On the Lithological and Structural Factors Controlling the Localization of Gold Ores in the Berikul Ore Field (Kuznetsk Alatau)

I. V. Kucherenko

(Presented by Professor A. M. Kuzmin)

A number of works carried out in the 1930s and 1940s were devoted to the problem of the structural conditions of localization of gold mineralization in the Berikul ore field (Gukovskii, 1932; Monich, 1937, 1939; Timofeevskii, 1947; Timofeevskii et al., 1952). Over the last two decades, as a result of intensive exploration and mining, considerable new factual material has been accumulated, relating in particular to the conditions of occurrence of mineralization at deep levels and on the flanks of the deposit; the need to generalize this material has recently become acute. On the other hand, the conclusions of the above-mentioned researchers differ substantially on questions concerning the genetic interpretation of various structural elements, the influence of the latter on the distribution of mineralization, the structural conditions of formation of metasomatic sulfide bodies, and so forth. The foregoing served as the basis for undertaking special investigations carried out by the author in 1965–1968; the materials of these investigations form the basis of the present article.

The territory of the ore field and its environs is composed of Cambrian sedimentary and effusive strata, which a number of researchers combine into two suites: the Lower Cambrian Belokamenka Suite, represented by marbled limestones, and the Middle Cambrian Berikul Suite, within which metamorphosed aphanitic effusive rocks are distinguished, confined mainly to the lower parts of the section, and plagioclase porphyrites with subordinate occurrences of volcanic breccias, volcanic glasses, and certain other varieties. The strata listed compose the southwestern limb of a large synclinal fold of northwestern strike, formed in connection with the Caledonian cycle of tectogenesis. The northeastern limb of the fold is cut off by the Dudet granitoid massif of the Martaigna complex, likewise of Caledonian age; in the western exocontact of this massif, small stocks and dikes of basic and alkaline rocks are known. In the opinion of some researchers, the small bodies mentioned are products of successive intrusive phases of the Devonian Kiya gabbro-syenite complex (Vasiliev, 1966); according to other views (Dovgal, Bognibov, 1965), the basic rocks and some alkaline varieties represent an ancient pre-Martaiga gabbro-syenite complex, while part of the alkaline rocks is connected with Devonian magmatism.

The mineralization of the ore field occurs in the effusive rocks of the Berikul Formation. The ore bodies are represented by a series of subparallel gold-bearing quartz veins, the statistical maximum of whose orientation corresponds to az. dip \(291^\circ \angle 45^\circ\), vein No. 4, which has a sublatitudinal strike with a gentle (up to \(20^\circ\)) dip to the south, and metasomatic sulfide deposits localized at the contact of limestones with effusives.

The leading types of structures determining the emplacement of dikes and ores are differently aged rupture structures that arose after the formation of the Caledonian fold system. Dikes of magmatic rocks, diverse in composition and of different ages, both pre-ore and post-ore, fill predominantly rupture fractures formed as a result of repeated impulses of tensile tectonic stresses. The tectonic processes of the pre-ore and ore stages were the most important for ore formation; as a result of these processes, under conditions of directed tangential compression (az. \(340^\circ\)), the Main Fault, three systems of shear fractures (vein and transverse northwestern structures), and one system of rupture fractures (vein No. 4) were formed. The fractures of each system are the result of repeated impulses of compression during the pre-ore and ore stages, and also partly during the early stage of the post-ore period.

In analyzing the distribution of economic concentrations of gold in the district, their confinement to the western exocontact of the Dudetsky granitoid massif is revealed; this was noted by E. A. Gukovsky (1932). On the other hand, work in recent years (Vasil’ev, 1966) indicates the constant confinement of ore fields also to small bodies of basic composition localized, as noted, along the western exocontact of the Dudetsky massif. At the same time, the rocks enclosing the mineralization differ substantially in different ore fields. Thus, at the Komsomol’skoye deposit, located 18 km from Berikul, the ore bodies occur directly in the basic rocks of one of the stocks; at the Berikul deposit, in effusives of the Berikul Formation in close proximity to stocks of basic rocks; at Novo-Berikul, located 6 km from Berikul, in those and other rocks; and finally, at Gavrilovskoye (12 km from Berikul), predominantly in basic rocks. The differing composition of the rocks enclosing the ore fields testifies to the absence of a clear regional lithological control of mineralization, whereas the linear arrangement of the ore fields must indicate the ore-controlling role of a major rupture structure of north-northwestern strike, to which the emplacement of the basic rocks, older than the mineralization, is also subordinate.

If the problem is considered on a more local scale, i.e., if the distribution of ore-hosting and associated rupture structures is analyzed at the scale of the Berikul ore field, a somewhat different picture emerges. All ore veins pinch out near the contact with limestones in the west, almost without extending into the latter, and in contacts with basic rocks in the east; moreover, in the gabbroids the veins pinch out at distances of the first tens of meters from the contact. Economic veins have not so far been found in the basic rocks or in the limestones, although small veins are known both in the gabbroids of the stocks and even in the granitoids of the Dudetsky massif near the line of continuation of the Main Fault. The data cited indicate that the ore-hosting and associated rupture structures underwent selective development, determined by the combination in this territory

...within relatively plastic limestones and gabbroids, on the one hand, and more brittle effusive rocks, on the other. However, this regularity is not manifested throughout the entire Berikul district, since ore mineralization in other ore fields is known in different rocks. Apparently, the local concentration of ore-bearing and associated fractures is a phenomenon that, other conditions being equal, is determined by a combination of the features of the distribution of pre-ore tectonic stresses in the district—namely, by the concentration and direction of such stresses in individual areas corresponding to ore fields—and by the lithological features of the rocks composing these areas.

It is assumed here that both factors are closely related, since the lithological heterogeneity of the medium cannot but influence the distribution of tectonic stresses. The expression of the release of such concentrated stresses, in particular at Berikul, is the major Main fault with its system of accompanying fractures. In other words, the ore-hosting and ore-controlling elements are tectonic elements that arise as a result of tectonic processes; and the character of their development in a given territory (i.e., their location, orientation in space to a certain degree, etc.) is in a number of cases determined by the lithological features of the rocks composing that territory. In this sense, the effusive rocks of Berikul should be regarded, using the term of N. I. Borodaevsky and P. S. Bernshtein (1967), as localizers of fractures on the scale of the ore field.

A substantial influence on the character of development in the ore field of ore-hosting and transverse shear fractures was exerted by the oriented anisotropy of the medium within the effusive sequence itself, caused by differences in the physico-mechanical properties of pre-ore dikes of magmatic rocks and the enclosing effusive rocks (submeridional and northeastern directions), on the one hand, and by the presence of weakened directions in the effusive rocks corresponding to the strike of the latter (northwestern direction), on the other. These causes, given a favorable pattern of pre-ore and ore deformations, largely determined the regular occurrence of ore veins at the contacts of pre-ore dikes and, in general, the regular orientation of veins in space. The aforementioned pre-ore dikes of magmatic rocks, consequently, were localizers of ore veins.

Special structural conditions were created during the formation of ore-bearing structures at the contact between limestones and effusive rocks. Large segments of the contact of these two media, sharply different in composition and physico-mechanical properties, were subjected to intense crushing chiefly where a system of major rupture fractures approaches the contact (vein No. 4). By the beginning of the activity of the ore-bearing solutions, these contact segments were structurally the most prepared, and precisely this circumstance had a decisive influence on the formation of metasomatic bodies, the configuration of which corresponds to the shape of the crushed zones, while their position in space is determined by the position of the line of intersection of vein No. 4 with the limestones (the principal factor), in combination with features of the limestone contact surface and certain other local factors. The pinching out of vein No. 4 with depth is accompanied by the disappearance of the metasomatic bodies.

The distribution of productive mineral associations and gold in the ore veins is characterized by features indicating the important, but dual, role of the structural factor in the localization of mineralization. Large pre-ore transverse dislocations, of the Main type, screened the mineralization, as is evidenced by the wide distribution of weakly...

ore-bearing zones near these disturbances in vein No. 4 and in a number of “steeply dipping” veins. On the other hand, ore shoots in ore veins are usually confined to places where the latter are intersected by pre-ore and ore transverse disturbances; more often the picture is obscured because of the peculiarities of intramineralization movements, which are affected by the degree to which the veins are disturbed by transverse faults, so that one may speak, in general, of the confinement of ore shoots in veins to the tectonically most disturbed sectors. In gently dipping vein No. 4 the enriched sectors, in rough approximation, have the form of triangles, with their acute angle directed down the dip of the vein and their base confined to the lines of intersection of this vein with the “steeply dipping” veins marking shear fractures. This fact indicates that the latter were channels that conducted ore-bearing solutions into rupture fractures.

In conclusion, the following main points should be emphasized:

1. Lithological and structural factors play an important role in the localization of mineralization in the Berikul ore field.

2. The influence of each of these factors on the localization of mineralization of different scales is not equivalent. The position of ore fields in the district is apparently controlled by rupture structures (hidden basement faults, according to B. D. Vasil’ev), whereas the confinement of ore bodies themselves in the Berikul ore field to particular rocks and to contacts between environments of differing lithological composition already depends on causes of an essentially lithological order, since the lithological anisotropy of the medium exerted a determining influence on the development of pre-ore structures, including the thickness of ore-hosting structural elements. In the distribution of productive mineral associations and gold in ore bodies, the decisive role, as compared with the lithological factor, is again played by the structural factor: ore shoots are confined to the tectonically most intensely crushed sectors.

REFERENCES

N. I. Borodaevskii, P. S. Bernshtein. On the structures of gold-ore deposits. Trans. TsNIGRI, issue 76, 1967.

B. D. Vasil’ev. Regularities in the distribution and forecasting of gold-ore deposits in the northern part of the Kuznetsk Alatau. Izv. TPI, vol. 151, 1966.

E. A. Gukovskii. Elements of the geology of the gold-ore deposits of the Mariinsk taiga. Vestn. Zap.-Sib. Geol. Trust, No. 5, 1932.

V. N. Dovgal’, V. I. Bognibov. The ancient gabbro-syenite complex of the Kuznetsk Alatau. In the collection Magmatic Formations of the Altai–Sayan Folded Region, Nauka, 1965.

I. V. Kucherenko, A. P. Gribanov. Interrelations of dike formations with gold-ore quartz veins in the Berikul ore field. Izv. TPI, vol. 134, 1968.

V. K. Monich. Geological sketch of the Berikul gold-mining district. Trans. “Zolotorazvedka” Trust and NIGRIzoloto Institute, issue 7, 1937.

V. K. Monich. The Koldedannye deposits of Berikul. Trans. TSU, vol. 96, 1939.

D. A. Timofeevskii. On the structure of the ore field of the Staryi Berikul gold-ore deposit. Trans. NIGRIzoloto Institute, issue 16, 1947.

D. A. Timofeevskii, P. I. Shcheglov, S. S. Borishanskaya. The Berikul gold-ore deposit. Trans. NIGRIzoloto Institute, vol. 5, 1952.