V. G. KOREL

CONTACT METAMORPHISM OF THE SHEREGESH IRON-ORE DEPOSIT IN GORNAYA SHORIYA

The Sheregesh deposit is the northernmost among the contact deposits of the Kondoma group. The geological structure of the area of this deposit is based on an effusive-sedimentary sequence, which includes albitophyres, their tuffs, tuffogenic sedimentary rocks, and limestones. In the lower parts of the sequence, pyroxene porphyrites occur, corresponding in chemical composition to rocks of the gabbro group. In the opinion of K. V. Radugin, the albitophyre sequence is Cambrian in age and correlates well with the pyrogenic-sedimentary Pecherkino formation of the Salair.

The Cambrian sequence is cut by an intrusion of syenites. A close spatial connection of all the Kondoma iron-ore deposits is established with the bodies of this intrusion. In turn, at the Sheregesh deposit the syenites are metamorphosed by granites of the Sarlyk massif.

Mineralization at the Sheregesh deposit is confined to tuff-sedimentary rocks enriched in carbonate material. It is represented by lenses of dense granular magnetites and skarn rocks. Among the skarns, pyroxene and garnet skarns are substantially developed, containing spinel, magnetite, amphibole, scapolite, zoisite, clinozoisite, and epidote in their composition. Owing to the presence of these minerals, the skarns form a number of intermediate varieties.

The country rocks (including the syenites) enclosing the ore bodies and skarns are subjected to hornfelsing and skarnification. Biotite, amphibole, augite-andradite-orthoclase or augite-andradite-plagioclase hornfelses and amphibolites are common.

Previously the Kondoma deposits were genetically connected with bodies of syenites, which were considered to be a differentiate of the Pustag and Sarlyk granite intrusions that had assimilated a large amount of carbonate material in the marginal part.

N. A. Batov [1] assigned the syenite intrusion to the Cambrian, while he synchronized the granites with the adamelites of the Telbes area, to which M. A. Usov [12] ascribed a Devonian age and with which he connected the iron-ore deposits. By analogy with this area, N. A. Batov connected the Kondoma iron-ore deposits with the granites of the Sarlyk and Pustag massifs. This investigator did not notice that, together with the syenites, the skarns and ores of the Sheregesh deposit are also subjected to contact metamorphism from the granites. After the work of N. A. Batov, the genetic connection of the Kondoma deposits with intrusions became uncertain, since it met with objections from some other investigators of this region [8].

The idea that the deposits of the Kondoma group were metamorphosed by a younger granite intrusion is found in the work of Mir-Ali Kashkai [5]. This author writes that the southern deposits “... were altered by the Pustag intrusion and accom-

by the orogenic movements that awaited it; namely, in the Tashtagol deposit this may have manifested itself in the formation of certain minerals—quartz and calcite—cementing the grains of high-temperature minerals—garnet and epidote.

The investigations carried out on the skarns and ores of the Tashtagol and Kochura deposits convince us that silicification and carbonatization constitute a single process with skarn and ore formation and appear at the final stage of this process. Therefore, the comparatively high carbonateness and siliceousness of the skarns and ores of Tashtagol and Kochura cannot serve as evidence that these deposits were contact-metamorphosed, especially since these deposits occur in non-hornfelsic rocks and at a considerable distance from the granite contact.

The study of the Kondoma deposits shows that contact metamorphism of these deposits did indeed take place, but it manifested itself only in the skarns and ores of the Sheregesh deposit, as the one situated closest to the granite intrusion. It was expressed in phenomena of recrystallization and in the emergence at this deposit of a complex of contact-metamorphic minerals.

First of all, the strong amphibolization of the skarns and country rocks of the Sheregesh deposit attracts attention. Characterizing the exocontact zone of the adamelite intrusion of the Telbes district, M. A. Usov [11] comes to the conclusion that “amphibolization and epidotization are the principal types of contact metamorphism—metasomatism caused by this intrusion. The amphibole described,” the author indicates, “is not the result of simple recrystallization of the enclosing rock. This situation is especially clearly manifested, for example, in felsitic porphyries and albitophyres; we see how the aggregate scattering of amphibole corrodes the microfelsitic groundmass of the effusives, penetrating mainly along clear zones of crumpling, but sometimes getting there even without the aid of visible cracks. At times amphibole also penetrates into the phenocrysts of feldspar, replacing it in clusters.”

Not only effusives and syenites, but also skarned rocks, various skarns, and magnetite ores undergo amphibolization of this kind at the Sheregesh deposit. In pyroxene skarns, amphibole occurs as thin, closely interlacing veinlets, penetrating along random cracks, cleavage cracks, and grain boundaries of pyroxene, and as accumulations. In the early stages pyroxene is preserved in the cores of newly formed idioblasts of amphibole, and when it is completely replaced, dark viscous rocks—amphibolites—arise in place of the pyroxene skarn; in them it is very rarely possible to see fragments of non-amphibolized pyroxene. Since analogous amphibolites are formed as a result of metamorphism by granites of pyroxene porphyrites, syenites, and effusives, it is very difficult, and for the most part impossible, to decide at the expense of which original rock a given amphibolite arose. In purely garnet skarns, as well as in all intermediate types containing garnet in their composition, amphibole develops along numerous fine cracks in garnet and pyroxene or forms small aggregate accumulations. Often, along with amphibolization, garnet decomposes into zoisite and clinozoisite, so that in place of the skarn amphibole-zoisite rocks are formed, in which only here and there are found portions of garnet that have escaped decomposition.

In all the cases considered, amphibole occurs in columnar and hexagonal crystals 0.05–0.4 mm in size, possessing uneven end faces, and in radiating aggregates. Measurements of a number of crystals of this amphibole established the following optical constants. Pleochroism: Ng—green, Nm—light, grass-green, Np—light yellow, \(2V = (-)84–88^\circ\),

angle with Ng = 20—25°, Ng—Np = 0.016. On the basis of these constants, the amphibole may be assigned to the series of sodium-iron hornblendes of the hastingsite type (after Larsen and Berman). The amphibole is closely associated with biotite, sphene, titanomagnetite and, both in optical properties and in association, differs little from the metamorphic amphibole that develops in the hornfelsed granites of the country rocks.

That the amphibolization of pyroxene is a contact-metamorphic process is indicated by a whole series of investigators. Textbooks of petrography and certain special works [10] give examples of such amphibolization of the pyroxene of basic rocks.

Biotitization of skarns and ores is expressed less intensely. Biotite is encountered in skarns and ores considerably more rarely than amphibole. This mineral appears as small (0.1 mm) plates, which form short veinlets, but most often are grouped into continuous areas characterized by a drusy or hornfels-like structure. Biotite is xenomorphic with respect to hornblende and usually fills the spaces between its crystals. At the same time it is idiomorphic with respect to sphene and titanomagnetite, the grains of which are situated between the biotite plates. Biotite develops in amphibole, garnet, and pyroxene. Rare plates of biotite occur in scapolite.

At the Tashelga iron-ore deposit (Gornaya Shoria), whose geological setting above all suggests contact metamorphism of this deposit by younger intrusions, this mineral attains such importance that P. I. Lebedev and V. A. Moleva [7] distinguish at this deposit a biotite-garnet and a biotite-magnetite-garnet type of skarns. These authors show that biotite also formed after hornblende and gives “small veinlike segregations” in the skarns.

Macroscopically, fine-crystalline aggregates of biotite are black in color. Under the microscope, biotite has sharp pleochroism from colorless, slightly greenish to deep green or greenish-brown. The birefringence of the mineral is on the order of 0.050. A chemical analysis of the biotite of the skarns of the Tashelga deposit, given in the cited work of Lebedev and Moleva, shows in it a content of 38.6% aluminosilicate and 50.9% orthosilicate constituents, with 10.5% residue, apparently attributable to water.

Zoisite and clinozoisite arise especially readily during the alteration of garnet skarns. They appear as irregular areas in the field of garnet or garnet-pyroxene rock, in close association with biotite and amphibole. Often in plane-polarized light it is quite impossible to distinguish zoisite from garnet, but under crossed nicols the areas composed of zoisite stand out sharply because of their anomalous interference colors. Both minerals are found only at Sheregesh.

As a result of the formation of the minerals listed, amphibolized pyroxene, amphibolized garnet, biotitized pyroxene, biotitized garnet, zoisitized garnet rocks, and amphibolites develop; these should be regarded as contact-metamorphosed garnet, pyroxene, and other skarns.

The garnet skarns of the Sheregesh deposit differ strongly from the skarns of other deposits in that they have now a compact, now a granular appearance. Compact skarns look like massive garnet rocks with a characteristic conchoidal or hornfels-like fracture. They gradually pass into areas of granular reddish-brown garnet skarn or zoisite rock. Often the reddish-brown garnet forms thin veinlets cutting both the main mass of ore and skarns and the country rocks.

To the conclusion that the reddish-brown garnet of the Sheregesh deposit is metamorphic, having arisen owing to the recrystallization of gra-

the primary skarns, we arrived from the study of the relationships of dike rocks associated with the granite intrusion to the skarns. The point is that dikes of aplite-pegmatites and feldspathic veinlets, in those cases when they cut the skarns, cause recrystallization of the skarn within the contact zone. In this process, the light-yellow (grossular-andradite) garnet nearer the veinlet acquires a dense yellowish-brown, reddish coloration and is transformed into polygonal and irregular small (0.05 mm) grains forming a narrow (0.5 mm) band along the given veinlet. A large part of the grains is turned toward the veinlet with well-formed faces and is idiomorphic with respect to the feldspars composing it. This garnet is entirely analogous to the garnet cutting the main mass of the skarns and magnetite ores, and also to the garnet of the granular reddish-brown skarns. The garnet of the veinlets has a darker color than the garnet of the main mass of the skarn. We regard this garnet, by analogy with the garnet developing at the contact of skarns with dikes, as the result of recrystallization of the skarn caused by intrusion of granites.

The frequently observed close association of reddish-brown garnet with magnetite, pyroxene, zoisite and clinozoisite, epidote, pyrite, and especially with fahlore makes it possible to assert that the listed minerals in this case are also metamorphic, recrystallized. This considerably complicates clarification of the normal sequence of deposition of the skarn minerals. Thus, there are cases where pyroxene veinlets cut magnetite ores, which gave N. A. Batova [1] grounds to consider this mineral later than garnet and magnetite. Further, cases are noted of cementation of fragments of magnetite ore by reddish-brown garnet. On the other hand, magnetite veinlets cut aggregates of reddish-brown garnet, or magnetite is deposited in the interstices between its grains.

Furthermore, the Sheregesh deposit is characterized by the presence of a large amount of pyrrhotite. This mineral is developed in the ores and skarns of the Sheregesh deposit and is absent at Shalym, Tashtagol, and Kochura. Pyrrhotite develops after pyrite and magnetite, penetrating these minerals along fine cracks. Sometimes it forms pseudomorphs after them. Pyrrhotite is always xenomorphic with respect to pyrite, magnetite, and chalcopyrite. The average size of pyrrhotite grains is 0.4–0.6 mm.

It is known that pyrrhotite can form from pyrite when the latter is heated (without access of air) to a temperature of 575°. Stevenson [14] described typical pseudomorphs of pyrrhotite after pyrite crystals in shales that had undergone strong heating at the contact with a dike of igneous rock. In all probability, the Sheregesh ores, like the ores of the other deposits of the group under consideration, contained only pyrite, while pyrrhotite arose in them owing to the contact metamorphism of this deposit by the granite intrusion.

In addition to those listed, there are also certain other facts indicating contact metamorphism of the Sheregesh deposit. For example, the ores of the Sheregesh deposit, in contrast to the ores of Tashtagol and Kochura, are recrystallized, dense or fine-grained, weakly martitized magnetites. Under the microscope the ore aggregate has a heteroblastic or allotriomorphic structure, and since differences in the size of magnetite grains for this deposit are usually insignificant, it is possible to speak of a granoblastic structure. It should also be added here that the pyroxene of the skarns and skarnized rocks (including syenites) often undergoes cataclastic granulation [3], breaking down into an aggregate of the finest grains of the very same composition.

The slight abundance of hematite at this deposit is connected with the fact that the ores of this deposit, compacted as a result of

of contact metamorphism, naturally underwent martitization with greater difficulty. On the other hand, hypogene hematite, which formed insignificant inclusions in quartz, may, during metamorphism, have been transformed into magnetite or have entered into interaction with other minerals or chemical elements present in the solution, with the formation of more complex compounds.

Further, attention is drawn to the almost complete absence of optically anomalous garnets and zonal magnetites at the Sheregesh deposit and to the wide development of these properties in the garnets and magnetites of Tashtagol and Kochura. If zonal magnetites do occur at the Sheregesh deposit, then the zonal structure is manifested in them by an alternation of fewer and more regular, broader bands with indistinct boundaries. The origin of optical anomalies is explained by isomorphous admixture during the layer-by-layer deposition of foreign ions in the lattice of the “host crystal,” changing the value of the specific volume of the substance for each layer, as a result of which internal stresses arise, altering the optics of the crystal [13]. S. I. Ivanov [4], using the example of sulfides of the polymetallic deposits of the Urals, showed that the disappearance of sulfide zonality is connected with phenomena of contact metamorphism and is an expression of the fact that the metamorphosed mineral tends to become purified of impurities, to equalize its chemical composition and its crystal structure [2]. From this point of view, the presence of zonal magnetites and anomalous garnets at Tashtagol and Kochura, and the almost complete absence of such at Sheregesh, may also testify to contact metamorphism of the Sheregesh deposit by a granite intrusion.

It should be noted that all skarns, ores, and country rocks metamorphosed in the manner described above occur adjacent to less altered rocks, skarns, and ores, or among them as separate bands or areas. The spatial arrangement of various types of hornfels and hornfelsed skarns and ores relative to the granite intrusion makes it possible to conclude that hornfels in the contact aureole of the intrusion are distributed extremely unevenly. This unevenness in the distribution of hornfelsed rocks, skarns, and ores, appearing both on a large and on a small scale (the greater part of the hornfels has a taxitic appearance), is apparently due to the fact that hornfelsing of the rocks proceeded not only under the action of heat, but also under the action of directed streams of emanations facilitating recrystallization. Therefore those authors [6] are right who say that for metamorphism a change in temperature alone is insufficient. “A characteristic feature of any metamorphism of rocks is the appearance of solutions and their passage through separate parts of the rock” [9]. The appearance of a large amount of sphene and apatite in the hornfels indicates that the emanations were enriched in phosphorus, titanium, and iron. In general, for the entire deposit a natural regularity is observed, consisting in the fact that the hornfelsed skarns and ores occur in that part of the deposit which is situated closest to the contact of the granite intrusion.

Summarizing the material presented concerning contact metamorphism of the Sheregesh deposit, we arrive at the following conclusions.

1. The siltstones, skarns, and ore bodies of the Sheregesh deposit are subjected to contact metamorphism from the side of the granites of the Sarlyk massif.

2. Contact metamorphism of the Sheregesh deposit manifested itself in the recrystallization of skarns and ores or in the formation within them of a complex of contact-pneumatolytic and metamorphic minerals. This metamorphism proceeded with the participation of solutions, owing to which the hornfelsed rocks, skarns, and ores are characterized by an uneven distribution.

1. The source of skarnization and ore formation is the syenite intrusion, and not the granite intrusion, as N. A. Batov supposed.

REFERENCES

1. Batov N. A. Geology and minerageny of the iron-ore deposits of the Kondoma group. Materials on the Geology of the West Siberian Territory, issue 18, Tomsk, 1935.

2. Grubenman and Niggli. Metamorphism of Rocks. Geolrazvedizdat, 1933.

3. Zavaritskii A. N. Mount Magnitnaya and its iron-ore deposits. Transactions of the Geological Committee, new series, issue 122, 1927.

4. Ivanov S. I. Study of pyrite grain growth zones in pyritic deposits of the Urals. Proceedings of the All-Union Mineralogical Society, 79, issue 2, 1950.

5. Kashkai Mir-Ali. On the mineralogy of the ore processes of the Kondoma group of iron-ore deposits. Materials on the Petrography and Geochemistry of the Kuznetsk Alatau and Altai, part 1, 1934.

6. Korzhinskii D. S. Equilibrium factors in metasomatism. Proceedings of the Academy of Sciences of the USSR, geological series, No. 3, 1950.

7. Lebedev P. I. and Moleva V. A. On the mineralogy of contact-metasomatic processes of the Tashelga iron-ore deposit (Gornaya Shoria), Transactions of Petrin, issue 6, 1934.

8. Molchanov I. A. On the age of the ore-bearing intrusions of Gornaya Shoria and the iron-ore deposits associated with them. Bulletin of the West Siberian State University, No. 4, 1941.

9. Niggli. Magma and Its Products, part 1, Gosgeolizdat, 1946.

10. Sokolov G. A. Alkaline metasomatism in the amphibolites of Mount Verblyuzhya. Transactions of Petrin, issue 6, 1934.

11. Usov M. A. The Telbes iron-ore district. Proceedings of the West Siberian Department of the Geological Committee, vol. VI, issue V, Tomsk, 1927.

12. Usov M. A. Phases and cycles of tectogenesis of the West Siberian territory, Tomsk, 1936.

13. Brauns R. Die optischen Anomalien der Kristalle. Preisscher Jablonowski ges., Leipzig, 1896.

14. Stevenson. Economic Geology, No. 3, 1937.