On the Role of Loose Products in the Study of Oxidation Zones of Sulfide Deposits

V. K. Cherepnin

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

Introduction

In many works devoted to the description of oxidation zones of sulfide deposits, we encounter one or another material relating to the characterization of loose products of the so-called leaching zone. In most cases these are data of a mineralogical character, comparatively rarely accompanied by considerations concerning the genesis and regularities of distribution of individual varieties of these distinctive rocks [3, 4, 5, 6, 7]. Special attention is paid to the loose components of oxidation zones in cases where the horizons composed of them are characterized by an increased content of certain useful components, especially valuable ones [1, 2]. The most striking example in this respect may be the monograph by V. M. Kreiter, V. V. Aristov, and others [1].

Finally, loose products are described quite rarely in works devoted, to one degree or another, to the development of criteria for assessing ore outcrops of sulfide deposits. Exceptions in this regard are a number of works by F. N. Shakhov [8, 9, 10], as well as, in part, the monograph by F. V. Chukhrov [6] on Central Kazakhstan.

The present article is devoted to the characterization of loose products of oxidation zones from the point of view of their possible use for evaluating ore outcrops. The work has been compiled from materials of the study of sulfide deposits of Western Siberia.

Characteristics of Loose Products

In the sulfide deposits of Salair, the Western and partly the Eastern Sayan, such loose formations as barite and quartz sands, as well as products of disintegration of the country rocks, are most widely developed.

Barite sands. These formations are very widely developed in the oxidation zones of a number of sulfide deposits of the Salair Range.

In their composition, the formations described are not homogeneous: in some cases they are barite sands, in others quartz–barite sands, and in still others barite or quartz–barite sands with jarosite and limonite.

Barite sands proper, in their typical expression, are developed in the deposits of the Ursk group. Macroscopically these are loose rocks, in most cases very heterogeneous in degree of comminution. Fine-grained material usually predominates over large fragments. Their coloration is also very varied. Light-gray or grayish-white varieties occur comparatively infrequently; more common are yellow, orange, and brown tones of various shades. Spectral analyses of the sands described, in addition to barium, always show the presence of lead and copper. Moreover, while copper is usually scarce (weak lines), lead is recorded in appreciable quantity (medium and, more rarely, very strong lines). In seven samples out of eighteen, zinc was registered, as a rule in the form of weak lines. In four samples the presence of silver was detected.

The presence of an elevated lead content in such intensely leached products of the oxidation zone as barite sands is very indicative and is fully consistent with the poor migrational capacity of this element under the conditions considered. Such a fact may have a quite definite prospecting significance, as an indicator of the presence of polymetallic mineralization. It should be noted that the mineralogical investigations carried out did not reveal any appreciable quantities of secondary lead minerals. Therefore, the bulk of the lead should apparently more correctly be associated with the fine dusting of relict sulfides.

In addition to barite varieties proper, quartz–barite and jarosite–barite sands are often encountered. The former differ from the formations described only in that quartz material plays a significant role in their composition (up to 50% of the total mass of the sand). The latter are characterized by a constant admixture of jarosite. They are most vividly represented at the Novoursk deposit, occur on the Kontaktovaya lens, and have been found in very small quantity in the oxidation zone of the Pesterevsk deposit. The sands described are usually characterized by a darker coloration. Brownish-red, reddish-brown, and orange tones are typical of them.

In the lower part of the thick sequence of leaching sands of the Novoursk deposit, a horizon has been found that contains a very distinctive, relatively coarse-crystalline jarosite, which in optical properties corresponds to the lead-bearing variety of this mineral.

The results of spectral analyses of these mixed sands, with respect to the content of the principal components, do not differ in any essential way from those of the purely barite varieties. It may only be noted that the horizon containing coarse-crystalline jarosite shows a somewhat elevated lead content.

Quartz sands. Quartz sands are most widely developed in the ore outcrop of the Main deposit. In Salair they are not widespread: they have been encountered on the Vasil-Popov lens of the Salair ore field proper and, in very insignificant quantity, here and there in the deposits of the Ursk group. In composition, among them, quartz sands proper, jarosite–quartz, and limonite–quartz varieties are clearly distinguished.

The former are distinguished chiefly by a light gray or brownish-gray coloration. The brownish tint is of secondary origin and is connected with the pigmenting effect of iron hydroxides. In character

the products described by comminution are also heterogeneous. Investigations have shown that, in addition to quartz and an insignificant quantity of other vein minerals, there is always a considerable admixture of ore material, in which dense limonite plays the main role, more rarely pyrite.

Jarosite–quartz sands were encountered by us in the Vasil’-Popov lens of the Salair deposit. These formations can be called sands only conditionally, since they are characterized by a clear predominance of large fragments (up to 5 cm in cross section) over the fine-grained mass. The large pieces are represented by quartz, usually finely porous, often spongy. The fine material consists of extremely small quartz grains and very fine-scaly jarosite. In the oxidation zone of the Pesterëv deposit of Salair, jarosite–quartz sand with abundant native sulfur was found in a very small quantity.

As for the limonite–quartz sands, developed mainly in the oxidation zone of the Main deposit, they differ from the preceding ones essentially only in that limonite is present in them instead of jarosite. Very typical of these formations is the sharp predominance of fine-grained limonite–quartz material, consisting of very small grains of vein quartz covered with a “shirt” of powdery limonite. The large pieces do not exceed 10–15 mm in cross section and in most cases consist of compacted fine-grained material or are pieces of limonitized quartz sponge. Noteworthy is the fact that among the powdery limonite there is always some quantity of jarosite material. Moreover, sometimes substantially limonitic nodules have a jarosite core inside. All this suggests that the limonite in the sands described is not a product of the direct oxidation of sulfides, but was formed through the subsequent oxidation of jarosite. Apparently, the limonite observed among the jarosite varieties of barite sands has an entirely analogous origin.

Spectral analyses of quartz sands show that lead occurs in them less often than in barite sands. The highest content of this element (medium and strong lines) is established in jarosite–quartz sands and in barite-bearing varieties. Zinc, however, is detected here much more frequently; only one third of the samples are devoid of this element. Usually, an insignificant amount of copper is constantly present.

Products of disintegration of the country rocks. Among the formations of this group one can clearly distinguish: 1) products of disintegration of bleached kaolinized rocks; 2) products of disintegration of bleached opalized rocks.

The former are most typically represented in the Novour and Pesterëv deposits of the Salair Ridge. They consist of loose, fine-grained, kaolin-like material of various shades of fawn coloration. It is greasy to the touch and very strongly soils the hands. In this mass occur larger (up to several centimeters in cross section) platy fragments of schistose, sharply bleached rocks. Detailed investigations showed that the indicated fine-grained formations consist of small plates of the very same bleached rocks, fragments of quartz grains, isolated pieces of barite, the finest quartz grains, and ore components immersed in the thinnest clayey material. The ore components are represented by pieces of dense limonite with an admixture of fragments of pyrite grains. Investigations of the fine-grained material by centrifugation showed that it consists mainly of hydromicas with an admixture of kaolinite.

The results of spectral analyses of individual samples of bleached kaolinized rocks from the Salair deposits show—

give strong barium lines, medium lead lines, and weak copper and zinc lines. In the sample with the highest barium content, the presence of traces of silver was established.

The study carried out on these rocks convincingly shows that they are nothing other than products of the disintegration of disseminated and sharply silicified quartz keratophyres and their tuffs. The individual quartz grains mentioned above, lacking the structures characteristic of hydrothermal quartz, should apparently be regarded as relics of porphyritic segregations of the original rock.

The latter are expressed rather clearly at the Mainskoye deposit. Macroscopically this is a grayish-white material, often with a noticeable yellowish tint. The degree of comminution is uneven. Along with a fine powdery mass, larger platy pieces up to five centimeters across are also present. Microscopic study of them showed that they are essentially a siliceous schistose rock in which, in places, the relict structure of effusives has been preserved. The principal components of the rock are quartz, opal, and finely dispersed clay-like material. The latter consists mainly of powdery opal with an admixture of montmorillonite and hydromicas.

Spectral analyses of the described products show the constant presence in them of copper and lead, predominantly in the form of weak lines. Zinc and silver were encountered twice.

Conclusion

The investigations carried out on the most widespread loose products from the oxidation zones of a number of sulfide deposits in Western Siberia make it possible to draw the following conclusions:

1. Leaching sands are distinguished by an especially wide distribution; they are the result of prolonged and vigorous processing of the oxidized zone by sulfate solutions. They could have formed only during the oxidation of massive or densely disseminated sulfide ores characterized by a high content of iron sulfides.

2. The presence among the leaching sands of abundant unoxidized pyrite grains, as shown by the study of the Mainskoye deposit, should apparently be linked with the presence in the primary ores of more readily oxidized pyrrhotite.

3. The presence, in the lower part of the leaching-sand sequence of the Novourskoye deposit, of a horizon enriched in the lead-bearing variety of jarosite should be regarded as the result of lead migration, which is fully consistent with the high pyritization of the primary ores, which caused a significant concentration of ferric sulfate within the oxidation zone.

4. Barite sands of ore outcrops can apparently be regarded as a reliable indicator of deep-seated sulfide ores of polymetallic character. The various varieties of these sands will only indicate one or another specificity in the composition of the primary ores.

5. Quartz sands, as may be judged from the example of the Mainskoye deposit, are probably more typical of the outcrops of deposits of pyritic character.

6. As for the products of disintegration of the wall rocks, their presence is always due to a significant reduction in the volume of the ore mass during its oxidation, and this, again, can occur only during the decomposition of massive or densely disseminated ores. Some idea

on the composition of sulfide ores will give the researcher spectral analyses of the indicated loose material. The studies carried out have shown that even the disintegration products of intensely bleached, opalized rocks preserve a definite complex of the principal elements characteristic of the primary ores of the given deposit.

References

1. V. M. Kreiter, V. V. Aristov, I. O. Volynskii, et al. The behavior of gold in the oxidation zone of gold-sulfide deposits. Moscow, Gosgeoltekhizdat, 1958.

2. I. P. Palei. Concentration of native selenium in the oxidation zone of a pyrite deposit. Geochemistry, No. 7, 1957.

3. S. S. Smirnov. The oxidation zone of sulfide deposits. Publishing House of the Academy of Sciences of the USSR, 1955.

4. V. K. Cherepnin. Mineralogy and genesis of secondary zones of sulfide deposits of the Altai. Proceedings of the Tomsk Polytechnic Institute, vol. 65, issue 2, 1950.

5. V. K. Cherepnin. Native sulfur in the selenium oxidation zone of the Maiskii copper-pyrite deposit. Transactions of the All-Union Mineralogical Society, 2nd series, part 87, 1958.

6. F. V. Chukhrov. Study of the mineralogy of the hypergenesis zone of ore deposits. Materials on the Geology of Central Kazakhstan, Publishing House of the Academy of Sciences of the USSR, 1940.

7. T. N. Shadlun. Mineralogy of the oxidation zone of the Blyava pyrite deposit in the Southern Urals. Transactions of the Institute of Geological Sciences, issue 96, Publishing House of the Academy of Sciences of the USSR, 1948.

8. F. N. Shakhov. On the question of organizing prospecting and exploration work in the Altai regions for polymetallic deposits. Bulletin of the West Siberian Geological Administration, No. 5, 1940.

9. F. N. Shakhov. Types of ore outcrops of polymetallic deposits in the Salair region. Proceedings of the West Siberian Branch of the Academy of Sciences of the USSR, geological series, No. 1, 1946.

10. F. N. Shakhov. Morphological features of oxidation zones. Transactions of the Institute of Geology and Geophysics, issue 4, Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, 1960.