NEW DATA ON NEPHELINE ROCKS OF THE KUZNETSK ALATAU

V. G. MIKHALEV

(Presented by the seminar of the Department of Petrography)

Nepheline rocks in the Kuznetsk Alatau have been known for a long time, but especially rich and interesting factual material on them has been accumulated in the last 4 years; it served as the basis of the present work.

I. Brief Geological Characterization of Certain Massifs of Nepheline Rocks

In the Kuznetsk Alatau, in 1957–1960 several more massifs of nepheline rocks were established: Kurgusul, Belogorsk, Uchkuropsk, Cheremushka, Solnechny, and the Inziyul group of bodies; during the same period, ideas about the previously known massifs also changed substantially (Goryachegorsk, Kiya-Shaltyr, Semenovka, and others). All the massifs occur among the Lower Cambrian sedimentary-metamorphic formations of the Kuznetsk Alatau or within the Lower Devonian strata of the Chulym-Yenisei depression. Within the zone where the two named regions meet, the position of individual massifs was determined also by certain local features (large faults, folded structures, etc.).

Until 1958, the nepheline rocks were known mainly in the form of stock- and sill-like bodies and dikes; now sheets have been added to them.

1. Goryachegorsk massif. Until 1957 it was assumed that the nepheline rocks of Mount Goryachaya composed two separate bodies; subsequently it was established that these bodies are connected and form a single massif. Previously, 3 varieties of intrusive nepheline rocks were distinguished in this massif: trachyto-syenites, both equigranular and porphyritic, and nepheline syenites; now 5 varieties are distinguished: leucocratic trachytes, trachyto-syenites, feldspathic urtites, nepheline syenites, aegirine and aplite-like; the endocontact zones of hybrid formations have received more definite expression; the variety of dike rocks has also increased. Features of all the newly distinguished varieties of nepheline rocks had been noted earlier as well, but at that time they did not find clear expression: geological maps and sections looked too schematic, and the massif was considered uniquely simple and homogeneous. This would still be true even now, had the Kiya-Shaltyr and Kurgusul massifs not been discovered, with an even simpler structure and homogeneous composition of the nepheline rocks.

Mode of occurrence and composition. The Goryachegorsk massif is a complex intrusive body of stock-like form with a steep dip to the northwest, cutting Lower Devonian paleobasalts and various porphyrites and, in turn, itself intersected by dikes. It is composed of alkaline rocks belonging both to the group of nepheline syenites and to alkaline gabbroids; in each of them there are also dike formations, comprising 2–3 varieties. Between the two groups of intrusive rocks there are transitional varieties; in addition, silicate–carbonate formations occur.

The massif is divided into four zones: a western—theralite—zone, a transitional—theralite-syenite—zone, a central—feldspathic-urtite—zone, and an eastern—nepheline-syenite—zone. In plan, the western zone has the form of an irregular semicircle, constitutes about one-third of the massif, and is composed predominantly of leucocratic varieties of theralites. In the endocontacts the latter gradually pass into hybrid formations that, in mineral composition, correspond to ordinary theralites; nepheline is not infrequently also observed in the near-contact parts of the enclosing porphyrites (nephelinization).

The transitional zone constitutes about one-tenth of the intrusive massif, is situated east of the theralite semicircle, and to the east passes directly into the central—feldspathic-urtite—zone. The transitional zone is composed mainly of theralite-syenites. Genetically it is not an independent structural-geological unit, but reflects only an intermediate stage of magma differentiation, possibly with some mixing of differentiates of alkaline-gabbroid and nepheline-syenite composition, emplaced successively one after another with a short interval of time.

The central zone (about one-fifth of the massif) is composed chiefly of feldspathic urtites. Along the eastern endocontact belt of the northern “arm,” the latter gradually pass into hybrid formations close in composition to theralites and teschenites.

The eastern zone is distinguished by the most complex geological structure: it is composed mainly of nepheline syenites with highly variable mineral composition and structure. Here varieties of nepheline syenites occur: gray and light pink, up to transitions into aplite-like alkaline varieties without nepheline; with even medium- and fine-grained texture and porphyroid varieties, comparatively fresh and intensely altered. In the north the eastern zone is composed of theralite-syenites.

Dikes make up about 3.5% of the massif and, in composition, are distributed in some correspondence with the intrusive rocks: in the theralite zone—predominantly theralite and normal porphyrites; in the nepheline-syenite group—nepheline syenite-porphyries and tinguaite. There are few bereshites and alkaline aplite-pegmatites, and therefore no regularities have been noted for them.

Interrelations between the nepheline rocks. In places, feldspathic urtites with sharp contacts form apophyses in leucocratic theralites. Consequently, the latter are older than the former. Feldspathic urtites are cut by veinlets of aegirine nepheline syenites or occur as xenoliths (together with fragments of the enclosing porphyrites) in a breccia whose cement is aegirine nepheline syenites. In most cases, however, the transitions between the various nepheline rocks are gradual, especially between theralite-syenites, leucocratic theralites, and feldspathic urtites. The same applies also to the hybrid theralites and teschenites, which have gradual transitions not only in the endocontact but sometimes also in the exocontact. Many dike bodies

nepheline syenite-porphyries are apophyses of aegirine nepheline syenites.

Silicate–carbonate formations. All the magmatic rocks are, to one degree or another, cut by veinlets of quartz, albite, and especially carbonates (more often jointly with one another). Their densest network is observed around dikes in zones of crushing. The thickness of individual veinlets varies from threadlike to several centimeters, but veins of predominantly carbonate composition 0.2–1.5 m thick are found, as are separate lens-shaped bodies up to 200 m long with swellings up to 10–20 m. In outward appearance and in composition such bodies resemble carbonatites.

Tectonics. The Goryachegorsk massif is confined to an ancient tectonic fault of meridional strike. Subsequent tectonic processes proceeded mainly in dependence on the position of this fault. This is evident from the following data.

Numerous dikes of magmatic rocks and, more rarely, silicate–carbonate bodies are, in essence, a reflection of tectonic disturbances filled with the corresponding material; in individual cases tectonic disturbances occur in the usual, unhealed form (crushing, mylonitization, slickenside grooves, etc.). The largest disturbance passes through the central—feldspar-urtite zone. This disturbance (possibly others as well) has a very long history of development: the factual data show that its formation preceded the intrusion of the magma that produced the leucocratic ijolites and feldspar urtites; both were again subjected to tectonic stresses; the disturbances that arose in this process served as a conduit and receptacle for the subsequent portion of magma that produced the nepheline syenites. After these had solidified, disturbances again arose in them, subsequently healed by differentiates of nepheline-syenite magma with the formation of dike bodies of tinguaite, and in some other places—of alkaline aplite-pegmatites. All the rocks listed above, within the limits of this disturbance, were in places again crushed and then partly or completely cemented by carbonate material, which has been preserved in a massive, uncrushed state up to the present time.

The tectonic disturbances were also accompanied by significant displacements (the more northern position of the eastern block of the feldspar-urtite zone, including the formation of the northern “arm”; blocks of country rocks thrust along the fault zone into the intrusive body: in the south of the western and north of the eastern blocks, where the nepheline rocks along the tectonic disturbance border on the country rocks, there are no hybrid ijolites and teschenites). The amplitude of relative displacement along the strike in plan is 350–500 m. Movements evidently occurred at all stages of the tectonic disturbances and had a multistage character.

From the foregoing it follows that the formation of the Goryachegorsk massif of nepheline rocks took place in several phases. The first phase was expressed in the manifestation of leucocratic ijolites, the second—of feldspar urtites, and the third—of nepheline syenites. Each of them was accompanied by intensive tectonic processes and ended with the formation of dikes and silicate–carbonate bodies.

2. Kurgusul massif. It is an elongated stock-shaped body with a small satellite and a number of apophyses, cutting Cambrian metamorphic formations and, in turn, cut by dikes. The northern and southern contacts dip toward one another at an angle of about 80°. The western and eastern contacts dip westward at an angle of 70–80°.

The large southeastern part of the massif is composed of urtite-syenites with transitions into nepheline syenites; the smaller northwestern part consists of theralites with transitions into leucocratic varieties and theralite-syenites. The country rocks are marbled limestones and metamorphosed effusive-schist formations.

The dike complex is divided into two groups: alkaline (with feldspathoids) and normal. The urtite-syenites are almost always porphyritic, the porphyritic character being due to large grains, predominantly of feldspars, in the central parts of the massif and of nepheline along its periphery. The urtite-syenites are close to the Goryachegorsk feldspathic urtites and differ from the latter in a smaller amount of nepheline (48% versus 61%). The Kurgusul theralites differ noticeably from the Goryachegorsk theralites and are somewhat closer to the Goryachegorsk theralite-syenites. Dike rocks. Alkaline: nepheline syenite-porphyries, tinguaites, monchiquites, teschenites, theralite-porphyrites; normal: porphyrites, diabase porphyrites, dolerites. The dikes make up about 3.5%, have mainly a SE strike, a southwest dip at an angle of 60–90°, and reach thicknesses of 0.5–5 m, rarely 10–15 m.

• Tectonics. * The Kurgusul massif is confined to the hinge of an anticlinal fold in ancient Cambrian metamorphic strata. The expression of the internal tectonics of the massif is provided by disjunctives, some of which are filled with magma, forming dikes. Movements took place along certain disjunctives. This applies especially to disturbances passing between theralites and urtite-syenites in the north and between urtite-syenites and their country rocks in the south of the massif. The amplitude of displacement reaches 100–200 m along the fault.

• Kiya-Shaltyr massif. In view of the published materials on the massif, only a few remarks should be made.

• According to the investigations of E. D. Andreeva and A. P. Bozin, the northeastern part of the massif belongs to gabbroids of normal composition, whereas according to A. M. Prusevich, who possesses all the primary materials from the documentation of exploration workings, the indicated part of the massif is represented by alkaline rocks—trachytoid leucocratic theralites (essexites), containing from 2 to 13% nepheline.

• E. D. Andreeva and A. P. Bozin consider that the formation of the Kiya-Shaltyr massif (and, according to A. P. Bozin, of the Goryachegorsk and Kurgusul massifs) took place in a single magmatic phase, and they regard the entire diversity of nepheline rocks as the result of alkaline metasomatism. The author of the present article, however, like A. M. Prusevich, assigns to this process only a local character; in the main, the diversity of nepheline rocks is explained by the multiphase formation of the intrusive body, although without a large break in time between individual phases. Confirmation of this in the Kiya-Shaltyr massif is provided by the presence of sharp contacts between urtites and theralites and by the identity of the optical properties of the corresponding rock-forming and accessory minerals of these rocks; in the Goryachegorsk massif the same applies to leucocratic theralites, feldspathic urtites, and nepheline syenites.

• Belogorsk massif. The Belogorsk massif, like the preceding two, occurs among effusive-schist-carbonate formations of the Lower Cambrian. It is represented by a stock-like body of nepheline rocks with rather large “bays” and remnants of the country rocks. The northeastern part of the massif is composed of nepheline syenites, the southwestern part of theralites. The nepheline syenites are comparatively

homogeneous, whereas the theralites are distinguished by great variability of composition and apparently also represent hybrid or metasomatic formations.

1. Semenovskii area. It is represented by complex lens- and dike-shaped bodies of hypabyssal type, composed of several varieties of alkaline rocks, chiefly of the nepheline-syenite group (alkaline syenites without nepheline and with nepheline, theralite-syenites, theralites) and by dikes of nepheline syenite-porphyries, bereshites, and nepheline dolerites. The bodies of the entire complex of alkaline rocks occur within a sequence of normal effusive rocks of the Lower Devonian. The relationships between the individual varieties of nepheline rocks in the hypabyssal bodies are rather complex.

2. Inzyul’skii area. It lies between the previously known bodies of nepheline rocks of the Andryushkina Rechka and Bazyrskii areas and forms with them a single zone about 3 km wide. Within this zone sill-like bodies and dikes of theralite-porphyrites, bereshites, nepheline syenite-porphyries, and a horizon of alkaline effusives have been found, the latter represented by beds of ijolite-porphyries, phonolites, and nepheline-analcime basalts. These beds attain a thickness of 60–100 m and, according to the succession, alternate with sheets of Lower Devonian effusives of normal composition. The total thickness of the horizon is about 300 m.

II. Material Composition of the Nepheline Rocks

The nepheline rocks, even within a single body, are more often represented by several varieties, among which there are representatives belonging both to the alkaline gabbroids and to the group of nepheline syenites. The mineral composition of the alkaline gabbroids is characterized by basic plagioclases (about 30%), augite, especially titaniferous augite (3–30%), and not infrequently olivine (up to 15%), on average 2.4%; for the group of nepheline syenites—acid plagioclases, potassium-sodium feldspars (together more than 40%), aegirine-augite (7%), aegirine (6%), and, to some extent, alkaline amphiboles (up to 5%). Among the accessory and ore minerals, nepheline rocks of both groups almost always contain apatite and magnetite; fluorite, sphene, zircon, titanomagnetite, and ilmenite occur more rarely.

In the chemical composition of the alkaline gabbroids, compared with the nepheline-syenite group, there is less silica (46% versus 49%) and alkalis (8.5 versus 11%), with relatively elevated contents of calcium (10 and 5.5%), magnesium (3 and 1%), and titanium (0.7 versus 0.5% dioxide). More or less significant deviations from the cited data are explained by various secondary endo- and exogenous processes. The latter were especially intense in the Goryachegorsk massif, where within the zones of crushing and near them the nepheline rocks are densely cut by silicate-carbonate veinlets, locally with noticeable sulfide mineralization; the rock-forming minerals have largely been transformed into secondary aggregates (cancrinite, thomsonite, albite, carbonates, chlorite, iron hydroxides, etc.); and in places, especially in the weathering zone, the contents of alkali-metal oxides decrease noticeably (by 1.4%), while the amounts of silica (by 0.2–2.4%) and iron oxides (by 0.7–2.1%) increase.

In the Kurgusul’ and Kiya-Shaltyr massifs the secondary alterations are analogous to those at Goryachegorsk, but are expressed much more weakly.

III. Some Technological Features of Nepheline Rocks

Some nepheline rocks constitute a valuable complex raw material for the production of alumina, cement, and soda products. Several methods have been developed for its use: one with preliminary beneficiation, others without beneficiation.

Beneficiation is aimed mainly at reducing the content of iron oxides; at the same time, the contents of alumina and alkalis in the concentrate increase somewhat. Until 1958, magnetic separation was regarded as the most profitable and rational beneficiation scheme. In 1959–1960, at the Sibelektrostal plant, very positive results were obtained using a flotation scheme, in which the content of iron oxides in the concentrate is reduced to 3.3–4.5%, regardless of their amount in the original ore, whereas with magnetic separation their reduction even to 7% is limited by a content in the original ore of not more than 10–11%.

The positive results of flotation characterize not only the poorer rocks of the Goryachegorsk massif, but also the bereshites of the Andryushkina River, which previously had generally been considered almost not amenable to beneficiation.

All this is clearly seen from Table 1 below.

Table 1

Processing of nepheline rocks without beneficiation is possible by sintering with alkali and limestone when the content of iron oxides is not more than 7%, or with the addition to them of a reducing agent, for example brown coals of the Nazarovo type, if iron oxides exceed 7%; a hydrochemical method is being developed that excludes the sintering process.

IV. Conclusion

1. The massifs of nepheline rocks of the Kuznetsk Alatau are confined to the area of its junction with the Chulym-Yenisei depression and reflect the regional character of the intrusion of magma of alkaline composition.

2. The formation of most of the massifs took place under hypabyssal conditions, in several phases, and was accompanied by processes of magma differentiation; the enclosing rocks were thereby subjected to assimilation and, to a lesser extent, nephelinization.

3. In the geological structure of most massifs, both alkaline gabbroids and rocks of the nepheline syenite group take part. This gives grounds to suppose that the nepheline rocks of, if not all, then most massifs (Goryachegorsk, Kiya-Shaltyr, etc.)

are genetically related to alkaline magma of gabbroid composition, during the differentiation of which, in the residual melt, there was a gradual increase, on the one hand, in silica, up to the formation of silica-saturated, nepheline-free alkaline rocks, and, on the other, in calcium carbonate, which produced (together with silica gel) silicate–carbonate bodies.

1. Nepheline rocks occur under various geological conditions and have diverse, sometimes complex, modes of occurrence: stock- and sill-like sheets, dikes, and complex zonal forms resembling layered intrusions.

2. Massifs of nepheline rocks were repeatedly subjected to tectonic stresses with the development of disjunctive structures, which were often accompanied by noticeable displacements.

3. Some nepheline rocks constitute a valuable complex raw material: several technological schemes have been developed according to which aluminum, cement, and soda products can be successfully obtained from such rocks; recently the question has even been raised of the possibility of utilizing beneficiation tailings for various purposes (obtaining iron, concentrates of rare and dispersed elements, etc.).

4. All the intrusive nepheline rocks which the author distinguished in the Goryachegorsk massif as western, transitional, and central zones were called teralite-syenites by I. V. Luchitskii in 1952; somewhat later he introduced for them the entirely new term “goryachites,” while A. P. Bozin calls feldspathoid urtites bazirites. Such an unstable situation with the terminology of alkaline rocks has arisen not only for the Goryachegorsk massif. For example, in teaching and theoretical literature, urtites are often characterized as rocks related to the group of nepheline syenites and containing aegirine; by both of these criteria the Kiya-Shaltyr urtites do not fit such a definition: they are related not to nepheline syenites, but to alkaline gabbroids, and instead of aegirine they contain titanaugite.

It is entirely obvious that the further—and essentially arbitrary—accumulation of new terms is inadmissible; but it indicates that the nomenclature of alkaline rocks often does not correspond to the rich factual material accumulated in recent years and, of course, needs to be put in order.