ON TWO SERIES OF GOLD-ORE FORMATIONS AND SOME FEATURES OF THE RELATIONSHIP OF GOLD MINERALIZATION TO MAGMATISM

P. F. IVANKIN, K. R. RABINOVICH (SNIIGGiMS)

At the present time, apparently, there are grounds for speaking of three series of gold-ore formations, distinguished according to the type of productive magmas: hyperbasite, basaltoid, and granitoid. The gold content of copper-nickel, chromite, and platinum deposits genetically associated with hyperbasite magmas has not been studied by the authors and is not considered in the present communication.

The features of the basaltoid series of gold-bearing ore formations may be examined on the materials of Rudny Altai, where these formations are represented very fully; among them there are skarn and high-, medium-, and low-temperature hydrothermal formations. Sulfide deposits are decidedly predominant, and all of them are gold-bearing to one degree or another. The main mass of nonferrous metals was deposited at the very end of the Hercynian tectonomagmatic cycle; moreover, the deposits of this late stage formed under conditions ranging in depth from near-surface to 4–6 km and at different temperatures of the mineralizing medium. The ore bodies in all thoroughly explored ore fields are paragenetically connected with a complex of small intrusions of quartz porphyries, albitophyres, porphyrites, and diabases. The principal ore fields have been explored to depths of 600–1000 m. Taking into account the change in mineralization with depth in the ore fields, as well as the different depths of their formation, it is possible to note a definite change in the types of ore formations with depth and a change in the degree of their gold content within the depth interval from near-surface levels to 5–6 km.

The reasons for the sharply different scales of accumulation of nonferrous metals and associated gold and silver in formations of different types depend in many respects on the conditions under which, and the manner in which, postmagmatic solutions separated from the parent magmas at different stages in the development of Rudny Altai. A special work was devoted to consideration of these reasons [6].

The presence of gold in numerous formations of different ages, spatially dispersed, testifies to a generally high potential gold content of the alkaline-earth (basaltoid) magmas that manifested themselves in Rudny Altai from the Eifelian through the end of the Upper Paleozoic inclusive. Analysis of tectonomagmatic relationships has shown that these magmas, unlike the granitic magmas of neighboring regions, had a deep-seated source and a primary composition probably close to andesite-dacite [5]. The principal factor in the concentration of gold and silver (as well as nonferrous metals) in certain products of these magmas is their deep-

...deep differentiation, apparently very prolonged, repeatedly interrupted by acts of intrusion of differentiates into the upper zone of the crust [7]. The comagmatic relationships of all Rudny Altai complexes testify to deep differentiation; as a rule, these complexes have a multiphase internal structure, with phases persistent to depth. Particularly productive for nonferrous metals and gold are those complexes that are represented by differentiates polar in composition—basic and acid, up to ultracid, alkaline-earth rocks. Such is the specificity of the metallogeny of gold in such a typical “femic” region as Rudny Altai. In general form, these conclusions apparently apply to the “greenstone synclinorium” of the Urals, to the gold-polymetallic belts of Kazakhstan, and to the “inner zone” of the Pacific belt of S. S. Smirnov. The paragenetic companions of the gold of basaltic magmas are silver, antimony, cadmium, selenium, tellurium, and the principal chalcophiles.

The most recent studies apparently confirm the views expressed by P. N. Kropotkin, F. N. Shakhov, and others that the primary source of gold in granitoid plutons is the enclosing deposits of increased basicity [9, 12]. Special investigations carried out by us in Southern Altai showed that those parts of the granitoid belt that arose under conditions of substantial iron-magnesian contamination of magmas proved productive for gold; areas with aluminosilicate assimilation of this same belt are not gold-ore-bearing [11]. Recently, on the basis of determinations of gold clarkes, these long-noted geological relations have received an in-depth geochemical interpretation in the works of Yu. G. Shcherbakov [13].

Questions of the place, time, and forms of separation of gold from granitoid plutons, which account for the well-known diversity of formational types of gold-ore deposits of this series, are highly important.

First of all, there are numerous data indicating that gold is capable of separating from granitoid magmas at different depths. Cases of the association of gold mineralization with ancient granite-gneiss complexes are well known. According to some data, gold is sometimes concentrated in zones of granitization and then is associated with albite, feldspar, apatite, tourmaline, muscovite, and rutile [4]. Gold-bearing quartz and quartz-feldspar veins associated with aplite-pegmatites and pegmatoid granites are widely distributed, which enabled A. Ya. Bulynnikov [3] to distinguish a special group of gold deposits. Especially numerous are gold-ore occurrences in the hypabyssal zone, where mineralization is localized both in apical parts and in contacts of granodiorite, diorite, and other plutons, and away from them, in the form of skarn, various hydrothermal quartz-vein and stockwork formations, differing in mineral associations, formation temperatures, stage relationships, the number and character of vein intrusions, and many other features. The existing classifications of gold-ore formations [2, 3, 10], taking these features into account, reflect the great diversity of formational types of gold-ore deposits.

This diversity can scarcely be understood if one proceeds from the assumption of some uniform relationship of gold to the parent granitoid rocks. Rather, it suggests to us the idea of different modes of formation of gold-bearing solutions. However, this question has so far been insufficiently studied and therefore deserves attention.

The usual criteria, which take into account the spatial-structural relations of mineralization to a magmatic body, mineral associations, the time of mineralization relative to thermal metamorphism, and the appear-

of vein intrusions make it possible to establish a general connection between mineralization and magmatism. As a rule, however, we do not have a clear conception of the specific forms of this connection, i.e., of the parts of this magmatic body from which, for what reasons, and in what form the gold-bearing fluids separated that formed a given deposit. In addition to the well-known difficulties associated with the fact that the process must be judged from its products, one may point to difficulties of another kind: they are caused by the schematic character of our ideas about the morphology of those mineralized volumes of rock—parts of geological structures—that are usually called ore fields. Some deepening of our knowledge in this area, as experience shows, can be obtained through a morphogenetic study of gold-ore fields. Considering an ore field petrologically as a specific part of a magmatic system (a pluton, a complex), and forming definite three-dimensional concepts of it down to the maximum explored depths, we obtain the possibility of tracing how the relationship between postmagmatic and magmatic processes changes not only in time but also in space. Comparative investigation of this kind of gold-ore fields of different formational types, first, reveals their important structural–morphological regularities—in particular, it makes it possible to identify characteristic types of ore-field figures with their particular relationship to magmatic plutons—and, second, provides new factual grounds for judging the probable scales of movement of gold-bearing fluids from the place of their separation to the place of discharge.

Work on the morphological study of gold-ore fields proper was begun by us 3–4 years ago, and therefore comparatively little has been done so far. On the basis of these data, insufficient as they are for a complete systematics of phenomena, the following groups of gold-ore fields can be distinguished among those studied:

I. Rootless or halo fields:

a) associated with acmolithic granite plutons in zones of folding;

b) associated with the apical parts of abyssal stocks of diorites and granodiorites.

II. Ore fields with intra-intrusive roots in diorite massifs and in plutons of varied composition:

a) shallow-rooted fields;

b) deep-rooted fields.

III. Ore fields with deep roots, without any visible connection with large plutons down to a depth of 1–1.5 km.

Let us consider some examples of fields of these types.

I. In zones of folding, in the area where granitic acmolithic bodies wedge out within envelopes of crystalline schists and migmatites, gold-bearing quartz and quartz–feldspar veins are often encountered. Such veins may give gradual transitions into aplites, microgranites, and pegmatites. In this case, the gold-bearing siliceous solutions separated already during the magmatic stage and, being localized at the front of the acmolith, did not undergo significant migration. The most probable mechanism for the separation of gold-bearing quartz in this case should probably be considered liquation. Scattered segregations of a substantially siliceous fluid may accumulate, owing to greater mobility, at the front of the acmolith and form a system of irregularly situated nests and small veins, or be squeezed out in the form of elongated bodies of injection character. In the zone of hybrid rocks such quartz crystallizes after the solidification of the dioritic and gabbroid parts of the pluton and therefore occurs as xenoliths in the subsequent more acidic granitic int—

injections. Gas distillation at the level of emplacement of gold-bearing acmoliths is apparently insignificant.

However, in the hypabyssal zone the role of distillation in the removal of gold from magma becomes substantial. It is apparently with distillation processes that one should associate the gold content of some skarn zones situated at the exocontact of magmatic bodies, and of sulfidized hornfelses around granitic bodies. Mineralization of this type may be developed over a considerable area, but spatially it is usually unstable and dies out comparatively rapidly with depth. It is often necessary to suppose that the zone of separation of solutions is the surface of the magmatic body, usually sharply complicated morphologically because of the definite influence of factors of heterogeneity in the enclosing environment. Magmatic bodies accompanied by distillation and by aureole-type mineralization are usually simple in structure and single-phase; split vein intrusions, if they are manifested, are often later in age. It may be considered that aureole mineralization around ore-bearing plutons is a widespread phenomenon and plays an important role, for example, in the formation of placers; however, it rarely reaches the intensity at which actual gold-ore fields of the rootless type are formed.

II. Ore fields with intra-intrusive roots differ sharply from those considered above, first of all morphologically. They represent various types of bundled systems of hydrothermal bodies (zones, veins, stocks, deposits) and vein intrusions that are late split magmatic injections in relation to the parent plutons.

In space, vein intrusions and ore bodies paragenetic with them interact in such a way that at higher levels they occupy relatively large areas, and at lower levels smaller ones; at a certain depth the horizontal dimensions of the ore field decrease sharply; at the same time the number of ore bodies, their thickness, and useful mineralization usually also decrease sharply; on the contrary, the relative amount of vein intrusions increases. Nodes of this kind at the base of ore fields, into which the ore zones descend down the dip and wedge out, we call the roots of ore fields. The extent of such bundled systems down the dip varies widely. From the example of the Kommunar, Olkhovskoye, Tsentralnoye, Dzhelambet, Bestobe, and some other deposits, it may be estimated from 200–300 m to 1–1.5 km, and possibly more, since the root parts of some extended ore fields remain insufficiently explored. Therefore, as a first approximation, ore fields with intra-intrusive roots may appropriately be divided into shallow-rooted and deep-rooted ones.

The Kommunar ore field may be regarded as a typical representative of the former. The vein-stockwork deposits “Severnaya Zone,” “Podlunny Golets,” and Maslovskoye are enclosed in diorite intrusions and are morphologically similar. Each of them is a flattened conical figure, the root parts of which (the apices of the conical figures) are situated at depths on the order of 200–250 m from the present-day surface. Taking into account the different degree of erosion of these deposits, their full vertical extent may be estimated at 300–400 m, their maximum strike length at 450–500 m, and the corresponding width at 70–100 m. The root parts of individual stockworks (deposits) are situated within diorite intrusions, usually at the level where a later phase of fine-grained diorites is manifested. The high-temperature character of the mineralization, the paragenetic association of gold-bearing quartz with amphibole in

within the stockwork and the noted confinement of the root parts to fine-grained diorites make it possible to state with considerable confidence that the source of the gold-bearing fluids was precisely the diorites—most likely their fine-grained, more deeply occurring varieties. Additional intrusions of beresitized porphyries penetrated to the level of the deposit from deeper parts of the massif after the ores had already been formed. Down-dip, these dikes pass into the root parts of the stockworks. Morphological analysis of the field confirms the previously expressed views of A. Ya. Bulynnikov, S. V. Mainagashev, and S. S. Ilyenko on the close genetic connection of the mineralization with the diorite intrusions of Kommunar [3, 8]. Judging from the textures of the ores, the stockworks considered were formed by capillary-fracture infiltration of solutions in solidified but still hot diorite, so that the moistened capillaries were accompanied by broad rims of amphibolitization. The path of the solutions could hardly have exceeded several hundred meters.

Good examples of gold-ore fields with more deeply situated intra-intrusive roots may be the Olkhovskoye, Tsentralnoye, Dzhelambet, Bestyube, and other deposits. It should be noted that, in structural–morphological terms, these deposits are diverse. However, if one generalizes the data from three-dimensional mapping of ore fields and operates not with the morphology of their outcrops on the erosional surface or with individual cross sections, but with three-dimensional figures of the ore fields, then this diversity is simplified. The gold-ore fields mentioned represent ore–magmatic bundles of conical (sometimes pyramidal) type, usually inclined to the horizontal at gentle and moderate angles. They differ from one another in the degree of flattening of the figures, angular parameters, and depth of occurrence of the root parts. They are also united by the fact that the root parts are always located within magmatic plutons, which, evidently, should be regarded as ore-generating. Vein intrusions, closely connected structurally and in age with the ore bodies, are characteristic elements of the fields and, down-dip, pass into their root zones. Some ore fields are situated entirely within intrusive plutons (Tsentralnoye, Dzhetygarinskoye, Komsomolskoye); others are, as it were, divided in half by the contact of the intrusive (Olkhovskoye); still others lie for the most part in the rocks of the roof (Berikul). Taking into account, in the above series, the specific composition of the enclosing magmatic plutons, one cannot fail to note a certain tendency of the root parts of the ore fields toward strongly hybridized rocks, acquiring the composition of diorites and even gabbro-diorites. Such a composition is found in the ore-bearing massifs Dzhelambet and Bestyube, which directly enclose the ore stockworks and veins. Similar relationships were noted above for the Kommunar field. At the Tsentralnoye deposit, by analogy, one may only suppose that such rocks will be encountered below the granodiorite, at depth, in the root part of the ore field. This is indicated indirectly by abundant lamprophyre dikes, as well as by xenoliths of hybrid diorites in the granodiorites. As is known, in many gold-ore fields dioritic rocks and granodiorite–plagiogranitic rocks, usually forming large massifs, spatially coincide. The question of which of these rocks served as the sources of the gold is sharply debatable. It seems to us that morphogenetic study of ore fields, which includes determining the location of their deep-seated root parts, will help resolve the question in each specific case.

III. Examples of ore fields with deep roots and without visible connection with large plutons down to a depth of 1–1.5 km may be the...

deposits such as Bakyrchik in Kazakhstan and Berezovsk on the Urals. A morphological analysis of the Bakyrchik ore field has shown that the genetic relationships of dikes and ores here do not change over a distance of more than 1 km down the dip of the ore zone. In this ore field it is reliably demonstrated that magmatic injections, which formed dikes of granite-porphyries, syenite-porphyries, and diorite-porphyrites, penetrated for at least one kilometer into the tectonic zone in the form of narrow lenticular bodies diverging fanlike from a common center. Constructions show that the root part of the dike bundle must be situated at a depth of 2.5–3 km. An analogous picture is established in the spatial distribution of ore bodies represented by zones of sulfidized shales and quartz veins. An important feature is that the dikes, lenticular in form, and the ore bodies, in longitudinal projection, form a single fanlike system, i.e., have a common root part. The nearest large exposures of granitoid plutons lie several tens of kilometers from the ore field. The depth of occurrence of the parent pluton beneath the ore field can tentatively be estimated from the level of the presumed root part of the field as 2 km below the erosional surface.

The specific character of this ore field lies in the fact that vein magmatic injections and the auriferous fluids that followed them after their consolidation developed under conditions of a structural-tectonic plan that changed very little, and therefore a system of parallel dikes and lenticular ore bodies was formed.

The equally unique Berezovsk ore field on the Urals [1] represents an example of diametrically opposite conditions, under which dikes that underwent fracturing became the host medium for steplike auriferous quartz veins. The ore-magmatic bundles in both cases probably have a similar nature, although they differ sharply in their internal structure.

Conclusion

1. The authors believe that in metallogenic studies of folded regions one should distinguish two series of gold-ore formations, connected respectively with the evolution of basaltoid and granitoid magmas. In the basaltoid series, gold occurs in paragenesis with silver, antimony, selenium, tellurium, and the principal chalcophiles. In the granitoid series, silver, as an important companion of gold, is not characteristic; here gold is sometimes accompanied, in one amount or another, by tungsten, molybdenum, and tin. The main mass of gold in the basaltoid series is contained in complex sulfide ores, especially in those that formed under relatively shallow-depth conditions.

For the granitoid series, proper gold-ore deposits are characteristic, in which gold occurs as the principal useful metal. The nonferrous metals, like the gold itself in this case, apparently were taken up by the magma from the enclosing sequences during processes of granitization.

1. The forms of association of gold with granitoid magmatic rocks are diverse and as yet little studied. Gold apparently separates both together with substantially siliceous and other liquates from hybrid dioritic magmas, and in the form of gaseous distillates that, upon condensation, turn into hydrothermal solutions. The forms of connection between mineralization and the magmatic source largely determine the morphogenetic features of ore fields.

1. Gold-ore fields are diverse in morphogenetic respect; moreover, as a first approximation three principal groups may be distinguished: 1) rootless or halo fields; 2) fields of varying vertical extent with intra-intrusive roots; and 3) ore fields with deep roots, having no visible connection with large plutons down to a depth of 2–2.5 km. The dimensions of ore fields and their economic importance generally increase in the sequence indicated above.

LITERATURE

1. Borodaevskii N. I. and Borodaevskaya M. E. Berezovskoe ore field. Metallurgizdat, 1947.

2. Borodaevskii N. I. Materials on methods for studying the structures and geological-prospecting evaluation of gold deposits. Proceedings of TsNIGRI, issue 35, 1960.

3. Bulynnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountain system. Proceedings of Tomsk State University, vol. 102, 1948.

4. Vasil’chenko S. F. The relationship between gold-ore veins and granitized enclosing rocks at the Kazakovskoe deposit. Author’s abstract of dissertation for the degree of Candidate of Geological-Mineralogical Sciences, Moscow, 1958.

5. Ivankin P. F., Vedernikov P. G. On the question of the systematics and petrochemistry of magmatic complexes of Southwestern Altai. Proceedings of AGMNIИ, vol. VIII, 1960.

6. Ivankin P. F., Inshin P. V., Kuzebnyi V. S. Ore formations of Rudny Altai. Publishing House of the Academy of Sciences of the Kazakh SSR, Alma-Ata, 1961.

7. Ivankin P. F. The relations of complexes within the comagmatic series of Rudny Altai. In: Magmatism, Metamorphism, Metallogeny of the Urals. Proceedings of the First Ural Petrographic Conference, Sverdlovsk, 1963.

8. Il’enok S. S. The most ancient gabbro-diorite complex of the eastern part of the Kuznetsk Alatau. In: Magmatic Formations of the Altai-Sayan Folded Region. Nauka Publishing House, Moscow, 1965.

9. Kropotkin P. N. Tectonics, stratigraphy, and metallogeny of Northern Kazakhstan. Tectonics of the USSR, vol. 1, part 1, Moscow, Publishing House of the Academy of Sciences of the USSR, 1948.

10. Petrovskaya N. V. The character of gold-bearing mineral associations and formations of gold ores of the USSR. Materials of the XXI Session of the International Congress. Reports of Soviet Geologists, Gosgeoltekhizdat, 1960.

11. Rabinovich K. R. Petrologic-metallogenic features of the area of junction of the polymetallic and rare-metal belts in Southern Altai. Author’s abstract of dissertation for the degree of Candidate of Geological-Mineralogical Sciences, Novosibirsk, 1965.

12. Shakhov F. N. On the origin of granitic magmas and ore deposits. In: Magmatism and the Mineral Resources Associated with It, Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.

13. Shcherbakov Yu. G. Correlation dependence of ore complexes associated with granitoids on the composition of the formations enclosing them. Reports of the Academy of Sciences of the USSR, vol. 156, no. 2, 1964.