USGS Professional Paper 1697 - Alaska Resources Library

during back-arc extension or transtension in an region continentward
of the subduction-related Coast continental-margin
arc. These two early Tertiary two metallogenic belts represent
the last stage of magmatism in the Coast-North Cascade plutonic
belt that constitutes most of the Coast continental-margin
arc. This period of regional extension or transtension in the
southern Canadian Cordillera is interpreted as the result of
either (1) a change from transpression to transtension at about
55 Ma caused by a change of obliquity of convergence of the
oceanic plate (Parrish and others, 1988), or (2) alternatively,
but likely, collapse of overthickened thrust units (Monger and
Nokleberg, 1996; Nokleberg and others, 2000). This origin is
analogous to the occurrence of back-arc magmatism forming
the Columbia River basalt to the rear of the Cascade arc in the
Miocene (Wells and Heller, 1988; England and Wells, 1991;
Nokleberg and others, 2000).
Skeena Metallogenic Belt of Porphyry Cu-Mo,
Porphyry Mo; Ag Polymetallic Vein and Au-Ag
Epithermal Vein Deposits (Belt SK), Central
British Columbia
The Skeena metallogenic belt of porphyry Cu-Mo, porphyry
Mo, Ag polymetallic vein, and Au-Ag epithermal vein
deposits occurs in central British Columbia (fig. 103; tables 3,
4) and is associated with several early Tertiary plutonic suites
in the Intermontane Belt in west-central British Columbia,
including the Nanika, Babine, Quanchus and Goosly (Woodsworth
and others, 1991; Carter, 1982). Coeval and spatially
related volcanic rocks support the interpretation that many
of the plutons represent the roots of volcanic centres. The
host plutonic rocks are the younger part of the Coast-North
Cascade plutonic belt and the related early Tertiary Kamloops
magmatic belt (Plafker and others, 1989; Nokleberg and others,
1994c, 1997c). The significant deposits in the belt are
(table 4) (Nokleberg and others 1997a,b, 1998) (1) porphyry
Mo deposits at Ajax, Bell Moly (Alice Arm), Kitsault (BC
Moly), Lucky Ship, Mount Thomlinson, Redbird, Roundy
Creek, Serb Creek, (2) porphyry Cu-Mo deposits at Berg, Big
Onion, Dorothy, and Nanika (DW, New Nanik), (3) polymetallic
and Ag polymetallic vein deposits at Capoose Lake, Equity
Silver (Sam Goosly), Nadina (Silver Queen), and Prosperity-
Porter Idaho, and (4) porphyry Cu-Au (Mo) deposits at Bell
Copper (Newman), Granisle, and Morrison. However the age
of the Equity Silver (Sam Goosly) deposit is unresolved.
Porphyry Mo and Cu-Mo Deposits Associated with
Nanika Intrusions of Nanika Plutonic Suite
The Eocene Nanika Plutonic Suite, which forms the most
widespread suite of postaccretionary granitoid plutons intruding
the Stikinia terrane, include the Nanika Intrusions south
of Bowser Basin (Carter, 1982) and the Alice Arm Intrusions
west of the basin (Woodsworth and others, 1991). The small,
multistage, calc-alkaline granitic to granodiorite stocks, dikes,
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 257
and sills, which were intruded along steeply dipping faults, are
interpreted to be the roots of deeply eroded volcanic centers,
perhaps of the Hazelton Group (MacIntyre, 1985). Several
major porphyry deposits associated with Nanika Intrusions
occur northeast of Hazelton, B.C. The largest of these deposits
are at Berg, Mount Thomlinson, and Redbird. Other significant
deposits in the Huston district are at Lucky Ship porphyry
Mo and Nanika Mountain porphyry Cu (Mo) deposits. The
Serb Creek porphyry Mo and the Big Onion porphyry Cu-Mo
deposits (Wojdak and Stock, 1995) occur east and west of
Smithers, respectively
Berg Porphyry Cu-Mo (Pb-Zn-Ag-Au) Deposit
The Berg porphyry Cu-Mo (Pb-Zn-Ag-Au) deposit
consists of chalcopyrite, molybdenite and pyrite with minor
sphalerite, galena, and arsenopyrite (Panteleyev, 1981; EMR
Canada, 1989; Dawson and others 1991). Estimated reserves
are 238 million tonnes grading 0.39 percent Cu, 0.05 percent
MoS2, 2.84 g Ag. The deposit occurs within a fine-grained
stockwork of quartz veinlets that are distributed in a broad
asymmetrical zone around a semicircular quartz-monzonite
porphyry stock of the Eocene Nanika Suite and within the
peripheral, contact-metamorphosed volcanic rocks of the
Hazelton Group. The most intense Mo concentrations occur in
the stock, whereas the most intense Cu concentrations occur
60 meter beyond the contact. A pyrite halo occurs from 300 to
600 m from the contact. Also occurring is extensive oxidation,
leaching, and secondary enrichment.
Mount Thomlinson Porphyry Mo Deposit
The Mount Thomlinson porphyry Mo deposit consists
of molybdenite, chalcopyrite, along with minor magnetite
and scheelite that occur in a stockwork of quartz veinlets near
the northwest contact of a circular, Eocene quartz monzonite
porphyry stock that intrudes argillaceous Jurassic sedimentary
rocks (Carter, 1982; EMR Canada, 1989). Estimated reserves
are 40.8 million tonnes grading 0.12 percent MoS 2. The
deposit occurs predominantly within intrusive rocks along the
northwest contact. The host sedimentary rocks are deformed
and metamorphosed into biotite, muscovite, cordierite, and
andalusite-bearing schists.
Redbird Porphyry Mo Deposit
The Redbird porphyry Mo deposit consists of molybdenite
and pyrite that occur in a stockwork of quartz veinlets
within peripheral, concentric alteration zones of a quartzmonzonite
porphyry stock of the Eocene Nanika Suite, which
intrudes Middle Jurassic pyroclastic rocks of the Hazelton
Group (EMR Canada, 1989; Dawson and others, 1991). Estimated
reserves are 63.5 million tonnes grading 0.17 percent
MoS 2.The stock is dominantly one phase, and hosts a peripheral
ring of molybdenum. Also present are potassic (K-feldspar),
silica-sericite, and kaolinite alteration are present. The
stock has a K-Ar isotopic age of 49.0 Ma.