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 themain
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 fromthe caldera are characterized by low3He/4He values (~3.5Ra), isotopically heavy
CO2 (δ13C N−2.6‰) and isotopically lightmethane (δ13C≤−40‰). This is a rare case when “chemical” (C-H-O)
temperatures are higher than the “isotopic” (CO2-CH4) equilibriumtemperatures. Trace element hydrochemistry
shows preferential congruent rock dissolution in ultra-acid steam-heated SO4 waters inside the caldera andmore
complicated water-rock interaction for other types of waters. The REE patterns for chloride-sulfate and sulfatechloride
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 fromthe Golovnin caldera is the Kipyaschee Lake (Cl-SO4 hot springs on the lake bottom
and at its shore). Solute output fromthe 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.