The Sirung volcanic boiling spring: An extreme chloride-rich, acid brine on Pantar (Lesser Sunda Islands, Indonesia)

The Sirung volcano — with a history of phreatomagmatic outbreaks — has its present active center within a 1000-m-wide subsidence crater. The Sirung Crater Lake (SCL) has 15 g l −1 Cl and 19 g l −1 SO 4 (pH = 0.75, temperature 40°C). A highly saline brine (Sirung boiling spring, SBS) boils to the surface, beside the lake: it contains 116 g l −1 Cl (pH = 1.4). Chemical geothermometers indicate a reservoir temperature of 240°C; the maximum depth of this reservoir is estimated to be 400 m. The stable isotope composition is δ D = − 12‰ and δ 18O = + 5.0 to + 6.0‰. Tritium in the SBS is 0.0 TU, in the SCL 4.6 TU. Evidence against invading seawater as a source is the negative δD-value, in addition to the six times higher Cl concentration of the SBS. Evaporites are rejected as a source of the Cl because of the geologic setting. The average Cl H 2 O ratio in Sirung volcanic glasses as estimated by microprobe is very close to the Cl H 2 O ratio of 0.047 for the SBS. The zero tritium level in the SBS, its high Cl concentration and the acid character are other evidences for magmatic water as source for the SBS. The acid SBS is poor in sulphur, which may have been precipitated at depth as sulphides. Theories on porphyry Cu formation in volcanic arc environments invoke highly saline brines as the transporting agent. The acid saline brines from the Sirung and White Island (New Zealand) craters, and Cl-rich crater lakes associated with andesitic volcanism in subduction zones indicate that such fluids occur more frequently at the surface than has been possibly realized hitherto.