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 (~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 megacrystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (>350°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 δ³⁴S_sulfide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from −7.1 to +1.4‰. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8‰. 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 ~480° to ~250°C.

Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite δ¹³C values range from −0.2 to −10.5‰, and δ¹⁸O values from 4.0 to 20.9‰. 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 ^206/204Pb values of 18.77 to 18.92, ^207/204Pb of 15.56 to 15.59, and ^208/204Pb 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 ^206/204Pb values of 18.96 to 19.04, ^207/204Pb of 15.57 to 15.59, and ^208/204Pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as ~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 δ¹³C-δ¹⁸O 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.