USGS Professional Paper 1697 - Alaska Resources Library

22 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
rhotite-galena-sphalerite vent complex that overlies a heavily
tourmaline-altered hydrothermal upflow zone. The remaining 30
percent of the deposit occurs in concordant laminated pyrrhotitesphalerite-galena
ore bands that extend eastwards from the vent
complex (Lydon, 1995). Ongoing hydrothermal activity from
marine brines generated successive chlorite-pyrrhotite-muscovite
and albite-chlorite-pyrite-sericite-calcite assemblages
in ore zone and hanging wall and footwall, all coincident
with gabbro dikes and sills. The deposit is hosted conformably
within folded, Middle Proterozoic turbidite of the Early
Aldridge formation of the Purcell Supergroup. The turbidites
fill an intracontinental extensional-rift marine basin, which
is extensively intruded by tholeiitic Mesoproterozoic Moyie
Sills series. The sulfide deposition is interpreted as predating
the Mine Sill series, part of the Moyie Sills series, and was
accompanied by extensive boron (tourmaline) alteration of
marine sedimentary origin. Related, smaller Zn-Pb-Ag deposits
are at Fors, Stemwinder, North Star, and Vine.
Origin of and Tectonic Setting for Purcell
Metallogenic Belt
The Purcell metallogenic belt of SEDEX Zn-Pb-Ag
deposits (fig. 3; tables 3, 4) is hosted in the Purcell Supergroup
that is as much as 10 km thick to the west, but thins eastward
Creston Fm.
Aldridge Formation
LowerUpper
Middle
W Rocky Mtn.
E
Purcell Mountains
Trench Hughes Range
into platformal sedimentary rocks. The Supergroup is overlain
by shallow marine and nonmarine rocks (Aitken and
McMechan, 1991), is underlain by basement rocks older than
1.7 Ga, and is intruded by the tholeiitic Moyie Sills with a
U-Pb zircon isotopic age of 1,467 Ma (Anderson and Davis,
1996). As much as 30 percent of outcrops of the turbidite
sequence consists of the tholeiitic sills that were emplaced
before significant consolidation of the sedimentary rocks.
The sills are interpreted as forming during rifting (Høy, 1989;
Lydon, 1995). The SEDEX Zn-Pb-Ag deposits of the Purcell
metallogenic belt are interpreted as forming at the beginning
of major period of mid-Purcell (middle Paleozoic) rifting,
which consisted of exhalation of Zn-Pb-Ag-bearing fluids
and associated hydrothermal alternation that was followed
by intrusion of the abundant Moyie Sills series. Several other
similar metallogenic belts of Mesoproterozoic stratiform
massive sulfide deposits occur in parts of the North American
Craton Margin and are interpreted as forming during a major
period of Middle Proterozoic rifting along the passive continental
margin of the North American Craton (Stewart, 1975).
These belts include (1) Churchill belt of Cu vein deposits, (2)
Clark Range belt of sediment-hosted Cu-Ag deposits, and (3)
Gillespie belt of SEDEX deposits. The SEDEX deposits are
interpreted as directly associated with mafic volcanic rocks
and hydrothermal activity.
1,000 m Approximate
scale
10 km
Moyie Sills
Gabbro, diorite
Creston Formation
Siltite, argillite,
subtidal to peritidal
Aldridge Formation
Upper
Argillite, siltite,
upward shoaling to
intertidal
Middle
Turbidite, sandy-silty,
deep water
Lower
Turbidite, silty,
deep water
Fort Steele Formation
Quartzite,
tidal marine or
braided stream
Sullivan Deposit
Contact
Figure 7. Sullivan sedimentary-exhalative Zn-Pb-Ag deposit, Purcell metallogenic belt, Canadian Cordillera. Schematic restored cross
section. Adapted from Lydon (1995). See figure 2 and table 4 for location.