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

Mount Milligan Porphyry Cu-Au Deposit
The Mount Milligan Porphyry Cu-Au deposit consists of pyrite, chalcopyrite, bornite and magnetite which occur as
disseminations and in quartz veinlets (Delong and others, 1991; McMillan, 1991; Nelson and others, 1991; Banie, 1993;
Sketchley and others, 1995). The deposit has estimated reserves of 298.4 million tonnes grading 0.22% Cu and 0.45 g/t Au. The
deposit is hosted in augite porphyritic andesite of the Witch Lake (informal) formation of the Late Triassic to Early Jurassic Takla
Group which is intruded by several small brecciated diorite and monzonite porphyry dikes and stocks. Cu-Au mineralization in the
Main deposit accompanied the emplacement of the MBX stock and Rainbow dyke; the Southern Star deposit surrounds the stock
of the same name. A U-Pb zircon isotopic age of 183 * 1 Ma is obtained for the Southern Star monzonite. Cu and Au minerals are
associated with moderate to intense potassic alteration around intrusive contacts. Potassic alteration, which is ubiquitous in
mineralized stocks and surrounding volcanic rocks, is surrounded by propylitic alteration which decreases in intensity outward
from intrusive. A well-developed mineral zoning consists of a biotite-rich core in the potassic zone which contains most of the Cu
and Au. Numerous polymetallic veins are hosted by the propyliyic alteration zone immediately beyond the limits of the porphyry
deposit.
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 subduction-related 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 which 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, 199 1; P. Holbeck,
Cordilleran Roundup, written commun., 1995; MlNFlLE, 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 1-3, 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% Cu, 0.16 g/t Au, and 0.63 g/t Ag (MINFILE, 2002). Estimated resources are 200
million tonnes grading 0.4% 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 which is often closely associated with bornite-chalcopyrite-pyrite-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; and 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,
197 1; Preto, 1972; Macauley, 1973; Fahmi 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; then both the stock and prograde