Hydrothermal system and acid lakes of Golovnin caldera, Kunashir, Kuril Islands: Geochemistry, solute fluxes and heat output

Golovnin caldera on the southernmost Kuril Island arc Kunashir Island is characterized by intense hydrothermal activity and thermal manifestations of different types inside and outside the caldera. In this paper we report our results of the 2015 field campaign together with already published data and discuss unusual geochemical features of the whole system. Acid chloride sulfate waters discharging inside the caldera are different from hot sulfate chloride waters discharging along the coast of the Sea of Okhotsk. The difference is in the ratios of the main conservative components (Cl, B, Na) and a high fraction of a Ca-SO4 enriched component in the coastal springs. Another unusual feature of the system is the existence of boiling Na-Cl springs outside the caldera, between the caldera thermal fields with Cl-SO4 and SO4 acid waters and SO4-Cl acid-to-neutral springs along the coast. Fumarolic and bubbling gases from the caldera are characterized by low 3He/4He values (~ 3.5Ra), isotopically heavy CO2 (δ13C > − 2.6‰) and isotopically light methane (δ13C ≤ − 40‰). This is a rare case when “chemical” (C-H-O) temperatures are higher than the “isotopic” (CO2-CH4) equilibrium temperatures. Trace element hydrochemistry shows preferential congruent rock dissolution in ultra-acid steam-heated SO4 waters inside the caldera and more complicated water-rock interaction for other types of waters. The REE patterns for chloride-sulfate and sulfate-chloride waters normalized by average rock show depletion in LREE caused, most probably, by co-precipitation of LREE with mineral assemblages characteristic for argillic and advanced argillic alteration. The only source of chloride in the drainage from the Golovnin caldera is the Kipyaschee Lake (Cl-SO4 hot springs on the lake bottom and at its shore). Solute output from the Golovnin caldera is lower than that from the other studied volcano-hydrothermal systems of Kuril Islands (5.7 t/d of Cl and 7.3 t/d of SO4). Natural heat output by hot water and steam discharges is estimated as 63 ± 20 MW.