Chemical Composition of Groundwater in the Konstantinovskoye and Medvezhye Gold Ore Deposits of the Olkhovsko-Chibizhek District (Eastern Sayan Mountains)
Abstract Generated abstract
This study examines the groundwater chemistry and gold migration forms at the Konstantinovskoye and Medvezhye gold ore deposits in the Olkhovsko-Chibizhek district of the Eastern Sayan Mountains. Based on chemical, trace-element, polarographic, microbiological, and filtration or dialysis analyses of mine-water samples, the authors compare waters circulating in sulfide-rich silicate rocks at Medvezhye with those in carbonate-bearing rocks at Konstantinovskoye. The deposits show distinct hydrogeochemical conditions: Medvezhye waters are more varied, commonly sulfate-bearing, richer in suspended matter, organic substances, metals, and bacteria, while Konstantinovskoye waters are mainly low-mineralized hydrocarbonate calcium magnesium waters. Gold occurs largely in suspended and colloidal forms at Medvezhye and more often in colloidal and ionic forms at Konstantinovskoye, supporting the potential use of hydrogeochemical indicators for detecting blind ore bodies in the region.
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CHEMICAL COMPOSITION OF GROUNDWATER OF THE KONSTANTINOVSKOYE AND MEDVEZHYE GOLD-ORE DEPOSITS OF THE OLKHOVSKO-CHIBIZHEK DISTRICT
(EASTERN SAYAN)
V. K. BERNATONIS, V. K. POPOV
(Presented by Professor P. A. Udodov)
The geological structure of the named deposits has repeatedly been covered in the literature [3, 5, etc.]. However, questions of the chemistry of groundwater and its geochemical characteristics have hardly been discussed, although without knowledge of them it is impossible to draw correct conclusions about the conditions of formation of oxidation zones, the migration and accumulation of gold within them, and also about the applicability of the hydrogeochemical method of prospecting in this district.
During the period from 1971 to 1972, in underground mine workings of the Medvezhye and Konstantinovskoye deposits, 39 samples were taken for abbreviated chemical analysis, 39 samples for determination of microcomponents by the TPI method [4], 75 samples for polarographic determination of gold, 18 samples for determination of natural organic substances, 29 samples for microbiological analysis, and a number of other samples. In addition, by dialysis and by filtration through crucibles with a porous bottom, the forms in which gold occurs were determined in 18 samples.
The chemical composition of groundwater in any district is governed by physicogeographical, hydrogeological, and geological-structural factors.
The physicogeographical conditions of the Olkhovsko-Chibizhek district are determined by its position in the southwestern part of the Eastern Sayan. The district is a mid-mountain dissected terrain with a fairly dense hydrographic network. The river network belongs to the system of the Dzhebi River, a right tributary of the Kizir River. The principal source of river recharge is atmospheric precipitation, the mean annual amount of which is 600–980 mm. Absolute elevations reach 100–1200 m; the heights of watersheds above valley floors are 200–400 m.
In hydrogeological terms, the district is an open, well-washed structure. However, in terms of water abundance and the rate of infiltration of groundwater, the areas of the deposits described differ from one another to some extent.
The hydrogeological conditions of the Konstantinovskoye deposit are determined by the fact that a significant part of its area is composed of carbonate karstifying rocks, to which the valley of the Chibizhek River, a left tributary of the Dzhebi River, is confined. The area of the Medvezhye deposit, located in the upper reaches of the stream of the same name, is composed of rocks with relatively impeded water exchange. All this has left its imprint on the hydrogeochemical setting of the deposits studied.
The Medvezhye and Konstantinovskoye deposits are confined to the exo- and endocontacts of granitoid rock protrusions of the Olkhovsky massif within tectonically disturbed host rocks represented by carbonate deposits of the Balakhtison Suite and effusive-sedimentary formations of the Osinovka Suite.
The ore bodies are represented by veins, lenses, deposits, and stockworks and are composed mainly of pyrrhotite, pyrite, quartz, carbonates, marcasite, chalcopyrite, arsenopyrite, sphalerite, chlorite, and sericite.
Oxidized ores are traced on average to a depth of 50–70 m from the surface and consist chiefly of limonite-clayey material with an admixture of disintegration products of the wall rocks. Recording the vertical change in metal contents made it possible to establish the presence of one or two horizons of secondary gold enrichment.
The chemical composition of the groundwater of the Medvezhye and Konstantinovskoye deposits proved to be substantially different. At the Medvezhye deposit, which is characterized by an exceptionally high sulfide content and by a predominance of silicate host rocks over carbonate rocks, the greatest development is shown by weakly mineralized (up to 1 g/l) groundwaters, mainly of hydrocarbonate–sulfate–calcium–magnesium, sulfate–hydrocarbonate–sodium–calcium, and hydrocarbonate–calcium–magnesium compositions, with alkaline, weakly alkaline, neutral, and weakly acidic reactions. The high contents in these waters belong successively to: $\mathrm{HCO_3'}$, $\mathrm{SO_4''}$, $\mathrm{Ca^{\cdot\cdot}}$, $\mathrm{Na^{\cdot}}$, $\mathrm{Mg^{\cdot\cdot}}$, and $\mathrm{Cl'}$. In insignificant amounts the following were found: $\mathrm{Si}$, $\mathrm{Fe^{\cdot\cdot\cdot}}$, $\mathrm{Fe^{\cdot\cdot}}$, $\mathrm{NH_4'}$, $\mathrm{NO_3'}$, $\mathrm{NO_2'}$, $\mathrm{S_2O_3''}$, and $\mathrm{SO_3''}$.
Acidic and very acidic waters of high mineralization, up to 10 g/l, with elevated contents of chloride and sulfate ions, as well as iron up to 200 mg/l, occur more rarely. Hydrogen-sulfide waters were encountered in the lower horizons of the deposit. The presence of hydrogen sulfide only in those samples in which sulfate-reducing bacteria were detected makes it possible to speak of its biogenic origin.
The groundwater of the Medvezhye deposit contains from 10.8 to 26.0 mg/l of organic matter, which is represented mainly by fulvic acids. The content of humic acids is low for most of the waters studied and only in a few samples reaches 4.3 mg/l. An insignificant part of the organic substances is represented by low-molecular-weight organic acids, apparently products of microbial synthesis, since microbiological analyses established the almost universal development of thionic bacteria.
A broad complex of metals reflecting the ore mineralization is associated with the groundwater of the deposit described. Spectral analyses of aqueous concentrates revealed the following trace elements (µg/l): Pb — 0.0–190.0; Cu — 0.37–67.9; Zn — 3.7–11.4; Ag — 0.0–2.68; Ni — 0.0–19.0; Co — 0.0–11.5; As — 3.7–6787.0; Sb — 0.0–7.97; Hg — 0.0–17.5; Sn — 0.0–1.2; Sr — 0.0–418.0; Ti — traces — 6.1; Cr — 0.0–5.7; Ba — 1.5–740.0; Mn — 1.6–1430.0.
Gold, according to polarographic analyses, was found in 65% of the samples studied in amounts up to 157.6 µg/l. Dialysis and study of the amount of gold in the fine mechanical suspension separated with the aid of crucibles with porous bottoms established that the principal amount of gold migrates in suspended and colloidal forms. Gold in ionic form was found only in two samples, in amounts of 1.97 and 39.4 µg/l.
M. N. Al’bov [1] pointed to the significant role of the suspended form of gold migration; he admitted the possibility of transport of wat—
by a water flow gold particles ranging in size from 0.01 to 100 µm. In addition, electron-microscopic studies of mine-water precipitates established the presence in them of colloidal gold particles on various clay minerals. The calculations performed show that, in the form of fine mechanical suspensions, gold can migrate over considerable distances only when its particles are smaller than 0.1 µm. Apparently, the main amount of gold in mine-water suspensions is not in a free form, but in a state sorbed by particles of clay minerals.
The amount of suspended matter in the groundwaters of the Medvezhye deposit reaches 1.2 g/l; therefore, on their surface they sorb practically all ionic gold and part of the colloidal gold. In the suspensions, electron-microscopic studies established the presence of hydromicas, kaolinite, halloysite, and hydrous iron oxides. Organic matter in the suspensions is present in amounts from 0.17 to 0.68%. Spectral analyses, in addition to gold, determined the following elements (%) in the composition of the suspensions: Mn—0.01–1.0; Al—0.03–0.05; Ba—0.001–0.007; Ti—0.001; Sr—0.07; As—0.01–0.5; Ni—0.001–0.002; Cu—0.0007–0.003; Ag—tr.
Gold particles from 0.01 to 0.001 µm in size are capable of forming colloidal systems, whose migrational capacity increases in the presence of protective colloids. The role of protective colloids is played by silica and hydrous iron oxide. Complex compounds of gold with humic and fulvic acids will also have the dimensions of colloidal particles. In addition, ionic gold may be transported by micelles of clay and other minerals.
As for the ionic gold detected by polarographic analyses in the dialysate of samples, the most probable forms of its migration are complex compounds with thiosulfates and low-molecular organic acids.
The presence of a considerable amount of limestones at the Konstantinovskoye deposit determines the chemical composition of the circulating waters, which are mainly hydrocarbonate–calcium–magnesium waters with a somewhat elevated content of chloride and sulfate ions. They are characterized by low mineralization (up to 0.4 g/l) and an alkaline, weakly alkaline, neutral, and more rarely weakly acidic reaction. Compared with the groundwaters of the Medvezhye deposit, these waters contain reduced amounts of suspended matter and organic substances. Thionic bacteria are also of limited distribution.
Despite the sulfide character of the mineralization, microcomponents in the groundwaters of the Konstantinovskoye deposit are also found in reduced quantities (µg/l): Pb—36.6–109.6; Cu—0.6–17.8; Zn—5.5–24.4; Ag—0.0–0.54; Ni—0.0–1.2; As—3.7–51.2; Sb—0.0–16.3; Sn—0.0–0.34; Sr—0.0–77.6; Ti—0.5–3.1; Cr—0.0–1.5; Ba—3.3–388.0; Mn—4.1–512.0.
Gold was found in 75% of all analyzed samples in amounts up to 492.4 µg/l. Owing to the reduced content of suspended matter, gold in the groundwaters of the Konstantinovskoye deposit migrates mainly in colloidal and ionic forms. Colloidal gold was found in 37.5%, and ionic gold in 50% of the samples studied.
From the foregoing it is evident that the chemical composition of groundwaters and the forms of gold migration in them are to a considerable extent determined by the composition of the enclosing rocks and ores. Consequently, the data of hydrogeochemical studies indicate the possibility of using this method in the given region to detect blind ore bodies. The following should be regarded as indicators of mineralization in this case [2, 4]:...
more complex in chemical composition waters (several types within a single area), an elevated content of the sulfate ion, an acidic reaction of the waters, an elevated content of heavy-metal ions in the waters, and the presence of thionic bacteria.
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