Isotope Geochemistry of the Northeast Zone, Mount Polley Alkalic Cu-Au-Ag Porphyry Deposit, British Columbia: A Case for Carbonate Assimilation

Mount Polley is a Late Triassic (similar to 205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.1 t Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of mega-crystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (> 350 degrees C), CO2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and calc-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The delta S-34(sulfide) isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from -7.1 to + 1.4%o. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8%o. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from similar to 480 degrees to similar to 250 degrees C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite delta C-13 values range from -0.2 to -10.5%o, and delta O-18 values from 4.0 to 20.9%o. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have Pb-206/204 values of 18.77 to 18.92, Pb-207/204 of 15.56 to 15.59, and Pb-208/204 of 38.22 to 38.32. Strontium isotope data (0.70331-0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate Pb-206/204 values of 18.96 to 19.04, Pb-207/204 of 15.57 to 15.59, and Pb-208/204 of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as similar to 100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the delta C-13-delta O-18 isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate.