USGS Professional Paper 1697 - Alaska Resources Library

as at Oz, Monster, and Tart, may also be related to this mafic
igneous activity. Similar mineralization ages occur at Blende
(1.4 Ga; Robinson and Godwin, 1995), Hart River (1.24 to
1.28 Ga; Morin, 1978), and Sullivan (1.43 Ga; LeCouteur,
1979). These ages and host rock setting indicate sedimentary
exhalation occurred in a distal sedimentary shelf facies and
was possibly related to a widespread Middle Proterozoic
event, including faulting, rifting, and associated mafic intrusion
(Dawson and others, 1991). This rifting is interpreted as
influencing sedimentation in the Wernecke and Purcell Supergroups
and the Muskwa Ranges Assemblage.
Wernecke Metallogenic Belt of U-Cu-Fe (Au-Co)
Vein and Breccia Deposits (Belt WR), Central
Yukon Territory
The Wernecke metallogenic belt of U-Cu-Fe (Au-Co)
vein and breccia deposits (fig. 3; tables 3, 4) occurs in the
central Yukon Territory and is hosted in the Early Proterozoic
Wernecke Supergroup in the North American Craton Margin.
In this area, the Early Proterozoic Wernecke Supergroup
consists of a thick sequence of dominantly fine-grained clastic
rocks (Delaney, 1981). More than 40 deposits of Cu, U, and
Fe are associated with extensive heterolithic breccias, veins,
and disseminations in the matrices and clasts and in adjacent
hydrothermally altered rock (Nokleberg and others 1997a,b,
1998). The significant deposits are at Dolores, Igor, Irene,
Pagisteel, Porphyry, and Slab (table 4). Chalcopyrite, brannerite,
hematite and magnetite are associated with alteration
assemblages of Na and K-feldspar, silica, chlorite and carbonate
(Dawson and others, 1991). No definitive tonnage and
grade data exist for the deposits and occurrences in the Wernecke
metallogenic belt; however, resource estimates exist for
two significant occurrences that contain varying proportions
of Cu, U, Au, Fe and Co. The Igor occurrence contains an estimated
resource of 0.5 million tonnes grading 1.0 percent Cu,
and the Pagisteel occurrence contains an estimated resource
of 1 tonne grading 29 percent Fe (Archer and others, 1986;
Hitzman and others, 1992; Abbott and others, 1994). Other
significant occurrences are at Slab, Irene, Porphyry, Dolores,
Athens, and Olympic (table 4).
Formation of the vein and breccia deposits in spatial
relationship to associated mafic dikes and minor diorite intrusions
was proposed by Abbott and others (1994). Similarities
between these deposits in the Wernecke belt and those
in the better known Kiruna-Olympic Dam deposit type were
discussed by Gandhi and Bell (1996), but evidence of coeval,
large-scale magmatic activity, regarded as an important feature
of the later deposit type, is lacking. A deep-seated magmatic
hydrothermal source the formation and mineralization of the
breccias was proposed for both the Kiruna-Olympic Dam
deposit type and the deposits in the Wernecke belt (Hitzman
and others, 1992; Thorkelson and Wallace, 1993). A recent,
unpublished U-Pb zircon isotopic age of 1.72 Ga for a postdeposit
dike suggests an Early Proterozoic age for both the
Proterozoic Metallogenic Belts (2500 to 570 Ma; figures 2, 3) 19
Wernecke Supergroup and the mineralization (D.J. Thorkelson,
personal commun., 1994).
Rapitan Metallogenic Belt of Sedimentary Iron Formation
Deposits (Belt RA), Central Yukon Territory
The Rapitan metallogenic belt of iron formation deposits
(fig. 3; tables 3, 4) occurs in the central Yukon Territory and
is hosted in the Rapitan Sedimentary Assemblage, the lowest
and easternmost unit of the Windermere Supergroup, which
is part of the North American Craton Margin. The Rapitan
assemblage is interpreted as forming in a rift environment
that exhibits rapid facies and thickness changes and contains a
suite of rift-related igneous intrusions and extrusions with isotopic
ages of about 770 Ma (Gabrielse and Campbell, 1991).
Diamictite, in part glaciogenic, occurs at several localities and
stratigraphic levels, notably at two well defined horizons in
eastern Mackenzie Mountains. The largest deposit of hematitejaspilite
iron deposit in North America occurs in one of these
horizons at Snake River (Crest Iron; table 4) (Nokleberg and
others 1997a,b, 1998).
Crest Iron Formation Deposit
The Crest Iron (Snake River) formation deposit consists
of a main zone of banded, laminated, or nodular jasper
hematite that occurs along a stratigraphic interval about 130-m
thick near the base of the ice marginal glacial diamictite complex
of the Shezal Formation. The richest part of the deposit
occurs in the top 80 m that contains little or no interbedded
sedimentary rocks. Estimated resources are 5.6 billion tonnes
grading 47.2 percent Fe. Numerous smaller regional occurrences
are also hosted in the ‘proglacial’ siltstone facies of the
underlying Sayunei Formation (Eisbacher, 1985; Yeo, 1986).
This type of banded iron formation mineral deposit is named
the Rapitan-type by Gross (1996). This type iron of deposit
exhibits distinctive lithological features, including association
with diamictites (tillite) that contain dropstone, sandstone,
conglomerate, and argillite. The Crest Iron deposit and the Jacadigo
iron formation in Brazil are interpreted as having been
deposited in Late Proterozoic or early Paleozoic rock grabens
and fault-scarp basins along the rifted margins of continents or
ancient cratons (Gross, 1996).
Origin of and Tectonic Setting for Rapitan Metallogenic Belt
An origin of marine exhalation along synsedimentary
faults was proposed for this type of hematite-jaspilite iron
formation by Gross (1965), with modifications by Yeo (1986)
to include brine transport by currents generated by the thermal
gradients between cold glacial and warm hydrothermal waters.
The iron deposits in the Rapitan assemblage are correlated
with hematite-jasper iron formation in siltstone and diamictite
of the Late Proterozoic Tindir Group near Tatonduk River in
eastern Alaska (Payne and Allison, 1981; Young, 1982). Dawson
and others (1994) correlate the iron deposits in the Rapitan