USGS Professional Paper 1697 - Alaska Resources Library

92 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Origin of and Tectonic Controls for Copper Mountain
(North) Metallogenic Belt
The Copper Mountain (North) metallogenic belt is hosted
mainly in intermediate-composition granitoid plutons in the
Copper Mountain suite, which are part of the subductionrelated
Quesnellia island arc (Nokleberg and others, 1994a,b;
Monger and Nokleberg, 1996; Nokleberg and others, 2000).
Emplacement of plutons was apparently along faults and
intersections of faults. In both the Copper Mountain (North)
and Copper Mountain (South) metallogenic belts, many of the
porphyry Cu-Au deposits occur in alkaline plutons. Isotopic
ages indicate intrusion of host granitoid plutons and formation
of associated mineral deposits from about 175 to 185 Ma in
the Middle Jurassic in the Copper Mountain (North) metallogenic
belt. This age represents the end of subduction-related
igneous building of Quesnellia island arc, just before accretion
of the Quesnellia terrane, along the with tectonically related
Stikinia island arc and Cache Creek subduction-zone terranes,
onto the North American Craton Margin (Monger and Nokleberg,
1996; Nokleberg and others, 2000).
Copper Mountain (South) Metallogenic Belt of
Porphyry Cu-Au Deposits (Belt CMS), Southern
British Columbia
The Copper Mountain (South) metallogenic belt of porphyry
Cu-Au deposits (fig. 32; tables 3, 4) occurs in southern
British Columbia and is hosted in the Copper Mountain
alkalic plutonic suite in the Quesnellia island-arc terrane. The
significant deposits are the Copper Mountain, Iron Mask area
(Afton, Ajax), and Mt. Polley (Cariboo-Bell) porphyry Cu-Au
deposits and the Lodestone Mountain zoned mafic-ultramafic
Fe-V deposits (table 4) (Nokleberg and others 1997a,b, 1998).
Copper Mountain (Ingerbelle) Porphyry Cu-Au Deposit
The Copper Mountain (Ingerbelle) alkalic porphyry
Cu-Au deposit (fig. 39) consists of mainly of chalcopyrite and
bornite that occur as disseminations and in stockworks in Early
Jurassic alkaline intrusive rocks of the Copper Mountain Suite
and similar age volcanic and volcaniclastic rocks of the Nicola
Assemblage (Preto, 1972; McMillan, 1991; P. Holbeck, Cordilleran
Roundup, written commun., 1995; MINFILE, 2002).
This and similar deposits in the Copper Mountain area occur
along a northwest trend for over 4 km. The main ore bodies are
the Copper Mountain Pits 1 to 3, Ingerbelle East, Ingerbelle,
Virginia, and Alabama. Production up to 1994 was 108 million
tonnes containing 770,000 tonnes Cu and 21.8 tonnes Au.
Estimated reserves are 127 million tonnes grading 0.38 percent
Cu, 0.16 g/t Au, and 0.63 g/t Ag (MINFILE, 2002). Estimated
resources are 200 million tonnes grading 0.4 percent Cu
equivalent. Significant values in Pt and Pd were reported from
assays of chalcopyrite- and bornite-rich concentrates.
The Copper Mountain (Ingerbelle) deposit consists
of a silica-deficient, chalcopyrite-pyrite-bornite stockwork
hosted almost entirely by fragmental andesitic volcanic rocks,
calcareous volcaniclastic rocks and minor carbonate strata
(Fahrni and others, 1976). Intrusive rocks are equigranular
diorite stocks and more siliceous dikes, sills, and irregular
plugs of the Lost Horse Intrusive Complex, a porphyritic unit
that is often closely associated with bornite-chalcopyritepyrite-magnetite
mineral deposits and occurrences. A K-Ar
biotite isotopic age of 197 to 200 Ma is interpreted as an Early
Jurassic age for the deposit. Alteration mineral assemblages at
the Copper Mountain Pits 1 to 3 and the Alabama and Virginia
ore bodies are early albite-diopside-epidote-calcite, potassium
feldspar-biotite-epidote-magnetite, and a later propylitic
assemblage of chlorite-pyrite-epidote-scapolite-calcite (Preto,
1972; Stanley and others, 1995).
At the Ingerbelle deposit, skarn-like ore and gangue mineral
zonation occurs along the contact of the Lost Horse stock
where it intrudes agglomerate, tuff, tuff-breccia, and sedimentary
rocks of the Nicola Assemblage (Sutherland Brown and
others, 1971; Preto, 1972; Macauley, 1973; Fahrni and others,
1976; and Dawson and Kirkham, 1996). In these areas, early
biotite hornfels was overprinted by prograde albite-epidote,
chlorite, andradite, diopside, and sphene; both the stock and
prograde skarn were then extensively replaced by retrograde
albite, potassium feldspar, scapolite, calcite and hematite.
Chalcopyrite-bornite ore, about 90 percent of which occurs
in andesitic volcanic rocks, occurs along contacts, apophyses,
and dikes of the Lost Horse stock.
Iron Mask (Afton, Ajax) Porphyry Cu-Au Deposit
The Iron Mask (Afton, Ajax) porphyry Cu-Au and
other deposits in the Iron Mask district are hosted in the Iron
Mask Batholith that consists of an Early Jurassic composite
alkaline intrusion emplaced into the Nicola Group. The Ajax
deposit occurs at the contact between two diorite phases of the
Iron Mask pluton, a hybrid diorite and the younger Sugarloaf
diorite (Ross and others, 1992, 1993; Ross and others,
1995). Pyrite, chalcopyrite, minor bornite, and molybdenite
are accompanied by main-stage albite and peripheral propylitic
alteration. Potassic and scapolitic alteration crosscuts
albite and propylitic alteration (Ross and others, 1992, 1993,
1995). The Afton deposit consists of a tabular-shaped body
of chalcopyrite and bornite that is hosted in fractured diorite
of the Cherry Creek pluton. A deeply penetrating supergene
zone contains native copper and lesser chalcocite. Aggregate
preproduction reserves and production for the Ajax East and
West deposits and for the Afton deposit are 66 million tonnes
grading 0.77 percent Cu and 0.56 g/t Au. The common occurrence
of picrite intrusions along faults that cut the Nicola Group
and their association with the porphyry deposits in the Iron Mask
pluton indicate that regional, steeply dipping faults controlled
emplacement of the plutons in the batholith and also served as
conduits for mineralizing fluids. An U-Pb zircon isotopic age of
207 Ma (Late Triassic) for the Cherry Creek pluton.
Deep drilling in 2001-2002 southwest of and below the
Afton orebody by DRC Resources Corporation has proven