute
the younger part of the Wrangellia sequence in southeastern
Alaska, which in this area overlies the early to middle Paleozoic
Alexander sequence; together the two sequences form the
Wrangellia superterrane in the region (Nokleberg and others,
1994c, d, 1997c, 2000)
In southeastern Alaska, in addition to marine basalt, the
Triassic part of the Wrangellia sequence contains siliceous
(meta)volcanic rocks (rhyolite and tuff), limestone, argillite,
and conglomerate in a relatively narrow belt on the eastern
side of the terrane (Gehrels and Berg, 1994). In contrast, in
southern Alaska, the Triassic part of the Wrangellia sequence
consists of thick unit of marine and subaerial basalt in the
Nikolai Greenstone, and lesser limestone (Nokleberg and oth-