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Kalacheva E.G., Rychagov S.N. and Belousov V.I.


Institute of Volcanology and Seismology FED RAS, Petropavlovsk-Kamchatsky, Russia.

E-mail: keg@kcs.iks.ru We have studies the macrocomponent composition of natural waters and reconstructed the hydrochemical zoning and dynamics of the surface and underground waters within the North-Paramushir hydrothermal-magmatic system. We have studies the areas of feeding and discharge of pressure waters of the upper water-bearing horizonts and complexes restricted to the isometric geological blocks 1.5-2 to 3.5-5 km across and to the ruptured tectonic structures of local and regional types. It is shown that in the Paramushir Island northern part all water types characteristics of the hydrothermal systems, related with the island-arc andesite volcanoes, are present. The composition of hydrotherms and gases is of a great meaning for the formation of deep-level flow structure and the evolution of hydrothermal and magmatic processes in the interior of North-Paramushir geothermal area. The availability of a great of carbon dioxide, hydrogen and other gases creates conditions for a high dynamics of physical-chemical and structure-forming processes.

1. Introduction The North-Paramushir geothermal area has attracted researchers for many decades (Nekhoroshev, 1960; Zelenov, 1972; Menialov et al., 1992; Rychagov et al., 2001). The interest of this area is conditioned by its insular position at the interface of oceanic and continental crust, by a great thickness of the system, by the presence of the active andesite volcano Ebeko in the central part and by the complex structure of the systems feed zone.

The North-Paramushir hydrothermal-magmatic convective system under study is an intrusive-volcanogenic complex, long-evolving (since Paleogene up to now) and large (a volume of its host rocks 700 km3) geological structure, now being in an infancy stage of island arc. A magmatic source drives heat and material transfer, the origin of which may be both primary (generation of melts at the upper mantle level a nearsurface magmatic center a subvolcanic body), and secondary (heat release in process of exothermic chemical reactions) (Belousov et al., 2002). The North-Kuril geothermal deposit is located in the eastern section of the hydrothermal-magmatic system. Estimated geothermal resources of the deposit exceed 100 MW of electric capacity. Apart from geothermal processes, modern mineral- and ore-formation is of a great interest: deep drilling showed deep-level formation of gold-polymetallic mineralization, and, possibly, the genesis of a copper-porphyry type of mineralization (Rychagov et al., 2002). Geological, 4: hydrodynamic, and geochemical studies allowed development of geological-geochemical and structural-hydrodynamic models of the North ore-generating hydrothermal-magmatic system that are of great importance for understanding the process of modern hydrothermal ore formation and for orientation of prospecting works to hydrothermal and mineral resources.

2. General geological- hydrogeological description of the North-Paramushir hydrothermal-magmatic system The Paramushir Island is a relatively elevated block of the earths crust and is considered a southern continuation of the South Kamchatka horst (Aprelkov, 1971). The northern end of the island is composed of volcanogenic-sedimentary, intrusive and volcanic erupted rocks of Upper-Miocene to modern age and of glacial deposits (Rychagov et al., 2001). The basement consists of tuffs and tuffites of the Okhotsk suite of UpperMiocene-Lower-Pliocene age. Thickness of the suite is within a range of 1400 to 3000 m.

Tuffs and tuffites of 900 to 1000 meters thick Oceanic suite of Middle-Upper-Pliocene age overlie the Neogene deposits. Rocks of Okhotsk and Oceanic suites are intruded by sills, dykes and other subvolcanic bodies of andesite-basalt and basalt composition. Large eroded subvolcanic bodies (for instance, Aerodromnoye Plateau) are up to 2500-meters across. The contacts of these bodies with host rocks consist of breccias and largeblocks. Contact zones ranging from 1-5 to 10-30 m and more are visible. Subvolcanic bodies are of interest as possible analogs of modern intrusive bodies, feeding a hydrothermal-magmatic system. Lavas of andesites and basalts of Upper-PlioceneHolocene age constitute a series of thick flows. Three groups of volcanoes are distinguished, that form the Vernadskogo Ridge which is a North to South extended volcanic-tectonic structure. There is an active volcano, called Ebeko. It is known to have cycles of activity in historical time in 1793, 1895, 1934-38, 1967-71 and in 1987-91.

(Menialov et al., 1992). The last eruptions were phreatic. Currently, the volcano is in a stage of intensive fumarolic activity. The Krasheninnikov volcano also shows some steamgas activity. The central part of the North-Paramushir hydrothermal-magmatic system is localized in a concentric morphotectonic ring structure, formed at the intersection of a north-northeast and west-northwest tectonic zones. A Negative gravity anomaly was identified within the Ebeko volcano area, which is attributed to a lesser density of rocks, forming a vertical cylindrical body of oval section with a size of 2 x 1 km. Rocks were altered to opalites and argillizites propagating to a depth of around 1 km. The reference Kalacheva E.G., Rychagov S.N. and Belousov V.I.

geological section of the hydrothermal-magmatic system is composed of four complexes of rocks (Rychagov et al., 2002). The bottom of the section (2500-1700 m) is composed of lithocrystalloclastic psepho-psammitic tuffs of andesite-basalt composition and intrusive tuffs (breccias), forming a block-breccia mantle of a large diorite body. The middle strata (1700-960m) are lithocrystalloclastic tuffs of andesite composition, brecciated in the upper part of deposits. The upper volcanogenic-sedimentary strata (960-140 ) consist of alternating bands of psepho-psammitic and aleuro-pellitic tuffs and tuffites; the bottom of the strata is dominantly manifested with tuffs; the upper part is mainly composed of tuffites containing relics of marine microfauna. Holocene age lavas of andesite composition overlay the section. The main portion of the middle strata acts as an aquifer; volcanogenicsedimentary rocks have water-confining properties. It is suggested that the block-breccia mantle of the diorite body hosts the most high-temperature and, possibly, ore-bearing therms. Between the middle and upper strata there is a higher permeability zone including a thick vapor-dominant system on the upper and lower borders of which ore geochemical barriers are formed. Complex of sulfide and oxide minerals is formed in rocks; native metals, their alloys and intermetallic compounds are deposited in fractures and pores.

In northern part of Paramushir island the following tipes of waters are represented Ultra acidic waters (pH 0-3) are discharged on Ebeko volcano. The waters are shaped at the expense of merging magmatic gases with meteoric waters. Acidic waters (pH 3-5.5) have compound cationic composition. They are shaped at the expense of interaction of underground waters with metasomatites. The neutral waters (pH 5.5-7.5) have speckled cationic composition. Waters are shaped at the expense of atmospheric precipitation. The alkaline waters (pH 7.5-8) are detected by drill-holls. It is deep-level chloride-sodium hydrotherms. Chloride-sodium deep-level hydrothermal fluids are typical waters for the lower water-bearing complex of the Kuril-Kamchatka region. Chloride-sulfate, sulfate and hydrocarbonate-calcium hydrotherms are formed in a zone of mixing of deep-level and meteoric waters. Sulfur discharge centers of deep-level hydrotherms are low-thermal chloride-sulfate waters with 3.5 5.5. Sea waters, penetrating 2.5 km inland to an axis zone of the Vernadskogo Ridge, significantly influence the composition of the hydrothermal fluids. In all, hydrotherms are formed by mixing of meteoric and sea waters, volcanic and hydrothermal gases and deep-level fluids.

3. Hydrothermal characteristics of the Northern end of the Paramushir Island There are various approaches for determining input to hydrothermal systems; the 4: water balance method is simplest. The method includes determination of moisture input and discharge over any period of time for the area under study. Setting up a simple equation of water balance for a separate area is the main difficulty. The best solution to this problem is to study a multiyear water balance (Kudelin, 1960). In this case the area is divided into balance sections corresponding to river basins. An equation of water balance for a separate area over a multiyear period is of the following form:

W=X Z, where (1) -YX calculated normal annual precipitation; Y calculated normal annual run-off; Z calculated normal annual evaporation; W- normal annual infiltration into deep waterbearing horizons within the feed zone, or calculated artesian run-off within the discharge zone, or difference between them.

3.1. Precipitation Cold waters of the Okhotsk Sea and of the Kuril Current shape the climate of the Paramushir island. The region is characterized by cold winters with frequent blizzards and chilly damp foggy summers. An average annual precipitation in the town of SeveroKurilsk is 1795 mm (table 1). But these data is their watershed. The drainage network is dense. Rivers are mainly cataracted with numerous falls and V-shaped valleys; alluvial valleys are developed only in estuary zones.

are correct for the meteorological station altitude (23 m above the sea level), whereas the area under study occupies territory with absolute points of 0 m to 1150 m (the Ebeko volcano). Using an altitude distribution correction factor 1.7, derived for the Pauzhetsky area (the South Kamchatka), situated short of the Paramushir island (Vakin, 1968), we can calculate normal annual amount of precipitation in the North Paramushir - 3123 mm.

3.2. Evaporation Evaporation in natural conditions is an intricate process depending on many meteorological factors: humidity, ambient temperature, wind velocity etc. Also, relief, ground water occurrence, structure and composition of soil-vegetation cover greatly influence evaporation process. Since no special study of evaporation in the area was Kalacheva E.G., Rychagov S.N. and Belousov V.I.

conducted, and meteorological data refer only to the urban areas, we will estimate evaporation by a graphic extrapolation method. An average annual evaporation value is 381 mm.

3.3. River run-off The area under investigation occupies an area of 215 km2. Volcanic, tectonic, intrusive and denudation processes by permanent and temporary water courses play the major role in the formation of the areas relief. Rivers of the area belong to the basins of the Okhotsk sea and Pacific ocean. The Vernadskogo ridge The north-east rivers are an exception: they have flat lengthwise profiles and relatively tranquil flows. Totally, there are more than 20 distinct water courses 3 to 15 km long with many small inflows.

Temporary water courses with episodical run-offs are widely spread on the slopes of volcanoes. Water collection for rivers takes place by means of atmospheric precipitation and underground water discharge. Surface volcanics are mainly highly-permeable rocks, through which surface run-off goes underground.

3.4. Description of balance sections For run-off calculation purposes, the area was divided into three large balanced sections () including several lesser ones. Section selection criteria included hydrological, geographical and other aspects, and also a level of study of the areas river basins. Since there are no water gauge stations on the island, hydrometric works were being done during summer mean water level except for rivers near the town of Severo-Kurilsk where regimen monitoring was organized in 2001. Normal annual runoff is the main estimated parameter of river run-off (an average annual run-off over multiyear period). Where there is a lack of field work data, the methods of approximated determination of normal run-off are used.

The following formula of Kritsky (Solomentsev, 1961) can be applied for the area in question:

=11/(d3d+11), where - normal annual run-off, mm, d filling deficit (See table 1) =11/(1.5431.54+11)=0.Proceeding from an amount of precipitation, normal annual run-off is determined for the separate river basin: Y=X * 0.708. Y=3123 * 0.708 = 2271 mm In this manner, all terms of the water balance equation were derived; the precision of the determinations is sufficient for a pre-assessment of the amount of infiltration feeding the underground waters of the North-Paramushir hydrothermal-magmatic system. According to formula1, W = 3123-2271-381=471 mm. W has a positive value, therefore underground water refilling in the area averages 471 mm a year. For the purpose of a detailed study of 4: water balance on the development territory of the hydrothermal-magmatic system, calculations were done for all the river basins of the identified balance sections.

3.4.1. The eastern section The eastern section is of a special interest because its interior hosts the presumed North-Kuril geothermal deposit. Water gauge stations were set up in estuary zones for regimen monitoring of first-order water-courses. Seeing that amount of precipitation in the northern end of the Paramushir island over the last hydrological year did not differ much from the mean annual value over the whole monitoring period, data on average annual river discharge can be used as the multiyear average. Table 2 shows the results of water balance calculations. Infiltration values for all the rivers are negative. But this increment is very low in the Kuzminka river basin: water balance calculations reveal neither absorption of atmospheric precipitation for feeding deep-level water-bearing horizons, nor significant discharge of underground waters, which allows this section to be considered an underground pressure area. Significant discharge centers of deep-level water-bearing horizons were determined by water balance calculations in basins of the rivers of Nasedkina, Ptichiya, Matrosskaya and Snezhnaya. Moisture excess in these sections is 9% (the Snezhnaya river) and 26% (the Ptichiya river) of normal annual precipitation values. Infiltration in the basin of the Gorodskaya river has also a negative value, but water excess is caused both by deep-level pressurized water discharge and by discharge of upper water-bearing horizons.

3.4.2. The Northern and Western sections In view of episodical hydrometric measurements at these sections, a value of run-off was determined by means of empirical formulas, Kalacheva E.G., Rychagov S.N. and Belousov V.I.

mentioned above. The other parameters were determined in the same manner as for the Eastern section (See Table 2). Infiltration (W) all over the territory ofthe Northern section has a positive sign, and a water balance equation is of the form X=Y+Z+W, which is typical of river basins, located within an underground water feed area. The calculations show that feeding of deep water-bearing horizons takes place with an equal intensity in all the river valleys (15-17% of normal annual precipitation). The lack of data on river profiles does not allow the delineation of feed zones at this section. However, the field studies we conducted in summer and fall of 2001, and literature data (Opit Kompleksnogo, 1966) indicate numerous vents of ground waters in downstream of the Savushkina, Zelenaya etc.

rivers on the contacts of lava flows and modern alluvium. Based on this data, fig. 1 shows the discharge zone of upper water-bearing horizons of the Northern water- balance section.

A negative increment of water balance at the Western section is observed in the Yurievaya and Gorshkovaya river basins. Moisture excess is due to numerous thermal water vents there. Infiltration is positive on the rest of the territory and, consequently, the basins of the Lozhkina, Lozovaya, Pereturpit and Burnaya rivers are situated in the underground water feed area; feed intensity of water-bearing horizons is practically equal (13-16% of normal annual precipitation).

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