PROCEEDINGS
of the Tomsk Order of the Red Banner of Labor Polytechnic Institute
named after S. M. Kirov

Vol. 151　　　　　　　　　　　　　　　　　　　　　　　　　　　　1966

ON THE COMPOSITION OF THE IRON-BEARING MAGMATIC COMPLEX IN THE AREA OF THE TASHTAGOL MINE

G. M. IVANOVA

(Presented by the scientific seminar of the Department of Petrography)

The area of the magmatic complex under consideration is located in the territory of Gornaya Shoriya, in the upper reaches of the Kondoma River. The iron-ore deposits of the district, being contact-metasomatic formations, are closely confined to the outcrops of the syenites of this complex. For this reason, in order to study the structure of the deposits, an investigation was carried out of the geological structure of the area and of the occurrence of syenite outcrops, which in general are poorly exposed. The earliest investigations of the district belong to M. A. Usov, I. A. Molchanov, I. K. Bazhenov, Yu. A. Asanov, A. S. Mukhin, and K. V. Radugin.

Detailed geological mapping and the study of the deposits were carried out by K. V. Radugin, A. I. Skomorokhov, G. L. Pospelov, S. S. Lapin, V. G. Korel, A. G. Volodin, V. I. Kaminskaya, F. D. Zeitsev, and others. The syenites of the Kondoma district attracted the attention of many investigators, and many chemical analyses of the rocks were performed; however, not a single petrographic map of the syenite massifs had been compiled. Until now the suite of rocks of the complex had not been shown, nor had the affiliation to it of the basic rocks of the gabbro series been proven. Metasomatic formations in the field of the syenite intrusion, apart from the iron-bearing skarns, had likewise been studied very poorly up to the present. All this has not made it possible to draw a clear picture of the conditions under which the complex formed, or to clarify the genetic connection with it of the ore-forming processes.

In 1963–1964, petrographic investigations were carried out by the author in this area, and the geological-petrographic structure of the Tashtagol syenite massif was detailed.

The enclosing rocks of the massif are metamorphic schists, tuffs, and effusive rocks of intermediate and basic composition; limestones of the Kondoma series of the Lower Cambrian; and, to a lesser degree, limestones, sandstones, albitophyres, and their tuffs of the Mundybash series of the Middle Cambrian. The deposits of these series compose the Kondoma graben-syncline, bounded on the east and west by major disjunctive dislocations from the strata of the Lower Cambrian and Upper Proterozoic. This structural zone bears signs of prolonged development and is accompanied by diverse magmatic formations.

At the beginning of the Lower Devonian, within the Kondoma graben-syncline, simultaneously with the formation of the Kuznetsk depression, northeast-trending reticulate-fracture zones arose, characterized in a number of works by G. L. Pospelov.

In plan view, the Tashtagol syenite massif is represented by a system of narrow, elongated apophysis-like bodies, parallel to one another, with a total area of 0.75 km², a maximum length of 2.0 km, and a width of 0.5 km.

Data from core drilling show that, in vertical section, the syenites form a series of dike-like or vein-like offshoots with steep or vertical dip, joining at depth. Their contacts with the enclosing rocks are cross-cutting, but quite often they are concordant with the structure of the country rocks, which indicates the adjustment of the intrusive bodies to the most stressed tectonic structures of the area.

Principal Elements of the Geology and Petrography of the Massif

In studying the intrusive formations in the area of the Tashtagol mine, three groups of rocks are clearly distinguished, which are closely related to one another spatially and, apparently, genetically:

1. Pyroxene and pyroxene–hornblende gabbro-porphyrites.
2. Syenites of several varieties.
3. Dike formations of diabases, diorites, and microsyenite porphyries.

Rocks of the first group usually occur as stocks (0.6–3.9 km²) and are spatially associated with the syenites, often forming a single massif with them. The pyroxene–hornblende and pyroxene gabbro-porphyrites cut through the enclosing sequence and are themselves cut by syenites. Such gabbro-porphyrites within the Tashtagol intrusive massif have been encountered on Mt. Otvesnaya south of the Tashtagol mine and farther south at the headwaters of the Aleksandrovka River, as well as along the Kondoma River. Active contacts of the syenites with the gabbro-porphyrites—locally sharp, locally with uneven assimilation boundaries—are observed on Mt. Otvesnaya and in the underground workings of the Tashtagol mine.

Similar gabbro-porphyrites have also been found within the Shalym and Sheregesh ore fields. In 1963, in the upper reaches of the Men Creek, southwest of the Shalym mine, core-drilling boreholes uncovered a gabbro intrusion in the form of a small body cut by syenites. Occurrences of gabbros have also been noted within the Kochurinsky syenite massif. This spatial association of syenites with gabbro and gabbro-porphyrites, as well as the similarity of the accessory elements present in these rocks and analogous features in the associations of basic and alkaline rocks in neighboring districts, indicate that the intrusions of basic and subsequently alkaline magma are most likely phases of a single magmatic process.

Within the Tashtagol area, among the rocks of the magmatic complex, syenites predominate; their small bodies belong to the hypabyssal facies of depth, as indicated by the uneven grain size of the rocks, porphyritic texture, and a comparatively narrow zone of contact alteration.

In composition and structural features, the syenites may be divided into alkaline and lime-alkaline varieties. In both groups, according to quartz content, quartz-free, quartz-bearing, and quartz syenites are distinguished; by structural features, microsyenites, microsyenite porphyries, syenite porphyries, and medium-grained syenites may be recognized.

The characteristic features of the syenites of the Tashtagol massif are as follows:

Fig. 1. Schematic petrographic map of the Tashtagol syenite massif. The enclosing rocks are shown according to data from the Tashtagol geological exploration party: 1 — alkaline microsyenite-porphyry, quartz-free; 2 — alkaline syenite-porphyry, quartz-free; 3 — alkaline syenite-porphyry, quartz-bearing; 4 — calcareous-alkaline microsyenite-porphyry, quartz-free; 5 — calcareous-alkaline syenite-porphyry, quartz-free; 6 — calcareous-alkaline syenite-porphyry, quartz-bearing; 7 — metamorphic schists (chlorite-albite-carbonate, chlorite-sericite, carbonate-chlorite, etc.); 8 — hornblende-pyroxene and pyroxene-hornblende porphyrites; 9 — conglomerate breccias, sandstones; 10 — diorites, diabases; 11 — ore; 12 — limestones; 13 — skarns; 14 — outlines of the syenite intrusion; 15 — boundaries between petrographic varieties of rocks.

Table 1

1, 2, 3, 4, 5, 6 — pyroxene and pyroxene-hornblende gabbro porphyrites; 7 — gabbro of the pionkinite type; 8, 9 — amphibole syenites; 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 — weakly alkaline and alkaline syenites.

1. The absence in them of dark-colored components. Only in insignificant amounts are varieties observed with pseudomorphs of chlorite and biotite after hornblende, which has very rarely preserved small relicts and the shape of the grains. This indicates the removal of iron from the rocks of the intrusion at the postmagmatic stage.

2. In the northern part of the Tashtagol massif, alkaline syenites have not been encountered at all; they appear farther south in boreholes of exploration lines Nos. 2 and 7. Moreover, alkaline syenites are observed, as a rule, in the upper part of the section, whereas at depth normal lime-alkaline syenites are exposed. The plagioclase content in the lime-alkaline

Fig. 2. Petrochemical diagram of rock compositions of the Tashtagol massif according to A. N. Zavaritskii. Points of chemical compositions correspond to Table 1.

syenites of the Tashtagol massif usually does not exceed 15–20%, and sometimes amounts to only 3–5%. The indicated distribution of syenites is probably connected with more favorable conditions for the accumulation of alkalis within the southern part of the intrusive field. Here there is a suite of rocks from gabbro-porphyrites to normal lime-alkaline syenites, which is not observed in the northern part of the massif.

1. Within the massif a regularity is observed in the change of structural varieties of syenites. At the contacts with the enclosing rocks and in the apical part of the massif, microsyenites, microsyenit-

porphyries and syenite-porphyries, whereas with depth and closer to the central part the porphyritic structure disappears.

1. With increasing distance from the contacts toward the central part of the body, and also with increasing depth, the size of feldspar grains invariably increases.

2. With increasing distance from the contact with the enclosing rocks, the shape of feldspar grains in the syenites changes both in the groundmass and in the porphyritic segregations. Thus, near the contacts elongated-prismatic and columnar grains are observed; with increasing distance from the contact the grain shape gradually becomes short-tabular.

3. Quartz-bearing syenites (the quartz content usually does not exceed 10%, and sometimes is only 3–5%) make up mainly the central part of the body; closer to its periphery quartz occurs more rarely.

Among the dike formations, the most widespread are dikes of diabase, diorite, diabase porphyrite, and microsyenite-porphyry. Their thickness ranges from several centimeters to 1.5–4 m.

All types of dikes may be divided into two groups: the first, more ancient group includes microsyenite-porphyries and diorites, while the second, younger group of dikes includes diabases and diabase porphyrites. Abundant sulfide mineralization is associated with the latter.

Petrochemistry of the Complex

In view of the presence of secondary alterations in the rocks of the complex described, samples for chemical analyses were taken from rocks in which these alterations were absent or insignificant. In the analyses, the content of CO₂ bound in carbonates was taken into account. It proved to be insignificant and ranges from traces to 0.19%.

All rocks of the complex are divided into two groups—the basic gabbro-diabase rocks and the rocks of the lime-alkaline and alkaline syenite series. Rocks of basic composition, petrographically characterized as gabbro-porphyrites, vary in chemical composition from olivine gabbros and diabases close to crinanites, with a value of \(Q = -11\text{–}13\), to rocks approaching quartz diabases (Table 1). If the more basic varieties of the rocks have the same content of feldspathic lime as the closely related intermediate types of olivine diabases according to Daly, then in the more acid varieties a reduced value of this quantity is observed. The most characteristic feature of the rocks consists in the value of \(n\), which, instead of 80 for the above-described intermediate rock types, more often decreases to 50, indicating the high role of potassium among the alkalis.

The syenite group of rocks possesses interesting chemical features. Here we have a continuous series of rocks from lime-alkaline syenites, close to Daly’s average type, to alkaline varieties of rocks, which are the most widespread.

The lime-alkaline syenites of the Tashtagol complex vary in composition from rock types almost coinciding with Daly’s average lime-alkaline syenites, or close to lime-alkaline hornblende syenites, to rocks chemically close to lime-alkaline trachytes, which link them with the alkaline varieties of rocks developed in the area.

Interesting rock varieties are the transitional formations linking the basic and lime-alkaline groups. These are shonkinites, which form an independent body in outcrops along

on the Kondoma River below the town of Tashtagol. They are probably connected with a separate phase of magma intrusion following the formation of the basic rocks. Thus, a relationship is indicated between the basic and alkaline differentiates of the magma as a result of its deep differentiation.

The alkaline varieties of syenites are very distinctive. In their chemical composition they correspond to pulaskites and nordmarkites. Some of them are close to the average alkaline syenite of the world, but a significant number of rocks fall on the diagram above the position of the noted rocks, i.e., in the field of aplite rocks close in their alkaline differences to paisanites. Indeed, their mineralogical composition is often characterized by an almost complete absence of dark-colored minerals. In the rocks a variable content of potassium and sodium is noted, which is connected with the uneven development of later albitization. The entire main mass of rocks belongs to the potassium series and is characterized by a value of “\(n\)” from 58 to 23 and even 7.6. In some rock varieties the content of \(K_2O\), in weight percent, reaches 11.7–13.4% with a \(Na_2O\) content of less than 1%.

The content of alumina, in weight percent, in all syenitic rocks ranges from 17.2 to 19.8, and in isolated cases to 16.7. Some of the rocks, according to calculations by the method of A. N. Zavaritsky, are oversaturated with alumina. This is explained by the development in the rocks of fine sericite. At the same time, in a number of cases the sodium content in such rocks increases relatively, which is connected with albitization.

The syenites of Tashtagol, as Yu. A. Kuznetsov noted earlier, belong to the \(\beta\)-varieties, with a high value of the \(FeO : MgO\) ratios, which indicates their basaltoid origin.

The gabbro-porphyrites described above, in our opinion, belong to an early phase in the formation of the Tashtagol magmatic complex. This is indicated not only by their spatial relationship, but also by the peculiarity of their chemistry in the form of the relatively high role of potassium among the alkalies, as well as by the similarity in the composition of rare and dispersed elements.

Conclusion

The Tashtagol gabbro-syenite magmatic complex, with which the iron mineralization of the district is associated, developed in a part of the region of completed folding that split, as a result of the development of large faults, into graben-synclines and horst-anticlines. The outcrops of intrusive rocks are elongated as a belt of northeastern strike within the narrow Kondoma graben-syncline. The features noted above of the petrogenetic similarity of the basic gabbroid and syenitoid rocks suggest that the formation of the magmatic complex is connected with the intrusion of basaltoid magma having a trachytic trend of differentiation. An indication of this is the presence of rocks of intermediate composition in the form of shonkinites, associated with magma of trachyandesitic composition.

The facies features of the complex are characterized by near-surface conditions during the intrusion of magma along narrow fissure zones. As a result of the comparatively rapid cooling of the magma, chilled zones are well expressed in the form of fine-grained rocks. Zones of alkaline replacement of rocks in the form of potassium metasomatism, grading into magmatic replacement, are widely manifested in the area.

During the replacement of basic rocks, as S. S. Ilyenok (1960) noted earlier, the iron released during the decomposition of silicates and oxides is discharged into the contact zone and goes into the formation of metasomatic ore occurrences.