Possible Mechanism of Native Gold Formation in Ice Lenses

S. L. Shvartsev

(Presented by Professor N. A. Udodov)

Cases have been described in the literature of the discovery of native gold in thin ice lenses, which are widely distributed in regions with permafrost. In particular, native gold was discovered by R. W. Boyle [7] in ice veins of the Giant-Yellowknife deposit (Canada). The ice containing gold was encountered at a depth of 25 m; the width of the ice veinlets averages 7.5 cm, and their length is 1 m. Gold in these ice veinlets occurs in two types: 1) tubular gold contained in calcite and pyrite, which form inclusions of small crystals in the ice, and 2) leaf-like gold forming thin films on the surface of ice lenses with an area of up to ten cm².

A detailed study carried out by R. W. Boyle allowed him to conclude that the gold was formed by precipitation from solutions in geologically recent, most probably post-Pleistocene, time, i.e., that it is associated with the time of formation of permafrost. However, the causes of the concentration of gold in ice lenses remain unknown and require further study; this applies especially to the mechanism of formation of thin gold films. In this connection, the author considered it possible to express his point of view on this question, which is of not only theoretical but also practical interest.

The proposed mechanism of formation of thin gold films in ice lenses, in the hypothesis considered by us, is based on two experimentally established facts: 1) the ability of gold to migrate in groundwater in the form of true solutions, including in the form of complex compounds, and 2) the appearance of an electric potential at the moment of freezing of a solution, i.e., at the interface solution—solid phase (ice). Let us first consider the second question.

A number of works [1, 2, 5, 8] have shown experimentally that at the moment water freezes, a potential difference of considerable magnitude arises between the ice and the liquid phase. At the same time, at the moment of freezing of distilled water or water containing NH₄Cl salts, the ice becomes positively charged relative to the water [2, 8], whereas in the presence of NaCl, CaCl₂, and other salts in the solution, the ice is always negatively charged [5].

Later work by V. P. Borovinskii [3] confirmed the presence of an electric potential also under natural conditions, in particular in the active layer of regions with permafrost at the moment of its freezing and thawing. In this case the ice, as well as frozen

soil, always had a negative charge with respect to the unfrozen water and soil.

Consequently, the occurrence of electric potentials in the process of water freezing is beyond doubt. The sign of the charge that arises in this case depends on a number of factors, but for groundwaters, which are complex natural solutions, the most characteristic case is apparently that in which the ice is charged negatively with respect to the water. In this case, ahead of the freezing front a potential barrier is formed from an excess of positive charges. Negatively charged ions will be repelled from the freezing front and will accumulate in the solution.

Owing to its good ability to form coordination compounds, gold in natural waters migrates in the form of various complex compounds. Thus, according to B. N. Peshchevitskii et al. [6], in seawater the principal form in which gold occurs is $\mathrm{AuCl}_2^-$, and less often $\mathrm{AuClBr}^-$. In fresh groundwaters, according to G. A. Goleva et al. [4], gold migrates in the form of the uncharged ion $\mathrm{Au}^0$, but in acidic waters and in waters with an elevated chlorine content the predominant form of migration is likewise $\mathrm{AuCl}_2^-$.

Consequently, in the process of freezing of groundwaters, for the reasons considered above, gold will, as it were, be repelled from the freezing front and accumulate in the solution as long as the geological and hydrogeological conditions permit—in particular, the size of the freezing system. Since under natural conditions the dimensions of the latter are limited, each time, as soon as the freezing front reaches the last portions of the solution, reactions of the following type will occur at the cathode, which in the present case is ice:

\[ \begin{aligned} \mathrm{AuCl}_2^- + e &= \mathrm{Au}^0 + 2\mathrm{Cl}^-, && E_0 = 1.119\ \mathrm{V},\\ \mathrm{AuBr}_2^- + e &= \mathrm{Au}^0 + 2\mathrm{Br}^-, && E_0 = 0.963\ \mathrm{V}, \end{aligned} \]

Thus, as a result of cathodic reactions, the complex compounds of gold are destroyed and it is deposited on the surface of the ice that has formed. When the size of the fissure changes, even if only as a result of the action of water expanding in volume upon freezing, this process may be repeated several times. Ultimately this should lead to an alternation of films of gold and ice, which is indeed observed in the ice veins of the Giant–Yellowknife deposit [7].

In the case of migration of gold in solution in the form of fine colloids or complex compounds with organic matter carrying a negative charge on its surface, the conditions for the formation of thin films of gold on the ice surface are preserved.

Thus, in our opinion, the mechanism of native gold formation in ice lenses is caused by electrochemical processes that arise at the ice–solution boundary during freezing of the latter. In view of the fact that the processes of water freezing in the active layer of permafrost rocks are still taking place at the present time, in ice lenses of areas of gold-ore deposits it is possible to discover thin films of gold of modern genesis.

In conclusion, the author expresses sincere gratitude to Candidates of Chemical Sciences A. A. Kaplin and N. A. Kolpakova for discussion of the questions raised here.

REFERENCES

1. Arabadzhi V. I. On the contact potential difference between water and ice. DAN SSSR, vol. 60, no. 5, 1948.

1. F. I. Bayandina. On the difference in potentials arising between the solid and liquid phases during the freezing of water. Izv. AN SSSR, ser. geofiz., No. 2, 1960.

2. V. P. Borovitskii. On the influence of natural electrical potentials on the migration of moisture and of the components contained in it in the active layer. Problems of Hydrogeology and Hydrogeochemistry. Irkutsk, 1969.

3. G. A. Goleva, V. A. Krivenkov, Z. G. Gudz. Geochemical regularities of the distribution and forms of migration of gold in natural waters. Geochemistry, 1970, No. 6.

4. R. I. Korkina. Electrical potentials in freezing solutions and their influence on migration. Processes of Heat and Mass Transfer in Frozen Rocks. Nauka, 1965.

5. B. N. Peshchevitskii, G. N. Anoshin, A. M. Ersenburg. On the chemical forms of gold in seawater. Doklady AN SSSR, vol. 162, No. 4, 1965.

6. R. W. Boyle. An occurence of native gold in an ice Lens-Giant-Yellowknife Gold mines, Yellow Knife, Northwest Territories. Econ. geolog., No. 2, 1951.

7. E. Y. Workman and S. E. Reynolds. Electrical phenomena occurring during the freezing of dilute aqueous solution and their possible relationship to thunderstrom electricity. Phys. Rev., No. 3, 1950.