FEATURES OF THE FORMATION OF QUARTZ VEINS IN THE ZNAMENITINSKY ORE FIELD

S. S. Ilyenok (TPI)

The Znamenitinsky ore field is located on the watershed of the Cherny Ius River and its tributary—the Izekiul River, on the eastern slope of the Kuznetsk Alatau. The area is composed of ancient effusive-sedimentary formations, with a predominance of diabases and porphyries, quartz keratophyres, and rarer horizons of sandstones, conglomerates, as well as lenses of limestones. Within this sequence there are intrusive massifs, represented in the northern part of the district by quartz monzonites, syenites, and granites, with weak development of gabbro-diorites and gabbros in the marginal zones; in the southern part, by quartz and quartz-free pyroxene-hornblende diorites.

Quartz ore veins are situated along narrow zones of fracturing of northeastern and nearly latitudinal strike, expressed in the field of effusive-sedimentary strata. At the intersection of these zones lies an ore cluster in the form of the richest gold deposit (the Znamenitaya, Pozdnyaya, and Pionerka veins).

A characteristic feature is the formation of ore bodies during the deposition of mineral matter as a result of a series of successive stages of mineralization, which are, in turn, connected with the development of fracturing. In this process, new fractures usually form along the selvages, sometimes within the vein itself, with brecciation of the early minerals and their cementation by new mineral matter. The greatest number of mineralization stages is distinguished for the Znamenitinsky ore cluster. This area of junction of fracture zones of two directions was the most mobile. Here 10 stages of mineralization are clearly distinguished, with which 4 generations of gold are associated.

The pulsational character of mineral deposition in the vein is determined by bursts of tectonic movements and the development of fractures, through which hydrothermal solutions penetrated, healing these fractures. Then, after a period of rest, new movements manifested themselves and a new stage of deposition of hydrothermal minerals began. From the character of the mineral associations that appeared, replacing one another, one may judge the evolution of the hydrothermal solutions. It should be noted that even after the completion of hydrothermal activity, tectonic movements continued to take place, which led to the appearance of fractures along the selvages with vein clay, in places to the crushing of minerals composing the body of the vein, and to slight deposition of carbonate from cold waters. Occasionally, workings revealed gaping fractures with a thickness of up to 20—

30 cm. All this determined the banded structure of the veins. In the composition of the latter one may encounter bundles of early quartz, parallel to which, through a false selvage with vein clay, lies a bundle of a later quartz–carbonate aggregate. The main mass of sulfides appeared after deposition of the main mass of the vein material. Therefore the sulfides form independent stringers either within the quartz vein or along the selvage. In places where the vein bends, the sulfides also follow independent fractures beyond the limits of the vein body. At the same time, the deposition of sulfides occurred in several stages, associated with separate stages in the development of the intermineralization fractures.

Tracing the individual bundles that make up the vein, the character of their pinching out, and the sequence of development makes it possible to identify the principal features of formation of the vein bodies. This makes it possible to see a regular connection between the morphological features of veins in their different parts. In particular, within a single ore district, the presence of both saddle-shaped and folded veins, as well as vein segments persistent linearly and in thickness with comb quartz and geodes, indicating the filling of cavities, finds its explanation.

Individual bundles of vein material form well-persistent bodies up to 100–150 m long. They terminate in areas distinguished by a peculiar character, for here the enclosing rocks are crumpled into schists with strong hydrothermal alteration. The original greenstone porphyrites have been transformed into sericite–chlorite formations with abundant quartz and calcite in the form of veinlets along the schistosity. Disseminated pyrite is uniformly developed. The schists are gathered into small multistep folds. They are localized only within the limits of vein fractures, at the place where these fractures terminate. The quartz filling the bent and folded parts of the vein in these areas is always later than the quartz of the main vein (Fig. 1). A little white calcite is usually associated with it. These vein minerals show similarity to the thin veinlets in the schists and are material released into fractures during hydrothermal alteration of the enclosing rocks. The formation of the schists is connected with residual elastic deformations localized at the place where the vein fractures pinched out.

The formation of the structure of the ore field began with tectonic movements and the development of linear fracture zones. Analysis of the fracturing indicates the manifestation of the principal pressure from the west [1]. Owing to heterogeneity in the composition and physical properties of the masses composing the district, zones of fracturing appeared that passed around blocks of rigid intrusive massifs. These zones are observed in the enclosing effusive–sedimentary rock sequence. In the area of the northern termination of the Bel’skii intrusive massif, heterogeneous movements (twisting stresses) took place because of the influence of a rigid buttress and the development of a series of en echelon fractures. It must be assumed that the physical properties of the effusives permitted the appearance of fractures prone to gaping. This can be seen from the results of mining operations at the Znamenitinskoe deposit. During excavation work in blocks where timbering was absent, the roof bent elastically, without loss of continuity, down to the floor by 2 m in height, having abutments in the form of pillars at a distance of 50 m (Fig. 2). This convincingly demonstrates the real possibility of opening gaping fractures, which were then filled with mineral matter. The presence in the veins of comb quartz, geodes, and also zoning in prismatic quartz grains, noticeable from changes in color and inclusions in the mineral, points to the manifestation of such fractures.

The places where fractures die out, i.e., the ends of the elastic arc of an opened fracture, are characterized by the presence of residual elastic deformations. Here the pressure does not exceed the strength limit of the rocks, overcoming co-

which is necessary for opening the fracture. Mineral solutions circulating along the fractures, in these areas, transform the rocks into schists. At the same time, the pressure here is partly relieved through the development of schistosity. If, in the segments where linear fractures are filled, the alteration of the enclosing rocks is weakly expressed, then at the places where they pinch out it is very sharply expressed. In addition to filled fractures, due to repeated delamination of the fractures, here there arise parting veins with replacement of the enclosing rocks.

Fig. 1. Morphological features of a quartz vein (1) at the place where it pinches out among schistose hydrothermally altered porphyries gathered into small folds. Face of the sublevel drift of block 7, level 7 of the Znamenitaya vein.

Fig. 2. Example of elastic sagging of the roof over the mined-out space in block 5, level 7 of the Znamenitaya vein.

Repeated development of fractures as a result of a new pulse of tectonic movements usually leads to the appearance of delamination fractures along the selvage of the hanging wall of the vein. The new opening of the fracture proceeds both parallel to the previously formed vein and farther along the strike. A new portion of mineral solutions forms deposits of a second vein packet, which along the strike extends far beyond the pre-

the parts where the early vein packet pinches out and lengthens the vein. Here, in the area of accretion of the vein, one can see the individual features of the mineral composition of the new stage of mineralization and the nature of alteration of the wall rocks. In the area where the new packet of the vein pinches out, the development of schists penetrated by quartz-carbonate veinlets is also manifested; among these are observed thicker parts of the vein, of saddle-like or folded form. For the ore node of the Znamenitaya and Pozdnyaya veins, after deposition of the early grayish-white quartz, the formation of veins and packets of white quartz and ankerite took place, accompanied by scheelite and pyrite (Fig. 3). In places between these two packets of the vein, lens-shaped xenoliths of hydrothermally altered rocks are observed.

Fig. 3. Example of accretion of a quartz vein in thickness and along strike: 1 — quartz vein of the 1st stage; 2 — quartz-carbonate vein of the 2nd stage of mineralization; 3 — local development of schists in places where vein-forming fractures pinch out. Eastern adit of the 6th level of the Znamenitaya vein.

Complication of the vein form occurs in areas of branching of pre-ore fractures, where bulges of ore bodies usually develop. Especially large bulges of veins are encountered in areas of knee-shaped bends of pre-ore fractures, caused by features of the structure of the enclosing rocks. The displacements that appear, usually from the side of the hanging wall rocks, lead to brecciation of the latter and to intensive replacement of their breccia mass by quartz (“perebutor,” in miners’ terminology). In the upper levels of the southern part of the Yakovlevskaya vein, the thickness of the vein in the bulging areas reaches 25 m. Such parts of the vein, as a rule, contain no mineralization.

Many generations of quartz following the formation of the principal vein mass of the bodies have a small volume and do not increase the mass of the vein to any significant degree. They have the character of fillings of thin fractures, often threadlike, and even microscopic in character.

The main stages of deposition of vein minerals are accompanied by a small quantity of ore minerals in the form of magnetite, hematite, pyrite, and scheelite. Gold of the first generation is associated with pyrite. As stated above, the principal mass of sulfides appeared in the form of several successive stages, after the formation of the vein mass of the bodies. They were deposited along independent fractures that pass either inside the vein, or along its selvage, or these fractures extend beyond the vein into the enclosing rocks. Three generations of gold are associated with the sulfide stage: low-grade gold with galena, high-grade gold with chalcopyrite, and also with minerals of the bismuth-tellurium group—bismuthinite, calaverite, and tetradymite.

It should be noted that the form of ore occurrences along fractures without accompanying vein material is widespread in the district in areas south and west of the Znamenitinskoye deposit and was described by the author earlier [2]. This fact is of great prospecting significance.

Thus, the mechanism of staged opening of fractures with the development of local areas of schists and folded veins makes it possible to identify

...the nature and genetic combination, in a single deposit, of veins formed by filling and by replacement, as well as the various features of their morphology. At the same time, for the Znamenitinsky deposit area, as for the other veins of the district, the leading process was the formation of fractures capable of gaping, and their filling with mineral matter.

REFERENCES

1. Il’enok S. S. A feature of the structure of the ore fields of the Znamenitinsky mining-ore district in Khakassia. Izv. Tomsk Polytechnic Institute, vol. 65, issue 2, 1950.

2. Il’enok S. S. The “Kustovoe” deposit in the Kuznetsk Alatau. Zolotaya promyshlennost’, No. 7, 1939.