ON THE RATE OF GOLD DISSOLUTION UNDER HYPERGENE CONDITIONS

A. D. MIKOV

(Presented by the scientific seminar of the Department of Mineral Deposits)

In the present article an attempt is made to determine the possible rate of gold dissolution in the oxidation zone of quartz–sulfide deposits of the Mariinskaya taiga (Kuznetsk Alatau), using the author’s experimental data (Mikov, 1965, 1970).

According to E. I. Parkhomenko (1965), the specific electrical resistivity of typical ores from deposits with a sulfide content (pyrite, chalcopyrite) of 20% is 30,000 ohm·cm and decreases to 90 ohm·cm in ore-vein segments (ore shoots) enriched in sulfides (pyrite—60%, chalcopyrite—5%, sphalerite—15%). In the oxidation zone, taking the cited data into account, the current density of the natural field is determined by us as \(10^{-4}—10^{-7}\) mA/mm². Close values of the current density of the natural field were determined by calculations of L. A. Zashchinsky (1967) for ore bodies of pyrite deposits—\(10^{-3}—10^{-5}\) mA/mm². In the following calculations a current density of \(10^{-5}\) mA/mm² is taken as the most probable characteristic of the natural field of ore shoots of quartz–sulfide veins.

Experimentally, under conditions close to natural ones, it was established that at a current density of 3 mA/mm², 0.0014 g of gold dissolved in 12 hours (Mikov, 1970). Comparing the data on the current density used in the experiment and the natural current density in the ore body, we attempt to estimate the quantitative aspect of the process of electrochemical dissolution of gold as applied to the natural conditions of the oxidation zone.

The rate of gold dissolution under the conditions of the experiment performed may be determined by the formula

\[ M_0=\frac{Q_0}{tS_0}, \]

where \(M_0\) is the rate of gold dissolution per unit area during the experiment;

\(Q_0\) is the amount of dissolved gold, g;

\(t\) is the dissolution time, days;

\(S_0\) is the area of the plate. \(S_0=40\ \text{mm}^2\).

Substituting the experimental data, we obtain

\[ M_0=\frac{14\cdot10^{-4}}{0.5\cdot40}\cdot365=0.025\ \text{g/mm}^2\ \text{per year}. \]

Taking into account the density of the direct current used in the experimental dissolution of gold (3 mA/mm²) and the possible value of its density adopted by us (\(10^{-5}\) mA/mm²) in the oxidation zone of quartz– ԥ

quartz-sulfide veins, one can make an approximate calculation of the rate of dissolution of gold under natural conditions by the formula

\[ M_1=M_0\frac{I_1}{I_0}, \]

where \(M_1\) is the rate of dissolution of gold per unit area under natural conditions;

\(I_0\) is the current density during the experiment, mA/mm\(^2\);

\(I_1\) is the current density in the ore body, mA/mm\(^2\).

Substituting the data, we obtain

\[ M_1=0.025\cdot\frac{1\cdot10^{-5}}{3}=8\cdot10^{-8}\ \text{g/mm}^2\ \text{per year}, \]

or

\[ 8\cdot10^{-8}\cdot10^6=8\cdot10^{-2}\ \text{g/m}^2\ \text{per 1 million years}. \]

The average size of a gold particle in quartz-sulfide deposits may be taken as equal to 0.01 mm. The surface area of such a gold particle, assuming its shape to be spherical, will be:

\[ S=4\pi r^2=4\cdot3.14\cdot0.005^2=12.6\cdot25\cdot10^{-6}=315\cdot10^{-6}\ \text{mm}^2. \]

Taking into account the decrease in the surface area of the gold particle as it dissolves, we shall take as the average surface area a value smaller by a factor of two (\(S_1\)), i.e., \(157.5\cdot10^{-6}\) mm\(^2\). From such an area, an amount of gold (\(Q\)) will dissolve:

\[ Q=M_1\cdot S_1=8\cdot10^{-2}\cdot157.5\cdot10^{-6}=1260\cdot10^{-8}\ \text{g per million years}. \]

Let us now determine the weight of the gold particle taken (\(P\)) as the product of its volume (\(V\)) and specific gravity (\(d\)), and we obtain

\[ P=V\cdot d=\frac{4}{3}\pi r^3=\frac{4}{3}\,3.14\cdot0.005^3\cdot19.3=10100\cdot10^{-9}=1\cdot10^{-5}\ \text{g}. \]

For complete dissolution of such an entire gold particle at an average solubility of \(1260\cdot10^{-8}\) g per million years, the time required (\(T\)) will be:

\[ T=\frac{P}{Q}=\frac{1\cdot10^{-5}}{1260\cdot10^{-8}}=0.795\ \text{million years}, \]

i.e., less than one million years.

We could not take into account a number of factors, including favorable ones—for example, the presence of chlorine—which to one degree or another will influence the course of the process of gold dissolution under hypergene conditions. A comparatively large gold particle was used for the calculation; therefore it is quite natural that the dissolution of smaller grains will proceed considerably faster.

From the materials presented, one may conclude that the effect of electrochemical dissolution of gold under the action of an external natural electric field—taking into account that the age of the oxidation zone of many gold-ore deposits is estimated at several tens of millions of years—is one of the important factors affecting the dissolution of gold under these conditions.

The data obtained are consistent with the calculations of M. N. Albov (1960) and make it possible to speak of the great duration of hypergene processes.

REFERENCES

1. Albov M. N. Secondary zonality of gold-ore deposits of the Urals. Gosgeoltekhizdat, 1960.

2. Zashchinskii L. A. On the question of the magnetic effect of constant natu-

...of natural electric currents. In the collection: Methods and Techniques of Exploration Geophysics. “Nedra,” 1967.

1. Mikov A. D. On the question of secondary zonality in the gold-ore veins of the Mariinskaya taiga (Kuznetsk Alatau), Proceedings of TPI, vol. 138, 1965.

2. Mikov A. D. Experimental studies on the electrochemical dissolution of gold. In the collection: Geology of Gold-Ore Deposits of Siberia. “Nauka,” Siberian Branch, 1970.

3. Parkhomenko E. I. Electrical Properties of Rocks. “Nauka,” 1965.