Authigenic and biogenic minerals in volcanic hot springs of Kamchatka, Russia

Abstract

The volcanic hot spring deposits from Kamchatka, Russia were examined by scanning electron microscopy and transmission electron microscopy equipped with energy dispersive spectroscopy. Those hot springs include Burlyashii, Oil pool, Zavarzin, Thermophile and Jen’s vents, which were previously reported for microbial ecology and mineralogy. Opal A spheroids are common in all hot spring sediments with particle size from <20 nm to 50 nm. Euhedral single quartz nanoparticles of 700 nm crystallized from Opal-A can be observed scattered on the surface of anhedral sediments. Anthigenic kaolinite of sharp faces can be observed in all hot springs with Si, Al and O as major compositions and having flaky- and scale-like structures. Iron sulfides are common with a wide spectrum of crystal sizes and morphologies. Small pyritic iron sulfides have octahedral shape with crystal size around 20-80 nm; while pyrite crystals of 200 nm to 500 nm and these >1 μm have cubic structure. The small pyrite crystals are also observed assembled to bigger cubic shapes, showing an ostwald ripening process. There are also spheroid shaped, 1 μm size pyritic aggregates with rough surface which showed a sulfate-reducing bacteria mediated formation of iron-sulfides. Fibrous and tabular gypsum crystals can be observed coexisting with sulfides, indicating local oxidation condition. Euhedral crystals of element sulfur of 30 μm can also be observed. Magnetic minerals are abundant in Jen’s vents, however, no magnetite can be detected, implying the existence of magnetic iron sulfides though thermophilic iron-reducing bacteria were isolated from some springs. Detrital quartz and silicates of larger sizes can be observed in some hot spring, showing water-rock interaction induced corrosion structures. The high abundance of iron sulfide minerals and the high percentage of menaquinones in sinter deposits indicated highly reduced conditions. The common existences of submicro- to nano-meter authigenic minerals, the abundant bacteria filaments, biofilms and diatoms indicate microbial mediation is a significant factor in the mineralization of those minerals in the hot springs.