Silica-Alkaline Metasomatism during the Emplacement of Small Diorite Intrusions in the Area of the Ilinskoye Deposit

G. V. Shubin

(Presented by Prof. A. M. Kuzmin)

In studying intrusive formations at the Ilinskoye gold-ore deposit, located in Eastern Transbaikalia, we observed phenomena of autometasomatism associated with the emplacement of small intrusions of dioritic composition that cut granites belonging to the Kyrinsky intrusive complex.

The diorites are medium-grained rocks with a hypidiomorphic-granular texture, consisting of intermediate and basic plagioclase (No. 49–55), ordinary hornblende; more rarely, among these minerals there occur isolated grains of potassium feldspar, quartz, apatite, sphene, and zircon.

The granites contain acidic plagioclase (No. 32), orthoclase, quartz, and, more rarely, biotite. In character, the granites have the appearance of leucocratic rocks, which, as investigations have shown, corresponds to apical parts of the intrusive massif, where the accumulation of potassium already in the magmatic stage led to the formation of well-expressed porphyritic segregations of large crystals of orthoclase and microcline.

Figure 1 presents a diagram of the chemical and material change of two contacting rocks in the course of autometasomatism associated with the emplacement of the diorites.

Considering the external features of alteration of the two contacting rocks, the following may be stated: 1) medium-grained, porphyritic granites toward the contact gradually reveal a compact texture, a fine-grained structure, and acquire features of rocks analogous to aplite-pegmatites and micropegmatites; 2) the diorites in the contact zone also undergo transformations. It is clearly seen how dark-gray, almost black, even-grained rocks closer to the contact with the granites acquire a pink shade and, in appearance, resemble rocks of the syenite type; directly at the contact itself the diorites become compact, with a barely discernible fine-grained structure and rare segregations of mafic minerals, acquiring a sporadic porphyritic character. As a result, rocks are formed that resemble fine-grained, pinkish-yellow granites.

The phenomena of autometasomatism were preceded by a certain recrystallization of the diorites in the near-contact parts, which was expressed in the blocky character of the plagioclase grains, with a simultaneous change in the optical orientation both of the twin structure and of individual blocks of the grains studied. Recrystallization was also expressed in the disappearance and blurred character of zoning and of the twin structure.

During the formation of the diorites, the zones of their contacts with the granites proved to be most favorable for the percolation of solutions from the deeper parts of the magma; these solutions produced changes in both the endocontact and exocontact zones of the diorites. We regard these solutions not as postmagmatic, but as solutions that accompanied the continuing crystallization of the dioritic magma, for even under conditions of formation of an apical or marginal shell we still cannot conclude that the solutions in question were of postmagmatic origin (except only in a narrow sense, with respect to the apical shell). Moreover, as will be shown below, the postmagmatic solutions had an entirely different character of mineral deposition.

Fig. 1. Diagram of the chemical and material change of granites (28, 29, 30) and diorites (31, 32, 33) (after Nakovkin):
Q — quartz, Fp — feldspar, Fm — dark-colored minerals, Pl — plagioclase, d — bulk density.

Fig. 2. Crossed nicols, ×14, thin section 540.

Penetrating along the most weakened contact zones, the solutions produced a number of characteristic changes both in the diorites and in the granites in contact with them within narrow near-contact zones. These changes were expressed in the broad development of quartz, orthoclase, microcline \((2V = 80)\), and, more rarely, potassic feldspar \((2V = -52)\), which, beginning to develop in the interstices between the prismatic grains of plagioclase in the diorites, gradually increased more and more, filling the volume of the rock at the expense of the gradually disappearing grains of plagioclase and dark-colored minerals, up to the formation of dense rocks with a microgranitic structure and having an aplite-like appearance (Fig. 2).

A similar picture, only on a smaller scale, is also observed by us in the contact zones of granites.

The quantitative ratios of the mineral components are shown in Fig. 1.

Tracing in thin sections the development of the quartz–feldspar mass, one can see the metasomatic character of its manifestation. Thus, quartz–feldspar aggregates almost entirely replace grains of plagioclase, as a result of which only relicts of the latter remain in the form of “skeletons” or individual, similarly oriented blocks (Fig. 3). In places it can be seen that hornblende also undergoes replacement of this kind.

Almost everywhere one can observe a micropoikilitic character of the manifestation of the quartz–feldspar mass. At the same time it should be noted that the feldspathic material was evidently deposited much earlier

Fig. 3. Pl—plagioclase, Q—quartz,
Ort—orthoclase.

than quartz. Thus, in a number of cases it is visible how prismatic grains of plagioclase become overgrown along the periphery and are replaced by feldspathic material, and then along these rims quartz develops, with the formation of micropoikilitic structures. In place of large orthoclase grains of the granites, structures of a micropegmatitic character arise, owing to which, in place of granite in the contact-adjacent part, the appearance of rocks analogous to micropegmatite or aplite-pegmatite is observed.

The above-described phenomena of rock alteration may be termed silica-potassic metasomatism. It is characteristic to note that this metasomatism is clearly observed not only in the contact zone, but also far beyond the contact (by 100–150 m) toward the diorite massif. True, here these processes, being of smaller scale, were expressed only in the weak development of quartz–feldspathic material in the interstices between grains of rock-forming minerals.

This may indicate that the action of solutions during the formation of the diorite magma manifested itself not only locally, at places of contact, but had the character of broad frontal impregnation.

As observations have shown, silica-potassic metasomatism is followed predominantly by sodic metasomatism, which can be explained by a sharp change in the composition of the solutions over time.

A change in the composition of the solutions toward an increase in their sodium content led to enrichment of the rocks in the immediate contact by albite, which develops in the early quartz–feldspar mass with the formation, in places, of cryptoperthites. Phenomena of sodic metasoma-

…compared with the earlier metasomatism had a local character of manifestation, i.e., only within the limits of the immediate contact. Simultaneously with the increase in albite content, there was also a decrease in the basicity of the plagioclase (from No. 55 to No. 30).

Somewhat later, in the contact zone of the diorites, biotite was deposited, exhibiting strong pleochroism from reddish-brown to colorless or weakly yellowish. Biotite develops not only in the form of independent idiomorphic grains, but also in the groundmass, penetrating into the intergranular space. Relict grains of hornblende are also subject to biotitization.

In conclusion, we note that the processes described above were accompanied by: 1) during the period of silica-potassium metasomatism—by a quantitative increase in the content of quartz and potassic, less commonly potassic-sodic, feldspar, with a progressive decrease in the content of plagioclase and mafic minerals; 2) during the period of predominantly sodic metasomatism—by a sharp decrease in the basicity of the plagioclase, the development of albite, and a further decrease in the content of mafic minerals and plagioclase.

The character of the behavior of elements during the processes of potassium-alkaline metasomatism is reflected in Fig. 1. The slight increase in the content of $\mathrm{Fe}^{\prime\prime\prime}$, Mg, and Ti in the contact area of the granites can evidently be explained by diffusion of these elements into the granite zone from the diorites, which was greatly favored by the alkaline solutions circulating in the contact.

It should be noted that no phenomena of remelting or assimilation of the enclosing granites by the dioritic magma were observed.

After recrystallization of the dioritic magma, in the zones of contact between the diorites and the granites, postmagmatic solutions manifested their action, which was expressed in the chloritization of hornblende and biotite with the simultaneous liberation of iron, which was concentrated in the form of small magnetite grains near biotite. Owing to the action of postmagmatic solutions, intensive tourmalinization took place, accompanied by the development of quartz and pyrite. All these processes correspond to later postmagmatic phenomena, which appeared at the deposit after the formation of small intrusions of dioritic composition and repeated tectonic disturbances affecting the near-contact zones of the diorites.