Magmatic Replacement and Metasomatism of the Host Effusive-Sedimentary Rocks in the Area of the Tashtagol Mine

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

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

In studying the petrography of the Tashtagol area of the gabbro-syenite iron-bearing intrusion, we drew attention to the widely manifested alteration here of the host rocks near the contact with the syenites. This alteration appears in the form of metasomatism and magmatic replacement, which may be combined under the term “syenitization,” since the final product of these processes is a pink fine- or medium-grained syenite. Indications of manifestations of syenitization of the host rocks are found in earlier works by G. L. Pospelov [1].

The studies we have carried out showed that all rocks are subjected to alkaline replacement: both the earlier intrusive formations in the form of gabbro-porphyries and diorite porphyries, and the rocks belonging to the Kondoma Formation of the Lower Cambrian and the Mundybash Formation of the Middle Cambrian. The Tashtagol acmolith-like intrusive massif is interformational; therefore, rocks of both formations undergo contact alteration. We have previously described the phenomena of syenitization of earlier basic intrusive rocks. In the present article, manifestations of alkaline metasomatism and magmatic replacement in limestones, chlorite-sericite-quartz schists, and effusive plagioclase porphyries will be noted.

Syenitization of Chlorite-Sericite-Quartz Schists

The indicated rocks are widely distributed in the area and are part of the Kondoma Formation. Relicts of the structures of lithoclastic tuffs, from which they were formed, are often encountered in them. The formation of the schists is associated with regional metamorphism of the rock sequence in connection with the manifestation of the Salair tectogenesis. Syenitization among these rocks appears locally in areas adjoining the syenite massif, and the replacement process is developed here most extensively in comparison with other rock types. Examples of alkaline replacement of rocks can be observed in various areas. The most interesting transitions to rocks with various degrees of replacement can be seen at the 140 horizon in crosscuts 20 and 21 of the Tashtagol mine. The unaltered original rock has a granolepidoblastic structure and the following composition: sericite 25–30%, chlorite 13–15%, quartz 50–55%, calc-

cite 8–10%. Quartz grains show undulatory extinction and appear to be enveloped by a scaly aggregate of sericite, chlorite, and fine-grained carbonate.

In areas of weak alkaline replacement, pink K-feldspar appears in the rock in the form of separate microscopic accumulations of a granoblastic aggregate. At the same time, the content of sericite and calcite usually increases as a result of redeposition.

Closer to the intrusive body, pink spots composed of an aggregate of K-feldspar appear in the schists. They have indistinct and uneven boundaries. In places the spots pass into irregularly arranged veinlet-like segregations of fine-grained syenite. The bulk of the rock between the indicated spots has a grayish-green color, but also acquires a faint pinkish tint as a result of the appearance of K-feldspar in it. The latter is detected under the microscope as a fine granoblastic aggregate of isometric grains with serrated margins. The properties of the mineral are as follows: $Ng = 1.525$; $Np = 1.520$; $2V = -75^\circ$. Sericite is widely developed in the rock; chlorite, epidote, and calcite occur in subordinate amounts; relict quartz is encountered. In some areas K-feldspar forms larger tabular grains.

Macroscopically, the pink spots are composed predominantly of K-feldspar. The structure of the rock here differs sharply from that described above. It is bostonitic, locally passing into a trachytic structure with a fluidal arrangement of crystals. In the marginal parts of the spots or schlieren there are transitions to an aplitic and granoblastic structure.

The veinlike segregations are already typical alkaline leucocratic microsyenites. Under the microscope, the structure of such rocks is tabular-granular; in places a fluidal arrangement of grains is well expressed. A granoblastic structure is entirely absent here. Later albite is developed over the K-feldspar. Redeposited sericite and calcite occur.

Thus, according to the intensity of alkaline replacement and the features of the structures, two types of rocks are distinguished. In rocks characterized by a weakly expressed replacement process, there is an abundance of relict rock minerals, in places noticeably redeposited. Granoblastic structures with fanciful serrated grain boundaries are developed here. Only individual, weakly expressed porphyroblastic grains acquire more or less regular tabular outlines. Such a syenitized rock has a metasomatic aspect.

The other type of rock, in the form of spots, schlieren, and veinlike segregations, is characterized by a fluidal texture, typical of a liquid melt capable of flow. The structure of the rock changes from bostonitic to tabular-trachytic. It is characteristic that the better the fluidality is expressed, i.e., the more mobile the melt, the more perfect the crystal faceting and the more clearly expressed the trachytic structure. The bostonitic structure of the rock characterizes a melt with weak mobility, just beginning to manifest itself. It is noteworthy that in areas of development of a mobile melt, in the marginal zones, an aplitic-like structure is observed, passing into a granoblastic structure in areas of metasomatic aspect.

The identical character of the mineralogical composition of the newly formed minerals in both types of rocks clearly indicates that the initial stages of alkaline replacement had the character of metasomatism. Progressive development of replacement led to the transformation of metasomatism into magmatic replace-

replacement. Nest-like segregations of liquid magma, minor movements of it, and the formation of vein-like segregations, together with the growth of these small centers, led to the complete replacement of the rock in areas adjoining the main massif.

Syenitization of Effusive Plagioclase Porphyrites

A significant place in the composition of the Kondoma suite is occupied by effusives of plagioclase and pyroxene-plagioclase porphyrite composition. In zones of crumpling they are locally altered to porphyritoids and greenish-gray chloritic schists. The syenite intrusion in a number of places cuts through the indicated porphyrites. At the same time the contacts of the intrusion vary in character—sometimes sharply expressed, but more often indistinct, in the form of broad intervals of zones in which the enclosing rocks are impregnated by syenitic magma.

Thus, in borehole 125 gradual transitions can be traced from unaltered and weakly syenitized porphyrites to rocks formed as a result of complete magmatic replacement. At a distance of 120–150 m from the contact the porphyrite has undergone weak potassic feldspathization. Macroscopically this is a greenish-gray, slightly schistose rock with a banded and barely noticeable spotted texture, due to the appearance of a pink aggregate of potassic feldspar. Under the microscope a primary porphyritic structure is observed, with plagioclase phenocrysts No. 25–28. The groundmass of the rock is strongly altered and contains relicts of plagioclase laths among secondary minerals in the form of sericite, calcite, albite, and leucoxene. The plagioclase of the phenocrysts is also strongly replaced by sericite and calcite. In the groundmass there are observed either separate xenomorphic grains of potassic feldspar or small accumulations of this mineral in the form of a granoblastic aggregate.

Over a distance of 100 m toward the intrusive body, the content of potassic feldspar in the rock increases continuously. Spots of a pink fine-grained aggregate of this mineral increase in size and merge with one another. Under the microscope a granoblastic aggregate of uneven, serrated grains of potassic feldspar is visible; it is partly replaced by albite, with rare relict minerals of the original rock. Here and there, as if shining through the rock, a relict porphyritic structure is visible. Against this background, schlieren and vein-like segregations of syenite with a trachytic structure and a fluidal arrangement of crystals are locally developed. The latter indicate nest-like development of a liquid melt capable of movement.

Farther on, over a distance of 10–20 m, the rock passes into a typical alkaline microsyenite—a porphyry with a bostonitic structure of the groundmass. In this macroscopically pink massive rock, porphyritic phenocrysts of tabular potassic feldspar and plagioclase No. 18 are observed. The groundmass contains no dark-colored minerals and is composed of potassic feldspar and albitized plagioclase. An admixture of secondary minerals in the form of chlorite and calcite is observed.

In the section of borehole No. 266, the development of alkaline replacement along the microfracturing of the rocks is visible, as a result of which the rock acquires a pseudobreccia texture. The mechanism of such a process was analyzed and demonstrated by the experimental studies of G. L. Pospelov, P. I. Kaushanskaya, and S. S. Lapin [2, 3]. Angular small blocks of rock are separated by pink vein-like areas of a granoblastic aggregate of potassic feldspar. Local growths of accumulations of the latter lead to the appearance of schlieren. In places, subsequent fragmentation of the rock and cementation by an aggregate of epidote and chlorite are observed. Relicts of po-

Geological section along line A–B

Fig. 1. 1 — tuffs, sandstones, conglomerates, breccias \((Cm_{2}pn)\); 2 — metamorphic shales with metasomatic breccias \((Cm_{1}kd)\); 3 — dike rocks (diabases, syenite-porphyries, diorites); 4 — alkaline quartz-bearing microsyenites \((\gamma\alpha D_{1-2})\); 5 — alkaline quartz-free microsyenites \((\gamma\alpha D_{1-2})\); 6 — alkaline leucocratic quartz-bearing syenite-porphyries \((\gamma\alpha D_{1-2})\); 7 — alkaline leucocratic quartz-free syenite-porphyries \((\gamma\alpha D_{1-2})\); 8 — lime-alkaline leucocratic quartz-free microsyenite-porphyries \((\gamma\alpha D_{1-2})\); 9 — lime-alkaline leucocratic quartz-free syenite-porphyries \((\gamma\alpha D_{1-2})\); 10 — lime-alkaline quartz-bearing leucocratic syenite-porphyries \((\gamma\alpha D_{1-2})\); 11 — pyroxene, pyroxene–hornblende gabbro-porphyrites \((\gamma\alpha D_{1-2})\); 12 — zone of “syenitization” of the host rocks; 13 — skarns; 14 — ore bodies; 15 — stratigraphic boundaries; 16 — boundaries between petrographic varieties of rocks.

rocks in weakly replaced, angular blocks contain either isolated grains of newly formed potassium feldspar, or accumulations of it making up about 15% of the area of the rock. Primary segregations of plagioclase are strongly replaced by carbonate, while the main mass of the rock is replaced by streaky aggregates of sericite, carbonate, and chlorite. Albite is widely developed both after potassium feldspar and as independent grains. Calcite and epidote are often developed after potassium feldspar.

In areas of large vein-like segregations of syenite, fluidality appears, as do more regular, elongated-tabular crystal forms, typical of syenite of magmatic origin.

In zones of more pronounced fracturing of the effusives, the syenites form sharp boundaries with the host rocks. Here syenitization of the host rocks is almost undeveloped.

Syenitization of limestones

At the contact of syenite bodies with limestones, their replacement is also manifested, with the development of small spots of syenitic composition and more dispersed potassium feldspathization. Apparently, owing to the compositional features of the rock, this process was weakly expressed, as indicated by the small thickness of such zones.

In the areas farthest from the contact, but weakly affected by replacement, the limestones acquire a pinkish tint because of the appearance of irregular grains of potassium feldspar ($N_g = 1.528$, $N_p = 1.523$). Closer to the contact, small spots of a potassium-feldspar aggregate appear in the limestones, locally forming rosettes and radial aggregates. Recrystallization of calcite is noticeable.

Finally, near the contact with the syenites, abundant spots of syenitized rock appear. They merge into indistinct schlieren and nests, as well as elongated vein-like segregations. In these areas the granoblastic structure of the potassium-feldspar aggregate is replaced by a radial and bostonitic structure with a fluidal arrangement of grains. Within such rock, islands and nests of recrystallized limestone are preserved. Albite and sericite show slight development after potassium feldspar.

Thus, the manifestation of alkaline replacement of the host rocks has general regularities. They are expressed in the following:

1. The process of syenitization is manifested in quite the same way in original rocks of very different composition. The syenite that appears by replacement does not differ in composition, whether it is formed after effusive plagioclase porphyrites, after limestones, or after other rocks. All this indicates the high temperature of the active melt-solution that caused the replacement of the rocks.

2. Depending on proximity to the contact with the syenite massif, the intensity and morphological features of alkaline replacement differ. In the areas most distant from the contact, individual grains of potassium feldspar and its aggregates are developed. Closer to the contact, spotted segregations of a pink granoblastic aggregate of potassium feldspar appear, contaminated by remnants of the replaced rock and superimposed products of metasomatism. Then the spots increase in size, and veinlet-like segregations of pink syenite appear. In this rock, weak fluidality, a porphyritic fabric, and a bostonitic structure are already observed. Breccia-like rocks, or breccias, are developed in places. Finally, near the contact, the spots expand and merge with one another. Fluidality and a syenitic structure of lath-shaped feldspar grains are manifested. Interestingly,

that with the development of fluidity, when crystallization of the moving melt occurs, fine-grained chilled zones arise in these areas.

1. Thus, in areas of magmatic replacement, a gradual transition into the replaced rock is observed. Where the syenite magma underwent significant displacement, we see sharp intrusive contacts and more distinct chilled zones composed of fine-grained rocks.

2. Syenites formed as a result of the separation of a mobile alkaline melt-solution, with the manifestation of magmatic replacement, have a leucocratic composition and belong to the alkaline syenites. Within the massif they form upper marginal zones with gradual transitions into calcareous-alkaline syenites, which make up the cores of the massifs (Fig. 1).

A very important fact is the confinement of contact metasomatic iron-ore bodies to alkaline varieties of syenites. At the Tashtagol deposit, the ore bodies are confined to a deep pocket of development of these syenites. In our opinion, the indicated regularity is genetic and makes it possible to use it for evaluating and predicting mineralization.

REFERENCES

1. G. L. Pospelov. On replacement phenomena in the formation of effusive-like rocks and on the features of certain ore-bearing volcanic complexes. In: “Magmatism and the Associated Mineral Resources.” Proceedings of the 1st All-Union Petrographic Conference, 1955.

2. G. L. Pospelov, P. I. Kaushanskaya, S. S. Lapin. On the crack-free formation of vein-like mineral bodies and breccia-like zones of mineralization. Geology of Ore Deposits, No. 2, 1961.

3. G. L. Pospelov, P. I. Kaushanskaya. Stages of development and types of crack-free vein formation. Geology and Geophysics, No. 9, 1962.