On the Phenomena of Syenitization in the Host Rocks of the Kondoma District

G. M. Ivanova, S. S. Il’enok, A. A. Mityakin

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

The massifs of the gabbro–syenite complex under consideration in the Kondoma district are located within Gornaya Shoria, in the Tashelgino–Kondoma tectonic zone in the upper reaches of the Kondoma River. The area lies within Tashtagol District of Kemerovo Oblast.

The host rocks of the complex are effusive-sedimentary strata of the Middle Cambrian (the Kondoma and Mundybash formations), as well as sandy-argillaceous Ordovician deposits and effusives of presumably Devonian age.

Contact-metasomatic iron-ore deposits and ore occurrences are closely associated with syenite outcrops in the district. In this connection, clarification of the conditions under which the syenites formed is of great interest. On this question we still have little factual data.

As a result of detailed investigations in recent years, the authors have obtained interesting material indicating a distinctive structure and composition of the intrusive bodies and a widespread development of metasomatic syenitization that passes into magmatic replacement within the host rocks. It is noteworthy that such replacement processes affect a variety of rocks: intrusive and effusive as well as sedimentary.

The present article describes the syenitization of intrusive rocks.

Syenitization of Gabbro

Alteration of gabbro under the influence of alkaline replacement is well expressed in the Chernichny area of the Shalym ore field. Especially interesting material is provided by the section of borehole No. 286, located 80 m southeast of a small syenite body of submeridional strike. This borehole exposes gabbro, within which five horizons of metasomatically altered rocks are observed; they have a fine-grained granoblastic structure and a composition corresponding to syenite. The thickness of each is small and varies from 3 to 10 m. Down to a depth of 142.80 m, the gabbro macroscopically has a banded-spotted texture owing to the presence of areas with different grain sizes. In one case the main mass consists of fine-grained por—

porphyritic gabbro, in which, either in the form of lenses with fairly distinct outlines, or in the form of irregular spots, and often isometric ones with indistinct boundaries, coarse-grained gabbro is observed. In another case, the relationship between fine- and coarse-grained gabbro is reversed. Under the microscope the coarse-grained gabbro reveals a panidiomorphic-granular texture and is composed of tabular grains more than 1 cm in size of labradorite No. 60 and augite (CNg = 48°, 2V = 40°).

Below a depth of 142.8 m only porphyritic gabbro was observed. In segregations of this rock there are elongate-tabular grains of labradorite No. 60. The groundmass has a sideronitic texture and is composed of augite, labradorite, and magnetite. The content of magnetite in the porphyritic gabbro amounts to 15–20% of the area of the thin section. Farther along the borehole section, syenites occur among the gabbro. At the contact with the latter, altered gabbro about 2.5 m thick is observed. Within the syenites themselves there are broad zones of quartz albitites, as well as silicified and sulfidized rocks. Macroscopically the syenites are pink fine-grained rocks. Under the microscope they reveal a granoblastic inequigranular texture and are composed of small isometric grains of potassium feldspar. Grains of potassium feldspar with a microperthitic lattice structure are encountered. Sometimes quartz is present in the interstices, its content in rare cases reaching 10%. There are transitions to areas with a coarser-grained texture.

The grains of potassium feldspar in such areas have an elongated form with very uneven serrated margins. Often such grains enclose, along their periphery, small grains of potassium feldspar. In separate areas the syenites have a banded-spotted texture, caused by the presence of bands and spots of pink color within the gray groundmass of the rock. In the field of the gray-colored part of the rock there are small flakes of biotite, sometimes constituting up to 5% of the total mass, as well as small grains of hornblende. Sometimes grains of hornblende of rather large size are observed in comparison with the granoblastic potassium-feldspar mass; at the same time it is clearly seen that they have irregular, embayed lateral outlines and are nothing other than relics of pyroxene grains replaced by hornblende.

In the banded syenites, lens-shaped bodies are observed, not persistent along strike or in thickness, enriched in biotite, where its content reaches 20%. The remaining share of the total composition falls to potassium feldspar and apatite. In separate areas of the rock the content of the latter is high and reaches 30%. Apatite is represented by small columnar grains forming a dense network of poikilitic inclusions in grains of potassium feldspar. In addition to apatite, sphene and rutile are also observed in abundance among the accessory minerals.

The gabbro near the syenites is strongly altered, especially in areas in the form of narrow bands that have remained among the syenites. They have been transformed into a fine-grained rock of granoblastic texture, consisting of albite (65%) and biotite (35%); relics of plagioclase are sometimes encountered. Macroscopically such rocks sometimes have a relict porphyritic texture.

Under the microscope, however, it is found that the plagioclase segregations are almost completely replaced by albite and biotite, while the groundmass is filled with these same minerals. With increasing distance from the syenite horizon, less active secondary alterations are observed in the gabbro. In such areas either relics or unaltered grains of pyroxene are preserved.

Pyroxene is replaced, as a rule, by biotite and amphibole, the former forming an outer rim. In this process, biotite flakes often grow perpendicular to the grain boundary, while in the central part amphibole develops in the form of columnar grains of pale-green color; here, too, there is a large amount of ore component forming a fine, dense dissemination. In such areas plagioclase is usually partly replaced by sericite and epidote; fine-grained albite sometimes develops between plagioclase grains, and quartz also appears.

Syenites are usually accompanied by horizons of quartz–albite rocks, which have a fine-grained granoblastic structure and consist of albite with a variable proportion of quartz from 10 to 30%. In such horizons, intense zones of fracturing and quartzization are observed, where the rocks are almost entirely composed of quartz with participation of sericite and albite. Sulfide mineralization is, as a rule, associated with these zones. Such quartz–albite horizons are the result of later hydrothermal activity, with which sodium metasomatism is connected. Albitization also affects the syenites; in this case the albite content in the latter reaches 30%.

The spatial association of syenites and quartz–albite rocks indicates that the same pathways, determined by favorable structural conditions, were used in the processes of potassic feldspathization and sodium metasomatism.

The metasomatic character of the syenites noted above is proved by the following: 1) the presence in the syenites of a fine-grained granoblastic structure; 2) the presence in the syenites of relics of pyroxene grains, sometimes replaced by hornblende, among the granoblastic aggregate of potassic feldspar and, as it were, corroded from the margins by potassic feldspar; 3) the presence of spotted textures of the syenites in areas of incompletely reworked primary rock, and the abundant development in such areas of rocks enriched in apatite; 4) the mode of occurrence of the syenites, which form a series of thin horizons and schlieren among gabbro.

Syenitization of gabbro-porphyrites

Pyroxene and pyroxene–hornblende gabbro-porphyrites occur in the southern part of the Kondoma iron-ore belt. The syenites cutting them have either sharp contacts or are bounded by zones of altered syenitized rocks. There are cases where the syenites follow dike-like bodies of gabbro-porphyrites, as if completely replacing them. Higher up they pinch out, and above them, along the line of an uneven, serrated contact, lie gabbro-porphyrites repeating the same elements of occurrence.

The manifestation of alkaline replacement along the broad boundary zone between syenites and gabbro-porphyrites is clearly visible in trench 1 on Mount Otvesnaya. The unaltered pyroxene–hornblende gabbro-porphyrites in this area are macroscopically greenish-gray in color and have a massive texture. Under the microscope, the rock has a porphyritic structure with phenocrysts of pyroxene and hornblende, which make up 25–35% of the volume. The sizes of their grains average 0.3–1.5 mm. The pyroxene is augite with \(2V = +60^\circ\), \(CNg = 40^\circ\). Hornblende forms prismatic pale-green crystals. The groundmass of the rock is composed of amphibole and plagioclase No. 33. Among the secondary minerals there is a large amount of epidote and chlorite, and to a lesser extent sericite and carbonate. Near the contact with the syenites, the pyroxene–hornblende porphyrites are quite strongly altered. Pyroxene and hornblende

hornblende grains are replaced from the periphery by secondary amphibole, chlorite, partly biotite, carbonate, and epidote. Closer to the contact with the syenites, a rock with a faint pinkish tint appears.

Under the microscope it is evident that the main mass of the rock has a hialopilitic structure—the plagioclase laths are immersed in a felsitic mass. In places, spots of a fine-grained granoblastic aggregate of K-feldspar are developed in the groundmass of the rock; this aggregate replaces plagioclase. Sometimes only fragments of the grains of this mineral remain, or the relict form of its grains is preserved. Closer to the contact with the syenites, the gabbro-porphyrites undergo more intense K-feldspathization. Macroscopically, the rock becomes greenish-pink; in it, green porphyritic phenocrysts stand out clearly, while the groundmass has a pinkish color. Under the microscope, in such a rock we observe porphyritic phenocrysts of pyroxene and hornblende, almost completely replaced by actinolite and later chlorite. The groundmass is wholly recrystallized. It has a granoblastic structure and consists of isometric grains of K-feldspar; only here and there are relicts of plagioclase grains faintly visible. Among secondary minerals in the groundmass, amphibole and epidote are observed. The total amount of K-feldspar progressively increases on approaching the syenites. In this process the rock acquires a spotted texture. Pink spots develop as schlieren with irregular outlines and as short veinlets. The remaining mass of the rock has a dark, greenish-gray color.

Under the microscope it is found that the pink spots have a blastoporphyritic structure. The phenocrysts are represented by large tabular grains of K-feldspar, often having the structure of a microcline lattice. The grains are elongate-tabular in shape. Optical properties: \(Ng = 1.525\), \(Nm = 1.522\), \(Np = 1.520\) \((0.001—0.002)\), \(2V = -78^\circ\), which indicates an admixture of up to 15% of the albite molecule. The groundmass is granoblastic and consists of isometric grains of K-feldspar with sinuous, uneven outlines. In places, together with the granoblastic structure, the structure acquires a tabular-granular character; the plates of K-feldspar, thin and elongate, are arranged either quite arbitrarily and disorderly, producing the character of a bostonitic structure, or subparallel, producing the appearance of a trachytic structure; in places it assumes a semblance of a radial-rayed structure, forming rosette-like forms.

The dark parts of the rock are characterized by a substantial predominance in their composition of magnetite and chlorite over K-feldspar, which accounts for the peculiarity of their color. Among secondary minerals, biotite and sericite are also present here. K-feldspar, as in the pink parts of the rock, is represented by individual large grains in the form of blastoporphyritic phenocrysts, and also by small grains of the groundmass, forming a granoblastic structure. The total content of K-feldspar in the greenish parts of the rock is 45—50%, whereas in the rock with a pink color the K-feldspar content reaches 80—85%.

Thus, the K-feldspathization of the gabbro-porphyrites described represents the result of metasomatic replacement, in the course of which a change in the mineral composition and structure of the rock takes place. At the same time, the redistribution of material in the replaced rock, accompanied by the introduction of alkalies and silica, leads to a spotted-schlieren structure. The development, among the granoblastic aggregate, of areas of syenite with a trachytic structure and normal faceting of grains testifies to the appearance of a liquid melt, from which crystalliza-

a magmatic-looking rock was formed, having a fluidal texture as a result of this melt’s ability to flow.

The study of the syenite massifs of the Kondoma iron-ore district shows the wide development of phenomena of alkaline metasomatism, which passes into magmatic replacement with the development of distinctive syenitic rocks. At the same time, in the marginal zones of high-temperature potassium metasomatism, owing to a decrease in temperature, the latter is replaced by sodium metasomatism. The latter also appears at late stages, superimposed on the potassium syenites.