ON THE QUESTION OF SECONDARY ZONATION IN GOLD-ORE VEINS OF THE MARIINSKAYA TAIGA (Kuznetsk Alatau)

A. D. MIKOV

(Presented by the scientific seminar of the group of departments of mineralogy and crystallography, petrography, geology, and prospecting for mineral deposits)

The question of the behavior of gold in the oxidation zone of gold-ore deposits and the problem of hypergene enrichment in gold have been discussed in the domestic and foreign literature for several decades. Whereas for such nonferrous metals as copper the processes of hypergene migration and their role in the formation of zones of secondary enrichment have been well studied, for gold these questions are far from resolved. Academician S. S. Smirnov [7] wrote: “The questions of the formation of the zone of secondary sulfide enrichment by copper have been excellently developed, although for gold and silver we still do not have a satisfactory solution to analogous questions.”

In recent years, a number of works in the domestic literature have been devoted to the behavior of gold in the hypergenesis zone [1, 3, 4, 6, 8, 9].

For gold-sulfide deposits at the present time there is considerable material confirming the migration of gold in the oxidation zone and indicating the presence of secondary horizons enriched in it. For quartz-sulfide deposits this question still remains unresolved [1, 4].

The present article gives the results of investigations on the question of secondary zonation in gold-bearing quartz-sulfide veins of deposits of the Mariinskaya taiga.

With respect to a number of deposits of this territory, certain data have long been known that indicate the presence of rich gold in the lower part of the oxidation zone of ore veins. V. K. Monich [5], at the Berikul’ deposit, noted the presence of a subzone rich in gold and connected in its origin, probably, with hypergene processes.

Investigations carried out by us at the Tsentral’nyi, Berikul’, and Komsomol’skii deposits showed that all ore veins are characterized by an increased gold content in the oxidation zone in comparison with primary ores. In the distribution of gold in the oxidation zone a definite regularity is observed, consisting in the confinement of enriched sections, as a rule, to its lower part. Such confinement makes it possible to distinguish a definite zone of secondary gold enrichment in the veins of the deposits studied.

Brief Geological Characterization of the Ore Veins

The ore veins of the Tsentral’noe, Berikul’skoe, and Komsomol’skoe deposits of the Mariinskaya taiga are typical fissure formations. At the deposits they are confined to two or three definite systems of fractures [2]. Thus, at the Tsentral’noe deposit there are veins of northeastern strike (the Tsentral’naya and Dmitrievskaya veins), east–northeastern, almost latitudinal strike (the Kavkazskaya and Kazanskaya veins), and almost meridional strike (the Severnaya and Suvorovskaya veins). The dip angle of the veins ranges from 40 to 75°. At the Berikul’skoe deposit the ore veins are confined to two principal systems: steeply dipping veins of southwestern strike (the Rossiiskaya and Prokop’evskaya veins), with dip angles of 40–80°, and gently dipping veins of northeastern, eastern, and northwestern strike (the Krutobaikal’skaya and Chetvertaya veins), with dip angles of 5–40°.

In general, for the ore veins of the deposits described, with an overall persistence along strike and dip, very characteristic features are the frequent alternation of swellings and pinch-outs that replace one another over short distances. Splitting of the veins into series of independent veinlets is also very characteristic.

The ore veins of the Mariinskaya taiga deposits belong to the quartz–sulfide vein type. The most widespread textures in the commercial ores are banded, spotted, breccia-like, and massive. The principal vein mineral is quartz. Calcite and ankerite are of subordinate importance. Their amount in the ore veins does not exceed 10–15%. Among other non-ore minerals, muscovite, chlorite, epidote, zoisite, apatite, and zircon may be noted.

The veins of the deposits described are characterized by a low content of sulfides and do not differ by any great variety of them. The predominant mineral is pyrite. In a number of veins arsenopyrite is almost not inferior to it in amount. In smaller quantities, but in almost all veins, galena, sphalerite, and chalcopyrite occur. In some veins there are also pyrrhotite, pale ore, bismuthinite, niccolite, gersdorffite, and other ore minerals.

Among the ore veins studied one may distinguish veins with a low sulfide content, up to 3–5% (the Tumashevskaya vein of the Tsentral’noe deposit), and veins with a moderate sulfide content of 10–15% (the Prokop’evskaya and Tatarskaya-2 veins of the Berikul’skoe deposit). The sulfides in the veins are distributed unevenly. Along with barren intervals there are intervals with sulfide contents of up to 40–60%.

Native gold is closely associated with sulfides and is distributed extremely unevenly within the plane of the veins. Visible gold is rarely encountered.

Change in Gold Content with Depth and Secondary Zonation of the Ore Veins

The opening of ore veins at the gold mines of the Mariinskaya taiga is carried out by adits and crosscuts along working levels at vertical intervals of 20–40 m. Samples for chemical analysis are taken from all mine workings. Accounting for changes in gold content, according to data from face chemical sampling of preparatory workings (adits, crosscuts, inclines, raises) or stoping workings (by floors), is the principal method whose materials are used in this article. We recorded the change in gold content with depth for individual veins along their entire length or within certain—

geological blocks—stable ore shoots. The calculation of the average grade and thickness was carried out by horizons and by layers of 5–15 m according to the levels of the stoping workings. The average distance between adjacent face samples along crosscuts and stoping workings is usually 2.0 m. The average metal content was calculated by the arithmetic mean method for a horizon or layer (hereafter we regard these concepts as equivalent), using from 15 to 150 or more individual face samples. In all, for individual veins the number of samples taken into account ranged from 150 to 5000–9000. The length of the veins in the deposits is 100–500 m, more rarely 1000–1500 m. Such a length considerably exceeds the horizontal length of the ore shoots in the veins, and the increased grade in them does not exert any appreciable influence on the value of the average sample for a horizon. For convenience in comparing average grades for different veins and deposits, we calculated average gold grades for the veins in meter-grams. The data obtained on average grades and thicknesses for a number of veins of the Tsentralny, Berikulsky, and Komsomolsky deposits are shown in the form of graphs of the variation of grade and thickness with depth (Figs. 1, 2, 3, 4).

Fig. 1. Graph of the variation of average grade and thickness along veins of the Tsentralny area of the Tsentralny deposit:
a) Shkolnaya vein; b) Promezhutochnaya vein; c) Tsentralnaya vein.

To determine the degree of concentration of the metal in one or another part of an ore body, in the practice of mine geology the index of intensity of metallic mineralization is used [1]. By this is meant the ratio of the average metal content in any part of the ore body to the average bulk content of this metal throughout the entire ore body. We determined the index of intensity of the horizon of gold enrichment as the ratio of the average gold content over the vertical interval—the zone of greatest gold enrichment—to the average bulk content according to face-sampling data for the entire exposed part of the vein.

In calculating the average gold content for a vein, we gave special consideration to rich samples. As the lower limit of a rich sample we took a content 4–5 times greater than the average metal content for the deposit. Accounting for rich samples by vein horizons made it possible to reveal the regularity, noted earlier by M. N. Albov [1], that hori-

the zone of secondary gold enrichment corresponds to the maximum yield of rich samples. Their number was determined as a percentage of the total number of samples from one or another level of the vein.

The oxidation zone at the deposits of the Mariinskaya taiga is not very clearly expressed. The depth of its development is not great. At the Tsentralnoe and Berikul deposits the oxidation zone is, on average, limited to a depth of 35–40 m. Although in a number of veins of the Tsentralnoe deposit it descends to 70–80 m (the Shkolnaya and Promezhutochnaya veins). Along the Potereinaya and Altaiskaya veins it is considerably higher—31–38 m, respectively, although the lower level is less regular and, in the latter, at the Sayanskaya mine, descends to 70 m. As a general rule, the lower boundary of the oxidation zone repeats the relief, smoothing it somewhat, which is manifested especially clearly in the veins of the Berikul deposit.

Along certain of the largest tectonic disturbances, oxidized ores occur at a depth of 100 m and more. The Komsomolskaya vein of the deposit of the same name is an original exception, since it is confined to a tectonic zone along which numerous post-ore movements took place, and oxidized ores in it occur throughout the entire exposed depth down to the 420 m level.

Within the oxidation zone at the deposits described, two subzones may be distinguished: the upper one—the subzone of intensive oxidation and leaching of the ores; and the lower one—the subzone of secondary gold enrichment.

The upper subzone is usually represented by brownish-red ocherous quartz with numerous cavities from the leaching of sulfides. Often the quartz is crushed and consists of small fragments. In low-sulfide veins, in ocherous-clayey loose weathered host rocks, there is weakly ocherous vesicular quartz with a lattice of pyrite impressions, sometimes with small nests of lead ocher and coatings of scorodite. Visible gold is encountered very rarely (the Dmitrievskaya vein of the Tsentralnoe deposit). According to stope sampling data, the gold content in this subzone is somewhat higher than in the primary ores.

The lower subzone is characterized by intensive accumulation of limonite. It is usually represented by ochers of yellowish or reddish-brown color, confined to cracks in the vein or to their selvages. Quartz and the host rocks in such sections are strongly ocherized. In most veins, within the composition of the subzone described there is no sharp boundary between oxidized and primary ores (the Magistralnaya and Prokopevskaya veins and others of the Berikul deposit). In some veins this subzone is confined to completely oxidized ores, in which, together with transported limonites, residual limonites are also present. The lower boundary of the oxidation zone in these veins is sharp (the Shkolnaya and Kavkazskaya veins of the Tsentralnoe deposit). The subzone described is noticeably enriched in gold. Microscopic studies have established a close association of gold in the subzone with transported limonites.

The practice of mining gold-bearing vein quartz-sulfide deposits of the Mariinskaya taiga also undoubtedly testifies to an increased concentration of gold in the oxidation zone. Our comparison of the average metal contents in the oxidation zone and in the primary ores for 14 veins with identical sulfide mineralization from the Tsentralnoe and Berikul deposits confirmed this fact. The obtained ratio of the average contents in the oxidation zone to the average content in the primary ores showed that it varies within the limits 1.25–4.23. From the obtained indicators it is evident that the

the gold content in the oxidation zone is considerably higher than in the primary ores. Moreover, the highest values of the indicated ratio, 2.10–4.23, are characteristic of veins with an elevated sulfide content (the Uralskaya veins of the Tsentralnoye deposit; Tatarskaya-2, Iksovaya, and Troitskaya of the Berikulskoye deposit).

At the deposits described, more than 20 quartz-sulfide veins were studied. In all veins an enriched gold belt was established, tending toward the lower part of the oxidation zone (the subzone of gold enrichment). Below is a brief characterization of the gold-enrichment belt for a number of veins of the Tsentralnoye, Berikulskoye, and Komsomolskoye deposits of the Mariinskaya taiga.

Characterization of the Gold-Enrichment Belt

At the Tsentralnoye deposit two areas are distinguished: Tsentralny proper and Yubileiny. In the Tsentralny area the following veins were studied: Shkolnaya, Promezhutochnaya, Tsentralnaya, Kazanskaya, and others.

The data from analysis of stope sampling of the Shkolnaya, Promezhutochnaya, and Tsentralnaya veins are given in Fig. 1 as graphs of the change in average content and thickness along these veins with depth. In the graphs presented, two maxima of high gold content are very clearly distinguished at depths of 20–25 m and 58–65 m from the surface. The first maximum corresponds to the gold-enrichment belt in the interval 15–40 m. The calculated indices of the intensity of gold enrichment for the first belt are low and are, respectively, for the Shkolnaya, Promezhutochnaya, and Tsentralnaya veins, 1.16; 1.20; and 1.44. The second gold-enrichment belt in these veins is expressed more distinctly in the interval 54–84 m. This belt is also characterized by higher intensity indices, equal to 2.32; 1.73; and 2.36, respectively. This belt is also characterized by an increased content of rich samples.

For the Kazanskaya vein, data from stope sampling have been preserved only from the 53 m level. At a depth of 55–75 m, as in the other veins of this area, a gold-enrichment belt is distinguished, with an intensity index of 1.73.

In the Yubileiny area we studied the Uralskaya, Petrovskaya, Kavkazskaya, Dmitrievskaya, Tumashevskaya, and other veins.

In the Uralskaya, Dmitrievskaya, and Petrovskaya veins, a well-expressed gold-enrichment belt is distinguished in the interval 12–35 m (Fig. 2 a, v). The intensity indices for the veins described are respectively equal to 2.83; 1.87; and 1.58. The indicated gold-enrichment belt is also readily distinguished by the content of rich samples.

In the Tumashevskaya vein the gold-enrichment belt is weakly manifested (Fig. 2, g) in the interval 8–20 m, with an intensity index of 2.09.

At the Berikulskoye deposit we studied the Rossiiskaya, Prokopyevskaya, Troitskaya, Krasnoarmeiskaya, Tatarskaya-2, and other veins. For the first three of these, graphs are presented showing the change in thickness and content with depth (Fig. 3). In analyzing the data from stope sampling, the presence of a gold-enrichment belt was established in each vein. In the Rossiiskaya and Troitskaya veins (Fig. 3 a, v) this belt is located in the interval 5–30 m from the surface. The indices of the intensity of gold enrichment in these veins are relatively high and are respectively 2.58 and 1.86. In the Prokopyevskaya (Fig. 3, b) and Krasnoarmeiskaya veins, the gold-enrichment belt is located in the interval 15–35 m, with intensity indices close in value, 1.46 and 1.50. For the Iksovaya and Tatarskaya-2 veins this belt

Fig. 2. Graph of changes in the average grade and thickness along veins of the Central area of the Yubileynoye deposit:
a) Uralskaya vein; b) Kavkazskaya vein; c) Dmitrievskaya vein; d) Tumashevskaya vein.

Fig. 3. Graph of changes in the average grade and thickness along veins of the Berikulskoye deposit:
a) Rossiyskaya vein; b) Prokopyevskaya vein; c) Troitskaya vein.

is located somewhat lower, in the interval 28–42 m. The intensity indices of gold enrichment for the indicated veins are close and equal, respectively, to 1.77 and 1.81. In the Rossiiskaya vein the belt of gold enrichment is also easily distinguished by the number of rich samples.

At the Komsomolskoye deposit we studied the Komsomolskaya, Parallelnaya, and Bolotnaya veins. In the Komsomolskaya vein three clearly expressed maxima were identified (Fig. 4), corresponding to belts of gold enrichment in the intervals 3–7 m, 25–50 m, and 84–92 m from the ground surface. All the indicated belts are characterized by high indices of the intensity of gold enrichment, equal, respectively, to 2.70, 3.44, and 2.57. Such high intensity indices are explained by the low average metal content in the vein because of the presence of considerable pinches.

Fig. 4. Graph of the change in average content and thickness along the Komsomolskaya vein of the Komsomolskoye deposit.

For the Bolotnaya and Parallelnaya veins, two belts of gold enrichment are distinguished. The first is in the interval 2–8 m, and the second in 18–24 m and 25–32 m, respectively. The intensity indices for the Bolotnaya and Parallelnaya veins for the first belt are 1.28 and 1.81; for the second, somewhat lower: 1.05 and 1.42. The belts distinguished in all the veins of the deposit are also marked by the content of rich samples.

Thus, in all the veins studied at the Tsentralnoye, Berikulskoye, and Komsomolskoye deposits we identified the presence of a belt of gold enrichment. At the Berikulskoye deposit and the Yubileinyi sector of the Tsentralnoye deposit, only one belt of gold enrichment was established, whereas in the veins of the Tsentralny sector of the Tsentralnoye deposit and of the Komsomolskoye deposit, two belts of gold enrichment were distinguished. The indicated belts are situated within the oxidation zone and tend toward its lower part.

Both belts, as noted above, are in our opinion due in origin to supergene processes. The first belt is developed in all veins and is confined to the zone of discharge of groundwater. The penetration of groundwater along steeply dipping longitudinal fractures in a number of veins led to the formation of a second belt of gold enrichment, which is sometimes very pronounced (the Shkolnaya and Promezhutochnaya veins of the Tsentralnoye deposit).

The investigations presented also established that the dip angle of ore veins has an undoubted influence on the position and development of the belt of secondary gold enrichment. The studied veins of the Mariinskaya taiga deposits with identical sulfide mineralization were divided into three groups according to their dip angles (Table 1).

Table 1

Distribution of ore veins of the Mariinskaya taiga deposits
depending on their dip angle

From the table presented it is evident that the zone of secondary gold enrichment is most clearly manifested in veins with medium dip angles of 40—60°. This confirms the conclusion drawn by M. N. Albov [1] for the veins of the Berezovskoe deposit (Urals).

Conclusions

1. Most of the studied ore veins of the Central, Berikul, and Komsomol deposits of the Mariinskaya taiga are characterized by a moderate (10—15%) content of sulfides in the ore. This is considerably lower than their content in the veins of the Urals studied by M. N. Albov [1], where it reaches 40% and even 50—70% (the Zelenaya vein and others of the Kochkar ore field).

2. The oxidation zone in the deposits we studied is not as sharply expressed as in the gold-bearing quartz-sulfide deposits of the Urals. Its depth for most ore veins of the Mariinskaya taiga is 30—40 m, and only at the Central deposit (the Shkolnaya, Promezhutochnaya, and other veins) does it reach 75—85 m. Only the Komsomolskaya vein of the deposit of the same name is oxidized to a depth of 240 m. Within the oxidation zone two subzones have been distinguished: an upper subzone of intense oxidation and leaching of ores; and a lower subzone, noticeably enriched in gold.

3. Despite the comparatively low content of sulfides in the ore and the weak development of the oxidation zone, the zone of secondary gold enrichment is manifested quite clearly in all veins. This zone of gold enrichment in the veins varies within wide limits, from 3—5 to 35—40 m, more rarely to 75—85 m, from the ground surface. It has been established that the most favorable for the manifestation of secondary gold enrichment are veins with medium dip angles of 40—60°. The average gold content in the distinguished zone of gold enrichment in the veins is 1.7 times higher than its average content in the exposed part of the deposits described.

References

1. M. N. Albov. Secondary zonation of gold-ore deposits of the Urals. Moscow, 1960.

2. E. A. Gukovskii. Elements of the geology of the gold-ore deposits of the Mariinskaya taiga. Vestnik Zap.-Sib. GRT, issue 5, 1932.

1. V. M. Kreiter, D. S. Kreiter. Geological evaluation of gold-ore deposits from outcrops. Soviet Geology, No. 17, 1946.

2. V. M. Kreiter, V. V. Aristov, I. S. Voldynsky, A. N. Krestovnikov, V. V. Kuvshinsky. Behavior of gold in the oxidation zone. Moscow, 1958.

3. V. K. Monich. Geological outline of the Berikul gold-mining district. Transactions of the Gold-Prospecting Trust and the NIGRIzoloto Institute, issue 7, 1937.

4. L. V. Razin, I. S. Rozhkov. On the geochemistry of gold in the weathering crust and biosphere of the permafrost province on the Aldan Shield. Transactions of the Yakut Branch of the Siberian Division of the Academy of Sciences of the USSR, Geological Series, issue 16, 1963.

5. S. S. Smirnov. Some general questions in the study of ore deposits. Bulletin of the Academy of Sciences of the USSR, Geological Series, No. 5, 1946.

6. F. V. Chukhov. On the migration of gold in the oxidation zone. Bulletin of the Academy of Sciences of the USSR, Geological Series, No. 4, 1947.

7. E. D. Shlygin, K. M. Mukanov, V. M. Grishin, S. G. Magomedov. On the hypergene concentration of gold at gold-ore deposits of Northern Kazakhstan. Bulletin of the Academy of Sciences of the Kazakh SSR, No. 8, 1963.