On the Role of the Processes of Regional Metamorphism and Ultrametamorphism in the Development of Gold Mineralization

V. A. Buryak, F. A. Letnikov
(Institute of the Earth’s Crust, Siberian Branch, Academy of Sciences of the USSR)

In the development of gold mineralization, especially that represented by ancient Precambrian formations, a close dependence on the character of regional metamorphism of the enclosing rocks, synchronous in time with the processes of mineralization, is manifested quite clearly. Elevated concentrations of gold that are of practical importance are usually localized in the zone of development of the greenschist facies of metamorphism, predominantly in the chlorite-sericite and, in part, in the biotite-chlorite subfacies (Obruchev, 1963; Radkevich, Moiseenko, 1966; Shcher, 1963; Buryak, 1964). More intensely metamorphosed rocks, especially under conditions of the amphibolite and granulite facies, in the overwhelming majority of cases lack significant concentrations of gold. This dependence of the character of gold content on the degree of regional metamorphism, synchronous in time with the ore-forming process, is expressed quite clearly (the Lena gold-bearing district, Northwestern, Western, and Southwestern Transbaikalia, the Biryusa district, Northern Kazakhstan) and can be successfully used in the practice of prospecting and evaluation work both for lode and placer gold, as well as in various kinds of prognostic and metallogenic studies. At the same time, it should only be borne in mind—and we emphasize this especially—that the matter concerns metamorphic zonality (processes of regional metamorphism) associated with one and the same stage of regional metamorphism, ultrametamorphism, and palingenic magmatism as the gold mineralization.

If the mineralization and the regional metamorphic zonality manifested in the enclosing rocks are substantially separated in time and are associated with different tectono-magmatic periods or stages, then the indicated dependence of the intensity of gold mineralization on the degree of metamorphism of the enclosing rocks may not be observed. Examples of this are gold-ore deposits developed among granite-gneisses and metamorphic sequences of the amphibolite facies (Kolar in India, the Middle Vitim mountainous region, the Sayan Mountains, the Far East). Gold in such deposits is associated with a new tectono-magmatic stage superimposed upon the preceding high-temperature regional metamorphism of the enclosing rocks. However, syn-ore metamorphic processes of alteration of the enclosing—

...of the host rocks (wall-rock alterations) and in this case correspond to the greenschist facies.^1

The presence of polyfacies development and of many stages in the evolution of metamorphism in a given territory substantially complicates the identification of the indicated relationship between mineralization and the degree of metamorphism of the enclosing strata. In all cases, rocks metamorphosed under conditions of the greenschist facies of metamorphism are potentially the most favorable for the development of elevated gold concentrations, and, all other conditions being equal, the overwhelming majority of deposits are localized among them, especially if they are considered in terms of their industrial significance.

As we have already emphasized (Buryak, 1967), a characteristic heterofacies column of regional-metamorphic formations is established—a metamorphic zonality in which gold occupies a quite definite place, determined by its geochemical properties. Thus, in the amphibolite-facies zone, mica-bearing, rare-metal, and ceramic pegmatites are formed; in the epidote-amphibolite facies, “barren,” non-gold-bearing quartz veins and, sometimes, rare-metal and rare-earth pneumatolytic-hydrothermal mineralization; in the biotite-chlorite subfacies, weakly gold-bearing quartz veins and pyrite-pyrrhotite mineralization in the host rocks; in the chlorite-sericite subfacies, the main mass of the richest gold-bearing quartz veins and a somewhat less developed, in comparison with the chlorite-biotite subfacies, gold-sulfide mineralization of predominantly pyritic composition. It is characteristic that, in passing from one facies of metamorphism to another, not only the gold content of vein quartz and the content in it of various minerals and elements gradually and directionally change, but also its color, pH values, the temperature of the transition of $\alpha$-quartz to $\beta$-quartz, and other chemical properties. Thus, in the chlorite-sericite subfacies it is predominantly gray and grayish-white in color, often with magnesian-ferruginous carbonates and albite; the sulfides in it are represented mainly by pyrite. In the biotite-chlorite subfacies the quartz is somewhat darker—dark gray and gray—and the sulfides in it are represented by pyrite and pyrrhotite, present in approximately equal proportions. In the epidote-amphibolite facies the quartz is often smoky, dark gray, usually without sulfides or with a small amount of pyrrhotite; disthene and small crystals of green muscovite are sometimes present. In the amphibolite facies, veins of mica-bearing granite pegmatites are characteristic.

Taking into account the belt-like, zonal arrangement of the indicated heterofacies metamorphic zones around deep-seated faults, it may be assumed that the noted zonality of mineralization, like the metamorphism of the host strata, is caused by the influx, along fault zones, of deep metamorphic (“through-magmatic,” according to D. S. Korzhinskii) solutions. In this case the noted zones with different intensities of gold mineralization may be regarded as constituent parts (different sections) of a single, long-developing column of metamorphic, more precisely, metamorphogenic-metasomatic formations. A fairly definite conclusion suggests itself regarding the close connection of mineralization with the processes of regional metamorphism and ultrametamorphism. If this connection were absent, we would not have such

^1 Hydrothermal alteration, especially of effusive-sedimentary strata (for example, propylitization), is very similar to rocks of the greenschist facies. But on this basis it is still impossible to identify hydrothermal alterations with regional metamorphism of the greenschist facies. This question has not been resolved and requires careful study (Editor’s note).

constantly clearly expressed confinement of gold to greenschist facies of regional metamorphism.

It should also be emphasized that the indicated metamorphic zonality formed during the progressive stage of the regional metamorphism under consideration, while the mineral associations formed during this stage may be progressive or regressive with respect to the mineral composition of the original rocks, depending on the degree of preceding metamorphism. Thus, for example, if the processes of amphibolite-facies metamorphism are superimposed on rocks previously metamorphosed under granulite-facies conditions, then the newly formed mineral associations will be regressive in relation to the original ones, although they, too, formed during the progressive stage of this new superimposed type of metamorphism¹.

A clearly expressed dependence of gold mineralization on the degree of metamorphism of the enclosing rocks is especially well manifested in the Lena gold-bearing district (Buryak, 1964). In general form, this zonality is also manifested on a regional scale in the zone of junction of the south of the Siberian Platform with the folded framing (Buryak et al., 1966). Here four principal different-facies metamorphic zones are noted, with mineralization typomorphic for each zone: in the amphibolite zone, various kinds of pegmatites are developed; in the epidote-amphibolite zone, rare-metal mineralization; in the greenschist zone, gold; and in the zeolite facies, polymetallic mineralization. Similar zonality is outlined in the Sayans and the Yenisei Ridge. Evidently, one may speak of a metallogenic specialization of different-facies metamorphic complexes.

The indicated close dependence of mineralization—in the case under consideration, gold mineralization—on the degree of regional metamorphism of the enclosing rocks may be conditioned, as it seems to us, by the following factors.

As has been shown in a whole series of works (A. Engel, D. A. Velikoslavskii, and others) and is confirmed by our studies, regional metamorphism is not isochemical and, at known grades, leads to accumulation of some elements in rocks and migration of others from them.

Progressive regional metamorphism under conditions of granulite and eclogite facies is directed toward an increase in the basicity of rocks, up to the disappearance of free silica; there occurs a sharp decrease in H₂O, K, Na, and a total increase in the contents of Mg, Ca, and Fe (Belousov, 1966). Under conditions of lower-temperature facies of metamorphism, directed migration of various elements also occurs. Thus, in the amphibolite-facies zone, in comparison with lower-temperature facies, the total content of ferric and ferrous oxides, as well as of magnesium and calcium, usually increases. At the same time, a decrease in the content of alumina is outlined.

The essence of these phenomena is as follows. It is generally known that, as the degree of metamorphism increases, the partial pressure of oxygen in the earth’s crust decreases. Against the background of an increasing oxygen deficit under conditions of high metamorphic facies, separation of elements into two groups must occur, depending on their affinity for oxygen. Elements with a high affinity for oxygen will create their own compounds (oxides or silicates), while those whose affinity is small under the given conditions must show a tendency to migrate into zones of weaker metamorphism.

characterized by lower values of $P$ and $T$ and by higher oxygen activity. A measure of the chemical affinity of elements is the value of the isobaric-isothermal potential of formation of a given compound ($\Delta Z$); this question was considered in greater detail earlier (Letnikov, 1965), when series of “native occurrence of elements” were constructed, characterizing the degree of affinity of ore elements for oxygen and sulfur. In the temperature interval from 100 to 1000°C, gold closes the native-occurrence series, which indicates its minimal chemical affinity for oxygen and sulfur. This circumstance predetermines the most active migration of gold from metamorphosed rocks with increasing $T$ and $P$. The forms of migration may evidently differ depending on the character of metamorphism. The predominant forms for ordinary regional-geothermal metamorphism¹ are: migration in pore solutions and migration along vacancies and dislocations in minerals. The small magnitude of the thermodynamic potential of gold ions causes their weak interaction with the medium at high $T$ and $P$ and low oxygen potential, so characteristic of rocks of high grades of metamorphism. That is, a long-lasting and stable process of metamorphism of rocks under conditions of the granulite and eclogite facies leads to depletion of the metamorphosed rocks in gold and to its migration into the region of lower $T$ and $P$. Deposition of the removed gold must take place in rocks of low grade of metamorphism, where the rate of diffusion is low, while $T$ and $P$ are not large and the nearly stagnant character of pore solutions predominates.

It is important to emphasize that the primary regional diffusion flow under conditions of regional-geothermal metamorphism cannot produce deposits; it leads only to a rather uniform enrichment in gold of a definite part of the section of metamorphic rocks, most often of the greenschist facies. However, this makes possible the subsequent secondary mobilization of gold when local thermal fields are imposed on this part of the section. Under conditions of regional-plutonic metamorphism, caused by the influx of heat flow along zones of deep-seated faults and by the development in the greenschist facies of zones of increased schistosity—anomalous thermal fields—in the late concluding stages of metamorphism (especially in the regressive stage), the distribution of gold in the zone of the greenschist facies will not be uniform. This may cause and probably does cause the formation of deposits without repeated mobilization. Such cases apparently occur in the Lena gold-bearing district, in the Baikal region, and also, possibly, in the Yenisei Ridge and in many other similar districts.

Under conditions of development of regional-geothermal metamorphism, the geochemical and thermophysical features of the processes of mineral formation are substantially different.

Ore occurrences or deposits of gold here may arise only in the event of repeated mobilization of gold when anomalous thermal fields are imposed on the greenschist part of the section. Such thermal fields arise in connection with the manifestation of magmatism or the development of local superposed tectonic structures (zones of increased schistosity, faults, etc.). Within such tectonic zones, hydrothermal activity may develop, leading to the formation of “ordinary” hydrothermal deposits. Let us consider this using the example of the Kokchetav block (Northern Kazakhstan). In the central part of the block, metamorphic rocks of amphibolite and

¹ The classification of types of regional metamorphism is adopted after Khoreva (1967).

partly retrograde-altered granulite and epidote-amphibolite metamorphic facies, which are bordered by rocks of the greenschist facies. Intrusions of granitoids are emplaced in these and other rocks, but gold ore occurrences are noted only with those of them that are emplaced in rocks of the greenschist facies (Fig. 1).

Fig. 1. Structural scheme of the Kokchetav block.
1—boundaries of the block; 2—major faults; 3—gold ore occurrences; 4—granitoids; 5—rocks of high grades of metamorphism; 6—rocks of the greenschist facies.

Metamorphism of Archean and Proterozoic rocks on the block, which lasted until the Devonian, caused a constant diffuse flux of elements from these strata into the overlying deposits of the greenschist facies; for gold, moreover, the process must have manifested itself most fully. In the event that a granitic melt interacted with rocks of low grades of metamorphism that differed substantially in composition from granite, it became hybridized and enriched in chalcophile elements and gold. Hence the connection in this region between gold mineralization and maximally hybridized granitoid melts becomes understandable, as does the confinement of gold deposits to the extreme members of the hybrid series—diorites and gabbro-diorites. It is important to emphasize that, for the formation of concentrated gold mineralization, the solutions must not be dispersed but concentrated in a comparatively small (local) volume of rock. The most favorable case is the presence of a small intrusion (stock) that is a satellite of a larger magmatic center. One of the obligatory conditions for the realization of a hydrothermal system in a deposit, within such a stock–fluid conduit (in the terminology of G. L. Pospelov), is the stability and duration of the thermodynamic regime most favorable for the formation of a deposit—

...deposition. That is, a prolonged and stationary influx of hydrotherms into the zone of concentration and realization is necessary. In light of this, it is of some interest to note the fact that the principal gold-bearing massifs of the Stepnyakovsky type are predominantly located in the exocontact zones of large granitoid massifs. That is, stocks of diorites and gabbro-diorites with gold mineralization formed in zones heated by enormous masses of granite melts, which predetermined the duration of the process of their evolution and, ultimately, the formation of deposits.

REFERENCES

Belousov V. V. The Earth’s Crust and the Upper Mantle of the Continents. Moscow, “Nauka” Publishing House, 1966.

Buryak V. A., Lobanov M. P., Khrenov P. M. The role of deep-seated faults in the distribution of metamorphic facies and endogenous Precambrian mineralization in the folded frame of the south of the Siberian Platform. Doklady of the Academy of Sciences of the USSR, vol. 168, no. 2.

Buryak V. A. On the influence of processes of regional metamorphism on the development of gold-sulfide mineralization in the central part of the Lena gold-bearing district. In: Physicochemical Conditions of Magmatism and Metasomatism. Proceedings of the III All-Union Petrographic Conference. Moscow, 1964.

Letnikov F. A. Selected Potentials of Mineral Formation. Moscow, “Nedra” Publishing House, 1965.

Obruchev V. A. Selected Works. Vols. III, IV. Moscow, Publishing House of the Academy of Sciences of the USSR, 1961–1963.

Radkevich E. A., Moiseenko V. G. Regularities of distribution and genetic features of gold mineralization in the Far East. In: Genetic Features and General Regularities in the Development of Gold Mineralization of the Far East. Moscow, “Nauka” Publishing House, 1966.

Sher S. D. Some questions of the metallogeny of gold of the African Shield. Proceedings of the Central Scientific-Research Geological Prospecting Institute, issue 56, 1963.

1. The author, unfortunately, does not indicate examples of such superpositions in nature (editor’s note). ↩