FEATURES OF THE INTERNAL STRUCTURE OF GOLD FROM ANCIENT PLACERS OF THE AMUR REGION

A. A. Saprykin, S. V. Yablokova (TsNIGRI)

In solving a number of problems in the geology of placers that involve establishing the connection with primary sources and the time at which the metal entered the placers, geological and geomorphological data are usually used. At the same time, insufficient attention is often paid to the gold itself and especially to its internal structure. Meanwhile, the study of the features of the internal structure of gold can provide very important information that contributes to solving a number of practical and theoretical problems.

As shown by the studies of N. V. Petrovskaya and A. I. Fastalovich [4, 5], gold in placers, while retaining the granular structure characteristic of ore segregations, acquires certain new features under the influence of factors acting under placer conditions. First, high-fineness shells with a fine-grained polyhedral structure appear on gold particles. The origin of these shells is caused by electrolytic corrosion, during which silver is leached out. Second, under the influence of the mechanical action of clastic material, deformation structures arise in the gold—slip lines, disintegration of primary grains, and twinning.

As is known from metallography [1], a deformed metal is in a nonequilibrium, unstable state. In metallurgy, deformation is removed by annealing the metal; in this process, in place of the deformed grains, new, more stable small grains arise—recrystallization takes place. N. V. Petrovskaya and A. I. Fastalovich [4] were the first to describe recrystallization structures in native gold and to prove the possibility of this process under placer conditions at low temperature. Thus, third, recrystallization structures are observed in gold from placers.

Attention should be paid to the fact that N. V. Petrovskaya, A. I. Fastalovich, and other authors regard the processes of electrochemical corrosion and recrystallization as unrelated, proceeding independently of one another. Meanwhile, the character of the recrystallization structures, according to their description, fully corresponds to the structures of the high-fineness shell, and the intensive development of these structures depends on one and the same factor—the duration of the residence of gold in the placer—and is observed mainly in ancient placers. These coincidences are not acci-

are not accidental and are probably due to the interrelation, under certain conditions, of the processes of recrystallization and electrochemical corrosion. It is known that deformations reduce the corrosion resistance of a metal and increase its chemical activity [1]; therefore, electrochemical corrosion of deformed placer gold proceeds more actively than that of undeformed ore gold, which is also confirmed by observations of gold from primary sources and placers. As indicated above, signs of transformation in a placer appear in the form of a high-fineness rim developing along the periphery of the gold particle, that is, where it undergoes the greatest deformation. In ore gold, however, electrochemical corrosion is manifested in the formation of intergranular high-fineness veinlets that penetrate the entire gold particle and, unlike the high-fineness rim, do not have a granular structure.

On the basis of the considerations presented, it seems to us that, under placer conditions, corrosion processes are associated with deformation and recrystallization of gold, and that the formation of a high-fineness rim should be regarded as the result of deformation, recrystallization developing along it, and electrochemical corrosion.

The study of these features of secondary transformation of gold in placers appears to be very important, since the degree of their development makes it possible to judge the conditions of transport and accumulation of gold, as well as the duration of its residence in the placer. Of greatest interest in this respect is gold from ancient placers.

The present article sets forth some results of a study of the internal structure of gold from ancient Neogene–Lower Quaternary placers of the Amur Region. As is known, the Amur Region is characterized by ancient Neogene–Lower Quaternary placers preserved on non-peneplainized surfaces. They are distinguished by intense weathering of the alluvium and by a characteristic composition of the heavy-mineral concentrate, in which minerals highly resistant to weathering predominate.

The features of gold most typical of Neogene–Quaternary placers were identified by us most clearly in the Yasnopolyanskaya and Maksimovskaya groups of placers of the Dambuki district. In the Yasnopolyanskaya group (Fig. 1), the ancient placer is confined to the Yasnopolyanskaya sequence of Lower Quaternary age, extending in an east-northeasterly direction along the flattened interfluves of the right bank of the Ilikan River. The sequence is represented by interbedded pebble beds, boulder beds, and sands with a thickness of about 40 m. As a result of weathering, most of the pebbles and boulders have been transformed into grus, and the heavy-mineral concentrate is dominated by minerals highly resistant to weathering—ilmenite, zircon, rutile, and others.

Morphologically, the gold of the Yasnopolyanskaya sequence is fairly uniform: these are flake-like, more rarely lump-like, isometric, medium- and well-rounded gold particles. The 1–2 mm size class accounts for about 50%. The surface is uneven and corroded; many gold particles are covered with reddish films of iron hydroxides; the fineness, according to assay analysis, ranges from 967 to 987. Most gold particles, as shown by reflectance measurements and determination of fineness on a touchstone, have a heterogeneous composition. The main part of the cross-sectional area of the gold particles is distinguished by a dark reddish color; its fineness is 960–980. Within this reddish high-fineness gold, small light areas stand out, as a rule confined to the central part of the gold particle; their fineness is 850–900. Structural etching of the gold showed that areas of low-fineness light gold are characterized by a coarse-grained structure, representing relicts

of the primary structure of ore gold. The peripheral high-fineness part of the gold grains is distinguished by a fine-grained polyhedral structure; the grains composing it, 0.008 mm and smaller in size, have irregular serrated outlines. A twinned structure is often observed, with growth twins that terminate within the grains.

Judging from the ratio of low-fineness and high-fineness gold, the latter represents a high-fineness shell, but, unlike the gold usually encountered in Middle–Upper Quaternary, as well as Holocene, placers, where the thickness of the high-fineness shell does not exceed 0.02 mm, in the gold of the Yasnopolyanskaya sequence the degree of transformation is so great that only small relics of primary gold are preserved. It should be noted that the observed fine-grained, polyhedral structure of the high-fineness shell has many features in common with recrystallization structures—namely, a reduction in grain size compared with the original dimensions, polygonality, the presence of twins, etc. Hereafter these structures of the high-fineness shell, similar to recrystallization structures, are called re-recrystallization structures. For greater clarity, histograms are presented of the percentage ratio of gold recrystallized to varying degrees. For this purpose, gold grains in etched mounted polished sections were divided into 6 classes (Fig. 1). At each placer point, from 20 to 60 such gold grains were examined.

In the Yasnopolyanskaya placer, gold grains recrystallized to varying degrees constitute 92–98% of the total number of examined gold grains. At the same time, gold recrystallized by 75–100% predominates (60%); 36% of the gold has been transformed to a weaker degree, and only 5% of the gold retains its primary structure. In accordance with the high degree of recrystallization of the gold of the Yasnopolyanskaya sequence, its fineness as a whole is very high and corresponds to the composition of the recrystallized high-fineness shell (967–987).

In the modern placer of the Dzhalta River, which cuts across the belt of distribution of the Yasnopolyanskaya sequence, gold similar to that of Yasnopolyanskaya is observed: 70% of it consists of size classes coarser than 0.75 mm; it is well to moderately rounded, and most of the gold grains have a flattened, leaf-like form. According to data from the study of the internal structure of gold from the Dzhalta placer, a significant part of it has been redeposited from the Yasnopolyanskaya placer. Most of the gold grains have a thick high-fineness shell with a fine-grained polyhedral structure. The amount and character of the transformed gold, and its fineness at any given point of the valley placer of the Dzhalta River, vary depending on position relative to the ancient Yasnopolyanskaya valley (Fig. 1). At the mouth of the Gorashchevsky Creek, recrystallized gold constitutes only 59%. Farther downstream, the Dzhalta River crosses the belt of development of the Yasnopolyanskaya sequence. This is immediately reflected in the character of the gold: its fineness rises to 960, and the amount of recrystallized gold to 72%. Below the belt of development of the ancient sequence, the content of altered gold reaches 90%. (The displacement of the maximum content of recrystallized gold along the Dzhalta River by several hundred meters below the belt of development of the Yasnopolyanskaya sequence is explained by the transport of gold downstream.) In addition to the amount of recrystallized gold, the supply of the Dzhalta River placer from the Yasnopolyanskaya placer is also indicated by similar histograms of the distribution of gold classes differing in degree of recrystallization. Altered gold occurs not only in the Yasnopolyanskaya placer, but also in the placers of rivers eroding it. It is natural to suppose that, at the same time, ancient alluvium was also being formed in other valleys, and that processes likewise occurred in it.

Fig. 1. Diagram of the distribution of gold of different structure in the Yaspolianskaya group of placers

Symbols:
1 — valley placers;
2 — Upper Quaternary terrace placers;
3 — Middle Quaternary terrace and watershed placers;
4 — Lower Quaternary terrace and watershed placers;
5 — inferred boundaries of the Yaspolianskaya Lower Quaternary valley;
6 — placer point with a predominance of unrounded gold;
7 — placer point with a predominance of rounded gold;
8 — gold sample;
9 — histogram of the percentage content of gold with different structure:
a) gold particles preserving the primary structure without traces of recrystallization;
b) with a thin recrystallized rim;
c) gold particles with a high-fineness rim making up 25% of the section area;
d) the same, 25–50%;
e) the same, 50–75%;
f) the same, 75–100%;
10 — diagram showing the ratio of (a) recrystallized gold (sum of b + c + d + e + f) and (b) unrecrystallized gold (sum of 9a + 9b).

Visible labels in the diagram include: r. Dambuki, r. Dikan, r. Amata, r. Gorashevskii, 939, 921, 954, 967, 982, 837, 906, A, B, Q₄, Qₐ.

recrystallization of the gold. During erosion of the ancient alluvium, the gold entered the younger placers. Such recrystallized gold has been found in insignificant quantities on the left terrace and in the valley placer of the Ilikan River above the mouth of the Dzhalty stream.

At the same time, in the modern placers of the Yasnopolyanskaya group under discussion, poorly rounded gold particles are encountered, often intergrown with quartz, indicating the proximity of ore sources of supply. In this gold the primary coarse-grained structure is preserved almost completely, while secondary alteration appears in the form of a thin, high-fineness shell. Its amount increases with distance from the known ancient placers.

In comparison with the Yasnopolyanskaya group of placers, where the ancient strata occupy a relatively large valley, in the Maksimovskaya group the Lower Quaternary placer is confined to the valley of a small creek and lies on a terrace 6–8 m high. In addition to the ancient placer, terrace placers of Upper Quaternary (near the settlement of Galchima) and Middle Quaternary (right Maksimovsky ridge) age are known here (Fig. 2).

In all the placers listed, gold with a high-fineness shell occurs in one quantity or another; this shell has a characteristic fine-grained polyhedral structure. This graininess develops against a background of coarser-grained segregations. It is clearly visible how individual high-fineness grains seem to float in a mass of primary crystals. The greatest amount of recrystallized gold (80%) has been recorded in the Lower Quaternary placer of the left Maksimovsky ridge.

As in the Yasnopolyanskaya group, in the valley placer of the Maksimovsky stream there is transformed gold redeposited from the ancient placer; and, as in the Dzhalta valley placer, its amount depends on the position of the placer site relative to the ancient one (Fig. 2). In the area where the ancient placer is being eroded, in the valley placer 44% of the gold particles bear traces of recrystallization, whereas downstream only 35% do. It should be noted that almost unaltered gold occurs in these placers more often than in the placers of the Yasnopolyanskaya group, and the degree of its recrystallization is lower—the predominant particles are those in which the high-fineness shell constitutes only 25% of the section area.

When comparing placers of the two groups, the dependence of the degree of transformation of gold in placers of the same age on the size of the valley containing the placer is striking. Indeed, in the Lower Quaternary Yasnopolyanskaya placer, formed by a relatively large river, recrystallized gold constitutes 92–96%, with 55–61% transformed almost completely (Fig. 1). In the placer of the left Maksimovsky ridge, lying 2–3 km from the head of the stream, 80% of the gold particles are recrystallized, but only 24% completely (Fig. 2). In small channel Neogene placers of the Tygda–Ulunginsky district, characterized by weak transport of alluvium and metal, the degree of recrystallization of gold is lower than in the described Lower Quaternary placers of the Dambukinsky district, even despite their more ancient age. Thus, in the placer of Sergievsky stream, all examined gold particles are affected by recrystallization, but to a comparatively low degree—up to 50% of the section area of the gold particles; while in the small placer of Terrasny stream, changes have been recorded in no more than half of the gold particles.

The study of unrounded gold occurring in intergrowths with quartz and other minerals in the placers under discussion showed that it comes from different types of primary sources. Thus, in placers

Visible labels in the figure: Molot Stream; Pichugina Hill; Maksimovsky Stream; Galchima Stream; Galchima settlement; A; B; \(Q_1\); \(Q_4\); \(Q_2\); 942; 940; 905; 889–929; 915; 913.

Fig. 2. Scheme of the distribution of gold with different structure in the Maksimovsky group of placers.
For legend, see Fig. 1.

of the Dambuki district, confined to areas of distribution of Proterozoic gneisses, varieties of gold have been identified that are associated with quartz veins, zones of diaphthoresis, and quartz and quartz–feldspar metasomatites. The gold is characterized by a high fineness (900–930). The sources of placer gold in the Tygda–Ulunga district are often genetically connected with small intrusions of Cretaceous age. The fineness of this gold does not exceed 850. However, in the placers of each of the districts, gold has been found both in a highly transformed state and preserving the primary structure of ore gold. Moreover, a considerable degree of alteration is observed both in gold whose relict cores have a high fineness (900–930—Yasnaya Polyana) and in lower-fineness gold (804—Tygda). In individual cases, relicts of primary gold had a fineness of 600–650.

Obviously, in the present case neither the fineness nor the genesis and age of mineralization determine the appearance in placers of recrystallized high-fineness gold.

The existence of this gold likewise cannot be explained by the influence of local physicochemical conditions, since significant fluctuations in the amounts of transformed gold are observed within the transverse profile of a small valley.

As indicated above, recrystallized gold occurs in the greatest amount in ancient Neogene–Quaternary placers, whose confinement to areas of stable or weak uplift best ensures the prolonged residence of gold particles in a definite environment.

An important factor contributing to the intense chemical transformation of gold from ancient placers is the warm, humid climate of these epochs. And finally, the size of the valley in which the placer was formed influences the degree of development of recrystallization processes, since it determines the character of the hydrodynamic conditions of gold transport and the sum of mechanical effects (deformations) received by a gold particle in the course of its horizontal or vertical displacement. Thus, the very close connection between electrochemical corrosion and deformation in the formation of a high-fineness shell is once again confirmed.

When ancient placers are eroded, recrystallized gold enters younger placers, and its quantity gradually decreases with increasing distance from the intermediate source. It should be noted that beyond the peneplain, in areas of intense uplift where ancient placers have not been preserved or did not exist, recrystallized gold is not encountered; the gold observed has an appearance characteristic of placers of Middle–Upper Quaternary and Holocene age: secondary transformation in it is manifested as a thin high-fineness shell, and it preserves the primary ore structure.

On the basis of our observations, it may be concluded that recrystallized Neogene–Lower Quaternary gold is preserved during its redeposition into younger placers only in those cases where these ancient placers, as an intermediate source, also existed in the Quaternary period.

Searches for ancient placers by the presence of transformed gold in channel placers will probably be more successful when a quantitative accounting of the varying degree of transformation of recrystallized gold is applied. Such an approach to the study of the internal structure of placer gold makes it possible to proceed to an estimation of the quantity of metal entering the placer at different stages of its formation, and to a comparative-

…to a correct assessment of the role of primary sources and intermediate collectors in the supply of the placer and to the determination of the age of the metal in the placer.

References

1. Lakhtin Yu. M. Fundamentals of Metallography. Metallurgizdat, 1957.

2. Nikolaeva L. A. Features of the internal structure of native gold from the placer on the B. Dogaldyn River (Lena district) as a criterion of the relative time of entry of gold into the placer. Proceedings of TsNIGRI, issue 25, book 2, 1958.

3. Perelyaev A. P. A criterion for determining the age of gold-bearing placers. Proceedings of the Geological Institute, Ural Branch of the Academy of Sciences of the USSR, issue 20, mineralogical collection, No. 2, Academy of Sciences of the USSR.

4. Petrovskaya N. V., Fastalovich A. I. Morphological and structural features of native gold. In: Materials on the Mineralogy of Gold, Ob"ed. Glavspetsvetmet, 1952.

5. Petrovskaya N. V., Fastalovich A. I. Changes in the internal structure of native gold under placer conditions. In: Problems of the Geology of Asia, vol. II, Academy of Sciences of the USSR, 1955.