FEATURES OF GOLD MINERALIZATION IN QUARTZ-VEIN ZONES

(A Case Study of One of the Gold-Ore Districts of the Yenisei Ridge)

A. M. Khazagarov (KTU)

In a number of districts of the Yenisei Ridge and the Altai-Sayan region, the principal type of gold mineralization consists of quartz-vein formations, morphologically represented by small, scattered bodies—lenses, veins, and veinlets. Among them, relatively large commercial deposits occur rarely. At the same time, under appropriate structural and lithological conditions these bodies are grouped into large vein zones. In the Yenisei Ridge one may encounter vein zones traceable along strike for up to 10 km, with widths of up to 100–150 m. Some vein zones with an increased concentration of gold correspond to commercial deposits of medium or large scale, such as Sovetskoye, Eldorado, and others. Therefore, vein zones are one of the promising types of gold-ore deposits, and their study at present represents an important task.

The principal gold-bearing areas of the North Yenisei district are concentrated along the crest part and, partly, the northeastern limb of the central anticlinorium of the Yenisei Ridge. According to L. V. Li and V. A. Kutsaenko, the axial part of this regional structure spatially coincides with a zone of deep-seated faulting, which is marked by a chain of linearly elongated bodies of Upper Proterozoic granitoid intrusions and the gold-ore areas associated with them. In the erosional section, the deep-seated fault is traced by the following features:

a) development of intense schistosity, fracturing, and small longitudinal ruptures with a general strike of 300–310°;

b) an en echelon arrangement of intensely dislocated segments, displaced relative to one another toward the northeast. As a result, the general direction of the deep-seated fault zone is 345–360°;

c) deep metamorphism of rocks within the zone, characterized by the development of biotite, tourmaline, and garnet in quartz-chlorite, quartz-sericite, and mica schists, and of graphite in phyllites;

d) an exceptionally broad development of hydrothermal quartz-vein formations carrying sulfide and gold mineralization;

d) confinement to a zone of superposed depressions filled with weakly dislocated terrigenous-carbonate deposits of the Lower Cambrian.

All these features, characterizing a long-developing structure, are inherent in a deep-seated fracture of the concealed type. According to I. N. Tomson (1965), such fractures of the basement within the upper structural level often break up into a series of individual, frequently en echelon zones of intense cleavage and fracturing. The latter, owing to their increased permeability, localize quartz-vein mineralization. Thus, the high concentration of mineralization, the morphology of the ore bodies, and also the large scale of the zones are determined by the deep-seated nature of the ore-controlling structures.

The weakened zone described above, within which the principal fields of the North Yenisei district are concentrated, is distinguished as the Enashiminskaya ore zone.

In its southern part lies the Eldorado ore field, and in the northern part—the Sovetskoye field. The former is situated in the crest part of the anticlinorium and is confined to a belt of quartz-chlorite, quartz-sericite, and biotite schists of the Gorbilok and Kordin formations, which make up the lower horizons of the Sukhoi Pit series. In these schists, biotite, staurolite, and garnet (almandine) are widespread; they indicate a rather high degree of metamorphism, corresponding to the boundary between the greenschist and amphibolite facies.

The Sovetskoye ore field is situated within a second-order fold that complicates the northeastern limb of the anticlinorium, and is composed of phyllites and phyllitized carbonaceous schists, locally containing graphite. In the stratigraphic section of the Sukhoi Pit series, this formation occupies a higher position than the Gorbilok formation.

Thus, the ore fields mentioned differ in their stratigraphic and structural position, as well as in the composition of the ore-bearing strata.

Among the vein zones, the Sovetsky and Eldorado types are distinguished, corresponding to the deposits of the same names. The Sovetskoye deposit has been studied in considerable detail by N. V. Petrovskaya, P. S. Bernshtein (1954, 1963), and V. A. Bogdanovich (1963), while the Eldorado deposit was studied by N. V. Petrovskaya, S. S. Ilyenko, A. N. Averchenko, T. S. Gulimova, and others.

The Sovetsky type of vein zones is characterized predominantly by large bodies of quartz ores, the thickness of which not infrequently reaches several tens of meters, and by their very complex form. Small veins and veinlets of quartz are of subordinate importance here. Large accumulations of quartz ore are usually confined to places of flexures, sharp complications of small folds, and local zones of crushing and crumpling of rocks. Small veins are subordinate to elements of schistosity and bedding joints. This type is characterized by a high concentration of quartz mineralization. The ore-content coefficient (OC), expressing the ratio of the total thickness of veins and veinlets to the width of the vein zone, reaches 20–50%, and in the blocks being mined, 40–60%.

The ore bodies consist of 90–95% vein quartz. The most widespread is white, light-gray, opaque, coarse-grained quartz. Among the ore minerals, pyrite is encountered most often, and considerably more rarely arsenopyrite, galena, sphalerite, bismuthinite, and others.

According to P. S. Bernshtein and N. V. Petrovskaya (1954), the formation of the Sovetskoye deposit took place under conditions of medium tempera-

tour. The distribution of gold in the ores, according to Yu. A. Kudryavtsev and V. V. Bogatsky (1968), is characterized by extremely high variability. In this case the coefficient of variation reaches 500–800%. The gold content in the ores varies from 0.1 to 14042 conventional units; the maximum values exceed the average contents of the zones by hundreds, and sometimes thousands, of times. The distribution curves of the contents are sharply left-asymmetric and are characterized by positive skewness of +16 to +53 and large excesses. The predominant part of the ores (about 90%) has a content below the average. A significant part of the metal (about 50%) is enclosed in rich ores, for which high contents are typical, reaching values of more than 1924 conventional units; at the same time, the samples characterizing these ores make up less than one percent of their total number.

For each zone there is a small group of “hurricane” values, which account for a large differential reserve of metal. Hurricane values exert a decisive influence on the magnitude of the average metal content. In one of the zones the average is 14.52 units, whereas without the hurricane samples (192) it decreases to 6.95, i.e., by more than 2 times. Hurricane values constitute 0.28–1.03% of the total number of samples. However, they account for 25–52% of the total metal reserve (Fig. 1).

The Eldorado type of vein zones is characterized by small, most often lenticular concordant veins and veinlets confined to cleavage cracks, bedding cleavage, and small longitudinal faults. Vein zones of this type are distinguished by an en echelon arrangement and greater length, sometimes up to ten kilometers. The ore-bearing coefficient varies from 10–15% to 30–35%, and in individual cases reaches 40–45%. The vein quartz is predominantly gray, dark gray, and bluish, medium- and fine-grained, translucent. Coarse-grained white quartz mainly composes rare large lenticular bodies. Among the ore minerals, arsenopyrite predominates; pyrite, pyrrhotite, molybdenite, and chalcopyrite occur much more rarely. The main feature of the internal structure of the vein zones is the abundance of small veins and veinlets densely penetrating the mass of biotite and sericite schists. On average, per 10 m of zone thickness there are 6–7, and in places of concentration 8–11, veins 5–10 cm thick, as well as a large number of small threadlike veinlets. Such densely veined intervals are often replaced by less saturated ones. The veins usually have small thicknesses. Thus, in vein zone No. 1 their average thickness is 0.24 m. Veins up to 0.5 m thick and less constitute 76%, and those more than 1 m thick constitute 4% (Fig. 2). Analysis of the distribution of average gold contents by thickness intervals (Fig. 3) shows that the highest contents correspond to small vein thicknesses—from 0.1 to 1 m. With an increase in vein thickness, the gold content decreases noticeably, and large ore bodies are significantly poorer in gold. However, in zones of the Sovetsky type the distribution of gold contents in the ores does not depend on vein thickness. As can be seen from Fig. 3, the distribution of gold contents here is characterized by a comparatively even line, although in intervals of small thicknesses there is a noticeable slight decrease.

As for differential reserves, their distribution depends mainly on the frequency of occurrence and the thickness of the veins themselves. From Fig. 2 it is evident that the highest reserve values correspond to thickness intervals from 0.5 to 1 m (21.4%) and from 1.0 to 1.5 m

Fig. 1. Distribution of gold grades and differential gold reserves in different types of vein zones

1. Distribution of gold grades in a vein zone of the Sovetsky type.
2. Same — Eldorado type, at deposit A.
3. Same — at deposit B.
4. Distribution of differential reserves in a vein zone of the Sovetsky type.
5. Same — Eldorado type.

Average grade of classes. Logarithmic scale.

Fig. 2. Distribution of ore veins by thickness in vein zones of the Eldorado type.

Distribution of ore veins by thickness in zones of the Eldorado type.

Thickness classes are equal to 0.05 units for values up to 2.0;
1.0 unit for values above 2.

Vertical axis: Frequency of samples by thickness classes, %.

Horizontal axis: vein thickness.

Marked values on the horizontal axis: 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0.

Annotation: maximum.

Legend

— Distribution of average contents in the vein zone of the Eldorado type
— Distribution of average contents in the vein zone of the Sovetsky type
▧ Distribution of differential reserves by thickness classes (ore classes 0.5 and 1.0 g/t) in zones of the Eldorado type
▨ Percentage ratio of differential reserves in zones of the Eldorado type

Vertical axis: Average contents and differential reserves by thickness classes, %

Horizontal axis: Thickness

Bar annotations: 12.1; 21.4; 27.5; 13.7; 9.1; 6.2

Fig. 3. Distribution of ore veins by thickness in vein zones of the Eldorado type.

(27.5%). Small veins—the most widespread and richest—account for only 12.1%; reserves in the less frequently encountered and relatively gold-poor veins of increased thickness amount to 9.1–16.2%. Observations show that in vein zones of this type the enclosing schists are more auriferous than in zones of the Sovetskoye type. Auriferous schists usually occur in areas where veins are concentrated. Gold in the schists occurs as fine grains in schistosity fractures; in addition, it is associated with sulfide disseminations (arsenopyrite).

The distribution of gold in vein zones of the Eldorado type is also uneven, as at the Sovetskoye deposit (Fig. 1). The difference is that the maximum of the curve (mode) for the Eldorado type is shifted to the right, while the maximum of differential reserves is shifted to the left. This indicates a comparatively lower variability in the distribution of gold in zones of this type. It follows from this that differences in the morphology of ore bodies and in the material composition of the ores do not have a substantial effect on the distribution of gold in the ores of the vein zones of the types considered.

Conclusion

1. Quartz-vein zones in the Yenisei Ridge, with which a significant mass of gold is associated, develop along regional zones of increased fracturing, cleavage, and small ruptural disturbances (a concealed deep fault).

2. Most vein zones among ancient metamorphic sequences are characterized by a quartz low-sulfide type of mineralization with free gold, distinguished by an extremely uneven distribution of gold in the ores. A relatively small quantity of ores with “hurricane” gold contents contains a large differential reserve of the metal. Hurricane values have a decisive influence on the magnitude of the average.

3. Such an uneven distribution of gold contents in ores often makes it difficult to assess the industrial significance of vein zones. Therefore, in each individual case, a careful analysis of sampling data and verification by bulk sampling are necessary.

4. In industrial terms, vein zones of the Sovetskoye type are more suitable as objects for selective underground mining. Owing to the small sizes of the ore veins, their increased auriferousness, and also the presence of gold in the enclosing schists, vein zones of the Eldorado type are more suitable for mass open-pit mining. In connection with the expansion of possibilities for bulk ore mining, vein zones of this type acquire important practical significance.

References

1. V. V. Bogatsky, Yu. A. Kudryavtsev, S. N. Tobin. Evaluation of the average value of gold contents for sharply asymmetric distributions. Proceedings of the conference “Experience in the Application of Mathematical Methods and Computer Technology in the National Economy of Krasnoyarsk Krai,” Krasnoyarsk, 1966.

2. P. S. Bernshtein, N. V. Petrovskaya. The Sovetskoye gold-ore deposit (Yenisei Ridge). Geology of the Principal Gold-Ore Deposits of the USSR, vol. VI, Moscow, 1954.

3. L. V. Li, V. A. Kutsayenko. Some results of studying regularities in the distribution of gold-ore fields in the Yenisei Ridge. Report at the Second Interuniversity Conference on the Gold Content of Siberia in Tomsk, 1968.

4. I. N. Tomson. Deep faults, their ore-controlling significance, and methods of studying them. In: Lithological and Structural Factors in the Distribution of Ore Deposition in Ore Districts. Nedra Publishing House, 1964.