ON THE GENETIC RELATIONSHIP OF GOLD MINERALIZATION WITH GROUPS AND COMPLEXES OF IGNEOUS ROCKS

A. Ya. Bulynnikov (TSU)

As is known, geologists genetically associate endogenous mineralization, and in particular gold mineralization, with magmatic rocks. Mineralization is formed chiefly in connection with postmagmatic processes. Indeed, within one or another ore-bearing territory, certain erupted rocks often enjoy development. They may be represented by large or small bodies of intrusive rocks, dike formations, or covers of effusive bodies. They may be of the same geological age, but more often they are formations of different ages. Many magmatic bodies may be merely the rocks enclosing ore-bearing capacity, while some of them are maternal or related to the mineralization. Of course, within one or another territory intrusive, effusive, and dike rocks may appear separately or jointly. One of the main tasks of the geologist-investigator studying an ore-bearing area is to establish the genetic relationship of mineralization with one or another erupted rock—the maternal rock. This task is not a simple one, even when studying areas of relatively small size, such as ore fields and nodes. This task becomes more complicated when considering the ore-bearing capacity of large regions and provinces.

In the descriptive petrography of erupted rocks, according to generally accepted classifications of these rocks, the geologist deals with groups of rocks. By a group of erupted (magmatic) rocks is meant a large classification unit. Thus, in the well-known textbook work of Academician A. N. Zavaritsky [4], the following groups of rocks are distinguished: 1) gabbro-basalt, 2) ultrabasic rocks, 3) diorite-andesite, 4) syenite-trachyte, 5) granite-rhyolite and granodiorite-dacite, 6) nepheline syenite-phonolite, 7) alkaline gabbroid and basaltoid rocks, and 8) dike rocks (in particular, lamprophyres). In investigations, geologists usually genetically linked ore-bearing capacity with one or another group of erupted rocks, or with their special varieties (such as monzonites, diabases, albitophyres, norites, etc.).

In recent decades another trend has begun to develop in the natural groupings of rocks. There arose the doctrine of petrographic provinces of magmatic rocks, related groups of rocks, series, associations, formations, and complexes of these rocks. Especially progressive should be recognized the doctrine of formations and complexes of magmatic rocks [8], of their metallogenic specialization [2]. Questions connected with this

section of petrology—metallogeny—are being actively discussed in the contemporary geological literature. This question is topical.

In the present article I shall attempt to elucidate the question of the genetic connection of gold mineralization with groups of igneous rocks, chiefly on the basis of material on gold mineralization in the Altai–Sayan mountain region. As for the connection of gold with complexes and formations of magmatic rocks, a separate article will be devoted to it.

As is known, gold is often encountered in ores in the free state, being included in vein quartz, barite, carbonates, and in many sulfides: pyrite, pyrrhotite, arsenopyrite, sphalerite, galena, and other sulfides. But gold does not form sulfur compounds (sulfides, sulfosalts). Only minerals are known that are chemical compounds of gold with tellurium, more rarely with selenium and bismuth. The well-known geochemist Goldschmidt believed that gold is essentially siderophile and thus, by its origin, is connected with basic magma. At the same time, he also noted the chalcophile character of gold and did not deny its connection with more acid magma. The remarkable Soviet geochemist A. E. Fersman believed that gold and its deposits are connected chiefly with acid magma (granitoids) and its products. It should be noted that many major scholars, experts on ore deposits—such as Academicians V. A. Obruchev, A. N. Zavaritsky, M. A. Usov, I. F. Grigoriev, and foreign scholars Emmons and Lindgren—linked gold mineralization substantially with granitoids of batholithic form of occurrence.

Another group of scholars of the Soviet school, whose outstanding representative is Yu. A. Bilibin, argues for a connection of gold with small intrusions of essentially intermediate composition—of diorite–diabase aspect. However, there is also a connection of it with more acid small intrusions—of the hypabyssal facies of depth of emplacement. It must be acknowledged that considerably earlier Academician M. A. Usov, in a number of his works [7], emphasized that ore-bearing capacity, including gold mineralization, is essentially genetically connected with intrusions of the hypabyssal facies of depth, i.e., with small intrusions. The material of my studies of gold mineralization in the Altai–Sayan region as early as the 1930s–1940s showed the important role of small intrusions in the manifestation of this ore-bearing capacity.

Gold may form industrially valuable deposits, or may occur as if in a dispersed state. Let us make a brief review of the connection of gold with groups of igneous rocks.

Let us begin with ultrabasic rocks, which include peridotites, pyroxenites, dunites, and the serpentinites derived from them. A general metallogeny connected with ultrabasic rocks is known. This is chromium, nickel, and platinoids. Gold is noted in them only as an admixture, usually together with manifestations of platinoids. Much information on the gold of serpentinites is given in the valuable monograph of V. N. Lodochnikov [6]; the examples he cites speak of gold as an admixture in association with platinoids. The connection of gold with platinoids has long been established in many placers of the Altai–Sayan mountain region and the Urals. The hyperbasites of the Altai–Sayan region, often transformed into serpentinites (of Cambrian age), are widely developed. They form belts in the Salair, Kuznetsk Alatau, Western Sayan, and Gorny Altai; in many gold-bearing placers of such serpentinite belts, accumulations of grains—occasionally nuggets—of platinum and platinoids were present together with gold (the systems of the Talanova, Uksunai, Talovka, Usa, and other rivers). It should be noted that in many such systems small intrusive bodies of diorites of Lower Paleozoic age are rather widely distributed; with them are associated quartz gold-bearing veins that served as the source of the gold; the latter also pass through serpentinites, often trans-

altered to listvenites. In addition to diorite massifs, in individual serpentinite belts younger granitoid massifs appear, and quartz-sulfide gold mineralization is also associated with them.

Thus, primary gold mineralization in ultrabasic rocks occurs extremely rarely; often it is superimposed, associated with younger intrusions of the hypabyssal facies, both of intermediate-basic and acidic composition. Gold occurs in them in the form of quartz veins with sulfides of ores. The enclosing rocks (serpentinites) are listvenitized.

As for the connection of gold with basic rocks, there is still much that is unclear in this question. Basic basaltic magma is parental for most groups of eruptive rocks. Kennedy distinguished two basaltic magmas: 1) olivine magma, through the evolution of which a series of rocks with an alkaline trend is formed (trachytes, phonolites), and 2) tholeiitic magma, through the differentiation of which rocks of the normal series are formed (andesites, dacites, liparites). Judging from a survey of the areas of development of rocks with an alkaline trend (islands of the Atlantic and Pacific oceans, forelands of Europe and Asia), it must be admitted that their gold content is not great. By contrast, tholeiitic magma, close to andesite-basalts, is exceptionally productive with respect to gold content, if one takes into account the area of its distribution (the Pacific ring of islands adjoining Asia and Australia, the Cordilleras and Andes of the American continent, individual regions of the Alpine system, etc.), in which large gold-ore deposits are present. However, gold mineralization is associated not with the basalts themselves, but with more acidic differentiates of basaltic magma (andesites, dacites).

Within the Altai-Sayan mountain region there are predominantly numerous small intrusive gabbro and diabase bodies. They occur quite often within ore fields, in particular gold-ore fields. Not infrequently gabbro and diabases pass into diorites. Under such names as “augite diorites” and “epidiorites” they have been described in the geological literature. In the Kuznetsk Alatau they occur over many areas, usually rich in gold placers, such as: the systems of the Kundustuyul, Talanova, Balyksa, Usa, Kondoma rivers, the Kommunar mine, etc. The same gabbro-diorite bodies occur in the gold-placer areas of the Western and Eastern Sayan. Many gold-bearing veins lie in intrusive rocks or their contacts. There is reason to connect gold mineralization with them. In the Salair Ridge, diorite-diabase intrusions are widely distributed; in the areas of their development rich gold-bearing placers are often developed. A number of primary gold deposits closely associated with diorites are known [3]. Larger gabbro bodies, in association with monzonites and syenites, occur in many places on the eastern slope of the Kuznetsk Alatau. Here, too, primary gold deposits are developed (chiefly quartz and more rarely sulfide and skarn deposits), as well as productive gold placers. Thus, basic rocks of gabbro and diabase composition, often with a transition into diorites, are significant with respect to gold ore. In other regions of the USSR it has also been noted, in particular by Yu. A. Bilibin, that small intrusions, predominantly of diorite-diabase aspect, play an important role in gold mineralization. Bilibin even advanced the theory of the independence (parental nature) of andesitic (dioritic) magma; apparently also of tholeiitic (andesite-basalt) magma, and assessed its role in gold mineralization.

Turning to the connection of gold with eruptive rocks of intermediate-basic composition of the normal series, i.e., with groups of diorite-andesite and syenite-trachyte rocks, let us note their weak distribution in the lithosphere. Only rocks of intermediate composition, andesite-basalts (pyroxene-

porphyritic ones) are widely distributed among effusive rocks. In addition, it should be noted that only the compilation of large-scale geological maps has shown the broad development of small intrusive bodies of diorites, monzonites, and, more rarely, syenites, which makes it possible to introduce a correction into the distribution of individual groups of rocks in the Earth’s crust (in particular, diorites and syenites).

The considerable role of diorites in gold content has already been discussed above; it is also necessary to note the relevance, in gold-ore formation, of root apparatuses of outpourings of lavas of intermediate composition (the Pacific Ring). In connection with diorites there occur many valuable gold deposits in the western part of North America, as noted by Buddington [1].

As is known, magnetite deposits (not containing titanium) are most often genetically associated with syenites and syenite porphyries. There are few data on gold content associated with syenitic magma. As for the Altai–Sayan mountain region, in the northeastern part of the Kuznetsk Alatau there passes a relatively broad belt of gabbro-syenite rocks. Here, gold-ore deposits (Udarnoye) and a number of gold-ore occurrences are developed with alkaline and normal syenites. Even greater importance in the gold-ore formation of this part of the Kuznetsk Alatau is held by monzonites, with which large deposits are associated (Znamenitoye, Komsomolskoye, and others). Unfortunately, the monzonites of this mountain region have been insufficiently studied. It should be noted that in the American literature a genetic connection is emphasized with them of numerous gold-ore quartz, sometimes with barite, carbonate deposits rich in sulfides.

What, then, can be said regarding the connection of gold with the group of granite and granodiorite and their effusive analogues? It should be noted that geologists’ views on the predominant connection of gold deposits with granitoids were clearly exaggerated (small-scale geological maps; the occurrence of ore bodies in granitoids of the near-contact zone). Intrusions of granodiorites passing into banatites, plagiogranites, quartz diorites, and quartz monzonites are indeed gold-bearing intrusions of the batholithic type. However, granites proper—biotite, muscovite, two-mica granites with greisens—are more characteristic of rare-metal ore formation and usually are not gold-bearing. Quartz gold-bearing veins developed in the near-contact zones of granites are genetically associated with small intrusions of intermediate composition.

Quartz albitophyres and quartz porphyries are often relevant in terms of gold content in the formation of large gold-bearing sulfide, in particular polymetallic, deposits. Examples are the copper and polymetallic deposits of Rudny Altai, the Salair Ridge, and the Urals. Granite pegmatites sometimes contain gold, but no deposits associated with them are established.

Of interest are the materials of I. S. Turkin, who studied the magmatism and ore formation of the Yenisei Ridge, where he definitely established a connection of gold-ore formation with granites of the hypabyssal facies. Distinctive granites are developed as a belt among metamorphic rocks, with broad development of granites of palingenic origin.

Turning to groups of igneous rocks of the alkaline series in the context of their gold content, let us first note their greater distribution in individual regions than had been established during small-scale surveys. Thus, within the Altai–Sayan mountain region, a considerable development of feldspathoid rocks has been definitely established in the Kuznetsk Alatau, Eastern Sayan, and Tuva. In the Kuznetsk Alatau there is already taking shape a distinctive province of alkaline rocks of both the leucocratic and melanocratic series (feldspathoid). Their role in gold content is very weak and has definitely not been established. One may think that

rocks of the alkaline groups are later than the gold-ore formations, since dikes, for example of essexites, cut the gold-ore veins of Sarala; this is also observed in other ore fields.

The well-studied ore-bearing character of nepheline syenites and other feldspathoid rocks, widely represented on the Kola Peninsula, has not shown appreciable gold content. For southern Ukraine—the Azov region, where alkaline rocks occur, no concentrated gold content is known, although Prof. Morozovich noted the gold-bearing character of mariupolites, rocks essentially composed of nepheline and albite. Everyone knows the gold mineralization of Aldan, where alkaline leucite rocks are widely developed.

As for dike rocks, in many gold-ore fields of the Altai–Sayan region they play an important role in the structure of fields and ore bodies. Many of them are pre-ore formations, being cut by ore veins and bodies (the area of the Znamenity mine, the Olkhovsky deposit). Another group of dike rocks cuts the ore veins, for example the proterobase dikes of the Saralinsk ore field, lamprophyre-odinites and vintlites of the Disler deposit of Chibizhek, and diorites of the Salair ore bodies. The relevance of the topic “dike rocks of gold-ore fields” must be acknowledged.

Thus, from this brief review one may conclude that alkaline magma, which produced groups of alkaline feldspathoid rocks, is generally weakly gold-bearing and in most cases does not yield industrially valuable gold-ore deposits. Although it may be noted that in North America the large Cripple Creek gold-ore deposit occurs in phonolites, and also, apparently, there are a number of deposits in British Columbia where feldspathoid rocks are widely developed.

Rocks of intermediate basicity, sometimes transitional to basic rocks—diorites, diabase-diorites, monzonites, quartz diorites—are relevant in gold mineralization. All of them belong to the group of small intrusions.

As for ultrabasic rocks, gold content is inherent in them, but superimposed, manifesting itself chiefly in altered serpentinites. With regard to rocks of the gabbro group (gabbro-norites, anorthosites), their gold content is very weak. Gold content is manifested more distinctly, with a low content in massive sulfide ores, often with platinoids, in lopoliths and intrusive deposits of norite or trap composition. And only diabases with a dioritic tendency are not without interest from the standpoint of gold content. Granitoids, especially those tending toward intermediate rocks—granodiorites, banatites, plagiogranites—are more favorable with respect to gold than leucocratic, potassium-rich granitoids. Substantially magmatic (not palingenetic) granitoids of hypabyssal and possibly mesoabyssal facies are gold-bearing.

REFERENCES

1. Baddington A. F. Correlations of types of mineralization with various types of igneous rocks. Geology of Ore Deposits of the Western States of the USA, ONTI, 1937.

2. Bilibin Yu. A. Metallogenic Provinces and Metallogenic Epochs. Gosgeoltekhizdat, 1955.

3. Bulynnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai–Sayan mountainous region. Proceedings of Tomsk State University, No. 102, 1948.

4. Zavaritsky A. N. Igneous Rocks. Publishing House of the Academy of Sciences of the USSR, Moscow, 1956.

5. Lindgren V. Mineral Deposits. Issue III, ONTI, NKTP, 1934.

6. Lodochnikov V. N. Serpentines and serpentinites, Ilchirskii and others. ON NKTP, 1936.

7. Usov M. A. A Short Course on Ore Deposits. Tomsk, 1932.

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