DEVONIAN MAGMATIC COMPLEX OF THE IRBINSKOYE IRON ORE DEPOSIT AREA

G. V. Polyakov

(Presented by Professor, Doctor Yu. A. Kuznetsov)

The Devonian magmatic complex is widely developed in the territory of the south of Krasnoyarsk Krai, being located chiefly along the eastern margins of the Minusinsk and Chulymo-Yenisei depressions. Possessing distinctive geological and petrographic features, this complex is of particular interest because the largest contact-metasomatic iron ore deposits in the krai are genetically associated with its intrusive representatives.

In the area of the Irbinskoye deposit, the Devonian magmatic complex includes both intrusive and effusive formations.

The effusive formations are represented by extensive sheets that transgressively overlap the strongly dislocated rocks of the Middle Cambrian. The sheets are gathered into broad, gentle monoclinal folds with gently dipping limbs, averaging \(25—30^\circ\), and are thus subordinated to the structures of the normally sedimentary Devonian of the Minusinsk basin.

The effusive sequence is conventionally divided into two parts: a lower one—of andesite porphyrites—and an upper one—of more acidic subalkaline effusive rocks.

The sequence of andesite porphyrites and their tuffs is most widely developed, cropping out in a broad belt of northeastern strike. The extreme points of this belt are the villages of Berezovskoye—Krasny Dar in the south and the villages of Znamenka—Uspenka in the north. In the south the belt extends farther, into the adjoining areas of the Minusinsk basin. The maximum visible thickness of the sequence of andesite porphyrites, determined in the section of the Gryaznushka River (south of the village of Kurskaya), is estimated at approximately 1 km. In petrographic composition, it includes labradorite, augite, and andesine porphyrites, which in a number of cases, under the influence of albitization, are transformed into albitophyres.

The labradorite porphyrites, which crop out chiefly in the section of the Gryaznushka River, are characterized by an exceptionally dense, massive texture and a dark-brown color. The phenocrysts are represented by thin matte-white prisms of plagioclase up to 3 mm long. The groundmass is macroscopically dense, almost aphanitic; microscopically it is hyalopilitic and consists of a fine felt of microlites of basic plagioclase. In the phenocrysts the plagioclase is usually strongly

destroyed. From individual relicts, the composition of plagioclase phenocryst inclusions was established on the Fedorov stage as ranging from No. 50 to No. 60.

The augite porphyrites occurring along the northern slopes of the town of B. Ural very much resemble labradorite porphyrites in external appearance, differing in the presence, in the porphyritic segregations, of small isometric grains of augite. The microstructure of the groundmass is intersertal.

The andesine porphyrites developed in the central part of the belt have a characteristic brownish coloration and a clearly expressed porphyritic, often glomeroporphyritic, structure. Under the microscope the phenocrysts are represented by thin tablets of plagioclase, the composition of which varies from No. 30 to No. 40. The structure of the groundmass is usually microlitic, often passing into allotriomorphic-granular. Such modification of the structure is, as a rule, connected with albitization of the andesine porphyrites, during which the latter are transformed into albitophyres.

Within the andesite porphyrites, in the form of separate thin flows and, possibly, sills, there occur well-crystallized diabases and diabase porphyrites, approaching non-olivine basalts in composition and degree of alteration. Under the microscope the structure is usually porphyritic; the groundmass is diabase. The quantitative mineralogical composition is: plagioclase—54%, pyroxene—21%, actinolite and chlorite—21%, accessory minerals (chiefly ore minerals)—4%. Plagioclase is represented by labradorite (No. 65–70), pyroxene by salite \((2V = 54^\circ,\ Ng = 1.706 \pm 0.002)\).

Stratigraphically and almost invariably hypsometrically higher, absolutely conformable with the andesite porphyrites, lie more acid subalkaline effusives, represented by orthophyres, trachyte porphyres, quartz albitophyres, and keratophyres. Within this sequence, thin flows of andesite porphyrites are observed, compositionally analogous to the porphyrites of the underlying sequence.

Orthophyres are most fully represented in the section of the right bank of the Tuba River between the village of Kuragino and the village of Oikha. In external appearance these are reddish-brown dense rocks, with prominently projecting prismatic phenocrysts of pink potash feldspar, reaching 8 mm in length. The groundmass is orthophyric, but quite often, undergoing silicification and albitization, it is transformed into microfelsitic. The potash feldspar of the porphyritic segregations is represented by cross-hatched microcline \((-2V = 85—87^\circ;\ \perp\ 001 : Nm = 17^\circ)\); in the most recrystallized varieties of orthophyres the microcline is slightly perthitized. Among the orthophyres of the Tuba River and in the area of the village of Berezovka there are varieties of porphyres with a trachyte and bostonite structure of the groundmass, which in most cases is transformed, under the influence of secondary processes—albitization and silicification—into allotriomorphic-granular and microfelsitic. It is probable that, in the course of such changes, most of the quartz keratophyres of the Gryaznushka River, the village of Berezovka, the town of Uspenskaya, etc., were formed.

One more variety of effusives of the Irba area should also be noted. In the porphyrite and orthophyre sequences there are amygdaloidal varieties; in the first case they are represented by porphyrites with amygdules of chlorite, epidote, and calcite, and in the second by trachyte porphyres and albitophyres with amygdules of quartz and albite.

Thus, the composition of the rocks of the entire effusive sequence varies over a wide range, from diabases to trachytes; moreover, the earliest eruptions are characterized by a predominance of basic varieties, close to diabases, whereas in the composition of the latest eruptions in time—

in the outcrops, acid effusives of the type of trachyte porphyries and quartz keratophyres predominate.

The age of the effusive sequence is Lower Devonian. As evidence, we cite the following data.

1. Judging from the character of its occurrence, this entire sequence is subordinate to the gentle, gently sloping folded structures of the sedimentary Devonian of the Minusinsk Basin.

2. In its geological structure and material composition the sequence is very close to the so-called Chilan Formation, whose Lower Devonian age has been proven (“Field Atlas of the Fauna and Flora of the Devonian Deposits of the Minusinsk Basin,” 1955).

3. In the section along the Gryaznushka River, among the effusives, a small, thin lens of yellowish-gray polymict sandstone was encountered, with poorly preserved remains of ostracods, presumably of Lower Devonian age.

At the same time, it should be noted that there have been attempts to assign such effusives to the Abakan Formation, whose age is determined as the Givetian stage of the Middle Devonian. Thus, in the “Field Atlas of the Fauna and Flora of the Devonian Deposits of the Minusinsk Basin” (1955), the following is stated: “A considerable part of the so-called effusive border of the Minusinsk Basin is provisionally assigned to the Abakan Formation, in particular the porphyrites, which occupy rather large areas in the northeastern part, where they are intruded by an alkaline-magma intrusion. In the opinion of some investigators, these porphyrites are older—Lower Devonian, corresponding to the Chilan Formation; only the red-colored rocks overlying them, possibly unconformably on a basal conglomerate, should be assigned to the Abakan Formation.” As can be seen, we adhere to the second point of view.

The Lower Devonian effusives are distinctly intruded by intrusive representatives of the magmatic complex. Intrusive formations in the area are represented by two plutons: the small, zonally structured Sidorovskiy Ural pluton and the Irbinsky massif, which is more considerable in size. At the contacts of the Irbinsky massif with the ancient carbonate sequence of the Middle Cambrian lies the Irbinskaya group of iron-ore deposits. The plutons formed in three stages: in the first stage, quartz alkaline syenites, close to nordmarkites, were formed; in the second—alkaline granosyenites and alkaline granites; and in the third—numerous dike rocks: alkaline granite-porphyries, syenite-porphyries, alkaline syenite-aplites, albitite-porphyries, and, finally, various porphyrites approaching trachyandesites and even trachybasalts in chemical composition. In the Sidorovskiy Ural pluton, alkaline syenites clearly predominate, whereas the quartz varieties occupy the smaller, central part of the body. The Irbinsky massif, on the contrary, is represented mainly by quartz varieties, among which alkaline riebeckite granites attain considerable development. In the endocontacts of the Irbinsky massif, very diverse hybrid rocks occur (syenodiorites, diopside syenites, quartz diorites, diorites, gabbro-syenites, and gabbro-diorites), formed as a result of assimilation by syenitic magma of the surrounding rocks, more basic in composition.

The nordmarkites of the Sidorovskiy pluton are divided into amphibole and biotite-amphibole varieties. Both varieties have approximately the same distribution and a very similar chemical composition. Their quantitative mineralogical composition is as follows: amphibole nordmarkite—microcline-perthite, 67%; albite-oligoclase (No. 15), 14%; amphibole, 8%; quartz, 9%; accessory minerals, 2%; biotite-amphibole nordmar-

K-feldspar–microcline–perthite — 64%, albite-oligoclase — 12%, amphibole — 4%, biotite — 5%, quartz — 12%, accessories — 3%. The perthites are very diverse in morphology. Among them the following are distinguished: fine-fibrous perthites close to cryptoperthites, spotted, patchy, mantled antiperthites, and, finally, vein-metasomatic perthites. The amphibole is represented by common hornblende \((-2V = 66—70^\circ,\ Nm = 1.670 \pm 0.004)\), more often by bluish-green subalkaline amphibole \((2V = -80^\circ,\ Ng = 1.666 \pm 0.002,\ Np = 1.655 \pm 0.002)\), and much more rarely by a typical sodic amphibole of the glaucophane-riebeckite series.

The biotite is typical brown \((Ng = Nm = 1.628 \pm 0.002)\). The accessories are represented by sphene, zircon, apatite, and ore minerals.

The more acidic representatives of the syenite intrusion are much more widespread and varied. They compose the central part of the Sidorovsky pluton and the entire Irbinsky massif. The following petrographic varieties are distinguished: granosyenites close to syenodiorites, alkaline granosyenites, leucocratic granosyenites, alkaline granites, and quartz albitites.

Granosyenites close in composition to syenodiorites are fairly widespread in the eastern part of the pluton, which tends toward the Irbinsky remnant. Their chief distinguishing feature is a relatively high content of plagioclase, whose basicity is higher than in the normal syenites of the pluton. Quantitative mineral composition: plagioclase (No. 18—25) — 19%, microcline and microcline-perthite — 55%, quartz — 14—20%, actinolite \((-2V = 82^\circ,\ CNg = 16^\circ,\ Ng = 1.668 \pm 0.002)\) — 3—7%, accessories (zircon, sphene, apatite) — 1—2%.

Alkaline granosyenites are the most widespread, main constituent part of the Irbinsky and Sidorovsky plutons. They mainly compose the central part of these plutons. In the field, two structural varieties are distinguished among them: medium-grained porphyritic granosyenites and fine-grained granosyenites. Both varieties often replace one another, and the transitions may be fairly sharp. The mineralogical composition of the varieties is approximately the same and is characterized by the following proportions: microcline and microcline-perthite — 63%, albite (No. 3—5) — 12%, quartz — 18%, amphibole (actinolite and subalkaline variety) — 6%, accessories — 1%.

A very close petrographic variety of the alkaline granosyenites is their leucocratic variety, in which the amphibole content falls to 1—2%.

Alkaline granites compose the summit and the northwestern slopes of Mt. B. Ural. We distinguish them as a petrographic variety of granosyenites, although the possibility is not excluded that they formed during an independent, later stage in the formation of the pluton. Among them, riebeckite and leucocratic biotite-riebeckite varieties are distinguished. Quantitative mineral composition of the riebeckite granites: weakly perthitized K-sodium feldspar — 55%, quartz — 37%, riebeckite \((2V = -80^\circ,\ CNg = 5^\circ,\ Ng = 1.696 \pm 0.002,\ Ng - Np = 0.004)\) — 8%, zircon — isolated small grains. The leucocratic granites contain very little riebeckite \((\approx 1\%)\) and equally little brown biotite.

Quartz albitites have a more limited distribution. They compose small crosscutting bodies that are located near the periphery of the Irbinsky pluton and, in all probability, are its satellites. Such albitite satellite bodies are noted at the mouth of the Poperechka River, where they actively affect Devonian effusives, and also at the mouth of the B. Burluk River and on the right bank of the Izyk River, where they clearly

cut through bodies of the more ancient granodiorite intrusion. In outward appearance the quartz albitites very much resemble fine-grained leucocratic micropegmatitic granites. Under the microscope they have an allotriomorphic-granular texture, close to an aplitic texture, and show the following composition: albite—52%, microcline—17%, quartz—27%, chlorite—2%, epidote, sphene, and magnetite—2%.

The rocks of the dike facies are divided into two groups: basic and subalkaline in composition. The first group includes diabases, diabase porphyrites, and diorite porphyrites, which in chemical composition are close, respectively, to trachybasalts and trachyandesites. The second group is more varied. Here one distinguishes syenite porphyries, syenite aplites, alkaline granite porphyries, and albitite porphyries. Most of the dikes occur within the ore field of the Irbinskoye deposit.

Within the deposit field there is a very variegated assemblage of hybrid rocks: diopside syenites, syenite-diorites, granodiorites, quartz diorites, diorites, gabbro-syenites, and gabbro-diorites.

Both plutons belong to the hypabyssal facies of depth, as indicated by the uneven grain size and porphyritic character of the rocks, the considerable development in them of micropegmatitic and microperthitic textures, and the relatively weak manifestation of metamorphism in the exocontacts (Yu. A. Kuznetsov, 1955). Especially indicative in this respect are the perthites, which in composition and morphology are an exact copy of the perthites characteristic of hypabyssal granites (O. F. Tuttle, 1952). The age of the quartz-syenite intrusion is determined as post–Lower Devonian on the grounds that these rocks distinctly cut and metamorphose the rocks of the Lower Devonian effusive sequence. At the same time, there are grounds for limiting the age of the intrusion to the Lower Devonian, since, according to A. Ya. Edelshtein and T. F. Vasyutinskaya, pebbles of the syenites representing this intrusion are present in conglomerates of the upper parts of the Lower Devonian (Chanysh town). Yu. A. Kuznetsov (1932) noted the presence of pebbles of nordmarkites of the “Stolbovskaya intrusion,” entirely analogous to the Irbinskaya intrusion, in conglomerates of the lower Kachinskaya Formation, to which a Middle Devonian age is now assigned. This point of view is confirmed by the recent work of A. L. Dodin (1956).

It is necessary to dwell especially on the considerable similarity of some intrusive and effusive rocks of the area of the Irbinskoye deposit. Particularly close are some rocks of the Irbinsky pluton to the trachytic porphyries and orthophyres of the upper parts of the effusive sequence. Thus, the orthophyres and trachytic porphyries of the area of Chalpán town, the villages of Berezovka, Kurskoye, and Buturtak, outwardly do not differ in any way from the dike syenite porphyries of the Irbinsky pluton. Both these and the other rocks are composed essentially of alkali-sodium feldspar and quartz, with an insignificant amount of subalkaline amphibole. The difference consists only in the fact that in the syenite porphyries of the pluton the alkali-sodium feldspars have undergone exsolution breakdown with the formation of fine and even coarse perthites, whereas in the analogous effusives perthites are encountered very rarely. However, this distinction is quite explainable. O. F. Tuttle (1952) drew attention to the fact that alkali-sodium feldspars of one composition manifest themselves differently in the extrusive-effusive and plutonic facies. In effusives they are represented by homogeneous grains of anorthoclase, whereas in plutons of the hypabyssal facies feldspars of the same composition are transformed, under the action of unmixing, into albitic perthites. The following may serve as evidence of the similarity of the alkali-sodium feldspars of the syenites of the Irbinsky pluton and the orthophyres of the effusive sequence. In the syenites of the pluton not all alkali-sodium feldspars

subjected to decomposition; some of them have retained homogeneity. In optical properties such varieties are completely analogous to the feldspars composing the phenocrysts of orthophyres, and conversely, some phenocrysts of effusive rocks, undergoing decomposition, become in every respect similar to the perthites of intrusive syenites.

The identity of the mineralogical composition leads to the fact that some of the better-crystallized structural varieties of orthophyres in the area of the villages of Kurskoye and Bugurtak become completely indistinguishable from the syenite-porphyries of the dike facies and the apical parts of the pluton.

In conclusion it should be noted that closeness in time of formation, a constant local connection, considerable similarity of the petrographic and mineralogical compositions, and the presence of transitional varieties between the syenites of the intrusion and the subalkaline effusive rocks of the area—all this has allowed us, and some earlier investigators (A. Ya. Bulynnikov, 1929; A. G. Vologdin, 1932; V. V. Bogatskii, 1956), to regard the intrusive and effusive formations as comagmatic and to assign them to a single magmatic complex.

The kinship of the effusive and intrusive rocks of the complex is especially well expressed in their chemistry. At our request, 40 chemical analyses of rocks of the complex were carried out in the laboratories of the Krasnoyarsk Geological Administration and of the West Siberian Branch of the Academy of Sciences (Novosibirsk). We recalculated the results of the analyses by the method of A. N. Zavaritskii (see the table of chemical analyses). Below is a characterization of the samples analyzed.

1 — Sample 49/54 — quartz alkaline syenite. Irbinskii pluton, right watershed of the Poperechka River. Analysis by the laboratory of the West Siberian Branch of the Academy of Sciences of the USSR.

2 — Sample 139/54 — alkaline granosyenite. Irbinskii pluton, right watershed of the B. Irba River. Analysis by the laboratory of the West Siberian Branch of the Academy of Sciences of the USSR.

3 — Sample D20/54 — nordmarkite. Krasnyi Dar, Lekarev Klyuch. Analysis by the laboratory of the West Siberian Branch of the Academy of Sciences of the USSR.

4 — Sample 476/55 — alkaline granite. Irbinskii pluton, Mt. B. Ural. Analysis by the TsNIL laboratory of the Krasnoyarsk Geological Administration (KGU).

5 — Sample 475/55 — alkaline granite. Irbinskii pluton, Mt. B. Ural. Analysis by the TsNIL laboratory of the KGU.

6 — Sample 448/55 — porphyritic granosyenite. Irbinskii pluton, Mt. B. Ural. Analysis by the TsNIL laboratory of the KGU.

7 — Sample 453/55 — porphyritic granosyenite. Irbinskii pluton, Mt. B. Ural. Analysis by the TsNIL laboratory of the KGU.

8 — Sample 648/55 — porphyritic granosyenite. Irbinskii pluton, Mt. B. Ural. Analysis by the TsNIL laboratory of the KGU.

9 — Sample PKZ 4/54 — quartz albitite. Southern margin of the Irbinskii pluton, mouth of the Burlyuk River. Analysis by the laboratory of the West Siberian Branch of the Academy of Sciences of the USSR.

10 — Sample 265/54 — quartz albitite-porphyry from a dike of the ore field of the Irbinskoye deposit. Analysis by the laboratory of the West Siberian Branch of the KGU.

11 — Sample 2172/54 — monzonite-porphyry from a dike of the ore field of the Irbinskoye deposit. Analysis by the laboratory of the West Siberian Branch of the Academy of Sciences of the USSR.

12 — Sample 458/55 — alkaline granite-porphyry from a dike. Irbinskii pluton, western slope of Mt. B. Ural. Analysis by the TsNIL laboratory of the KGU.

13 — Sample borehole 103/55 — granosyenite-porphyry from a dike. Ore field of the Irbinskoye deposit, borehole 103, interval 225.7–226.0 m. Analysis by the TsNIL laboratory of the KGU.

14 — Sample borehole 106/55 — diabase porphyrite. Ore field of the Irbinskoye deposit, borehole 106, interval 267 m. Analysis by the TsNIL laboratory of the KGU.

15 — Sample 140/55 — alkaline syenite-aplite. Sidorovskii pluton, southern slope of Mt. Sidorovskii Ural. Analysis by the TsNIL laboratory of the KGU.

20 — Sample borehole 103/55 — quartz syenodiorite. Ore field of the Irbinskoye deposit, borehole 103, interval 241 m. Analysis by the TsNIL laboratory of the KGU.

21 — Sample borehole 66/55 — diopside quartz syenite. Ore field of the Irbinskoye deposit, borehole 66, interval 315 m. Analysis by the TsNIL laboratory of the KGU.

22 — Sample borehole 66/55 — quartz diorite. Ore field of the Irbinskoye deposit, borehole 66, interval 387 m. Analysis by the TsNIL laboratory of the KGU.

23 — Sample borehole 101/55 — diorite. Ore field of the Irbinskoye deposit, borehole 101, interval 333 m. Analysis by the TsNIL laboratory of the KGU.

Table 1

Chemical analyses of rocks of the Devonian magmatic complex

Numerical characteristics of A. N. Zavaritskii

Fig. 1. A. N. Zavaritsky diagram for the effusive and intrusive rocks of the Irba area.
1—granosyenites and alkaline granites of the Irbinsky pluton. 2—nordmarkites of the Sidorovsky pluton. 3—effusive rocks of the complex.

24 — Sample, borehole 92/55 — gabbro-diorite. Ore field of the Irbinskoye deposit, borehole 92, interval 355 m. Analysis by the TsNIL KGU laboratory.

25 — Sample 2174/54 — quartz diorite. Ore field of the Irbinskoye deposit. Analysis by the TsNIL KGU laboratory.

26 — Sample TKK11 — granodiorite. Talovskoye deposit. Analysis by the laboratory of the ZSFAN USSR.

27 — Sample 254 — gabbro-syenite. Bezymyannoye deposit. Analysis by the laboratory of the ZSFAN USSR.

31 — Sample 666/55 — orthophyre. Right bank of the Tuba River near the village of Kuragino. Analysis by the TsNIL KGU laboratory.

32 — Sample 672/55 — orthophyre. Right bank of the Tuba River near the village of Kuragino. Analysis by the TsNIL KGU laboratory.

33 — Sample 623/55 — trachyte porphyry, village of Berezovskoye. Analysis by the TsNIL KGU laboratory.

34 — Sample 1501/54 — quartz keratophyre, Mt. Uspenskaya, right watershed of the Buruluka River. Analysis by the laboratory of the ZSFAN USSR.

36 — Sample 724/54 — porphyrite. Left watershed of the B. Burluk River in its mouth area. Analysis by the laboratory of the ZSFAN USSR.

37 — Sample 577/55 — quartz keratophyre. Mouth of the Gryaznushka River. Analysis by the TsNIL KGU laboratory.

38 — Sample 2063/54 — diabase porphyrite. Left watershed of the B. Irba River north of Mt. Bezymyanka. Analysis by the TsNIL KGU laboratory.

39 — Sample 405/55 — diabase. Western slope of Mt. B. Ural. Analysis by the TsNIL KGU laboratory.

46 — Sample 523/55 — nordmarkite. Sidorovskii pluton. Analysis by the TsNIL KGU laboratory.

47 — Sample 535/55 — nordmarkite. Sidorovskii pluton. Analysis by the TsNIL KGU laboratory.

48 — Sample 547/55 — granosyenite. Sidorovskii pluton. Analysis by the TsNIL KGU laboratory.

50 — Sample 234/55 — nordmarkite. Bes pluton. Analysis by the TsNIL KGU laboratory.

51 — Sample 364/55 — quartz alkaline syenite. Bes pluton, Mt. Balaban. Analysis by the TsNIL KGU laboratory.

52 — Sample 172 — nordmarkite. Kedrovyi pluton. Analysis by the TsNIL KGU laboratory.

53 — Sample 120/55 — nordmarkite. Kedrovyi pluton. Analysis by the TsNIL KGU laboratory.

55 — Sample 4/55 — alkaline syenite. Saibar pluton. Analysis by the TsNIL KGU laboratory.

It is evident from the table that, in chemical composition, the quartz keratophyres, trachyte porphyries, and orthophyres from the sheets of the effusive sequence of the Lower Devonian are completely identical to the nordmarkites and alkaline granites of the Sidorovskii and Irbinskii plutons. This is very clearly illustrated by the diagram constructed according to the method of A. N. Zavaritskii (Fig. 1). In the diagram, the vectors corresponding to the orthophyres and trachytes of the village of Berezovka and the village of Kuragino are superposed on the cluster of vectors corresponding to the quartz syenites and alkaline granites of the Irbinskii pluton; moreover, the Berezovka effusives prove to be very close to the dike rocks of the pluton. The magnitude and orientation of the vectors are also similar. In chemical composition, the quartz keratophyres, which form small sheets among the more basic varieties of the lower division of the effusive sequence, approach the nordmarkites of the Sidorovskii Ural.

The chemical affinity of the intrusive and effusive rocks of the area is also manifested in other additional characteristics. Thus, the values of \(Q\), \(a:c\), and \(\mathrm{MgO}:\mathrm{FeO}\) prove to be almost equal for the orthophyres and trachyte porphyries of the effusive sequence and the quartz syenites and alkaline granites of the Irbinskii pluton. The same is observed for the keratophyres of the Gryaznushka River and the nordmarkites of the Sidorovskii pluton (see the table of chemical analyses).

The basic varieties are significantly developed among the effusive and dike rocks of the complex; moreover, the basic effusives are the earliest formations. All this gives grounds for regarding the entire complex as derivative of a basic magma. The subalkaline varieties of the effusives, in chemical properties, are very close—

to trachytes of a series of oceanic islands having a basaltoid origin. On A. N. Zavaritskii’s diagram, the vectors corresponding to the trachyte porphyries and orthophyres of the Irba area almost coincide with the vectors of the trachytes of Auvergne and the Far East, genetically related to typical basalts (Fig. 4).

The intrusive rocks of the magmatic complex also have a number of features indicating their genetic connection with basic magma. It should be noted that the question of the connection of the syenites of the Irbinskoye deposit area with basic magma is not being raised for the first time. Comparatively recently, suggestions to this effect were made by Yu. A. Kuznetsov (1955) and V. V. Bogatskii (1956). There are many examples of the formation of magmatic rocks analogous to the Irba rocks in the course of differentiation of magma that is basic in composition, and all investigators unanimously agree that the principal features of the chemistry of the final products of differentiation of basic magma are the following two:

1. A high value of the ratio FeO : MgO (greater than two), although the absolute amounts of both are small.

2. An increase in the alkali content, with the potassium content increasing relatively more rapidly.

Fig. 2. Triangular diagram for effusive and intrusive rocks of the Irba area.
1—granosyenites and alkaline granites of the Irba pluton, 2—nordmarkites of the Sidorovo pluton, 3—effusive rocks of the complex.

For the syenites of the Irbinskoye deposit area these features are very characteristic (see the table of chemical analyses). For the rocks of the Irba pluton, for example, the FeO : MgO ratio averages 6, not falling below three. The relationship between the contents of FeO, MgO, and the sum of the alkalies Na₂O + K₂O in the intrusive rocks is reflected in the triangular diagram (Fig. 2). From the diagram it is evident that all rocks of the Sidorovo and Irba plutons are characterized by an exceptionally high content of alkalies (Na₂O + K₂O) and a very small amount of MgO; moreover, these features are more strongly expressed for the latest, in time of formation, alkaline granites and albitites.

In order to understand better the genesis of the Devonian magmatic-complex intrusion of the Irba area, it is necessary to compare it with certain other analogous intrusions whose basaltoid origin has been proved.

The Irba and Sidorovo plutons are completely identical in material composition and chemistry. In turn, the alkaline syenites of the Sidorovo Urals are completely identical to the alkaline syenites of analogous plutons of neighboring regions (Bes, Vysokaya, Kedrovyi, Saibar) and to the nordmarkites of the “Stolbovaya intrusion” in the Krasnoyarsk area. The latter, in the opinion of Yu. A. Kuznetsov (1955), are derivatives of basic magma.

Fig. 3. A. N. Zavaritsky diagram for syenites of Irba and several other regions

1 — granosyenites and alkaline granites of the Irbinskoye pluton; 2 — hybrid rocks of the Irbinskoye pluton; 3 — syenites of the Slyudorov pluton; 4 — syenites of the Bes’ pluton; 5 — syenites of the Kedrovy pluton; 6 — syenites of the Isov pluton; 7 — syenites of the Kushva complex (Urals); 8 — syenites of the environs of the city of Krasnoyarsk; 9 — syenites of the Morotu district (South Sakhalin).

Fig. 4. Direction of differentiation in the Irba effusive series and its comparison with certain natural associations of volcanic rocks.

1—Irba effusives; 2—effusives of the alkaline series of the Far East; 3—effusives of the Cantal volcano, Auvergne region.

The nordmarkites of Krasnoyarsk Krai, and in particular the syenites of the Irba deposit area, are very close in age, petrographic composition, and chemistry to the syenites of the Kushva complex of the Urals, whose derivation from basic magma was demonstrated by E. A. Kuznetsov (1950). In addition, the syenites of Irba, in their petrographic features and chemical composition, are analogous to the Tertiary syenites of Sakhalin, which are genetically connected with effusions of basaltic composition (Yagi, 1953). The identity of the chemistry of all the syenites listed above is reflected in the table of chemical analyses and in A. N. Zavaritskii’s summary diagram (Fig. 3).

Thus, both the effusive and intrusive rocks of the Devonian magmatic complex of the area of the Irba deposit probably originated from a single parent magma of basic composition.

It is necessary to dwell especially on the questions of the character and direction of the differentiation of the magma, in the course of which the complex of interest to us was formed.

At present the opinion has been developed (V. Sobolev, 1936; V. M. Kupletskii, 1947; N. A. Vebman, 1950; E. A. Kuznetsov, 1950; Yu. A. Kuznetsov, 1955; T. Barth, 1956) that differentiation of a magma of basic composition may proceed in two directions: either toward the accumulation of a mainly acidic residue, or toward the accumulation of an alkaline residue. Accordingly, two series of differentiates of basic magma are distinguished—the alkaline-earth and the alkaline. Most proponents of the two directions of differentiation believe that magnetite deposits are usually genetically associated with the alkaline branch.

In terms of its rock association, the magmatic complex of the Irba area corresponds more closely to the second, alkaline direction of differentiation of basaltic magma. In the composition of the intrusive and effusive rocks of the complex, rocks such as nordmarkites, alkaline granosyenites, alkaline granites, albitites, trachytes, keratophyres, and albite porphyries clearly predominate; according to the classification of E. A. Kuznetsov (1956), these belong to the alkaline varieties of the syenite–trachyte family. In analogous complexes of neighboring areas (the Bes’ pluton, Vysokaya, Saibar), typical alkaline rocks are noted—nepheline and analcime syenites. The dike rocks of the Irba area are likewise distinguished by increased alkalinity, approaching trachy-andesites and even trachy-basalts in chemical composition. Judging from the fact that the most acidic and alkaline rocks of the complex were formed predominantly last, it may be assumed that differentiation proceeded toward a decrease in basicity and an accumulation of alkalis. The final products of differentiation are, in the effusive series, orthophyres and trachyte porphyries, and in the intrusive series—alkaline granites and albitites.

The alkaline character of the differentiation was manifested especially clearly in the effusive series of the complex. The change in the chemical composition of the rocks corresponding to the various stages of magma evolution is illustrated in diagrams (Figs. 4 and 8).

In the diagram (Fig. 4) it is evident that the vectors corresponding to the rocks of the effusive series are arranged on the \(SAB\) plane in a single straight line with a steep inclination toward \(SB\) and a characteristic bend at the upper end toward the vertex \(S\). Comparing the diagram of the chemical properties of the effusives of the Irba area with the summary diagram of the types of natural associations of volcanic rocks (A. N. Zavaritskii, 1950), we come to the conclusion that the effusive series of the Irba area, in its chemistry, occupies an intermediate position between typical Pacific calcareous-alkaline and typical alkaline series (Fig. 5). In this diagram it is evident that the curve of the Irba effusive series (EA) occu-

occupies, on the \(SAB\) plane, an intermediate position between the curves of the lime-alkaline (3) and typical alkaline (7) series and comes very close to the curve of the transitional series of the Etna type (4); on the \(SBC\) plane, the variation curve of the effusives of the Irba area is closest to the curve of the alkaline series.

Using the data of A. N. Zavaritsky, we come to the conclusion that the effusive rocks of the Irba complex are very close, in their association of rocks and in their chemistry, to the alkaline series of the Far East and of some regions of Western Europe (Fig. 4). A similar resemblance is also indicated with some other series described in the literature. In particular, the Irba series in many respects recalls the famous alkaline series of the Oslo environs, and the similarity is manifested not only on A. N. Zavaritsky’s diagram but especially on the variation diagram. The variation diagram of the Irba series (Fig. 6) proves to be almost entirely similar to the analogous diagram compiled by A. Harker for the rocks of the Oslo environs. This diagram, but averaged (Fig. 7), approaches the variation diagram for the rocks of the classical alkaline magmatic series of the island of Mull (A. N. Zavaritsky, 1950).

Fig. 5. Position of the Irba effusive series on the summary diagram of types of natural associations of volcanic rocks.
3—lime-alkaline series, 7—alkaline series, 4—transitional type (Etna), EA—effusive series of the Irba area.

The alkaline tendency of the Irba series is also emphasized by several other diagrams. Thus, on the triangular diagram \((\mathrm{MgO}, \mathrm{FeO}, \mathrm{Na_2O}+\mathrm{K_2O})\), the left, alkaline branch of the variation line, approaching substantially the \(\mathrm{Na_2O}+\mathrm{K_2O}\) apex, is more strongly developed than the right, essentially lime branch (Fig. 8).

The “bond factor” diagram, constructed in rectangular coordinates

\[ \frac{\mathrm{Mg}}{\mathrm{Mg}+\mathrm{Fe}} \]

—the “bond factor” (A. Poldervart, W. E. Elston, 1954), also proves to be typical of the differentiation of basaltic magma (Fig. 9).

Having examined the character of differentiation on the example of the effusive series, let us determine the position in this sequence of the intrusive rocks of the complex. Judging by their petrographic features and chemistry, they are the last differentiates of the magma. They, as it were, complete the differentiated branch of the effusive series. We have already drawn attention to the fact that the rocks of the Irba and Sidorov plutons, in petrographic composition, are very close to the latest-formed subalkaline effusives of the complex. For them, as also for the effu-

rocks, a general tendency is observed toward an increase in the content of quartz and alkali feldspars. The similarity of the intrusive rocks and the effusive rocks of the complex that are latest in time of formation is especially well expressed in their chemistry. In the table (see the table of chemical analyses) and in the diagrams (Fig. 1 and Fig. 2) it is evident that the earliest representatives

Fig. 6. Variation diagram for the Irba effusives.
39—numbers of chemical analyses.

of the intrusive group of rocks—nordmarkites and alkaline granosyenites—are superposed on the latest rocks of the effusive series and even repeat the “alkaline hook” of the variation line characteristic of the effusive series; and the rocks of the complex that followed them and were last in time of formation—alkaline granites and quartz albitites—

Fig. 7. Averaged variation diagram for the Irba effusives.

as it were continue and complete the whole series.

On the basis of what has been said above, the following assumptions may be made about the formation of the Devonian magmatic complex of the Irbinskoye deposit area. At the time of the inception of the Minusinsk depression...

… in connection with the beginning of the downwarping, numerous fractures formed in its marginal parts; and the Irba area occupies precisely a marginal position. Along these fractures magma began to be extruded onto the surface, its composition changing with time. At first there was an outpouring of magma of basic basaltic composition; toward the end, magma of a more acidic subalkaline composition was poured out onto the surface, forming the uppermost

Fig. 8. Direction of differentiation in the effusives of the Irba area.
38—numbers of chemical analyses.

covers, composed of orthophyres and trachyte porphyries. The melts of approximately the same and of a more acidic composition that followed, encountering the resistance of the solidified effusive roof, did not reach the surface, but solidified at a shallow depth within the

Fig. 9. Direction of differentiation of the effusive series of the Irba area.
39—numbers of chemical analyses.

lower horizon of the effusive sequence and at its boundary with the underlying Cambrian deposits. As a result, hypabyssal bodies of syenites were formed, representing intrusive analogues of the latest rocks of the effusive series; moreover, granosyenites and alkaline granites were formed last.

As is evident from the foregoing, the Irbinsky massif is composed of the final products of differentiation of a basic magma that had a clearly expressed alkaline trend and thus belongs to a genetic type of intrusions prospective for iron-ore mineralization (Yu. A. Kuznetsov, 1955). The Irbinsky pluton differs from related, substantially normative intrusions of neighboring regions in that the most extreme rocks of the complex predominate in it—alkaline granosyenites, alkaline granites, and quartz albitites, formed at the final stage of magma differentiation. The features of the mineralogical composition of the pluton rocks (the development in them of perthites, microperthites, alkaline amphiboles, acicular apatite, and certain other minerals), considerable autometasomatic changes (albitization, hematitization, etc.), and a comparatively strong manifestation of metasomatism in the exocontacts—all this also distinguishes the Irbinsky pluton from the related intrusions of neighboring regions and permits one to assume a significant participation of volatile components in its formation. Taking into account the fact that a large deposit of iron ores occurs in the contacts of the Irbinsky pluton, while no such deposits have been found in connection with other related intrusions, the Irbinsky type of intrusions, with all the petro genetic and mineralogical-petrographic features noted for it, should be considered the most promising for iron-ore mineralization.

REFERENCES

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Corrections and Noted Misprints

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