MINERALOGY AND GENESIS OF THE SECONDARY ZONES OF A SULFIDE DEPOSIT IN ALTAI

V. K. Cherepnin

Introduction

In recent times much attention has been devoted to questions of secondary zoning in sulfide deposits. The deposit described below has one very characteristic feature, namely that its ore body emerges not at the present-day surface, but in a buried valley. Also characteristic is the presence of an intensively developed and well-preserved cementation zone. This latter circumstance made it possible to trace the spatial distribution of the ores of the zone mentioned and to clarify their relationships both with the primary ores and with the oxidized ores.

Material was collected directly at the deposit twice, during field trips in 1944 and 1945. The most detailed collections were made in that part of the ore body which had undergone the action of secondary processes. Here the author tried to obtain the most complete sections. It is much to be regretted that we were unable to penetrate into the caved-in extreme southeastern part of the ore body, since later, during processing of the material, this often made itself felt.

The principal method used by the author during the laboratory treatment was the mineragraphic study of ores in reflected light, with extensive use of qualitative microchemical reactions. In all, more than two hundred polished sections were studied by this method. Much less often we resorted to purely mineralogical investigations, sometimes with the aid of a blowpipe (determination of the composition of ocherous biverite ores).

Brief information on the geomorphology of the deposit area

The relief in the area of the deposit consists of gentle, insignificant elevations and represents a weakly expressed piedmont low-hill terrain, gradually rising to the north. The spaces between individual elevations, with numerous gentle ridges, are usually covered by loose deposits of varying thickness. The hills themselves in most cases are devoid of a mantle of loose deposits and are monadnocks of a buried relief.

The ore site is situated in a small basin elongated in a northwestern direction and surrounded by several low elevations. In itself it is a plain covered by a thick mantle of loose deposits.

Thus, in geomorphological respect, the area of the earth’s surface that interests us represents a typical peneplain, levelled—

covered by loose deposits. It is a fairly ancient formation, since in its main mass it is buried beneath younger loose sediments. Beneath this mantle of loose deposits, within the territory of the deposit, four well-developed buried valleys have been discovered, rather deeply incised into the surface of the ancient peneplain.

Fig. 1. Schematic longitudinal section of the ore body. 1—primary horizon of primary pyritic ores overlain by calybazone; 2—horizon of primary pyritic ores not overlain by calybazone; 4—horizon of the richest calybazone ores; 5—horizon of cupriferous ores; 6—horizon of cupritized “variegated” ores; 7—horizon of barren ores; 8—buried valley; 9—level of the modern surface.

The southernmost buried valley displays extremely interesting relationships with the ore body described here, namely: the head of the latter is cut off by this valley (Fig. 1). This fact accounts for the entire specificity of the development of secondary processes in our ore body.

We have no data directly indicating the time of formation of the buried valleys. It seems to us that, in this respect, the valleys mentioned may be compared with the ancient valleys of Central Kazakhstan studied by N. G. Kassin. The difference between these two types of valleys, in our opinion, lies only in the fact that the ancient valleys of Central Kazakhstan are still clearly expressed in the relief and modern rivers often follow them, whereas the ancient valleys of our area are buried beneath a thick cover of alluvium. With regard to the time of formation of the ancient valleys of Central Kazakhstan, N. G. Kassin indicates that the incision of these valleys took place later than the Lower and even Middle Jurassic and ended by the Upper Cretaceous. Approximately the same view on this question is held by V. P. Nekhoroshev, E. D. Shlygin, and V. A. Obruchev—all of them believe that the relief of Central and Northern Kazakhstan was formed mainly in pre-Tertiary time and was only smoothed by marine

and by continental deposits of the Tertiary period. One of the reasons why this pre-Tertiary relief has been preserved in Kazakhstan to the present day, V. A. Obruchev tends to see in the absence there of Quaternary uplifts capable of rejuvenating erosion. As for Altai, then, as is known, its development as a mountain structure took place precisely in the Quaternary period. The Southwestern Altai that interests us probably at that time was nothing other than a foothill area of the rising Altai mountain system, from which an enormous quantity of clastic material was carried down into the foothills, gradually filling ancient valleys and burying them beneath a many-meter thickness of deposits.

General Information on the Geology of the Deposit

The deposit under consideration is situated within the Irtysh zone of folding of Southwestern Altai, which is reflected in the considerable metamorphism of the rocks composing the area of the deposit and in their northwestern strike. Among the rocks developed in the area of the deposit, both sedimentary and igneous varieties are known. Both bear the features of more or less significant dynamic action and of alteration under the influence of ore-bearing solutions.

The sedimentary rocks, as it seems to us, may be divided into two formations: lower and upper.

The lower formation is composed of phyllite-like carbonaceous-clayey, clayey, and sandy-clayey schists. Varieties rich in carbonaceous matter are colored black, while those lacking it have a gray or greenish-gray color. These schists are almost always thin-bedded, and dense varieties occur only rarely.

The upper formation is composed of rocks in which calcareous material plays a very large role. Depending on the amount of the latter, we have all transitions from pure limestones to marly rocks. Under the action of metamorphism, the rocks of the formation have been transformed into marmarized limestones, marbles, marly schists, and chlorite-calcareous schists. Among the marbles there are varieties containing a large quantity of crinoid ossicles. In their stratigraphic position, the varieties mentioned are the uppermost member of the formation being described. In addition to the enumerated sedimentary rocks, tuffs and tuffaceous schists are developed rather widely in its composition. The tuffs are usually of porphyritic composition, but more acid varieties—porphyries—also occur.

The rocks of the upper formation lie unconformably upon the lower formation, as was established earlier by the geologist D. N. Logvin during the geological survey of the deposit area.

Under the influence of ore-bearing solutions, the above-described schists of both formations are transformed into sericite-chlorite-quartz, chlorite, chlorite-quartz, and sericite-quartz schists. As for the tuffs, the latter also not infrequently pass into chlorite schists, while their more acid varieties, as D. M. Korulin and S. A. Bukeikhanov indicated, are often transformed into sericite-quartz schists.

The question of the age of the metamorphosed sedimentary rocks of the deposit area remains open at present, owing to the almost complete absence of paleontological evidence. The youngest deposits—crinoid limestones—D. N. Logvin provisionally assigns to the Middle Devonian. The oldest rocks—carbonaceous-clayey and clayey schists—D. M. Korulin and S. A. Bukeikhanov are inclined to regard as Cambro-Silurian.

Of the igneous rocks developed in the area of the deposit, the chief role is played by albitophyres, among which G. L. Padalka distinguished two varieties: 1) quartz albitophyres and 2) felsitic albitophyres; moreover, the banded varieties of the latter were distinguished by him as porphyroids. It is quite beyond doubt that some of the albitophyres mentioned are effusive formations. Their main mass, however, consists of hypabyssal intrusions occurring in the form of sheetlike veins and intrusive bodies among the sedimentary rocks. Under the influence of hydrothermal solutions the albitophyres underwent various alterations, at times so considerable that they can scarcely be distinguished from sericite-quartz schists. In addition to the albitophyres, in the northeastern part of the deposit area there occur veins of strongly altered basic rocks and dikes of felsites transformed into secondary quartzites. Besides the igneous rocks listed, one other occurrence is known: a diorite porphyrite, which was described by G. L. Padalka from old dumps.

As regards the tectonics of the deposit area, D. M. Korulin and S. A. Bukeikhanov distinguished here three local zones of crumpling, parallel to the main Irtysh zone. Two of them are weakly expressed and are associated with insignificant manifestations of mineralization. The third zone, passing through Rudnichnaya Hill, is the largest, and the mineralization of the deposit described is connected with it. Within this zone the rocks are gathered into a large anticlinal fold of northwestern strike. The limbs of this anticlinal structure are complicated by additional folding of several orders, down to plaiting. The axes of the secondary folds are mutually parallel and have a northwestern strike of 310–320°, plunging to the northwest at angles of 15–25°.

The ore body that is the object of our study is situated in the southwestern limb of the main anticlinal structure and has the character of a deposit. The ore deposit usually follows the structure of the enclosing rocks, repeating the latter in all its details. We consider entirely just the opinion expressed by A. A. Amiraslanov, D. M. Korulin, and S. A. Bukeikhanov that the main features of the form of the ore bodies at the deposit are predetermined by the geological-tectonic structure of the enclosing rocks, and that the receptacles for the ore bodies were essentially prepared as a result of plicative dislocations. The role of disjunctive dislocations in the ore field of the deposit is negligible, and their manifestation, in the opinion of the above-mentioned investigators, belongs to the post-ore time. The ore body, being situated in the southwestern limb of the main anticline, has a northwestern pitch and a predominant southwestern dip, with dip angles of 45–70°. In addition to the predominant southwestern dip, there are parts of the ore body with a reverse, northeastern dip, which are distinguished at the deposit under the name “northeastern limb.”

Mineralogical Characterization of the Ore Body

Primary Ores

Ore Texture

On the basis of textural features, the primary ores of the deposit may be subdivided into two groups: 1) massive ores, 2) disseminated ores. The distinction between massive and disseminated ores is purely conventional, since the ores studied are characterized by a high content of vein minerals and by the almost constant presence of inclusions of sometimes even only slightly replaced schist. Very rarely do massive ores contain vein-

…of non-ore minerals is less than 6%; usually, however, they are present in amounts from 10 to 25%, often increasing considerably. In the latter case the ore occupies an intermediate position between massive and disseminated ore. Nevertheless, it seems to us that, for the convenience of characterizing the ores, such a purely conventional distinction may be applied.

Massive ores, in turn, are subdivided into two varieties: 1) ores of massive structure and 2) ores of banded structure. The first variety does not make up large masses, but is more often encountered in the form of lenses and irregular patches among ores of other textural varieties. The second variety is characterized by an alternation of bands enriched in various ore components. This textural variety predominates among the massive ores of our deposit.

Disseminated ores are characterized, in general, by a predominance of non-ore material over ore material. The ore minerals form either individual inclusions, or, more or less uniformly—in the form of a sprinkling—are dispersed through the enclosing rock, or else, finally, develop in the form of irregular accumulations of veinlets and bands. Among the ores of this group, in turn, two subgroups may be distinguished: 1) ores richly mineralized with poor sulfides, and 2) disseminated ores proper. The ores of the first subgroup are intermediate formations between massive and typical disseminated ores. These ores are characterized by an abundance of vein minerals, indicating rich mineralization and insignificant development of ore components. According to the content of particular sulfides, one or another variety is distinguished among the ores of the subgroup described. The second subgroup includes the pyrite disseminations widely developed in the deposit, representing the result of weak mineralization.

Mineralogical Composition

Among the primary ores of the deposit described, the following ore minerals were established: pyrite, chalcopyrite, sphalerite, pale ore, galena, and arsenopyrite. In addition, among the vein minerals the following were recorded: quartz, barite, sericite, and chlorite.

Pyrite is the most widespread ore mineral. In disseminated ores, especially in poor ones, pyrite is often the only ore mineral. Grain sizes vary from 0.05 to 2.4 mm, and in poor disseminations crystals measuring 5–6 mm on an edge are encountered. Grain forms are usually extremely varied; idiomorphic forms are rare in massive ores, while in disseminations they occur more often and predominate in very poor disseminated ores. In the overwhelming majority of cases, pyrite grains are considerably corroded by other ore minerals and, more rarely, by vein minerals, with the formation of typical replacement structures (up to and including skeletal and crushed structures). In massive ores, pyrite is almost always distributed uniformly and only in some cases is separated into various accumulations and bands, forming massive pyrite ores similar to the true sulfur-colchedan ores of the Urals. In disseminated ores there is a certain tendency for pyrite grains to be distributed along the schistosity. In addition, many grains bear traces of evident dynamometamorphism in the form of a whole system of cracks arranged across the elongation of the grains.

The internal structure of pyrite grains, revealed by etching, displays the form of a cube, which was also noted by B. I. Veid.

Chalcopyrite is the second most widespread mineral. In massive ores chalcopyrite occurs in the form of variously sized…

spots, areas, and entire fields composed of an uneven-grained aggregate. The sizes of individual grains range from 0.03 to 0.2 mm.

In disseminated ores it is developed either as spots of various sizes and of the most diverse shapes, or as veinlets, very irregular and often interlacing with one another.

Being often the predominant mineral, chalcopyrite constitutes, as it were, a cementing mass in which all the other ore components are dispersed. The greatest number of such inclusions usually belongs to sphalerite, with which chalcopyrite is always in a very close association, indicating very close periods in time for the formation of both minerals. Quite often it is associated with pale ore, locally replacing it intensively.

Sphalerite, yielding in abundance to the preceding mineral, occupies third place. As a rule, it appears as spots and inclusions of the most varied sizes and diverse shapes, representing an aggregate combination of grains. The sizes of individual grains, revealed by etching, range from 0.04 mm to 0.5 mm. In massive ores the mineral described often forms whole fields or is distributed in bands. In disseminated ores it usually develops in the form of chains consisting of separate inclusions arranged along the schistosity. In sphalerite, inclusions of chalcopyrite, galena, more rarely pale ore, and very rarely single grains of arsenopyrite, of different shapes and sizes, are often observed; however, regular inclusions of an emulsion-like character were not encountered in any polished section.

Galena is developed in the ores described in a sharply subordinate quantity. Galena occurs both in massive and in disseminated ores. In massive ores it develops in the form of inclusions of diverse shape, representing either allotriomorphic aggregates of several grains or single grains. The sizes of individual grains range from 0.05 to 0.1 mm. In most cases galena is closely associated with sphalerite, forming in it a typical subgraphic texture. In etched polished sections it is clearly visible that in this case the galena fills the intervals between the idiomorphic grains of sphalerite, which also explains the peculiar shape of its inclusions. Much more rarely the mineral described develops among chalcopyrite, where it forms inclusions of a more isometric shape. Extremely characteristic is the very close association of galena and pale ore, especially manifested in the fields of chalcopyrite development.

In disseminated ores galena occurs in larger inclusions, subordinate in their position to the schistosity of the enclosing rocks. Individual grains here are also larger and attain sizes on the order of 0.4 mm.

Pale ore is present in a very limited amount among the ore components of the deposit. The results of microchemical studies showed that both arsenic and antimony are present in its composition, the former, judging by the intensity of the reaction, having a definite predominance.

The mineral described occurs both among massive ores and among disseminated ores. In the first case it is developed in the form of more or less isometric inclusions ranging in size from 0.03 to 0.4 mm. Less commonly, inclusions of the most irregular shape are encountered. Among the disseminations, pale ore has the form of the most diverse spots, usually arranged along the schistosity and often associated with galena.

Arsenopyrite is present in the ores in extremely insignificant quantities. The mineral almost always occurs as well-formed crystals of prismatic or pseudodipyramidal shape.

The sizes of individual crystals range from hundredths to one and, rarely, one and a half millimeters. Most often they are situated among the vein minerals, chiefly quartz, and more rarely among chalcopyrite and gray ore. With the exception of pyrite, all the other ore minerals corrode arsenopyrite to one degree or another.

One cannot fail to note a very interesting fact, sometimes observed in polished sections: among areas composed of chalcopyrite, there is in places a pattern that in outward appearance is very reminiscent of a graphic texture. This pattern is produced by extremely fine grains of arsenopyrite, very densely scattered in the chalcopyrite (Fig. 2). Noteworthy is the fact that such accumulations of arsenopyrite appear only in those parts of the chalcopyrite where the latter corrodes gray ore. This is especially clearly seen in thin veinlets of chalcopyrite penetrating into inclusions of gray ore, where arsenopyrite develops only within the limits of these veinlets.

It seems to us that the process of formation of this structure may be interpreted in two ways. First, it is possible that the above-mentioned accumulations of fine arsenopyrite grains arose at the moment when gray ore was being replaced by chalcopyrite, during which arsenic was released and crystallized in the form of arsenopyrite. On the other hand, it is very possible that the above-mentioned accumulations of arsenopyrite are not new formations at all, but are relics left during the process of replacement of arsenopyrite by gray ore. In favor of this supposition is the presence of small spots of gray ore around many of the finest arsenopyrite grains; in places, moreover, clear traces of corrosion of arsenopyrite by gray ore are visible. In addition, here and there along the periphery of accumulations of arsenopyrite “dust” there occur separate, larger grains of arsenopyrite, still only slightly replaced. Thus, it seems to us that the second explanation of the structure described is more probable and corresponds more closely to reality. As for the frequent confinement of the fine arsenopyrite grains to chalcopyrite veinlets cutting gray ore, it is quite possible that, in the process of its replacement, chalcopyrite developed along those directions in the gray ore where arsenopyrite grains were present, making use of the weakened directions that always exist at the contact of different minerals.

Fig. 2. Graphic texture of arsenopyrite (white) in chalcopyrite (black). Bl—gray ore; Q—gangue. Magn. 400.

Paragenesis and sequence of deposition

In the course of studying the primary ores of the deposit, two paragenetic associations were clearly indicated in them, sharply differing in age. Each of these paragenetic groups included two minerals. The first group—pyrite and arsenopyrite—forms a pair in which the minerals are very closely associated with one another. Nevertheless, study of their mutual relations permits pyrite to be regarded as an earlier mineral than arsenopyrite. The second group—chalcopyrite and sphalerite—is also extremely

closely associated with one another minerals, and the question of their relative age could be resolved only with the aid of structural etching. We were able to establish that a considerable part of the chalcopyrite was deposited earlier than the sphalerite, but, undoubtedly, the reverse picture also occurs.

In general, the order of deposition of the ore minerals appears as follows: pyrite was deposited first; then, in the order of deposition, follow arsenopyrite, gray copper ore, chalcopyrite (part of it is deposited after sphalerite), sphalerite, and galena (Fig. 19).

It seems to us highly probable that the deposition of the mineral complex described took place in two stages. In the first stage, pyrite and arsenopyrite were deposited; in the second, all the remaining ore minerals. Between the deposition of the minerals of the first and second stages there was crushing, traces of which, as indicated above, we encounter on many pyrite grains.

Ores of the Cementation Zone

Ore textures

With respect to the textural features of the ores of the zone described, it should first of all be noted that in its lower part—that is, where the processes of secondary sulfide enrichment are weakly expressed—the textural types of the primary ores are still preserved. In the higher horizons of the zone, however, which constitute the arena for a powerful development of the processes of secondary sulfide enrichment, we encounter the following textural types: 1) ores of massive composition, 2) ores of earthy composition, 3) ores of banded composition, 4) ores of crustiform composition.

The first type is the most widely developed. To it belongs the greater part of the chalcocite ores of the middle part of the cementation zone. Ores of the second textural type, representing earthy masses, also sometimes make up considerable areas. As for the banded composition, this textural type is fairly widely distributed among the ores of the first two textural varieties, and it is perhaps better regarded as a variety of the latter. Crustiform textures are most widely developed among the ores of the oxidation zone; here they appear only in the upper horizons and have a comparatively limited distribution.

Mineralogical composition

Mineralogically, the ores of the zone under consideration consist essentially of chalcocite and covellite. In addition to the sulfides mentioned, especially in the upper part of the zone, such minerals as limonite, native copper, malachite, cerussite, and smithsonite are often developed, and there is also always some quantity of primary sulfides.

Chalcocite is the principal mineral of the zone of secondary sulfide enrichment. Its content in the ores is very variable—from isolated veinlets developing among the mass of primary sulfides to continuous areas.

In the very lowest part of the cementation zone we encounter chalcocite exclusively among chalcopyrite, in the form of thin, isolated, threadlike veinlets, no more than 0.02 mm thick, forming so-called threadlike structures (Fig. 3). As for the other primary sulfides, they still bear absolutely no signs of replacement.

It has been established that the threadlike veinlets of chalcocite in chalcopyrite usually follow along the boundaries of individual grains of the latter or make use of

by other such weakened directions, such as, for example, twin seams.

Somewhat higher, hypsometrically, the number of chalcocite veinlets in the chalcopyrite increases considerably. The veinlets themselves constantly increase in thickness, branch repeatedly and, intersecting one another, create a reticulate pattern of varying density, usually called a reticulate replacement structure (Fig. 4). Here one sometimes observes the first, extremely weak replacement of galena.

In higher horizons of the cementation zone we already see traces of vigorous replacement of sphalerite, portions of which are intersected by rather numerous chalcocite veinlets, also forming a reticulate structure, though incomparably coarser than that in chalcopyrite (Fig. 5).

Fig. 3. Filiform replacement structure of chalcopyrite (Cp) by chalcocite (veinlets). Magn. 400.

Fig. 4. Reticulate replacement structure of chalcopyrite (Cp) by chalcocite (black). Sl—sphalerite; Gn—galena; Py—pyrite; Q—vein mineral. Magn. 180.

Finally, in still higher parts of the zone described, the chalcocite veinlets, increasing in thickness, intersecting and joining together, break up the sphalerite areas into separate scattered “patches,” forming a somewhat distinctive looped structure (Fig. 6). With further replacement, these “patches” either disappear completely or only the thinnest relics remain in their place. In general, the ore from this horizon consists essentially of chalcocite, among which are scattered a few relics of primary sulfides. Of the latter, pyrite, chalcopyrite, and sphalerite are usually present. It is striking that the replacement of pyrite proceeds worst of all. This is expressed in the fact that even in the very upper part of the cementation zone there are abundant grains of pyrite that show no traces of replacement by chalcocite. Arsenopyrite is likewise poorly replaced. As for the pale ore, any signs of replacement in it begin to appear only in the upper part of the cementation zone.

Unreplaced galena is encountered only very rarely. Usually in its place we find a fine white powder, the shape of whose accumulations corresponds to the shape of the replaced galena grain (Fig. 7). In places the replacement proceeds so intensively that even the aforementioned powder does not remain.

The chalcocite developed in the ores of the deposit is heterogeneous, and within it two varieties can clearly be distinguished: metacolloidal and crystalline.

Metacolloidal chalcocite is the most widespread variety and is characterized by the following features:

Fig. 5. Net-like replacement structure of sphalerite (Sl) by chalcocite (black). Cp—chalcopyrite. Magn. 150.

Fig. 6. Loop-like replacement structure of sphalerite (Sl) by chalcocite (black). Cp—chalcopyrite; Py—pyrite; Bl—pale ore. Magn. 275.

1) it polishes easily and well; 2) its reflectivity is medium and decreases markedly in oil; 3) its color is grayish white with a distinct bluish tint, distributed in patches; 4) there is no reflection pleochroism; 5) in polarized light, phenomena of aggregate polarization are observed; 6) under the action of HNO₃ it reacts vigorously and turns blue; 7) it is etched beautifully, structurally, by HNO₃ (1:1) within 15–20 seconds. Etching does not reveal a granular aggregate with any internal structure of individual grains. Instead, an irregular pattern appears, formed by the finest cracks, in all probability shrinkage cracks (Fig. 8).

Fig. 7. Replacement of galena (white) by chalcocite (black). Sl—sphalerite; Bl—pale ore; Q—vein mineral. Magn. 375.

Fig. 8. Cracked structure of metacolloidal chalcocite. Magn. 195.

Crystalline chalcocite is a less widespread type and occurs in the form of separate, usually insignificant areas.

...patches amid fields of metacolloidal chalcocite. In a polished section, under ordinary illumination, the variety described has the same physical properties as the first. For this reason it cannot be distinguished at all from the preceding one without the use of polarized light. In polarized light this variety reveals its crystalline structure. At the same time a distinct change in the coloration of individual grains occurs. The color changes from light bluish-gray to dark grayish-blue. The crystalline nature of the chalcocite described is still more clearly brought out in structure-etched polished sections. After fifteen to twenty seconds of etching with fifty-percent nitric acid, a beautiful granular structure and distinct cleavage in two directions within individual grains are revealed (Fig. 9). The size of individual grains ranges from 0.05 to 0.06 mm.

No regularity has been established in the distribution of crystalline chalcocite or in its relationships with the metacolloidal variety. It may only be noted that in the lower levels of the cementation zone the variety described by us was not encountered, and its occurrence pertains approximately to the middle levels. Likewise, we have absolutely no material on the basis of which the varieties described could be assigned to two different generations of chalcocite. It seems to us that crystalline chalcocite is the product of further recrystallization of colloidal material—metacolloidal chalcocite.

Fig. 9. Crystalline chalcocite. Boundaries of individual grains and cleavage. Disseminated-vein mineral. Magn. 300.

As for the heterogeneity in the coloration of our chalcocites, this fact has repeatedly been noted as a characteristic feature inherent in Altai chalcocites in general. At the same time, the bluish tint was explained by the presence of a submicroscopic admixture of covellite. In addition, experimental studies of the solid phases of the Cu₂S—CuS system, carried out by Berger, showed that low-temperature chalcocites are capable of containing a considerably larger amount of covellite than high-temperature ones. It seems to us that the scattered areas and small specks of blue coloration in our chalcocite, which in all their properties do not differ from the surrounding chalcocite mass, represent areas enriched in extremely fine covellite material, occurring in almost the same colloidal state as the chalcocite.

The relationships of chalcocite with covellite will be considered below in the description of the latter. Limonite and malachite usually distinctly corrode chalcocite, which is especially well seen in the uppermost levels of the cementation zone. As for the relationships of chalcocite with smithsonite, crusts of the latter were sometimes encountered on the surface of essentially chalcocite ores. Relationships of chalcocite with cuprite, native copper, and cerussite could not be observed.

Covellite is much less widespread in the ores described than chalcocite, approximately in a ratio of 1:3. The only exception is the very lowest levels of the cementation zone, where it predominates over chalcocite.

Studies have shown that not all covellite is homogeneous, and that, for this reason, two different types can be clearly distinguished within it. The first type of covellite is developed chiefly in the very lowest horizons of the cementation zone and is characterized by an extremely fine grain size. The finest grains can be distinguished only with difficulty under strong magnification, while the coarsest rarely reach a size of 0.01 mm. Their color is distinctly blue. In the largest grains, reflective pleochroism is visible. The color changes from dark blue to white with a delicate bluish tint. In crossed nicols the mineral is distinctly anisotropic. The change in color ranges from bright orange to reddish brown. In oil no difference is observed. It boils rapidly from aqua regia. From KCN a violet-red precipitate is formed, which could be observed only where there are more or less substantial accumulations of grains of this mineral. All other reagents

Fig. 10. Threadlike structure of replacement of chalcopyrite (Cp) by covellite (black). Sl—sphalerite; Py—pyrite; Q—vein mineral. Magn. 375.

Fig. 11. Covellite (black), developing at the boundary of chalcopyrite (Cp) with gangue material. Sl—sphalerite; Gn—galena; Bl—fahlore. Magn. 300.

do not act. Like chalcocite, covellite at first, i.e., in the very lowest horizons of the zone under consideration, appears in chalcopyrite, developing most often along the boundary of the latter with gangue material or forming rare and thin veinlets (Fig. 10). Somewhat higher up, covellite veinlets begin to appear along the boundary of chalcopyrite with inclusions of galena. Finally, in still higher horizons of the cementation zone, traces of replacement of sphalerite by covellite appear. The replacement of sphalerite that began proceeded, apparently, very effectively. For this reason, a concentration of covellite is almost always observed around inclusions of sphalerite.

Replacement of chalcopyrite by covellite proceeds especially intensively in cases where separate areas of the former are enclosed in gangue material. Here vigorous replacement develops only from the periphery of the indicated inclusions, while within them replacement veinlets almost never arise (Fig. 11). As for the relations of this type of covellite with chalcocite, the latter, overgrowing areas composed of covellite, replaces it intensively (Fig. 12), creating a certain semblance of a mesh structure. Relations of the described covellite with other secondary minerals, developed mainly in the upper parts of the cementation zone, were not observed.

The second type of covellite is developed in the upper part of the cementation zone. It occurs as small areas or continuous masses and, more rarely, as veinlet-like formations. Unlike the first type, the grain size of the covellite here reaches 0.2 mm. The shape of individual grains resembles leaves or scales elongated lengthwise.

Its color is the same distinctly blue as in the preceding variety, and it likewise polarizes effectively in bright tones. As for reflection pleochroism, here it is manifested many times more distinctly than in the previous case. The change in color proceeds from bluish-white to dark blue. In oil this coloration is preserved, only the colors become somewhat brighter and lighter. Diagnostic etching is in no way different from that of the first variety. As for the relations of the covellite described here with chalcocite, unlike the first variety this covellite is not corroded by chalcocite, but itself intensely corrodes it, surrounding unreplaced areas of the latter in the form of loops (Fig. 13). Among other relations, it was possible to observe only its relations with malachite and smithsonite. Often one can see malachite, developing later than the covellite described here, occupying the central parts of the latter’s “veinlets” (Fig. 13). As for smithsonite, the latter sometimes forms crusts on the surface of ore consisting essentially of covellite of this type.

Fig. 12. Replacement of first-generation covellite (white) by chalcocite (black). Cp—chalcopyrite; Sl—sphalerite. Magn. 375.

Fig. 13. Looped structure of replacement of chalcocite (white) by second-generation covellite (Cov.). Black—malachite. Magn. 190.

The data presented above allow one to consider that the two types of covellite differ from one another not only morphologically, but also genetically, representing formations of different ages, i.e., independent generations.

Such minerals as native copper, cuprite, malachite, smithsonite, cerussite, and limonite will be characterized in the description of the ores of the oxidation zone, as having their greatest development there.

Paragenesis and Order of Deposition

As is evident from the description, covellite and chalcocite are minerals very closely connected paragenetically, with chalcocite very often associated with covellite of either one or the other generation—

whereas covellines of different generations were never once encountered together. With respect to the vertical distribution of these minerals in Altai, at present there are two diametrically opposed opinions. Representatives of the first point of view believe that chalcocite is characteristic of the lower part of the cementation zone of the Altai deposits, and covelline of the upper parts. The second point of view, expressed by G. L. Padalka, is that, in the opinion of this investigator, covelline is predominantly developed in the lower horizons of the cementation zone. In general, however, most investigators are inclined to regard covelline, in its main mass, as a younger formation than chalcocite.

As a result of our investigations, the regularity in the distribution of these two minerals for the ores of the deposit described has the following form: in the very lowest parts of the cementation zone, where the processes of secondary sulfide enrichment are developed only in an incipient state, covelline of the first generation is predominantly developed. In the middle part of the zone chalcocite predominates, which in the uppermost part is replaced by covelline of the second generation. Thus, the most ancient secondary sulfide in our ore body is fine-grained covelline of the first generation. Next in time of formation comes chalcocite, and the last is covelline of the second generation. The earlier formation of covelline of the first generation in relation to chalcocite can often be observed in thin secondary veinlets, in which covelline of the first generation is situated along the selvages, while chalcocite, cutting it, occupies the central parts. In addition, cases are often observed in which areas composed of covelline of the first generation are, as it were, overgrown by chalcocite and are cut by numerous veinlets of the latter (Fig. 12). The latest formation of coarse-grained covelline of the second generation is established from the facts of its cutting chalcocite (Fig. 13). Consequently, it turns out that chalcocite, in time of its formation, separates covelline of two generations. As regards the relationships of the secondary sulfides described with malachite, the latter is a younger formation than chalcocite and covelline of the second generation (Fig. 13). The same may be said of limonite and smithsonite.

Ores of the Oxidation Zone

Directly at the deposit we studied only the lower half of the oxidation zone. Its upper horizons proved to be completely inaccessible, and the small amount of material from this part that we have at our disposal was taken from drill-hole core.

Textures of the Ores

Among the ores of the oxidation zone we established the development of the following textural types: 1) ores of massive structure, 2) ores of earthy structure, 3) ores of banded structure, 4) ores of crustiform structure.

The first textural type is widely developed among the ores of the lower part of the zone described, rich in limonite.

The second textural type predominates among areas composed of ocherous biverite-bearing ores. As for the banded structure, it is most distinctly expressed in the lower part of the zone, among dense oxidized ores. In this case, just as in the ores of the cementation zone, the indicated texture is a reflection of the primary banding of the sulfide ores. Some banding is also observed—

also in higher horizons, where rather often there is an alternation of ocherous biverite-bearing bands with bands composed of brown-iron ores.

Crustiform texture is very widely developed among oxidized ores. Excellent examples of such a texture are crusts of malachite, cerussite, and especially smithsonite, developing on the surface of essentially limonitic ores.

According to the character of the composition of the crusts themselves, three varieties may be distinguished: 1) amorphous crusts, 2) crusts with a botryoidal structure, and 3) crusts with a drusy structure. The first variety is characteristic of malachite. The second occurs in malachite, cerussite, and especially effectively in smithsonite; finally, the last variety is formed by cerussite.

Mineralogical Composition

Among the ores of the oxidation zone of the deposit, we established the following ore minerals: native copper, cuprite, limonite, malachite, azurite, cerussite, smithsonite, and biverite with an admixture of undifferentiated hydrous sulfates of the brochantite—chalcanthite type. In addition, in the very lowest part of the zone we always have some quantity of relics of secondary sulfides, chiefly chalcocite, and, finally, relic pyrite is also encountered here. Of the vein minerals, barite and carbonates are developed to the greatest degree, and quartz in considerably smaller amount.

Native copper is rather widely distributed in the very lowest part of the oxidation zone. Its content in polished sections is very small. The sizes of individual inclusions reach 0.15 mm and, as an exception, 0.4 mm. The color of the mineral in freshly polished sections is pinkish, and the mineral is distinguished by an exceptionally high reflecting power. In air it quickly tarnishes and yellow tones appear. The mineral polishes well, but the surface always bears a certain number of scratches. Diagnostic etching revealed no deviations from the norm.

Usually native copper is closely associated with cuprite, and cases of its occurrence separately are extremely rare. Cuprite mostly develops along the periphery of inclusions of native copper and much more rarely forms some semblance of intersecting veinlets (Fig. 14). Very often copper is closely associated with limonite, developing in the form of veinlet-like aggregates consisting of a large number of very closely spaced individual inclusions. Nest-like accumulations of fine inclusions of copper are also not uncommon, developing most often among carbonate areas. Relationships of native copper with such minerals as malachite, smithsonite, cerussite, and biverite were not observed.

Cuprite is a more widespread member of the very lowest part of the oxidation zone than native copper. As was indicated, it is often associated with native copper, but it is also not infrequently encountered separately, in the form of inclusions of the most varied shape. The sizes of individual inclusions rarely exceed 0.3 mm. The polishing hardness of the mineral is considerable, but it polishes well. The color is light bluish. Next to native copper the mineral appears dark blue. In non-ore material it appears grayish-white with a barely noticeable bluish tint. Reflection pleochroism is not noticeable. In crossed nicols it is weakly anisotropic and in holes shows a dark-red internal reflection. With HNO₃ 1:1 it effervesces vigorously; native copper appears for a moment, then turns black; with HCl 1:1 it quickly turns black, forming

brown coating; from FeCl₃ it darkens somewhat, and a brownish shade appears. The other reagents have no effect. Like native copper, cuprite develops both among limonite and among carbonates; in the former case it never forms more or less significant accumulations. No interrelations of cuprite with other ore minerals were observed.

Limonite is the most widespread mineral of the oxidation zone. It is developed everywhere, both in its lower and in its upper horizons. It usually composes either continuous fields, serving as a kind of cement in which the other ore minerals are disseminated, or densely impregnates the barite-rich rock. Both in its physical properties and genetically, the limonite developed in our ores may be divided into two varieties.

The first variety occurs chiefly in the very lower part of the oxidation zone. In this case the limonite is formed exclusively at the expense of pyrite, which it replaces while completely preserving the outlines of the grains of the latter (Fig. 15). A characteristic feature of this limonite is the absence of a metacolloidal structure. The hardness for polishing is

Fig. 14. Native copper (Cu) with cuprite (Cup) in barren material. Magn. 95.

Fig. 15. Limonite (lm) after pyrite (black) and after chalcocite. White—chalcocite. Magn. 300.

sufficiently high, but the mineral polishes well. Its reflectivity is moderate. Its color is grayish white or light gray; in oil it becomes considerably darker. In crossed nicols no distinct anisotropy is observed; a brownish-red internal reflection is visible. It is not etched diagnostically. Some investigators have described this variety of limonite as goethite. In view of the fact that the mineral was not subjected to thermal analysis, then, despite its actually considerable similarity in physical properties to goethite, it seems to us more correct not to distinguish it as an independent mineral, but to regard it as a variety of limonite. As for the interrelations of this limonite with other minerals of the oxidation zone, these were observed only with malachite and with the second variety of limonite. The character of the former always indicates replacement of the mineral described by malachite; the latter will be considered below.

The second variety of limonite is predominant and composes considerable masses. In contrast to the variety described above,

... it is everywhere characterized by a clearly expressed metacolloidal structure. The polishing hardness is similar to that of the first variety, or even somewhat higher. The reflectivity is lower. The color is gray, dark gray in oil. In crossed nicols the mineral is isotropic; a sharp yellowish-brown internal reflection appears. It is likewise not attacked diagnostically by etching.

In the very lowest part of the oxidation zone, the mineral described intensely replaces chalcocite, portions of which are often encountered among the oxidized ores. On these same levels, in places, a rather intense replacement by the limonite described is observed in relict grains of pyrite. The replacement process proceeds either from the periphery of the grains, forming a looped structure (Fig. 16), or along a network of rare veinlets, also taking in the inner parts of the grains. With considerable intensity of the replacement process, even skeletal forms are produced in pyrite (Fig. 17). Very often the metacolloidal limonite described develops at the expense of limonite of the first variety, distinctly replacing the latter. In turn, the variety described is corroded by malachite and azurite, being cut by veinlets of both minerals. As for its relations with cerussite and smithsonite, both minerals often form crusts on essentially limonitic ore, and cerussite even forms cutting veinlets. The relations of limonite of this variety with biverite indicate a later deposition of the latter.

Fig. 16. Looped structure of replacement of pyrite (Py) by limonite (ln); Cc—chalcocite. Magn. 100.

Fig. 17. Limonite (ln), corroding grains of pyrite (Py). Cc—chalcocite. Magn. 100.

Malachite is a fairly widespread mineral of the zone described. Appearing in insignificant quantity in the upper parts of the cementation zone, it has the form of thin branching veinlets and negligible earthy smears. Within the oxidation zone both these and other forms attain an incomparably broader development. In addition, crystals of malachite developing among druses of cerussite were encountered here several times. Some of the malachite crusts, in their coloration, very closely resembled aurichalcite, but microchemical tests for zinc always gave negative results. As for the relations of malachite and azurite, both often develop within one and the same veinlet, with malachite tending toward the selvages, while azurite occupies the axial parts of the veinlets. It is very difficult to judge the relations of malachite and cerussite...

difficult, since they are often very closely intermixed with one another, and only in one place was it possible to observe a crust of cerussite developing over malachite. No relationships of malachite with beaverite and smithsonite were observed.

Azurite develops only in the very uppermost parts of the oxidation zone. The few polished sections at our disposal were made from drill-core material. It usually develops in the form of veinlets cutting limonite, often together with malachite. No relationships with cerussite, smithsonite, or beaverite were observed.

Cerussite develops exclusively in the very lowest part of the zone being described. Diagnosis of the mineral presented no significant difficulties, since in all its forms of occurrence it dissolved very readily in dilute nitric acid and gave a distinct reaction for lead iodide. The cerussite studied is heterogeneous and, according to its form of occurrence, may be divided into two varieties.

The first variety occurs as crusts of a turbid light- or dark-gray color, in association with malachite and limonite.

The second variety of cerussite is clearly crystalline, water-transparent, and colorless. It develops as thin crystalline brushes covering the walls of numerous leaching cavities or growing on the parting planes of the ore. In addition, whole veinlets composed of cerussite of this variety are not infrequently encountered. The habit of the crystals is short-prismatic, with rhombic cross-section, and very much resembles that of the crystals described by P. P. Pilipenko from the Vostochnaya deposit of the Belousovka ore field. Twinned intergrowths are extremely common. In the character of its occurrence, our crystalline cerussite very much resembles the cerussite described by F. V. Chukhrov from the lead deposit of Kurgasyn, where it is the result of recrystallization of previously deposited flowstone masses of the same mineral. The relationships between cerussite and smithsonite will be considered in the description of the latter.

Smithsonite. On the basis of the investigations carried out, it may be considered that the mineral described is fairly widely developed in the ores of the deposit under discussion, although it never forms more or less large accumulations. It is developed chiefly in the very lowest part of the oxidized zone and, by its character, may be divided into two varieties.

The first and most widespread variety occurs as crusts of flowstone texture, white, light-gray, or yellowish-gray in color, developing on the parting planes of the ore. These crusts effervesce vigorously with hydrochloric acid and give a positive reaction for zinc. The relationships of smithsonite of the variety described with limonite are quite clear and were already noted in the description of the latter. As for its relationships with cerussite, one often observes that cerussite veinlets, substantially cutting the limonitic ore, are covered on the parting planes by crusts of smithsonite.

The second variety was encountered by us only twice among limonite-rich areas of the lower part of the oxidation zone. This variety develops as thin spots (visible only under the microscope) among limonite. Macroscopically, the ore areas containing inclusions of the carbonate described have the same brown color as the surrounding limonitic mass, revealing themselves only by vigorous reaction with hydrochloric acid. In transparent sections this carbonate likewise appears impure and is filled with iron hydroxides, which are often concentrated along grain boundaries. Microchemical tests of this material for zinc gave positive results. Apparently, a mineral very close to this carbonate is the brown smithsonite described by P. P. Pilipenko from the ores of the Zyryanovsk deposit, and it is quite possible that both

minerals are analogous formations. It seems to us highly probable that the variety of smithsonite described here is nothing other than its ferruginous variety—monheimite. In this case the constant contamination of the mineral by iron hydroxides becomes especially understandable. No direct interrelations of the smithsonite of the described variety with limonite, cerussite, and beaverite were observed. It could only be seen that ore areas containing this carbonate had coatings of malachite on the surface and were cut by veinlets of cerussite.

Beaverite is still considered a very little widespread mineral, occurring in the oxidation zone of sulfide deposits. In the USSR beaverite was described for the first and only time by F. V. Chukhrov from deposits of the Dzhezkazgan-Ulutau district in Kazakhstan.

The mineral encountered by us in the ores of the deposit is characterized by the following properties: its macroscopic color is ocher-yellow, more rarely brownish-yellow. When rubbed between the fingers it gives a soft, greasy sensation. The luster is silky, but noticeable only on a fresh fracture. Under the microscope the mineral appears as equilateral hexagonal plates (Fig. 18) of pale-yellow color or almost colorless. In addition to the forms indicated, very thin prisms are also encountered, representing a side section of the hexagonal plates. Incidentally, it is interesting to note that the color of our mineral under the microscope varies greatly depending on the quantity of material. The color of individual crystals was indicated above; if abundant accumulations of the latter occur, the color of these accumulations will be bright yellow and, finally, still more considerable masses of the mineral described appear yellowish-brown under the microscope. The size of the hexagonal plates mentioned above usually ranges within 0.006 mm in cross section; smaller ones are often encountered, and only rarely do their dimensions reach 0.12 mm. The extraordinary fineness of the grains is apparently a very characteristic feature of beaverite.

Fig. 18. Idiomorphic beaverite grains in ginerite. Dark shapeless masses—accumulations of beaverite grains. Magn. 400.

Another characteristic feature of the mineral is the presence, in almost every crystal, of some fine indefinite inclusions which possibly belong to silicic acid. In the larger grains of the mineral described, a weak pleochroism can be noticed: \(x\)—pale yellow, almost colorless; \(z\)—denser, brownish-yellow. The mineral is optically negative. It dissolves in hydrochloric acid, but with difficulty. In nitric acid the material under study could be dissolved only after preliminary fusion with soda. From the resulting nitric-acid solution abundant lead iodide was readily precipitated and a quite distinct reaction for copper was obtained. The material fused with soda gave an excellent reaction for liver of sulfur.

In the form of its crystals, their size, and color, our mineral very strongly resembles jarosite, described by P. P. Pilipenko from many deposits of Altai. However, the morphological similarity does not continue further, since the mineral described differs considerably in chemical composition from jarosites in general and from Altai jarosites in particular. This difference, on the one hand, is manifested in the almost complete absence of potassium and sodium in the material investigated (the spectroscope revealed only traces of these elements). On the other hand, it is characterized by a substantially increased lead content.

and copper, which, even if they do occur in Altai jarosites, are always present in negligible quantities.

Linarite and caledonite, which are close in chemical composition, differ from the mineral described by their blue and bluish-green color, partial solubility in nitric acid, and crystal habit. Plumbojarosite, extremely close both in morphological features and in certain optical properties, differs in the absence of copper and in strong pleochroism; finally, the mineral beaverite, close to plumbojarosite, differs from the mineral described by the absence of copper and by its optical sign.

The diagnostic features given, it seems to us, make it possible to conclude that we are dealing not with some other hydrous sulfate, but specifically with beaverite.

The development of beaverite in the oxidation zone is in no way at variance with the primary composition of the ores; on the contrary, for such a primary complex, according to views currently existing, beaverite is a more characteristic mineral than, for example, jarosite or plumbojarosite.

The mineral described is widely developed in the lower part of the oxidation zone of the deposit, composing, together with limonite, considerable masses and occurring in the form of a distinct horizon.

Paragenesis and sequence of deposition

Establishing the true paragenetic relationships of the minerals of the oxidation zone proved to be many times more difficult than was the case in resolving a similar question in the primary ores and in the ores of the cementation zone. These difficulties are due, to a significant degree, to the extremely close associations of minerals, on the one hand, and to the presence of several varieties of one or another mineral, often existing at different stages of the oxidation process, on the other.

Of the entire complex of minerals of the zone, the earliest in time of formation are native copper and cuprite. The question of their relative age is extremely complex. As was indicated in the characterization of these minerals, native copper very rarely appears without cuprite, whereas the latter is often encountered independently. In those places where both minerals develop together, cuprite usually rims the copper inclusions at the periphery, sometimes even forming crosscutting structures (Fig. 14). In these cases we believe that cuprite formed at the expense of copper during the process of its oxidation. Instances of such replacement of native copper by cuprite were recorded repeatedly at the Belousovsk deposit. They were also noted by P. P. Pilipenko for the territory of the Rudny Altai in general.

The appearance of the copper itself in these cases we regard as the result of direct oxidation of chalcosine without the formation of any intermediate products. Lindgren believes that such a process may take place, but only provided that the circulating waters contain a more or less considerable amount of ferric sulfate. Since the chalcosine ores of our deposit are distinguished by a high content of relict pyrite, it may be assumed with sufficient certainty that in the first stages of oxidation of these ores a considerable quantity of the necessary ferric sulfate would be formed.

Thus, we believe that some part of the native copper formed directly at the expense of chalcosine and represents an earlier formation than cuprite. But, taking into account the fact that the oxidizing environment at the boundary between the cementation zone and the oxidation zone does not

everywhere the same, although, depending on the structural features of one or another sector, it is quite probable to assume that at the same time, while native copper is forming in some places, cuprite arises in others at the expense of the same chalcocite. In the latter case, apparently, those nest-like accumulations of cuprite grains among the non-ore material, described above, were formed. The cuprite formed in this way also, apparently, is a source of native copper, which, according to currently existing views, in most cases forms as a result of the reduction of cuprite by ferric sulfate.

Following the formation of the minerals described above, limonite was deposited. Part of it formed at the expense of pyrite, pseudomorphically replacing the latter (Fig. 15), but its main mass arose during the replacement of chalcocite. Somewhat earlier than the limonite described, the ferruginous variety of smithsonite was formed; isolated finds of it in the lower horizons of the zone were noted at the time. In transparent polished sections it is clearly visible how brown iron oxides locally impregnate the indicated carbonate rather densely, penetrating into it along grain boundaries. Further, after the limonite of this stage, the next mineral in the order of deposition is malachite; it represents a later formation, since in the form of films and crusts it develops along the parting planes of ores of the lower part of the oxidation zone impregnated with limonite. Malachite is very closely associated with cerussite, especially with its incrusting variety. In this case the minerals are so closely intermingled with one another that one can speak only of their simultaneous formation. As for the relationships of malachite with crystalline cerussite, such relationships were observed extremely rarely, and only in one place was a crust of crystalline cerussite found over incrusting formations of malachite. The next mineral in the order of deposition is smithsonite, whose crusts develop on the parting surfaces of limonite-rich ores of the lower part of the oxidation zone, in places covering cutting veinlets of cerussite. As for beaverite, it is already a mineral of the second stage, deposited after the minerals considered above. Macroscopically it is often clearly visible how this mineral substantially replaces limonitic sectors in the lower part of the oxidation zone. It seems to us that the formation of beaverite occurred at the expense of cerussite under the action upon it of ferric sulfate in the presence of copper sulfates. The ferric sulfate necessary for this was formed in sufficient quantity during the oxidation of relict pyrite. There was also no deficiency of copper sulfate, since in the lower part of the oxidation zone there are frequent relics of both primary and secondary copper-bearing minerals. Naturally, not all the ferric sulfate that arose was consumed in the formation of beaverite; undoubtedly, part of the sulfate hydrolyzed and was precipitated as limonite, finely intermixed with beaverite. And, finally, in the last stage of supergene mineralization, limonite was again deposited, concentrating in the form of considerable masses in the upper part of the oxidation zone. In a number of samples one can see a gradual transition of beaverite ores into the brown iron ores lying above. It seems to us that these iron ores are a further product of the oxidation of beaverite ores. In addition to limonite, in this last stage malachite and azurite were deposited. These minerals, in the form of numerous veinlets, cut the limonitic sectors; moreover azurite, being a later formation, always occupies the axial parts of the veinlets, while malachite is distributed along the selvages.

Thus, as a result of the investigations carried out, the mineralogical composition of the deposit, differentiated by zones, is represen-

is presented in the following form: 1) primary ores—pyrite, chalcopyrite, sphalerite, fahlore, galena, and arsenopyrite; 2) ores of the cementation zone—covellite of two generations and chalcocite; 3) ores of the oxidation zone—native

Fig. 19. Scheme of the order of deposition of ore minerals.

copper, cuprite, limonite, malachite, azurite, cerussite, smithsonite, and biverite. The paragenetic relationships of the listed minerals are given in the accompanying scheme (Fig. 19). Among the vein minerals, quartz, barite, carbonate, talc, sericite, and chlorite have been recorded.

Subdivision of the Secondary Zones and the Genetic Relationships of Individual Horizons

As a result of the investigations carried out, it has been established that in the distribution of individual minerals within the secondary zones there exists a definite regularity, reflecting the character of development of the secondary processes and making it possible, within the indicated zones, to distinguish the following six horizons (from bottom to top):

1) the horizon of primary pyrites with covellite; 2) the horizon of primary pyrites with chalcocite; 3) the horizon of sooty chalcocite ores; 4) the horizon of cuprite ores; 5) the horizon of ocherous biverite ores; 6) the horizon of brown-ironstone ores.

The horizon of primary pyrites with covellite directly borders the primary sulfide ores and constitutes the very lowest part of the cementation zone (Fig. 1). The transition to the primary ores is gradual, which to a considerable degree makes it difficult to draw a more or less exact boundary between the primary ores and this horizon. Incidentally, it should be noted that in general the most deep-

the penetration of secondary sulfides into the primary ores occurred along the hanging and footwall sides of the ore body.

An extremely interesting picture is observed in the area of one of the crosscuts, where, suddenly, above the horizon described above, fresh primary ores appear, entirely devoid of any traces of secondary sulfides¹. It seems to us that the occurrence of such a picture is explained by the presence at this place in the ore body of a dome-shaped bulge rising considerably above the line of the entire hanging wall as a whole. It is quite natural that, in such a position, the supergene solutions moving through the ore body from the side of the buried valley did not enter the aforementioned bulge, and consequently no deposition of secondary minerals could take place there.

The horizon described is represented by sulfide ores that appear entirely fresh, both massive and disseminated. The presence of secondary sulfides is established only under the microscope. Among the secondary minerals, fine-grained covellite is predominant, and chalcocite develops in subordinate quantity. The amount of secondary sulfides is usually very small, and their content in polished sections does not exceed 2–3%. They develop exclusively within areas composed of chalcopyrite, in the form of very rare thin veinlets and stringers. Secondary sulfides develop especially readily at the boundary of chalcopyrite with inclusions of other minerals, more often non-ore minerals.

The presence of secondary sulfides exclusively among chalcopyrite, as it seems to us, is evidence that the descending solutions during the formation of this horizon were significantly impoverished in copper, as a result of which the secondary sulfides mentioned could form only in those places where the supergene solutions borrowed copper from the primary minerals.

The horizon of primary pyrites with chalcocite lies immediately on the preceding horizon (Fig. 1) and represents a further development of the processes of secondary sulfide enrichment. Its width is not constant and fluctuates within broad limits. The processes of secondary sulfide enrichment in the ores of this horizon are developed incomparably more strongly than in the preceding case, which was reflected to a considerable degree also in the external appearance of the ores, which in their main mass acquired a bluish-gray coloration. The amount of secondary sulfides in polished sections of the ores ranges from 4 to 25%.

The predominant secondary mineral is chalcocite. Covellite is also present quite often, but always in sharply subordinate quantity (up to 2%). Among the other secondary minerals in the ores of this horizon, malachite sometimes appears, developing chiefly in the form of rare crusts in the upper parts of the horizon.

Here the secondary sulfides replace not only chalcopyrite, but also sphalerite and galena. As for pyrite and pale ore, these minerals almost always remain unreplaced.

If in the pale ore it is nevertheless possible here and there to note the presence of isolated secondary veinlets, then in pyrite any manifestations of them, even the most insignificant, are always absent.

The horizon of sooty chalcocite ores, lying on the preceding one (Fig. 1), also has an inconstant width. The greatest depth at which sooty ores were encountered is 67.5 m from the daylight surface; the smallest is 50 m.

The horizon is composed for the most part of loose varieties of earthy texture, dark-gray or bluish-black in color. The ores of this horizon

represent the highest degree of development of the processes of secondary sulfide enrichment. Incidentally, it should be noted that most previous investigators, in general, assigned only these sooty ores in Altai to the zone of secondary sulfide enrichment.

Mineralogically, the ores of the horizon described consist essentially of chalcocite. In addition, coarse-grained covellite of the second generation is fairly widely developed; it formed later than the chalcocite and at the expense of the latter. Of the other secondary minerals, a considerable amount of limonite, malachite, and some smithsonite are always present.

The replacement of primary minerals in the ores of this horizon took place most completely; nevertheless, rather numerous relics of the sulfides of the primary ores are almost always encountered. First place among these relics belongs to pyrite, the grains of which usually show no signs whatever of replacement by secondary sulfides and only in the very upper part of the horizon are partly replaced by limonite.

As for the above-mentioned covellite of the second generation, it is now believed that its appearance in chalcocite ores already indicates the beginning of the oxidation process in the cementation zone. The formation of this covellite is most often regarded either as the result of aqueous-atmospheric oxidation of chalcocite, or as the result of its oxidation under the action of ferric sulfate. It seems to us that, with the abundance of pyrite in the primary ores of the deposit, the second mode of formation of covellite apparently played the principal role. Besides covellite, the presence here of limonite, malachite, and smithsonite also indicates that the oxidation process of the sooty ores had begun.

In concluding the characterization of this horizon, I should like further to note the striking discrepancy between the insignificant width of the belt of sooty ores and the intensity with which the processes of secondary sulfide enrichment developed in it. The fact is that ordinarily cementation zones that have been intensively worked over have a thickness on the order of several hundreds of meters, whereas in our case the width of the horizon described is very small. It seems to us that such a discrepancy can be understood only by assuming that repeated oxidation destroyed the greater part of the cementation zone.

The horizon of cuprite ores could not be traced throughout the entire section. It is reliably known only in the middle part of the section (Fig. 1). This horizon reaches its greatest dimensions in the “northeastern wing.” In the principal ore body with its southwestern dip, this horizon was encountered only in one place, and then in the form of a very narrow strip. The greatest depth at which ores of this horizon have been found is 60 m from the surface.

Mineralogically, the horizon is composed of material usually called brick-red copper ore. The principal component is limonite, among which a considerable amount of cuprite, native copper, and malachite is always present.

A very characteristic feature of this horizon is a considerable concentration in it of the lead carbonate—cerussite. Among the ores of the horizon, insignificant areas or “spots” of secondary sulfides of the cementation zone are always observed in the form of relics. Of the primary ore minerals, unaltered pyrite occurs in fairly considerable quantity.

The boundary between the cuprite horizon and the underlying sooty ores is very indistinct. The transition of one ore into another takes place extremely gradually.

The horizon of ocherous beaverite ores lies either on the cuprite horizon or directly on the sooty ores (Fig. 1). The larg—

the greatest width is observed in the middle part of the section. It is interesting to note that this horizon is found only in the main ore body. In the “northeastern wing,” directly upon the thick cuprite horizon, brown iron ores occur at once.

The ores consist of ocherous material of varying density, more often earthy. Mineralogically, they represent a mass of extremely fine grains of barite, cemented by limonite, beaverite (a complex sulfate of lead, copper, and iron) and undifferentiated hydrous copper sulfates of the brochantite–chalcanthite type.

There is one more highly characteristic feature distinguishing this horizon from all the underlying ones: the complete absence in it of relics of primary sulfides, even pyrite.

The boundary of the beaverite horizon with the underlying cuprite horizon is very indistinct. Usually one observes a very gradual transition from one horizon to the other. Finally, I should like to note a certain regularity characterizing the relations between the beaverite and cuprite horizons. As a rule, in those parts of the oxidation zone where the ocherous beaverite ores have great thickness, the cuprite horizon is developed extremely weakly or is altogether absent. Where, on the other hand, there are no ocherous ores or their thickness is negligible, the cuprite horizon is developed most strongly.

The horizon of brown iron-ore ores is the uppermost member of the secondary zones studied by us (Fig. 1). We were unable to study this horizon directly at the deposit because of its inaccessibility. Therefore the configuration of the horizon is given by us entirely in the form in which it is represented by the geologists working at the deposit.

Mineralogically, the horizon consists chiefly of limonite with some amount of malachite and azurite.

Thus, from the characterization of the individual horizons it is clear that the first three horizons (the lower ones) belong to the cementation zone, representing different degrees of development of the processes of secondary sulfide enrichment. The fourth—the cuprite horizon—undoubtedly arose as a result of direct oxidation of the ores of the cementation zone.

As for the horizon composed of ocherous beaverite-bearing ores, while it, like the cuprite horizon, belongs to the oxidation zone, it is an extremely peculiar formation, by its appearance seemingly disrupting the general course of development of the oxidation zone. The point is that, according to the views now established, the process of development of an oxidation zone should proceed from sulfates to carbonates. Consequently, under normal development of the process, we should always have sulfates below and carbonates above. Here, however, the reverse picture is obtained, since the sulfate beaverite horizon overlies the cuprite horizon, which is rich in carbonates.

Such a reversed sequence in the formation of oxidized products, it seems to us, could have occurred only under special conditions of development of the oxidation process, whose specificity was caused chiefly by the fact that it was not the primary sulfide ores that were subjected to oxidation, but the chalcocite ores of the cementation zone, with a large quantity of relict pyrite.

In the initial stage of oxidation of such ores, the intensely oxidizing chalcocite largely protected the pyrite grains from oxidation. This, probably, is the reason why the cuprite horizon that arose at the expense of the sooty ores contains a fairly significant amount of almost completely unreplaced pyrite.

Then, after the main mass of chalcocite had already oxidized and the cuprite horizon had formed, the relict pyrite began to oxidize intensively. This inevitably produced a considerable quantity of ferric sulfate, which, together with copper sulfate—undoubtedly present in the solution—acting upon previously deposited cerussite, could have led to the formation of a complex hydrous sulfate of lead, copper, and iron of the beaverite type, just as, in the opinion of S. S. Smirnov, in the absence of copper in solutions the mineral plumbojarosite is formed. Thus, it seems to us that the beaverite horizon is a further product of oxidation of the cuprite horizon rich in cerussite. This is in full agreement with the fact that, where the ocherous beaverite ores have great thickness, the cuprite horizon is developed as a very thin band or is altogether absent, having been replaced by beaverite ores. Conversely, where ocherous ores are absent or their thickness is negligible, the cuprite horizon is most strongly developed.

As for the horizon of limonitic ores, it appears to us to be the result of further oxidation of the ores of the beaverite horizon.

General Conclusions

Summing up the results of the investigations carried out on the secondary zones of the deposit, we may draw the following conclusions:

1. The observed fact that the ore body emerges into a buried valley is indisputable evidence of the antiquity of the processes that created the secondary zones.

2. The formation of such a complex secondary zonation as we see at the deposit could have occurred only under an extremely specific set of conditions, with one process replacing another, and apparently must be characteristic of ancient secondary zones.

3. The gradual transition between primary ores and sooty ores, which made it possible to distinguish two additional intermediate horizons, compels us to suppose that the formation of the cementation zone took place under conditions of an arid climate with a very unstable groundwater level.

4. The noted discrepancy between the intensity of development of the process of secondary sulfide enrichment and the insignificant width of the belt of sooty ores, as well as the appearance of a rich cuprite horizon, indicate that here there was a repeated superposition of the oxidation process upon the ores of the cementation zone, during which a considerable part of the latter was destroyed.

5. Finally, from all that has been said above, there are many grounds for thinking that the normal, originally formed oxidation zone is absent at the deposit. The present horizons of oxidized ores represent the product of repeated oxidation of the ores of the cementation zone.

1. The place mentioned is marked by a cross in Fig. 1. ↩