Structural Conditions for the Formation of Gold Mineralization in the Berikul Ore Field (Kuznetsk Alatau)

I. V. Kucherenko (TPI)

Many researchers have devoted attention to elucidating the structural conditions of ore formation in the Berikul ore field (Gukovsky, 1932; Monich, 1937, 1939; Timofeevsky, 1947; Timofeevsky et al., 1952; and others). However, apparently, the complexity of the problem and, most importantly, the relatively weak exploration of the ore field in the past led to contradictions in the conclusions drawn in different years. Researchers assessed in different ways the role of ore-hosting structural elements, transverse faults in the localization of mineralization in veins, the conditions of formation of metasomatic sulfide deposits, the significance of the gently dipping vein No. 4, and so on. As a result of the substantial volumes of exploration and mining work carried out over the last two decades, it has become possible to study the problem under consideration on the basis of more extensive factual material, which has made it possible to clarify a number of propositions and to give a different genetic interpretation to certain aspects of the problem.

The Berikul ore field (as well as the Komsomolsk, Novo-Berikul, Gavrilovsk, and others) is confined to the western exocontact of the large Dudetsk granitoid massif of Caledonian age. The exocontact is composed of thick volcanogenic and sedimentary sequences of the Cambrian, gathered into large folds of northwestern strike and containing relatively small bodies (stocks and dikes) of basic and alkaline rocks. The age of the latter, according to most researchers, is Devonian; there is no consensus regarding the age and origin of the former. Most justify the genetic affiliation of the basic rocks with Caledonian magmatism; according to the conclusions of other researchers (Vasiliev, 1966, and others), these rocks represent products of the first phase of the multiphase Kiya complex of the Devonian. It must be emphasized that an important feature of the localization of gold mineralization in the district is the constant confinement of commercial ore accumulations either to these rocks (Komsomolsk, partly Novo-Berikul, Gavrilovsk ore fields), or to their immediate vicinity (Berikul, Novo-Berikul ore fields).

Within the Berikul ore field, the ore bodies occur in volcanogenic formations of the Berikul Formation of the Middle Cambrian, which in the west overlie the Lower Cambrian marbled limestones of the Belokamenka Formation. In the east and south, the volcanogenic rocks contain stocks and dikes of the above-mentioned basic rocks. All the listed rocks make up a southwestern, relatively steeply dipping—

to the northeast (up to 70°) wing of the Berikul syncline. The southwestern contact of the largest eastern stock of basic rocks dips to the southwest at steep angles (up to 70°). In connection with the opposed dips of the contacts of the stock and the limestones, the effective ore-hosting sequence in the northern and central sectors is gradually wedged out with depth. The ore bodies are represented by a series of gold-bearing quartz veins with the orientation of the statistical maximum for most veins: az. dip 291°, ∠45° (Fig. 1, diagram V), by gently dipping vein No. 4 dipping south at angles up to 20°, and by sulfide bodies confined to the contact of vein No. 4 with the limestones.

In the ore-hosting sequence and in the stocks of basic rocks, discontinuities are developed that are filled with magmatic and hydrothermal products of varied composition and age. The overwhelming majority of fractures, with the exception of those filled with skarns, arose after the formation of the Berikul syncline and the stocks of basic rocks (Kucherenko, Gribanov, 1968); consequently, their formation is not connected with Caledonian folding processes, but was caused by the manifestation of later tectonic dislocations¹.

In the history of the tectonic life of the ore field, pre-ore, ore, and post-ore periods are distinguished, each of which, in turn, is subdivided into a number of stages.

The structure of the earliest stage is the Berikul syncline, which, as mentioned, took shape as a result of folding processes of Caledonian time. This stage is characterized by the relatively wide development of small folded marginal fractures, irregular in configuration, predominantly concordant with the strike of the rocks folded into folds and subsequently filled with skarns. The fractures under consideration are almost absent in the stocks of basic rocks and are developed in the ore-hosting effusive sequence.

The following stage of structural preparation preceded the intrusion of acid magma and was characterized by tensile stresses, as a result of which regularly oriented, relatively small discontinuities (up to tens, more rarely the first hundreds of meters along strike) arose in space; these were not accompanied by fine fracturing and cut the veinlets of skarns. The fractures are marked by dikes of aplites and pegmatites. Diagram A (Fig. 1) shows that the main mass of these fractures has az. dip 296°, ∠56°; a relatively small number of fractures dip steeply to the northeast and southwest. Fractures of other directions occur episodically. By the end of the stage, the medium acquires clearly expressed features of oriented anisotropy.

The stage of structural preparation preceding the intrusion of diabase magma (diabases of the 1st generation) is characterized by a noticeably greater intensity of tensile stresses: discontinuities of considerable extent (hundreds of meters) are formed. Fine fracturing is not characteristic. The time of formation of the fractures is determined on the basis of the fact that they cut dikes of acid rocks and themselves are cut by vein fractures that formed later. In addition, diabases of the 1st generation bear distinct traces of near-ore alteration. The predominant orientation of the fractures (Fig. 1, diagram B) is meridional and northeastern. From a comparison of diagrams A and B it is evident that

Fig. 1. Orientation diagrams of structural elements of the Berikul ore field

A—dikes of aplites and pegmatites. 121 measurements. \<0.5—1.0—1.5—2.5—5.0—8.0—11.0\<%.
B—dikes of diabases of the first generation. 29 measurements. \<1.0—5.0—10.0—15.0—25.0\<%.
C—quartz veins. 601 measurements. \<0.25—0.5—1.0—1.5—2.5—5.0—8.0—11.0—14.0\<%.
D—transverse fracture disturbances. 1193 measurements. \<0.25—0.5—1.0—1.5—2.5—3.5—5.0\<%.

that with time the anisotropy of the ore-hosting sequence increases. Dikes of pre-ore diabases of the second generation are rare, but their presence indicates that the tensile stresses were pulsating in character.

As a result of tectonic processes of the pre-ore stage, structural conditions for ore deposition were prepared. Tectonic activity at this stage is defined by the development, under conditions of directed tangential compression, of the Main fault (reverse-strike-slip fault, az. dip 215°, ∠80°), three systems of shear fractures, and one system of tensile fractures (Fig. 1, diagram C, maxima I, II, IIᵃ; tensile fractures are not shown on the diagram). The industrial veins chiefly fill shear fractures corresponding to maximum I (az. dip 291°, ∠45°); these fractures are often developed at the contacts of acid-rock dikes and diabases of the first generation, i.e., they inherit the earlier structural directions as the most weakened ones in the ore-hosting sequence.

(Kucherenko, Gribanov, 1968). In this sense, the dikes mentioned play the role of fracture localizers, in the terminology of N. I. Borodaevsky and P. S. Bernshtein (1967). At the deep levels of the ore field, systems of industrial veins are distinguished that correspond exactly in orientation to maxima I and II in diagram B (on the summary diagram V these maxima were not reflected). The rupture fractures (vein No. 4) have, as a whole, a sublatitudinal strike and a gentle (up to 20°) dip to the south. The principal fracture systems of this stage are matched by fine fracturing of the enclosing rocks.

From analysis of displacements along major faults and of the spatial position of nearly contemporaneous shear and rupture fractures, it is possible to obtain an idea of the orientation of the principal deformation axes in the pre-ore stage. The compression axis C was oriented at an azimuth of approximately 340°, with a slight inclination toward the south-southeast; axis A was oriented toward the ancient ground surface with a steep (not less than 75°) inclination toward the north-northwest. The rupture fractures (vein No. 4) are oriented approximately perpendicular to axis A; the wedging out of these fractures with depth was caused by an increase in rock pressure in this direction. The formation, under such a deformation plan, of theoretically possible shear fractures with a southeastern dip (conjugate with the fractures of maximum I, diagram V, Fig. 1) was hindered by the fact that the enclosing rocks had already been cut by dikes of acidic rocks and early diabases.

At the contact of effusive rocks with limestones, crushing of the rocks took place, as a result of which favorable conditions were created for the formation, during the ore stage, of metasomatic sulfide bodies. Outside the sphere of influence of the fractures of vein No. 4, large accumulations of ore at the contact are not known. Thus, in Fig. 2 it is seen that the position of the chain of deposits in space corresponds to the conjugation of the limestone contact with the fractures of the vein No. 4 system, and the wedging out of the latter at deep levels is accompanied by the wedging out of metasomatic deposits. The intense crushing of rocks necessary for the broad development of processes of metasomatic replacement occurred only in those parts of the contact where fractures of vein No. 4 were formed. This is the leading factor in the localization of sulfide deposits. Other factors noted earlier (Monich, 1939; Timofeevsky et al., 1952; and others), namely favorable bends of the limestone contact and the presence of a series of closely spaced “steeply dipping” vein fractures, have subordinate significance: they control the position of the richest ore accumulations (Fig. 2, b).

The orientation of the tectonic rupture structures of the ore and early post-ore stages is shown in diagram G, Fig. 1. Comparison of diagrams V and G shows that in the stages under consideration the deformation plan as a whole was preserved. However, the number of transverse fractures (maxima II and IIa, diagram G) considerably exceeds the number of fractures belonging to the vein system (maximum I). Along with the appearance of new fractures, renewal of old fractures also took place, including vein fractures. Judging from the number of stages of mineralization, at least five successive impulses of compression may be noted in the ore stage. The behavior of productive mineral associations and gold in the ore veins reveals a definite dependence on the degree to which the latter were disrupted by pre-ore and ore transverse ruptures. The influence of transverse disturbances is twofold. On the one hand, near some major faults (the Main fault and others) mineralization in the vein fractures dies out (Fig. 3). At the same time, rich ores in places tend toward areas of intense disruption.

Labels in the figure:

• NW
• line of the horizon
• a-1
• line of the horizon
• a-2
• SE
• NW
• section along I–I
• section along II–II
• section along III–III
• section along IV–IV
• N
• S
• 85°
• b
• 6th horizon, plan
• vein No. 4

Legend:

Fig. 2. Sections across the strike of vein No. 4 (a-1) and along the dip of metasomatic sulfide bodies (a-2). Example of the localization of a rich sulfide body in contact with the limestones of vein No. 4 and a series of closely spaced ore cleavage fractures (b). Based on materials of the Berikul Ore Administration.

Legend: limestones; quartz veins; transverse faults.

Fig. 3. Projection of vein No. 4 onto a horizontal plane. The worked-out areas are hatched; oblique transverse faults are shown by dotted lines. Near the Main Fault, over a considerable distance, the ores are represented by crumpled hydrothermally altered host rocks with disseminated sulfides and a total gold content. An example of the shielding of mineralization by a large pre-ore transverse fault. Based on materials of the Berikul Mine Administration.

veins by transverse fractures. Examples are the ore shoots in the Zabytaya, Tatarskaya, Magistralnaya, Bikul’tovskaya, and Iksovaya veins, located along the line of intersection of these veins with one and the same transverse fault. A similar picture was noted by V. K. Monich (1937); however, regarding these faults as post-ore, he gave a different explanation for these facts. In the gently dipping vein No. 4, the enriched areas, as A. I. Kuchin noted in one of his manuscript works, resemble triangles in shape, with the acute angle directed down the dip of the vein and the broad base confined to the line of intersection of the fractures of vein No. 4 with the oblique vein fractures.

For the late stage of the post-ore period, a waning of tectonic activity and the predominance of pulsating tensile stresses are characteristic. The resulting rupture fractures are filled by dikes of post-ore diabases of no fewer than two generations.

In conclusion, it is necessary to emphasize the following.

1. The principal structural elements of the ore field, hosting diverse dikes and ores, are rupture faults of different ages, formed after the Caledonian folding².

1. Dikes of magmatic rocks, diverse in composition and age, almost always fill rupture fractures. Fractures of shear occurring contemporaneously with them are practically not observed. This is characteristic both of pre-ore and post-ore dikes. Therefore it may be considered that impulses of extension of a segment of the Earth’s crust immediately preceded the intrusions of magmatic material and possibly were caused by such intrusions. A different picture is observed in the formation of ore-hosting and accompanying structures: here tectonic compression is pulsationally replaced by extension. Heterochronous shear fractures of several systems are formed, corresponding to individual compression impulses; the rupture fractures, however, are subordinate and reflect only tectonic extension acting from below upward under horizontal compression. At the same time, for structural elements of the pre-ore and ore stages, inheritance of structures is exceptionally characteristic; it consists in the frequent development of ore-hosting, in part transverse, fractures in the contacts of dikes of acidic rocks and early diabases, i.e., rocks whose physical-mechanical properties differ sharply from those of the enclosing volcanogenic formations. This circumstance determined the regular orientation of gold-bearing veins, corresponding to the character of the anisotropy of the medium³.

2. The distribution of ore bodies in the ore field is governed by factors of lithologic-structural control, consisting in the preferential development of ore-hosting and accompanying structures in effusive rock formations that differ in their physical-mechanical properties from the surrounding limestones and gabbroids. In turn, the distribution of productive mineral associations and gold within individual ore bodies indicates a regular confinement of ore nests and shoots to those areas that were most prepared structurally.

LITERATURE

1. Borodaevskii N. I., Bernstein P. S. On the structures of gold-ore deposits. Proceedings of TsNIGRI, issue 76, 1967.

2. Vasil'ev B. D. Regularities of the distribution and prediction of gold-ore deposits in the northern part of the Kuznetsk Alatau. Izvestiya of the Tomsk Polytechnic Institute, vol. 151, 1966.

3. Gukovskii E. A. Elements of the geology of gold-ore deposits of the Mariinsk taiga. Bulletin of the West Siberian Geological Trust, No. 5, 1932.

4. Kucherenko I. V., Gribanov A. P. Relationships of dike formations with gold-ore quartz veins in the Berikul ore field. Izvestiya of the Tomsk Polytechnic Institute, vol. 134, 1968.

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

6. Monich V. K. Pyrite deposits of Berikul. Proceedings of Tomsk State University, vol. 96, 1939.

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

8. Timofeevskii D. A., Shcheglov P. I., Borishchanskaya S. S. The Berikul gold-ore deposit. Proceedings of the NIGRIzoloto Institute, vol. 5, 1952.

1. It should be borne in mind that the district is located in the area of manifestation of Early Caledonian (Salair) tectogenesis, while formation of the Dudetsky pluton took place in the Late Caledonian stage (P. Ivankin). ↩

2. Folding in the area is of Early Caledonian (Salairian) age. Therefore the author’s conclusion concerning the post-folding age of the Berikul mineralization does not contradict the generally accepted view of the Late Caledonian (properly Caledonian) age of the principal gold mineralization of the Mariinsk taiga (P. Ivankin). ↩

3. The division of fractures into shear and rupture fractures, as is known, is a very complex problem, the solution of which usually requires special investigations. The author perhaps does not sufficiently take into account the fact, repeatedly confirmed, in particular at vein gold-ore deposits, that the origin of fractures and their subsequent filling (by magma or hydrothermal solution) occur under entirely different stress-tectonic states of the continent. Shear fractures that arise under a condition of compression, during the formation of vein intrusions, for example, behave as rupture fractures. However, it would be a mistake to judge the genesis of fractures by their behavior during periods of filling with vein material (P. Ivankin). ↩