Geological-geochemical model of the Akademii Nauk caldera hydrothermal system

Thus according to the geochemical data, after the sub aqueous eruption in 1996, two independent hydrothermal waters (near-alkaline Cl-Na waters of the Akademii Nauk and acidic waters of complex Cl-SO₄-Ca-Na composition in Lake Karymskoe northern sector) co-exist simultaneously in the Akademii Nauk caldera. The latter are genetically similar to Lake Karymskoe acid waters. What explanation can be found for such condition from a geological viewpoint? We have included all presently available geological and geochemical information about the area under study and our genetic constructions in Figure 16. It is distinctly seen that the source of discharge of boiling Cl-Na thermal waters obviously originating from the deep magmatic chamber thermal-mass-flow is confined to the Akademii Nauk volcano slope. Apparently, it can be logically supposed that the Akademii Nauk water-dominated hydrothermal system is localized in the strata of basaltic tuffs and subjacent

volcanogenic-sedimentary rocks underlying dacites of the volcano structure (Figure 7). Evidently, propylitized rocks serve as the host of this hydrothermal system because occasional fragments of these rocks were found in products of phreatic-magmatic eruption in 1996. The fact that there are few such fragments, is the evidence of propylite pinching out northward from the caldera. Judging from the location of the funnel of the explosion occurred in 1996, the peripheral magmatic chamber (or its apical part), which initiated the eruption in 1996, is located under the northern sector of Lake Karymskoe. This chamber is the source of fluids dominated by the gases: H₂O, HCl, HF, SO₂, H₂S, CH₄ and CO. Magmatic gases are partially neutralised as a result of solution-rock reaction taking place while passing through the zone of fracturing caused by magmatic-tectonic action. Later when deep fluids are mixed with infiltrating cool waters of surface origin, reduced gases are oxidized, dissolve and form hydrothermal solutions.

Figure 15 - Correlation diagram of δ O – Cl and δ D – Cl in thermal water of the Uzon caldera and Lake Karymskoe area in Kamchatka. The legend is the same as in Figure 12.

Thus, both gas and solute compositions of newly formed hot springs in the northern sector of the Akademii Nauk caldera and Lake Karymskoe water mineralization are affected by the flow of magmatic fluids from a recent peripheral chamber. It can be supposed that the pH differences of thermal spring waters and cool waters of the Karymsky Lake are caused by discharge of less neutralised acid fluids carrying sulphur (from our calculations - more than 70 t/day) as sulphate on the bottom of Lake Karymskoe and by the fact that surface discharges are in their way “processed” by flow through zones of argillization and silicification.

Figure 16. A schematic geologic-geochemical model of the hydrothermal system in the Akademii Nauk caldera (Kamchatka)

Legend: 1 – basalts of the 1996 eruption; 2 – andesites; 3 – the area of oxidation of gases, argillization and silicification of rocks; 4 – the area of propylitization; 5 – limits of mixing of surface and deep waters; 6 – the level of neutralisation of acid magmatic fluids; 7 – isothermal lines; 8 – limits of separation of lithologically different rocks; 9 – non-consolidated bottom deposits; 10 – atmospheric precipitation (and volcanic ash); 11 – paths of migration of infiltration waters; 12 – flows of magmatic fluids; 13 – flow of neutralized (hydrothermal) fluids; 14 – the zone of relatively permeable rocks; 15 – tectonic fractures; 16 – thermal springs; 17 – rocks enclosing magmatic chambers.

These conclusions are totally logical, however, it is quite possible that there is another mechanism of forming sub-neutral and moderate-acid solutions if we accept that the former are steam condensates of high-temperature (more than 300^oC) ultra-acid magmatic fluids and the latter are the result of mixing ultra-acid solutions and infiltrating surface waters. Then it follows that the level of steam separation (boiling zone) is located not very deep because, on the one hand, such depth is enough for enrichment

of solutions with main components – Na, K, Ca and Mg (owing to reaction with rocks during their movement) and, on the other hand, this path is not very long (based on the low total mineralization of these solutions shown in Table II). The great quantities of boron and chlorine indicate an endogenic component. Sulfate is provided by oxidation reactions of sulphur-containing compounds.

4. Conclusions

The preceding data and arguments show that saline compositions of thermal springs and acid lakes in caldera hydrothermal systems genetically connected with “andesite” volcanism form in different ways.

Two main types of hydrothermal water types co-exist in calderas of the “Karymsky” type having magmatic chambers with continuing intense intrusive-explosive activity. Some hydrothermal waters, the so-called “geyser” type of Cl-Na composition, are connected with abyssal fractures and delimit the area of generation of thermal-mass-flow from abyssal zones of volcanic-tectonic structures. These flows are manifest in zones of joint linear and circular fractures within ancient magmatic structures.

In this case, acid solutions and acid lakes appear in zones of resurgent magmatic activity and are connected directly with a flow of acid fluids from apical zones of a peripheral magmatic chamber. In thermal waters of Cl-Na content, the saline compositions are formed from abyssal fluids that contain high NaCl at temperatures of 800–900˚C and pressures 1-8 kbar and react with deep country rocks – neutralizing their predominantly acid compositions. Finally, these rocks undergo propylitization.

Infiltrating surface waters contribute greatly to waters feeding “geyser” type thermal waters.

The main source of mineral components in acid waters has recently erupted fresh magmatic rocks that are mainly of basic composition (basalts). An acidic contribution to these solutions is supplied by oxidation of sulphur-containing compounds and by halide (HCl, HF) gases in fluids separated from shallow magmatic chambers. In addition, a large part of water acidification results from extraction of acid components from fresh ashes entering lacustrine basins. The water component of such sources and basins is mainly meteoric.

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