Hydrothermal alteration and stable isotope systematics of the Babine porphyry Cu deposits, British Columbia; implications for fluid evolution of porphyry systems

Abstract

The Eocene Babine Intrusive Suite of west-central British Columbia hosts a number of porphyry copper deposits, most significantly Bell, Granisle, and Morrison. All deposits feature central potassic zones containing the ore zones and peripheral propylitic zones. In addition, Granisle and Bell feature superimposed sericite-carbonate zones between the potassic and propylitic zones, and the Bell deposit also possesses a superimposed phyllic stockwork zone, which hosts most of the ore.Calculated fluid compositions from potassic zone biotites of all deposits yield a range from delta 18 O (sub H 2 O) = 6 to 8 per mil, whereas plagioclases yield delta 18 O (sub H 2 O) = 1 to 7 per mil. Since plagioclase samples yield more 18 O-depleted fluids than coexisting biotites and delta (sub plagioclase-biotite) values are very small, it appears that isotopically lighter fluids were present in later stages of potassic alteration. Oxygen isotope values from the sericite-carbonate and phyllic alteration zones yield fluid compositions more 18 O-depleted than magmatic fluids, requiring the incorporation of isotopically light ground waters.Calculated hydrogen isotope values of potassic, propylitic, and sericite-carbonate fluids fall in the range of -40 to -110 per mil. These results, in combination with the delta 18 O data, suggest that the alteration events were caused by mixtures of magmatic and meteoric waters. Phyllic and argillic alteration fluids (delta D (sub H 2 O) = -100 to -130 and -140ppm, respectively) indicate fluids dominantly of meteoric origin.The stable isotope data, in combination with the petrologic and field relations, suggest that early alteration fluids were derived from the magma, and in peripheral areas fluids evolved at very low water/rock ratios. Incorporation of external fluids in later alteration was responsible for the texturally destructive, cation-leaching alteration and remobilization of the ores.Differences in the stable isotope systematics and salinities of porphyry copper deposits compared to meteoric hydrothermal systems and epithermal ore deposits indicate that the systems are fundamentally distinct. The strong density contrast between the high-salinity magmatic fluids and low-salinity meteoric waters most likely inhibits mixing, resulting in separation of the hydrothermal systems.