USGS Professional Paper 1697 - Alaska Resources Library

66 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Nick SEDEX Ni-Zn-PGE-Au Deposit
The Nick SEDEX Ni-Zn-PGE-Au deposit consists
of pyrite, vaesite, melnikovite-type-pyrite, sphalerite and
wurtzite that occur in a gangue of phosphatic-carbonaceous
chert, amorphous silica and intergrown bitumen (Hulbert and
others, 1992; Yukon Minfile, 1992). The deposit has reserves
of 900,000 tonnes grading 5.3 percent Ni, 0.73 percent Zn,
and 0.8 g/t PGE, along with minor Au. The deposit forms a
thin, conformable unit at the contact between Middle and Late
Devonian Earn Group. The deposit extends laterally over a
80 km 2 basin (Hulbert and others, 1992). The host rocks are
the basinal sedimentary part of a Devonian and Mississippian
clastic wedge exposed in an east-west trending syncline. The
basin is interpreted as a local trough or embayment on the
eastern margin of the Selwyn Basin. The only known deposits
similar to this rare SEDEX deposit are the Ni-Mo sulfide beds
of the Yangtze Platform, China (Coveney and others, 1994).
Origin of and Tectonic Controls for Dempster
Metallogenic Belt
In the Middle Devonian, a dramatic change in sedimentation
patterns occurred throughout the North American Craton
Margin when continental shelf platform assemblages of
carbonate and clastic rocks were drowned and starved of clastic
sediments before being inundated by mainly turbidite and chertrich
clastic rocks derived from the west and north. The abrupt
change from passive-margin to variably coarsening-upward
clastic sedimentation represented by the Earn Assemblage is
interpreted as the result of local block uplift as a consequence of
regional extension or strike-slip faulting (Gordy, 1991; Gordey
and Anderson, 1993), or as interpreted herein, related to syndepositional
faults that bounded a westerly trending, rift-related
trough. The formation of the Dempster metallogenic belt and
the similar Macmillan Pass and Gataga metallogenic belts is
interpreted as occurring during deposition of the clastic wedge.
Macmillan Pass Metallogenic Belt of Zn-Pb-Ag-Ba
SEDEX Deposits, Central Yukon Territory (Belt MP)
The Macmillan Pass metallogenic belt of SEDEX Zn-Pb-
Ag-Ba deposits (fig. 17; tables 3, 4) occurs in the central Yukon
Territory and is hosted in the Devonian and Mississippian
sedimentary rocks of Earn Group, part of the North American
Craton Margin in the northern Canadian Cordillera. The significant
deposits are at Cathy (Bar, Walt, Hess), Gravity (BA),
Jeff (Naomi, Baroid), Macmillan Pass (Tom, Jason Main, Jason
East), Moose (Spartan, Racicot), Oro (Buc, Mar, Dar, Tang),
and Tea (Brock) (table 4) (Nokleberg and others 1997a,b, 1998).
Tom, Jason Main, and Jason East Pb-Zn-Ag-Ba SEDEX
Deposits
Tom, Jason Main, and Jason East SEDEX Pb-Zn-Ag-
Ba deposits occur in two or more stratigraphic intervals in
the Middle to Late Devonian lower Earn Group, interpreted
as part of a Devonian and Mississippian clastic wedge in the
MacMillan Pass area. The deposits are interpreted as spatially
related to syndepositional faults bounding a rift-related trough
filled with fine- to coarse-grained siliceous turbiditic clastic
rocks (MacIntyre, 1991; Mining Review, 1992). Estimate
reserves are 9.3 million tonnes grading 7.5 percent Pb, 6.2 percent
Zn, and 69.4 g/t Ag for the Tom deposit, and 14.1 million
tonnes grading 7.09 percent Pb, 6.57 percent Zn, and 79.9 g/t
Ag for the Jason deposits (MacIntyre, 1991; Mining Review,
1992). The distribution of ore facies consists of (1) a Cu- and
Ag-rich footwall stockwork that is overlain by Pb- and Zn-
rich massive sulfide facies, (2) an upward and lateral gradation
into a Zn- and Fe-rich, laminated sulfide facies, and (3) a distal
gradation into Ba-rich ore. This distribution is interpreted
as forming during a zonal deposition from low-temperature
brines exhaled into an anoxic subbasin (McClay and Bidwell,
1986; Large, 1983). The Jason deposits, 5 km southwest of
Tom, possess similar ore facies but occur closer to a graben
margin and are characterized by slump and debris flows and
discordant, replacement ore textures.
Moose Ba SEDEX Deposit
The Moose Ba SEDEX deposit consists of finely laminated
barite that occurs in two beds from 25 to 45 meters
thick and exposed for 200 to 250 meters along strike (Dawson
and Orchard, 1982; Yukon Minfile, 1992). The deposit has
estimated reserves of 3.0 million tonnes grading 84 percent
BaSO4, and 12 to 14 percent SiO2. The deposit occurs near the
base of a shale member of the Middle to Late Devonian lower
Earn Group, immediately above an underlying chert pebble
conglomerate. The host rocks are interpreted as part of a
Devonian and Mississippian clastic wedge.
Origin of and Tectonic Setting for MacMillan Pass
Metallogenic Belt
The MacMillan Pass metallogenic belt of Zn-Pb-Ag-Ba
SEDEX deposits is hosted mainly in Late Devonian (Frasnian)
units of the North American Craton Margin (Dawson and
Orchard, 1982). Most of the SEDEX deposits in the metallogenic
belt are related to syndepositional faults that bound
a westerly trending, rift-related trough filled with turbiditic
siliceous clastic rocks of the lower Earn Group (Abbott,
1986b). The Devonian and Mississippian Earn Group, which
hosts the metallogenic belt, represents a dramatic change in
sedimentation patterns. The change consisted of drowning of
shelf carbonate-clastic platforms and subsequent inundation
by turbidite- and chert-rich clastic rock derived from the west
and north (Gordey and others, 1991). The abrupt change from
passive continental margin sedimentation to variable, coarsening-upward
clastic sedimentation is interpreted as the result
of local block uplift as a consequence of regional extension
related to rifting or strike-slip faulting (Gordey, 1992) or as a
consequence of ensialic arc magmatism, uplift, and foreland
clastic wedge deposition (Gabrielse and others, 1982).