USGS Professional Paper 1697 - Alaska Resources Library

Origin of and Tectonic Controls for Northern Cordillera
Metallogenic Belt
The deposits in the Northern Cordilleran metallogenic belt
are classic Southeast Missouri Pb-Zn deposits composed of
sphalerite, galena, and pyrite, with a gangue of dolomite, quartz,
calcite and barite, and lesser gypsum, fluorite, chalcopyrite, and
pyrobitumen. These minerals occur in vugs, pores, burrows,
various sedimentary and tectonic breccias, and minor to major
fractures. Secondary dolomite commonly accompanies mineralization.
Zn: Pb ratios average 10:1, and Ag and Fe contents are
low. The deposit form is highly irregular and usually discordant
on a local scale, but stratabound on a district scale. The deposit
sizes range from a few tens of thousands to about 10 million
tonnes, and grades range from 3 to 10 percent combined Zn and
Pb in larger deposits and to about 50 percent combined Zn and
Pb in small bodies (Dawson and others, 1991). Remote location
and lack of infrastructure has limited drilling and development to
only a few of the several hundred known occurrences.
The Late Proterozoic to Middle Devonian passive part of
the North American Craton Margin consists of a miogeoclinal
sedimentary prism that is segmented into two contrasting
facies belts. To the northeast are shallow water sandstone,
dolostone and limestone that define the Mackenzie Platform,
whereas to the southwest are turbiditic sandstone, deep-water
limestone, shale and chert that define the Selwyn Basin. Sedimentary
lithofacies exerted a primary control upon the localization
of preaccretionary sediment-hosted mineral deposits.
Minor occurrences of Southeast Missouri Pb-Zn deposits are
a common feature of carbonate rocks of all ages in the North
American miogeocline; however, significant deposits commonly
are localized along the carbonate-shale facies changes
near the tectonically unstable, western margin of Late Proterozoic
to early Paleozoic platformal carbonate successions.
The apparent spatial relationship of mineralization to extensional
structures suggests formation of the Southeast Missouri
Zn-Pb deposits during major rifting and basinal subsidence along
the passive North American Craton Margin. These structures
include rift-induced, synsedimentary and block faulting, uplift,
basinal subsidence, resultant facies changes, reefal development,
development of karsts, brecciation, and basinal brine migration
(Dawson and others, 1991). The common association of
hydrocarbons with Zn-Pb deposits in carbonate rocks suggests a
genetic relationship between mineralization and oil maturation,
migration, and entrapment (Jackson and Beales, 1967). Although
the timings of mineral deposition commonly are not well known
(Sangster, 1986), the two major age groups of Southeast Missouri
Zn-Pb deposits are herein interpreted as forming during
two major periods of incipient rifting of the North American
Continental Margin in the Late Proterozoic and early Paleozoic.
Other metallogenic belts in the Canadian Cordillera, which
contain stratiform or stratabound massive sulfide deposits that are
hosted in parts or rifted fragments of the North American Craton
Margin, are (in order of decreasing age) (1) Monashee belt of
Late Proterozoic SEDEX deposits, (2) Redstone belt of Late Proterozoic
sediment-hosted Cu deposits, (3) Cathedral belt of Cam-
Middle and Late Devonian Metallogenic Belts (387 to 360 Ma; figures 16, 17) 65
brian Southeast Missouri Zn-Pb deposits, (4) Churchill belt of
Late Proterozoic Cu vein deposits, (5) Kootenay belt of Cambrian
SEDEX deposits, and (6) Anvil belt of Cambrian through Silurian
SEDEX deposits. An important distinction occurs between some
of the metallogenic belts. Many metallogenic belts with SEDEX
deposits are directly associated with mafic volcanic rocks and
hydrothermal activity, whereas the metallogenic belts containing
Southeast Missouri Zn-Pb deposits are not.
Dempster Metallogenic Belt of SEDEX Ba,
Sedimentary-Exhalative (SEDEX), SEDEX Ni-Zn-
PGE-Au, and Kuroko Zn-Pb-Cu Massive Sulfide
Deposits (Belt DE) Northwestern Yukon Territory
The Dempster metallogenic belt of SEDEX Ba, SEDEX
Ni-Zn-PGE-Au, and Kuroko Zn-Pb-Cu massive sulfide deposits
occurs in the northwestern Yukon Territory (fig. 17; tables
3, 4) (Nokleberg and others, 1997b, 1998). The belt is hosted
in the North American Craton Margin in a sequence Devonian
and Mississippian clastic strata, which are part of the Earn
assemblage in the region north of Tintina Fault and south of
Dawson Fault. The significant deposits are the Rein SEDEX,
Marg Kuroko massive sulfide, and the Nick SEDEX deposits.
Rein SEDEX Ba Deposits
The Rein and several other large SEDEX deposits in
the region contain barite, barytocalcite, and witherite and are
hosted in Early to Middle Devonian (late Emsian to early
Eifelian) sedimentary rocks that crop our near Dempster
Highway (M.J. Orchard, written communication, 1985). None
of the occurrences has measured reserves. The upper Earn
Assemblage includes beds between the Tombstone and Robert
Service thrust faults previously interpreted to be Mesozoic,
including the Keno Hill quartzite, which host the large
polymetallic silver-vein district of Keno Hill in the Tombstone
metallogenic belt.
Marg Kuroko Volcanogenic Zn-Pb-Cu Deposit
The Marg kuroko Zn-Pb-Cu-Au-Ag massive sulfide
deposit consists of pyrite, sphalerite, chalcopyrite and galena
with minor arsenopyrite and tetrahedrite that occur in a quartz
and barite gangue (Eaton, written commun., Archer, Cathro,
and Associates, 1989; Yukon Minfile, 1991). The deposit
occurs in four stacked massive sulfide lenses that occur at the
contact of quartz-sericite-chlorite phyllite and graphitic phyllite.
The deposit contains an estimated 2.097 million tonnes
grading 5.0 percent Zn, 2.7 percent Pb, 1.8 percent Cu, 65
g/t Ag, 1.2 g/t Au. The host rocks are tectonically interleaved
with and overlain by the Keno Hill quartzite of the Late Earn
Assemblage, part of a Devonian and Mississippian clastic
wedge (Mortensen and Thompson, 1990; Turner and Abbott,
1990). The felsic metavolcanic rocks are interpreted as part of
a Carboniferous continental-margin arc form along the North
American Craton Margin.