SOME FEATURES OF THE GEOLOGICAL STRUCTURE AND COMPOSITION OF THE MUSTAG PLUTON (MOUNTAIN SHORIA)

E. I. LISENKOV

(Presented by the scientific seminar of the departments of mineralogy, petrography, and mineral resources)

In the Kondoma iron-ore district of Mountain Shoria, granitoids of the so-called Mustag intrusive body are widely developed. At various times the intrusive body was visited by A. M. Kuzmin [7], Yu. A. Kuznetsov [6], N. A. Batov [1], A. G. Volodin [3], V. I. Kaminskaya [4], and a number of other geologists, whose works contain only brief information on the Mustag pluton. The presence, in the exocontact zone of the northern part of the intrusive body, of ore occurrences of piezo-optical raw material, as well as indications of a genetic connection between polymetallic mineralization and granitoids [2], naturally increase interest in a more detailed investigation of the intrusive body.

The Kondoma district is characterized by a very complex geological structure, which is due to the position of the district at the boundary of two major structures—the Biya massif and the Kondoma–Lebed’ mobile zone—separated by a regional northeast-trending fault [5, 4]. The principal structural elements of the district are the Spasskaya anticline (within the eastern margin of the Biya massif), composed of rocks of Proterozoic age [8] (marbleized bituminous limestones, quartzites, dolomites, effusives of basic composition), and the Kondoma graben-syncline (the northeastern part of the Kondoma–Lebed’ mobile zone). Within the latter are distributed effusive-sedimentary formations of the Lower and Middle Cambrian, sandy-clayey shales and siltstones of the Ordovician, and tuffs and tuff lavas of presumably Devonian age [4]. The general trend of the main structures is northeastern.

The largest substantially granitoid body in the district, the Mustag massif (area about 290 km²), is confined to the axial part of the Spasskaya anticline; its long axis is elongated in accordance with the main trend of the district’s structures (Fig. 1). Several smaller stock-like satellites of the main intrusive body (Sarlykskii, Ienskii, Verkhnekamenushinskii) are located within the Kondoma graben-syncline.

The Mustag massif appears as a linearly elongated body about 40 km long and 5–10 km wide. Its western contact is generally concordant with the structure of the enclosing rocks and has a steep (80–90°) dip, as evidenced by its almost rectilinear character in plan under conditions of strongly dissected terrain and by the insignificant width of the hornfels zone (tens and first hundreds of meters). A broader

Fig. 1. Schematic geological map of the Mustag pluton (compiled using materials of V. I. Kaminskaya, A. I. Skomorokhov, K. V. Radugin, B. A. Blyuman):
1 — carbonate–silicilite sequence (Pr—Sn?);
2 — metamorphic schists, schistose basic effusives and their tuffs, limestones (Cm₁);
3 — acid effusives and their tuffs, sandstones with limestone interbeds (Cm₂);
4 — siltstones, silty sandstones, quartzites (O);
5 — lithoidal dark tuffs, felsites and tuff lavas (D?);
6 — gabbro-porphyrites, plagioclase–hornblende porphyrites;
7 — syenites and granosyenites;
8 — hornblende gabbro, gabbro-diorites, diorites;
9 — coarse-grained biotite granites, granodiorites;
10 — alaskite and granophyric granites;
11 — main faults.

The contact aureole (locally up to 2 km) is observed along the eastern contact, where the roof of the massif apparently dips considerably more gently. A gentler dip of the surface of the eastern contact is also indicated by its sinuous plan form, the presence in the granitoids of xenoliths of the enclosing rocks, and the confinement of stock-like satellites to the eastern exocontact. The dip angles of the contact directly in outcrops vary from 35 to 70°. The character of the contact is everywhere sharp and intrusive. In direct contact with the enclosing rocks, the granites often acquire a porphyritic structure and a banded texture, expressed in the alternation of leucocratic bands and bands enriched in biotite, 3–5 cm thick. The width of the chilled zone usually does not exceed 0.5–1.0 m.

Contact metamorphism of the country rocks was expressed mainly in their recrystallization. Marbles are formed at the expense of limestones, sometimes containing a minor admixture of diopside and forsterite. Ordovician sandy-shaly deposits and effusive-sedimentary formations of the Cambrian strata have been transformed into various kinds of hornfelses, in whose composition the most widespread minerals are albite, actinolite, epidote, chlorite, biotite, quartz, and, occasionally, diopside and andradite. Diopside-garnet skarns [4] and diopside-wollastonite rocks are encountered extremely rarely in the immediate contact. The metasomatic effect of the granitoids was weak and locally was expressed in the introduction of a small amount of potassium (biotite, K-feldspar), silica, and boron (tourmaline).

In the history of formation of the pluton, a number of successive phases can be distinguished, established by observations of crosscutting relations of rocks in outcrops. The first phase of the intrusive is the most complex in composition and structure. It is represented by various rocks of intermediate-basic composition, of which hornblende gabbros, gabbro-diorites, diorites, and quartz diorites are widely distributed.

As is evident from the available chemical analyses (Tables 1, 2), the rocks of the first phase belong to the compositions of the normal series, and in terms of their silica and alkali contents they encompass varieties of rocks that are saturated and, more rarely, slightly undersaturated with silica and poor in alkalis (after A. N. Zavaritsky). On the petrochemical diagram (Fig. 2), the plotted points of the analyzed rocks are located near the points of the average compositions of gabbro and quartz gabbro (after R. Daly). The more acidic varieties of rocks of the first phase correspond in composition to Daly’s diorite. In comparison with Daly’s average types of gabbro and quartz gabbro, the corresponding rocks of the first intrusive phase are distinguished by elevated contents of feldspathic lime (characteristic “c”) at the expense of alkalis (“a”). The sum of alkalis is usually less than 3.5%, and the ratio of the molecular amounts of potassium and sodium ranges from 0.11 to 0.33.

A petrographic characterization of the rocks of the pluton will be given in a special article. Here we note only that the data available to the author definitely indicate the hybrid character of the rocks of the first phase and make it possible to relate the genesis of the latter to granitic magma contaminated through intensive deep assimilation of carbonate and effusive-sedimentary rocks of the Proterozoic and Cambrian. This conclusion is also confirmed by certain mineralogical features common to the rocks of the first phase and to the granites of the second phase (both contain quartz, potassic-sodic feldspar, biotite, hornblende, and zircon), and by the close spatial association of the gabbro-diorites with the granites of the Mustag pluton (outside the massif, rocks of the first phase are absent).

Table 1

Chemical compositions of rocks of the Mustag intrusion (wt. %)

Note. 1st phase: I—hornblende gabbro-norite, average of 4 analyses; II—hornblende gabbro-diorite, average of 4 analyses; 625—leucocratic gabbro; 517—diorite. 2nd phase: III—coarse-grained biotite granite, average of 21 analyses; 330—hybrid plagiogranite; 696-Г—hybrid plagiogranite; 84-А—hybrid quartz diorite (after V. I. Kalinin); 286—granodiorite. IV—alaskite granite, average of 5 analyses; 520—granite-porphyry dikes; 245-А—aplite; 508—riebeckite granite-porphyry. Analyses were performed in the chemical laboratory of ZSGU, published for the first time.

The second intrusive phase, whose rocks make up about 80% of the area of the Mustag massif and the stock-like satellites, is represented by coarse-grained biotite, more rarely hornblende-biotite microcline granites and by slightly widespread granitoids of increased basicity of the endocontact facies. The granites of

Table 2

Numerical characteristics of the rock compositions of the Mustag intrusion
(after A. N. Zavaritsky)

Note: the numbering of the samples is the same as in Table 1.

this phase are characterized by a high content of alkalies (7.5–8.4%) with a considerable predominance of potassium, a low content of lime (0.4–1.8%) and magnesia (0–1.0%), and a sharp predominance of ferric iron over ferrous iron (Tables 1, 2). The granites under consideration belong to rock compositions oversaturated with aluminum (“a” is usually equal to 25–30). According to the classification of A. N. Zavaritsky, these rocks belong to the second class (silica-oversaturated), to the group rich in alkalies. The figurative points of the compositions of the granites of the second phase lie mainly between the points of the average compositions of granite and alaskite (after R. Daly). From the average types of granites of R. Daly, the granites of the second phase differ markedly in their alumina oversaturation (the gently sloping position of the vectors directed to the left in the plane \(a \leq b\)) and in their lower content of feldspathic lime.

The rocks of the third intrusive phase compose the northwestern part of the Mustag massif and are represented by fine- to medium-grained alaskite granites, granophyric granites, and granite porphyries, for which a miarolitic texture is extremely characteristic. The basis for distinguishing these rocks as an independent phase is the fact that, in the upper reaches of the Kara-Su River, they cut the biotite granites of the second phase. In chemistry, the granitoids of the third phase are, in general, close to the rocks of the second phase and differ from the latter in their more leucocratic composition and smaller amount of magnesia (not more than 0.16%). On the petrochemical diagram the figurative points of the granitoids of the third phase are grouped around the point of the average composition of the alaskite of R. Daly (Fig. 2). The small value of coefficient “c” (0.5–0.8) brings the granitoids under consideration closer to the group of rocks oversaturated with alkalies.

Dike rocks are not very diverse. Most often there occur thin dikes of northeastern or northwestern strike, corresponding in composition and structure to aplites, granite porphyries, and felsites. Much more rarely there occur dike-like bodies with a thickness

up to 50 meters of riebeckite granite porphyries. The chemical characteristics of the dike rocks are represented by chemical analyses 246-A and 508 (Tables 1, 2 and Fig. 2).

Note: point 625 is actually located 5 cm to the left.

Fig. 2. Vector diagram of the chemical compositions of rocks of the Mustag pluton (after A. N. Zavaritskii): 1 — rocks of the first phase of the intrusive; 2 — rocks of the second phase of the intrusive; 3 — rocks of the third phase of the intrusive; 4 — chemical compositions of rocks, after R. Daly.

Thus, analysis of the chemical and petrographic composition and of the spatial distribution of the rocks of the Mustag pluton makes it possible to distinguish three temporally distinct groups of rocks corresponding to successive phases in the formation of the intrusive.

References

1. N. A. Batov. The Kondoma group of iron-ore deposits. In: “Mineral-raw-material base of KMK,” Tomsk, 1933.

2. V. A. Bakhrushev. On the genetic relationships of iron and polymetallic mineralization with intrusions in the Kondoma district of Mountain Shoria. Notes of the All-Union Mineralogical Society, ser. 2, 37, 1958.

3. A. G. Volodin. On the age of the granites of the Mustag and Sarlyk plutons in Mountain Shoria. Proceedings of the Mining-Geological Institute of the West Siberian Branch of the USSR Academy of Sciences, issue 17, 1956.

4. V. I. Kaminskaya. Geological features of the Kondoma iron-ore district. Geology and Geophysics, No. 3, 1961.

5. V. A. Kuznetsov. Geotectonic zoning of the Altai-Sayan folded region. Questions of the Geology of Asia, vol. 1, Moscow, 1954.

6. Yu. A. Kuznetsov. On the intrusions of the Kuznetsk Alatau and their ore potential. Bulletin of ZSGRT, issue 3–4, 1932.

7. A. M. Kuzmin. Geological overview of Mountain Shoria and the district of the Stalin Plant. In: “Mineral-raw-material base of KMK,” ZSGRT Publishing House, Tomsk, 1933.

8. K. V. Radugin. Elements of the stratigraphy and tectonics of Mountain Shoria. Materials on the Geology of the West Siberian Territory, issue 37, 1936.