DISTRIBUTION OF GOLD IN THE VEINS OF THE KOMSOMOLSKOYE DEPOSIT

(KUZNETSK ALATAU)

Yu. P. DENISOV (Berikul Mine, Zapsibzoloto Trust)

The Komsomolskoye deposit belongs to the Berikul group of vein-type gold-ore deposits and is localized in a gabbro-diorite stock with an area of about 15 sq. km, a northern satellite of the Dudet granodiorite batholith. The Komsomolsky stock cuts through a sequence of Proterozoic and Cambrian marbles, and in the northwest—intrusive bodies of porphyrites and ophitic gabbro of Middle Cambrian age. The internal linear-zonal structure of the stock was created by successive injections of differentiates of basic magma, which, in composition and structure, are subdivided into two series. Rocks of the first series form the foundation of the stock and are represented by gabbro-diorites of closely similar composition, differing in structure. The core of the intrusion, saturated with xenoliths of marbles and dioritized porphyrites, is composed of gabbro-diorites of prismatic-granular structure; from the core toward the contacts of the intrusion there is a successive alternation of discontinuous zones of gabbro-diorites of poikilitic structure, prismatic-granular, poikilitic, hypidiomorphic, and gabbro-diorites with flaky biotite, etc. Bodies of younger hypersthene-bearing rocks—gabbro-norites, hypersthene diorites, quartz monzonites, and quartz diorites—are situated in the endocontact zone, locally cutting across the northeastern structures of the stock created by rocks of the first series. All these rocks belong to the Martaiga complex, are cut by numerous dikes of its latest varieties—granites, aplites, and pegmatites—and are dissected by gold-bearing quartz veins and a series of post-ore diabase dikes belonging to the Kiya complex.

A generalization of data on determination of the absolute age of intrusive rocks of the Kuznetsk Alatau shows that the gold mineralization associated with the Martaiga intrusive complex and preceding the formation of the alkaline rocks of the Kiya complex was formed in the interval from 425 to 415 million years ago, which corresponds to the Lower Devonian.

In the ore field of the Komsomolskoye deposit, a large number of tectonic disturbances and about 150 gold-bearing quartz veins are known, the main mass of which is concentrated in the southeastern zone.

stock 1000–1500 m thick. The most productive placers have also been worked here. The veins are grouped into ore nodes; in each node there are one or two principal veins, which are of greatest interest. The largest veins—Komsomolskaya, Klyuchevskaya, Bolshaya, and Zdorovaya—have a northwestern strike with an eastward dip at an angle of 30–55°; they are traced from the contact of the intrusion with the marbles through the massive endocontact zone for 1–1.5 km and die out in the core of the intrusion, saturated with xenoliths and unfavorable for the formation of persistent fractures because of its heterogeneity. In the northwestern part of the stock there are not only no large veins analogous to Komsomolskaya, but also no northwestern-oriented faults capable of accommodating such veins. Here even small veins are rare, and samples with a gold content of more than 1 g/t are isolated. The southeastern part of the field, in the immediate vicinity of the contact of the intrusion with the marbles, is distinguished by the greatest number of veins.

The causes of the asymmetric distribution of veins within the stock may be considered to be those features of the geological structure of the massif that determined the uneven distribution of stresses during the emplacement of the ore-bearing fractures: (a) the different character of the southeastern and northwestern contacts; (b) the presence in the northwest of a fault of northeastern orientation; (c) the heterogeneous composition of the intrusion and, above all, the brecciated structure of its core. The temperature of the enclosing rocks, and the temperature of the solutions dependent on it, also had a significant influence on the distribution of gold in the ore field. In studying the distribution of gold and sulfides in the veins and the hydrothermal alteration of the enclosing rocks, a horizontal zoning directed from the contacts inward into the intrusion was established. In the endocontact belt, about 800 m thick, all veins are accompanied by zones of intense sericitization and pyritization, and all reserves of commercial ores are concentrated here. Then, over a distance of about 200 m, the wall-rock alteration is expressed in silicification with weak pyritization and hematitization of the gabbro-diorites, and among the ore minerals in the veins only pyrite occurs. Finally, in the inner zone, explored in detail on the northern continuation of the Klyuchevsky ore node, vein formation is accompanied by ankeritization and chloritization of the gabbro-diorites, and the ore minerals in the veins are represented by disseminated pyrite. The character of the zoning is usual for zones of listvenitization and leaves no doubt that we are dealing with formations of different temperatures and that, with respect to the Komsomolsky stock, the zoning is anomalous: the lowest-temperature alterations are located closer to the core of the intrusion. Certain features of the structure of the zones of hydrothermal alteration—the constancy of composition; the absence of any dependence of composition on the thickness of the zone or of the quartz vein, on the presence in it of particular sulfides, or on the structure of the enclosing rocks—serve as evidence of the stability of the temperature conditions of alteration of the walls of the vein fissures throughout the entire process. Such stability cannot be achieved by heating cold rocks with hot solutions of different temperatures, and was ensured by the influence on the process of the high temperatures of the enclosing rocks. The most heated at the time of ore formation were the gabbro-diorites of the southeastern contact, probably warmed by hot or still molten masses of the granitoids of the Dudetsky batholith. Quantitative estimation of the temperatures is difficult, but at 500° for the rocks of the endocontact of the Komsomolsky stock the gradient was 250–300° per 1 km. It may be assumed that during the period of ore formation there existed a zone of temperatures favorable for mineralization, corresponding to the distance

2.5–4 km from the contact of the granitoids in the Dulet batholith, and the Berikul group represents a typical example of halo distribution of deposits.

The vertical extent of mineralization in the Komsomolskoye ore field depends on the direction of plunge of the gabbro-diorite stock, tectonic conditions, and displacement of the boundaries of the productive temperature zone. The available materials show that mineralization at a depth of 650 m does not differ in character from mineralization in near-surface zones: the composition of the veins, gold content, paragenetic associations, and type of wall-rock alteration are preserved. The columnar character of the distribution of gold, established in all the major veins of the deposit, is also preserved at depth. The parameters of the columns are measured in tens and hundreds of meters, while their orientation is characterized by considerable variety. The ore column of the Pionerskaya vein is oriented in a west-northwesterly direction, that of the Bolotnaya vein in a north-northwesterly direction, that of the Komsomolskaya vein in a submeridional direction, and veins No. 35 and No. 60 in a northeasterly direction. The distribution of gold has been studied better than elsewhere in the exploited veins of the Komsomolskoye ore cluster—Komsomolskaya, No. 35, and No. 60. The Komsomolskaya vein has been explored from the surface to the 420 m level, and within it the ores of two parallel ore columns have been mined; these are clearly confined to places where the vein flattens. The upper column has been mined for 800 m along the strike of the vein, from the surface to the 115 m level. The dip angle of the vein in this segment is about 30–35°. Then the vein twists to 40–45° and is represented by a zone of mylonitized rocks with separate quartz lenses. From the 270 m level the vein flattens to 30°, and in the depth interval of 270–370 m it contains ores with commercial gold grades. Below the 370 m level the vein again acquires a dip angle of 40–45° and has not been mined. The dependence of the distribution of commercial ores in the vein on its dip angle is also confirmed by statistical data. V. A. Rubanov (TPI) constructed rose diagrams of the dip angles of the Komsomolskaya and No. 35 veins, both for all measurements and for that part of the measurements that were made in areas of commercial ores. The Komsomolskaya rose of angles has a two-petaled structure, with maxima at 32° and 42°; measurements of commercial ores are confined to the upper part of the upper petal (angle 20–25°). The rose of angles for vein No. 35 has a more complex structure, but the position of commercial ores is fixed by the same dip angle of 25–27°. Vein No. 35 is located in the hanging wall of the Komsomolskaya vein and dips eastward more gently, at an angle of only 18–30°; vein No. 60, situated above it, has a dip angle of 10–25°. The ore columns of veins No. 35-south, 35-north, and No. 60 plunge in a northeasterly direction, intersecting in plan with the ore columns of the Komsomolskaya vein. The ore column of the 35-south vein has been mined for 800 m and traced for 1200 m down the plunge, from the 85 m level to the 620 m level, with a width from 70 m to 250 m. It is situated between the contact of the intrusion with marbles and a narrow zone of carbonate xenoliths parallel to the contact, which serves as the northern boundary of the column. At the same time, the upper ore column of the Komsomolskaya vein cuts the zone of xenoliths without substantially changing its parameters. This is explained by the greater tectonic activity of the Komsomolskaya fissure in the pre-ore period, which led to rupture of the xenoliths. Displacements along vein No. 35 were so insignificant that the fissures could not cut through the xenoliths, forming within the zone and on both sides of it a series of small overlapping segments. The different orientation of the ore columns of conjugate veins proves that the position of the columns was not decisively influenced by factors that, with respect to the vein, may be called external—

by them: tectonic disturbances, dikes, zones of fracturing, zones of xenoliths, etc. The position of commercial ores in the vein is controlled by the morphological features of the vein fissure itself. On the one hand, this is demonstrated by the above-described relationship between the dip angles of commercial and barren segments of the vein. It is also clearly demonstrated by comparison of isograms of thicknesses and grades, constructed for mined-out segments of the veins. On the grade isograms, the internal structure of the ore shoots is revealed, formed by ore streams up to 10 m wide, producing a complex looped pattern; a similar pattern is revealed on the thickness isogram. It should be noted that the thickness isogram, constructed from production sampling data, suffers from large errors caused by the method used for sampling veins and altered rocks over the “bulk” thickness. Numerous attempts to establish a direct dependence between the thickness of a vein and the gold content in it by methods of mathematical statistics have failed. Nevertheless, comparison of the isograms shows a general regularity in the distribution of the indicators, although the maxima do not coincide, and permits the assumption that the identical character of variability is caused by the same factors. Since the veins of the deposit belong to typical filling veins, the principal cause may be the relationship between the morphology of the vein fissures and the direction of post-vein movements, which determined the shape, dimensions, and position of the opening cavities and the sites of greatest crushing.

The connection of mineralization with post-vein tectonics becomes unquestionable when the composition and structure of the ores are studied. Reticulate, breccia-like, veinlet, and banded textures of quartz-sulfide ores are widely developed, with numerous slickensides in the salbands and intensive replacement of crushed minerals of early generations. Gold is associated with sulfides represented by pyrrhotite, pyrite, sphalerite, chalcopyrite, arsenopyrite, and galena, and in terms of the time of its deposition in the ores it is one of the latest. The ore process begins with metasomatic alteration of the walls of the fissures by solutions, with the formation of hydrothermally altered rocks. Then follows the opening of cavities and their filling with quartz, the superposition of pyrite and pyrrhotite on quartz, and the formation of pyrite-pyrrhotite bodies during replacement of carbonate xenoliths. The productive phase of the process is accompanied by intra-ore movements and by the formation of quartz-calcite veins, deposition of sphalerite, then arsenopyrite, galena, scheelite, and gold. The general regularity in the distribution of gold and other ore components is confirmed by high coefficients of correlation of gold with silver (0.739), lead (0.625), and arsenic (0.621), obtained in processing the sampling data on an electronic computer.

After its deposition in the ores, gold does not undergo any noticeable redistribution. Numerous diabase dikes, cutting the fully formed ores, cause only slight regeneration of the principal components. In the calcite veinlets cutting the dikes and ores, pyrite and tabular pyrrhotite, sphalerite, and galena are encountered only rarely. Equally rare are aggregates of crystalline and colloform retinite, which completes the hypogene process, cutting calcite veinlets or growing on brushes of transparent calcite crystals.

No redistribution of gold occurs during oxidation of the ores either. Construction of isograms of gold content in oxidation zones, study of oxidized ores and vein outcrops at the surface, and comparison of placer-

and primary gold convinces us that the transport of gold in the oxidation zone is too insignificant to affect the industrial value of the veins. At the same time, the well-known effect of “secondary enrichment” of veins in the oxidation zone can be explained by a decrease in the volumetric weight of the ores during the leaching of sulfides, without any change in the total quantity of gold. According to an approximate calculation, with a sulfide content in the primary ores of 15%—the average figure for the Komsomolskoye deposit—a coefficient of enrichment of 1.2 may be observed in oxidized ores; with 50% sulfides, the coefficient of enrichment reaches 2.0. The enrichment coefficients established by A. D. Mikov for deposits of the Berikul group are in fact estimated at 1.2–1.4.

Thus, the distribution of gold in the ore field and in the veins of the Komsomolskoye deposit was influenced by:

1. The structure of the gabbro-diorite stock and of the enclosing sequence;
2. The temperature gradient created by the Dudetsk batholith;
3. Various elements of the morphology of the quartz veins, above all their orientation, dip angles, and thickness.