Assoc. Prof. G. P. Bolgov

SULFIDES OF SALAIR. THE URSKA GROUP OF POLYMETALLIC DEPOSITS

INTRODUCTION

The developing nonferrous metallurgy of Western Siberia is, at present, far from being supplied with cheap local raw material. The ore reserves of the Salair deposits proper amount to somewhat more than 200 thousand t, which, in essence, only guarantees the depreciation period of the already operating Belovo zinc plant. As for the reserves of ore-bearing rocks or ores of the disseminated type, although the figure for the latter exceeds 400 thousand t, all these reserves are in category C; moreover, the question of their beneficiation has not yet been finally resolved. Such a situation in no way provides for the Kemerovo electrolytic plant now under construction, whose annual capacity is planned at 50 thousand t of zinc. In this connection, the study and search for new deposits of polymetallic ores are an urgent task of the present day.

And so, in order to approach more or less correctly the prospective evaluation of the district, and also to provide some guiding and orienting material for subsequent geological prospecting work, the question arose of the immediate study of the genesis of the polymetallic deposits. In this direction, the Belovo zinc plant showed great activity; in addition to carrying out routine exploration work, it undertook an analysis of the accumulated substantial factual material. The execution of this task fell partly to me, in connection with which, in the summer of 1934, I collected material for mineralogical investigations.

If sufficient attention has been paid in recent years to the group of Salair mines, then the Urska group, with its largest deposit “Lense 2,” remained in the shadows, owing to the poorly developed exploration work. But even the material that was obtained from five boreholes cutting through the deposit at the 50, 100, and 200 m levels was sufficient to characterize the structure, mineralogical composition, and to reconstruct the history of its formation. Attaching especially great importance to the deposits of the Urska group, I set myself the task, first of all, of studying the latter with sufficient completeness, which has found its expression in the present work.

I take this opportunity to express my deep gratitude to P. M. Lopushinsky, head of the GRB of the Belovo plant, and

his deputy—the geological technician S. S. Gering, who placed at my disposal graphic and rock material on the deposits of Salair.

The laboratory treatment of the material was carried out by me in 1934–1935 at the Tomsk Industrial Institute under the direct supervision of Prof. F. N. Shakhov and Prof. M. A. Usov, whose valuable guidance I made use of repeatedly, and for which I express to them my sincere gratitude.

I. A BRIEF GEOLOGICAL SKETCH OF THE DEPOSITS OF THE URSKA DISTRICT

The polymetallic deposits of the Urska group are confined to a complex of metamorphic formations, the original composition of which is not always susceptible of precise determination. In the bulk of their mass, the rocks of this sequence are represented by albitophyres, quartz porphyries, and, in places where hydrothermal processes were intensively manifested, by porphyroids, quartz-sericite and sericite-chlorite schists formed at their expense.

In the form of a comparatively narrow belt this metamorphic sequence extends in a NW–SE direction and is enclosed among Archeocyathid limestones, the age of which, according to the latest data of A. G. Vologdin¹, is determined as Lower Cambrian or the base of the Middle Cambrian. Observations on the relationship of these formations lead A. G. Vologdin to the conclusion that the rocks composing the sequence of metamorphic schists owe their origin to the appearance in the Middle Cambrian period of abundant flows and sheets of quartz albite porphyries, interbedded with tuffaceous and tuffaceous-sandy material. Along with these outpourings of acid effusives, in the author’s opinion, intrusions of the same composition probably also occurred, which at present cannot be distinguished because of their subsequent intensive alteration. Personally, however, it seems to me that such an acid, and even ultra-acid, magma could hardly have produced abundant flows. Most probably, during their eruption the lavas did not reach the surface, but solidified at some depth, forming sills and stock-like masses.

But if we do not have sufficient data for resolving this question, then, turning to the conditions of occurrence and to the composition of the effusive rocks composing this sequence, we shall find certain material that makes it possible to express more or less probable assumptions about the sequence of their eruption. First of all, one may make use of the relation of the masses of rocks of different—

of a given composition to one another. And indeed, if, within the limits of the metamorphic belt, where the predominant role in its composition is played by albitophyres, which for the most part have the character of various schists, true quartz porphyries appear in the form of thin veins and stock-like masses, then it is natural to think that the appearance of the latter was preceded by the formation of those rocks in which they occur, despite the fact that by our time the sharpness of the contact boundaries between them has proved to have been smoothed out. Thus, it seems to me that there is every reason to speak of a relatively younger age of the quartz porphyries as compared with the albitophyres enclosing them. At the same time, there is no doubt that the moments of the successive eruption of these rocks were geologically close.

In addition to the porphyry rocks described above, which have a fairly ancient aspect, younger formations of the composition of gabbro-diabases or diorite-diabases are also encountered in the district. Thus, for example, G. S. Labazin notes finds of fragments of gabbro-diabases near the Samoilovsk deposit. According to A. G. Vologdin, who carried out detailed geological surveying in the district, it turns out that these rocks are fairly widespread, forming large dikes and even intrusive bodies; with this gabbro-diorite intrusion, as we shall see below, the author attempts to connect the formation of the pyritic deposits of the Ursk district.

In general, the history of the formation of the metamorphic sequence described may be represented as follows. In the Middle Cambrian period, vigorous movements of magma began in the district; the magma soon found an outlet for itself, piercing the Lower Cambrian limestones that had formed by that time. Such easy penetration of this magma was evidently promoted by the large zones of faults that existed here, as is emphasized by the persistent distribution of volcanic rocks in a definite direction. These structural lines, which predetermined the sites of the magma’s emergence, in the opinion of B. F. Speransky¹, were laid down as early as Precambrian time and readily opened in subsequent revolutionary epochs along these weakened seams. This opinion is also supported by A. G. Vologdin². The period of magma eruption was apparently not especially prolonged, but undoubtedly it proceeded intermittently, in several episodes, of which we have the possibility of recording two. The first moment is characterized by the eruption of masses corresponding in composition to albitophyres, which solidify near the surface in the form of sills. After a certain interval of time, quite probably sufficient for the magma to have had time to differentiate somewhat, possibly along

¹ B. F. Speransky, Structures of the Paleozoic Formations of the Ob–Tom Interfluve. Collection on the Geology of Siberia, 1933.
² A. G. Vologdin and A. A. Predtechensky, p. 162.

along the same feeder channels there penetrates the most acidic part of the magma, which, upon solidification, gave rise to typical quartz porphyries. In individual cases it is possible that this portion of the magma broke through to the surface, but we have no material for such an assertion, since erosion has left us only the roots of these channels in the form of veins, dikes, and stock-like masses.

After the complete cessation of extrusive activity, the rocks that had formed were subjected to repeated tectonic pressure and, as a result of dynamo-thermal metamorphism, underwent significant changes, being transformed into typical schists. Then dike-type formations of diorite composition were again intruded into the rock mass. At this point the history of the formation of the sequence, in the sense of the appearance of new rocks, breaks off. But in speaking of this, we must find a place in this history for those deposits that occur within the sequence described. For the reasons set forth in the last chapter, their connection precisely with the Middle Cambrian quartz porphyries appears to us the most probable.

II. DESCRIPTION OF INDIVIDUAL DEPOSITS

The Urska group of pyrite deposits includes the following: 1) “Lens 2,” 2) Beloklyuchevskoe (“Lens 1”), 3) Samoilovskoe, 4) Kopennoe, and 5) Urskoe. The last four deposits are described in considerable detail in the work of G. S. Labazin, “Ore Deposits of Salair,” and therefore I shall dwell on their characteristics only very briefly, noting only the features of greatest interest in them. I devote the main attention to the “Lens 2” deposit, which at that time was not yet known to Labazin and therefore has received little treatment in the literature. This deposit requires a more detailed description also because, at the present time, it has been explored more than the others, both by surface workings and by core drilling.

1. THE “LENS 2” DEPOSIT

The “Lens 2” deposit was discovered in 1932 during detailed geological surveying of the area of these deposits. In the course of trenching, at a distance of 1.7 km south of the Beloklyuchevskoe deposit, on the slope of a small elevation, a zone of barite sands was discovered. In view of the appreciable gold content in these sands, the zone of barite “sands” was subjected to detailed exploration by Soyuzzoloto, as a result of which it proved possible to outline the deposit at the surface; it was found that the length of this zone is about 400 m, with an average thickness of 15 m. To a depth

“Syshchka” was explored to 18 m by two shafts, later connected by an adit.

The fairly persistent thickness of the deposit in outcrops on the surface led to the supposition that primary pyrite ores, with a possibly significant zinc content, were present at depth. Therefore, in 1933 the geological-exploration base of the Belovo zinc plant initiated core drilling.

Conventional symbols

• Cross-hatched ore
• Zinc-bearing ore
• Pyritized sandstone
• Barite-bearing vein
• Sericitized schists mineralized
• The same, oxidized
• Albitophyres, weakly recrystallized
• Chlorite-sericite schists
• Sericite schists
• Quartzification of schists

Fig. 1. The “Lens 2” deposit. Section along the strike of boreholes Nos. 47–48.
Scale 1:500.

The very first borehole (No. 47) intersected primary ores at the 50 m horizon. It was found that, in the composition of the ore, the zinc content in individual sections reaches 10%, and averages up to 4% over the ore body. The remaining four boreholes were laid out in order to clarify the nature of the mineralization at depth, as well as along the strike. On the basis of the drilling work carried out, interrupted in the autumn of 1934, three vertical sections were compiled, giving a clear idea of the dimensions, thickness, and character of the enclosing rocks of this deposit (Figs. 1, 2, 3). As can be seen from the appended sections of boreholes Nos. 47, 48, 52, 49, and 50, the thickness of the deposit increases sharply with depth, while on one and the same horizon it decreases toward the ends of the ore ...

body. If at the 50 m level the thickness of the ore body is 35 m, then at a depth of 200 m it reaches almost 50 m. Such a gradual but steady increase in thickness with depth, taking into account the experience of similar massive-sulfide deposits of the Urals, indicates a considerable extension of the ore body in depth. We are not yet able to judge the more exact form

Legend: 75 m level

• alluvium
• sericite-quartz and quartz-sericite schists
• the same, with disseminated pyrite
• chlorite-sericite schists
• the same, with disseminated pyrite
• albitophyres
• barite sand
• sulfur-pyrite and zinc-sulfur-pyrite ores
• sulfur-pyrite sand

Fig. 2. M-mine “Lens 2”: Section along borehole No. 49.

of the ore deposit, other than that which appears to us on the basis of drill holes and surface prospecting. We can only say that it is a large lens-shaped body, gradually pinching out as it approaches its ends. However, the surface work of Soyuzzoloto in the summer of 1934 exposed, at the southern end of the deposit, two thin, diverging small lenses, represented at the outcrop by strongly leached porous formations (“sukhari”) and resembling in themselves corre-

corresponding porous quartz-barite rocks of the Salair deposits proper. In all probability, these small lenses are nothing other than a continuation of “Lens 2,” branching into “fingers” as it pinches out. In addition, drilling shows the presence, within the ore body, of interbeds of sericite schists.

Thus, there is no doubt that in reality the configuration of the deposit is more complex, if one also takes into account

Dip angles of the borehole

Depth	Angle
0 m	85°
25	83°
50	80
60	72°
85	69°
100	58°

Legend

Symbol	Explanation
dotted pattern	alluvium
diagonal hatching	sericite-quartz and quartz-sericite schists
diagonal hatching with inclusions	the same, with disseminated pyrite
small chevrons	chlorite-sericite schists
tick marks	weakly altered albite porphyries
blank	barite sand
blank	sulfur-pyrite sand
black fill	sulfur-pyrite ore and zinc

Fig. 3. M mine, “Lens 2.” Section along borehole No. 50. Scale 1:500

also the structure of the ore bodies of the Salair mines proper.^1

As for the mineralogical composition of this deposit, here we shall note only the following. In the bulk of its ore

^1 The work of 1935 and 1936 clarified the structure of the deposit.

the “Linza 2” deposit constitute a massive fine-grained aggregate of pyrite, to which, in one or another—though always sharply subordinate—amount, sphalerite, chalcopyrite, pale ores, and galena are admixed; moreover, a certain zonality, or banding, is observed in the spatial distribution of the individual minerals. Thus, sphalerite, and usually galena together with it, are concentrated in gigantic bands, most often confined to the sides of the ore body, and more rarely appearing in the middle. In such areas sphalerite, galena, and their constant companion—barite—play a significant role in the composition of the ore. At the same time, not infrequently, owing to the alternation of substantially sphalerite-barite bands with pyritic ones, banded ores are formed. A more detailed characterization of the mineral composition, as well as of the enclosing rocks, is given below.

2. THE BELOKLYUCHEVSKOE DEPOSIT

The Beloklyuchevskoe deposit, “Linza 1,” was discovered in 1931 by a prospecting party of the Belovo Zinc Combine as a result of the tracing of electrical anomalies. It is located 1.7 km from “Linza 2” and 2.4 km from Mount Kopna, occupying the highest part of its immediate surroundings. At the surface this deposit presents an almost complete analogue of the “Linza 2” deposit, with the only difference that here we have a typical “iron cap,” represented by brown ironstones of various structure, lying directly upon the barite “loose zone.” The thickness of this cap does not exceed 4 m in horizontal section and 7–8 m in vertical section. The ore body occurs in the form of a lens-shaped vein and can be traced for 130 m, having a maximum thickness of about 9.5 m. As a result of testing the barite “loose zone,” it showed quite satisfactory results for gold, which served as the basis for designing a barite plant here as well. At present the deposit has already been considerably worked from the surface by open cuts, and the shaft sunk at the southern end of the lens is at a depth of about 50 m, without, however, having passed out of the zone of the barite “loose zone.” The latter circumstance compels one to doubt G. S. Labazin’s conclusions about the pinching out of the deposit at a depth of 45 m. In all probability, the two drill holes that were set down (Nos. 39, 36) deviated down-dip, without passing through the zone of the “loose zone.”¹

The deposit lies among quartz-sericite schists, which, away from the ore body, are replaced by green schists. In addition, borehole No. 39, as well as two surface pits, uncovered schistose quartz porphyries closer to the ore body, where processes of sericitization were intensely manifested, passing into quartz porphyroids.

The primary character of the ores is unknown, unless one counts the thin bed, encountered in borehole No. 36 and represented exclusively by pyrite, as the root of the ore body.

3. SAMOILOVSKOE DEPOSIT

The Samoilovskoe deposit is located between Beloklyuchevskoe and “Lens 2,” on the left bank of the Zvonchikha River. The deposit was discovered as early as 1840 and, in the subsequent period, was prospected several times. According to archival data, it consists of two quartz–barite veins situated along one line, 147 m apart from one another, and having a rather steep dip to the SW. The thickness of the veins ranges from 0.5 to 0.4 m. For one of the veins it is stated that it has been traced to a depth of 17 m and along strike for 64 m. According to the exploration data of G. S. Labazin, mineralization at the surface is expressed in the presence, among sericite schists, of a zone of ferruginized schists, silicified and baritized rocks, and also secondary quartzites, weakly mineralized. The length of this mineralized zone reaches 220 m, and its width is about 20 m. The secondary quartzites form separate, comparatively small (20 and 40 m) outcrops.

The deposit is confined to a rather considerable (250 × 450 m) zone of quartz–sericite schists, which, away from the sharply manifested mineralization, pass into albite–chlorite and quartz–chlorite schists, undoubtedly also formed here at the expense of albitophyres. Quartz porphyries and porphyroids occur in a very subordinate relation in the southern part of the deposit.

The primary composition of the ores of this deposit is unknown, but, judging from analyses of samples from the oxidation zone, it must be supposed that such sulfides as pyrite, sphalerite, galena, etc., undoubtedly occurred in the primary ores—in general, apparently, the entire assemblage of minerals characteristic of the “Lens 2” deposit. In addition, minerals such as hematite also occur, appearing in the quartzites in a disseminated state or in the form of thin elongated crystals. In the oxidation zone, besides the vein minerals (barite, quartz, sericite), limonite and hydrohematite are common, forming either encrusting forms or thin films lining cavities in vesicular quartz–barite rocks.

4. KOPENNOE DEPOSIT

The Kopennoe deposit was discovered as early as 1798; it was then subjected to surface prospecting in 1827, after which, owing to very weak mineralization, it was abandoned. Only in 1928–1930 was it again examined and additionally explored by shallow mine workings, core drilling, and electrical prospecting.

According to archival data, as well as according to the works of G. S. Labazin, the Kopennoe deposit consists of secondary quartzites, limonitized and ferruginized to varying degrees, which make up Mount Kopna, situated between the Zvonzikha and Kopennaya streams, left tributaries of the Ur River. Along its strike the body of secondary quartzites can be traced for 1.3 km, with a maximum thickness in the northern part of more than 100 m. This rather large quartzite lens lies among quartz-sericite and sericite-quartz schists, reaching a thickness of up to 300 m on the hanging-wall side (on the western side of the lens) and about 100 m in the footwall. By analogy with the preceding deposits, here too we observe a gradual transition of sericite schists into green ones (albitophyres). In outcrops at the surface, the secondary quartzites are a coarse-platy or massive rock, usually gray in color (when mineralization is absent in them), or yellowish and brownish owing to the abundant occurrence in them of brown iron ore. The abundance of cavities and their character testify to the presence of pyrite in the weathered varieties. The latter is fully confirmed by a drill hole put down on the deposit in 1930 for the purpose of checking an electrical anomaly. The hole reached a depth of 130.1 m and, along its course, intersected sericite schists in the interval 0–35.46 m, pyritized secondary quartzites from 35.46 to 115.89 m, and then again passed through quartz-sericite schists for the remaining part. Among the mineralized quartzites, three interbeds with an abundant content of pyrite are distinguished, with a total thickness of 31.75 m. Chemical analyses of the pyritized quartzites give a very low content of copper, silver, and gold, which is in full agreement with the mineralogical composition, in which, apart from pyrite and microscopically small grains of chalcopyrite, usually forming inclusions in the pyrite, nothing is recorded.

In a detailed microscopic study of the secondary quartzites, one can very often observe a relict porphyritic structure, expressed in the preservation of phenocrysts of quartz scattered among a fine-grained aggregate of secondary quartz. This circumstance was also emphasized by G. S. Labazin as one of the direct proofs of the primary character of the rocks—namely quartz porphyries subjected to intensive metasomatism.

The Ursk deposit consists of a series of quartz veins of comparatively small thickness (up to 5 m) and, according to G. S. Labazin’s conclusion, has no value.

III. MINERALOGICAL COMPOSITION OF THE ORES

As was noted above, we have at our disposal too little material for a complete characterization of the mineralogical composition of the individual deposits. We do not know the composition of the primary ores of the Samoilovskoe and Beloklyuchevskoe deposits—

deposits; the material on the Kopennoye deposit is rather poorly represented. Therefore the characterization of individual minerals of the primary-ore zone given below will be based chiefly on material obtained from the “Linzа 2” deposit.

A. THE PRIMARY-ORE ZONE

The mineral composition of this zone is not especially varied. The sulfides occurring here are represented by simple sulfur compounds of heavy metals in the form of pyrite, chalcopyrite, sphalerite, arsenopyrite, galena, and fahlores. Iron oxides occur only rarely, in the form of hematite.

The description of the minerals listed below, established in the ores of the “Linzа 2” deposit, is given in the order of their successive separation from the solutions.

Pyrite decidedly predominates over the other sulfides. Suffice it to say that, in the entire ore mass, it occupies (on average) about 60%. Thus, according to chemical analyses of the core from borehole No. 47, the average pyrite content is 54.68%; for borehole No. 48 it is 57.87%, and in individual intervals of these boreholes it reaches 72.54%. There is no doubt that there are also places with a low pyrite content (18–20%), but such intervals characterize a zone of pyritized schists, and not the ore body as such. Hence it is clear that, if vein minerals are excluded, only a very small percentage remains for the other sulfides.

Being, therefore, the sharply predominant mineral of the primary-ore zone, pyrite at the same time clearly exhibits its usual properties and is readily recognized macroscopically. In bulk it forms a fine-grained aggregate of pale brassy-yellow color, with a massive and sometimes almost compact structure. In individual parts of the ore body, as a result of the leaching of vein minerals by katagenetic waters, well-prepared drusy structures may be seen; here the pyrite is represented by more or less large crystals, most often of cubic form, but not infrequently also as combinations of the cube with the pentagonal dodecahedron. Apparently, by virtue of its enormous crystallizing capacity, pyrite very often forms well-developed crystals both in such drusy portions and among the enclosing sericite-quartz schists.

Incidentally, pyrite associated with the wall rocks, unlike the pyrite of the ore bodies, always occurs as disseminations, and the assemblage of minerals associated with it is sharply different from the assemblage of minerals occurring together with pyrite of the ore bodies. First of all, in the zone of impregnation such sulfides as fahlores, sphalerite, and galena are almost never encountered, especially at some distance from direct contact with the ore body. Its usual companions are

—and then in very small quantities—chalcopyrite and arsenopyrite, which also quickly disappear with distance from the ore body. The size of individual pyrite crystals among the enclosing rocks rarely reaches 2–3 mm; more often they measure fractions of a millimeter and are visible only with a hand lens. A characteristic feature of the smaller crystals is that among them there predominate combination faces of the pentagonal dodecahedron with the cube, and sometimes weakly developed faces \((110)\) also appear, whereas the larger ones are almost always represented by the simple and usual form of the cube.

Fig. 4. Euhedral forms of pyrite (white) and its relationship with sphalerite (gray); black—vein mineral. Reflected light.

Being disseminated among the mass of vein minerals of the country rocks, pyrite not infrequently has the ability to concentrate into lenses, chains, and thin veinlets extending along the planes of schistosity. Very often the pyrite crystals bear traces of tectonic effects, expressed in crushing or in fine splitting and smearing of them along the planes of schistosity. Such deformed pyrite zones occur both in the hanging wall and in the footwall of the ore body of the “Lens 2” deposit and are the result of fairly sharp subsequent tectonic movements.

Owing to the general fine-grained character of the ore, and also to the very small content in it of other sulfides, the relationships of pyrite with the latter cannot in any way be resolved macroscopically.

...differ. In this respect it should only be noted that there is a certain spatial separation and, in individual specimens of essentially pyritic ore, the appearance of sphalerite and chalcopyrite (and rarely arsenopyrite) in the form of small lenses and bands, often alternating with one another and giving rise to characteristic banded textures (see below). All the remaining features

Fig. 5. Skeletal forms of pyrite (white), formed during its intensive replacement by sphalerite (gray); black is the vein mineral. Reflected light.

and the mutual relations of pyrite with the sulfides are resolved on the basis of microscopic observations, from which it is established first of all that pyrite is one of the earliest ore minerals. It almost always has clearly expressed euhedral forms and occurs both as isolated crystals and as intergrowths forming aggregates of closely adjoining grains. It should be noted, however, that the regular crystallographic outlines are very often disrupted by metasomatic replacement of it by other sulfides, the chief role among which should be assigned to sphalerite. As a result of such processes the crystals acquire irregular outlines, and sometimes preserve only the outer thin walls, while inside they are completely replaced by sphalerite. In the end, typical skeletal structures are obtained (Fig. 5). But where a continuous field of pyrite is subjected to a similar intensive process of replacement, dispersed replacement structures are usually formed. The latter are especially characteristic in the replacement of pyrite by chalcopyrite (Fig. 6).

The size of pyrite grains in the ore varies within considerable limits; the predominant grains, however, are those with a diameter from 0.01 to 0.8–1.0 mm; larger grains occur much more rarely and characterize the impregnation zone and portions of drusy structures. The pyrite of the ore bodies proper, for the most part, proves not to have been affected by tectonic movements, but extremely rarely, both in polished and in transparent sections, one can see its distinct crushing, with subsequent filling of the cracks formed in it by quartz or barite, as well as by later sulfides (Fig. 7). It is interesting to note, in transmitted light, the observed overgrowth of pyrite by rims of finely prismatic quartz, by flakes of sericite, chlorite, and, more rarely, barite.

Fig. 6. Crushed replacement structure of pyrite (white) by chalcopyrite (gray); black—vein mineral. Reflected light.

Thus, for the pyrite of the deposits of the Urska district, the following features are characteristic:

1) In all deposits of the group described, pyrite, among the other sulfides, is sharply predominant in relation both to sphalerite and to the vein mineral, represented in most cases by barite.
2) Constituting the principal mass of the ore bodies, pyrite is almost the only sulfide mineral of the impregnation zone, where it forms sparse disseminations in the country rock, composed chiefly of quartz and sericite.
3) Without speaking of the difference in the conditions of occurrence of the pyrite of the ore bodies and the pyrite of the impregnation zone, it should be noted

some difference between these pyrites in crystallographic respect. Thus, the pyrite of the impregnation zone, owing to its great individualization in comparison with the pyrite of the ore bodies, is always better crystallized and is most often represented by a combination of the cube with the pentagonal dodecahedron, and sometimes weakly developed octahedron faces are also added to them. For crystals

Fig. 7. Fragmented pyrite grains (white) with subsequent filling by quartz (dark gray) of the cracks formed in it. Reflected light.

of the pyrite of the ore bodies proper, the simple cube form is characteristic; more rarely, very weakly developed faces of the pentagonal dodecahedron appear. 4) Qualitative chemical analysis of pyrite from the impregnation zone gives a reaction for gold. 5) Pyrite of the impregnation zone very often proves to be crushed and deformed, whereas the pyrite of the ore bodies, in its mass, proves to be fresh. 6) Undoubtedly, the pyrite of the ore bodies appears in the form of several generations corresponding to the respective stages of mineralization.

Arsenopyrite is the rarest mineral of the primary-ore zone. Until now none of the investigators of Salair had recorded it for the polymetallic deposits. And only in the “Lens 2” deposit of the Ursk area does arsenopyrite appear in insignificant quantity both in the ore body itself and in the impregnation zone. Like all the other sulfides (except pyrite), it is most often found in schli—

facies, but sometimes it can also be noticed macroscopically. In the latter case arsenopyrite is very often concentrated in lenses and veinlets of barite, having, in this case, more or less large and well-formed crystals reaching 3–4 mm in length and 1–2 mm in cross section. Prepared crystals make it possible to establish the presence of faces of a prism of the third order,

Fig. 8. Forms of arsenopyrite crystals under the microscope.

which is found in combination with the basal pinacoid. But a number of cross sections listed in Fig. 8 and well observed under the microscope give grounds to speak of the development of faces of a second pinacoid, which in individual crystals is sometimes weakly, sometimes quite well developed. All arsenopyrite crystals visible to the naked eye are elongated along the \(Z\) axis and on the faces \((110)\) have fine vertical striation. Incidentally, a rather characteristic circumstance in the spatial arrangement of arsenopyrite crystals is that in areas where barite appears in the form of thin veinlets and small lenses, the arsenopyrite associated with it is oriented with its long axes in one direction, being concentrated mainly in the selvages. Where, however, arsenopyrite penetrates farther into the interior of massive pyrite, the orientation of its crystals becomes arbitrary.

Among the enclosing rocks, arsenopyrite was found only in one borehole, No. 52, on the footwall side of the ore body. The character of its crystals and their size are ordinary, and the crystals are almost always oriented parallel to the schistosity.

Arsenopyrite can be seen much more often under the microscope, where it forms crystals of various sizes with well-formed faces. It appears, undoubtedly, in several generations, each of which is characterized by a corresponding assemblage of associated minerals. Thus, it appears among solid fields of pyrite; against the background of a fine-grained aggregate of the latter, because of the coarseness of its crystals, it forms, as it were, porphyritic inclusions; and, as was already noted above and is clearly visible macroscopically, it also appears in considerable quantity among essentially baritic areas, where, in this way, it is in close association with the latter, as well as with galena and sphalerite. In the first case it undergoes slight corrosion by pyrite, which partly penetrates, more rarely enters into individual crystals of it, but usually passively adapts itself and grows upon the faces of the latter. As for the other sulfides, and especially sphalerite, in the field of development of which its abundant crystals are often encountered, the latter shows not even weak signs of replacing arsenopyrite, and only in rare cases (borehole No. 48, depth 83.1 m) can etching of its faces be observed.

The size of arsenopyrite crystals ranges from 0.01 to 1–2 mm in sections perpendicular to the third axis. In places a weakly expressed cleavage along (110) is manifested.

Sphalerite, in its quantitative proportion in the ore body, is second after pyrite, although it almost never reaches even the minimum percentage content of the latter. Analyses from boreholes Nos. 47 and 48 make it possible to calculate the average ZnS content as, respectively, 5.5 and 6.1%. The maximum content in borehole No. 48 over the interval 81.1–87.71 m is 14.8%, and in borehole No. 52 over the interval 172.2–175.78 m, up to 21.47%. From the data presented it is evident that the distribution of sphalerite in the mass of the ore body is extremely uneven. It should be noted, however, that in almost all sections along the boreholes, sharply expressed sphalerite-enriched bands are outlined, which are confined to different parts of the ore body. In the section through boreholes Nos. 47, 48, and 52, these sphalerite-rich bands are confined to the hanging and footwall sides of the ore body, whereas its central part always gives a meager zinc content. Conversely, in the section through borehole No. 49, similar bands also appear in the central part of the deposit. Such gigantic macro-banding of the ore is apparently due to the same causes that produced the formation of the ordinary, frequently observed banded textures here. We shall discuss the genesis of the latter below. A characteristic feature of sphalerite is also that, in contrast to the other sulfides (pyrite, chalcopyrite, and arsenopyrite), it never appears in the impregnation zone. In the ore body itself, where it

reaches appreciable quantities; it is usually represented by a weakly ferruginous variety of brownish or brown color, with a greasy luster, owing to the compact character of the aggregates. In those places, however, where it appears among pyrite in the form of narrow bands, it almost always assumes a black coloration, while having an adamantine luster.

Fig. 9. Fragmental structure of replacement of sphalerite (gray) by galena (white); black—vein mineral. Reflected light.

Such a fairly sharp difference in the character of sphalerite, as well as its spatial separation, indicates, on the one hand, the presence of at least two generations, and, on the other, its later appearance relative to pyrite. Its relations with other minerals are resolved under the microscope in polished sections, but we shall dwell on this in greater detail below. For the present we shall note the most general and characteristic features for it observed in reflected light.

It cannot be said that sphalerite is an invariable constituent part of every section. Very often in individual sections it is entirely absent, and sometimes this absence is explained by its replacement by covellite. Where sphalerite does appear in one quantity or another, its usual association is barite—pyrite—sphalerite. More rarely, but also quite readily, galena, chalcopyrite, and tennantite are associated with it. The character of its appearance under the microscope varies, depending on the quantitative proportions of the minerals associated with it.

In polished sections, where its accumulations reach appreciable quantities, it forms irregularly shaped areas, spots, and veinlets, representing an aggregate of fine grains that are clearly revealed only after etching with acid potassium permanganate; at the same time, a characteristic twinning

Fig. 10. Fine intergrowth of sphalerite (gray) with galena (white), with the formation of a structure closely resembling subgraphic; black is barite. Reflected light.

intergrowth structure also always appears. Owing to the abundant appearance of sphalerite, the earlier-deposited sulfides are subjected to rather strong replacement by it. Conversely, where sphalerite is concentrated only as insignificant admixtures, it forms very small grains filling the spaces between pyrite grains. Occurring quite often also in association with pale ore, chalcopyrite, and galena, sphalerite undergoes marked replacement by these minerals, with the formation of characteristic corroded (Fig. 9) or subgraphic (Fig. 10) structures.

Tennantite. In the zone of primary ores of the deposits described, pale ores have been found only in the “Lens 2” deposit. The most frequently encountered mineral of this group is tennantite, although alongside it there is undoubtedly also its antimonious analogue—tetrahedrite. The latter circumstance is indicated by microchemical reactions carried out on individual grains of pale ore in polished sections taken from various horizons. On the basis of optical

properties, it is impossible to distinguish these two minerals from one another. Therefore, relying exclusively on qualitative chemical characterization, it proved possible to establish in these ores the presence of these two minerals, connected, apparently, by gradual transitions.

In the spatial distribution of gray copper ore there is no strictly expressed regularity. Its amount is rather variable in different parts of the deposit. However, a certain tendency is observed for gray copper ore to be concentrated on the footwall side of the deposit and, in general, in areas where the sole ore mineral is pyrite. At the same time it should be noted that tetrahedrite most often appears in association with pyrite, whereas tennantite appears with sphalerite, chalcopyrite, and galena. In the first case, gray copper ore, in the form of irregularly shaped grains of various sizes, fills the spaces between pyrite crystals, usually passively adapting itself to the faces of the latter or appearing within them as small inclusions. In very rare cases it shows a distinctly expressed reactive capacity with respect to pyrite. As for gray copper ore associated with sphalerite and chalcopyrite (tennantite), its character differs somewhat from the preceding one. The appearance of gray copper ore together with sphalerite leads to the formation of very irregular, patchy grains, caused by its noticeable replacement by chalcopyrite and galena.

In turn, gray copper ore (tennantite) is itself subject to noticeable swelling and eating-away by areas of sphalerite, with the formation of crushed structures. The size of grains of gray copper ore ranges from 0.005 to 0.3 mm. It is interesting to note, incidentally, that sometimes gray copper ore forms well-defined little crystals. Thus, in one of the polished sections (52/209), several such crystals, 0.01 to 0.035 mm in size, were found, usually in a combination of two tetrahedra or in a combination of a cube with a rhombic dodecahedron.

Chalcopyrite is also distributed extremely unevenly through the mass of the ore body. Most often it is found only under the microscope, but at times it forms accumulations visible even macroscopically, reaching 10–15 cm in thickness. In such areas, under the microscope chalcopyrite forms very large grains (up to 1–2 mm), filling the free space between pyrite grains; its reactive capacity in different areas is variable: it either passively adapts itself to the faces of pyrite crystals, or rather intensively swells and replaces them. Here, too, one can often see the appearance within it of veinlets of bornite and covellite, as well as the characteristic crushed structures of sphalerite resulting from its replacement by chalcopyrite. In places where chalcopyrite occurs only slightly, it is always more fine-grained and is found in association with pyrite or gray copper ore. It is also found rather often in

association with galena, gray copper ore, and sphalerite; of these, the first forms intergrowths with it with straight contact faces, whereas in intergrowths with gray copper ore and sphalerite, owing to processes of mutual replacement, the outlines of their contours are always capricious. Not infrequently, especially where chalcopyrite occurs as separate isolated grains in the mass of a vein mineral, it forms well-defined crystals. Judging from the accompanying sections (Fig. 11), observed under the microscope, one may think that we have here combinations: 1) of two unequally developed sphenoids (a, b, v, d) and 2) of a bipyramid of the first order with a bipyramid of the second order and, possibly, a basopinacoid (b).

Fig. 11. Forms of chalcopyrite crystals under the microscope.

Galena, in quantitative terms, is sharply inferior to all the other sulfides of the primary-ore zone and is distributed extremely unevenly through the mass of the ore body. This is shown by a series of chemical analyses of boreholes, from which a considerable fluctuation of lead at different horizons and intervals is evident. Thus, in borehole No. 47 the average PbS content is 0.10%, with a maximum in the interval 85–88 m of 0.29%; in borehole No. 48 the average PbS content is noticeably higher.

Thus, on the basis of the data from these boreholes, the impression is obtained of a noticeable increase in the content of galena with depth, which I had already expressed in 1934.¹ However, the data from borehole No. 52, sunk to a depth of 200 m, did not confirm such an assumption, and it is quite clear why. In 1933 I had at my disposal material from only two boreholes, which drilled the 50 and 100 m horizons. Mineralogical analysis of the cores showed that secondary sulfides very readily replace sphalerite and galena. Naturally, in the higher horizons this process had proceeded much more strongly; and, consequently, the already low content of galena and sphalerite was sharply

decreased in comparison with the 100 m horizon. A relative increase of these two minerals with depth was obtained, which led to the expression of such not entirely correct ideas.

Almost always appearing where sphalerite is concentrated in one quantity or another, galena forms rather close intergrowths with it, sometimes closely resembling subgraphic structures (see Fig. 10). When galena is abundant, it shows an energetic capacity for replacing all the previously separated sulfides, thereby indicating its later deposition. With particular readiness it sometimes replaces fahlores and chalcopyrite with sphalerite, swelling them out with the formation of cracked structures. It is interesting, incidentally, to note here the spatial arrangement of galena with respect to sphalerite where they occur among pyrite in the form of sharply expressed bands. Galena is usually concentrated in the very contact with pyrite, whereas sphalerite constitutes the central part of the incrustation band. The impression is produced of banding, though not always symmetrical, since galena is not infrequently confined to one side or another. Analogously to chalcopyrite, galena very often forms well-developed crystals, 0.03 to 0.06 mm in size, in the form of a cube (б, в) or in combination with the octahedron (а, д, ж, з), or a rhombic dodecahedron (г, и, к), as is shown by the corresponding sections observed under the microscope (Fig. 12). In the zone of impregnation it appears extremely rarely.

Fig. 12. Forms of galena crystals under the microscope

Hematite is an extremely rare mineral of the deposits described. Its insignificant accumulations appear in the secondary quartzites of the Samoilovskoe deposit and are found—

veins in the hanging wall of “Lens 2.” In the first case it occurs in a finely disseminated state or forms elongated crystals; these small grains are concentrated not only between individual quartz grains but very often are included within the latter. Its relationship with the sulfides could not be observed, owing to the absence of the latter (with the exception of rare pyrite grains) in these quartzes. In the “Lens 2” deposit, hematite was encountered at depths greater than 200 m in pyritized quartz–sericite schists. As a result of finely impregnating the latter, it imparts to them a reddish coloration. The thickness of the hematitized zones does not exceed 10–15 cm. Its character is analogous to the hematite of the secondary quartzites of the Samoilovskoye deposit. Under the microscope it is evident that it appears somewhat earlier than pyrite, which is explained by its independent orientation with respect to pyrite.

Gold in the primary ores could not be observed in any of the polished sections, but it must be supposed that it occurs in the native state. Some indirect evidence may serve as confirmation of this. First of all, in their genesis the deposits of the Ursk district are analogous to the Salair deposits proper, in the ores of which, in 1934, I found small inclusions of gold in sulfides¹. In addition, during the concentration of Salair ores, gold is always concentrated in the free state with the lead concentrates.

B. ZONE OF SECONDARY SULFIDE ENRICHMENT AND OXIDATION

In the deposits of the Ursk group we do not have deep exploitation workings exposing the direct contact between the oxidation zone and the primary ores; and the number of boreholes is far from sufficient for a complete characterization of this zone. Therefore one has to draw conclusions on the basis of the available material, according to which we can nevertheless say that the phenomena of secondary sulfide enrichment in these deposits are weakly expressed. The typical supergene minerals of this zone are: covellite, chalcocite, and bornite. Their occurrence is entirely sporadic and reaches the depth known to us—200 m. It should be noted, however, that in the highest horizons they are more widely developed.

Covellite is the most frequently occurring mineral among the supergene sulfides. Its quantitative relations with the primary minerals are quite variable, depending on the horizon taken by us, and within the latter—on the mineralo-

...of the mineralogical composition of the ore. In the highest horizons, and especially in areas where such minerals as sphalerite, chalcopyrite, and fahlores are appreciably developed, covellite not infrequently becomes an essential constituent. In the deeper horizons its accumulations are extremely insignificant in the total mass of the ore body. Possessing considerable reactivity, covellite attacks sphalerite, galena, chalcopyrite, and fahlores with particular avidity. The replacement of all these hypogene minerals usually begins at the margins of individual grains, which often leads to the formation of characteristic rims of covellite around grains of the primary sulfides. But in more intensive processes it readily penetrates inward and wholly displaces the mineral being replaced. Sometimes it appears in the form of very thin veinlets, often branching and forming loop-like structures. It should be noted that in individual cases it is preceded by some other supergene mineral (most often bornite), which advances as a narrow band and is itself, in turn, replaced by covellite. This latter circumstance gives some grounds for speaking of its later deposition as compared with bornite. In the processes of replacement of hypogene sulfides by covellite, a well-expressed selectivity of the process is observed, consisting in the fact that covellite shows different reactivity with different sulfides. Thus, for example, in the sphalerite—galena association it first replaces galena; in the pyrite—tennantite—chalcopyrite association it replaces the fahlore with greater avidity. A process is observed that is, as it were, the reverse of the order of deposition of the minerals.

Bornite is a rather rare mineral in the deposits described. Its small amounts are recorded only in drill cores from the deposit “Lens 2,” chiefly in parts of the ore with an appreciable content of chalcopyrite or fahlore. In most cases bornite develops from the periphery of individual grains of the indicated minerals, eating into the latter and, sometimes penetrating inward, gradually displacing them. Much more rarely it forms clearly expressed veinlets cutting chalcopyrite grains in various directions. In the pyrite—chalcopyrite—fahlore association, as already noted above, bornite first replaces the fahlore and is itself subjected to replacement by covellite. In all probability, bornite is the earliest supergene mineral of the stage of secondary enrichment.

Chalcocite is common in association with covellite, from which it is sometimes difficult to distinguish by color. Quantitatively, however, it is sharply subordinate to covellite, apparently owing to its intensive displacement by the latter. Incidentally, it should be noted that with depth its amount increases somewhat together with the decrease in covellite. It is quite possible,

that this is indeed explained by the conditions of formation of these two minerals, of which chalcocite requires a more neutral medium, which fully corresponds to the deeper horizons, where the descending surface solutions are weakly acidic.

The zone of oxidation (the actual “iron hat”) and the zone of leaching are mineral associations that differ sharply not only in their composition, but also in color and structural features.

Fig. 13. Colloform structure of limonite (gray); black—vein mineral. Reflected light.

If within the zone of primary ores we had the development of sulfurous compounds of various metals, then in the oxidation zone oxidized compounds, chiefly compounds of iron, acquire predominant importance; if a distinctly crystalline, granular structure and the metallic luster of most of the minerals composing them are characteristic of the primary ores, then for the oxidation zone the most characteristic and frequently encountered structures will be powdery, incrustational, cellular, and porous; finally, the predominant color for this zone will be yellow and brown, with various variations.

It would seem that, in view of the sufficient number of elements entering into the composition of hypogene minerals, one might expect an exceptional richness in mineral species in the oxidation zone. In reality, however, if vein minerals are not counted, we see that the highest horizons of our de-

deposits consist almost exclusively of iron compounds in the form of oxides and hydroxides. Even in this case, if in some deposits and outcrops we record primary minerals, here too these will be pyrite or hematite, i.e., likewise iron compounds.

The most widespread mineral of the zone described is limonite in the usual sense of this term. Microscopic study of polished sections makes it possible to establish that, in addition to limonite proper, hydrohematite and acicular iron ore also take part in the structure of the iron cap; these are connected by noticeable transitions and often, being deposited successively layer upon layer, form characteristic colloform structures (Fig. 13).

Silver and gold are associated with the brown iron ores of this zone; the former, apparently, is the result of the destruction of galena and pale ores, in which it may be present as an isomorphous admixture, while the latter, occurring in native form in the sulfides, upon the destruction of the latter is concentrated in the oxidation zone in more appreciable quantities (mechanical enrichment).

B. VEIN MINERALS

The vein minerals of the deposits described are represented by barite, quartz, sericite, and chlorite. All of them are very widely distributed; in individual deposits quartz or barite are almost the only minerals of the ore bodies.

Along with quartz, apatite is present in some deposits. Thus, for example, microscopically, in the secondary quartzites of Kopna Mountain it is recorded in a fairly noticeable quantity, most often in areas of considerable pyrite concentration. Undoubtedly being a hydrothermal mineral, apatite appears at the earliest stage of mineralization, separating out somewhat earlier than quartz. In the form of prismatic crystallites characteristic of it, sometimes concentrated in groups, apatite is always included in quartz grains. Its relationship with pyrite is difficult to establish.

IV. TEXTURE OF PRIMARY ORES

A banded texture is quite characteristic of the ores of the deposits described. It is not only manifested in the massive pyritic ores of the “Linza 2” deposit, but is also found in the secondary quartzites of the Kopennoye deposit, as well as in impregnation zones. In each individual case its character is distinguished by a certain peculiarity, but in general it is determined by the alternation of bands of different mineralogical

...of the mineralogical composition of both ore and vein minerals. However, such banding is often caused by differences in the grain size of the same minerals, which affects the character of the coloration that in this case emphasizes this texture.

Upon closer study of banded textures, it proved possible to distinguish the following four types.

I. Banded ores caused by the alternation of bands consisting essentially of pyrite with bands whose minerals are sphalerite, galena, and an insignificant admixture of other sulfides (Fig. 14). A characteristic feature of this type is the peculiar spatial distribution of some of the minerals composing these bands. Thus, for example, under the microscope, within the sphalerite bands one can often observe an asymmetric zonal structure, expressed in the fact that galena is almost always confined to one particular side. Where a noticeable accumulation of pale ore appears, the latter penetrates deep into the pyrite bands and is situated at some distance parallel to the sphalerite bands.

Fig. 14. Character of a banded ore formed by the alternation of bands of sphalerite (gray) with bands of pyrite (white) and other sulfides; black—vein mineral. Reflected light.

II. Banded ores caused by the appearance of bands consisting of pyrite of various grain sizes (Fig. 15). The size of the grains within the coarse-grained bands ranges from 0.03 to 0.65 mm, whereas within the fine-grained bands their size does not exceed 0.03 mm. The selvages of individual bands are sometimes expressed rather sharply, but most often, owing to the penetration of fine-grained pyrite to the side, the outlines acquire a diffuse character.

III. Banded ores caused by the alternation of pyrite bands with pyrite-chalcopyrite bands. In the latter, besides these

of the two minerals; under the microscope an insignificant amount of sphalerite and fahlore is found, always undergoing intensive replacement by chalcopyrite. Pyrite is also subject to the same replacement, and in such areas it is often found to be crushed, which apparently provided good pathways for the abundant penetration

Fig. 15. Character of banded ore formed by the alternation of bands of coarse-grained pyrite (white) with bands of fine-grained pyrite; the dark material is a vein mineral. Reflected light.

of chalcopyrite here. Therefore, the most characteristic replacement structures of pyrite are shattered ones. The size of the pyrite and chalcopyrite grains is always larger in comparison with the grains of the pyrite bands.

IV. Banded ores (“disseminated ores”), whose banding is caused by the alternation of sericite–quartz bands with bands of sulfides, chiefly pyrite (Fig. 16).

With regard to certain general features characteristic of banded textures, it should be noted, first, that the bands that produce them most often run parallel to one another, persisting over a considerable distance, although not infrequently, especially in ores of type IV, they take the form of rapidly pinching-out lenses. The width of the bands is in general insignificant and is measured in millimeters, rarely in centimeters.

The genesis of the textures described is not sufficiently clear. However, the study of this question leads us to the more or less probable conclusion set forth below. Indeed, turning to the lean ores or weakly mineralized country rocks, where the processes of mineralization and their sequence are expressed more sharply than in massive ores, we find fundamental, although at first glance contradictory, material.

Fig. 10. Character of a banded ore of the inclusion type, caused by the alternation of quartz–sericite bands (white) with bands of pyrite (black). Transmitted light.

Thus, for example, if we take some random specimen of mineralized country rock, with well-expressed banding, and observe the behavior of the ore minerals (in particular pyrite), their distribution in the mass of the given rock, then upon careful examination we can discover that the pyrite is distributed in the rock rather uniformly, while the observed bands of pyrite prove to be false and appear to us as such only because the primary, uniformly mineralized rock was subsequently broken up by pressure into a series of small lenses and bands, with the formation along the slip planes of abundant sericite, which also isolated the pyritized portions from one another, thereby emphasizing the linear and, in a certain way, oriented arrangement of the lenses. It must be thought that banding of this kind formed together with the schistosity of the rock and cannot, of course, serve as evidence of replacement by ore solu—

...the frames of the shales, just as it also cannot serve as material for explaining the banding observed in the ores. But, on the other hand, sometimes macroscopically, and especially under the microscope, more or less regular bands and lenses of pyrite can be seen, representing continuous veinlets or chains of individual crystals, arranged predominantly in the direction of cleavage, although cases are not rare in which they are situated at a certain angle to the schistosity (Fig. 17). It is characteristic, among other things, that here too sericite accompanies the pyrite, appearing as a brightly polarizing veined variety, usually confined to the periphery of these pyrite bands.

Fig. 17. Pyrite veinlets (black), cutting the schistosity at an acute angle. Transmitted light.

This kind of banding also becomes quite explicable if we imagine the full complexity of the conditions under which our deposits were formed. First of all, anticipating somewhat, we must take into account the order of deposition of the vein and ore minerals. Undoubtedly, the first, earliest stage of mineralization was accompanied by the deposition of quartz, chlorite, and sericite, the time of whose appearance was very close, if not simultaneous. Along with this, pyrite also formed, but only in the quantity that could be obtained from the iron contained in the primary rocks. Naturally, this pyrite was distributed more or less uniformly through the rock. Its appearance could have produced only a pseudo-banding, explained—

understand, as we saw above, by subsequent tangential pressures. As for pyrite, which forms clearly expressed bands and lenses in the shales, it owes its origin to the second stage of mineralization, during which its principal mass was deposited; moreover, the paths for the penetration of these sulfide solutions were facilitated by that preliminary emanational activity which led to the formation of an abundant quantity of sericite and chlorite—these two platy minerals which, at the very first onsets of stress, promoted the easier formation of an entire system of, undoubtedly, more or less parallel fractures. The solutions became localized along the latter, depositing the corresponding sulfides along their path. Each new stage of mineral formation was undoubtedly preceded by periods of compression and subsequent extension; in connection with changes in the physicochemical conditions within the magmatic source itself, the composition of the mineralizing solutions also changed, and from them, at each given moment, ever new complexes of minerals were precipitated. The latter, concentrating in separate bands, produced a sharply expressed banding. Even in those cases where the banded ores are represented by a single sulfide (most often pyrite), it is always clearly evident that this banding owes its origin to the appearance of the given sulfide in the form of bands with different grain sizes, corresponding to different generations of the respective stages of mineralization.

Thus, as a result of the vibration of unilateral pressure, which helped prepare new pathways for discontinuously moving mineral solutions, there occurred a gradual replacement of the enclosing rocks with the formation of the banded textures described above.

In general, the processes of formation of these textures were extremely complex and could have depended on a whole series of causes; but we cannot reduce them to a simple replacement of shales: first, because these banded textures, in their mass, are due to the presence of bands of differing composition, and, second, because the shales, as such, could have appeared only after they had previously been subjected to significant hydrothermal alteration, which in our area is always manifested in connection with ore mineralization.

V. ENCLOSING ROCKS AND THE PROCESSES OF THEIR THERMAL ALTERATION

The country rocks of the Ursk group of pyrite deposits are erupted porphyritic formations of the type of quartz and quartz-bearing albitophyres. Depending on the degree to which dynamo- and hydrothermal metamorphism is manifested in them, they have lost their primary appearance and, in the mass, their ...

they are now represented by various kinds of schists, and in individual cases, as, for example, in the Kopennoye deposit, they have proved to have been transformed into secondary quartzites. A detailed study of the rather extensive material from drill holes, chiefly from the “Lens 2,” Kopennoye, and Beloklyuchevskoye deposits, has made it possible to distinguish the following principal groups of these metamorphic rocks: 1) albite-chlorite-quartz, 2) chlorite-sericite-quartz, 3) sericite-quartz, 4) quartz-sericite schists, and 5) quartzites. All the above-listed varieties of schists are connected both with one another and with albitophyres by gradual transitions; the albitophyres, as a rule, compose the areas farthest removed from the ore deposits, whereas the quartz-sericite and sericite-quartz schists, as well as the quartzites, occur in immediate proximity to the latter or along their continuation. In addition, it should be noted that the sericite-quartz schists are concentrated with particular regularity in the hanging wall of the ore bodies and, conversely, the quartz-sericite (and sometimes the sericite proper) schists are usually confined to the footwall.

Without yet going into the details of the relations between the ore bodies and the enclosing rocks, I shall for the moment dwell on the characterization of the latter, and at the same time, on the basis of the available material, try to analyze the processes of their hydrothermal alteration.

Albitophyres. The primary composition of these rocks can be established only under the microscope, on the basis of the well-preserved relict porphyritic structure in them. Macroscopically, these are usually grayish-green, coarsely schistose rocks, on the planes of schistosity of which flakes of chlorite, rarely of sericite, always appear. In individual areas they are sometimes silicified and, in that case, acquire a more massive structure. In some hand specimens one can see a system of thin quartz-carbonate veinlets cutting the rock and, together with them, rare disseminations of pyrite; in most cases, however, sulfides in these rocks are not recorded macroscopically.

Under the microscope, the albitophyres are composed of a fine-crystalline aggregate of grains of quartz and albite, which constitute the main mass of the rock; against this background there almost always appear rare, but sometimes fairly large, phenocrysts of feldspars. On closer study of the latter, in all cases they prove to be albite, and only rarely are separate grains encountered which, in their optical properties (extinction angle 7—10°), approach oligoclase-albite. The phenocrysts of albite give well-expressed simple twins according to the second pinacoid (010), more rarely perpendicular to (001), and still more rarely form untwinned grains, in sections of which perfect cleavage along (010) is visible. Almost all of them have thick-tabular-

silvery-white as one approaches the ore-bearing areas. This phenomenon is a natural consequence of the progressively increasing amount of sericite, which possesses its own characteristic color. The microscope reveals to us far more details.

If, as the object of our observations, we take a rock from the most distant zone of hydrothermal alteration, then before us will be albite-sericite-chlorite schists, which in the present case constitute the first transitional stage from albitophyres to quartz-sericite schists. They differ from the former by a more perfect schistosity and by the appearance, macroscopically visible on the planes of schistosity, of plenosericite. Under the microscope, rather substantial changes are evident, consisting, first, in a fairly strong fragmentation of the albite phenocrysts, the individual particles of which are often displaced relative to one another and enclosed by the groundmass, represented by quartz, chlorite, and sericite. The latter, while in general still subordinate in relation to chlorite, develops not only in the groundmass of the rock, but also, to a considerable degree, affects the albite phenocrysts, forming in them small scales, usually arranged along the planes of schistosity. With further movement toward the ore body, the appearance of sericite becomes so abundant that the affiliation of individual grains to albite is established only on the basis of preserved outlines, visible under complete extinction. Parallel with the development of sericite, quartz is also formed, and these two minerals ultimately completely displace not only the primary minerals, but also the secondary ones, especially chlorite and epidote, as a result of which the rocks acquire a very simplified composition: quartz and sericite.

However, the characterization of the newly formed minerals would be incomplete if we did not say that, along with this, abundant disseminations of sulfides appear, among which the predominant role belongs to pyrite. The latter not infrequently, in the form of the finest dust-like particles, permeates the mass of the rock. But for the most part it forms clearly visible, and sometimes fairly large, crystals, scattered either irregularly or in the form of chains, lenses, and bands, often elongated in one direction, parallel to the schistosity. On the faces of pyrite cubes there always appear characteristic rims of quartz or sericite (Fig. 18), with quartz producing fine-prismatic, often strongly elongated and curved grains, while sericite grows on the faces of pyrite always in such a way that the basal pinacoid of its flakes is oriented parallel to the face on which this overgrowth takes place.

Thus, the phenomenon of hydrothermal alteration of the enclosing rocks bears the clearly expressed character of propylitization, passing near the ore deposits into sericitization. This zonality in the distribution of chlorite and se—

the new mass, almost unaffected by secondary alteration, consists of an aggregate of small grains of quartz and albite. And often, in field conditions, one can trace their gradual transitions into porphyroids, especially where the processes of sericitization near the ore bodies also encompass these porphyritic masses.

The principal types of eruptive rocks described above, under the influence of mineralizing solutions connected with the general course of mineralization, undergo various stages of alteration, always more intense as one approaches the ore deposits. As a result of these preliminary alterations of the primary composition of the given rocks and the subsequent, repeatedly superimposed phases of tangential pressure, the albitophyres are transformed into various kinds of chlorite and sericite schists, differing from one another in the relative quantity of particular constituent parts contained in them. All of them are characterized by a very perfect schistosity and by a green or silvery-white coloration characteristic of the end members of this series, namely the chlorite and sericite schists. The intermediate varieties, naturally, also have an intermediate coloration. The degree of their impregnation by pyrite gradually decreases with distance from the ore bodies, and thus the quartz-sericite and sericite-quartz schists always prove richer in disseminations of sulfides than the albite-chlorite or chlorite-sericite ones.

In general, the description given above of the various schists already characterizes sufficiently clearly those most typical processes of hydrothermal alteration which led these schists to their formation. These processes reduce to the following: 1) chloritization, 2) sericitization, and 3) silicification.

The most widespread and typical process accompanying all the deposits of the district is sericitization. In the description of individual deposits it has already been indicated that the near-ore border zone of sericite schists often reaches 300 m in thickness. This latter circumstance testifies to the manifestation of intensive hydrothermal processes that accompanied mineralization; moreover, the predominant role among them apparently belonged to the earliest stage of thermal metamorphism, when the temperature was still sufficiently high for such complete recrystallization of feldspars into a quartz-sericite aggregate. At the same time, sericitization affects not only the main mass of the primary porphyritic rocks, but also the phenocrysts of feldspars, with almost complete preservation of the quartz phenocrysts. Indeed, if we trace this process, beginning with typical albitophyres, we shall see the following picture.

First of all, even upon a cursory inspection of drill cores, an extremely gradual change in the color of the enclosing rocks from green to the characteristic ...

...of Mg¹ and Fe, necessary for the formation of these two minerals. On the other hand, the mineralogical composition of the quartz-sericite schists also suggests that the processes of sericitization were accompanied not only by the usual rearrangement of elements, but also by a considerable introduction of potassium and silica, with simultaneous removal of sodium.

Clark gives the following as the most probable equation for the reactions taking place in this connection:

\[ \begin{gathered} 3\,\mathrm{KAlSi_3O_8}+\mathrm{H_2O} = \mathrm{KH_2Al_3Si_3O_{12}}+\mathrm{K_2SiO_3}+5\,\mathrm{SiO_2} \\ (\text{orthoclase})\quad(\text{water})\quad(\text{sericite})\quad(\text{potassium silicate})\quad(\text{quartz}) \end{gathered} \]

A similar formula is given by S. F. Mashkovtsev, in discussing the processes of sericitization and the formation of secondary quartzites at the Kounrad deposit².

But since in our case the primary rocks contain a considerable amount of primary albite (at least in the form of phenocrysts), it must evidently be assumed that, in the course of this reaction, the liberated potassium silicate entered somewhat later into reaction with the albite phenocrysts, displacing the sodium in them, which was carried away in the form of the corresponding silicate, while sericite developed in place of the albite.

At the same time, however, for the formation of nearly pure sericite schists the amount of potassium that existed in the primary rocks was undoubtedly insufficient, and this is fully confirmed by mineralogical observations as well. Under the microscope, in the quartz-sericite schists, besides the finely scaly sericite scattered among the aggregate of quartz grains, one can see its appearance in the form of clearly expressed veinlets and, in such a case, usually of coarse-flaky character.

A less widespread, but also very typical, process of alteration of the country rocks is silicification. In those areas where this process is weakly expressed, under the microscope one can observe how very small grains of quartz gradually replace the main mass of the rock and then also develop in the feldspar phenocrysts, forming in their place a granular quartz aggregate. With a more intensive process, quartz replaces all other minerals, converting the rock into secondary quartzites. If quartz porphyries are subjected to this process, then their origin, even with complete silicification, is easily recognized owing to the preservation—

¹ On this point there are several contradictory statements in the literature. Thus, for example, Emmons, referring to Rober’s investigations, notes that in the processes of propylitization the amount of magnesium always decreases. Lindgren repeats approximately the same thing, although he immediately cites a chemical analysis of fresh and altered rocks from the Tonopah deposit, from which a fairly sharp increase in Mg (from 0.88 to 2.60%) is clearly evident.

² S. F. Mashkovtsev, “Petrology of the Kounrad Copper Deposit,” Problems of Soviet Geology, No. 7, vol. III, 1934.

...sericite is expressed in the “Lens 2” deposit, where at a distance of 95–100 m from the ore body we have only weakly altered and roughly disseminated albitophyres, which 75 m from the deposit already show a sharp increase in chlorite content, and in places calcite together with it. Phenocrysts are still preserved, but are already considerably destroyed. At 30–25 m the rock is almost entirely re-

Fig. 18. Overgrowth of pyrite crystals (black) by rims of sericite (gray). Transmitted light.

crystallized into an aggregate of quartz grains with chlorite and sericite. Owing to the abundant development of chlorite and sericite, the rock acquires a complete schistosity. Finally, beginning at 15 m and up to the ore body itself, the rock consists, as was already noted above, exclusively of two minerals—quartz and sericite.

Thus, the processes described sharply alter the mineralogical composition of the primary igneous rocks, but along with this a noticeable change also occurs in their chemical composition. We have already established above that, in their unaltered form, the primary rocks corresponded in composition to acidic formations of granitic magma, which, as is known, are distinguished by a rather pronounced predominance of the quartz–feldspar component over the mafic component. And since this is so, it is natural to think that the abundant development in the schists of chlorite and pyrite is due to the introduction of a noticeable quanti-

we have a sharply expressed leaching zone, represented by barite rubble, whereas in the Kopennoe and Samoilovskoe deposits it is entirely absent. Likewise, the oxidation zone (iron gossan) in some deposits (“Lens 2”) has been destroyed by denudation.

Iron gossan, in its typical manifestation, is found in the Beloklyuchevskoe and Samoilovskoe deposits. In depth it extends, in both the former and the latter case, to a maximum of 20–22 m. The question of the ratio between the thickness of the iron gossan and that of the primary ores remains unresolved, because in those deposits where iron gossan is present the primary ores have not been studied. The iron gossan of the deposits mentioned is a compact, cellular, or breccia-like mass consisting essentially of limonite and other hydrous iron oxides, among which hydrohematite plays the chief role. Depending on the predominance of one or another ironstone, the gossans acquire a brownish, yellow, or reddish coloration. Often both limonite and hydrohematite form ironstones with a loose earthy structure. Almost always, in one quantity or another, fragments of vein minerals and country rocks occur among the iron oxides. In the latter case the ironstones possess a characteristic breccia-like structure.

Chemical analyses of brown ironstones were not carried out, owing to a lack of funds. But some idea of their composition is given by the partial analyses cited by G. S. Labazin, which testify to the complete leaching of lead, zinc, and copper; this is also confirmed by the mineralogical composition of the iron gossan, since in no case was it possible to establish the presence of sulfides or carbonates of the corresponding metals.

With increasing depth the iron gossan rapidly diminishes in thickness and passes into barite rubble, the transitional interval being characterized by the fact that hydrous iron oxides here play a subordinate role, appearing only in the form of a cement binding barite grains of various sizes. And already within the rubble itself there are still often found only insignificant concretions of brown ironstone of the concretionary type.

Barite rubble, as already indicated, in the Beloklyuchevskoe deposit lies directly beneath the iron gossan, while in the “Lens 2” deposit it comes to the surface. In the latter case the iron gossan was apparently destroyed by denudation.

In origin it is the result of complete leaching of the sulfur compounds of the primary ores and is a rather loose mass of barite grains, colored to varying degrees by iron oxides. Quite charac—

...of quartz phenocrysts. An example of this may be the secondary quartzites of Mount Kopna, where the porphyritic structure is almost always clearly expressed under the microscope. Like the processes of propylitization and sericitization described above, silicification is accompanied by strong pyritization; here too, rims of finely prismatic quartz appear around the grains of pyrite.

From the reaction given above it is clearly seen that, when feldspars are altered, quartz is formed along with sericite, the ratio between them being such that for one molecule of sericite there are 5 molecules of SiO₂. It is therefore natural that sericitic shales always contain a large quantity of quartz in their composition. But, apparently, where high temperature existed during these processes—which, as is known, promotes the energetic dissolution not only of silica—it must be assumed that, with an excess of water, dissolution of the sericite molecule also occurred; owing to the high internal pressure of this phase as compared with the external pressure, it could easily be carried into the overlying horizons and, in this way, promote the formation of rocks consisting essentially of quartz. Undoubtedly, rocks of more acidic composition were most readily subjected to such a process of silicification, which is what we in fact observe.

VI. SECONDARY ALTERATIONS OF THE DEPOSITS AND OF THE ROCKS ENCLOSING THEM

Leaving aside for the time being an analysis of the causes that brought about the alteration of the deposits in their outcrops at the surface, let us now note the most characteristic morphological features of the latter. If we attempt to compare the deposits of the present district with the deposits of the Salair mines proper, we see a substantial difference between them, consisting in the fact that here, for most deposits, there is a very strong development of the zone of leaching, as was already noted above, situated between the iron cap and the primary ores. Unfortunately, we are not able to cite, as an example, even a single deposit where a complete vertical section could be seen. However, the set of features inherent in the individual deposits is mutually supplemented when they are considered together. On this basis, for the Urska deposits, we can record the following zones from the surface downward: 1) the “iron cap,” 2) the barite “cap,” 3) pyrite sand, 4) pyritic ores with traces of secondary enrichment, and 5) primary pyritic ores. Depending on a number of specific conditions, and chiefly on the primary character of the ores, the position of these zones is very variable. Thus, for example, in the “Lens 2” and Beloklyuchevskoe deposits we...

but beneath the iron cap there occur more densely colored varieties of friable ore; with further deepening, however, the various colors are often superimposed upon one another.

Besides iron oxides, which in quantitative terms do not exceed 8–10%, the barite friable ore contains an appreciable content of gold, silver, and arsenic¹. In comparison with the content of these elements in the primary ores, a rather sharp enrichment of them is observed; at the same time, their ratios to one another also change. Thus, for example, there is on average 7–10 times more gold in the barite friable ore than in the primary ores, 5–7 times more silver, and 2–3 times more arsenic. The ratio of silver to gold in the primary ores is 30 : 1, whereas in the barite friable ore it sometimes reaches 100 : 1.

All the above-mentioned elements are very finely disseminated in the barite sand, as a result of which they are not extracted at all by washing. Small veinlets of quartz are often encountered among the friable ore, as well as well-formed crystals of it. Barite, too, occurs in the form of large and perfectly transparent crystals, sometimes as pseudomorphs after carbonate, as indicated by their rhombohedral form.

In the lower horizons, the barite friable ore must be supposed to pass into pyrite sand; this is confirmed, for example, at the “Lens 2” deposit, borehole No. 47, which encountered, in the middle part of the ore body, a zone of weakly cemented aggregate of pyrite grains with barite and an insignificant amount of covellite; chalcopyrite, sphalerite, and other ore minerals are absent. The thickness of this zone in the given section is 9.93 m.

The zone of secondary enrichment and of primary ores has been characterized by us above.

Apparently, simultaneously with the alteration of the ore deposits proper, the enclosing wall rocks also undergo alteration. The abundance of sulfuric acid, formed in connection with the oxidation of sulfides, makes the surface waters strongly acidic; penetrating into deeper horizons, these waters react with sericite and convert the latter into pyrophyllite and then into kaolin. As a result, the quartz-sericite schists acquire a lightened appearance and are transformed either into pyrophyllite schists (often taken in the archival literature for talc schists) or into pure kaolin, with an appreciable content of quartz grains. The structure of the rock is usually preserved. Kaolinized sericite schists are sometimes encountered at quite considerable depth. In outcrops at the surface, however, as a result of the leaching of pyrite, the schists acquire a yellowish or brownish coloration, and the pyrite crystals are often preserved as pseudomorphs of limonite. In the event that the quartz-sericite—

It is characteristic that the color of this loose material is so varied that, with careful selection, it proved possible to distinguish 23 different colors. Of these, essentially four colors predominate: brown, red, yellow, and white; all the others are intermediate, with gradual transitions into one another. At first glance it would seem that the intensity is directly dependent on the quantitative ratio of iron oxides and barite particles. However, a chemical analysis made of 8 samples that differ most sharply in their color shows the following content of ferrous and ferric iron (Table 1).

Table 1

Sample No.	Color of the loose material	FeO (in %)	Fe₂O₃ (in %)
1	Orange-red with a brownish tint	2.18	0.21
2	Orange-yellow	0.49	0.79
3	Orange-red	0.17	0.58
4	White	0.08	0.09
5	Reddish-brown	0.45	7.78
6	Brownish-red	0.23	2.86
7	Light yellow	0.22	1.18
8	Gray-yellow, quickly darkening in the light	0.30	0.29

From the table it is evident that this dependence of coloration cannot be derived solely from the quantitative Fe content in the loose material. Apparently, a more important factor is the character of the ferruginous material, as well as the fineness of the substance and the degree of its distribution. All these factors, taken together and complementing one another, create that abundance of colors which we observe in the zone of barite loose material. Incidentally, it is interesting to note the coarse banding, often observed in workings, in the arrangement of loose material of different colors; these bands, or more precisely lenses of irregular shape, are situated almost horizontally, without, of course, having any sharp boundaries or any corresponding regularity in the gradual change of color with depth. True, in the “Lens 1” deposit, directly

1. Pyritization and hematitization of quartzites, with corresponding enrichment of them in Au and Zn (Mount Kopna), are one of the manifestations of mineralization in the district, undoubtedly connected with the migration and deposition nearby of polymetals.

2. The bedding of the ores corresponds exactly to the orientation of the schistosity of the rocks of the adjacent district. The latter argues for the relict preservation in the ores of traces of the schistosity of the rock replaced by the ores, i.e., of the same schistose porphyries. This conclusion is confirmed by observation at the II Salair mine of the gradual transformation of the schistosity of the enclosing rocks into the banded structure of the ore.

3. The ore bodies are confined to the hanging wall of the enclosing sequence where the beds dip normally, and to the footwall in the case of an overturned fold; moreover, these areas of mineralization are always pressed against the SW edge of the belts of development of rocks of the Pecherkino Formation.

4. Derivatives of dioritic magma are always present in areas of mineralization.

5. In addition to zones of hydrothermally altered rocks, within the Pecherkino Formation there are developed belts of rocks that have undergone weak mineralization in the form of disseminated sulfides, the development of a magnetite placer, etc. Along with this, there are grounds for considering the albitophyres as having a massive content of zinc and gold (my spacing — T. B.), as has been established at a number of points by I. V. Derbikov.

6. Polymetallic mineralization generally associates precisely with porphyries in their vein and intrusive facies, as has been established in Altai.

7. According to data from the same Altai, the concentration of ores is connected with a zone of crumpling and fractures, which ensure the localization and concentration of the depositing ore masses.

8. Within the area studied, apart from rocks of the dioritic intrusion, no other intrusions have been found.

9. The only dioritic intrusion in northeastern Salair is defined by researchers, with respect to its role in metallogeny, as gold-bearing. Among other emanations, one can indicate only rare cases of nonindustrial copper mineralization.

Here are the main theses (in a somewhat abridged form) on the basis of which the authors arrive at a rather simple conclusion, namely: they believe that the primary mineralization (in disseminated form) is connected with the quartz albitophyres of the Middle Cambrian, after which, in the epoch of Caledonian diastrophism, which led to the formation from the albitophyres of sericitic and sericite-chlorite schists, a dioritic intrusion was emplaced; the hydrothermal phase of this intrusion led, on the one hand, to the leaching of all metallic compounds from the quartz albitophyres and, on the other, concentrated them at certain points in the form of ore bodies.

the slates were intensively pyritized; while preserving their general appearance, they are subject to strong ferruginization. A typical example of such formations may be the quartz-sericite mineralized slates of the Samoilovskoye deposit.

Apparently, the formation of gypsum is also closely connected with the processes of kaolinization; it was recorded at a depth of about 200 m in the cores of boreholes of the “Lens 2” deposit, in the footwall. Its occurrence in the form of relatively thin (up to 1–1.5 cm) veinlets cutting the ore and the strongly pyritized slates at various angles to the schistosity proves its supergene origin. Under the microscope it is seen how individual fibrous aggregates of gypsum penetrate along the cleavage of sericite flakes and replace them entirely. Macroscopically it always appears as a fibrous variety with a silky luster, and its individual fibers are always arranged perpendicular to the selvages, like asbestos.

VII. GENESIS

Thus, the factual material set forth above makes it possible to detail somewhat and clarify the history of the formation of the Salair deposits that I set out as early as 1934.^1

But before turning to the substance of this question, I would like to make several critical remarks concerning the new point of view on the mineralization of Salair advanced by A. G. Vologdin and A. A. Predtechensky in their recent work, “Geological Sketch of a Part of Northeastern Salair.”

The following are put forward as the initial propositions which the authors cite in developing their theory:

1. All the polymetallic deposits of northeastern Salair are developed in the zone of a metamorphic sequence.

2. Mineralization was preceded by the regional transformation of the rocks of the ore-bearing sequence into a complex of sericite and sericite-chlorite slates (regional metamorphism).

3. The ore lenses are confined to an area of additional hydrothermal alteration of the rocks of the sequence, expressed in kaolinization, hematitization, desilicification, etc.

4. Along with desilicification of the enclosing rocks—for example, the leaching of excess quartz from crushed sericitized quartz albitophyres and their transformation into sericite slates—in the zone of hydrothermal processes there is observed the development of coarse and fine silicification of the same porphyroids (Mount Kopna). There is no doubt that the quartzites preserve traces of schistosity. A large part of the quartzites has a banded structure.

^1 G. P. Bolgov, Mineralogical Investigations of the Polymetallic Ores of the Salair Deposits. Materials on the Geology of Trans-Siberia, issue 15, 1934.

the principal mass of the quartz porphyries into an aggregate of fine-grained quartz and sericite; nevertheless the quartz phenocrysts remained wholly unaffected, often preserving their crystallographic outlines. There is no more or less significant indication of the leaching of quartz. On the contrary, there are facts of the opposite order, when a considerable introduction of silica takes place with the simultaneous removal of alkalis and alumina; even in this case the primary quartz of the porphyroids is preserved in the form of phenocrysts, as the authors had occasion to convince themselves from the example of the quartzites of Mount Kolba. And it is indeed difficult to imagine that, under conditions of not especially high temperatures of hydrothermal solutions, appreciable dissolution of quartz and its complete removal could occur, with only sericite left in place.

Thus, on the one hand, the assertion is advanced that the rocks of the ore-bearing sequence, under the influence of regional metamorphism, are transformed into a complex of sericite and sericite-chlorite schists; and on the other hand, it is said that these schists are formed as a result of leaching from albitophyres. The authors do not indicate what the difference is between sericite schists of the first type and those of the second—and undoubtedly because there is no factual material for this.

In general, in trying to prove that ore formation was preceded by the formation of schists, they imperceptibly fall into a number of contradictory positions and thereby weaken their own position.

The fifth point raises great doubts. Where did the authors obtain the information that the quartzites of the Kolenny deposit contain zinc? According to the data of G. S. Labazin, who carried out drilling work at the deposit, it is clear that even those portions of the quartzites that are strongly pyritized contain not even traces of zinc.

Finally, the ninth and most decisive point, on which the whole hypothesis is essentially built, proves unable to withstand any criticism. How could it have been openly asserted that “the quartz albitophyres in their mass contain zinc and gold”? Where is the list of chemical analyses on the basis of which such a conclusion is drawn? Where, finally, and in what article by I. V. Derbikov is it written that he established the presence of zinc in quartz porphyries at a number of points? It is no accident, of course, that the authors cite neither the one nor the other.

Such is the state of affairs with the “factual” material that was made the basis of this new theory. Its untenability is clear from what has been said above. However, it proves more than dubious even if one assumes that all the propositions advanced by the authors are correct. Indeed, how is one to imagine the mechanism of formation of ore bodies under conditions in which hydrothermal solutions coming from an intrusive...

Let us examine somewhat more closely the grounds on which this hypothesis is built. First of all, the second point draws attention to itself; from it it clearly follows that the formation of sericite schists preceded ore formation. What grounds did the authors have for such a statement? In my opinion—none. For anyone who has been in Salair, especially within the area of development of the rocks of the Pecherka Formation, the picture of the spatial distribution of the sericite schists is quite clear. Above, in describing the individual deposits, I especially emphasized sharply that all of them are accompanied by a fairly typical zone of quartz–sericite schists, which, with increasing distance from the ore bodies, pass through intermediate varieties (sericite–chlorite schists) into coarsely schistose albitophyres, with well-preserved phenocrysts of albite. In the drill cores from the “Lens 2” deposit it is clearly visible how, as the ore body is approached, the albitophyres gradually turn into quartz–sericite schists. The latter gives every basis for considering sericitization to be a process inseparable from the general course of formation of the ore bodies. Being, in this way, a characteristic feature of hydrothermal alteration, it played a significant role in Salair as a prospecting indicator; moreover, this indicator was apparently known as early as the eighteenth and nineteenth centuries, since all the old surface workings, so abundantly scattered over Salair, are always concentrated on the outcrops of sericite schists. Contrary to the established opinion, the authors now raise the question that sericitization is a regional process and is connected with the general metamorphism of the rocks of all the oldest formations of the district. By this, the role of one of the most important prospecting elements—one that has repeatedly justified itself—is reduced to zero; for, according to this theory, in places where sericite schists are developed one cannot see the manifestation of hydrothermal processes in the given area.

It is, of course, impossible even to entertain the thought that the authors could have failed to notice such a very characteristic feature. And if this is so, then one gains the impression of a complete disregard of the facts for the sake of a new theory—probably one created in the study.

Having stopped at this point, it should further be noted that the authors also have another thesis (the fourth), which is in obvious contradiction with the preceding one. If above the authors asserted that the sericite schists formed before ore deposition under the influence of regional metamorphism, then in this thesis they speak of the formation of the same schists at the expense of the “leaching of quartz (T. B.) from compressed sericitized quartz albitophyres” (not schists, but albitophyres). First of all it is necessary to recall—although this is, of course, known to the authors—that the processes of sericitization at the deposit of the II Salair mine manifested themselves extremely intensely, transfor-

stocks, veins, etc. This circumstance gives every reason to say that by the time of the intrusion of these ultra-acid rocks (quartz porphyries), the albitophyres already existed, having been pierced by later portions of differentiated magma. These masses of quartz porphyries, in individual cases, apparently broke through to the day surface, while in others they were compelled, without reaching it, to solidify beneath the firm mass of the albitophyres. Naturally, during this solidification the abundant metallic compounds, which the residual and most acid part of the magma always concentrates within itself, rushed upward along the fissured zones that had arisen by this time in connection with the cooling of the albitophyres, or in connection with possible disjunctive disturbances that could readily manifest themselves in this weakened zone of the earth’s crust. And so, where we are dealing with stocks of volcanic masses not yet exposed by denudation, as a rule the deposits in these areas occur in albitophyres; conversely, where typical vents of quartz porphyries are before us, there the deposits are always concentrated in the latter. An example of the first type of deposits may be “Lens 2,” and of the second—Kopennoe.

Thus establishing the genetic connection of our deposits with quartz porphyries in their vent and stock facies, we shall now try to outline the history of the formation of these deposits.

As a prerequisite for the further exposition it should be noted that in recent years the view has become fairly firmly established in the literature¹ that the formation of any deposit with its characteristic mineral complex is never completed in a single episode; that in this formation tectonic movements undoubtedly play a significant role: at moments of compression the movement of emanational streams ceases and, conversely, at moments of pressure weakening or extension these streams strive to find an outlet. As a result of such a combination of tectonic displacements with hydrothermal processes, we often obtain either a rather complex mineral complex of individual deposits, or, conversely, within one and the same ore district we encounter deposits sharply different in their composition. And within the district studied, already upon a cursory inspection of all its deposits, this difference in the mineral composition of the latter is conspicuous. Here we may see ore bodies represented exclusively by gray pyrite (Kopennoe), bodies composed of hematitized quartzites with traces of sulfides (Samoilovskoe, the group of the Zolotogorsk deposits, situated nearby but not described by me in the present work), and polymetallic bodies (“Lens 2”). Taking

¹ N. N. Gornostaev, “New Principles for the Classification of Emanational Deposits.” Collected Works of the All-Union Trust “Zolotorazvedka”, issue 2, 1936, 3–44.

the source, prove to be barren and become enriched in metals only when they pass through the mass of sericite schists? Why do these solutions, leaching metals throughout the entire mass of the quartz porphyries, move toward a single point and deposit them there in the form of various sulfides? If, on the other hand, they move only along those fractures, or even fracture zones, in which we now see our ore bodies, then the question arises: what must have been the content of these metals in the primary mineralized rocks in order to yield such massive bodies, with reserves amounting to tens of millions of tons? A simple arithmetical calculation shows the impossibility of such an idea. For example, let us imagine that, in their movement, the hydrothermal solutions embraced a zone of strongly fractured rocks 100 m thick, 1/2 km long, and 2 km high; taking into account the ore reserves, for example, of the “Lens 2” deposit, one may say that the initially mineralized porphyries would have had to contain, per 1 ton of barren rock, about 40–50 kg of ore minerals, which amounts to 4–5% ore content in them. If we subtract 1.5–2% for the sulfur contained in the ore minerals, the total content of polymetals is expressed by a figure of 2–3%. Under such conditions our quartz porphyries could have been regarded as ores; but, unfortunately, this is not so in reality. Finally, how from this point of view can one explain the banded character of the ores and the existence, within one and the same district, of separate deposits of different mineralogical composition? All these questions, which are of not only theoretical but also practical interest, can hardly be explained by this theory.

Thus, the proposed hypothesis, built for the most part not on facts but on speculative conclusions, gives us not the slightest grounds for abandoning our views and adopting a conciliatory point of view. We continue to maintain that the mineralization is connected with the quartz porphyries, and specifically with their “vent” and “stock” facies.

The material collected by us in the district of the Ursk deposits permits us to put forward the “stock” facies as the most probable and promising one with respect to mineralization. The following considerations lead us to this. First of all, together with A. G. Vologdin, we believe that quartz porphyries in the strict sense of the term, and quartz-bearing (for the most part) albitophyres, constitute a definite stratigraphic horizon of the Middle Cambrian. While establishing beyond doubt the presence of these two types of rocks, which in the Ursk district are sharply distinguished by the form of their occurrence, and observing their relation to one another, it proved possible to establish that rocks of the albitophyre type are widely distributed and, upon their outpouring, apparently formed thick sills. Conversely, quartz porphyries appear within the mass of these albitophyres in a sharply subordinate relation, always having the form of small

Fig. 19. Scheme of mineralization in the deposits of the Urska district in Salair.

Labels in the figure:

• Phases of mineralization
• Hypogene epoch of mineralization
• I stage
• II stage
• III stage
• Epigenetic breaks in time
• Hypergene epoch of mineralization
• Acid waters
• Alkaline waters

Minerals:

1. Hematite
2. Actinolite
3. Quartz
4. Sericite
5. Chlorite
6. Arsenopyrite
7. Pyrite
8. Blende
9. Sphalerite
10. Tetrahedrite
11. Tennantite
12. Chalcopyrite
13. Galena
14. Gold
15. Barite
16. Chalcocite
17. Covellite
18. Silver
19. Limonite
20. Gypsum

and, naturally, enter into reaction with water, thereby forming sericite and quartz. The excess silica formed in these processes (in an amount of up to 30%) is deposited in place or, dissolving in water, may be carried into the overlying horizons. Above we have already noted that, under the corresponding conditions, mo-

isolated individual types of these deposits, it would be possible to assign to each of them its own genesis, since identifying them with one another on the basis of a surface examination would be very difficult. Only a detailed study of the entire aggregate of geological and ore material for all the deposits makes it possible to connect them with one and the same parental magma and to assign them to a single hypogene epoch, though to different phases (stages) of its mineralization. All the factual material set forth above finds its concrete expression in the scheme presented in Fig. 19, to a detailed analysis of which we shall now turn our attention.

In 1934 I proposed a scheme of mineralization for the “Lens 2” deposit. At that time I had at my disposal material from only two drill holes, and, undoubtedly, on its basis I could not give a complete idea of the processes taking place during the formation of this deposit. The more extensive material I have studied makes it possible to somewhat detail this scheme and to generalize it with the other deposits of the district, attempting thereby to reconstruct the natural course of events.

There is no doubt that all the deposits of the district were, in the main, formed in a single hypogene epoch of mineralization. At least, at present we have no material on the basis of which we could judge the superposition of mineralization of a second epoch, separated from the first by sharp unconformity and by a different character of ore formation. Therefore we say that the ore bodies and the processes accompanying ore formation were completed in one epoch. However, within the latter three rather sharply expressed phases (stages) of mineralization are discernible. The earliest of them is characterized by the deposition of quartz, sericite, chlorite, hematite, and in places even magnetite. The pyrite appearing in this stage is deposited in insignificant quantity. The temperature conditions, apparently, were not especially favorable for the removal of sulfur and its compounds; therefore, in the presence of iron and of the amount of sulfur that was in the primary rocks, pyrite was formed, which is thus an inseparable part of the process of propylitization and sericitization. We believe that in this stage the principal formation of quartz bodies and the accompanying ore bodies takes place—typical alterations of the enclosing rocks, i.e., sericitization and chloritization. These three principal processes proceeded almost simultaneously, and the final result of their action, fixed in the form of sericite, chlorite, and secondary quartz (quartzites), is due to the same hydrothermal waters.

The physicochemical system in these processes consisted chiefly of two phases, namely: a solid phase, represented by feldspars and the quartz of albitophyres, and a liquid phase—in the form of hot hydrothermal waters. Feldspars, as is known, under these conditions are a very unstable compound.

of the last stage is confined chiefly to the selvages of bodies formed earlier, which we see in the example of the “Lens 2” deposit, where essentially sphalerite ores, in the form of gigantic bands, appear in the hanging or footwall side.

At this point the formation of the ore bodies comes to an end. It is quite possible that within the last stage there also existed inter-mineralization movements, but their amplitude was apparently extremely weak, and therefore they are not detected in the material that we have at present.

Considerably later, after the complete completion of the cycle of hypogene processes of mineralization, owing to erosional activity the deposits are brought to the ancient surface and are subjected to processes of weathering. As a result of the prolonged action of atmospheric precipitation upon the primary sulfides, a new complex of minerals appears, which we assign to the supergene epoch of mineralization.

CONCLUSION

Thus, on the basis of the study of a considerable amount of ore material, it proved possible to outline three rather sharply defined phases of mineralization, each with its own characteristic mineral complex. This fully explains a number of those unclear questions that arise upon a cursory examination of these deposits. But such a conception of the formation of the ore bodies is not only of theoretical, but also of great practical interest. Indeed, if we establish that the formation of secondary quartzites is connected chiefly with the first phase of mineralization, in which, as is evident from the scheme, no sulfides of value to us appear, then it is natural to think that industrial accumulations of ores can hardly be encountered in the field of development of these quartzites. However, the presence of these quartzites indicates the manifestation of intense hydrothermal processes and that near the outcrops of these quartzite bodies there may also be accumulations of sulfide ores, owing their origin to later stages of mineralization. Of course, in individual cases there may be superposition of stages of mineralization. Therefore this question must be approached more cautiously. An example of this, as has already been indicated, may be the “Lens 2” deposit, which, as we see, is chiefly pyrite-chalcopyrite and is due to the manifestation of the second phase of mineralization, but at the same time a complex of minerals of the last stage appears in it in the form of separate bands. And it is still difficult to say what the character of this deposit will be at a still greater depth than that which is now known to us. In any case, the last borehole shows that at the 200 m horizon the body has a tendency to widen, that the bands with a content of

the sericite molecule ceases to dissolve and is likewise removed. And so, depending on what the temperature conditions were at a given stage in the development of the process, either secondary quartzites or substantially sericitized rocks are formed. Along with this, chlorite also formed, but already farther away from the zone of the freest movement of hydrothermal waters, where the temperature of the latter was high for its formation. Confirmation of all this is provided by the zonality in the spatial distribution of these rocks. Thus, for example, quartzites are always concentrated near the ore bodies or themselves constitute them; farther away come quartz-sericite schists, and still farther away sericite-chlorite schists.

Thus, as a result of these processes, thick deposits of secondary quartzites are often formed, usually with disseminated inclusions of pyrite, hematite, and, rarely, magnetite; zones of strongly altered rocks, sericitized and impregnated with pyrite, are formed.

At this point the first stage of mineralization is interrupted by episodes of stress. In the period between compression and subsequent extension, differentiation of ore solutions occurs in the magmatic chamber, and a new stage is characterized, alongside the continuing deposition of quartz and sericite, by the appearance of an abundant mass of pyrite and of hematite, arsenopyrite, and accompanying tetrahedrite and chalcopyrite, which somewhat precede it. The solutions of this stage find new, more accessible pathways. Undoubtedly, such pathways were those fracture zones which had been prepared by the preceding episodes of stress; naturally, under relatively weak stress these fractures were most readily formed in rocks not silicified by the solutions of the first stage. Therefore the main mass of minerals of this new phase of mineralization is concentrated to the side of the monolithic quartz bodies, forming independent ore deposits. In the secondary quartzites we are able to record only a system of thin quartz veinlets, often with rosettes of hematite and a weak dissemination of pyrite.

Thus, apparently, the pyrite lenses of the Urska district were formed in the main. At this point tectonic movements and the further formation of ore bodies do not cease. However, the intensity and amplitude of the new episode of stress were much weaker. Nevertheless, as a result of these movements, new pathways are prepared, and the solutions differentiating at depth yield a new segment of the mineral spectrum, this time represented by such minerals as barite, sphalerite, tenantite, galena, and others. Since the weakening stress was not capable of substantially disrupting the ore bodies composed of minerals of the first stages, it is clear that these movements were discharged mainly at the contact of these ore bodies and only occasionally produced a system of fractures in the latter. Therefore the assemblage of minerals

zinc has not yet wedged out; and, since this is so, one may hope that in this rather thick lens there will be found such sectors as will be quite profitable for development as a polymetallic deposit.

In conclusion it should be noted that, in our view, the role of the diorite intrusion—in the light of the data at our disposal—is somewhat different from that represented by authors who associate with it the principal polymetallic mineralization of the district. Without denying that, in the aggregate, the appearance of these rocks gave rise to the gold mineralization of the Salair Ridge, one cannot fail to note a genetic connection with them of a whole series of other elements which not infrequently form industrially valuable accumulations. In particular, as early as 1932, on the Suenga River a fluorite deposit was discovered, which was exploited by the Kuznetsk Metallurgical Plant and whose genetic connection with the diorite-diabase intrusion is undoubted. Alongside fluorite, pyrite, chalcopyrite, and galena appear as the principal emanational minerals. This series is supplemented by the composition of the quartz-carbonate veins accompanying the deposit of the II Salair Mine and likewise associated with the diorite-diabases, where, in addition to the minerals mentioned, sphalerite and gray ores occur.

Thus, in terms of their composition, the emanations proceeding from the diorite-diabase magma were also polymetallic, as were the emanations of the quartz porphyries. Their essential difference consists only in the fact that the former were extremely poor in such metals as zinc, lead, copper, etc., and therefore could not have provided any more or less significant concentrations. In connection with this, the form of the ore bodies is that of thin, though sometimes rather complex, veins with a quartz-carbonate base. The appearance of the diorite-diabase intrusion is separated from the polymetallic mineralization proper by a considerable interval, within which the formation of the principal schistosity of the rocks of the Cambrian formation is included. By the time these rocks appeared, the ore bodies of the polymetallic deposits had already been formed, as is evidenced by the circumstance that dikes of diorite-diabases cut through the ore bodies, as has been established, for example, in the “Quartz Vein” deposit and in the “II Salair Mine” deposit. At the same time, the dikes and their emanational bodies are always confined to disjunctive (fault) fractures, the formation of which preceded this new phase of volcanism. Confirmation of the latter is provided by observations made and recorded on the plans of individual levels of the II Salair Mine, where it is clearly seen how, for example, quartz-carbonate veins are confined to zones of displacement of the ore body (field drift, level 100.3 m).

1. Recently, a considerable amount of mercury has been found in the friable ore, probably associated with the faded primary ores. ↩↩↩↩↩