ON THE ROLE OF PRE-QUATERNARY PROCESSES IN THE FORMATION OF GOLD PLACERS OF THE MODERN DRAINAGE NETWORK

G. V. NESTERENKO (IGiG SO AN SSSR)

Among the Meso-Cenozoic gold placers known at the present time, valley and terrace placers of the modern drainage network of Quaternary (Middle–Upper Pleistocene and Holocene) age predominate. The question of the role, in the formation of Quaternary placers, of exogenous processes that took place at earlier stages of the Meso-Cenozoic history of the continental development of territories remains open. Recently, works have begun to appear [3, 6, 9] in which the supposition is expressed that the main part of the gold found in placers of the modern drainage network is not synchronous with the placer-bearing deposits, but was released from primary sources earlier. The majority of researchers, however, directly or indirectly deny any substantial role of more ancient processes in the formation of these placers and essentially do not consider them.

In the present brief communication an attempt is made to approach the solution of this question from the standpoint of considering the general course of continental development of a territory, using as an example the southern marginal areas of the West Siberian Lowland and the adjacent areas of its mountainous framing.

Here, in the Meso-Cenozoic geological history, which is well recorded in sediments beginning with the Jurassic, a number of epochs of maximum placer formation are distinguished [2, 7]. In the section, a number of stratigraphic horizons are established, to which the main part of the known placers of ilmenite, zircon, monazite, and other heavy minerals stable under hypergene conditions is confined. These epochs, as a rule, are characterized by the maximum development of processes forming chemical weathering crusts, the reworking of which led to the emergence of placers in the corresponding facies. Most clearly distinguished here are the Aptian–Albian (Fig. 1), Paleogene, and Upper Pliocene–Lower Quaternary epochs of placer formation. However, in other periods as well, placers of heavy stable minerals were formed, and in the process of destruction of rocks an enormous quantity of these minerals was released. Thus, in a certain sense it may be said that the process of placer formation (or, more precisely, the process of release of placer minerals from primary sources) proceeded continuously throughout the entire history of the destruction of mountain structures and the reworking of the products of this destruction.

Text visible in Fig. 1: Epochs; Index; Duration, million years; Crust formation; Mountain building; Humidity; Tectonics; Denudation and accumulation; Kaolinite crusts, alkaline crusts; [[unclear:]]; Gold. Quaternary period; Pliocene; Miocene; Oligocene; Eocene; Paleocene; Upper Cretaceous; Lower Cretaceous; Jurassic; Triassic. Krasnoyarsk; kaolin crust. Legend: epochs with known manifestations; presumed.

Fig. 1. Scheme of placer formation in the south of Western Siberia

The Quaternary epoch is the least favorable for the formation of placers of ilmenite and rare-metal minerals, which is explained by sharp cooling and the suppression of processes of chemical weathering. At this time only small placer ore occurrences are formed, mainly through the erosion of more ancient Mesozoic and Tertiary productive deposits and placers, and also as a result of the reworking of pre-Quaternary weathering crusts. Moreover, this fact is well reflected in the composition of the newly formed placers, and first of all in the mineralogical composition of the heavy fractions. In them the content of minerals resistant to chemical weathering increases, while the content of less resistant minerals decreases.

Below we shall attempt to assess the probable role of pre-Quaternary processes in the formation of Quaternary gold placers. It is known that gold differs from the minerals noted above in a number of features and, first of all, in its high specific gravity and weak capacity for re-

is carried in the river flow. The consequence of this is that, whereas ilmenite–zircon and monazite placers are removed to considerable distances from their sources and are situated more often in areas of accumulation, gold placers are located near bedrock sources, i.e., in areas of denudation. Furthermore, when ilmenite–zircon and monazite placers are reworked by a new placer, the latter to a significant degree inherits the mineralogical composition of their heavy fraction, since the behavior of these minerals in a river flow essentially does not differ from the behavior of other heavy minerals. Gold is a different matter. Gold, especially the coarser, more “passive” gold, is concentrated mainly in the near-bed layer of gravels and in bedrock bottoms, and during transport tends to penetrate rapidly downward along the vertical. It is easy to imagine a situation in which, during areal wash from a terrace, almost all the gravel may be washed away while the gold remains. Subsequently the gold is reworked into a new placer, which, in this way, may inherit nothing from its parent placer except gold and, in essence, have no mineralogical paragenetic associates.

It follows from the above that the absence, in the Quaternary gold-bearing alluvium of the region under consideration, of mineralogical indications of the reworking of ancient placers is not yet proof that such reworking did not occur. On the contrary, it is difficult to suppose that the entire long pre–Middle Quaternary history exerted no influence, and that all the gold of the placers under consideration is syngenetic with the enclosing alluvial deposits and was released from its sources in that brief moment, whereas the gold released earlier disappeared without trace—especially if one takes into account its high chemical stability. The fact that gold was released from bedrock sources earlier as well, in Mesozoic and Tertiary time, is confirmed by both direct and indirect data. Thus, placer gold is noted in Jurassic conglomerates on the western slope of the Kuznetsk Alatau, and in Cretaceous and Tertiary rocks of the northwestern slope of the Kuznetsk Alatau. Indirectly, by the location of modern placers, the placer gold content of watershed Paleogene gravels of the Southern Altai is confirmed, and by direct sampling, that of Neogene deposits of Kalba [2]. In the Kuznetsk Alatau and in northern Altai, placers of the ancient hydrographic network are rather widely distributed [2, 7], their age being dated as Upper Pliocene–Lower Pleistocene (i.e., preglacial). Taking into account the fact that the modern hydrographic network in mountainous regions very often inherits the ancient Upper Pliocene–Lower Quaternary network, which in turn repeats a still more ancient network, it is easy to imagine the scale of possible placer transformation.

Indirect data confirming that gold was released from its sources rather intensively in pre-Quaternary time are provided by the results of geochemical sampling for gold of Meso-Cenozoic sedimentary rocks along the southern margin of the West Siberian Lowland. Sampling showed the presence of gold in approximately equal quantities throughout the entire section of the sedimentary cover of the foothills of the Kuznetsk Alatau (Table 1) and Altai, with some increase as one approaches the gold-bearing districts.

Thus, from the material presented, with a certain degree of probability the following assumptions may be made.

A large part of the gold contained in the Middle–Upper Pleistocene and Holocene placers of the territory under consideration in the south of Western Siberia was released from bedrock sources in pre–Middle Quaternary time. Thus, these placers are mainly transformed, more ancient ones. This, naturally, does not exclude that

At the present time, gold continues to enter the placer from primary sources; however, in most cases this influx is superimposed on an already established gold content.

Judging from the works of A. P. Sigov [9], E. Z. Gorbunov [3], O. V. Kashmenskaya and Z. N. Khvorostova [4], a similar picture is observed in the Urals and in the Northeast.

However, the role of pre-Middle Quaternary (pre-glacial) processes must be different for different regions, being determined by the course of their geological development in the Meso-Cenozoic. In young structures, such as the Caucasus and the Pacific mobile belt, it will be minimal. In areas of prolonged tectonic stability (Eastern Transbaikalia, Aldan), the role of these processes is probably substantially different and is manifested mainly through the development of oxidation zones, which in a number of cases also bears directly on the question of the formation of gold placers, since in oxidation zones, according to many investigators [1, 5, 8, 10, etc.], redistribution, concentration, and enlargement of gold may take place.

In Middle Quaternary–Holocene time, depending on geomorphological (tectonic) conditions and relief, ancient placers were buried, redeposited, or destroyed, and they were also replenished through the destruction of primary sources.

REFERENCES

1. Al'bov M. N. Secondary Zoning of Gold-Ore Deposits of the Urals. Gosgeoltekhizdat, Moscow, 1960.

2. Geology of Placers of the South of Western Siberia. (Collection of articles). “Nauka,” Moscow (in press).

3. Gorbunov E. Z. Features of the development of the hydrographic network and questions of placer gold content in the Northeast of the USSR. Soviet Geology, No. 2, 1963.

4. Kashmenskaya O. V., Khvorostova Z. M. Geomorphological Analysis in Prospecting for Placers. Editorial and Publishing Department, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk, 1965.

5. Kreiter V. M., Aristov V. V., Volynskii I. S., Krestovnikov A. N., Kuvichinskii V. V. Behavior of gold in the oxidation zone of gold-sulfide deposits. Gosgeoltekhizdat, Moscow, 1958.

6. Lapin S. S. On the concept of “placer” and the age of gold placers. In: Geology of Placers, “Nauka,” Moscow, 1965.

7. Nesterenko G. V., Tsibul'chik V. M. Sources of Supply of Titaniferous Deposits in the Southeast of Western Siberia. Siberian Branch of the Publishing House “Nauka,” Novosibirsk, 1966.

8. Razin L. V., Rozhkov I. S. Geochemistry of gold in the weathering crust and biosphere of gold-ore deposits of the Kuranakh type. “Nauka,” Moscow, 1966.

9. Sigov A. P. Historical continuity of placers. In: Geology of Placers, “Nauka,” Moscow, 1965.

10. Shlykin E. D., Mukanov K. M., Grishin V. M., Magomedov S. G. On hypergene concentration of gold at gold-ore deposits of Northern Kazakhstan. Bulletin of the Academy of Sciences of the Kazakh SSR, No. 8, 1963.

Table 1

Gold content in Meso-Cenozoic deposits of the northern territories of the Kuznetsk Alatau.*

* Analyses were performed from material smaller than 0.1 mm by V. G. Dymbaev at the IIG SO AN SSSR by the spectrochemical method.