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

Cassiar Metallogenic Belt of Porphyry Mo-W;
W Skarn, Zn-Pb-Ag Manto, Sn Skarn, and
Au Skarn Deposits (Belt CA) Northern British
Columbia and Southern Yukon Territory
The Cassiar metallogenic belt of porphyry Mo-W; W skam, Zn-Pb-Ag manto, Sn skam, and Au skarn deposits (fig. 62;
tables 3,4) occurs in northern British Columbia and southern Yukon Territory. The belt contains a variety of mineral deposit types
which are related to granitoid plutons of the mid-Cretaceous Cassiar Plutonic Suite which forms a narrow, linear belt of
dominantly biotite granite and granodiorite plutons which is part of the Omineca-Selwyn plutonic belt (fig. 62). The significant
deposits in the belt are a Porphyry W-Mo deposit at Logtung (Logjam Creek), a Pb-Zn-Ag skarn and manto deposit at Midway
(Silver Tip), a W skam deposit at Risby (Cab), a Zn-Ag polymetallic vein deposit at Logan, and a Sn skarn deposit at JC or Viola
(table 4) (Nokleberg and others 1997a, b, 1998). The Logan polymetallic Zn-Ag vein deposit is hosted by the Marker Lake
Batholith (Dawson, 1996a).
Logtung Porphyry Mo- W Deposit
The large Logtung (Logjam Creek) porphyry Mo-W deposit consists of disseminated scheelite, molybdenite and
powellite with minor associated fluorite and beryl in garnet-diopside skam, quartz vein stockwork and fractures (EMR Canada,
1989; Dawson and others, 1991; Yukon Minfile, 1991; Noble and others, 1995). The deposit contains estimated reserves of 230
million tonnes grading 0.104% W03, 0.05% MoS2. The deposit is hosted in a large quartz porphyry dike which is related to a
nearby mid-cretaceous quartz monzonite stock with a K-Ar isotopic age of 109 Ma which is part of the Cassiar Plutonic Suite.
The Logtung deposit is typical of a group of deposits (as at Stormy and Molly) which contain a molybdenite-rich stockwork in a
granitoid pluton and a scheelite-rich garnet-diopside skam assemblage in the wall rocks (Dawson, 1996~). At the Logtung deposit,
early quartz-scheelite veins are related to a monzogranite stock, a later stage of quartz-scheelite-molybdenite-pyrite-fluorite veins
is related to a felsic dyke complex, and a final stage of polymetallic W-Mo veins which form a large zone is centered on the dyke
complex (Noble and others, 1984). Both mid-Cretaceous intrusives are part of the Cassiar Plutonic Suite. The wall rocks are
Mississippian and Pennsylvanian chert, argillite, and quartzite of the Cassiar continental margin terrane.
Risby Skarn W Deposit
The significant, but undeveloped Risby (Cab) skarn W deposit consists of two diopside-garnet skarns which occur in
Early Cambrian carbonates which are intruded by granitoid sills of the Cassiar Plutonic Suite (EMR Canada, 1989; Yukon
Minfile, 1990; Mining Review, 1992). The No. 1 zone has a high pyrrhotite and low chalcopyrite content, whereas the No. 2 zone,
which is intruded by a sill, has higher WO, mineralization and low sulfide content (Sinclair, 1986). The deposit has a drilled
resource of 3.2 million tonnes grading 0.82% W03. The mineral assemblage is typical of W skam deposits associated with plutons
of the Omineca-Selwyn plutonic belt.
Midway (Silvertip) Manto Pb-Zn-Ag Deposit
The Midway (Silvertip) manto Pb-Zn-Ag deposit consists of generally coarse-grained assemblage of sphalerite-galena-
pyrite which contains elevated Ag, and minor Au, Sb and Bi values which occur as irregular, pipe-like, open-space filling and
replacement bodies in limestone of the Middle Devonian McDame Group of the Cassiar continental margin terrane (Bradford and
Godwin, 1988; EMR Canada, 1989). The nearest intrusive rock, interpreted as the source of mineralizing fluids, is a group of
quartz-feldspar porphyry dikes which are about 2 km from the deposit. The dikes intrude clastic sedimentary rocks of the Earn
Group which unconformably overlie the McDame Group. The wall rocks exhibit sericite alteration. The dikes exhibit a K-Ar
isotopic age of 66 Ma. Estimated reserves are 2.6 million tonnes grading 8.8% Zn, 6.4% Pb, and 3258 g/t Ag according to drilling
and underground exploration (Bradford and Godwin, 1988; Mining Review, summer 2000; Peruvian Gold/Imperial Metals
release, February 10,2000). The deposit does not contain a calc-silicate gangue typical of skarn Zn deposits, and exhibits only
minor amounts of a silica and carbonate gangue adjacent to replacement bodies, similar to the alteration in the large Ag-Zn-Pb
manto deposits of northern Mexico. At both the Midway and the nearby YP manto deposits, a crude zonation in metal distribution
exists wherein Au is concentrated commonly in massive sulfide zones which are rich in Fe, Cu, and Zn, rather than in more distal
Pb- and Ag-rich zones (Dawson, 1996a). An origin similar to which of Sa Dena Hes Zn-Pb-Ag manto deposit near Watson Lake,
Yukon is proposed, where mantos are developed distally to an intrusion inferred to underlie the deposit at depth (Dawson, 1996a).
Ketza River Manto Au Deposit
The Ketza River manto Au deposit consists of Au-quartz-sulfide veins and massive pyrrhotite-arsenopyrite-pyrite-
chalcopyrite-Au mantos which occur in a central part; and (2) mantos and veins of similar mineralogy which contain lower
amounts of sulfide minerals which occur in a outer part; and (3) Ag-Pb veins and mantos which occur in the periphery (Cathro,
1990). The deposit is hosted by Early Cambrian sedimentary rocks of Cassiar continental margin terrane. The district is interpretec
as underlain by a mid-Cretaceous pluton of the Cassiar Plutonic Suite which is interpreted as emplaced along the Ketza-Seagull
Arch, a major structural feature (Abbott, 1986a). Homfels underlies the deposit and exhibits a K-Ar whole-rock isotopic age of