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

Origin of and Tectonic Controls for
Monashee Metallogenic Belt
The SEDEX Zn-Pb deposits in the Monashee metallogenic belt, which are interpreted as Broken Hill type Pb-Zn-Ag
deposits by Hay (2001), consist of extensive, thin, sulfide layers which are folded and metamorphosed along with their
predominantly calcareous and schistose host rock (Fyles, 1970; Hsy, 1982a, 2000; Dawson, and others, 1991). A correlation of
the SEDEX Zn-Pb deposits and host rocks of the Monashee terrane with those in the Kootenay terrane may exist (Dawson and
others, 1991; Hsy, 2001). The Monashee terrane is interpreted as a displaced fragment of the North American Craton which
consists of a core of basement paragneiss (with a Late Archean and Paleoproterozoic isotopic age of 2.8 to 1.96 Ga), intruded by
orthogneiss (2.1 Ga), and mantled by paragneiss which is intruded by a syenite pluton which may be as old as 1,852 Ma
(Scammell and Brown, 1990; Nokleberg and others, 1994c, 1997c; Crowley, 1997). In the area of the Mount Copeland porphyry
Mo deposit, minor occurrences of pyrochlore and columbite-tantalite in carbonatite associated with syenite gneiss (McMillan,
1973) suggest a genetic relationship of these alkaline intrusions to a rifting event which resulted in the separation of the Monashee
terrane from the craton in the Late Proterozoic (Monger and Nokleberg, 1996).
Purcell Metallogenic Belt of SEDEX
Zn-Pb-Ag Deposits (Belt PR)
Southern British Columbia
The Purcell metallogenic belt of sedimentary exhalative (SEDEX) Zn-Pb-Ag deposits (fig. 3; tables 3,4) occurs in the
southern Canadian Cordillera and is hosted in sedimentary rocks of the Mesoproterozoic Purcell Supergroup. The sedimentary
rocks of the supergroup comprise a dominantly passive margin depositional sequence of fine-grained, basinal clastic rocks at least
11 km thick. This basinal sequence thins eastward into platformal sedimentary rocks toward the North American Craton (Aitken
and McMechen, 1991). The Purcell Supergroup is correlated with the Belt Supergroup in the western and northern USA. The
major SEDEX deposit is Sullivan; other significant deposits in the belt are the Moyie (St. Eugene) and Vine Ag-Au polymetallic
vein deposits (table 4) (Hsy, 199 1, 2001 ; Nokleberg and others 1997a, b, 1998).
Sullivan SEDEX Zn-Pb-Ag Deposit
The Sullivan SEDEX Zn-Pb-Ag deposit (fig. 7) consists of a laminated sulfide assemblage of galena, sphalerite and
pyrite which has undergone metamorphic recrystallization, and tectonically-induced mechanical and chemical remobilization
(Leitch and Turner, 1991, 1992; Lydon, 1995). The deposit occurs near a north-trending rift axis at an intersection with the east-
west-trending, proto-Kimberley fault. The Sullivan deposit originally contained 170 million tomes of ore with a grade of 5.5% Zn,
5.8% Pb, and 59 glt Ag. About 70% of the deposit occurs in a massive pyrrhotite-galena-sphalerite vent complex which overlies a
heavily tourmaline-altered hydrothermal upflow zone. The remaining 30% of the deposit occurs in concordant laminated pyrrhotite-
sphalerite-galena "ore bands" which extend eastwards fiom 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 which
ranges up to 10 km thick to the west, but thins eastward into platformal sedimentary rocks. The Supergroup is overlain by shallow
marine and non-marine 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). Up to 30% of outcrops of the
turbidite sequence consists of the tholeiltic sills which were emplaced before significant consolidation of the sedimentary rocks.
The sills are interpreted as forming during rifting (Hsy, 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 which 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.