COMPOSITION OF ORE-FORMING SOLUTIONS AND TEMPERATURE OF ORE FORMATION AT THE LYUBAVINSKOYE GOLD DEPOSIT

(based on the study of gas–liquid inclusions in minerals)

V. M. Volkov

(Presented by Professor A. M. Kuzmin)

The Lyubavinskoye gold-ore deposit is located in the south of Central Transbaikalia, within the Daur mobile zone, characterized by intensive development of Mesozoic magmatism and block dislocations. The geological structure of the ore field includes stock- and dike-like bodies of granitoids of the Kyrin complex and a complex of dike rocks that cut Permian sandy-shale deposits. Hydrothermal formations at the Lyubavinskoye deposit are represented by quartz–feldspar veins, sometimes with scheelite, molybdenite, and chalcopyrite, and by quartz–gold-ore veins with sparse sulfide mineralization. All intrusive and hydrothermal formations are confined to a zone of increased fracturing, the so-called Lyubavinsky deep fault of sublatitudinal strike.

The deposition of mineral formations during the formation of the ore bodies took place from hydrothermal solutions in several stages, of which four are clearly established: arsenopyrite, quartz, gold–polymetallic, and carbonate. Among the ore minerals, arsenopyrite and pyrite are most commonly encountered; less often—galena, sphalerite, chalcopyrite, and very rarely—tetrahedrite, lead sulfoantimonides, and antimonite. In the distribution of mineral associations both in individual ore bodies and within the ore field as a whole, according to the classification of D. O. Ongoev (1963), the following morphological types of zonality are observed: a) vertical zonality in individual ore veins; b) longitudinal horizontal zonality along the main ore-controlling fault. Vertical zonality is expressed in the replacement of quartz–arsenopyrite ores in the upper parts of veins by quartz–polysulfide ores on the lower horizons. Longitudinal horizontal zonality at the deposit is manifested in the replacement of quartz–arsenopyrite ores in veins on the eastern flank of the ore field by quartz–polysulfide ores on the western flank. In general, both longitudinal and vertical zonality should be regarded as “zonality of repeated tectonic ruptures,” caused by the “opening of the ore cavity as a result of successive deformations and the filling of this cavity by new portions of mineral matter of a different composition” (Smirnov, 1960), in combination with intrastage zonality of deposition (Timofeevsky, 1972).

Simultaneously with the process of ore formation, under the influence of hydrothermal solutions, metasomatic alteration of the country rocks took place. The successive superposition of processes of wall-rock alteration associated with different stages of ore formation leads to a complex assemblage of metasomatic transformations of the country rocks, which is further complicated by differences in their primary composition and physical properties. In general, the process of alteration of the enclosing rocks leads to the formation of beresites, noted by many researchers as a characteristic type of wall-rock-altered rocks of medium-temperature deposits at shallow depths (Kurek, 1954; Borodaevsky, 1960; Zharikov and Omel’yanenko, 1965, et al.).

Below, on the basis of the study of gas-liquid inclusions in vein quartz and in wall-rock-altered rocks by means of aqueous extracts, the results are presented for determining the composition and properties of mineral-forming solutions. The temperature of ore formation was determined mainly by the decrepitation method and partly by homogenization of gas-liquid inclusions.

For the preparation of aqueous extracts the author selected 29 samples from vein quartz and altered wall rocks (Table 1). Samples weighing 100 g each were first crushed in air (+1—2 mm), and then ground in bidistillate (pH = 6.2—6.5) (in order to reduce losses of the gas phase of inclusions) to a particle size of 0.1—0.05 mm and kept in a drying oven for 1—1.5 hours in 250 ml of bidistillate at a temperature of 50—70° C. The solutions obtained were purified in a centrifuge and analyzed for the following components: Na⁺, K⁺, Ca⁺², Mg⁺², Fe⁺², Fe⁺³, NH₄⁺, F⁻, Cl⁻, HCO₃⁻, SO₄⁻², NO₃⁻, NO₂, CO₂. Alkalis were determined by flame photometry, and the remaining components by methods of analytical chemistry (Reznikov et al., 1963; Sushevskaya, 1968). At the same time, the pH of the aqueous extracts was determined. In calculating analyses of gas-liquid inclusions, corrections for mineral solubility were introduced by preparing repeated extracts from the material of the same sample according to a known procedure (Khitarov, 1968; Myaz’ et al., 1968). Studies by A. F. Korobeinikov and A. D. Nazarov confirm the possibility of applying this method and show that the “water-soluble complex in the extracts, when the solubility of the host mineral is taken into account, is not a function of dissolution of the rock minerals, but gives a general idea of the salt composition of the original mineral-forming solution.” Aqueous extracts reflect not only the qualitative composition of ore-forming solutions, but also give approximate quantitative ratios of ions. However, in interpreting the results of aqueous extracts it must be taken into account that we are dealing with a bulk composition formed as a result of the successive superposition of hydrothermal solutions of later stages of ore deposition upon previously deposited mineral assemblages. The composition of the enclosing rocks has no substantial effect on the chemistry of gas-liquid inclusions (Masurenkov, 1968, 1969; Korobeinikov, 1971).

Table 1 gives the results of analyses of aqueous extracts which, taking into account the chemical and mineral composition of the ores and wall-rock-altered rocks, make it possible to clarify some features of the ore-forming hydrotherms. From Table 1 and Fig. 1 it is evident that the composition of the solutions is determined by the ratio of the principal ions Na⁺—K⁺—Ca⁺², Cl⁻—SO₄⁻²—HCO₃⁻ and, to a lesser extent, Mg⁺², Fe⁺³, F⁻. At the same time, certain regularities are outlined in the change in the composition of the hydrotherms in time and space, both in the pre-ore period and during the period of ore deposition.

For the study of gas-liquid inclusions in wall-rock-altered

Results of chemical analyses of aqueous extracts of gas–liquid inclusions

Sampling sites: 1 — Novo-Evgrafovskaya-3 vein, quartz (II level, 3 samples); Novo-Evdokievskaya, quartz (I level); 5 — same place (II level); 6 — same place (III vein with sulfides and sulfosalts); 9 — same place, quartz vein without sulfides (not [[unclear: end of parenthetical text]]); 11 — altered granites from the exocontact of the Novo-Evdokievskaya vein; 12 — M. area, quartz vein with antimonite; 14 — quartz–feldspar vein without sulfides (2 samples); 16 — altered alevrolites from the exocontact of a quartz–feldspar vein (2 samples);

Fig. 1. Diagrams of anion and cation ratios in aqueous extracts. For sample numbers, see Table 1.

Table 1

…and pH of hydrothermal formations of the Lyubavinskoye deposit

Cations (mol. %)

2—same locality (III level, 3 samples); 3—the Sekushchaya vein, quartz (1 sample); 4—the [[unclear: vein]] level); 7—same locality (IV level); 8—the Evgrafovsky area, quartz–carbonate, gold-bearing; 10—altered schists from the exocontact of the Novo-Evgrafovskaya-3 vein; 12—the Fedorovsky area, zone of beresitization and silicification of aleurolites; 13—the Yuzhny area, [[unclear: samples]]; 15—a quartz–feldspar vein with chalcopyrite and molybdenite (2 samples); 17—a pegmatite vein.

Fig. 2. Change in the composition of aqueous extracts from wall-rock-altered rocks (A) and ores (B): 1—beresites from the supra-ore part of a blind vein; 2—wall-rock-altered rocks near the Novo-Evgrafovskaya-3 vein (substantial pyrite–arsenopyrite mineralization); 3—wall-rock-altered rocks near the Novo-Evdokievskaya vein (polysulfide mineralization); 4—barren quartz vein; 5—the Novo-Evgrafovskaya vein; 6—the Novo-Evdokievskaya vein; 7—quartz–carbonate vein.

In the rocks, three samples were taken: from the zone of beresitization above a blind quartz vein (intense pre-quartz pyrite–arsenopyrite mineralization) (sample no. 12, Table 1); wall-rock-altered rocks from the exocontact of the Novo-Evgrafovskaya-3 vein (mainly pyrite–arsenopyrite mineralization with gold, a small admixture of polysulfides, and fairly significant later carbonatization) (sample no. 10, Table 1); from the exocontact with the Novo-Evdokievskaya vein (substant…

…apparently gold–polymetallic mineralization) (sample No. 11). With a certain degree of conventionality, it may be assumed that the samples were taken from a vertical column of wall-rock altered rocks that has been explored at present, where sample No. 12 corresponds to the upper (above-ore) part, sample No. 10 to the middle part, and sample No. 11 to the lower part.

The solutions from the wall-rock altered rocks are sulfate–hydrocarbonate, with a small content of chlorine and fluorine. The following features are observed: the contents of chlorine and of the hydrocarbonate ion increase from the upper part of the metasomatic column to the lower part (Fig. 2). The fluorine content and the sulfate character of the solutions decrease in the same direction. Among the cations, sodium, potassium, calcium, and magnesium predominate; \(NH_4^+\) and \(Fe^{+3}\) occur much more rarely. The \(Na:K\) ratio changes from 2.3:1 in the upper part of the section to 1:5.3 in the lower part. In the same direction, the content of \(SO_4^{--}\) in the solutions increases.

To study the composition of gas–liquid inclusions in the ore bodies, 15 samples were taken (Table 1): from barren quartz (sample No. 9); ore quartz from the Novo-Evgrafovskaya vein with substantial pyrite–arsenopyrite mineralization, with gold, a small admixture of polysulfides, and rather strongly carbonatized (samples Nos. 1 and 2); ore quartz from the Novo-Evdokievskaya vein with mineralization of the gold–polymetallic stage (samples Nos. 4–7); from a quartz–carbonate vein with lead sulfoantimonites (sample No. 8); and, finally, one sample each was taken from the Sekushchaya vein (galena–sphalerite mineralization) (sample No. 4) and from a vein with rich antimonite mineralization (sample No. 13).

From the results of analyses of aqueous extracts of the above-listed samples it follows that the contents of chlorine and sodium in the solutions decrease from the early stages to the late ones, whereas the sulfate character and the potassium content, on the contrary, increase toward the end of the ore process; here the sulfate character of the solution reaches a maximum during the period of deposition of the gold–polymetallic stage (Fig. 2). The content of \(HCO_3^-\) also increases, while \(NH_4^+\) decreases from the first stages toward the end of the ore process.

It is interesting to note that in the ore veins of the Evgrafovsky area and in the Novo-Evdokievskaya vein an increase with depth in the role of potassium in the solutions is observed (Fig. 3). There is also a tendency for the sulfate character of the solutions to increase in the same direction.

Fig. 3. Dependence of the \(Na:K\) ratio on the depth of veins in aqueous extracts from quartz of the Evgrafovsky (1) and Bol’she-Fedorovsky (2) areas.

In an aqueous extract from a vein with rich antimonite mineralization, a chloride–fluoride–sulfate composition was found, with a high content of calcium, magnesium, and sodium (Table 1).

Analysis of aqueous extracts from pre-ore quartz–feldspar and pegmatite veins (samples Nos. 14–17) shows that the solutions in them are chloride–bicarbonate, with a small content of fluorine and sulfate ion. The latter appears in veins with insignificant sulfide mineralization, represented by chalcopyrite and molybdenite. Among the cations, on the whole, the ratio observed is \(Na^+ > K^+ > Ca^{++} > Mg^{+2}\) (Table 1).

To determine the temperature of ore formation, a set of geothermometric methods was used: homogenization and decrepitation of gas–liquid inclusions, as well as the study of mineral geothermometers. To elucidate the most general regularities concerning the temperature of ore deposition at the deposit, the decrepitation method was chosen as the principal one, as the most rapid method and one that yields statistical results. The work was carried out by the author on an apparatus with a piezoelectric sensor and automatic recording of temperature and intensity of cracking. Vein quartz (fraction \(+0.1\)—0.25 mm) was studied in the temperature interval \(20\)—\(550^\circ\)C. In order to eliminate random pulses and interference, no fewer than three weighed portions from a single sample were analyzed. The material obtained is shown in the form

Fig. 4. Decrepitograms of quartz from veins of the Evgrafovsky area (I), Sekushchaya (II), Bolshe-Fedorovsky area (III), Antimonitovaya (IV)

summary decrepitograms (Fig. 4), onto which pulses confirmed in no fewer than two analyses were plotted.

For all the decrepitograms obtained in this way, a sharply contrasting multipeak character is typical, reflecting the superposition of mineral associations in several stages (Fig. 4). On the decrepitogram of quartz from veins of the Evgrafovka sector, composed mainly of quartz of two generations and early sulfides, the maximum peak corresponds to a decrepitation temperature of 380–400°C; for quartz from veins of the Bolshe-Fedorovka sector, with mineralization mainly of the gold–polymetallic stage, a maximum at 200–220°C is characteristic; quartz with abundant antimonite mineralization gives decrepitation maxima at 60–80°C and at 240–260°C. Quartz from the Sekushchaya vein, with mineralization of early sulfides and a noncommercial gold content, is characterized by the absence of decrepitation pulses in the interval 140–300°C.

Interpretation of the thermometric results makes it possible to distinguish the following intervals of intense decrepitation of gas–liquid inclusions in vein quartz: the interval 60–180°C corresponds to deposition of mineral aggregates of the carbonate stage; 200–320°C, to deposition of the gold–polymetallic stage; and 320–420°C corresponds to formation of the main mass of quartz and early sulfides. The higher-temperature peaks (above 440°C) noted on the decrepitograms probably correspond to explosions of the smallest, deeply situated gas–liquid inclusions, whose overheating, as V. B. Naumov (1968) notes, reaches 170°C or more. In turn, the distinguished intervals have two maxima each, with distinct weakenings of intensity between them, which probably corresponds to the deposition of different minerals (quartz and carbonates in the carbonate stage) or their generations (two generations of quartz in the quartz stage), with intrastage interruptions caused by local intrastage tectonic movements.

Direct study of the gas–liquid inclusions under the microscope showed that in the overwhelming majority of cases the inclusions are too small for determining the temperature of mineral formation by the homogenization method (diameter on the order of the first thousandths of a millimeter). In the case of larger inclusions, in most of them three phases can be distinguished: gas—carbon dioxide—aqueous solution. Homogenization of the inclusions occurs at temperatures fully comparable with their decrepitation. Thus, studies of the homogenization temperatures of gas–liquid inclusions in quartz veins of the Lyubavinskoye deposit enabled Yu. V. Lyakhov and A. V. Piznyur (1970) to determine the temperatures of formation of early sulfides as 270–240°C at a pressure of about 800–1000 atm; of the gold–polymetallic stage as 260–205°C at a pressure of 560–750 atm; and of the carbonate stage from 220–200°C to 80–50°C at a pressure equal to 200–400 atm. These data are quite comparable with the temperatures obtained by us by the decrepitation method.

There are divergent opinions on the applicability of the homogenization methods and especially decrepitation for determining the temperatures of mineral formation. A number of experiments on the comparability of the results of the homogenization and decrepitation methods both with one another and with the real temperature of formation of artificial minerals, carried out by Soviet and foreign researchers, give different results—from complete coincidence of temperatures (Takenouchi, 1962) to an overstatement of the decrepitation temperature relative to homogenization by 70–80°C (Kashiwagi et al., 1955) and even, for some inclusions, by 170°C or more (Naumov, 1968). On this basis, we have no right

to identify the results we obtained with the temperatures of formation of various mineral associations; however, the combined application of these methods helps to determine more clearly the sequence of mineral deposition and makes it possible to estimate approximately the temperature intervals of ore formation.

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