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

underlain by serpentinized mafic volcanic flows and tuffs. An incremental Ar age of 2 1 1 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 which overlies the early to middle Paleozoic
rocks of Alexander sequence. Both are part of the Wrangellia superterrane (Nokleberg and others, 1994c, 1997~). The host roc
are tightly folded into a southeast-plunging, overturned antiform. The deposit is interpreted as forming during marine exhalati
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).
Volcanic rocks
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).
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% barite. Sulfide-rich layers contain up to
5% 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 sub-alkaline composition, and abundant interleaved, craton-derived clastic sedimentary rocks which are
more characteristic of a Besshi-type deposit. Hosting the Greens Creek and other kuroko massive sulfide deposits are suites of
Triassic metasedimentary rocks, arglllite, 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 others, 1994c, 1997~). In
southern British Columbia on Vancouver Island, similar thick marine basalt forms the Karmutsen Volcanics (Nokleberg and
others, 1994c, 1997~).
The Triassic strata and contained massive sulfide deposits of the Alexander metallogenic belt are interpreted as forming
in a back-arc rift environment on the basis of (Dawson, 1990; Gehrels and Berg, 1994; Nokleberg and others, 1994c, 1997~): (I)
the presence of bimodal volcanic rock (basalt and rhyolite); (2) a variety of deposit types (Cyprus to Besshi, kuroko, and
carbonate-hosted massive sulfide deposits, and bedded barite deposits) which are generally related to rifting; and (3) the