USGS Professional Paper 1697 - Alaska Resources Library

supergene blanket are interpreted as an Early Jurassic event that
occurred in Early Jurassic volcaniclastic and epiclastic rock
(Diakow, 2001). The Late Cretaceous sedimentary rock of the
Sustut Group are interpreted as capping the supergene zone.
The Kemess South porphyry Cu-Au deposit is hosted by
the relatively flat-lying Maple Leaf quartz monzodiorite sill.
Higher Cu and Au grades correlate with zones of intense quartzpyrite-chalcopyrite
stockwork that contains intensely developed
K-feldspar vein selvages and magnetite stringers. A supergene
zone, which formed contemporaneously with the Late Cretaceous
Sustut Basin, forms about 20 percent of the deposit, and
contains elevated Cu grades and contains chalcocite and native
Cu. Production commenced at Kemess South in 1998, based on
estimated reserves of 442 million tonnes of hypogene and supergene
ore grading 0.23 percent Cu and 0.4 g/t Au (Rebagliati and
others, 1995; Northern Miner, March 10, 2003).
The Kemess North porphyry Cu-Au deposit is hosted in
potassic-altered, mafic volcanic rocks of the Takla Group. The
deposit is centered on Early Jurassic porphyritic monzodiorite
dikes. Higher grade Cu-Au mineral assemblages in volcanic
host rocks are associated with hydrothermal biotite alteration,
whereas potassium feldspar and propylitic alteration decrease
zonally outwards from these centers, along with decreasing Cu
and Au. Estimated reserves are116 million tonnes grading 0.19
percent Cu and 0.38 g/t Au (Rebagliati and others, 1995; El
Condor Resources Ltd., news release, July 19, 1993).
80°
Cliff Creek
Zone Duke's
Ridge Zone
10°
10°
Duke's Ridge
Lawyers Ridge
Attorney Fault
60°
80°
Early Jurassic Metallogenic Belts (208 to 193 Ma; fig. 42) 105
Origin of and Tectonic Controls for Toodoggone
Metallogenic Belt
The Toodoggone metallogenic belt is hosted in the
Toodoggone Formation, a Early Jurassic succession of subaerial,
intermediate, calc-alkaline to alkaline, predominantly
pyroclastic rocks, and coeval and comagmatic calc-alkaline
plutons of the Black Lake Suite that occur on the outer limb
of the oroclinal warp of the Stikinia and Quesnellia island arc
and associated terranes. The transition from alkaline to calcalkaline
magmatism is interpreted as forming during the final
stages of the oroclinal warp in the Early Jurassic, before accretion
of the Stikinia-Quesnellia arc and associated Cache Creek
subduction-zone terrane in the Middle Jurassic (Mihalynuk
and others, 1994; Monger and Nokleberg, 1996; Nokleberg
and others, 2000). The volcanic rocks of the Hazelton Group
and correlative units, such as the Toodoggone Formation, and
associated granitoid plutonic rocks, represent emergence of the
arc and a transition from marine to subaerial deposition in the
Early Jurassic. The Toodoggone Formation consists exclusively
of high-K, calc-alkaline volcanic rock that was deposited
in high-energy subaerial flows, associated air-fall tuff, and
lesser lava flows bertween about 200 to 190 Ma.
The Stikinia island-arc terrane is interpreted as forming
on the deformed continental-margin strata of Yukon-Tanana
terrane, which may be a rifted fragment of the North Ameri-
Amythest Gold
Breccia Zone
30°
80°
85°
Di Fault
10°
0 500 m
Contact
Fault
Mineralized breccia zones
Pyroxene basalt
Upper andesite
Upper trachyte flows
Lapilli tuff and graywacke
Trachyandesite
Welded tuff
Quartz andesite
Figure 46. Lawyers Au-Ag epithermal vein deposit, Toodoggone metallogenic belt, Canadian Cordillera. Schematic
geologic map showing various zones. Adapted from Vulimiri and others (1986). See figure 42 and table 4 for location.