SOME FEATURES OF THE MORPHOLOGY AND FORMATION OF THE OXIDATION ZONE OF THE MAINSKOYE COPPER-PYRITE DEPOSIT

V. K. Cherepnin

(Presented by Professor A. M. Kuzmin)

Introduction

This article presents the results of a study of the oxidation zone of the Mainskoye deposit. The latter is a very favorable object, since it is characterized by fairly good exposure and by the presence of a large number of the most diverse secondary products. In addition, a sufficiently branched network of underground workings makes it possible to correlate the surface outcrop with the primary mineralization; moreover, the literature contains some factual material on the structure and composition of the middle levels of the oxidized zone, which are inaccessible at present [1, 6].

It should also be noted that the author of the present article proceeds from a broad interpretation of the concept “oxidation zone,” fully sharing in this respect the views of F. N. Shakhov [10, 11].

With respect to the general geological position of the mineralization, it should be noted that the ore bodies of the deposit occur within a weakened zone of northwest strike, passing through the porphyrites of the Lower Monok Formation \((Cm_2^1)\).

The porphyrites within the indicated zone are intensely schistose, chloritized, and silicified. As a result, they have been transformed into chlorite, chlorite-quartz, and quartz-chlorite schists.

CHARACTERISTICS OF THE OXIDATION ZONE

Study of the surface outcrop of the central part of the deposit has made it possible, within its area, to distinguish a number of the following diverse elements (Fig. 1): 1) bleached rocks; 2) rocks enriched with hydrates of iron oxide; 3) brown iron ores; 4) oxidized analogues of poor disseminated ores; 5) opalized rocks; 6) a series of loose products of the oxidation zone.

Bleached rocks

These formations are sharply whitened rocks. They are expressed most effectively in the hanging wall of the ore zone. Noteworthy is the fact that rocks subjected to bleaching usually retain their former texture, and at times also their structure, as is the case, for example, in porphyrites.

Under the microscope, the lightened rocks are essentially siliceous formations consisting of quartz, chalcedony, opal, and finely dispersed clayey material. Study of the latter by centrifugation followed by staining with organic dyes, according to the method of N. E. Vedeneeva and M. F. Vikulova [2, 3], showed that it consists mainly of hydromica and montmorillonite, with an almost constant presence of kaolinite and a rare admixture of beidellite. These data agree well with the results of the study of lightened rocks of several polymetallic deposits in Central Kazakhstan given by N. N. Velikaya [4, 5].

The chemistry of the lightening process is at present not fully clear. However, the available facts show that it proceeds with vigorous decomposition of dark-colored components and, in part, of feldspars. Below we give the results of chemical analyses¹ of two relatively fresh rocks from the area, taken within the ore zone, and their lightened analogues. For comparison, we give the results of analyses of similar formations from the work of N. N. Velikaya [4] for one of the polymetallic deposits of Central Kazakhstan².

Analyzing the cited data of chemical analyses, one can see that the process of lightening proceeds with vigorous removal of calcium, magnesium, alkalis, and ferrous iron. With regard to the alkalis, N. N. Velikaya indicates that only potassium is removed, while the amount of sodium either remains unchanged or even increases noticeably. As for ferric iron, in our view its content is not indicative, since in places the lightened rocks may be intensely limonitized (as in the analyzed sample of quartz porphyry).

The analyses given above do not provide a clear picture regarding the behavior of alumina and silica. In two cases out of three there is

there is a significant increase in silica and a decrease in the amount of alumina. In one case the picture is exactly the reverse (quartz porphyry). It seems to us that this discrepancy follows from the course of the process of bleaching itself and is explained as follows. At the initial and middle stages in the development of the bleaching process, a certain accumulation of alumina occurs, which together with silica forms a considerable quantity of hydrous aluminosilicates. In the final stages of the process, sulfate solutions decompose the previously formed hydrous aluminosilicates and remove alumina. As a result, rocks enriched in silica and impoverished in alumina are formed.

Finally, from these same analyses it is evident that the highest content of titanium dioxide is observed in porphyrites, that is, in rocks considerably enriched in dark-colored components, which should be regarded as carriers of titanium. Attention is also drawn to the fact that bleached porphyrites contain almost three times more titanium than their unaltered varieties. This is apparently explained by the very poor migrational capacity of this element under hypergene conditions, when its accumulation occurs at the expense of the removal of other components.

Stem-like quartz, which frames numerous leaching cavities and limonitic pseudomorphs, is very widely developed in the bleached areas. In places, jarosite develops among the limonitized areas.

It is known that under certain specific conditions, as a result of regional weathering, bleached rocks may arise that are similar in appearance to the formations just described. However, the broad development of opal, leaching cavities, limonitic pseudomorphs, and jarosite in the bleached rocks of the ore zone does not allow us to confuse these formations.

Rocks Enriched in Hydrates of Iron Oxide

Such rocks appear differently in different places. In some cases these are bleached rocks bearing weak film-like limonitization on schistosity planes. In others, they are already varieties rather densely impregnated with limonite. Finally, we often have to deal with veinlets of dense limonite forming, in the enclosing rocks, a network of one or another density. The formations described are developed both within the ore zone itself and in its hanging and especially footwall sides. Within the ore zone, thin impregnating limonite or veinlets of dense limonite are predominantly manifested. The latter most often occur near its footwall side. As for film-like limonitization, it is usually observed beyond the limits of the ore zone, developing in the form of thin crusts and films on parting planes of porphyrites and chlorite schists. In places, jarosite appears among intensely limonitized areas of the ore zone. It is especially intensely developed in the hanging wall of the zone, within a sharply bleached dike of quartz albitophyres, which in places has been transformed into an ocherous jarosite-limonite rock. In the overwhelming majority of cases the mineral mentioned is relict in character, being distinctly replaced by limonite. However, within the ore zone, in one of the ditches, areas were encountered where, among rocks impregnated with limonite, isolated microscopic veinlets of jarosite were found. Dense limonite developed in the axial parts of each such veinlet. If the veinlets mentioned are regarded as a later formation relative to the enclosing limonitic mass,

then it must be assumed that, somewhere, already in the final stages of development of the given oxidation zone, there was a sharp increase in the acidity of the sulfate solutions. This could have been connected only with the oxidation of new portions of primary sulfide ores. However, we have found no other indications anywhere that such a phenomenon took place. Therefore, apparently, it is more correct to regard the aforementioned veins of jarosite as peculiar relicts of replacement.

In addition to the foregoing, the fact mentioned is of interest to us in another respect. At present it is well known that jarosite can form only at a comparatively considerable depth from the surface, where there is a deficiency of water and free oxygen. Its discovery directly in the ore outcrop indicates that the upper part of the oxidation zone has by now been removed by erosion.

As for the question of using this morphological feature to evaluate ore outcrops, it should be borne in mind that the literature describes [7] cases of occurrence not only of film limonitization, but even of veinlets of dense limonite during processes of regional weathering of iron-bearing rocks. We believe that a good criterion for distinguishing one limonitization from another will be the presence of jarosite, which will always indicate that the given limonite arose through oxidation of sulfides.

Brown Iron Ores

Massive brown iron ores within the modern ore outcrop are very poorly developed, occurring as separate pieces and blocks among loose ocherous material. Most often they are found in the extreme southeastern part of the area described. In most cases the brown iron ores are porous to one degree or another. The porosity is characterized by an uneven, nest-like distribution. Areas with dense porosity have a typical slaggy texture and not infrequently pass into normal sponges.

Spectral analysis of such brown iron ores showed the presence in them of copper (up to 0.01%), silver (up to 0.005%), and titanium (up to 0.03%).

The limonite of the brown iron ores is represented by a cryptocrystalline variety with a well-expressed metacolloidal structure. In optical properties it belongs to the lepidocrocite series. The limonite described often, in porous areas, lines the walls of cavities and forms a network of veinlets of varying density. Spectral analysis detected in this material: copper (up to 0.01%), titanium (up to 0.03%), and molybdenum (up to 0.001%).

Genetically, these brown iron ores should apparently be regarded as oxidized analogues of rich disseminated and, to a lesser extent, massive sulfide ores.

Oxidized Analogues of Poor Disseminated Ores

These formations are a rock containing pores from the leaching of sulfides. The form of these pores is mostly rounded or even square, more rarely irregular. Very often they are surrounded by stalk-like quartz. It has been noted that in some cases the aforementioned pores are empty, while in others they are filled with limonite with the formation of pseudomorphs.

Within the ore outcrop, the indicated porosity is almost everywhere confined to the area of development of lightened rocks. It is distributed

it is uneven and usually occurs as nests, patches, or vein-like formations. In places the pores are so densely distributed that such areas take on the character of a sponge.

Opalized rocks

Opalization is observed almost everywhere in the belt of lightened rocks of the hanging wall. Here the opal finely impregnates the rock, giving it a characteristic porcelain-like appearance. Owing to the comparatively weak limonitization of these rocks, it is white in color or completely colorless.

The process being described is also rather widely developed in the gossans. In this case the opal is always, to one degree or another, colored reddish-orange. It impregnates the most porous parts of the gossans, filling all the cavities present there. In dense varieties of gossans, opal is developed in the form of isolated veinlets, the thickness of which reaches several centimeters.

Under the microscope it is clearly visible how opal corrodes limonite, representing an evidently later formation. With such corrosion, a fine dusting of limonite usually remains in the opal, causing its reddish coloration. Spectral analysis has established the presence in the opal of hundredths of a percent of copper and titanium.

Genetically, opalization should be associated with silicic acid released during the decomposition of feldspars of the wall rocks under the action of sulfate solutions. The time of opal formation apparently coincides with the period of greatest concentration of sulfate solutions within this part of the oxidation zone, since only in this case could the decomposition of the wall rocks and the release of a large amount of silicic acid proceed most completely. This, in turn, is connected with the vigorous decomposition of the primary sulfide mass, essentially of pyritic or pyrite-pyrrhotite composition. In the opinion of S. S. Smirnov, this process will proceed most completely under conditions of an arid climate [8]. The development of opal over already formed gossans leads to the conclusion that the sulfate solutions formed at this locality were not involved in creating the opalization developed here. Apparently it is more correct to place the latter in genetic dependence on solutions that descended from hypsometrically higher horizons of the oxidation zone.

It is still completely unclear at present what the magnitude of the time gap was between the formation of the gossans and their opalization. On the one hand, proceeding from the fact that opal replaces already fully formed gossans, one may suppose that this interval was considerable. However, it is then difficult to explain the appearance of highly concentrated sulfate solutions after the sulfide mass of this horizon, and still more of the overlying horizons, had long since been transformed into gossans. On the other hand, there may have been no substantial gap between the formation of the gossans and the opals. It is quite possible that their formation represents two sides of one and the same process. Owing to structural features, in one place the ore may oxidize somewhat earlier than in another. Consequently, sulfate solutions arising in one place could cause opalization only of the gossans formed nearby (hypsometrically lower).

On the basis of a general analysis of the available material, the second view appears to us more probable.

Loose Products of the Oxidation Zone

The abundance of loose products constitutes one of the most characteristic features of the oxidation zone described. In terms of composition and genesis, the following groups can be distinguished among these products: 1) barite–quartz sands; 2) quartz sands; 3) limonite–quartz sands; 4) disintegrated porous brown iron ores; 5) products of disintegration of bleached rocks; 6) pyrite loose material.

Barite–quartz sands are developed as a single small lens in the footwall of the ore zone, among limonite–quartz sands (between the outcrops of two magnetite bodies).

Macroscopically they are a very fine yellowish-gray material with numerous fragments of bleached rocks. Many fragments are distinctly schistose and, to one degree or another, limonitized. Microscopic study of the fine material showed that it consists of irregular quartz grains, platy barite grains, and a minor admixture of finely dispersed kaolin-like material. Quartz predominates over barite by approximately ten times in volume. It often has a porous structure and in places contains nest-like distributed pseudomorphs of limonite after pyrite. A detailed study of the finely dispersed material by centrifugation and staining with organic dyes showed that it consists of hydromica with an admixture of montmorillonite.

Spectral analysis of the indicated material showed the presence of the following components (in %):

Separation of this material into fractions with subsequent spectral testing showed that gallium is concentrated in the light fraction, in the finest dust-like material. An elevated zinc content (more than 0.1%) was also noted there. In general, everywhere in the light fraction the zinc content exceeds 0.02%. The heavy fraction is enriched in lead (up to 0.02%).

Genetically, we regard the described barite–quartz sands as a product of leaching of rich disseminated, or even massive, sulfide ores of the pyrite type, probably containing a noticeable amount of sphalerite, chalcopyrite, and galena.

Quartz sands were also encountered as a low-thickness lens, but not in the footwall; rather, in the hanging wall of the ore zone, exactly at the boundary of bleached and opalized rocks with limonitized porphyrites.

Macroscopically this is a fairly homogeneous light-gray sandy material, in which rare small pieces of schistose bleached rock occur. Under the microscope it is seen that the material described consists mainly of quartz grains, whose mode of occurrence is similar to those in the barite–quartz sands. Their noticeable limonitization, usually of a film-like character, is somewhat specific. Spectral analysis established in this material only the presence of thousandths of a percent of copper.

Genetically, the sands described are apparently analogues of the preceding ones, with the only difference that here, in the original material, the admixture of copper, zinc, and lead sulfides was probably negligible.

Limonite–quartz sands are the most widely distributed loose products of the ore outcrop, developed chiefly in the axial part of the zone. By the character of their occurrence, morphology, and genesis, two principal varieties may be distinguished among them.

The first of these is composed of ocher-yellow clastic material. Individual pieces or fragments consist essentially of quartz rock literally impregnated with ocherous limonite material. Most of these fragments, when strongly pressed with the fingers, disintegrate into quartz grains and a fine earthy ocherous material. Under the microscope it is seen that the fine material consists of the smallest quartz grains, covered with a limonite “shirt” of varying thickness, which strongly distorts the true shape of the grains.

On careful examination of this limonite material, the presence of a certain amount of jarosite is not always detected. In individual grains, pseudomorphs of limonite after pyrite are visible here and there. Spectral analysis established in this material the presence of 0.1% copper and approximately the same amount of zinc, apparently associated with the ocherous limonite mass.

The described formations occur as a thin lens in the footwall of the zone, in direct contact with barite–quartz sands.

The second variety is represented by an orange or yellowish-brown earthy material, among which occur fragments of bleached rocks and rare pieces of porous brown ironstones, locally opalized. All fragments are coated at the periphery with a shell of ocherous earthy limonite.

Microscopic study of the earthy material showed that, as in the preceding case, it consists of small quartz grains covered with a “shirt” of ocherous limonite with an admixture of jarosite. Spectral analyses of the earthy material showed the presence of copper (up to 0.01%) and thousandths of a percent of silver.

These formations are developed in the very axial part of the ore zone, as well as in some places near its hanging and foot walls.

Genetically, both varieties of limonite–quartz sands apparently represent nothing other than the result of disintegration of a quartz, jarosite-rich sponge. In the course of the further process of oxidation, the jarosite was replaced by limonite.

Disintegrated porous brown ironstones. The material described is represented by a red-brown or yellowish-brown clastic rock, externally somewhat resembling limonite–quartz sands. It consists of fragments of brown ironstones, often opalized; small pieces of dense limonite with a double internal structure (radial and concentric); pieces of bleached schistose rock; and small fragments of quartz, sometimes cemented into rather large pieces. All this is enveloped by a thin ocherous jarosite–limonite material. In pieces of bleached rocks, one or several veinlets of dense limonite, developing along the schistosity, are almost always visible. In fragments of quartz, areas of cellular limonites are often found.

In the fine clayey-limonite material, spectrography revealed the presence of copper (on the order of 0.005%), silver (up to 0.01%), and

zinc (up to 0.1%). Some samples showed a sharply elevated copper content.

These rocks are developed as a well-sustained band traced through the entire area described. In addition, in the northwestern part of the area, along the axial part of the zone, a second, pinching-out band of similar formations is indicated. It was precisely within these bands that pieces of massive opalized brown iron ores, described above, were encountered.

In genetic terms, the formations described, in our opinion, are nothing other than the result of the destruction of highly porous brown iron ores under the load of the rocks of the hanging wall.

Products of disintegration of bleached rocks. In the lower part of the oxidation zone, at a depth of about one hundred meters from the surface, a yellowish-white loose material was repeatedly encountered in mine workings. In degree of comminution it is heterogeneous. Along with a very fine powdery mass, there are fragments reaching five centimeters along the long axis. All fragments, both small and large, have a platy character and are pieces of completely bleached schistose rock.

Microscopic study showed that this material is a substantially siliceous rock consisting of quartz, opal, and some amount of finely dispersed clayey products. Among the latter there are montmorillonite and hydromica. In places, slight limonitization is developed. In addition, it has been established that all fragments on the schistosity planes bear fine, more or less evenly distributed porosity. The average pore size along the long axis is 0.25 mm. In most cases they are filled with native sulfur. In places it forms thin films on the schistosity planes of the rock. A very large amount of sulfur is also contained in the fine loose mass, accounting for the yellowish tint of the latter. More detailed descriptions of the sulfur mentioned and of the conditions of its formation are given in a separate note [9].

The loose material described in one place overlies pyrite sand, and in another—from the hanging wall—borders sharply bleached banded rock of substantially siliceous composition, possessing a relict porphyry structure.

The foregoing allows us to consider that the loose material described is the result of collapse of bleached rocks of the hanging wall over a sulfide mass decreasing in volume during its transformation into pyrite sand. The process of formation of native sulfur should also be associated with this same phenomenon of intensive decomposition and leaching of sulfide ores in the lower part of the oxidation zone.

Pyrite sand. This material was encountered by us twice in the lower part of the zone described. In one place, sharply bleached rocks occur above it in the hanging wall. In another, it is overlain by the disintegrated products described above. Here it is clearly visible how, over one and a half meters, the pyrite sand was gradually replaced downward by massive pyrite ore. The microscope showed that this ore is a pyrite aggregate broken by a large number of fractures. In the latter, vein material and isolated inclusions of chalcopyrite have been preserved here and there. The impression is created that the indicated pyrite ore is not fully primary, but is already a substantially leached formation. Considering that the primary ores of the deposit contain a sufficient amount of pyrrhotite, it is easy to suppose that it was the first to undergo leaching. If this is so, then one may consider that pyri-

the accumulation of fragments at the deposit most readily arose in those parts of the ore bodies that contained a significant amount of pyrrhotite. True, we have no direct evidence on this point.

Fig. 1. Scheme of the geological structure of the ore outcrop in the central part of the Mainskoye deposit: 1 — porphyrites; 2 — quartz albitophyres; 3 — chlorite, chlorite-quartz, and quartz-chlorite schists; 4 — magnetite bodies; 5 — film limonitization; 6 — veinlets of dense limonite; 7 — bleached rocks; 8 — limonite-quartz sands; 9 — barite-quartz sands; 10 — quartz sands; 11 — disintegrated brown ironstones; 12 — pits; 13 — dumps; 14 — geological boundaries; 15 — trenches; 16 — shafts and prospecting pits.

Conclusion

Analysis of the material presented above shows that, over the mapped area, the outcrops of the ore bodies proper correspond to belts of disintegrated porous brown ironstones. This indicates that at depth the ore bodies are represented mainly by rich disseminations, among which massive ores occur only locally. The presence of the latter is revealed by finds on the surface, among the loose mass, of individual pieces and blocks of massive brown ironstones. This conclusion is fully confirmed by observations in the primary-ore zone.

The limonite-quartz sands widely developed at the surface are a product of the disintegration of a siliceous-limonitic sponge, which is the oxidized analogue of poorer disseminated ores.

As for the barite-quartz and quartz sands, they apparently represent products of leaching of rich disseminated or even massive, essentially pyritic ores. The high concentration of sulfate solutions here ensured the almost complete removal of iron. The presence of zinc, gallium, lead, and silver in the barite-quartz sands indicates that sphalerite and galena were present as impurities in the primary ores.

The oxidation zone described is an ancient formation that, after its formation, underwent a number of subsequent changes. One of the irrefutable proofs of the existence of such changes may be the appearance, in the very lowest part of the oxidation zone of our deposit, of a narrow, discontinuous horizon with cuprite and native copper. In the literature, similar formations are regarded as products of the oxidation of chalcocite–covellite ores of the cementation zone, arising as a result of a sharp lowering of the groundwater level. The small thickness and poor continuity of the indicated cuprite horizon make it possible to suppose that the formerly existing cementation zone was weakly expressed. At present, at the Mainskoye deposit there is no cementation zone as such at all; only in places, at the boundary between primary and oxidized ores, are small areas with intensive development of covellite encountered.

The widespread development, in the upper part of the described zone, of ocherous limonite, which is a product of the further alteration of jarosite, is also evidence of the antiquity of the Mainskoye oxidation zone.

REFERENCES

1. I. K. Bazhenov. Report on geological investigations in the area of the Mainskoye copper deposit, Minusinsk uezd. Publ. of the Siberian Branch of the Geology Committee, vol. IV, issue 1, 1924.

2. N. E. Vedeneeva and M. F. Vikulova. Organic dyes in the application to analysis of clays. Collection “Weathering Crust,” issue 1, 1952.

3. N. E. Vedeneeva. A method for studying clay minerals with the aid of dyes and its application in lithology. Gosgeolizdat, 1952.

4. N. N. Velikaya. The bleached zones of polymetallic deposits of Central Kazakhstan and their prospecting significance. Information collection of VSEGEI, No. 3, 1956.

5. N. N. Velikaya. Hypergene alterations of rocks in the oxidation zone of the Kuzhok-Adyr polymetallic deposit in Central Kazakhstan. Materials on the geology and mineral resources of Altai and Kazakhstan. Gosgeolizdat, 1956.

6. M. E. Nekipelov. The Mainskoye sulfur-pyrite copper-zinc deposit. Materials on the geology of the Western Siberian Territory, No. 16, 1934.

7. V. N. Razumova. The weathering crust of the northwestern part of the Kazakh upland. Collection “Weathering Crust,” issue 2, Publ. of the Academy of Sciences of the USSR, 1956.

8. S. S. Smirnov. The oxidation zone of sulfide deposits. Publ. of the Academy of Sciences of the USSR, 1951.

9. V. K. Cherepnin. Native sulfur in the oxidation zone of the Mainskoye copper-pyrite deposit. Proceedings of the All-Union Mineralogical Society, series 2, part 7, 1958.

10. F. N. Shakhov. Morphological features of the oxidation zone of the Belousovskoye deposit in Altai. Collection of abstracts on research works completed in 1946, Tomsk Polytechnic Institute, 1947.

11. F. N. Shakhov. Morphological features of oxidation zones. Transactions of the Institute of Geology and [[unclear: works]], Tomsk Polytechnic Institute, 1947.

1. The analyses were carried out by the chemical laboratory of the Tomsk Integrated Expedition of the Novosibirsk Territorial Geological Administration. ↩

2. From the analyses of two samples of unaltered rocks, the average content was taken. ↩