MAGMATIC COMPLEXES AND GOLD MINERALIZATION

S. S. Il’enok (TPI)

Considering magmatic complexes as a source of gold mineralization, one should distinguish different types of relationships with them. Thus, gold may be associated with magma that is of deep origin but, in the process of interaction with the Earth’s crust, becomes contaminated to a considerable degree by its material during assimilation. Hydrothermal solutions separating from the magma may be a source of gold mineralization.

At the same time, the magmatic rocks that are formed contain primary disseminated gold. During massive leaching of them by hydrothermal solutions, extraction of this gold and its accumulation in solutions are possible. In this process, alkalis play a major role. In the early stages, potassium played a major role as a mobilizer of gold. It is capable of forming solutions of alkali-metal chlorides with the formation of stable ions of potassium chloroaurate (Remy, 1966). The role of sodium increases sharply in the later stages of development of the solutions, when sodium displaces in them potassium fixed in molecules of feldspars.

Considering magmatic rocks as a source of disseminated gold, one should distinguish complexes of rocks that did not yield up their gold during leaching by solutions, as well as rocks that yielded it up to one degree or another. Such solutions may be residual or postmagmatic formations arising during crystallization of magmatic rocks. Taking into account the factors indicated above, magmatic complexes may be divided into a number of groups whose composition and conditions of formation reflect their possible gold-bearing character.

The first group comprises complexes of ultrabasic and basic rocks without sharply expressed magmatic differentiates. Ultrabasic rocks stand out sharply by their high content of disseminated gold (Shcherbakov, 1967; Shcheka, 1968). Thus, S. A. Shcheka notes a gold content of \(n \cdot 10^{-7}\) for rocks of the dunite–harzburgite formation of the Far East; \(n\) in this case is characterized by a high value, reaching 35–40, whereas in gabbroids not contaminated by sulfides it is 3–7. Many believe that hyperbasite magma is rich in volatiles, although no primary endogenous gold deposits are associated with hyperbasite massifs, which can be explained only by the absence, in the composition of the volatiles of this magma, of the necessary mobilizers of gold. Occurrences of rich deposits are known

gold among hyperbasites that underwent hydrothermal reworking by later intrusions. These include the gold-bearing listvenites of Mount Zelenaya, recently discovered in the Kuznetsk Alatau (Goncharenko, 1968). Similar deposits are known in the Urals, Altai, and other regions. Probably the main source of gold in them was the hyperbasites, while the mobilizers of the gold were hydrothermal solutions of granitoid intrusions.

A comparatively high gold content is also noted in basic rocks. At the same time, as emphasized by Yu. G. Shcherbakov (1968), the intrusive analogues of these rocks contain less gold than the effusives. This can be explained by the longer process of the crystallization stage of the former and by the formation of postmagmatic solutions that wash the crystalline aggregate of the rocks. Earlier we emphasized the preferential development of endogenous gold deposits of the Kuznetsk Alatau within effusive sequences of basic and intermediate composition (Ilyenok, 1957) and the loss of gold-bearing capacity of ore-bearing fracture zones when they pass into carbonate sequences.

The lowest gold content is manifested in syenites and granites. Such a direct dependence of the gold content in rocks on the composition of the complexes is most likely determined by the quantity and composition of postmagmatic solutions, as well as by the duration of their action, accompanied by the alteration of rocks. Therefore, multiphase plutons, in which the duration of the process of magma differentiation, as well as changes and accumulations of postmagmatic solutions, is reflected, are the most ore-bearing.

The second group includes magmatic complexes predominantly of subvolcanic and hypabyssal type. They have a complex composition and are characterized by the formation of magmatic differentiates rich in volatiles. The process of development of the magmatic column is accompanied by a process of magmatic replacement of local character, with contamination of the departing solutions by material from the replaced rocks. These are gabbro-diorite, gabbro-syenite, and diorite-monzonite complexes, widely developed in the Kuznetsk Alatau, Salair, Eastern Sayan, and other regions. The complexes include feldspathoid rocks, which may arise under certain conditions from magma oversaturated with silica. The multiphase nature of these complexes, their comparative complexity, and the duration of their development are usual. Characteristic is the prolonged accumulation of volatiles in narrow channels. In view of the features of cooling and crystallization of the magma, with the pulsating nature of the inflow and movement of volatiles, the ore-forming process is manifested as prolonged and multistage. The types of gold-ore deposits are diverse, including deposits of secondary quartzites, skarns, quartz veins and stockworks, and disseminated sulfide zones. Often various types of mineralization are associated with one and the same complex. Thus, with the magmatic complex of Belsky Golts on the eastern slope of the Kuznetsk Alatau, both gold-bearing quartz veins and skarns are associated. With the subvolcanic gabbro-syenite complex of Gornaya Shoria, secondary quartzites in certain areas containing gold are associated, as well as skarns with known weak gold-bearing capacity.

Granitoid complexes differ sharply in the character of their manifestation and ore mineralization. Among them, first of all, plagiogranite and granodiorite plutons, usually belonging to the mesoabyssal facies, should be noted. These are massifs of variable, variegated composition. Transitions between diorites, tonalites, plagiogranites, granodiorites, and other rock varieties of the complex are more often...

gradational. Rocks associated with individual independent phases of magma emplacement are also distinguished, but they are not always clearly expressed. In the Kuznetsk Alatau and the Gorny Altai this type of gold-bearing complexes is widely developed (Tsentralny, Berikul, Balakhchino, Olkhovka, the Lebed massif, etc.). The principal and most highly developed type of deposit is represented by quartz veins with gold-bearing sulfides. A metasomatic skarn type of deposit also occurs, with diverse mineralization and, at times, abundant sulfides. Rather extensive fields and zones of development of quartz veins are observed, forming groups or series. Individual series have considerable strike length.

The main area of gold mobilization for this type consists of large fields of the magmatic complex, where processes of magmatic replacement of early basic rocks are widely manifested. The latter are represented by gabbro, by more widely developed diorites, and bear traces of extensive replacement by granitoids of various composition. In this process, rocks of sodium specialization are formed. A regional potassium metasomatism, sometimes very intense, is usually superimposed on them, and its role in ore formation is also significant. Thus, for example, in the Lebed massif of the Gorny Altai, iron-ore and simultaneously gold-ore skarns have two distinct stages of development. One of them is associated with the early process of magmatic replacement of the enclosing basic and intermediate effusives during the formation, in the endocontact zone, of monzonites and syenites. The other is associated with intense regional potassium metasomatism.

Gold deposits confined to granite stock-batholithic complexes have a somewhat different character of genetic relationship. They are more typical of ancient Precambrian regions. Their composition is distinguished by considerable homogeneity. An example of such a complex is the Kalamin batholith of the North Yenisei taiga, to which systems of ore fields of great extent are confined. They are subordinated to linear structures in the folded sequence.

The distribution of ore mineralization, zonality, the structure of ore fields, and the absence of dike series in them indicate a certain independence of the deposits from magmatic bodies. The ore deposits are connected with them in a roughly paragenetic manner. The principal source of emanations is the deeper zones of regional granitization of rocks. Solutions are localized along linear fracture zones of considerable extent. Along these zones more intense folding is manifested; during periods of stress decline, the development of fractures and ore deposition is observed. Regional metamorphism of progressive character noticeably increases toward the areas of ore zones. Characteristic are veins filling flexural bends, saddle-shaped veins, zones of lenses and stringers of the horsetail type, and others. Migration of gold was carried out by highly mobile compounds; an abundance of such a volatile element as arsenic is characteristic, producing arsenopyrite. Chalcopyrite, pyrite, pyrrhotite, galena, and some other minerals are of subordinate importance. It is noteworthy that rare cassiterite and garnet are recorded in the composition of gold-bearing quartz veins; albite is not uncommon, and coarse-platy biotite occurs along the selvages.

The outlined rough scheme of the features of the relationship between gold mineralization and magmatic complexes shows the possibility of revealing the character of the leading processes of ore formation in connection with the study of the composition and conditions of formation of magmatic complexes, as well as the processes of regional and local metamorphism.

References

1. Goncharenko A. I. New data on the gold-bearing potential of the northern part of the Kuznetsk Alatau. In: Questions of the Geology of Gold Deposits of Siberia, Tomsk, 1968.

2. Ilyenok S. S. Features of the metallogeny of the ophiolite formation of the Kuznetsk Alatau. In: Proceedings of the 7th Scientific Conference Dedicated to the 40th Anniversary of October, issue 4, Tomsk, 1957.

3. Shcherbakov Yu. G. Distribution and Conditions of Concentration of Gold in Ore Occurrences. Moscow, Publishing House “Nauka,” 1967.

4. Shcheka S. A., Moiseenko V. G. On certain regularities in the distribution of gold in basic and ultrabasic rocks. In: Proceedings of the 3rd Interuniversity Conference on the Study of Gold Deposits of Siberia, Tomsk, 1968.