FEATURES OF THE MORPHOLOGY OF THE OXIDATION ZONE OF THE USKANDINSKOE POLYMETALLIC DEPOSIT IN SALAIR

V. K. Cherepnin, B. M. Levashov

(Presented by the scientific seminar of the Departments of Mineralogy, Petrography, and Mineral Resources)

Introduction

By the present time, a fairly extensive literature has already accumulated on questions of the geology and mineralogy of polymetallic deposits of the Salair Ridge. A number of these works present concrete material on the mineralogy and conditions of formation of oxidized products [1] and some general data on the oxidation zones of individual deposits [3, 4, 5]. Over the entire period of study of the Salair deposits, essentially only two works have appeared that are devoted to the question of interest to us [1, 2, 6]. In them, various types of ore outcrops are distinguished and a brief characterization of them is given. Any detailed description of the morphological features of the oxidation zones of individual deposits is entirely absent from the literature. Possibly, the main reason for this is the poor exposure of the area, which greatly hampers detailed study of ore outcrops. The conduct of such investigations for the territory under consideration should be regarded as absolutely necessary, since it will make it possible to develop criteria for a comparatively rapid prospective appraisal of new ore occurrences.

Wishing to fill the indicated gap to some extent, we here give a description of the morphological features of the ore outcrop of the Uskandinskoe deposit, located in the southern part of the northeastern slope of the Salair Ridge, at the very line of overthrust of Lower Cambrian rocks onto Silurian–Devonian rocks.

Brief Geological Characterization of the Area

The above-mentioned overthrust line divides the ore field of the deposit into two areas differing in geological structure: the southwestern area, composed mainly of rocks of the Pecherkinskaya suite of the Cambrian, and the northeastern area, in the structure of which Krekov limestones and sandy-shaly deposits of the Middle Devonian take part.

The southwestern part of the deposit is the most interesting in terms of the manifestation of secondary processes. It served as the object—

volume of our investigations. The Pechenga suite, which composes the described area, is represented mainly by quartz keratophyres and their tuffs (Fig. 1). Among them are three lens-shaped bodies of limestones, with thicknesses of 10, 15, and 60 m, traceable along strike for 120, 150, and 160 m, respectively. In addition, the area contains

Fig. 1. Scheme of the geological structure of the southwestern part of the ore outcrop of the Uskandin deposit.

1—quartz keratophyres; 2—quartz keratophyres and their tuffs; 3—tuffs of quartz keratophyres; 4—limestones; 5—quartz-sericite schists; 6—quartz-chlorite rocks; 7—amygdaloidal porphyrites; 8—diabases; 9—silicified rocks; 10—breccias; 11—old mine and dump; 12—bleached rocks; 13—strongly lightened rocks; 14—noticeably lightened rocks; 15—pits; 16—massive brown iron ores; 17—intensely ocherized rocks; 18—lithological boundaries of the primary rocks and boundaries of the various products of the oxidation zone.

quartz-sericite and quartz-chlorite rocks, as well as formations of the secondary-quartzite type.

In addition to the rocks listed, in the easternmost part of the area there is a comparatively large body of amygdaloidal porphyrites. Numerous diabase dikes are scattered over the entire area. With a small thickness, on the order of 2–3 m, they extend for up to 150 m and, in most cases, occur conformably with the enclosing rocks. The general strike of the rocks of the area is north-northwest; the dip is to the

west-southwest at an angle of 40–50°. All rocks of the area, to one degree or another, are, in general, rather intensely dislocated.

The ore bodies intersected by drill holes are lens-shaped areas mineralized with sulfides of copper, lead, and zinc, consisting of hydrothermally altered quartz keratophyres and their tuffs. The character of the primary mineralization is veinlet-disseminated, with a rare development of massive-ore areas.

Morphology of the Ore Outcrop

Within the ore outcrop of the Uskandinskoye deposit we encounter traces of a rather intense action of sulfate solutions on the rocks enclosing the mineralization. The result of this action is a whole complex of specific formations, among which the following can be clearly distinguished: bleached rocks, “sponges,” rocks rich in iron, and relics of primary mineralization.

Bleached Rocks

Bleached rocks are the most widely distributed morphological element of the oxidation zone in the area. In general outline, such bleached rocks are macroscopically characterized by white and grayish-white coloration, by obscured structural and textural features of the primary rock, by the presence of clayey products with varying degrees of limonitization, and also by the development of pores from the leaching of sulfides.

Microscopic study of these formations shows that secondary clayey products develop exclusively along the feldspars of quartz keratophyres and their tuffs, to one degree or another obscuring the primary structure of the rock.

The porosity characteristic of the rocks described is distributed very unevenly. The dimensions of the main mass of pores rarely exceed 1 mm in cross section. The largest of them reach 3 mm. In the overwhelming majority of cases the shape of the pores is highly irregular, but at times pores with regular square outlines are encountered. Around some pores there are rims of vein quartz with a feathery or comb-like structure. In some cases the pores are completely empty; in others their walls and bottoms are lined with limonite; and, finally, pores completely filled with limonite are often encountered.

Depending on the intensity of the alteration process that has taken place, three following varieties may be distinguished among the bleached rocks: a) whitened rocks, b) strongly bleached rocks, c) noticeably bleached rocks.

a) Whitened rocks have a limited distribution. An area of well-expressed whitening is located in the central part of the area described, precisely at the northern end of the middle body of limestones. Here it has the form of a small lens 40 m long and up to 6 m thick. Small areas of these rocks are also not infrequently encountered elsewhere among the bleached formations.

Macroscopically, the whitened rocks are dense or earthy formations of white or yellowish-white color, in most cases having completely lost the textural features of the original rock. They are greasy to the touch, often crumble under light pressure from the finger, and externally resemble a kaolin mass. Research-

...of this clayey material by centrifugation in heavy liquids, with subsequent staining by organic dyes, showed that it consists of minerals of the hydromica group. Pigmentation of the described formations by iron oxide hydrates is very insignificant. Only in isolated cases can weak impregnating limonitization and the spread of limonite along cracks be noted.

Microscopic studies show that the entire principal mass of the rock is a clayey substance, among which separate quartz grains occur.

b) Strongly bleached rocks occupy a rather extensive area within the deposit. They extend through the area described in the form of a meridional belt. The belt has its greatest width, of about 100 m, in the central part of the area. On the southern and northern flanks the width of the belt decreases to 40–55 meters. In addition to this main field, the rocks described occur as small lenses up to 10 m thick in other parts of the area, for example, in the hanging wall of the eastern limestone body and on the southern slope of Mt. Kopna. Areas of strong bleaching, like areas of whitening, always develop among fields of quartz keratophyres and their tuffs and, as a rule, are confined to the most fractured varieties of these rocks.

Macroscopically, the strongly bleached rocks are dense formations, rough to the touch, readily broken by hammer blows, with a partially preserved primary texture. The bleaching is expressed in a change of the gray and gray-green color of the quartz keratophyres and their tuffs to white and grayish-white as a result of the development of clayey material. Study of the latter using centrifugal analysis and subsequent staining showed that, as in the preceding case, it consists of minerals of the hydromica group.

The strongly bleached rocks are characterized by varying degrees of limonitization. Occasionally, completely unpigmented varieties with a white color occur. In most cases limonite develops as coatings and films along the planes of schistosity of the rock. Much more rarely, areas completely impregnated with limonite occur. It has been established that such intensive limonitization is always observed in rocks especially rich in finely dispersed clayey material. Evidently, this is connected with the high adsorption capacity of the latter.

Among the dense, strongly bleached rocks, porous varieties also occur, the pores usually preserving the form of sulfide accumulations. The pore sizes are very variable and sometimes reach 2–3 mm in diameter. Their distribution in the mass of the rock is highly uneven. At times they are so densely situated that the rock in such areas assumes the character of a sponge. The bottoms and walls of the pores are often lined with limonite.

Under the microscope it is clearly visible that abundant clayey products readily develop after the feldspars of the original rock. Where the process is highly intensive, the entire thin section is completely clogged with these newly formed substances, among which entirely fresh grains of porphyritic quartz segregations occur.

Under the microscope, the pores for the most part have irregular outlines, but sometimes they preserve the form of leached pyrite grains. They, as indicated above, are often bordered by rims of quartz. Along the edges...

limonite is present, which colors the nearby clayey material in yellow-brown and brownish-brown tones. Pseudomorphs of limonite after pyrite are encountered very rarely.

c) Noticeably lightened rocks have the widest distribution in the area in comparison with all the other morphological elements of the ore outcrop. They are distributed on the western and eastern flanks of the deposit, and also extend as a meridional band almost through the middle of the area, enclosing Mt. Kopna on two sides. Reaching its greatest width, about 160 m, in the north of the area, this band gradually narrows to the south. In addition to these principal fields, the formations described make up a number of lenses in different parts of the area.

The variety described includes lightened rocks that have still substantially retained their original textural and structural features. They are always dense and usually rough to the touch. The gray-green and gray color of the quartz keratophyres changes here only to whitish-gray. This is most closely connected with the sharply limited amount of finely dispersed clayey matter in the rock. In composition, the clayey products are represented, as in the preceding cases, mainly by hydromicas.

The limonitization of the rocks described, in comparison with the strongly lightened varieties, is also less significant. It usually has a film-like character, developing along the schistosity of the rocks and along planes of separation. Porosity is rare; spongy areas are almost never encountered. The sizes and shapes of the pores are the same as in the preceding cases.

Under the microscope these formations appear considerably fresher than the strongly lightened varieties. The mineralogical composition and structure of the primary rock are established without particular difficulty. The always-present limonite is distributed mainly along cracks. In porous areas, pseudomorphs of limonite after primary sulfides are encountered fairly often.

Sponges

Sponges have a very insignificant distribution in the area. They were encountered within the field of silicified quartz keratophyres and their tuffs on the hanging wall of the eastern limestone body and on its footwall, among ocherous and limonitized rocks. Limonitized rocks occurring within the central limestone body also in places have a spongy texture.

Macroscopically, the sponges are a porous rock, to one degree or another impregnated with limonite. The pores of the sponges have the most varied configuration. Their sizes vary within rather wide limits; the largest reach 3–4 mm in cross-section. The skeleton of the sponges is composed of quartz; the thickness of the partitions between pores rarely exceeds 1 mm. Limonite impregnates the skeleton of the sponges, lines the bottom and walls of the pores, and only in rare cases fills them completely. The coloration of the limonite is characterized by great variety, but brown, brownish-brown, and yellow-brown tones predominate.

In transparent thin sections it is evident that the sponges consist of quartz and limonite. Quartz forms the skeleton of the sponge and has mosaic, flamboyant, and comb structures, with grain sizes up to 0.2 mm in cross-section. In addition, quartz not infrequently forms irregularly shaped aggregates reaching 4–5 mm in cross-section. Limonite, as indicated, develops throughout the entire mass of the sponges, impregnating their skeleton and lining the walls

rocks. Here it is clearly visible that impregnation of the framework proceeds by the penetration of limonite along the boundaries of quartz grains. In cavities and pores, limonite forms well-pronounced colloform structures.

In addition to limonite, jarosite is often encountered in the spongites, and in fairly large quantities. It develops in cavities in the form of irregularly shaped spots and is replaced by limonite. More rarely, jarosite veinlets are observed.

Iron-rich rocks

These formations are characterized by exceptionally intense limonitization. They stand out clearly among all the other morphological varieties of the ore exposure by their rusty-brown, brownish-brown, or yellow coloration. In the area described there are two more or less significant sectors composed of these formations. One of them is situated in the hanging wall of the central limestone body, the other in the hanging wall of the eastern limestone body. The first is traced along strike for 110 m, with a thickness of 35 m; the second extends for 65 m and has a thickness of 8 m. East of the first of the indicated sectors, in a field of strongly bleached rocks, one more small lens of the same formations was encountered. Among the iron-rich rocks two varieties may be distinguished: a) ocherous rocks; b) massive brown iron ores.

a) Ocherous rocks constitute the main area of the sector of the morphological type being described. Macroscopically they are bleached rocks very intensely impregnated with limonite. They are characterized by variegated and spotted coloration, which depends on the intensity of the development of limonitization in different parts of the rock. Uneven development of limonite often leads to coloration in separate bands, creating the appearance of a banded texture. Among these formations, ocherous earthy material is widely developed. The rocks are often porous. The porosity is usually distributed in a nest-like manner. The pore dimensions do not exceed 1 mm across; their form is highly varied, and in most cases they are filled with limonite.

Under the microscope it is seen that the ocherous rocks described consist mainly of clayey material colored by limonite. The primary structure of the rock is in most cases unrecognizable. In places the amount of limonite is so great that it completely impregnates the clayey material, and the rock as a whole becomes limonitic.

Among the ocherous rocks there occur varieties bearing traces of silicification. Quartz develops as separate areas composed of xenomorphic grains up to 0.5 mm across. Characteristic are flamboidal and mosaic structures of quartz, which indicate its hydrothermal origin.

b) Massive brown iron ores occur in the fields of development of the formations described above as small areas. The largest area reaches 10 m in length, with a width of about 4 m.

Macroscopically these are heavy, dense rocks, having a reddish-brown or black color. They are usually porous to one degree or another. In most cases the pores are slit-like in character, sometimes reaching 2 cm in length. The bottoms and walls of the pores are, as a rule, lined with encrusting limonite of reddish-brown or brown color.

Among the massive brown iron ores developed in the sector of ocherous rocks near the eastern limestone body, there is fairly widely distributed...

porous formations, intensely impregnated with manganese hydroxides and containing rather abundant azurite. The latter most often develops in cavities in the form of well-defined kidney-shaped or spherical aggregates. At high magnification it is evident that all these kidneys and spherules are accumulations of the finest crystallites. In addition to the form of occurrence described, azurite in places forms thin veinlets cutting the main mass of the rock, as well as coatings on its surface. Everywhere it is clearly visible that azurite is a later formation than the iron and manganese hydroxides.

Microscopic study showed that the brown ironstones described are not homogeneous formations. Their main mass consists of limonite, and its reflectivity, even within a single thin section, varies somewhat. As is known, most investigators relate this to the amount of adsorbed water. Metacolloidal structures are developed quite widely.

This limonitic mass is cut by numerous, very irregular veinlets of goethite, which probably developed along dehydration cracks. The largest of the veinlets reach a thickness of 0.12 mm. In addition, goethite lines the walls of numerous cavities. It should be noted that the color of the internal reflection of the mineral described is not entirely typical of goethite; in most cases it is distinctly reddish. Pure yellow or light-brown areas are observed rarely.

In addition to goethite, small areas composed of lepidocrocite occur in the limonite field. The latter, together with goethite, forms colloform formations of extremely small size and independent tangled-fibrous aggregates. The finest prismatic grains, often bent and probably consisting of tabular crystals set on edge, show distinct reflection pleochroism and bright anisotropy. The internal reflection is reddish-brown, but not very dark; with dry objectives it is clearly observed only in places of fresh fractures.

As for the manganese formations, they are represented by psilomelane and its sooty variety—wad. Both varieties commonly compose colloform structures.

With regard to the genesis of the iron-rich rocks described above, two views are possible. On the one hand, they may be regarded as a normal emergence at the surface of sulfide ore bodies. On the other hand—taking into account their confinement to the contact with limestones and the constant presence of oxidized products in the form of pieces and blocks—the formations described may prove to be redeposited material of the oxidation zone, accumulated in karst depressions that developed in the footwall of the limestones at their contact with quartz keratophyres. To resolve this question, in the course of work at the deposit we drilled a borehole beneath the brown-ironstone rocks occurring in the footwall of the central lens of limestones. The borehole mentioned, at a depth of 42 m, passed through the footwall of the limestones and encountered quartz keratophyres containing an extremely poor pyrite dissemination, zones of chlorite and sericite, and also rare calcite veinlets. It is quite obvious that such mineralization could not, upon oxidation, have produced the brown ironstones and ocherous rocks described above. Thus, apparently, the second view should be recognized as the most correct concerning the genesis of these ferruginous formations. The source area of the oxidized material was a belt of strongly illuminated rocks, representing the ore outcrop proper.

Relicts of Primary Mineralization

Ore-bearing solutions that give rise to industrial concentrations of one or another metal, as is known, produce a number of characteristic changes in the enclosing rocks. Such altered rocks usually appear in the literature as hydrothermally altered formations. When secondary processes are superimposed, many of these rocks are only weakly altered, which makes it possible, in studying the ore outcrop, to regard them as relict forms of primary mineralization. Within the oxidation zone of the Uskandinsky deposit, such relict formations are silicified rocks, quartz-sericite schists, and quartz-chlorite rocks.

Silicified rocks. Silicification is developed to one degree or another everywhere in the ore field of the deposit. In any bleached rock we encounter veinlets, spots, and even whole areas composed of hydrothermal quartz. Silicification has particularly strongly affected the quartz keratophyres and their tuffs in the area of the hanging wall of the eastern limestone body. Here the silicified rocks form a lens whose length reaches 200 m, with a maximum thickness on the order of 60 m.

Macroscopically these formations are rocks of the secondary quartzite type, of white, grayish-white, and brownish tones. In their general mass they bear clear signs of bleaching.

Microscopic studies have shown that they consist mainly of quartz with flamboyant, comb-like, and feathery structure. The largest grains reach 0.3 mm along the long axis. The primary rock, occurring as separate areas among the quartzites, is so altered that its affiliation with quartz keratophyre or its tuff can be established only with difficulty. The rocks described locally carry sparse disseminated pyrite; pseudomorphs of limonite after pyrite are more often visible. Isolated leaching pores or even whole spongy areas are not infrequently encountered.

Quartz-sericite schists were encountered in the southern part of the area described, within the field of unbleached quartz keratophyres. They are developed as a lens about 70 m long and 7–8 m thick.

Macroscopically, the quartz-sericite schists are an intensely schistose rock of gray and whitish-gray color, with a silky luster along the schistosity planes. In the rock, knobby formations are observed, which are nothing other than relicts of porphyritic quartz phenocrysts of the original rock. These grains are predominantly rounded in form and reach 3–4 mm across. In places the rock contains pores from the leaching of sulfides. More often their form is highly irregular, but cavities with square outlines also occur. A small amount of white clayey material develops in these rocks. Limonitization of the quartz-sericite schists is very weak and extremely uneven.

Under the microscope the rock described consists mainly of sericite and quartz, among which completely decomposed grains of feldspars are encountered. The rock has a porphyroblastic structure, with a granolepidoblastic structure of the groundmass.

The amount of quartz varies widely; its grains have an irregular, generally rounded form, often possess clouded extinction, and are surrounded by flakes of sericite. The latter locally changes into a clayey substance. Sometimes, around rare pores, rims of hydrothermal quartz with comb-like or feathery structure are encountered. Thus, from the material presented it is quite clear—

it is clearly evident that the rock described is nothing other than a schistose and hydrothermally altered quartz keratophyre.

Quartz-chlorite rocks have been encountered in the footwall of the eastern limestone body, where they are developed as a lens about 60 m long and with an average thickness of about 13 m.

Macroscopically, these formations are a schistose rock of gray-green color, consisting of quartz, sericite, and chlorite. The quartz grains, as in the preceding case, are most readily distinguished in a cross fracture relative to the schistosity of the rock.

Under the microscope they differ from quartz-sericite schists by the extensive development of chlorite and, apparently, are likewise schistose and intensely reworked by hydrothermal solutions quartz keratophyres or their tuffs.

Conclusions

As a result of studying the characteristics of the composition and distribution of the morphological elements of the oxidation zone of the Uskandinsky deposit, the following conclusions may be drawn.

1. The quartz keratophyres and their tuffs underwent the most intense alteration under the action of sulfate solutions.

2. Bleaching, which is the principal morphological feature of this ore outcrop, is associated with the decomposition of the feldspars of the host rocks and is due to the development of finely dispersed products of the hydromica type. The broad development of bleached rocks within the ore outcrop reflects the abundant content of pyrite in the primary ores.

3. The sponges, which in most cases have pores unfilled by limonite and usually contain some amount of jarosite, should be regarded as oxidized analogues of dense sulfide dissemination poor in copper.

4. Areas with intense ochering, containing solid brown iron ores, are redeposited products of the oxidation zone which, apparently, were at one time carried down from a band of intensely bleached rocks and accumulated in karst depressions of the limestones.

5. Within the fields composed of individual morphological varieties, considerable heterogeneity is observed, which apparently reflects the extreme unevenness in the distribution of hypogene veinlet-disseminated mineralization.

Literature

1. Bolgov G. P. On the method of prospecting for polymetallic ores of Salair. Vestn. ZSGRT, No. 5, 1933.

2. Bolgov G. P. Mineralogical investigations of polymetallic ores of the Salair deposits. Mat. po geol. Zap. Sib. kraya, issue 15, 1934.

3. Derbikov I. V. The Novo-Ursk polymetallic deposit. Mat. po geol. Zap. Sib. kraya, issue 42, 1937.

4. Derbikov I. V. Sulfide ores of Salair and ways of their industrial development. Tr. konf. po izucheniyu proizvodit. sil Sibiri, vol. 2, 1940.

5. Usov M. A. Formations of deposits of useful minerals of Salair. Vestn. ZSGRT, No. 6, 1933.

6. Shakhov F. N. Types of ore outcrops of polymetallic deposits in the Salair region. Izv. Zap. Sib. FAN SSSR, geology series, No. 1, 1946.