STRUCTURE OF THE BERIKUL ORE FIELD

(KUZNETSK ALATAU)

I. V. KUCHERENKO

(Presented by the scientific seminar of the Gold Laboratory)

The problem of the structural conditions of ore formation in the Berikul ore field has repeatedly been discussed by researchers in print (Gukovskii, 1932; Monich, 1937, 1939; Timofeevskii, 1947, 1952). Over the last two decades, owing to the large scale of exploration and mining operations, the territory of the ore field accessible for study has increased considerably, mainly at the expense of the southern sectors and deeper levels. Analysis of the materials obtained by the author in the course of detailed study of the deposit, and of the archive materials of the Berikul mine, has made it possible to approach the explanation of the complex phenomena of structure formation in the ore field in a different way than had been proposed by earlier investigators. The present article is devoted to a brief exposition of a new scheme of the structural development of the ore field.

The ore veins of Berikul occur in effusives of the Berikul Formation of Middle Cambrian age, which in the west are in contact with Lower Cambrian limestones of the Belokamensk Formation. In the east and south the effusives are cut by comparatively small stocks of gabbro-diorites, assigned by V. K. Monich (1937) and others to an intrusive complex of Caledonian age, and by B. D. Vasil’ev (1966) to the Kiya Devonian complex. The rocks listed make up the southwestern limb of the Berikul syncline of northwestern strike. In connection with the opposing steep dip of the contacts, the ore-bearing effusive sequence gradually pinches out with depth.

The geological position of the ore field in the general structure of the Berikul district is characterized by the fact that, together with other gold-ore fields—namely, Komsomol’skoe in the north, Novo-Berikul’skoe, Gavrilovskoe, and others in the south—it is localized in a belt of north-northwestern strike, extending along the western boundary of the large Dudets granitoid massif of Caledonian age. This feature of the spatial position of the ore fields in the district was pointed out as early as by E. A. Gukovskii (1932) and V. K. Monich. The latter, in particular, believed that the Berikul district is divided by a major Devonian fault of northwestern strike into two parts: a western part—a graben composed of Cambrian sequences and gabbro-dioritic, more rarely syenitic bodies—and an eastern part—a horst in which the deep-seated parts of the Dudets massif are exposed; in this, the gold-ore deposits are localized in the graben. Both researchers, as well as the overwhelming majority of others, regarded the gold mineralization as genetic-

... associated with the Caledonian granodiorite intrusion. In recent years, interesting results have been obtained by B. D. Vasil’ev (1966). He established that the position of the gold-ore fields is controlled by northwestern structures (zones of emplacement of Phase I gabbroids of the Kiia complex) and northeastern structures (the position within the folded structure of the Devonian stage), while the zonality of mineralization (a formation of gold–copper–bismuth ores, on the one hand, and a formation of gold–arsenic ores, on the other) is determined by the depth of formation of the Devonian intrusions and of the mineralization genetically associated with them.

Structurally, the ore field took shape as a result of tectonic processes of different ages, differing in nature and scale. The stages of structure formation are named according to the features of the structural preparation of subsequent geological phenomena, such as, for example, the intrusion of magmas and the formation of dike rocks of various compositions, the deposition of ore material, etc., since it is assumed that the scale of these phenomena, the forms of their manifestation, and other features depend on the degree and character of the corresponding structural preparation, while the latter, in turn, depends on the characteristics of the deep-seated life of the Earth’s crust. It is thus assumed that tectonic processes, on the one hand, and magmatic activity and ore-deposition processes, on the other, are closely interconnected and mutually conditioned.

In the history of the tectonic life of the ore-field territory, pre-ore, ore, and post-ore periods are distinguished.

The earliest structure is the Berikul syncline, formed as a result of folding processes of Caledonian time. Apparently, the small rupture fractures, predominantly concordant with the strike of the strata of northwestern strike, which are widely developed in the effusive rocks and were later healed, during the formation of intrusive massifs, by skarns, are also coeval with the folding.

The next stage of structural preparation, preceding the intrusion of acid magma, is characterized by the formation of rupture fractures which, in known cases, cut the skarns. The fractures have a length of many tens of meters and are regularly oriented in space: they predominantly dip along azimuth 296° at an angle of 56°. The ruptures under consideration formed as a result of extension of the territory and were healed by pegmatites and aplites.

The stage of formation of rupture fractures that also arose as a result of tensile stresses and are marked by first-generation diabases is characterized by a noticeably greater intensity of tectonic processes, which is confirmed by the larger scale of the fractures—their length is many tens, more often the first hundreds of meters. The predominant orientation of the fractures is nearly meridional (dip az. 270° ∠ 50°) and north-northeastern (dip az. 315° ∠ 46°). The time of formation of the ruptures is determined on the basis of the fact that they cut dikes of acid rocks, on the one hand, and are cut by vein fractures and other ruptures formed later, in the pre-ore stage, on the other hand. In addition, in all known cases the pre-ore diabases are hydrothermally altered near the ores. Dikes of pre-ore diabases of the second generation, according to preliminary data, are weakly distributed. Their presence indicates that the tensile stresses were pulsating in character.

The pre-ore stage of the ore period is characterized by the highest intensity of structure formation. As a result of the tectonic processes of this stage, structural conditions were prepared for the depo-

mineralization. Tectonic activity at this stage is defined by the inception of the Main fault (dip azimuth \(215^\circ\) at an angle of \(80^\circ\)), three systems of shear fractures, and one system of rupture fractures. The main mass of commercial veins marks the shear fractures of north-northeastern strike (dip azimuth \(291^\circ \angle 45^\circ\)); these fractures are usually developed in the contacts of dikes of acid rocks and first-generation diabases, i.e., they inherited earlier structural trends that were the most weakened in the ore-bearing sequence. It appears that, for this reason, the vein fractures took shape somewhat earlier than the bulk of the northwestern-striking shear fractures of the other two systems. The northwestern fractures (dipping at an angle of \(70\text{—}90^\circ\) to the northeast and southwest) of the pre-ore stage are less developed than analogous fractures of the ore stage. The latter are filled with the final products of hydrothermal activity—carbonates, locally with nickel and cobalt arsenides, and gold. The rupture fractures (vein No. 4) are contemporaneous with the shear fractures of the pre-ore stage; on the whole they have a nearly latitudinal strike and a gentle (to \(20^\circ\)) dip to the south.

From an analysis of displacements along major faults and of the spatial position of approximately contemporaneous shear and rupture fractures, ideas follow concerning the orientation of the principal axes of deformation during the pre-ore stage. Axis \(C\) was oriented in a north-northwestern direction with inclination toward the south-southeast, and axis \(A\) toward the ancient day surface with inclination toward the north-northwest. The wedging-out of rupture fractures (vein No. 4), which formed as a result of the reaction of the host rocks to tensile stresses along axis \(A\), is due at depth to an increase in this direction of the pressure of rock masses, which compensated at deeper levels for the tensile stresses mentioned above.

The formation of rupture and shear fractures was accompanied by intense brecciation of the effusive rocks at their contact with limestones, as a result of which favorable structural conditions were created for the formation of metasomatic sulfide bodies.

In the ore stage, while the general pattern of deformations of the pre-ore stage was preserved, the formation of shear fractures of northwestern, and to a lesser degree northeastern, strike continued under compression, along with the renewal of old ruptures.

The post-ore period in the early stage of compression is characterized by weak development of northwestern-striking shear fractures and, subsequently, under conditions of extension, by the formation of rupture fractures filled with later diabases.

From the foregoing, the following principal conclusions follow:

1. In the history of formation of the structure of the ore field, several stages are distinguished. The early stage of the pre-ore period is characterized by compressive tectonic stresses that caused the formation of folded structures and concordant rupture fractures subsequently filled with skarns. In the late stages of the pre-ore period, repeated tensile forces occurred, as a result of which rupture fractures arose, each impulse of fracture formation being accompanied by intrusion of magma of the corresponding composition; in this, the earlier fractures were filled with acid rocks, and the later ones with diabases of no fewer than two generations. Dikes of acid rocks and first-generation diabases have a uniform, predominantly west-northwestern dip at angles of \(40\text{—}60^\circ\); on the other hand, there existed weakened directions corresponding to the northwestern strike of flows of effusive rocks. These circumstances determined an oriented anisotropy of the medium by the pre-ore stage and, to a certain extent,

largely determined the formation at this stage, under the corresponding plan of deformation, of shear fractures analogous to the aforementioned directions of orientation.

1. In the pre-ore stage of the ore period, under conditions of compression in a north–northwestern direction, shear fractures took shape, predominantly with a west–northwestern dip and, to a lesser extent, a southwestern and northeastern dip, as well as rupture fractures dipping gently to the south. The stresses of the ore stage, with the deformation plan of the pre-ore stage preserved, were relieved chiefly through the formation of fractures of northwestern strike and the renewal of old ones.

2. The post-ore period is characterized by the waning of tectonic movements. After ore deposition there arose a small number of shear fractures of northwestern strike, cutting the ore bodies, as well as rupture fractures that accommodated the post-ore diabases.

3. In the history of the tectonic life of the territory there was an increase in the intensity of tectonic processes by the stage of ore deposition and a weakening after it.

REFERENCES

1. B. D. Vasil’ev. Regularities in the distribution and prediction of gold-ore deposits in the northern part of the Kuznetsk Alatau. Izv. TPI, vol. 151, 1965.

2. E. A. Gukovskii. Elements of the geology of gold-ore deposits of the Mariinsk taiga. Vestnik Zap. Sib. geol. tresta, No. 5, 1932.

3. V. K. Monich. Geological sketch of the Berikul gold-mining district. Tr. tresta “Zolotorazvedka” i in-ta NIGRIzoloto, issue 7, 1937.

4. V. K. Monich. Pyrite deposits of Berikul. Tr. TGU, vol. 96, Tomsk University Press, 1939.

5. D. A. Timofeevskii. On the structure of the ore field of the Staryi Berikul’ gold-ore deposit. Tr. in-ta NIGRIzoloto, issue 16, 1947.

6. D. A. Timofeevskii, P. I. Shcheglov, S. S. Borishanskaya. The Berikul gold-ore deposit. Tr. in-ta NIGRIzoloto, vol. 5, 1952.