STRUCTURAL FEATURES OF THE KOCHKAR GOLD-ARSENIC DEPOSIT (SOUTHERN URALS)

V. M. Yanovskii (TsNIGRI)

The Kochkar ore field, in structural terms, is usually regarded as a fracture structure, where sulfide-quartz and quartz veins constitute several conjugate systems of shear and tension fractures in a granitoid massif.

The study of the deep levels of the deposit in the interval of 200–350 m by mine workings and to depths of 800–900 m by boreholes, carried out by the author in 1962–1966, made it possible to distinguish in the structure of the deposit two new elements that in many respects determine the distribution of gold mineralization: brachyanticlinal folds forming a granite-gneiss tier in the Plast massif of plagiogranites, which are to a considerable extent of metasomatic and palingenic origin, and also tectonic blocks in the northern part of the massif, formed during the pre-ore and ore stages of deformation.

Within the industrial part of the deposit, systems of faults of different ages, dikes, elements of brachyanticlinal folds, and tectonic blocks are combined. The tectonic blocks play the principal role in the localization of gold mineralization; they determine the distribution of ore sulfide-quartz veins and productive mineral associations in particular parts of the deposit. This provides grounds for regarding the structure of the Kochkar deposit as blocky, despite the presence of other deformational elements as well.

The Plast massif of plagiogranites (massif area 150 km²) occupies the main part of a large regional block. The regular spatial position of pipe-like plagiogranites and granite-gneisses, which form large brachyfolds in the Plast massif (with wing spans up to 2.5–3 km), as well as the gradual transitions often observed in mine workings from granite-gneisses to banded and then to massive plagiogranites, and the features of the mineral composition and chemistry of the rocks, indicate the significant role of the granitization process in the formation of the massif [3, 4]. Structurally, the massif may, somewhat conventionally, be divided into two tiers: a tier of brachymorphic folds formed by rocks granitized to varying degrees, and a tier represented by massive, in large part palingenic, plagiogranites. Early dikes of plagio-

of granitic composition have clear cross-cutting relationships with the banded rocks; these relationships indicate that the massif formed under conditions of plastic deformation, which had ended by the time of emplacement of the dikes of plagiogranite aplites, plagiogranite pegmatites, and vein plagiogranites.

In the history of formation of the Kochkar deposit, we distinguish five stages of deformation in the development of its complex structure: pre-dike, dike, pre-ore, ore, and post-ore, differing in the conditions of deformation, the predominant type of deformational elements, and other features (see the scheme).

The pre-dike stage of deformation includes plicative dislocations that complicated the brachifolded structure of the gneiss–schist sequences and accompanied the formation of the synorogenic massif of plagiogranites before the consolidation of the massif, as well as ruptures preceding the emplacement of early dikes, filled with vein plagiogranites, plagiogranite aplites, and plagiogranite pegmatites.

The following dike stage corresponds in time to the emplacement of groups of dikes of different composition, from felsites and quartz porphyries to diabases [1, 2], transformed within the ore field into the so-called tabashki. On the basis of structural data on the tabashki dikes, it is assumed that formation of the dike framework preceding mineralization took place in two folds.

The pre-ore stage of structure formation is the stage of formation of systems of ore-controlling ruptures and a group of parallelepipedal blocks. Associated with this stage is the widespread manifestation of metasomatic complexes: epidote–amphibole, biotite, feldspar–biotite in composition along the dikes, and of the greisen type along plagiogranites and granite-gneisses. The stage ends with the formation of high-temperature quartz–feldspar and quartz veins of the pre-ore formation. The distinctive character of the pre-ore stage in the Kochkar ore field consists in the formation of specific structures—blocks—and in the maximal manifestation of hydrothermal metasomatism.

The ore stage of deformation is characterized by further development (activation) of the system of pre-ore blocks; the ruptures bounding the blocks determine the position and morphological features of the fissures of ore time. The hydrothermal process is manifested by the formation of a successive series of mineral associations composing the group of sulfide–quartz veins of the ore formation.

In the post-ore stage of deformation, relatively gentle thrusts and normal faults develop, inheriting early structural elements, including blocks, to a very small extent.

The principal ore bodies of the deposit are located within two large brachyanticlinal folds. The width of the granite-gneiss zones in the limbs of the brachystructures exposed by mine workings is up to 250–500 m. The limbs of the brachystructures dip at angles of 40–60° to the west and east, while the periclinal parts are complicated by additional small folds with gentle (15–20°) dip angles of the banded rocks. The influence of plicative forms on the subsequent development of the structure was expressed chiefly in the placement of early dikes of plagiogranitic composition in the granite-gneiss tier of the massif, where these dikes fill ruptures coupled with folds. The anisotropy of the rocks of the upper tier of the massif is associated with the formation of systems of small ruptures that complicate the shapes of dikes and of some ore veins, which have relatively simple forms in massive plagiogranites.

In the following stage of deformation, the dike stage, dikes of co-

SCHEME OF THE DEVELOPMENT OF THE STRUCTURE OF THE KOCHKAR ORE FIELD

composition from felsites and quartz porphyries to diabases. Dikes of east-northeast, latitudinal, and rare dikes of northwest and submeridional strike form a framework that predetermines the position of a considerable part of the subsequent fractures. The dike systems form several zones of concentration up to 30–40 m wide. The morphology of the dikes (rectilinear and smoothly curving contacts, gradual pinch-outs), the pattern of fractures, and the character of the linkage of east-northeast- and latitudinal-striking dikes indicate the shear nature of most of the fractures filled by dikes.

Within the plagiogranite massif, three large blocks of pre-dike emplacement are distinguished, separated by steeply dipping, extended fractures of NW strike. The mechanism of formation of the dike framework appears to have been multistage: an early stage—the development of large fractures bounding the blocks and filled by early dikes; and a subsequent stage—the development of fractures within the blocks and their filling by late dikes.

The pre-ore stage is the stage of formation of post-dike fractures and the emplacement of a series of parallelepipedal blocks in the structure of the deposit.

At the 300 m level, within the Northern and Southern mines, nine blocks have been distinguished, bounded by fractures of ENE and submeridional strike. The age of these deformational elements is determined by their relation to dikes and to ore sulfide–quartz veins, and the synchronicity of movements along the fractures bounding the blocks—by the near-fracture metasomatites corresponding to the stage of pre-ore quartz–feldspar veins and, in some cases, also by hydrothermal products of the ore stage (sulfide mineralization, quartz and sulfide–quartz ore veins).

The complex of near-fracture hydrothermal alterations—greisenization, beresitization, tourmalinization, silicification, actinolitization—is most strongly manifested in the zones of fractures of pre-ore emplacement that bound the blocks. Often the bounding fractures are accompanied by veins of pre-ore and ore quartz. Intersections and displacements of dikes by these fractures and their relationship with ore veins, which occupy positions diagonal to the zones of ENE strike and are bounded by submeridional ones, all these facts definitely indicate the time of emplacement and development of the blocks.

The fractures of the pre-ore stage proper include extended zones of schistosity, brecciation, and mylonitization of rocks of ENE (az. strike 60–70°, southeast dip, angle 60–85°) and WNW strike (az. strike 105–110°, northeast dip, rarely southwest, dip angle 85–90°). Shear fractures of the type of gently dipping faults and steeply dipping normal and reverse faults of meridional strike are less strongly manifested. The largest fractures of the pre-ore stage are a system of shears of east-northeast strike, which generally repeats the position of the dike concentration zones; the second system of fractures—submeridional—does not depend on the position of the dikes. Thus, the blocks emplaced during the pre-ore stage are a partially inherited element in the structure of the deposit. The deformation conditions and differences in the degree to which preceding elements were inherited by pre-ore fractures were expressed in the fact that fractures of east-northeast strike appeared as large, extended (up to 3–4 km) zones of schists, mylonites, and breccias, whereas other systems of fractures of pre-ore age are developed fragmentarily. Data on the amplitudes of relative displacement of the walls of pre-ore fractures are few; judging from the observed displacements of dikes

and in terms of the total thickness of the feathering tension veins accompanying the ENE disturbances, the horizontal component reached several tens of meters.

The ore stage of deformation is characterized by further development of the blocks (activation of the block system). At this time a successive series of mineral associations was formed: arsenopyrite–quartz and quartz–pyrite, sphalerite–chalcopyrite–pyrrhotite, gold–tetradymite, and the late-ore quartz–carbonate–galena association, which make up the ore sulfide–quartz veins. During the ore stage, near and in the zones of faults—block boundaries, distinguished by the maximum development of pre-ore mineral associations, mineral associations of the ore stage were deposited; however, the character of the distribution of these associations in adjacent blocks is different. The relative displacements of the blocks represent left reverse strike-slip movements along ENE faults (in the preceding, pre-ore stage the displacement in this system was a right normal strike-slip), with low-amplitude displacements of blocks along meridional boundary faults.

In the form of bundles, series of en echelon or net-like ruptures of the shear type, faults of the actual ore time are located within the blocks. These are systems of steeply dipping sublatitudinal, ENE, and WNW ruptures, partly occupying an inherited position (renewal of secondary ruptures of pre-ore age and contacts of dikes), and partly cutting earlier elements and having the character of superposed fractures.

Sulfide–quartz veins (as a rule, hundreds of meters long and with thicknesses in the first tens of cm) are regularly situated within the blocks, occupying the position of feathering fractures with respect to the ENE boundary faults. The number, position, and some morphological features of the veins differ in adjacent blocks.

The submeridional system of boundary faults is not accompanied by feathering fractures of ore time, acting as transverse limiting structures with respect to the sulfide–quartz ore veins. Near these boundaries, within 30–40°, the strikes and dip angles of the ore-bearing ruptures change; the ore-bearing zones of schistosity die out at the boundary or, conversely, sharply increase in thickness (for example, the Uspenskaya, Elizavetinskaya, Parallelnye, and South Aleksandrovskaya veins).

In addition to heterogeneities connected with differences in the position of dikes and granite-gneiss “packets” in adjacent blocks, the distribution of ruptures of ore time and their intra-ore renewal is affected by the relative displacement of blocks along boundary faults during the ore stage of deformation.

Despite the identical set of veins and mineral associations in all the blocks we studied, there is a certain individualization of the blocks during the ore stage. Adjacent blocks that have common bounding faults differ in the number of sulfide–quartz veins, in the predominance of systems of ruptures—latitudinal or east-northeastern (for example, the blocks of shafts No. 82, 116, Flangovaya, etc., horizons 300, 350, 242 m). Convergent sublatitudinal veins often have a northerly dip in one block and a southerly dip in another (the Tsentr shaft, horizons 185 and 225 m). The blocks of the K. Marx, No. 116, and Flangovaya mines are relatively uniformly saturated with ruptures of ore age, whereas other blocks are distinguished by a concentration of ruptures in marginal parts near the boundary faults. Differences in the movements of blocks during the intra-ore time lead to an asymmetric distribution of successive mineral complexes relative to boundary faults—

limiters: for example, the veins of all systems east of the meridional limiting fault at Shaft Center contain minerals of the sphalerite–chalcopyrite–pyrrhotite association, whereas west of this fault they do not. Similar relations between zones of intraruptural renewal of faults and block boundaries are characteristic of the entire deposit; adjacent blocks differ either in the set of successive mineral associations or in their content in the ores. Within the group of blocks, the blocks of the K. Marx, 116, and Flank shafts stand out as being relatively uniformly saturated with minerals of productive associations (levels from 190 to 350 m; deep boreholes—intervals of 700–1000 m), as do blocks in which these associations are concentrated directly at the limiting faults.

In the final stage of deformation, relatively gentle (dip angles of 30–40°) thrusts and normal faults of northeastern and northwestern strike develop; they inherit earlier structural elements, including faults bounding the blocks, only to a very slight degree. Ore and pre-ore faults are fragmentarily reactivated, and late quartz and quartz–carbonate veins are formed. Large late faults consist of several subparallel branches represented by breccias, mylonites, and tectonic clay containing fragments of quartz and sulfides. The amplitudes of displacement along late faults do not exceed 15–20 m (for example, faults along the Pokrovskaya, Uspenskaya, Sretenskaya, and Aleksandrovskaya veins). From the foregoing it follows that, in the process of formation of the structure, a considerable part of the deformation elements develops by inheritance; the stages of emplacement and the stages of activation of the blocks differ in the orientation of stress fields, and the stages of block activation correspond to the formation of new fault systems within the tectonic blocks. Thus, the main features of the structure of the Kochkar ore field may be summarized as follows.

1. The ore-hosting Plastovsky massif of plagioclase granites has a two-tiered structure: an upper tier of brachyform folds composed of rocks granitized to varying degrees; and a lower tier of relatively homogeneous massive plagiogranites situated in the cores of brachystructures. The ore bodies are located in the plagiogranite massif within two brachyanticlinal folds.

The connections between fault and fold forms are expressed in the regular position of faults filled by dikes of plagiogranites, plagiogranite aplites, and plagiogranite pegmatites, and also, probably, in the position of faults bounding large pre-dike blocks. During the ore stage of deformation, the influence of the granite–gneiss tier was manifested in the spatial position of the arsenopyrite–quartz association.

1. The structure of the deposit is determined by the combination of folded forms in granite–gneisses and metasomatic plagiogranites and of a dike framework with a system of tectonic blocks that formed later. Dikes of rocks of variegated composition, transformed into tabashki in the form of series of en echelon and parallel dikes, often branching and including lenses and plates of rocks of the massif, within the deposit form a framework that predetermined the position of a considerable part of the subsequent faults.

2. The system of blocks is a structural element synchronous with the main stages of the hydrothermal process, pre-ore and ore. Considering that the principal role in the localization of mineralization is played by the blocks, the structure of the deposit should be termed blocky. With the development of blo-

... are determined by the position and concentration of fractures of ore time and by the morphological features of the ore bodies.

During the intra-ore time, four mineral associations with differing gold content were successively formed; the industrial value of the ores depends on their combination: the arsenopyrite-quartz association, the quartz-pyrite association, the chalcopyrite-sphalerite-pyrrhotite association, and the gold-tetradymite association.

In the distribution of the last three mineral associations productive for gold, distinct spatial relationships are found between areas of intra-ore rejuvenation of the ore-bearing fractures and the fractures bounding the blocks. Movements of the blocks during the ore stage were the principal structural cause of the uneven distribution of gold in the veins of the Kochkar deposit.

References

1. Borodaevskii, N. I. The Kochkar gold-ore deposit. In: Geology of the Principal Gold-Ore Deposits of the USSR, vol. 3, 1952.

2. Borodaevskii, N. I. et al. New data on the vein rocks and tabashkas of the Kochkar ore field in connection with questions of their genesis and classification. Second Ural Petrographic Conference. Abstracts, Sverdlovsk, 1966.

3. Yanovskii, V. M. Typomorphism of plagioclase granites of the Plast massif in the Southern Urals. Second Ural Petrographic Conference. Abstracts of reports, Sverdlovsk, Ural Branch of the Academy of Sciences of the USSR, Ural State University, 1966.

4. Yanovskii, V. M., Tugolesov, L. D. Features of the composition and structure of the Plast massif of plagiogranites (Kochkar ore field). Soviet Geology, No. 5, 1967.