USGS Professional Paper 1697 - Alaska Resources Library

with a 0.25 percent cut-off. The deposit occurs in an altered
Late Cretaceous quartz monzonite stock of the Surprise Lake
Suite, with a K-Ar isotopic age of 87.3 Ma, and in adjacent
contact-metamorphosed argillite of the early Paleozoic Nasina
Assemblage of the Yukon-Tanana terrane. The stock is complex,
exhibits a classical alteration pattern, and is intruded by
barren quartz diorite.
Surprise Lake Polymetallic and Epithermal Au-Ag Veins.
Ag-Pb polymetallic vein deposits are associated with
an Late Cretaceous pluton at Boswell River, Yukon Territory
(Lees, 1936), and near the Red Mountain porphyry Mo
deposit. Several Au-Ag polymetallic vein mines at Venus,
Montana Mountain, and Wheaton River are hosted in or near
the Late Cretaceous Carcross pluton, which occurs south of
Whitehorse, Yukon Territory (Hart, 1994). Au veins at the
Engineer Mine, southwest of Atlin, B.C., are probably related
to Eocene magmatism that formed both the volcanic rocks and
late stocks (Mihalynuk, 1990). The deposit at the Engineer
Mine are cut by the major Lewellyn Fault. The Au epithermal
veins at the Engineer mine are transitional between mesothermal
and epithermal, low-sulphidaiton type.
Origin of and Tectonic Controls for Surprise Lake
Metallogenic Belt
The Surprise Lake metallogenic belt is associated with
the Surprise Lake Plutonic Suite that consists of stocks and
batholiths, having mainly felsic, high-level, Si- and lithophile
element-rich rocks, and has K-Ar isotopic ages of 84 Ma
1,500 m
1,450
1,400
1,350
NW
Adera Fault
.10
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 249
.15
.15
(Woodsworth and others, 1991). Plutons in the suite intrude the
Cassiar, Stikine, Yukon-Tanana, and Cache Creek terranes. The
major plutonic units are the Surprise Lake batholith, near Atlin,
B.C., and the Needlepoint Pluton, near Cassiar, B.C. (Woodsworth
and others, 1991). The Surprise Lake Plutonic Suite is
part of the extensive Late Cretaceous and early Tertiary Coast-
North Cascade plutonic belt that occurs along the western and
central parts of the Canadian Cordillera and into East-Central
Alaska (fig. 103) (Nokleberg and others, 1994c, 1997c; Monger
and Nokleberg, 1996). Probable coeval, extrusive equivalents
for the Surprise Lake Plutonic suite are the Windy-Table
Volcanics to the east and west (Mihalynuk, 1999; Mihalynuk
and others, 1992), and the Carmacks Assemblage to the north
(Grond and others, 1984), and unnamned tuffaceous units in
the Sustut Basin to the south (Woodsworth and others, 1991).
The Coast-North Cascade plutonic belt forms the major part of
the Coast continental-margin arc in the region.
Central-Southeastern Alaska Metallogenic Belt
of Porphyry Mo and Cu Deposits (Belt CSE),
Southeastern Alaska
The central-southeastern Alaska metallogenic belt of
porphyry Mo and Cu deposits (fig. 103; tables 3, 4) occurs
along the western coast of southeastern Alaska and is hosted in
(Nokleberg and others, 1995a) (1) a belt of late Oligocene and
younger Cenozoic granitoid stocks of the Glacier Bay magmatic
belt, and (2) the Tkope-Portland Peninsula volcanic-plutonic
belt. These igneous belts intrude the Wrangellia superter-
.10
.10
SE
.10
.10
.10
.15
.10
.15
TERTIARY AND QUATERNARY
Overburden
CRETACEOUS
Mount Leonard Stock
Coarse-grained
quartz monzonite
Coarse-grained to
aplitic "hybrid"
quartz monzonite
Mafic quartz
monzonite porphyry
Sparse quartz
monzonite porphyry
Fine-grained quartz
monzonite porphyry
Silicified quartz
monzonite porphyry
Fault and shear
.10
Contact
Molybdenum contours
at 0.10 and 0.15 percent
Figure 118. Adanac-Adera porphyry Mo deposit, Surprise Lake metallogenic belt, Canadian Cordillera. Generalized cross section
through high-grade zone. Adapted from Pinsent and Christopher (1995). See figure 103 and table 4 for location.