Contact-Metasomatic Formations in the Area of the Iron-Ore Deposits of the Krasnokamensk Group

(Eastern Sayan)

G. V. Shubin

(Presented by Professor A. M. Kuzmin)

The geological structure of the area within which the deposits under study are located includes rocks of the Lower and Middle Cambrian, represented by metamorphosed limestones, amphibole–quartz–feldspar schists, quartz sandstones, siltstones, quartz keratophyres and their tuffs. All these rocks have been intruded and partly assimilated by a granitoid intrusion, the marginal parts of which, as a result of assimilation of the country rocks, acquired a hybrid character and are composed of granodiorites, quartz diorites, diorites, and gabbro-diorites.

The tectonic structure of the area is determined by a second-order synclinal fold, complicated by small folds of the third and fourth orders.

During the emplacement of the granitoid intrusion, the enclosing rocks underwent profound alteration, with the formation around the intrusive body of a contact aureole about 300–400 m wide.

These profound alterations of the enclosing rocks, which also affected the apical parts of the intrusive formations, were mainly metasomatic in character and were expressed in silicification, biotitization, amphibolization, muscovitization, and pyroxenization.

During the silicification of amphibole–quartz–feldspar schists, the latter, within the contact-aureole zone, lose their schistose texture and pass into dense rocks with a characteristic conchoidal fracture.

The silicified rocks consist of a fine-grained quartz aggregate with a mosaic structure, strongly sericitized feldspar, and a fine dissemination of amphibole and magnetite.

As for the quartz keratophyres, within the contact-aureole zone they are likewise, in places, subjected to silicification, sericitization, amphibolization, chloritization, epidotization, and propylitization. It should only be noted that the indicated alterations are genetically connected with the hydrothermal phase of postmagmatic processes and with the contact metamorphism of the granitoid intrusion. In outward appearance, the altered quartz keratophyres look quite varied: from weakly altered ones that have preserved

their principal features, to massive, dense, hornfels-like rocks.

Marbles are also subjected to silicification. In initially pure marbles there is no more than one percent quartz, whereas in altered marbles the quartz content reaches 40–50 percent. In limestones transformed into quartz-carbonate formations, fine-grained aggregates of quartz are distributed more or less uniformly.

Materials from drill holes at the Rudnyi Kaskad deposit show that not only the enclosing rocks but also the intrusive rocks undergo silicification. In the latter, under the microscope one can clearly trace how the minerals of the intrusive rocks are, to varying degrees, replaced by a quartz aggregate. At the same time the dark-colored components and feldspars gradually begin to disappear, and in their place abundant quartz develops, represented by more or less isometric grains forming aggregates with a mosaic structure. As a result of such changes, as investigations show, elements such as iron, magnesium, aluminum, potassium, and sodium are removed from the rock; the rock is enriched in silica, and all this ultimately leads to the formation of quartzite-like rocks.

As for the processes of biotitization, amphibolization, and muscovitization, as a rule they are superimposed on silicification and condition the emergence of dark metasomatic hornfelses.

The processes of pyroxenization are later and, in turn, develop in dark hornfelses and quartz-carbonate rocks, leading to the formation of light metasomatic hornfelses. With the appearance of pyroxene, the development of new types of metasomatic formations begins, united under the general name—skarns.

All the diversity of the enumerated metasomatic rocks that arose in the zone of the contact aureole of the granitoid intrusion and in the place of the intrusion itself may, on the basis of mineralogical-structural and textural features, be divided into two groups:

1. Metasomatic hornfelses. They are characterized by dense composition and a hornfels structure.

2. Skarns proper—the product of metasomatic replacement of hornfelses, sedimentary and igneous rocks with the participation of hot solutions that came from the deep parts of the intrusive masses and introduced part or often all of the elements necessary for the formation of new minerals.

At present, materials coming from the skarn deposits of the southwestern Siberian region increasingly show that all the processes occurring at the indicated deposits can be divided into three stages strictly separated in time: a) formation of metasomatic contact hornfelses, b) the skarn stage proper, c) the stage of ore deposition.

In this respect, the material collected by us in the area of the iron-ore deposits of the Krasnokamensk group is of considerable interest.

In the present article we shall accordingly dwell on the analysis of both the early metasomatic processes (formation of metasomatic hornfelses) and the later ones—skarn formations proper, with a partial characterization of the processes of ore formation.

Metasomatic hornfelses

Metasomatic hornfelses may be divided, according to the character of their coloration, into two types: a) dark hornfelses, b) light hornfelses. In the first part

of this section we shall mainly dwell on the description of those dark hornfelses that form in place of intrusive rocks.

Dark hornfelses in their distribution are closely associated with intrusive rocks. In the field these hornfelses are sometimes indistinguishable from intrusive rocks, and only the microscope makes it possible to distinguish them; moreover, as a rule, the intrusive rocks are usually silicified to one degree or another. Under the microscope it is possible to trace clearly how the silicification of the intrusion that took place at the beginning transforms it into typical silicified (hornfelsed) rocks of the quartzite type. Often, among the fine-grained mass of the hornfelses, coarser-grained areas occur—relicts of the structure of the original rock. The mineralogical composition of the dark hornfelses is characterized by the presence of quartz, feldspar, biotite, muscovite, more rarely amphibole, magnetite, sphene, and apatite.

Biotite forms small greenish-brown, brown flakes with well-pronounced pleochroism from yellowish-brown to pale yellow or pale green. In addition to biotite, small accumulations of green hornblende may be encountered, with constants: \(CNg = 23^\circ\), \(2V = (-)72^\circ\); pleochroism according to \(Ng\)—green, \(Nm\)—greenish-yellow, \(Np\)—pale greenish. In the main quartz-feldspar mass one can often observe relict plagioclases characteristic of the intrusive rocks of the district, apatite, magnetite, and extremely abundant sphene, which sometimes forms thin veinlets clearly distinguishable macroscopically.

All the principal minerals listed above form specific veinlets and “bands,” imparting to the quartzites, in whose place they develop, a spotted and banded texture. As a rule, in places of biotite mineralization the structure of the hornfelses becomes microgranular, with a streaky distribution of quartz and flakes of biotite, more rarely muscovite, between which the relicts of quartzite fragments are also elongated in the direction of the streakiness.

Thus, banding, streakiness, and replacement of the substance of the quartzites are later formations, and this latter fact points to metasomatic processes that took place during the formation of the dark hornfelses.

As a result of such replacement of quartzites, we have new formations, among which, depending on the quantitative composition of one or another mineral, the following varieties may be distinguished: a) quartz-feldspar hornfelses, b) quartz-biotite hornfelses, c) biotite hornfelses, d) muscovite-quartz-feldspar hornfelses.

Apparently, the minerals considered above were deposited by hot solutions in fractures and cataclased zones that arose after the formation of the quartzites. In places the cataclasis is expressed so distinctly that characteristic breccia-like textures may be encountered (Fig. 1). In the photograph, irregularly shaped fragments of quartzites cemented by biotite, hornblende, and more rarely magnetite are clearly visible. Sometimes chlorite also appears in these areas in the form of thin veinlets.

Magmatic solutions, seeping through the mass of quartzites, deposited biotite, muscovite, and more rarely amphibole. At the same time, the deposition of these minerals was controlled not only by structural elements, but also by a definite physicochemical environment. Thus, the development of biotite occurs only in quartzites formed in place of intrusions, and nowhere did we observe phenomena of penetration of biotite mineralization into quartz-carbonate rocks. In this same environment, hornblende is deposited and muscovite develops.

Light-colored hornfelses usually occur among limestones (in zones of development of quartz–carbonate rocks), often in the form of vein-like bodies, sometimes with sinuous outlines; in some cases gradual transitions can be traced from light-colored hornfelses to the enclosing quartz–carbonate rocks. Light-colored hornfelses are readily distinguished macroscopically from the other metamorphosed rocks of the contact zone.

Fig. 1.

Hornfelses have a characteristic conchoidal (porcelain-like) fracture and great hardness. No relict structures could be found in the light-colored hornfelses. Under the microscope they reveal a rather uniform picture of typical hornfelses, differing, however, from one another in grain size. The mineralogical composition of these hornfelses is characterized by the presence of quartz, carbonate, and pyroxene. The latter, judging from thin sections, forms a dense dissemination of small, more or less isometric white and light-greenish grains in a mainly quartz–carbonate matrix. In places one can observe prismatic individuals up to 0.025 cm in size with well-developed cleavage.

The results of the study of pyroxene in immersion liquids and on the universal stage are given in Table 1.

Table 1

In addition to these varieties of pyroxene, in the pyroxene hornfelses one may also encounter another pyroxene of the hedenbergite type, which forms entirely independent veinlets associated with a later, skarn stage proper. Its constants are: \(Ng = 1.740\), \(Nm = 1.723\), \(Np = 1.715\), \(Ng - Np = 0.025\), \(CNg = 48^\circ\), \(2V = 65^\circ\), CaFeSi\(_2\)O\(_6\) content = 72%. According to Tröger, this pyroxene corresponds to ferrosalite.

The formation of pyroxene in the light-colored hornfelses took place as a result of an exchange reaction between the solution and the quartz–carbonate rock, where the environment (the presence of calcium) was clearly favorable to the appearance of pyroxene.

It should be noted here that the formation of the light-colored hornfelses was preceded by the crushing not only of the limestones enclosing them, but also of the dark hornfelses. This is indicated by phenomena of penetration of diopside mineralization into the dark hornfelses in the form of small veinlets, apophyses, etc.

As a result, the dark hornfelses acquire a characteristic spotted and banded texture typical of metasomatic formations.

These circumstances undoubtedly indicate the different times of formation of the dark and light-colored hornfelses.

An essential distinction of the hornfelses under consideration is the feature that the light-colored hornfelses play the role of host rocks for the skarn-ore deposits, which cannot be said of the dark hornfelses.

The skarnization of the light-colored hornfelses proceeded, as observations show, along the same tectonic zones as the diopside mineralization of the light-colored hornfelses.

Analyzing the products of the early metasomatic processes, we arrive at the conclusion that: 1) during the formation of quartzites and quartz–carbonate rocks, there was an intensive introduction of Si and, to a lesser extent, K, Na; 2) during the formation of the dark hornfelses, Mg, K and, to a lesser extent, Fe, Al took an active part; 3) during the formation of the light-colored metasomatic hornfelses, there was an introduction predominantly of Mg; 4) a characteristic feature of the metasomatic hornfelses is their structure (banded-hornfelsic, microgranoblastic mosaic), texture, mineralogical composition, and form of occurrence, which clearly distinguishes them from other metasomatic formations; 5) the contacts between the above-described types of metasomatic formations are quite distinct, which is sharply emphasized not only by their spatial and temporal position and genetic features, but also by their mineralogical composition and corresponding textures.

As K. N. Ozerov (1944) indicates, the theory of the contact-metamorphic (metasomatic) origin of rocks of this type not only gained fairly wide recognition, but was also supported by the works of numerous studies, by a large amount of factual material from the regions of Central Asia, the Caucasus, Transcaucasia, and by a number of additional conclusions and generalizations.

Metasomatic hornfelses are widely distributed formations in contact-metasomatic deposits. This was pointed out by A. N. Zavaritskii (1927) for Mt. Magnitnaya, where he distinguishes the so-called calcareous-silicate hornfelses formed at the site of intrusive rocks as a result of metasomatic processes. Similar phenomena are indicated by E. F. Ziv (1939); E. N. Zykov and G. A. Ivankin for the eastern slopes of the Kuznetsk Alatau; V. N. Shugurov for the southwestern slopes of the Kuznetsk Alatau; Goldschmidt for the environs of Christiania; and D. S. Korzhinskii (1948) for the Tur’insk skarn copper deposits, assigning them to the high-temperature stage of near-cont—

contact metamorphism, and L. Glanzho (1955) for the Zakkar iron-ore deposit.

In conclusion, let us dwell on the question of the connection and position of the stage of metasomatic hornfelses in relation to skarn-ore bodies, using the Rudnyi Kaskad deposit as an example. On this question the following may be stated:

1) quartzites, quartz-carbonate rocks, and dark and light hornfelses are formed at an earlier stage of metasomatic formations and precede the skarn stage proper and even the emplacement of dike rocks of acidic composition;

2) skarn formations arise, as a rule, in the place of light metasomatic hornfelses that have undergone crushing of varying intensity. Similar phenomena can be encountered in the core materials of literally all boreholes of Rudnyi Kaskad;

3) in light metasomatic hornfelses one can often also see isolated, small-scale ore concentrations in the form of a series of magnetite veins without the prior formation of typical skarns.

Skarns

By skarns in our deposits we mean rocks consisting of garnet (of the grossular–andradite series) and pyroxene (of the diopside–hedenbergite series), formed in the process of high-temperature metasomatism of limestones, hornfelses, and intrusive rocks, proceeding along tectonically weakened zones under conditions of intermediate depths.

Mineralogically, the skarns are extremely poor. The most widespread among them are: garnet skarn, pyroxene skarn, and garnet-pyroxene skarn.

Garnet skarn is a heavy, dense, more rarely drusy rock of brown, reddish-brown, greenish-yellow, brownish-black, and black color. Well-formed garnet crystals can often be seen, in the form of rhombododecahedra and, more rarely, combinations of a rhombododecahedron with a tetragontrioctahedron. With respect to subsequent concentrations of magnetite mineralization, this type of skarn is the most important.

Pyroxene skarn may be of two kinds: 1) green, greenish-black, dirty green, composed of hedenbergite and constituting a dense fine-grained rock; 2) light, greenish-gray, light green, composed of diopside, more coarse-grained, sometimes gathered into radial-fibrous aggregates, occurring much more often than hedenbergite skarn.

Garnet-pyroxene skarn is a dense, often banded rock of greenish-brown color, consisting of garnet and pyroxene.

The spatial distribution of the indicated skarns at the Margoz deposit, at the contact of the intrusion with a large remnant of limestone, makes it possible to outline a certain regularity for deposits of the contact type. This regularity consists in the fact that garnet skarns most often tend toward the contact with the enclosing rocks, while pyroxene skarns tend toward the intrusive rocks.

As for the regular distribution of skarns at such deposits as Odinochnoe and Rudnyi Kaskad, here the pathways for the “skarnizing solutions” were extensive zones of fracturing and crushing, confined to rocks of various genesis. As a result—

…as a result, the skarns have sharp contacts with the enclosing rocks, and their composition does not depend on the composition of the rocks being replaced.

For the skarn fields under consideration, the principal geological condition for their formation is the presence of fractured enclosing rocks. As is evident from what has been said above, for the formation of the skarns of the Krasnokamensk group of deposits, channels were necessary along which the introduction of the corresponding components could take place, as well as corresponding structural conditions ensuring preparation of the medium for the migration of elements, ultimately leading to the formation of skarn minerals.

Undoubtedly, in this connection, the forms and elements of occurrence of skarns will be very diverse and are determined mainly by the tectonics of the rocks enclosing them, and by the form and dimensions of the pre-skarn zones of fracturing and crushing. Incidentally, as F. N. Shakhov (1947) indicates, the development of skarns is always local; they arise where movement of solutions took place, and therefore we may speak of the form of skarn deposits.

On the basis of the available material, we may propose the following classification of the forms of skarn deposits for the Krasnokamensk deposits (Table 2).

Table 2

As regards the textural-structural features of the skarns, here we may note the following. Of the structures, the most widely developed are: granoblastic, granoblastic with elements of panidioblastic, poikiloblastic, dentate, hypidioblastic, heteroblastic, lepidogranoblastic, and replacement structures. The textural varieties of skarns are represented by the following types:

a) massive texture, characteristic of garnet and pyroxene skarns;

b) drusy texture, characteristic mainly of garnet skarns;

c) radial-fibrous, characteristic mainly of pyroxene skarns; it is encountered comparatively rarely;

d) looped texture, characteristic of garnet skarns formed in more or less intensive zones of crushing of metasomatic hornfelses;

e) banded textures are widely distributed and are typical of transitional varieties of skarns, and also of skarn-magnetite formations.

A rather interesting and important question, from the standpoint of understanding the entire course of development of metasomatic processes, is the interrelationship of skarns and dike rocks of acidic composition.

In a number of cases (the Margoz and Odinochnoye deposits), dikes of aplites and aplite-like granites cut the skarns; consequently, it is necessary to…

it possible that part of the skarns preceded the dike complex in their formation. However, along with this there are cases (the Rudnyi Kaskad deposit) where aplites and aplite-like granites themselves undergo skarnization, being replaced by such minerals as garnet and pyroxene, which often form small veinlets in aplites and aplite-like granites.

Consequently, one must admit the existence of skarns of two ages: earlier skarns formed before the dike rocks of acidic composition, and later ones—after the intrusion of the dike rocks. It should be noted that these two skarns of different ages are also characterized by different compositions. If the first skarn is characterized by salite, diopside, diopside-salite, and garnet of the grossular–andradite series, then the second skarn is characterized by ferrosalite and garnet of the schorlomite type. Naturally, such a difference is not accidental and reflects certain changes that occurred in the general chain of a single process.

Ore Formation

After the formation of the skarns, intense brecciation took place; moreover, the accumulating stresses were released mainly along the same directions as the pre-skarn zones of brecciation and fracturing. As a result, the latter circumstance led to the joint occurrence of metasomatic hornfelses and skarns with magnetite (a phenomenon of superposition of one mineral association upon another). This brecciation caused the skarns, metasomatic hornfelses, and locally also intrusive rocks (granodiorites, quartz diorites, and aplites) in individual areas to become readily permeable to ore-bearing solutions.

A horizontal plan (Fig. 2) of the Rudnyi Kaskad deposit is an extremely vivid confirmation of this idea. A cursory glance at this plan is sufficient to picture the entire pattern of ore formation under definite structural conditions. As is evident from the plan, all ore bodies are confined to zones of brecciation and fracturing, predominantly of northeastern strike.

The principal forms of magnetite concentration are as follows: (a) disseminations, more or less uniformly distributed; (b) nests and lenses; (c) crosscutting veins and veinlets.

The time of formation of the ore deposits is also associated with phenomena of wall-rock alteration, which were expressed in the formation of insignificant rims of vermiculite.

Later magnetite mineralization at the deposits is replaced by sulfide mineralization, which was also preceded by the widespread development of fracturing. The most widespread sulfide minerals are pyrite and chalcopyrite; pyrrhotite, sphalerite, and galena occur more rarely.

The development of silicified rocks, recrystallization of limestones, and formation of metasomatic hornfelses, skarns, and ores—all this constitutes an intermittent-continuous series, having been caused by one and the same action, i.e., the transfer of heat and matter from the intrusive body into higher parts of the Earth’s crust that were in contact with the granodiorites.

Paragenetic Associations, Sequence of Mineral Formation, and Questions of Chemistry

In analyzing the mineralogical composition of the skarns of the iron-ore deposits, we sought to reflect the principal regularities in the sequence of mineral formation, which we have shown in Fig. 3.

Fig. 2

Mineral names

1. Quartz I
2. Biotite
3. Pyroxene
4. Garnet
5. Epidote
6. Zircon
7. Apatite
8. Rutile
9. Chlorite
10. Quartz II
11. Vermiculite
12. Ordinary hornblende
13. Actinolite
14. Magnetite
15. Pyrrhotite
16. Pyrite
17. Galena
18. Sphalerite
19. Chalcopyrite
20. Calcite
21. Chalcosine
22. Covellite
23. Hematite
24. Limonite

Hypogene epoch

• Stage of metasomatic aureoles
• Skarn stage proper
• Magnetite stage
• Epidote–quartz–sulfide stage

Vertical annotations within the hypogene epoch:

• Composition
• Granular porphyroblastic composition

Hypergene epoch

Vertical annotations:

• Overgrowth
• Along fissures

Fig. 3. Paragenetic scheme of mineral formation at the iron-ore deposits of the Krasnokamensk Group.

Table 3

This allows us to form a more complete idea of the sequence of the process of mineral formation at the iron-ore deposits of the group.

The interrelations of minerals deposited in one or another epoch of mineral formation show that these minerals were not always deposited in strictly successive order, but more often overlapped one another in time, forming a number of generations.

In determining the relative age of two minerals, only the intersection by veinlets and clear corrosion of one mineral by another were taken into account; in some cases, inclusions of one mineral in another.

Analyzing the scheme of mineral formation presented, we see that at the beginning of the hypogene epoch aluminosilicates were deposited, and at the skarn-formation stage the formation of dike rocks also took place. Toward the end of the hypogene epoch the composition of the mineral-forming solutions changes sharply, and a large number of diverse minerals are deposited, including magnetite, which is the principal useful mineral.

The associations of minerals deposited after the skarn stage proper make it possible to distinguish, following it, two more stages: magnetite-

Table 4

tous and epidote-quartz-sulfide. All characteristic paragenetic associations of minerals at the various stages have been shown by us in Fig. 3.

In conclusion it is necessary to dwell on the most important question: the changes that occur during the formation of skarn in the second, properly skarn, stage, and to attempt to give a qualitative, and where possible also a quantitative, characterization of these changes. For this purpose we used T. Barth’s method of recalculating chemical analyses for the quantitative accounting of the movement of substances during metasomatic processes.

The chemical analyses of intrusive rocks, skarns, and rocks in whose place the skarns are formed are given by us in Table 3.

Having the formulas of all rocks that took part in the processes of contact metasomatism (Table 4), we can visualize the picture of introduction and removal of elements for each deposit of the group, which can be clearly represented in the form of graphs (Figs. 4, 5, 6).

Fig. 4. Course of change in rock composition during contact metamorphism at the Odinochnoye deposit

Analyzing the above formulas of the rocks and the diagrams, we come to the conclusion that:

1) during the formation of skarns at the Odinochnoe deposit, Ca, Mg, Mn, Fe, and Al were introduced;

Fig. 5. Course of change in rock composition during contact metamorphism at the Margoz deposit.

2) during the formation of skarns at the Margoz deposit, Mg, Mn, Fe, Al, Si, and an extremely small amount of Ca were introduced;

3) during the formation of skarns at the Rudnyi Kaskad deposit, Ca, Mg, Mn, and Fe were introduced.

It should probably be assumed that the introduction of Si and Al for the Rudnyi Kaskad deposit, and of Si for the Odinochnoe deposit, more or less compensates for removal.

Thus, not only Fe, H₂O, CO₂, and Mg possess significant mobility, as D. S. Korzhinskii (1948) believes, but also Ca, Al, and Si.

In addition to the listed components, the following elements have been detected in the composition of the skarns: Ge, Cu, Pb, As, Ga, Ni, Ti, Mo, Co, Zn, Na, Be, Sn, Zr.

A characterization of introduction and removal during the subsequent, post-skarn stages of mineralization, on the basis of the available materials, can for the time being be made only in the most general terms and only from a qualitative

side. The composition of the mineral associations of the magnetite and epidote–quartz–sulfide stages indicates the introduction and accumulation of F, Cl, Zr, Ga, Ba, Mo, Co, Zn, Cs, Nb, V, Be, Ge, Cu, Ni, Pb, Sn, As, and also S. The listed elements are not part of the rocks in place of which the skarns formed, and therefore their introduction is beyond doubt. In addition, such elements as Ca, Fe, Si, H, O, P, Ti, Mn, Na, Mg, and Al also participated in the formation of the minerals of these stages.

Fig. 6. Course of rock alteration during contact metamorphism at the Rudnyi Kaskad deposit.

It follows from the foregoing that the material introduced into the zone of contact metamorphism is characterized by rather great diversity (Table 5).

Table 5

As is evident from the table, the elements of the upper and left lower fields predominate. Such lithophile elements as Si, Al, Ca, Mn, Mg, and others play a very large role, being widely distributed as constituents of the principal skarn-forming minerals and of most minerals of the magnetite and epidote–quartz–sulfide stages.

Among the chalcophile, siderophile, and atmophile elements, S, Cu, Zn, Fe, and Mg are the most widespread; the remaining elements play a very minor role, being rare in most cases. These include: Be, F, Cl, Ti, V, Zr, Nb, Ba, Ge, Ga, Se, Sn, Pb, P, As, Co, Ni, Mo. These elements, in one quantity or another, enter into the composition of various minerals corresponding to one or another stage of mineralization.

All the material presented above gives us the necessary information on the basis of which we may speak of the contact-metasomatic genesis of the skarn deposits. In this connection it is necessary to note the following.

During the formation of the Odinochnoye deposit, postmagmatic solutions ascended along a fracture zone in quartz keratophyres, which possessed a certain volume of open space. Such a setting conditioned the linearly elongated, in plan, body of skarn-ore formations.

During the formation of the Margoz deposit, postmagmatic solutions ascended along the contact zone of a limestone remnant with an intrusion. In this case the contact zone represented a favorable place for fracturing during the adjustment of the rocks to their new position, and to the temperature difference between the magma and the sedimentary rocks; recrystallization of the sedimentary rocks and their mutual adjustment could also have further promoted fracturing. In plan, the form of the skarn-ore fields fully corresponds to the form of the contact and is characterized by the presence of swellings, constrictions, and apophyses.

During the formation of the Rudny Kaskad deposit, postmagmatic solutions ascended along a series of fractures and zones of crushing both in roof remnants and xenoliths, and in intrusive rocks, more rarely along the contact zone of the latter. Depending on the morphology of the enclosing structural elements, vein-like and nest-like skarn-ore bodies are formed.

All these circumstances indicate that, in terms of their character and the mechanism of movement of postmagmatic solutions, these deposits “in essence do not differ from the mechanism of formation of lower-temperature hydrothermal deposits” (F. I. Wolfson, 1952).

L. C. Graton and E. F. Ziv also emphasized that it is time openly to regard contact-metamorphic (skarn) deposits, which form at shallow depth, as a separate member of the hydrothermal group.

It should also be noted here that the theory of bimetasomatism is of little use for explaining the factual data cited above, and D. S. Korzhinskii himself cannot deny that “usually the most valuable are contact-infiltration skarns associated with the most powerful action of postmagmatic solutions in fractured zones” (D. S. Korzhinskii, 1953).

Thus, the formation of metasomatic skarn deposits is, to a considerable extent, a problem of the formation of skarn-forming solutions.

References

1. Volfson, F. I. Problems in the study of hydrothermal deposits. Moscow, Publishing House of the Academy of Sciences of the USSR, 1952.

2. Glanz, L. Iron ores of Algeria, “Iron-Ore Deposits of the World,” vol. I, 1955.

3. Greiton, L. S. The nature of ore-forming fluid, 1946.

4. Zavaritskii, A. N. Mount Magnitnaya and its iron-ore deposits. Trudy Geolkoma, issue 122, 1927.

5. Ziv, E. F. Scheelite mineralization of the skarns of the eastern slope of the Kuznetsk Alatau. Trudy Vsesoyuznogo issled. in-ta min. syr’ya, issue 145, 1939.

6. Korzhinskii, D. S. Petrology of the Tur’insk skarn copper deposits. Trudy In-ta geol. nauk AN SSSR, issue 68, series of ore deposits, no. 10, 1948.

7. Korzhinskii, D. S. An outline of metasomatic processes. Principal problems in the doctrine of magmatogenic ore deposits. Publishing House of the Academy of Sciences of the USSR, 1953.

8. Ozerov, K. N. Current problems in the petrology of secondary quartzites. Izv. Kaz. FAN, geological series, issue 2–3, 1944.

9. Chetverikov, S. D. A guide to petrochemical recalculations, 1955.

10. Shakhov, F. N. On the theory of contact deposits. Trudy Gorno-geol. in-ta, ZSF AN SSSR, issue I, 1947.

Corrections and Noted Misprints

In S. A. Stroitelev’s article “Investigation of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.