USGS Professional Paper 1697 - Alaska Resources Library

occur in a 30-km-long belt along irregular contacts of the
Whitehorse Batholith, which consists of a composite calcalkaline
granodiorite pluton with a diorite margin (Meinert,
1986; Dawson and others, 1991; Yukon Minfile, 1991). The
calcic or magnesian skarn mineralogy is a largely a function
of the skarn protolith (Morrison, 1981). The bulk of sulfide
minerals and highest average Au and Ag grades occur in
structurally controlled retrograde skarn alteration assemblages
(Meinert, 1986). From 1898 to 1982 estimated production was
142,000 tonnes Cu, 7,090 kg Au and 90,000 kg Ag from 5.2
million tones ore. Combined production and reserves are 13.2
million tonnes grading an average of 1.4 percent Cu, 0.7 g/t
Au, and 8.9 g/t Ag (Dawson and others, 1991; Watson, 1984).
Two small skarns are the Cu (Mo-Ag-Au) Hopkins and
the Zn (Cu-Ag) Sekulmun skarns (Dawson and others, 1991)
, which occur in carbonate rocks of the Cambrian through
Devonian Nasina Assemblage of the Yukon-Tanana terrane
where intruded by the Nisling Range granodiorite pluton
(Dawson and others, 1991). Significant Au-Ag polymetallic
vein deposits, which occur at Mount Nansen and Mount
Freegold (Dawson and others, 1991) are hosted in intermediate
to felsic volcanic rocks of the Mount Nansen Group, which are
War
Eagle
Pueblo
0
Rabbit's Foot
Anaconda
Copper
King
Best Chance
Grafter
Arctic Chief
5 km
Whitehorse
Big, Middle,
Little Chiefs
North Star
Keewenaw
Black Cub
Late Early Cretaceous Metallogenic Belts (120 to 100 Ma; figs. 61, 62) 175
Quaternary
Miles Canyon Basalt
Middle Cretaceous
Whitehorse Batholith
Diorite, granodiorite
Lower to Middle Jurassic
Laberge Group
Clastic sedimentary rock
Upper Triassic
Lewis River Group
Limestone, dolomite, and
local skarn
Clastic sedimentary rock
Anticline axis
Contact
interpreted as coeval and probably comagmatic the Whitehorse
Suite (Woodsworth and others, 1991). Small, polymetallic Au-Ag
veins are hosted by granitioid plutons in the Moosehorn Range on
the border between Alaska and the Yukon Territory.
Origin of and Tectonic Controls for Whitehorse
Metallogenic Belt
The Whitehorse metallogenic belt is hosted by the
Whitehorse Plutonic Suite, which has an isotopic age range
103 to 112 Ma. The suite includes the Coffee Creek Plutonic
Suite south of Dawson (Mortensen and others, 1994). The
Whitehorse Plutonic Suite is part of the collisional mid-Cretaceous
Omineca-Selwyn plutonic belt, which extends from the
southern part of the Canadian Cordillera, across Interior Alaska,
and northwestward into the Russian Northeast (figs. 61, 62).
The belt consists chiefly of granodiorite, granite, quartz syenite
and minor syenite plutons of Early to mid-Cretaceous age (110-90
Ma; Monger and Nokleberg, 1996; Nokleberg and others, 1994c;
2000). The spatial location of the belt, about 200 km west of the
eastern limit of Cordilleran deformation, and chemistry suggests an
anatectic origin of partial melting of cratonic crust during thickening
caused by Cretaceous contraction (Monger and Nokleberg,
Yukon River
Cowley
Park
Lewis
River Mines
Figure 78. Whitehorse Copper Belt of Cu skarn
deposits, Whitehorse metallogenic belt, Canadian
Cordillera. Schematic map showing location of
major Cu-Fe skarn deposits. Adapted from Dawson
and Kirkham (1996). See figure 62 and table 4
for location.