On the Role of Local Structures in the Distribution of Gold-Ore Deposits of Northern Kazakhstan

V. M. Grishin (IGN, Academy of Sciences of the Kazakh SSR)

Northern Kazakhstan is distinguished by the wide distribution of endogenous gold-bearing formations. The leading factor determining their distribution and industrial value is structure. However, the significance of particular structural elements is unclear. Some researchers assume the primary role of deep-seated faults [17, 19, 20], while others believe that the distribution of gold-ore deposits is subordinate to structures of local significance [10, 13, 15]. In recent years, as a result of detailed geological mapping of deposits and the synthesis of the results of prospecting and exploration work, important features have been revealed in the structure of deposits and in the structural control of mineralization.

The deposits considered by us—Bestyube, Dzhelambet, Aksu, and Stepnyak—are enclosed in deposits assigned to the Ordovician. The latter are represented by terrigenous-sedimentary and volcanogenic-sedimentary formations [1]. Porphyrites forming sheets and subvolcanic bodies confined to interstratal detachments are of subordinate importance. These rocks are complexly dislocated. Steeply (50–60°) and vertically set beds are often subjected to intense fold deformations. Therefore, the direction of dip of the beds along the strike often changes to the opposite. This is especially sharply expressed in sequences with frequent interbedding of sandstones and shales. One of the most specific features of the folded structures is S- and Γ-shaped bends along the strike (Fig. 1). In these structures, intrusions of intermediate and basic composition are widely manifested; Yu. A. Bilibin assigned them to the Stepnyak gold-bearing complex [4]. Structurally, formations of two stages are distinguished among them. Intrusions of the first stage are represented by sills and stocks, and rarely by dikes, with outcrop areas of up to one to three square kilometers. Their distribution is controlled by faults and fracture zones developed in direct dependence on the lithological features of the enclosing rocks and on folding. Among them are distinguished faults and fracture zones concordant with bedding, cutting it at an acute angle, and arranged across the strike of the bedding. The formations of the second stage are represented by typical fissure intrusions [19].

granite-porphyries, diorite-porphyries, spessartites, diabase porphyrites, etc. Spatially they tend toward the intrusive bodies of the first stage.

Fig. 1. S- and Γ-shaped bends of the enclosing Ordovician deposits at the ore fields of Bestyube (A), Stepnyak (Б), Aksu (В), and Dzhelambet (Г):

1 — strike of the bedding of the Stepnyak complex; 2 — intrusions

The post-intrusive fault structures are complex in character. Local faults are of particular interest. The first system includes disturbances along bedding, represented by cleavage zones with a thickness of up to several meters, more rarely tens of meters. Disturbances of the second system, which are referred to as shear faults [2, 3], are the most highly developed. Usually these are zones of crushing or cleavage of rocks with a thickness from several tens of centimeters to two to five meters. In the horizontal plane they form, with bedding, an angle of 5–10° (Bestyube) to several tens of degrees (Dzhelambet), and dip in the opposite direction relative to the bedding. The most extended faults are spaced from one another at definite intervals. At the Bestyube deposit this interval is 240–260 m, at Dzhelambet 155–175 m, and at Stepnyak 480–550 m. The third system comprises disturbances developing in tectonic blocks bounded by disturbances of the second system. In plan they adjoin them from the sides at an acute angle; at the Stepnyak deposit this angle is 25–30°, while at the Bestyube deposit it decreases to 15–20°. It may be thought that the disturbances of the third system represent tension fractures arising during deformation of tectonic blocks bounded by disturbances of the second order. Investigations have shown that in one case (Bestyube and Stepnyak) the character of movement along disturbances of the second order was reverse–strike-slip, and in another (Dzhelambet) reverse. This accounts for the different spatial orientation of disturbances of the third system. Disturbances of the fourth system are weakly expressed, are oriented across the strike of the enclosing rocks, and develop in areas of sharp bends of folding. Their length rarely exceeds 100–200 m along strike; their thickness varies from several tens of meters to 100–150 m. The fracture surface is uneven, characteristic of tension fractures [3]. The fifth system of gently dipping and horizontal fractures is morphologically analogous to the preceding one.

Systems of local faults are developed unevenly. For the most part they tend toward intrusions and zones of hornfelsing of the rocks. Around large intrusions the faults tend toward the exocontact zones.

Regional faults are established 5–10 km from the Bestyube deposit. Situated roughly parallel to the strike of folding, they form a stepped normal fault with vertical displacement of blocks up to 1.5 km (materials of investigations by R. A. Borukaev, ...).

V. S. Zvontsova et al., 1959). At some deposits (Aksu, Stepnyak), faults that dissect the deposits into tectonic blocks are more widespread; at other deposits (Bestyube) they are practically absent. At the Stepnyak deposit such faults are more often situated across the strike of the folding, arising in places where horizons composed of tuffaceous rocks bend sharply along strike, or they develop within intrusions and hornfels zones, which are distinguished by greater tensile and shear strength. They often appear along rock contacts. The insignificant amplitudes of displacement in the planes of the faults and their confinement to rocks with particular mechanical properties indicate their local origin [16]. The absence along them of hydrothermal alterations and halos of chemical elements associated with gold indicates their post-ore age. Much more rarely are they inter-ore, as may be observed at the Dzhelambet deposit (materials of investigations by N. A. Fogelman, 1953, V. I. Danilov, 1966, and others).

The structures under consideration are characterized by a broad distribution of hydrothermal formations in several successively developing stages. Skarns, quartzites, and quartz veins are distinguished. The skarns are of mineralogical interest only, developing along a fine network of fractures among hornfels and intrusions. Secondary quartzites are more extensively developed [18]. They form extended zones and are accompanied by chloritized, carbonatized, and sericitized rocks containing disseminated inclusions of gold-bearing pyrites. The largest “quartzified zone” of the Dzhelambet deposit can be traced along strike for 1500 m and has a thickness of up to 50–60 m. Coinciding with the bedding of the rocks, it is confined to a schistosity zone developing along a series of interstratal dikes and sheets of andesitic and dacitic porphyries (materials of investigations by V. I. Danilov). At the Aksu deposit, similar ore bodies (“Quartzite Hills”) are confined to thick (30–50 m) interstratal zones of rock schistosity developing along the contact between volcanogenic-sedimentary and sedimentary sequences. Much more rarely, quartzites develop along secant faults. Usually they are of less significant dimensions (up to 200–300 m along strike and 10–15 m in thickness). Quartzification often develops within the outline of intrusive bodies, apparently characterized by increased fracturing [5]. The quartzite zones are older than the faults that dissect the deposits into tectonic blocks. They are often torn and displaced along them.

Quartz gold-bearing veins belong to the latest hydrothermal gold-bearing formations. Along strike the veins do not exceed 600–800 m, and down dip 400–500 m. At the Bestyube deposit, blind ore bodies are established at depths from 200–300 to 1200 m. The maximum thickness of the ore bodies does not exceed 2–3 m. Quartz veins are confined to local fault disturbances developing in intrusions and hornfels (Fig. 2). Upon leaving them, the veins pinch out or acquire a complex morphology, representing a series of complex lenses.

Comparing the features of the distribution of mineralization, it is evident that: 1) at the Bestyube deposit, faults and veins are less commonly concentrated in intrusions and their contact-adjacent parts, and more often occur in unaltered tuffaceous-sedimentary rocks; 2) at the Dzhelambet deposit, faults and veins are localized in intrusions and, more rarely, in hornfels; 3) at the Aksu deposit, faults and veins are located to a greater extent in intrusions.

Fig. 2. Features of the distribution of quartz veins.

A — geological section through the Zapadnyi sector at the Bestyube deposit; B — section through the area of Shaft 6, and V — the Yuzhnyi sector at the Dzhelambet deposit (section V is borrowed from the data of A. A. Zailer).

It is not excluded that this peculiarity in the distribution of local fractures and mineralization reflects structural zonality [14]. As is known, Yu. A. Bilibin believed that the Aksu deposit is characterized by the deepest erosional level, and the shallowest by the Bestyube deposit.

Fractures belonging to one system and, consequently, developing under the conditions of a single tectonic plan, often differ sharply in their industrial value. Thus, in the Zapadnyi ore sector of the Bestyube deposit, quartz veins are confined to fractures of the second and third systems, whereas in the Tsentralnyi ore sector, situated in more plastic rocks—schists and sandstones—the veins are confined only to fractures of the third system. At the same time, a definite regularity is manifested in the frequency of the distribution of veins in local fractures. Thus, quartz veins are more common in fractures of the second system.

At the Dzhelambet deposit (area of Shafts 5–6), where transverse inter-ore faults are observed, characterized by significant displacement amplitudes, quartz veins are confined to fractures of the first system, representing typical bedding-plane partings in blocks cut off by practically barren fractures of the second system. In the Yuzhnyi sector of this same deposit

along transverse displacement ruptures is not established, and here quartz veins develop only in disturbances of the second system.

Thus, the investigations carried out have shown that the studied gold-ore deposits of Northern Kazakhstan are confined not to regional faults or to the fractures accompanying them, but to ruptures of local significance, developing in S- and Γ-shaped bends of folding in Ordovician deposits. The important role of S- and Γ-shaped structures in the distribution of ore deposits was considered, using the example of Altai and the Dzungarian Alatau, by A. K. Kayupov and Yu. I. Kazanin [12], and also by researchers of Siberia [6]. In such structures, mineralization may have a multi-level character of occurrence [8], as can be seen from the example of the Bestyube and Dzhelambet gold-ore deposits. At the same time, deposits with a similar geological structure are characterized by a considerable vertical extent of mineralization (the Kolar deposit in India, Morro Velho in Brazil, a number of deposits in Australia, etc.). Taking these data into account, we are inclined to arrive at the following conclusions.

1. The association of the most important, industrially significant gold-ore deposits with S- and Γ-shaped folded structures indicates the need in Northern Kazakhstan for a careful verification and reassessment, first of all, of those ore occurrences and deposits that are characterized by an analogous structural setting; in particular, this concerns the known gold-ore fields of Ushbulak, Zhulubai, Zhanatyube, and others.

2. The ore-hosting structures considered, with intense development of pre-ore and inter-ore block tectonics, are less favorable for ore localization, since in this case the S- and Γ-shaped bends lose their ore-controlling significance. The appearance of block tectonics indicates that the character of deformation has changed [9, 10, 11], and then only blocks favorable in their tectonic setting prove to be important, since the ruptures that form them delimit the areas of distribution of gold-bearing hydrothermal solutions [7, 10].

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