HYDROTHERMAL ALTERATION AND STABLE-ISOTOPE SYSTEMATICS OF THE BABINE PORPHYRY CU DEPOSITS, BRITISH-COLUMBIA - IMPLICATIONS FOR FLUID EVOLUTION OF PORPHYRY SYSTEMS

The Eocene Babine Intrusive Suite of west-central British Columbia hos ts a number of prophyry copper deposits, most significantly Bell, Gran isle, and Morrison. All deposits feature central potassic zones contai ning the ore zones and peripheral propylitic zones. In addition, Grani sle 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. Calc ulated fluid compositions from potassic zone biotites of all deposits yield a range from deltaO-18(H2O) = 6 to 8 per mil, whereas plagioclas es yield deltaO-18(H2O) = 1 to 7 per mil. Since plagioclase samples yi eld more O-18-depleted fluids than coexisting biotites and DELTA(plagi oclase-biotite) values are very small, it appears that isotopically li ghter fluids were present in later stages of potassic alteration. Oxyg en isotope values from the sericite-carbonate and phyllic alteration z ones yield fluid compositions more O-18-depleted than magmatic fluids, requiring the incorporation of isotopically light ground waters. Calc ulated hydrogen isotope values of potassic, propylitic, and sericite-c arbonate fluids fall in the range of -40 to -110 per mil. These result s, in combination with the deltaO-18 data, suggest that the alteration events were caused by mixtures of magmatic and meteoric waters. Phyll ic and argillic alteration fluids (deltaD(H2O) = -100 to -130 and -140 parts per thousands, respectively) indicate fluids dominantly of meteo ric origin. The stable isotope data, in combination with the petrologi c and field relations, suggest that early alteration fluids were deriv ed from the magma, and in peripheral areas fluids evolved at very low water/rock ratios. Incorporation of external fluids in later alteratio n was responsible for the texturally destructive, cation-leaching alte ration and remobilization of the ores. Differences in the stable isoto pe systematics and salinities of porphyry copper deposits compared to meteoric hydrothermal systems and epithermal ore deposits indicate tha t the systems are fundamentally distinct. The strong density contrast between the high-salinity magmatic fluids and low-salinity meteoric wa ters most likely inhibits mixing, resulting in separation of the hydro thermal systems.