Some Relationships Between Gold Ore Deposits and Valley Alluvial Placers in Northeastern Transbaikalia

V. L. Fisher (ChGU)

Northeastern Transbaikalia is part of the North-Daurian zone of Mesozoic activation [3], situated in the area where the Mongol-Okhotsk belt and the Aldan Shield join. More than 70% of the territory is composed of granitoids of different ages. Metamorphic and sedimentary formations are less widespread.

From a metallogenic point of view, the greatest interest is represented by Middle–Upper Jurassic and Upper Jurassic subvolcanic and volcanogenic formations, as well as by small intrusions, with which the gold ore deposits of the region are paragenetically associated.

In recent years a number of new gold ore fields and deposits have been identified in Northeastern Transbaikalia [8]. They are confined to belts of increased fracturing of sublatitudinal extent, spatially coincident with belts of small intrusions, subvolcanic, and volcanogenic formations. Individual ore fields are confined to junction areas between zones of increased fracturing of sublatitudinal extent and rupture structures of other trends. In such areas an increased amount of subvolcanic and volcanogenic formations and small intrusions is also observed.

The gold ore deposits of the region differ from one another in geological structure, structural position, material composition of the ores, morphology of ore bodies, processes of hydrothermal metamorphism, temperatures of formation, and industrial significance, which makes it possible to divide them into three groups: skarns, impregnation zones, and quartz veins poor in sulfides.

At the same time, they have much in common. Representatives of the different groups are often located in unified structures, are characterized by an Upper Jurassic age, are close in the number of productive stages of mineralization in quartz veins, and—what is very important for processes of gold migration under exogenous conditions—by the predominance of fine gold (microscopic and submicroscopic in size).

Therefore, processes of gold migration under exogenous conditions may be considered in a unified framework. Probably, at deposits of different types, and even within individual sectors of one and the same deposit, several different conditions could have existed, but

they are not of primary importance and do not hinder the identification of the general factor that determines the processes of exogenous migration of gold.

The principal feature of gold-ore deposits of various morphological groups is the fine size of the gold. Technological studies of ores, microscopic study of polished sections, and macroscopic observations in the ore bodies of the Itaka, Aleksandrovsk, and other deposits of the district have established that fine gold with sizes of less than 0.1 mm is chiefly distributed. In different deposits the amount of gold coarser than 0.1 mm varies within certain limits, but does not exceed 10% of the weight quantity of gold. At the Klyuchevskoye deposit only about 1% of the gold has sizes greater than 0.1 mm [6]. On average, only 5% of the gold in the district’s deposits has sizes greater than 0.1 mm.

Clarification of the relationship between gold-ore and placer deposits is connected with the period of release of gold from primary ores and its entry into valley alluvial placers.

Gold-ore deposits are dated to the Late Jurassic. The history of the development of Northeastern Transbaikalia indicates that by the beginning of the Lower Cretaceous period a qualitatively new stage occurred in the tectonic life of the territory. It was dissected into large tectonic blocks. Some of them acquired a tendency toward predominant uplift and toward transformation into horst-anticlines, coinciding with future watershed areas. Other blocks rose at a lower rate, as a result of which Lower Cretaceous clastic formations accumulated within them. The latter form graben-synclines, with which the depression basins coincide.

The emergence and further development of horst-anticlines was accompanied by significant uplift of the territory. Intense erosion took place, owing to which at least the majority of the gold-ore deposits were brought to the surface. For example, on the eastern flank of the Itaka gold-ore deposit, Lower Cretaceous formations lie on the eroded surface of quartz–gold-ore veins. In turn, the clastic deposits of Lower Cretaceous age that overlap the ore bodies contain gold in free form in the cement. Its quantity reaches tens of milligrams per 1 m³ of rock mass. Gold of size 0.1–0.2 mm predominates. Grains of gold 0.5–0.6 mm in size are observed more rarely. The gold is mainly slightly rounded, which indicates its insignificant transport. The presence of free gold in the cement of Lower Cretaceous formations has also been established on the slopes of the valley of the Amundzikan River, in the area of the town of Mogocha, and in other localities. Gold-ore bodies have also been established in these areas.

The gold content of the cement of Lower Cretaceous clastic formations indicates that, by the beginning of the Lower Cretaceous period, the gold-ore bodies had not only been exposed by erosion processes but were also being intensively destroyed with the aid of weathering processes. The appropriate conditions for this existed. The character of the spore-pollen assemblages in the deposits of Lower Cretaceous age indicates a humid and warm climate, which promoted not only mechanical but also chemical destruction of gold-ore formations and the liberation of gold.

The sharply dissected relief facilitated the transport of free gold and its deposition, in particular, in the cement of loose deposits of Lower Cretaceous age confined to depression basins of graben-like type, where it was preserved.

In the process of gold deposition in depressions, it is dispersed over considerable areas of lake basins, accompanied by differentiation of the metal in accordance with granulometric composition; coarser gold is deposited closer to the bedrock sources. Redeosition of gold is accompanied by an increase in its fineness, which is associated with the leaching of impurity elements.

The absence in the region of Upper Cretaceous, Paleogene, and Neogene formations makes it impossible to study not only the processes of surface migration of gold during these periods, but even to characterize the main features of the geological history. At the same time, the periods preceding the Quaternary in Siberia and Transbaikalia, in particular, were characterized by insignificant tectonic movements, as well as by a warm and humid climate. In all probability, such conditions must have led to intensive disintegration of the bedrock and to the development of a weathering crust [1]. It is possible that certain areas of kaolinized rocks, established in the channel-side parts of the largest valleys of the region, indicate remnants of a pre-Quaternary weathering crust.

The beginning of the Quaternary period is characterized by a revival of tectonic activity. A qualitatively new stage of incision of the main valleys takes place; these valleys inherit the negative relief forms established in pre-Late Cretaceous time. The weathering crust, developed to a greater or lesser extent on watersheds and slopes, undergoes intensive erosion.

The revival of tectonic activity is accompanied by redeposition of significant masses of material and of the gold occurring in it in a free state. This is evidenced by the distribution of gold in valley placers, which in general corresponds to its granulometric composition.

The repeated redeposition of gold-bearing material before its entry into valley placers is also indicated by the fineness of the gold. The latter is always higher in areas that are fed by terrace gold. Such a phenomenon becomes understandable if one recalls that, before entering valley placers through the terraces, gold must undergo additional reworking; accordingly, impurities are leached from the near-surface parts of the gold particles.

The fact that gold enters alluvial placers mainly in free form has also been established in the Northeast of the USSR [2, 4].

The revival of tectonic activity at the beginning of the Quaternary period was accompanied by intense cooling. However, the process of liberation of gold from primary ores does not cease. This is evidenced by the phase composition of secondary dispersion halos.

Secondary gold dispersion halos include mechanical and salt components. The mechanical part of the halo sharply predominates; 85% of it is represented by native gold.

Secondary gold dispersion halos arise under cold climatic conditions owing to intensive soil-forming processes occurring in the active layer in both summer and winter months [5]. Exogenous migration of gold, however, takes place mainly during the warm periods of the year.

Since secondary gold dispersion halos are represented mainly by free gold, they should be regarded as corresponding mechanical placers. Depending on their position in the relief, secondary gold dispersion halos are eluvial, deluvial, proluvial, and other placers.

Between gold-ore deposits, secondary halos

direct links are established between dispersion halos and valley alluvial placers. This is evidenced by their joint occurrence within zones of increased fracturing. The same is indicated by comparison of the mineralogical composition of sulfides in the primary ores, of the qualitative and quantitative composition of indicator elements in the secondary dispersion halos of gold, and also of the composition of admixtures in the gold of the corresponding placers.

At the same time, the granulometric composition of gold in secondary dispersion halos, in comparison with the primary sources, undergoes certain changes. The amount of gold in the size classes less than 0.01 mm and 0.01–0.1 mm decreases, while in the class greater than 0.1 mm it increases. Such “coarsening” is possible only when a considerable quantity of gold of sizes less than 0.1 mm is carried beyond the limits of the mapped secondary dispersion halos. It is not excluded that the finest gold is concentrated along the periphery of secondary dispersion halos and, at a sensitivity of the goldometric method of 0.01 g/t, is not detected by analyses. Therefore, increasing the sensitivity of goldometric analysis is one of the most important tasks, since in this case it would be possible to fully link gold-bearing placers with their sources.

From the secondary halos, gold migrates into valley placers, which is facilitated by the solifluction processes widely developed in the region. In the active layer of valleys, whose thickness rarely exceeds 1.5–2.0 m, sorting of gold-bearing material continues. The coarser gold settles, while the fine gold is transported down the valley. This determines the relatively great length of valley placers. The finest gold, being floatable, is carried far beyond the limits of the placers, and possibly of the district. By the amalgamation method applied to gold-bearing material, it has been established that panning fails to recover only up to 1–2% of the gold from its total amount.

The granulometric composition of gold in the process of exogenous migration from secondary dispersion halos to valley placers undergoes further change, as set forth in the table, which shows the percentage ratios of the different classes of gold.

Table 1.

Thus, placers are characterized by a sharp increase in the amount of gold coarser than 0.1 mm, which on average constitutes 5.0% of its gross quantity. At the same time, the 0.25–0.5 mm class is most widespread in placers. In this connection, the placers of Northeastern Transbaikalia should be regarded as the result of concentration of the coarsest gold from ore deposits. The concentration in placers of the coarsest gold and the removal beyond their limits of its fine varieties have also been established in the study of other gold-ore deposits and the placers associated with them [7].

At the same time, one must not disregard the possibility of mechanical enlargement (clumping) of gold in placers. In particular, this is evidenced by...

attests to the formation of conglomerate-like “new” gold. But the scale of this phenomenon is insignificant. Therefore it does not contradict the general conclusion.

Consideration of Table 1 indicates that, for the formation of a placer with reserves, for example, of 1000 kg of gold, it would have been necessary to erode ore sources whose reserves amounted to about 20 tons of metal. In Northeastern Transbaikalia about 50 valley alluvial placers are known. Therefore, for their formation it was necessary to erode primary sources whose reserves exceeded theirs by a factor of 20. All this indicates the wide distribution in the past of gold-ore deposits. But this still does not answer the question of the district’s present prospects for lode gold, since they may have been eroded to a greater or lesser degree.

The geological structure of the known gold-ore deposits attests to the wide distribution within them of volcanogenic-sedimentary and subvolcanic formations of Middle–Upper Jurassic age, as well as Upper Jurassic small intrusions (Arsikoi, Itaka, Amazarkan, Uryum, and many others). In a number of ore fields, clastic formations of Upper Jurassic–Lower Cretaceous age have been preserved, the thickness of which usually does not exceed hundreds of meters (Itaka, Amundzhikan). All these data indicate a relatively insignificant degree of erosion of the territory, including of the gold-ore deposits, which does not exceed the first hundreds of meters.

The considerable prospects of the district for lode gold are also indicated by the results of work at known gold-ore deposits, within which ore bodies have been established at depths of 400–600 m.

In recent years, searches for gold-ore deposits in Northeastern Transbaikalia have been conducted only in the area of certain placers. And even in this case, gold-ore deposits relatively large for Transbaikalia have been identified. Therefore, a sharp increase in the scope of prospecting and exploration work for lode gold in Northeastern Transbaikalia is undoubtedly justified and necessary. It may lead to the discovery of new rich ore deposits suitable for industrial development.

REFERENCES

1. Bogolepov K. V. Mesozoic Tectonics of Siberia. Nauka Publishing House, 1967.

2. Gorbunov E. Z. On the formation of gold placers under permafrost conditions. Soviet Geology, No. 6, 1962.

3. Gerzhevsky D. I., Kozerenko V. N., Fogelman N. A. On metallogenic zoning of activation zones. Regularities in the Distribution of Mineral Resources, Vol. VIII, Publishing House of the Academy of Sciences of the USSR, 1967.

4. Kashmenskaya O. V., Khvorostova Z. M. Geomorphological analysis in the search for placers, using the example of the gold-bearing districts of the Kolyma and Indigirka. Second Conference on the Geology of Placer Mineral Deposits, Abstracts of Reports, Moscow, 1964.

5. Nogina N. A. Soils of Transbaikalia. Nauka Publishing House, 1964.

6. Petrovskaya N. V., Andreeva M. G. Mineralogical features of the Klyuchevskoye gold-ore deposit (Eastern Transbaikalia). Transactions of TsNIGRI, Issue 25, 1958.

7. Sinyugina E. Ya., Volarovich G. P., Yablokova S. V. On the connection of alluvial gold placers with primary sources. Transactions of TsNIGRI, Issue 76, 1967.

8. Fisher V. L. Gold-ore deposits of Northeastern Transbaikalia. In: Geology and Mineral Resources of Transbaikalia, Issue 3(5), 1968.