Relationship of Gold and Gold-Silver Mineralization with Propylites, Secondary Quartzites, and Quartz Metasomatites

(Based on a Group of Subvolcanic Deposits in the Eastern USSR)

A. I. KAZARINOV, M. S. MIKHAILOVA, P. S. FOMIN (TsNIGRI)

Recently, in connection with the problem of subvolcanic gold and gold-silver deposits of the eastern USSR, a group of geologists from TsNIGRI studied a number of deposits and ore occurrences of this type. Table 1 lists these deposits, subdividing them into a group of gold deposits proper, with a ratio of gold to silver contents of 1:10, and deposits that are more gold-silver in character, for which this ratio is less than 1:10 and reaches 1:100 or less. One of the distinctive features of these deposits is the broad development, within their ore fields, of hydrothermally altered rocks.

At present, some investigators show a definite tendency to regard these altered rocks as various facial varieties of propylites or secondary quartzites, among which gold mineralization occupies a definite place in their zonality [1], or else as being superimposed upon altered rocks without relation to their zonality [5, 6]. Other investigators [2] assign these rocks to facies of propylites and hydrothermal metasomatites in the sense of D. S. Korzhinsky, and avoid the question of their belonging to the secondary quartzites of N. I. Nakovnik. According to these investigators, gold mineralization is superimposed upon hydrothermally altered rocks.

As our studies have shown, the postmagmatic hydrothermal activity with which the formation of subvolcanic gold and gold-silver deposits and altered rocks is associated represents a prolonged process. It is subdivided into three stages: pre-ore, early-ore, and ore proper. In the first stage, hydrothermal metamorphism actually proceeds in the direction of the formation of propylites and secondary quartzites. In the second stage, intense near-fissure metasomatism takes place, with the formation of quartz and other metasomatites (hydrothermal quartzites, according to D. S. Korzhinsky). In the third stage, the intensity of metamorphism decreases sharply, while its character remains close to that of the second stage.

In the table presented, the studied deposits are also divided, according to their relation to a definite type of magmatism, into two groups: 1—deposits occurring in the closest temporal relation to subintrusive and subvolcanic bodies of intermediate and basic com—

composition, and 2—the same, of acid and alkaline composition. This division is based on the well-known proposition of N. I. Nakovnik [4] concerning the correspondence between the character of hydrothermal metamorphism and the composition of the rocks. As is shown below, such a grouping reveals a quite definite regularity in the character of the hydrothermal alterations of the early, pre-ore stage of metamorphism.

As an example of deposits of the first group, let us consider the Mnogovershinnoe deposit, studied in detail by M. S. Mikhailova. The ore bodies of the deposit are represented by thick quartz veins occurring within a sequence of cover Upper Cretaceous andesites underlain by a sequence of siltstones. Both sequences form the roof of a large granitoid massif that metamorphoses the rocks of the roof. Among other geological features it is necessary to note the development of pre-ore subvolcanic bodies of diorite porphyries, younger than the granitoid massif.

For the pre-ore stage of hydrothermal metamorphism of the andesites hosting the mineralization, the development of medium- and low-temperature propylitization is characteristic; it was manifested in two stages, the first of which has a relatively broad areal distribution, and the second a more local, linear distribution (in connection with a fault zone). Attention is drawn to the fact that propylitization in the same facies is also manifested in the granitoids. If one traces the character of the pre-ore hydrothermal alterations at other deposits of the first group, it is easy to notice that the enclosing rocks (or rocks lying close to the ore bodies), irrespective of their composition, undergo low- and medium-temperature propylitization.

In the pre-ore stage of hydrothermal alterations, within the zone of the same faults with which the earlier propylitization is associated, veinlike bodies of quartz-sericite and quartz metasomatites develop over shorter intervals. The sharp difference in the forms of manifestation of metamorphism, its intensity, and the structural discordance between the outlines of the metasomatites and the zonality of the first stage indicate the presence of a new stage of alteration. Unlike the first, the altered rocks of the second stage are formed with a substantial introduction of silica and potassium, which is unambiguously established by analytical and petrographic methods. The metasomatites have one and the same, in general, composition regardless of the rocks in which they develop. The composition of the wall rocks is reflected only within narrow peripheral zones, affecting the features of relict areas.

Alterations associated with the ore stage proper are of low intensity and are manifested in disseminated adularization, sericitization, silicification, and ore mineralization of the near-selvage parts of the gold-bearing veins. The superposed, staged character of the mineralization and of the related near-ore alterations is established analogously to the preceding stage by the same criteria.

It should be noted that between the time of formation of the rocks of the pre-ore stage and the final steps of the ore stage there occurred the intrusion of granite-porphyry dikes, whose connection with the complex of pre-ore subintrusions is not entirely clear.

At the remaining deposits of the group under consideration, a picture similar to that described is observed. In the pre-ore stage, the alterations have a propylitic character regardless of the composition of the rocks being altered; in the pre-ore stage, quartz-sericite and quartz metasomatites are formed in all rocks; and in the ore stage proper there is weak alteration of the selvages, with siliceous-potassic metasomatism and deposition of ore minerals.

Of the second group, we have studied in greatest detail the Belaya Gora deposit, the hydrothermal metamorphism of which was partly considered by N. I. Nakovnik [3] and, more recently, by T. A. Yuga [6]. The deposit is represented by a number of small quartz-veinlet auriferous stockworks occurring in rocks of an Oligocene volcanic cone, composed of trachytic lavas and tuffs of mixed composition, as well as of a subvolcanic trachytic neck confined to the volcanic vent. The paleovolcano rocks cut through a cover of Eocene basalts and andesite-basalts.

In the pre-ore stage, the trachytic rocks of the paleovolcano underwent moderate acid leaching with the formation of hydromica-bearing secondary quartzites¹. The basaltic rocks adjacent to the deposit underwent, at this stage, low-temperature propylitization of moderate intensity. The pre-ore hydrothermal stage is separated from the pre-ore stage by a temporary resumption of eruptive activity, with the formation of magmatic and explosive breccias in subsidiary craters and fractures. The breccia fragments are represented by rocks altered in the pre-ore stage.

In the pre-ore stage, quartz–hydromica, quartz–dickite, and monoquartz metasomatites develop both in previously altered and in fresh trachytic rocks; in individual areas these have a zonal relationship to one another that in no way conforms to the plan of distribution of the rocks altered in the pre-ore stage. In contrast to the secondary quartzites of the first stage, the metasomatites, judging from analyses and microscopic data, form with the introduction of silica and with a greater or lesser local introduction of potassium, possibly owing to its redistribution among facies zones. The nature of the original rocks is not reflected in the composition of the metasomatites, and the metasomatites developed after basalts differ only in their characteristic relict basaltic structure.

The strictly wall-rock alterations of the ore stage have the same character as in the first group of deposits considered above. The mineralization and the alterations accompanying it are again superimposed in character, which is clearly established by their confinement to areas of altered rocks of different stages and phases of hydrothermal metamorphism.

For the remaining deposits of the second group we do not have detailed materials; however, the field observations and literature data available to us do not contradict the proposed scheme.

As a characterization of the gold content of hydrothermally altered rocks, we shall note that commercial concentrations of gold are formed only in connection with the deposition of productive mineral associations of the ore stage proper. At the same time, the ore bodies are composed to a considerable degree of rocks of the pre-ore stage of metamorphism. At the same time, the altered rocks themselves, judging from secondary halos of gold in the areas of their development, are characterized by an elevated gold content in comparison with the local clarkes of gold in unaltered rocks—altogether by approximately one order of magnitude.

The materials presented make it possible to draw the following conclusions:

1. Hydrothermally altered rocks in ore fields with Meso-Cenozoic near-surface gold and gold–silver deposits within the volcanogenic belts of the East of the USSR are formed in the course of prolonged hydrothermal activity,

which begins even before the cessation of the eruptive activity of paleovolcanoes or the intrusion of subvolcanic bodies and dikes of the pre-ore magmatic complex, and ends after the cessation of the magmatic activity proper of the magmatic centers.

1. The hydrothermal process is stadial in character. The metamorphism of the individual stages differs in the form and intensity of manifestation, in chemistry and in the nature of the process (leaching, regrouping of elements, metasomatism), and to varying degrees in the structural pattern of its manifestation. One may distinguish a pre-ore, an early ore, and an ore stage proper of metamorphism.

2. The pre-ore stage is manifested over comparatively broad areas, the outlines of which are not entirely clearly determined by the structural elements of the ore fields. At this stage, different chemistries of the metamorphosing solutions are manifested in the two distinguished groups of deposits. The manifestations of magmatism in the form of pre-ore intrusions of intermediate composition (the first group) are evidently associated with weakly alkaline hydrotherms that produce propylitization both in intermediate and in acidic rocks. The manifestations of magmatism in the form of acidic and alkaline volcanics (the second group) are evidently associated with weakly acidic hydrotherms, which alter acidic and alkaline rocks into various secondary quartzites, and basic and intermediate rocks, again, into propylites. The latter is evidently connected with neutralization and alkalization of the hydrotherms in the environment of these rocks.

3. The early ore stage at deposits of both groups is manifested in the same way: in the formation of continuous, massive metasomatites, the distribution of which is distinctly subordinate to a definite structural pattern. Proceeding from generally accepted concepts, it may be considered that the solutions at this stage were alkaline in character, were distinguished by a high concentration of silica, and contained potassium. The composition of the metasomatites is not related to the composition of the rocks being replaced.

4. The ore stage proper at deposits of both groups is manifested in near-vein silicification, sericitization, adularization, and disseminated sulfidization. The scale of hydrothermal activity in general, and of hydrothermal alterations, is sharply reduced. The alkaline character of the solutions at this stage is evidently preserved. The structural pattern of manifestation of the ore stage proper does not always inherit the preceding one, and in this case its superimposed character is clearly expressed.

5. Hydrothermally altered rocks bearing poor gold mineralization over a broad area are a prospecting criterion for identifying ore fields. The most favorable criterion is the early ore metasomatites. Under especially favorable conditions, according to the zoning of the metasomatites, one may guide the search for ore bodies.

REFERENCES

1. Vlasov G. M. Conditions of formation of gold-silver deposits of the propylite formation and their relation to intrusive and effusive rocks. In: Ore-bearing Capacity of Volcanogenic Formations, “Nedra,” 1965.

2. Eremin R. A., Naiborodin V. I., Sidorov A. A. Hydrothermal metamorphism in the gold-ore fields of the Okhotsk-Chukotka volcanogenic belt. Transactions of SVKNII, issue 30, Magadan, 1967.

3. Nakovnik N. I., Nikitin D. B. New data on the mineralogy of gold-bearing rocks of the Belaya Gora district and secondary quartzites of the Far East. Soviet Geology, No. 3, 1944.

4. Nakovnik N. I. Vertical zonality of the products of postmagmatic metamorphism and the place in it of the formation of secondary quartzites and propylites. Proceedings of the All-Union Mineralogical Society, part 92, issue 4, series 2, 1963.

1. Plyashkevich, L. N.; Sidorov, A. A. Preliminary information on gold–silver mineralization in the northern part of the Okhotsk–Chukotka volcanic belt. Materials on the Geology and Minerals of the Northeast of the USSR, issue 16, Magadan, 1963.

2. Yugai, T. A. Hydrothermal metamorphism and gold mineralization at the Belaya Gora deposit. Geology and Geophysics, No. 5, 1967. Nauka, Siberian Branch.

1. The discussion of the first stage is somewhat simplified: we do not touch here on the question of early potassium alteration. ↩