USGS Professional Paper 1697 - Alaska Resources Library

ers, 1994c, 1997c). In southern British Columbia on Vancouver
Island, similar thick marine basalt forms the Karmutsen
Volcanics (Nokleberg and others, 1994c, 1997c).
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, 1997c)
(1) 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) that are generally related to rifting, and (3) the occur-
0 200 m
Host Rocks (Triassic)
Metamorphosed argillite and calc-argillite
Quartz-pyrite-sericite-carbonate phyllite
Chlorite-calcite phyllite
Greenstone
Sulfide Assemblages & Related Units (Triassic)
White ore carbonate and locally barite rich
Massive sulfide, base-metal rich
Massive sulfide, fine-grained chiefly pyrite
White massive sulfides, silica-rich
Siliceous, carbonate breccia
Contact
> 0.4% Cu
+ _ Au, Zn
< 0.2% Cu
?
?
?
Late Triassic Metallogenic Belts (230 to 208 Ma; fig. 32) 89
rence of turbiditic clastic rocks. On the basis of geochemical
data, either back-arc rifting or hot-spot activity is interpreted
to have formed the widespread basalt fields of the Nikolai
Greenstone and Karmutsen Formation and coeval mafic and
ultramafic sills and plutons (Barker and others, 1989; Richards
and others, 1991; Lassiter and others, 1994). Herein, back-arc
rifting is interpreted as the tectonic environment for the Triassic
strata and contained massive sulfide deposits. The rifting is
tectonically linked to the coeval Bonzana-Talkeetna island arc,
which occurs along the length of the Wrangellia superterrane
for several thousand km (Nokleberg and others, 2000).
Cu < 0.2%
0
30 m
Figure 38. Greens Creek kuroko massive sulfide deposit, Alexander metallogenic belt, southeastern
Alaska. Schematic, predeformation, cross-section model based on drill core logging and underground
and surface mapping. Modified from Newberry and others (1997). See figure 32 and table 4 for location.