Sandimirov S.S. Geochemisrty of water-rock interaction in the area of the Khibiny alkaline massif. Geochemisrty International. 2007, T.54, №11, p. 1103-1110.

GEOCHEMISTRY OF WATER-ROCK INTERACTION 1109 Si02 content of the industrial water is 9-10 mg/l [7]. This fact is confirmed by numerous analyses obtained in different years ([6, 7], Table 1, an. 7 and 8) and can be explained on the basis of natural observations. White crusts consisting mainly of silica (spectral analyses) were described in the beds of small streams in the Khibiny Mountains already in the 1930s [9]. White foamlike sediments were repeatedly observed by us on the banks of Khibiny streams. They were analyzed twice, and it appeared that they consist of almost pure silica in an X-ray amorphous state. In some cases, this sediment was observed on the banks of streams whose beds contained rock with “pocks" at the place of nepheline crystals. This leads to the conclusion that sil­ ica released during slow nepheline dissolution in the beds of perennial streams is transported as a gel by water flows. The same phenomenon must take place during the opening of natural stagnant reservoirs. The possible existence of underground reservoirs of stagnant waters in the Khibiny Massif is suggested by the occurrence of gently sloping infiltration weathering crusts (“pockets") [2]. During their exposure and ero­ sion by atmospheric waters, alkalis and Al colloids could be transformed into dissolved species, while sil­ ica is removed as an amorphous phase. This may be responsible for the short-term anomalous increase in the contents of major rock-forming elements of nepheline syenites and foidolites in the waters. In the relatively calm setting of lacustrine waters, silica gel could form the amorphous component of the upper sed­ imentary layer. The anomalous contents of S 04and Cl disagree with the compositions of underground waters and the results of our experiments (Table 3). It can be supposed that S04 and Cl were accumulated in a natural way as salt sediments in the pores of rocks, but the influence of anthropogenic factors is probably more significant. The snow cover is the main concentrator of anthropogenic atmospheric sulfur under the conditions of the Far North. There are sources of industrial sulfur in the Khibiny area. Significant amounts of S04 and Cl can be supplied by mines, concentrating mills, towns, and mining settlements. Mining and concentrating plants significantly affect the content of alkalis and aluminum in waters (Table 1). In such a case, the role o f interaction between fine mineral particles (primarily, rock- forming nepheline, which is suggested by the obtained data) and water increases dramatically. The main anthropogenic sources o f fine mineral particles are the tailings o f the concentrating mills. Mill tailings containing the finest fraction after ore crushing are transported to the depository of sands remained after apatite extraction and to the settling tanks of industrial waters. The colloidal phases con­ tained in them cause local effects of quicksand. Owing to the disturbance of dams or during floods, the concen­ tration of colloids in the settling tanks may cause satu­ ration of surface and underground waters in the major rock-forming elements. At the same time, colloidal phases play a positive role as sorbents accumulating heavy and trace elements [6]. The following conclusions can be drawn. 1. Under the conditions of retarded weathering in the Far North, the slow dissolution of rock-forming nepheline and feldspathoids in the Khibiny alkali mas­ sif plays a significant role in the water-rock interaction. Intense processes of disintegration in the mountain mass and mining activity suggest that the rapid dissolu­ tion of dusty particles of some minerals strongly affects the composition of waters. The liquid and easily soluble phases extracted from rock pores, which were previ­ ously formed under different geologic conditions, could also affect the composition of underground and surface waters. 2. It was shown that nepheline particles smaller than 0.02 mm are transformed into amorphous phases at t= 18°-20°C and pH = 7.5-8.1. This process becomes observable already in the first day and continues for months. It is possible in natural stagnant water reser­ voirs and much more pronounced in the settling tanks of concentrating plants. This is confirmed by the com­ parison of experimental data with the phase composi­ tion of the upper layer of lacustrine sediments at the site of industrial water discharge. 3. Using the alkaline rocks of the Khibiny Massif as an example, we can conclude that the interaction of minerals with a water environment is controlled by a combination of various exogenic processes proceeding with different rates. The study of these processes is of great importance for the protection of ecosystems and for the utilization of wastes, in particular, for the addi­ tional extraction of trace and noble metals. ACKNOWLEDGMENTS The study was supported by the Russian Foundation for Basic Research, project no. 03-05-96174. REFERENCES 1. S. R. Krainov, B. N. Ryzhenko, and V. M. Shvets, Geochemistry o f Underground Waters. Theoretical and Ecological Aspects (Nauka, Moscow, 2004) [in Rus­ sian], 2. M. D. Dorfman, Mineralogy o fPegmatites and Weather­ ing Zones in the Ijolite-Uriites o f Yukspor Mount, Khib­ iny Massif (Izd. AN SSSR, Moscow and Leningrad, 1962) [in Russian], 3. О. B. Dudkin, L. I. Kozyreva, and N. G. Pomerantseva, Mineralogy o f the Apatite Deposits o f the Khibiny Tun­ dras (Nauka, Moscow, 1964) [in Russian], 4. E. E. Kostyfeva-Labuntsova, В. E. Boratskii, M. N. Soko- fova, et af., Mineralogy o f the Khibiny Massif (Nauka, Moscow, 1978), Vof. 2 [in Russian], 5. A. R Khomyakov, Mineralogy o f Ultra-Agpaitic Alka­ line Rocks (Nauka, Moscow, 1990) [in Russian], GEOCHEMISTRY INTERNATIONAL Vol. 45 No. 11 2007