ON THE MIGRATION CAPACITY OF GOLD IN ENDOGENOUS PROCESSES

N. V. VILOR, V. N. SARAPULOVA (IZK SB AS USSR, IG SB AS USSR)

Gold, possessing minimal chemical affinity for all strong oxidizing agents, forms a very limited range of compounds. At present 44 of them are known. Moreover, the overwhelming majority have been obtained artificially. Under natural conditions only native gold with a variable amount of silver, tellurides, and a compound of the sulfosalt type are formed.

Thermochemical constants have been calculated for only a few gold compounds (Latimer, 1954). Since entropy \((S)\) is a function of thermodynamic probability \((\omega)\), it is evidently possible, using \(\Delta S^\circ_{298}\), to determine the degree of possibility of the existence of gold compounds. From Table 1 it is evident that thermodynamically the most probable compounds are those with the largest \(\Delta S^\circ_{298}\).

Table 1

\(\Delta S^\circ_{298}\) of gold compounds

Using the equation

\[ \Delta Z_T^\circ \text{ of reaction} = \Delta H^\circ_{298} \text{ of reaction} - T \Delta S^\circ_{298} \text{ of reaction} \tag{1} \]

one can approximately estimate the magnitude of the isobaric-isothermal potential of formation of gold compounds at various temperatures (Garrels, Christ, 1968; Letnikov, 1965) and determine their thermal stability.

From the graph in Fig. 1 it is evident that the earliest and most [[unclear: line continues on next page]]

Fig. 1. Change in \(\Delta Z\) of formation of gold compounds with temperature.

Stable at temperatures from 700° to 900°K will be \(\mathrm{Au(OH)_3}\), \(\mathrm{AuCl_3 \cdot 2H_2O}\), and \(\mathrm{AuCl_4^-}\). But, probably, these forms arise and exist at an elevated oxidation–reduction potential of the medium, as shown in the works of A. P. Pavlov (1968) and D. Phillips (Garrels, Christ, 1968). At 25°C and 1 atm total pressure they arise under conditions of \(Eh\) greater than \(+0.8\)—1, near the upper limit of water stability.

Ogryzlo and a number of other researchers studied the stability of chloride complexes of gold at high temperatures and pressures in solutions with considerable ionic strength (Ogryslo, 1935).

A very important form of gold transport is the complex anion thioaurate \([\mathrm{AuS}]^1\). Theoretical studies by D. Phillips (Garrels, Christ, 1968) showed that at \(T = 298^\circ\mathrm{K}\), \(P = 1\) atm, and a concentration of dissolved sulfur equal to \(10^{-1}\), this form is stable under reducing conditions within the pH range from 2 to 14. The stability of this transport form at high temperatures and pressures in the presence of the hydrosulfide anion was demonstrated by Smith (1943), Ogryslo (1935). The existence of complex thioaurates of gold of the type \([\mathrm{AuS_3}]^-\) is assumed (Shcherbina, 1964), but studies of their stability are not yet known, and thermochemical data for them are absent.

We have begun a study of the formation and stability of \(\mathrm{Au(OH)_3}\) under hydrothermal conditions. The reaction proposed by N. G. Tyurin and I. A. Kakovsky (1960) was taken as the basis:

\[ 4\mathrm{Au} + 6\mathrm{H_2O} + 3\mathrm{O_2} = 4\mathrm{Au(OH)_3}. \tag{2} \]

At \(T = 298^\circ\mathrm{K}\) and \(P = 1\) atm, the reaction is endothermic. \(\Delta H^\circ_{298}\) of the reaction \(=\)

\[ = +10\ \frac{\mathrm{kcal}}{\mathrm{mol}} \]

in the case of gaseous \(O_2\). In the case of aqueous \(O_2\), \(\Delta H^\circ_{298}\) of the reaction is \(+21.4\ \frac{\mathrm{kcal}}{\mathrm{mol}}\). The decrease in free energy for the first variant is \(+37.0\ \frac{\mathrm{kcal}}{\mathrm{mol}}\); for the second variant it has not been established, since \(\Delta Z^\circ_{298}\) for \(O_2\) in water is unknown. The equilibrium constant \((K_a)\) under the indicated standard conditions is \(1 \cdot 10^{-27}\).

From equation (1) we calculated the change in \(K\) as a function of temperature (Fig. 2). Up to \(1300^\circ\mathrm{K}\) \((1023^\circ\mathrm{C})\), \(\lg K_a\) increases almost linearly (\(K_a\) is \(1 \times 10^{-14}\)), and at higher temperatures it approaches a limiting value equal to \(1 \times 10^{-11}\).

We carried out an experimental verification of the calculation. Film-type, finely dispersed gold preparations were taken as the object of study. They were obtained by calcining the crystalline hydrate \( \mathrm{HAuCl_4 \cdot 4H_2O} \) on the walls of quartz crucibles and in boats of unglazed porcelain. The calcination temperature was \(600\)—\(700^\circ\). The resulting preparations and distilled water, previously boiled for 30 minutes, were placed in titanium inserts with a capacity of 100 ml at a filling coefficient of 0.5. Heating was carried out in autoclaves designed by the Special Design Bureau of the Institute of Crystallography of the Academy of Sciences of the USSR, with a body made of 1Kh18N9T steel. The interval between series of experiments was \(50^\circ\). The minimum heating was to \(100^\circ\), the maximum to \(550^\circ\), and its duration was 3 hours. After heating, the autoclaves were quenched. In all, 39 experiments were performed. The resulting solutions were analyzed by the chemical-spectral method at the Institute of Geochemistry, Siberian Branch, Academy of Sciences of the USSR. The initial and final pH values were measured on an LPU-01 potentiometer.

Fig. 2. Calculated solubility curve of \( \mathrm{Au(OH)_3} \) in water

The experimental results differ substantially from the theoretical ones. When working with quartz crucibles (Fig. 3), the amount of gold in solution increases sharply from \(6.68\ \gamma/\mathrm{l}\) at \(100^\circ\) to \(67.5\ \gamma/\mathrm{l}\) at \(300^\circ\), gradually decreasing to \(34.1\)—\(45.5\ \gamma/\mathrm{l}\) at \(550^\circ\). In the case of porous porcelain, the concentration of gold in solution slowly increases from

Fig. 3. Dependence of the solubility of gold in water on temperature

3.3 g/L at 150° to 10.0 g/L at 300°C, and then gradually decreases to 0.5–0.8 g/L at 550°C.

The difference in the amount of gold leached from the surface of quartz and porcelain boats is probably explained by the fact that in the latter case, during ignition, gold sinters with the porcelain. The heating temperature is evidently insufficient for activation of the gold atoms and their transfer into a soluble state. The cause of the appearance of the region of retrograde solubility of gold with increasing temperature above 300°C has not been clarified. It is possible that thermal decomposition of some portion of \( \mathrm{Au(OH)_3} \) takes place, with reduction of the gold to the metal.

The results of the experiments show that gold dispersed in sedimentary rocks during sedimentation may, in the process of progressive metamorphism*, be leached from them by dilute aqueous solutions, transported, and precipitated when the \(P\) and \(T\) conditions of the filtering thermohydrocolumns change.

* The authors have experimentally reproduced the setting of a possible dissolution of gold by an aqueous solution when its temperature is raised. The experimental results show that the solubility of gold increases with increasing temperature up to 300° and then decreases. It is extremely difficult to draw other conclusions of a geological nature on the basis of these data (editor’s note).

References

1. Garrels, R. M.; Christ, Ch. A. Solutions, Minerals, Equilibria. Mir Publishers, 1968.

2. V. Latimer. The Oxidation States of the Elements and Their Potentials in Aqueous Solutions. Foreign Literature Publishing House, 1954.

3. Letnikov, F. A. Isobaric Potentials of Mineral Formation (Chemical Affinity) and Their Application in Geochemistry. Nedra Publishers, 1965.

4. Pavlov, A. L. The influence of the evolution of acidity—alkalinity of ore-forming systems on the behavior of their components. In: Endogenous Ore Deposits. Moscow: Nauka Publishers, 1968.

5. Tyurin, N. G.; Kakovskii, I. A. On the behavior of Au and Ag in the oxidation zone of sulfide deposits. Proceedings of Higher Educational Institutions, “Non-Ferrous Metallurgy,” No. 2, 1960.

6. Shcherbina, V. V. Do thiosulfates exist in hydrothermal solutions? Geology of Ore Deposits, No. 3, 1964.

7. Stephen Ogryslo. Economic Geology, Vol. 30, 1935.

8. F. G. Smith. The alkali sulfide theory of gold deposition. Economic Geology, Vol. 38, 1943.