Geological Setting and Characteristics of the Red Chris Porphyry Copper-gold Deposit, Northwestern British Columbia

Exploration & Mining Geology, Vol. 6, No. 4, 1997

The Red Chris Cu-Au deposit (522.7 Mt at 0.35% Cu and 0.27 g/t Au),
northwestern British Columbia, is one of several pre-accretionary porphyry
systems in the Cordillera of western Canada. It is hosted by an Early Jurassic
hornblende monzonite to quartz monzodiorite porphyry which forms part of a suite
of dikes and stocks that intrude Late Triassic volcanic strata. Major and trace
element chemical data suggest that these Early Jurassic intrusions have
compositions which fall on the boundary between alkalic and calc-alkalic
compositions. Alteration and mineralization in the Red Chris deposit are more
typical of calc-alkaline porphyry systems with abundant potassic, sericitic and
argillic assemblages and quartz-rich stockworks. It lacks Na- and Ca-bearing
silicate assemblages which characterize the alkalic porphyry deposits of British
Columbia. One of the more unusual features of the deposit is the abundance of
carbonate alteration and veins which occur throughout the hydrothermal history
of the deposit. This type of alteration and veining is more typical of alkalic
porphyry deposits. The composition of the carbonates vary both temporally and
spatially with different alteration zones. Potassic zones, which are
characterized by orthoclase±albite±quartz±chalcopyrite±bornit±
magnetite±hematite and Au/Cu ratios of 1:1 (g/t Au:% Cu), contain minor siderite
veins and alteration. Quartz-sericite-carbonate (QSC) zones overprint earlier
potassic alteration and comprise quartz+ankerite+sericite+pyrite+chalcopyrite
with Au/Cu ratios of 0.5:1 (g/t Au:% Cu). Ankerite is the main carbonate in this
stage but has a variable composition with a more Fe-rich variety associated with
mineralized veins. Localized chlorite-carbonate alteration zones, characterized
by chlorite+carbonate+quartz±kaolinite±pyrite±gypsum and high Au/Cu ratios [2 to
3.5:1; (g/t Au:% Cu)], contain predominantly calcite. Late stage
carbonate-gypsum veins have an ankerite to dolomite composition which is very
similar to the regional carbonate alteration and veins.
Keywords: Red Chris deposit, Cu-Au deposit, Porphyry, Carbonate
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