PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE, AWARDED THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR,
NAMED AFTER S. M. KIROV

Vol. 252　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

MORPHOLOGICAL TYPES OF ROCKS IN THE OXIDATION ZONES OF SULFIDE ORE OCCURRENCES OF THE KOLYVAN–TOMSK ZONE

B. M. LEVASHOV

(Presented by Prof. A. M. Kuzmin)

In recent years, prospecting work using geochemical and geophysical methods has yielded a large number of anomalies. In most cases, however, these anomalies do not indicate the exact position of the ore outcrop and require further detailing. One of the methods of detailing is the study and mapping of morphological rock types in the oxidation zones of sulfide deposits, on the basis of the principles developed by F. N. Shakhov (1947, 1960). Experience in studying and mapping oxidized outcrops of sulfide ore bodies in Salair (Shakhov, 1946; Cherepnin, 1961; Cherepnin, Levashov, 1962), Rudny Altai (Roslyakov, Roslyakova, 1965), and other regions shows that this method has great practical significance for the rational direction of exploration work aimed at discovering hypogene ores at depth.

Our study of the oxidized outcrops of the Baranovskoye, Mitrofanovskoye, and a number of other smaller sulfide ore occurrences of the Kolyvan–Tomsk zone made it possible to identify the principal morphological types of rocks that arose under the influence of sulfate weathering, and to develop a legend for these rocks that can be used in interpreting anomalies identified within its territory. In compiling the legend we were guided by the propositions set forth in the above-mentioned works of F. N. Shakhov.

The field-work methodology consisted in taking samples of rocks differing in the degree and type of hypergene alteration from natural outcrops and existing mine workings in areas where sulfide mineralization occurs, both along profiles and between them. In the laboratory period, all collected samples were subjected to microscopic examination (transparent and polished sections, binocular microscope) and spectral studies. In determining the secondary minerals found, the microchemical method was widely used, and, in individual cases, X-ray structural analysis. The composition of finely dispersed material in the samples was studied with an electron microscope.

As our investigations have shown, within the complex of specific formations of the oxidation zones of sulfide ore occurrences of the Kolyvan–Tomsk zone, the following main types can be distinguished: bleached rocks; rocks enriched in iron hydroxides; rocks with cavities from the leaching of sulfides (oxidized poor disseminated ores, gossans); rocks containing secondary ore minerals; and relicts of hypogene mineralization.

Bleached Rocks

Bleached rocks are the most widely distributed morphological type among the oxidized varieties. Their distribution in plan coincides with the extent of the mineralization zones. This group includes rocks whose primary minerals have been replaced by hydromicas, montmorillonite, kaolinite, halloysite, and similar silicates. As a result of the replacement process, the rocks acquire a light, at times white, coloration. The degree of bleaching depends on the mineralogical composition of the rocks, their spatial position relative to the ore outcrop, and the saturation of the rocks with sulfides. Effusive and pyroclastic varieties of rocks of acidic composition are bleached most intensively. The oxidation process develops somewhat more weakly in clayey and calcareous sedimentary rocks. Rocks rich in dark-colored minerals are least subject to bleaching. The differing susceptibility of rocks to the bleaching process is explained by F. N. Shakhov (1960) as the result of the heterogeneous resistance of rock-forming minerals to sulfate weathering.

Depending on the intensity of the bleaching process, three varieties may be distinguished among the rocks described: weakly bleached rocks; strongly bleached rocks; disintegrated, intensely bleached rocks.

Weakly bleached rocks are the most widely distributed at the ore occurrences studied, and this type of alteration encompasses nearly all rocks in the fields where sulfide mineralization has developed. They occur as bands or separate lenses and correspond chiefly to areas with insignificant sulfide mineralization.

This morphological type comprises macroscopically compact, grayish-white rocks, with areas of the rock’s primary coloration, that have preserved their initial textural and structural features. Microscopically, both minerals of the original rock and newly formed minerals are identified in them. The amount of finely dispersed clayey material in the rock is insignificant and amounts to 10–15%. It is composed predominantly of hydromicas; kaolinite and halloysite occur more rarely.

Limonitization of the rocks described occurs sporadically and is very weak. It usually has a film-like character, developing along the schistosity of the rocks and along planes of parting. It is also encountered in rare pores formed by the leaching of sulfides and as pseudomorphs after pyrite.

Strongly bleached rocks are considerably less widespread than weakly bleached rocks. In their position they correspond to areas of intense sulfide mineralization and occupy mainly the central parts of the ore outcrop. Macroscopically, strongly bleached rocks are dense and homogeneous in structural-textural features, with a partially preserved primary texture. Their color is chiefly white, very rarely with a greenish or pinkish tint. The transformation of the primary composition of the rock proceeds so deeply that it is sometimes impossible to determine its original composition.

Under the microscope it is established that, in the mass of clayey material, mainly quartz, barite, rarely chlorite, hornblende, and very rarely sericite, muscovite, carbonates, and feldspars remain from the primary composition of the rock. The composition of the clay minerals, whose content in strongly bleached rocks reaches 70–80%, depends on the mineralogical composition of the transformed rock. In acidic effusives, kaolinite, hydromicas, and, to a lesser extent, halloysite undergo preferential development; in clay shales, in addition to the minerals listed,

clays, a large amount of montmorillonite is observed; in the shabazes there develop chiefly montmorillonite, hydromicas, nontronite, and kaolinite in subordinate amounts.

Strongly bleached rocks are characterized by varying degrees of limonitization. In most cases limonite develops in the form of coatings and films along the planes of schistosity of the rocks. Much more rarely, areas entirely impregnated with limonite are encountered. Such areas are confined to rocks especially rich in finely dispersed clayey material. In addition, in strongly bleached rocks the newly formed minerals include calcite, barite, goethite, lepidocrocite, and secondary minerals of lead, copper, and zinc. Among these rocks porous varieties are noted, the pores often preserving the shape of sulfide grains. The bottoms and walls of the pores are most often lined with limonite.

Disintegrated, strongly bleached rocks are of minor extent, occurring as small areas among strongly bleached rocks. In outward appearance these rocks are compact or earthy formations of white or yellowish-white color, in most cases having completely lost the textural features of the original rock. They are all greasy to the touch, are easily destroyed when touched, and in general resemble white clays. In the composition of these rocks hydromicas and kaolinite predominate, with relics of individual quartz grains. Limonitization of these rocks is insignificant. Only in isolated cases is weak impregnating pigmentation observed in the form of unevenly distributed spots of brownish-yellow color.

To this same variety one may apparently assign the loose materials encountered in one of the boreholes of the Baranovsk ore occurrence in the lower part of the oxidation zone. They consist mainly of quartz, fragments of strongly bleached rocks up to 2–4 mm in size, and rare relics of sulfides and limonite.

Rocks enriched in iron oxides and hydroxides

This morphological group includes rocks characterized by visible accumulations of secondary minerals of oxidized iron. According to the quantity and features of occurrence of the iron minerals, and also according to the textures in the rocks described, a number of varieties are distinguished.

Rocks enriched in unredistributed (local) iron hydroxides and oxides. These are rather compact rocks, enriched to varying degrees in iron hydroxides and oxides, developing as separate areas within fields of development of bleached rocks. The content of secondary iron minerals in the rock varies within wide limits (from 10% to 60%). They are represented by goethite, hydrogoethite, lepidocrocite, jarosite, and hematite. The saturation of the rock with these minerals and, correspondingly, its textural appearance and coloration depend on the amount of primary sulfide material present in it before oxidation.

Where rare pores from leached sulfides are present, the rocks are most often characterized by a compact texture and by cherry-brown and brown coloration in the form of spots against a general light-gray background. The spotted coloration is due to the distribution of secondary iron minerals and to the impregnation by them of the clayey material of the rock around the pores.

Rocks with abundant development of pores are characterized by more intense development of secondary minerals of oxidized iron and are entirely impregnated by them, acquiring a reddish, brown, and dark-brown monotonous coloration. Sometimes the banded distribution of pores from leached sulfides is accompanied by a banded distribution of secondary iron minerals.

sulfides and finely dispersed material, which is readily subject to impregnation, leads to the emergence of secondary banded textures. Along with dense varieties, earthy and friable ones are also found among these rocks.

Microscopic studies show that among the secondary minerals of ferric iron in these rocks, goethite, lepidocrocite, and hydrogoethite are most highly developed; hematite and jarosite are very rarely present. They occur mainly as small irregular accumulations closely associated with clay minerals. Colloform and radial-fibrous aggregates are also noted, confined mainly to porous areas.

Rocks enriched with introduced iron hydroxides differ from the first variety by the absence of cavities formed by leached sulfides. In areas of ore outcrops they are distributed unevenly and are observed in places where light-colored rocks with a high content of clay material are developed, or in silicified fractured rocks. In the first case, the rocks have a rusty-brown, brownish-brown, or yellow uniform or mottled coloration from the iron hydroxides impregnating them. The morphology of accumulations of iron-bearing minerals is mainly irregular, very rarely spherulitic. The sizes of the accumulations are fractions of a millimeter and smaller. Their distribution in the rock is highly uneven; their content is characterized by great variability, reaching 10–20%. In the second case, iron hydroxides fill fractures of varying thickness (up to 0.5 cm) that cut the rock in different directions. The iron hydroxides are represented predominantly by goethite, hydrogoethite, and lepidocrocite. The listed minerals form mainly granular aggregates; colloform and radial-fibrous ones also occur.

Brown ironstones. The principal diagnostic features of brown ironstones occurring in the place of rich sulfide ores are well known and require no explanation. In the mineralogy, morphology, and textures of these formations, very important features inherent in the underlying sulfide bodies are preserved. There is a large body of work in which criteria have been developed and examples are given of the use of these features in brown ironstones for reconstructing ore outcrops (Blanchard, Boswell, 1925; Blanchard, Boswell, 1928; Blanchard, Boswell, 1937; Boswell, Blanchard, 1929; Zocke, 1926; Smirnov, 1965; Shakhov, 1946, 1947, 1960, etc.). In evaluating brown ironstones, consideration is given to the shapes of cavities formerly occupied by sulfides, the texture, structure, and color of iron oxides and hydroxides, the amount of clay minerals, and, finally, the presence of products of sulfide oxidation. Particular attention is paid to the configuration of boxwork textures (the reticulate texture often repeats the structure of leached primary sulfide aggregates).

At the sulfide ore occurrences studied, brown ironstones are not widely distributed. They occur among bleached rocks and rocks enriched with local hydroxides as specific formations in isolated cases. This is apparently connected with the fact that the ore outcrops are represented only by poor disseminated ores. In samples of these rocks, typical boxwork textures are almost never encountered. The exception is the brown ironstones of the Romanovskoye ore occurrence, among which samples with boxwork textures formed in the place of leached pyrite are noted. In general, however, the brown ironstones studied are characterized by dense, cellular, porous, and cavernous textures. In individual cases, secondary minerals are identified in them: malachite, azurite (Ognevozaimskoye ore-

occurrences), native copper in microscopic studies (Vlaskovskoe ore occurrence). Very often pyrolusite and psilomelane are associated with iron hydroxides and oxides in brown iron ores.

Rocks containing crusts, coatings, and films of iron hydroxides are very widespread at ore occurrences. As the data of N. A. Roslyakov (1965) show, the morphology and development of these formations make it possible to judge their genesis. In some cases they have sharp boundaries with the enclosing rocks and arose through precipitation from solutions rich in iron compounds on parting planes and in fractures of rocks resistant to chemical weathering (silicified, baritized, etc.). In others they arose as a result of diffusion of iron through the rock mass during weathering, so that the boundaries have a diffuse character. For the rocks of the sulfide ore occurrences studied, the development of the first variety of crusts, films, and coatings within ore outcrops is the most indicative.

Rocks with Cavities from the Leaching of Sulfides

Within this morphological group of rocks, two main varieties can be distinguished: oxidized lean disseminated ores and sponges.

Oxidized lean disseminated ores, or “cappings” (Anderson, 1958), include rocks containing cavities from the leaching of sulfides with a considerable number of pseudomorphs of secondary minerals after the latter. Very often the pores preserve the outlines of previously existing sulfides.

Oxidized disseminated ores in areas of sulfide mineralization of the Kolyvan-Tomsk zone are encountered rather rarely. Usually this morphological type is developed in the central parts of fields of rocks bleached or enriched with local iron hydroxides and oxides.

Sponges form from rocks characterized by the presence of dense sulfide dissemination. As a result of the leaching of these sulfides, porous formations arise, in the structure of whose framework both primary and secondary minerals take part. According to the composition of the framework, among the sponges one can distinguish quartz (Baranovskoe, Ognevozaimskoe, Romanovskoe ore occurrences), barite (Ognevozaimskoe ore occurrence), and limonite (Vlaskovskoe ore occurrence) sponges. The cavities in sponges have various shapes and sizes and are very often filled with limonite; more rarely, jarosite and secondary ore minerals occur in them. At the ore occurrences studied, quartz sponges are the most widespread, but on the whole they occur rather rarely and at times cannot be distinguished as a specific element of the oxidation zone among other morphological rock types.

Rocks with the Presence of Secondary Ore Minerals

Such rocks are always a valuable indicator of sulfide ores, and the content of secondary minerals in them must be noted when studying an ore outcrop. Among the oxidized varieties of rocks of the sulfide ore occurrences of the Kolyvan-Tomsk zone, secondary minerals, apart from iron and manganese minerals, occur in isolated cases and do not form large accumulations. They are represented by native elements, oxides, sulfates, carbonates, silicates, arsenates, and sulfides. The secondary ore minerals established by us

of the oxidation zones of sulfide ore occurrences of the Kolyvan-Tomsk zone and their distribution are given in the table below.

Despite the poverty of the ore outcrops in secondary minerals, some of them can be identified macroscopically (barite, malachite, azurite, scorodite, cerussite).

Table 1

Secondary ore minerals of the oxidation zones of sulfide ore occurrences of the Kolyvan-Tomsk zone

Relicts of hypogene mineralization

Ore-bearing solutions that give rise to commercial concentrations of one metal or another are known to produce a number of characteristic changes in the enclosing rocks. In the literature such rocks figure as hydrothermally altered formations. When secondary processes are superimposed, many of these rocks are only slightly altered, which makes it possible, in studying an ore outcrop, to regard them as relict forms of the primary mineralization. Relicts of hypogene mineralization also include primary sulfides preserved from oxidation.

Among the principal relicts of hypogene mineralization, represented by hydrothermally altered rocks, within the Kolyvan-Tomsk zone we have included silicified and baritized rocks, quartz-sericite and quartz-chlorite schists.

Of these, silicified rocks are widespread and constitute the principal morphological type. In any of the areas one may encounter veinlets, spots, and even entire fields composed of hydrothermal quartz. Such areas of altered rocks are often accompanied by formations of the type of secondary quartzites. Characteristic of the morphological type being described is its position on the hanging-wall side of the ore outcrop, as, for example, at the Baranovskoye ore occurrence. Very often the quartzites contain primary sulfide mineralization, chiefly pyrite, as well as sphalerite, galena, and arsenopyrite, preserved under hypergene conditions owing to the dense, undisturbed texture of the enclosing rock.

Quartz-sericite and quartz-chlorite schists occur considerably more rarely. They are localized as separate small areas in zones where rocks of analogous composition transformed by hypergene processes are distributed.

Baritization appears either as small areas among bleached rocks (the Opnevozaimskoye ore occurrence) or as veinlets together with quartz (the Mitrofanovskoye and Baranovskoye ore occurrences).

We shall not dwell on the mineralogical and textural-structural features of the listed relicts of hypogene mineralization, since they are well known from the relevant literature on hydrothermally altered rocks of ore deposits.

Relicts of primary sulfide mineralization are observed very rarely. They are confined for the most part to areas only weakly affected by sulfate weathering of the rocks and to areas with intense hydrothermal alteration (silicification, baritization). Pyrite is the most frequently encountered relict of hypogene ore mineralization; in isolated cases magnetite, sphalerite, galena, pyrrhotite, chalcopyrite, antimonite, and cinnabar are identified.

Thus, studies of the oxidation zones of a number of sulfide ore occurrences of the Kolyvan–Tomsk zone show that, within oxidized ore outcrops, definite morphological types of rocks are distinguished, differing in their structural-textural features and mineralogical composition, which arose under the influence of sulfate weathering, and also in their distribution and mutual relationships depending on the character and type of mineralization. All the indicated features have been made the basis of the proposed legend for these rocks.

Large-scale geological mapping using this legend, in combination with geochemical studies, will facilitate outlining and reconstructing the ore outcrop, which will make it possible to direct exploration work effectively. This is demonstrated by our experience in studying the oxidation zone of the Baranovsk ore occurrence (Levashov, 1969).

Undoubtedly, as prospecting and exploration work expands in the territory of the Kolyvan–Tomsk zone, the developed legend of morphological types of rocks of oxidation zones will be refined and elaborated in greater detail.

LITERATURE

Ch. Anderson. Oxidation of copper sulfides and secondary sulfide enrichment. — In: Problems of Ore Deposits. Izd-vo IL, Moscow, 1958.

B. M. Levashov. On the experience of studying the mineralogical composition of oxidation zones in combination with hydrogeochemical investigations. Proceedings of the interuniversity conference on hydrogeochemical and paleohydrological methods of investigation for prospecting for mineral deposits. Izd-vo TGU, Tomsk, 1969.

N. A. Roslyakov, N. V. Roslyakova. A method of large-scale mapping of oxidation zones of sulfide ore occurrences of Western Altai. — In: Geology and Geochemistry of Ore Deposits. Siberian Branch, Nauka Publishing House, Novosibirsk, 1965.

S. S. Smirnov. The Oxidation Zone of Sulfide Deposits. Publishing House of the Academy of Sciences of the USSR, Moscow, 1955.

V. K. Cherepnin. On the methodology for studying ore bodies at sulfide deposits. “Exploration and Protection of the Subsurface,” 1961, No. 8.

V. K. Cherepnin, B. M. Levashov. Features of the morphology of the oxidation zone of the Uskandin polymetallic deposit in Salair. Izvestiya TPI, Vol. 120. Tomsk, 1962.

F. N. Shakhov. Types of ore outcrops of polymetallic deposits in the Salair region. Izv. ZSFAN SSSR, 1946, No. 1.

F. N. Shakhov. Morphological features of the oxidation zones of the Belousovsk deposit in Altai (correlation review). Collection of abstracts of scientific research works completed in 1946 at TPI, Tomsk, 1947.

F. N. Shakhov. Morphological features of oxidation zones. Tr. IG i GSO AN SSSR, issue 4, 1960.

Blanchard R. and Boswell P. F. Notes on the oxidation products derived from Chalcopyrite. Econ. Geology, No. 7, v. 20, 1925.

Blanchard R. and Boswell P. F. Status of Zeached outcrops investigation. Eng. Min. Journ., No. 7, 9, 1928.

Blanchard R. and Boswell P. F. Additional Zimonite Types of galena and sphalerite Derivation. Econ. geology, No. 7, v. 29, 1937.

Boswell P. F. and Blanchard R. Cellular structure in limonite. Econ. geology, No. 8, v. 24, 1929.

Zocke A. Zeached outcrops as guides to copper ore. Baltimore, 1926.