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Stable isotope studies on Bougainville and in Matupi Harbour, New Britain, Papua New Guinea

Summary Two case histories are presented in which light stable isotope ratios (D/H and 18O/16O) place important constraints on the origins of ore-forming fluids in the Panguna porphyry copper deposit (upper Pliocene) and on the nature of the volcanic exhalative environment at Matupi Harbour, near Rabaul. At Panguna the overlap in δD values between local meteoric water and inferred primary magmatic waters poses problems in the interpretation of secondary biotite and sericite isotope data. The range of hydrogen and oxygen isotope ratios of the secondary biotites associated with the mineralized areas is essentially identical to those from other magmatic hydrothermal deposits, including porphyry copper deposits. The sericites from alteration haloes adjacent to mineralized quartz veins have δ18O values of 5.4 to 8.6‰ and δD values of −61 to −77. A combination of thermodynamic and fluid inclusion techniques indicates that ∼500°C is a realistic average temperature for much of the mineralization-quartz veining-biotitization process. The isotopic composition of the fluids causing this mineralization and alteration was δD⋍ −20 to −45‰ and δ18O ⋍ 6 to 7‰. If a meteoric water component existed at this stage of alteration, meteoric water/rock ratios were extremely small (less than 0.05 at %). Assuming that sericitization took place at temperatures down to 300°C, as suggested by the fluid inclusion data, the fluids responsible for this process had a composition of δD = −30 to −50‰ and δ18O of ⋍ 3.5‰. These fluids probably would be largely of meteoric hydrothermal origin, produced by 18O shifting of groundwaters during the formation of the propylitic zone and with a water/rock ratio of about 0.01 at % for the entire propylitic zone. Quartz δ18O values from veins associated with the different alteration assemblages increase away from intrusive centres, consistent with an outwards temperature decline, a decrease in δ18O content of associated water and a progressive increase in the meteoric water component associated with the later and (?) lower-temperature sericitization. There is no isotopic evidence to suggest that sea waters or connate waters formed a recognizable component of the hydrothermal system at any stage. Calculations with possible δ18O depletions of 0.5–1.0‰ in whole rocks from the Panguna region indicate that a meteoric leaching model for the genesis of the Panguna copper deposit is theoretically possible, but unlikely because of the small water/rock ratios. The area around Matupi Harbour includes the dormant volcanoes Rabalanakaia and Tavurvur, set in loosely consolidated, Quaternary ash deposits. Tavurvur last erupted in 1941 and 1942 and has since reverted to a quiet solfataric condition. D/H and 18O/16O ratios were determined on fumarole condensates, hot springs and meteoric waters. The data are grouped into distinct areas close to the meteoric water line and demonstrate that the thermal springs away from the shoreline are of meteoric origin and that sea water only enters the springs at the shoreline. If allowance is made for the vapour-liquid separation at the temperature of collection (100°C), the fumarole results from Tavurvur may be interpreted as steam separating from a water phase at ⋍ 100°C. The calculated water-phase isotopic compositions suggest that evaporation and possibly a small amount of mixing with sea water are involved. These conclusions are in conflict with those drawn from anion ratio and trace metal contents inferred by previous authors to be consistent with an hypothesis of modified sea-water origin. We contend that the chemistry is a reflection of the later history of these acid, mineralized, geothermal waters and does not necessarily give a correct picture of their ultimate origin. The markedly enhanced Fe, Mn and Zn values of the Matupi springs are purely a function of the leaching potential of geothermal fluids at elevated temperatures and of the chemistry of the porous rocks through which they pass.