SOME PROBLEMATIC ISSUES IN RESEARCH DIRECTIONS FOR GOLD ORE DEPOSITS

F. N. Shakhov (Siberian Branch, USSR Academy of Sciences)

Eight years ago I had occasion to speak at a conference with a similar report. It must be said that since then considerable changes have taken place in ideas about the genesis of deposits in general and of gold deposits in particular. Significant advances have also been made in research methods, especially geochemical ones. For scientific research in the field of the geochemistry of gold, substantial prospects are opening up, and I shall allow myself to consider some directions in this report.

Comparatively recently, as late as 1960, we could only dream of accurate and sensitive methods for determining gold in rocks and ores. The absence of such methods hindered the study of the geochemistry of gold and, in particular, the formulation and development of geochemical methods of prospecting. At present new methods have been proposed and are being used, and they are improving with extraordinary speed. Several reports on this subject are included in the program of our conference. It can now be said with confidence that in science and practice a situation has arisen that makes it possible to undertake broadly the study of the distribution of gold in rocks.

Small-scale geochemical surveys may be regarded as realistic; with their aid it will be possible to detect rock formations with high gold clarkes and to consider them as areas favorable for establishing ancient gold-bearing placers and possible later concentrations of gold by endogenous processes. Establishing the cycle of gold in the Earth’s crust has a real basis.

There is also a real basis for revealing the causes of the phenomenon of “inheritance” of gold by ore-bearing melts and solutions over time. The point is that in many regions, often even within a single ore belt, gold deposits of different ages are observed. A good example is provided by the regions of northeastern Siberia, where gold ore deposits are encountered that arose in the Precambrian, Jurassic, Cretaceous, and Cenozoic [10]. One gets the impression that in one and the same region, magmatic processes of different ages are accompanied by ore-forming solutions that are equally specialized in composition and that create gold-bearing ores. No serious studies have been conducted to establish the causes of this inheritance. One can only

only, taking the very fact into account, that such a phenomenon is encountered in regions with the most complex history of geological events. Where there are intrusions of different ages, and it should be characteristic of Precambrian shields, where large gold-ore deposits are also often encountered. From this one can only conclude that the geological complexity of a region may be regarded as a favorable factor for the formation of concentrated mineralization.

The phenomenon of inheritance proves to be characteristic not only of gold. Quite recently a work by Schuiling [13] appeared in print, in which the tin belts of the Atlantic Ocean are considered. The author asserts that ores of different ages are observed within the belts, and comes to the conclusion that such “inheritance” by endogenous solutions of tin can take place only in the event that it is assimilated by solutions from the rocks of the sialic crust. He regards as proof of this conclusion the confinement of commercial deposits to zones of rocks primarily rich in tin. We encounter a similar statement in M. I. Itsikson [3] for tin deposits of the Pacific continental belts.

At one time I expressed the supposition that the birth of gold-ore deposits is connected with the gold content in the rocks of passive sequences [9]. But I did not seek the depths as sources of solutions, believing that gold from the rocks is assimilated equally by palingenic magmas and by solutions departing from them. This supposition at that time was to a certain degree speculative, but now it already has the right to be tested by geochemical investigations.

Very close to the idea of “inheritance” is the thought of the migration of gold, of its circulation from sediments into magma and again into sediments. This problem can also now be solved with the aid of thorough geochemical investigations. This question was posed more than thirty years ago by P. A. Kharitonov [7], but it did not find noticeable responses in print.

Strange as it may be, the question of the sources of gold in ore deposits has become a problem. The opinion of Niggli and Schneiderhöhn, that solutions of “magmatogenic” origin remove ore substance from magma, is widely accepted. In light of the propositions stated above, these views require additional discussion.

Until now, the view that the source of the gold contained in copper-nickel ores of the Sudbury type is the gabbroid magma itself has not been subjected to doubt. Some doubt in this is raised by the article of Shene and Lange [11], who came to the conclusion that sulfide ores of the Sudbury type arose when gabbroid magma assimilated sulfur from country rocks.

If one allows a palingenic origin of the magmatic melt, it is also necessary to allow that the remote source of the metals in the solutions must be the rocks or the substance of the substratum.

But gold and other elements may be borrowed not only by magmas, but also by solutions issuing from them, from rocks of the passive sequence. Regions are known, for example, where uranium mineralization is developed among rocks with a high Clarke of uranium. A similar connection is also noted for gold-ore deposits. Therefore it seems quite natural to think that hot magmatogenic solutions may cause regroupings and additional concentrations of gold in rocks where it had previously been present in a dispersed state.

Some investigations (G. N. Anyushin), carried out in our laboratory, permit one to think that even solutions containing no gold at all possess the capacity to dissolve and redeposit the gold of rocks through which they circulate. Investigations

N. A. and N. V. Roslyakov’s observations in the ore fields of the Mariinsk Taiga and Transbaikalia undoubtedly prove the migration of gold during hydrothermal metamorphism and its subsequent concentration in quartz veins.

The material presented makes it possible to think that gold in ores may have various sources. The possibility is not excluded that, in nature, ores may arise with gold brought in from magma and from the country rocks enclosing the deposit. It becomes necessary to distinguish such deposits, because deposits of different genesis may also differ economically, since the dimensions of the gold concentrations may be different.

Of great interest for understanding the genetic connection of ores with parent plutons are the mechanism and time of separation of ore-forming solutions from the magmatic melt. The most widespread view, once supported by A. E. Fersman [6], is that volatile solutions separate from magma as its temperature decreases and are partly concentrated in residual melts. But geologists have always known that in pegmatite bodies the concentration of gold and nonferrous metals is manifested only very rarely and very weakly.

With this idea of the cooling of magma as the principal cause of volatile separation, Barth’s conclusion [1] does not fit, namely that volatiles are removed from the magmatic melt with greater difficulty the greater the distance from its chamber to the Earth’s surface. True, A. E. Fersman also believed that pressure hinders the separation of volatiles from the magmatic melt, but he did not consider a decrease in pressure as a cause of the separation of volatiles from the magmatic melt.

We have previously had occasion to note that the separation of volatiles from the magmatic melt is connected with the development of fractures in the walls of the chamber ([9], p. 93). In this process there must occur a decrease in pressure, injection of magma into fractures, escape of volatiles, and crystallization of the magmatic melt deprived of them. In this case the volatiles accompany the movement of the magmatic melt. Spurr’s opinion seems correct: where the dikes are, there are the ores. Today, in such cases, one speaks of a paragenetic connection between ores and dikes. The spatial associations of ores with so-called small intrusions are well known to all, and at times with extensive belts of dikes, i.e., with movements of magma caused by the emergence of belts of faults and by changes in pressure.

But there are cases of concentration of volatiles in apophyses and dikes, especially in their upper parts. Thus, the pipes of the La Colorado deposit in Cananea are well known; the release of volatiles from them led to the formation of explosion breccias. Recently, N. A. Solodov, in describing rare-metal pegmatites, noted their tendency, with depth, to pass into rocks very close in composition to granites [4]. In these cases the source of the volatiles is the small intrusions themselves. The impression is created that in magma moving in a region of low pressures, separation of volatiles—boiling—takes place. Thus, the decrease in pressure may be caused not only by the formation of fractures reaching the magmatic melt, but also by the melt itself reaching regions of low pressure. From these positions, attention is drawn to the entirely indisputable richness of granite domes in volatiles. Beneath them, Emmons once drew a dead line, below which no ore deposits are formed. It may be supposed, with support from a number of geological data, that the unevenness of the roof of the batholith is due to movements of the melt. Domes, like apophyses, are ...

by the mobile parts of the granite batholith. This is evidenced by observations of various geologists.

A study of the surface of the granite batholith of the Rudny Mountains by Tischendorf and others, German geologists [14], revealed that the known ore-bearing massifs exposed by erosion are domes of an enormous granite body. On the map compiled by them, attention is drawn to the subordination of the base of the batholith to structures other than the domes—sometimes having the form of ridges and following in their direction other, probably later, structures. All the ore fields are confined to the domes or ridges of this massif.

The movement of magma in domes has long been known. Fragments of hornfels or fragments of fine-grained granites in granites of late stages are interpreted not as a local phenomenon indicating local movement, but as evidence of batholith intrusion. It is sufficient to accept that the magma of a dome is moving, and the crossing of the granite boundary by bodies of quartz veins and pegmatites in a number of cases makes clear the genesis of “pre-batholithic” ores.

The movement of magma in a dome may precede—or, more precisely, be superimposed on—the products of earlier activity of volatiles. In this respect, the material cited by Yu. N. Trushkov [5] for the regions of Yakutia is noteworthy. According to his data, within a single tectonic block the apical parts of domes are situated approximately at one hypsometric level. Tin-ore deposits are confined to the tops of domes, whereas gold deposits are located hypsometrically lower, by 100–500 m. The author believes that the source of the gold was “deep-seated centers.”

What has been set forth compels us to raise the question that a decrease in pressure is the principal factor promoting the separation of volatiles from the magmatic melt. It may occur when fractures or ruptures of the surface of the magmatic body are reached, but it may also cause boiling in the melt if, in its movement, it reaches regions of low pressures. From this one may conclude that ore deposits in the overwhelming majority of cases are genetically connected in time with movements of granitic magma, or, in other words, with bodies whose magma experienced movement.

This conclusion is, to some extent, consistent with experimental studies published by N. I. Khitarov [8]. According to his data, only a small amount of water separates from magma in the deep parts of intrusive bodies as the temperature decreases. The main mass of volatiles separates as the pressure decreases in moving magma entering regions of low pressures. He believes that “differentiation (of magma—F. Sh.) with the separation of water as a result of boiling” should be regarded as a local process, possible only “in the upper parts of the magmatic chamber” ([8], p. 1281).

Thus, the separation of volatiles from the magmatic melt is connected with a change (decrease) in pressure. But it turns out that a decrease in pressure is also effective during the precipitation of mineral matter from ore-bearing solutions. Laffitte [12], in one of his recent works, notes the undeserved neglect by geologists of pressure decrease in solutions during the formation of veins and the assignment of a greater role to temperature. In his studies he shows that the precipitation of matter from hot solutions is often connected with a decrease in pressure. A very large body of material collected by P. F. Ivankin [2] indicates that the deposition of matter from ore-bearing solutions begins at a shallow depth, possibly corresponding to their reaching a pressure region that causes the solution to boil. This direction in the study of the volumetric structures of ore fields requires the most serious attention.

Summarizing the points considered, we may think that the formation of gold-ore deposits may be connected with hydrothermal solutions of magmas of various compositions. In the case of assimilation and migration of gold in the rocks of passive strata, hot solutions may initially be poor in gold and even genetically only weakly connected with the magmatic melt. However, I can regard this question only as a problem requiring solution, especially if one includes here the solutions that cause the greenschist alteration of rocks—an alteration often associated with a special facies of regional metamorphism.

Fewer changes have taken place in the study of placers. To this day, a painful question in prospecting for primary deposits by means of placers is the character of their connection, the possible sources of gold, their number, and intermediate collectors. It is possible that the application of very subtle and precise analyses will help establish the path and mechanism of gold migration from primary ores to placers.

In conclusion, I would like to note that the cornerstone of the gold industry is technology. The improvement of known methods for extracting gold from ores, or the establishment of new ones that make it possible to lower cutoff grades, will reveal reserves far greater than the discovery of even rich deposits.

REFERENCES

1. Barth, T. F. Theoretical Petrology. Moscow, 1956.

2. Ivankin, P. F. Morphogenesis and Depth of Ore Fields. 1968. (in press).

3. Itsykson, M. I. Geological and geochemical types of tin-ore formations of mobile regions of tectono-magmatic activation. “Sov. Geol.,” No. 11, 1967.

4. Solodov, N. A. Criteria for evaluating rare-metal granitic pegmatites. International Geological Congress, XXII Session. Reports of Soviet Geologists, Nedra Publishing House, 1965.

5. Trushkov, Yu. N. Conditions of formation and regularities of distribution of placers in the Mesozoides (Yakutia). Abstract of a doctoral dissertation, 1968.

6. Fersman, A. E. Pegmatites. Selected Works, vol. VI, Moscow, 1960.

7. Kharitonov, P. A. Sedimentary gold deposits as a geochemical problem. “Probl. Sov. Geol.,” No. 7, 1936.

8. Khitarov, N. I., Kadik, A. A., Lebedev, E. B. Basic regularities of the separation of water from magmatic melts of granitic composition. “Geochemistry,” No. 11, 1967.

9. Shakhov, F. N. Main directions of scientific research in gold-bearing regions of Siberia. “Geol. i Geof.,” No. 10, 1961.

10. Shilo, N. A. Geological structure and primary sources of the Yano-Kolyma placer gold-bearing belt. Proceedings of VNII-1, issue 63, Magadan, 1960.

11. Cheney, E. C. and Lange, J. M. Evidence for sulfurization and origin of some Sudburytype ores.—Miner. Deposita, No. 2, 1967.

12. Laffitte, P. Mechanism and duration of vein deposit formation.—Econ. Geol., No. 4, 1962.

13. Schuling, R. D. The belts of the continents around the Atlantis Ocean.—Econ. Geol., No. 4, 1967.

14. Tischendorf, G., Wasternach, J., Bolduan, H. und Bein, E. Zur Lage der Granitoberfläche im Erzgebirge und Vogtland.—Zeitschrift für angew. Geologie, Bd. 11, Hf. 8, 1965.