Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

Origin of and Tectonic Setting for
Gillespie Metallogenic Belt
The Gillespie metallogenic belt SEDEX deposits is hosted in the Wernecke Supergroup which is part of a shelf
assemblage, at least 14 km thick and is part of the North American Craton Margin. The assemblage consist of fine-grained,
turbidite clastic rocks which grade upward into carbonate rocks, and is broadly correlated with the Purcell Supergroup of the
southern Canadian Cordillera (Young and others, 1979). The major SEDEX occurrences in the Gillespie metallogenic belt at
Blende (and the Carpenter Ridge prospect) may be related to Proterozoic gabbro-diorite sills (Robinson and Godwin, 1995), or
may be possibly related to correlative overlying mafic volcanic flows as at Hart River (Abbott and others, 1994). Other SEDEX
Zn-Pb occurrences, as at Cord, which is hosted by the upper Gillespie Lake Group in the Wernecke Mountains, and other
stratabound Zn-Pb-Ag vein occurrences, 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 (I .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, rifling, 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
UGu-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, 198 I).
More than forty deposits of Cu, U and Fe are associated with extensive heterolithic breccias, as veins. and dissemi nations in I the
matrices and clasts, and in adjacent hydrothemally altered rock (Nokleberg and others 1997a, b, 1998). The signi ficant dep osits
are at Dolores, Igor, Irene, Pagisleel, Porphyry, and Slab (table 4). Chalcopyrite, brannerite, hematite and magneti te are ass( ~ciated
.c-:*:..with
alteration assemblages of Na and K-feldspar, silica. chlorite and carbonate (Dawson and others, 1991). No de~~n~uvc tonnage ..
and grade data exist for the deposits and occurrences in the Wernecke metallogenic belt; however, resource estimates exist for two
significant occurrences which contain varying proportions of Cu. U, Au, Fe and Co. The lgor occurrence contains an estimated
resource of 0.5 million tonnes grading 1.0% Cu, and the Pagisteel occurrence contains an estimated resource of 1 tonne grading
29% Fe (Archer and others, 1986; Hitnnan 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 lo 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 (1 996), 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 for 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 post-deposit dike suggests an Early Proterozoic age for both the Wernecke Supergroup and the mineralization (D.J.
Thorkelson, personal communication, 1994).
Rapitan Metallogenic Belt of Sedimentary
lron 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 which 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, 199 1). 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 hematite-jaspilite iron deposit in North America
occurs in one of these horizons at Snake River (Crest; table 4) (Nokleberg and others 1997a, b, 1998).
Crest lron Formation Deposit
The Crest lron (Snake River) formation deposit consists of a main zone of banded. laminated or nodular jasper hematite
which 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 which contains little or no interbedded sedimentary rocks.
Estimated resources are 5.6 billion tonnes grading 47.2% 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