OLIVINE HORNFELSES ON THE KATEGIR RIVER

(Western Sayan)

M. I. Yudin

(Presented by the scientific seminar of the Department of Petrography, Mineralogy, and Mineral Resources)

In the summer of 1957, in the valley of the Kategir River (a left tributary of the Yenisei), in the zone of contact between serpentinites and a granitoid intrusion cutting through them, the author discovered almost monomineralic olivine hornfelses. Below is given a brief petrographic description of the latter, and considerations are set forth concerning their origin.

Macroscopically, the hornfelses mentioned are fine-grained, light- or dark-gray rocks. In places, their light-gray and dark-gray varieties alternate with one another, forming typical banded textures with bands from 2–3 cm to 5–7 cm wide. In this case the strike of the banding coincides with the strike of the schistosity in the serpentinites and with the direction of their contact with the granitoids (Fig. 1). The banding in the hornfelses is especially well expressed on the surface prepared by the current of the river. In places the boundaries of the bands seem so sharp and rectilinear that their texture appears ribbon-like (Fig. 2). However, gradual transitions between bands are observed just as often; in this case the bands usually pinch out, acquiring the form of lenses of varying length. As the microscope shows, the light and dark bands of the banded hornfelses are respectively identical in mineralogical composition to the light-gray and dark-gray olivine hornfelses. The lighter and darker coloration in both cases is explained, respectively, by a lesser and greater content of fine disseminated magnetite.

Fig. 1. Sketch of the geological structure of a section of the Kategir River valley near Bolshoi Porog.
1—schistose serpentinites, 2—banded hornfelses, 3—granitoids, 4—elements of occurrence of the banding.

It should be noted that, in addition to bands of the indicated composition, in the banded hornfelses, which consist essentially of olivine, rarer black bands 2–3 cm wide are encountered. They are relatively coarse- מערך, relatively coarse-
grained and consist, in addition to olivine, of a significant amount of magnetite.

granular, with grain sizes up to 2 mm. The isometric or slightly elongate shape of the grains and the gleaming cleavage create the impression that these bands are composed of pyroxene. However, study under the microscope shows that the pyroxene in them has been completely replaced by magnetite and tremolite, forming pseudomorphs after it. The mineralogical composition of these bands, as is evident, is no longer olivine-bearing. The principal minerals are: tremolite—50–70%, magnetite—12–15%, talc—10–12%; accessories: olivine, chromspinelides, rarely serpentine and chlorite.

Fig. 2. Banded olivine hornfels.

Fig. 3. Relict pseudomorphic microstructure of the black bands.
Black—magnetite; white—tremolite and talc, replacing pyroxene. Parallel nicols, magn. 33.

Tremolite and magnetite form a relict pseudomorphic structure [3], whose features are as follows. The iron released in the process of replacement of pyroxene was, for the most part, driven toward the periphery of the grains, where, in the form of dusty aggregates of magnetite, it formed a continuous black rim, usually with a sharp outer boundary and a very dissected and uneven inner boundary. The remaining part of the former pyroxene grains was filled chiefly by tremolite, forming either single crystals or fan-shaped aggregates of strongly elongated grains, the ends of which either abut against the magnetite rim and in this case are, as it were, evenly cut off (Fig. 3), or extend beyond it, in which case the shape of the pyroxene grains is usually poorly preserved.

The olivine hornfelses proper, including the banded varieties, consist of 70–80% olivine and talc (10–15%). The constant accessory constituents are

are tremolite, magnetite, serpentines; chlorite and diopside are encountered rarely. Their microstructure is hetero-granoblastic. Most olivine grains have sizes of 0.02–0.05 mm, and only a few of them reach 0.5–0.7 mm. Sporadically, in thin sections there occur areas composed entirely of olivine grains 1–2 mm in size. Small olivine grains are usually arranged in clusters. In view of the extremely small size of these grains, often less than the thickness of the thin section, their aggregates appear as accumulations of small “grains.” The sizes of the clusters consisting of olivine grains vary. Situated at some distance from one another and joining with one another, they form irregularly shaped interspaces filled with the finest scales of talc, among which inclusions of serpentine and, very rarely, chlorite can sometimes be seen. A characteristic feature of these fine-grained areas is also that peculiar olivine veinlets are very often encountered in them, cutting both the olivine aggregates and the areas composed of talc. The length of these veinlets, with a width of 0.04–0.08 mm, reaches only 1–2 mm. In some places they have a subparallel arrangement; in others no orientation is observed; sometimes, bending, they form loops similar to those formed by chrysotile in antigorite serpentinite or olivine (Fig. 4). Just as in chrysotile veinlets, along the middle there passes a crack; in its place, in the described olivine veinlets, a narrow brownish stripe is everywhere visible. Olivine veinlets of another type are encountered more rarely. They are larger, pass through the entire thin section, and are composed of olivine grains that, as a rule, occupy the entire width of the veinlet. Measurements of the angle of the optical axes of olivine from the veinlets give values of \(+88^\circ\), which corresponds to olivine with a content of the fayalite molecule of about 10% [4]. Here one more, in our view, curious structural feature should be pointed out. In thin sections there are areas of rock consisting almost entirely of olivine grains, which in crossed nicols, in the position of maximum illumination, appear rather large (1–2 mm), while upon rotation from

Fig. 4. Fragments of the microstructure of olivine hornfelses

A — area of a thin section with intersecting olivine veinlets; white — talc; black: 1 — brown pelitomorphic substance inside the olivine veinlets; 2 — ore grains; crossed nicols, magn. 33. B — area of a thin section of olivine hornfels enriched in tremolite. Tr — tremolite, Sp — serpentine, Ol — olivine; crossed nicols, magn. 30.

from this position by a small angle (10–15°), some of these grains “break up” into an aggregate of small (about 0.1 mm) grains having a similar, but at the same time noticeably different, optical orientation. Thus, it is found that one group of adjacent grains has gray and related interference colors, another green, and so on. The small olivine grains are grouped, as can be seen, into separate “nests.” There is reason to see in the features of this structure phenomena of recrystallization. The small grains that arose at an early stage of thermal metamorphism underwent recrystallization during the stage of metasomatism, chiefly under the influence of volatiles that favored the formation of large grains at the expense of small ones. No noticeable differences were found in the magnitude of the angle of the optical axes of olivine in small and large grains. Only considerable fluctuations from \(+74^\circ\) to \(90 \pm 1^\circ\) were established (here only measurements made from the exits of the two optical axes are taken into account). The mean value of the angle of the optical axes from twenty measurements is \(+82^\circ\).

Serpentine of the antigorite type constitutes only a rare impurity. Its small flakes make up small isolated areas squeezed between aggregates of olivine grains, or between areas composed of talc and olivine. In all cases the serpentine aggregates have the form of remnants of serpentine that survived its recrystallization into talc and olivine; moreover, some of them are pierced in various directions by thin needles of tremolite.

Clinochlore occurs still more rarely than serpentine. It can be found in association with talc or magnetite. Around magnetite grains it often forms continuous rims 0.1 mm wide, composed of small fanlike flakes. In areas composed of talc it occurs as aggregates consisting of 3–4 flakes 0.2–0.3 mm in size. Wherever it was observed, it appears colorless, with characteristic anomalous brownish-gray or rusty-brown interference colors. In two thin sections, small diopside grains were found in association with talc.

Tremolite is distributed unevenly. Most of it is contained in the olivine aggregates along the boundary with the tremolite–magnetite bands. Here it forms large elongated crystals with split, splintery ends (Fig. 4b). In both massive and banded olivine hornfelses, subparallel tremolite veinlets 0.02 to 0.05 mm wide can often be seen. These veinlets leave no doubt that the tremolite in the main part formed after olivine and talc, and contemporaneously with the minerals of the black bands—tremolite and magnetite—which replaced pyroxene. Finer and rarer needles of tremolite can often be seen apart from the veinlets. They occur in talc–olivine aggregates. Some of them are visible in large olivine grains and appear as poikiloblastic inclusions.

When thin sections are examined in transmitted light, the presence of dark-brown, sometimes almost black or yellowish-brown concentrations attracts attention. At high magnifications and under strong illumination it is clearly seen that one or, more rarely, several olivine grains show through beneath them, and that they consist of a dustlike dissemination of ore mineral and of pelitomorphic material with a brownish-yellow color. Some of them, especially those formed only by ore dust, show features of a relict pseudomorphic structure, making it possible to suppose that they originated as a result of the replacement of pyroxene. However, most of these spots have an irregular form and indistinct, diffuse outlines. Olivine grains (possibly chondro-

dite), which appear almost entirely covered by a brownish or yellowish pelitomorphic substance; only a narrow colorless rim remains along the periphery. The mineral thus seems to be colored. It is characteristic that in a number of these grains a phenomenon resembling pleochroism is observed: the color, when the microscope stage is rotated, either disappears or becomes paler, then appears again, and the mineral acquires a brownish or orange-yellow color.

The geological position of the rocks described, and their structural and textural features, convince us that they arose under conditions of thermal metamorphism of serpentinites with the active participation of postmagmatic solutions. In genesis and mineralogical composition they are therefore close to the magnesian hornfelses of F. D. Turner [2]. Their origin, in general outline, may be represented as follows. Under conditions of general heating of the serpentinites in the contact zone up to the upper limit of serpentine stability (500°C according to H. Bowen and O. Tuttle), the latter proved unstable and dissociated according to the scheme:

\[ 2\mathrm{H}_4\mathrm{Mg}_3\mathrm{Si}_2\mathrm{O}_9 \rightarrow 3\mathrm{Mg}_2\mathrm{SiO}_4 + \mathrm{SiO}_2 + 4\mathrm{H}_2\mathrm{O} \]

\[ \underbrace{\phantom{3\mathrm{Mg}_2\mathrm{SiO}_4}}_{\text{forsterite}} \qquad \underbrace{\phantom{\mathrm{SiO}_2}}_{\text{solution}} \]

The siliceous solutions thus formed, heated to a temperature exceeding 500°, circulated along cracks in the serpentinites that were undergoing schistosity and, entering into chemical interaction with them, led to the formation of a parallel system of pyroxene veinlets. The latter thus represent “traces” of departing silica.

\[ \underbrace{\mathrm{H}_4\mathrm{Mg}_3\mathrm{Si}_2\mathrm{O}_9}_{\text{serpentine}} + \mathrm{SiO}_2 \rightarrow \underbrace{3\mathrm{MgSiO}_3}_{\text{enstatite}} + 2\mathrm{H}_2\mathrm{O}. \]

In those parts of the rock where open cracks were absent, the silica remained in the pore solution; as a result, talc formed together with olivine. The possibility of enstatite formation in these areas was apparently limited by the excess of water vapor, under whose conditions, according to H. Bowen and O. Tuttle [1], enstatite is unstable. The composition of the pore solution and the physical conditions at this stage of metamorphism were evidently such that a certain amount of iron entered the pyroxene and olivine.

Thus, during the first stage of metamorphism, which corresponds to the stage of hornfels recrystallization at the expense of serpentine, olivine, talc, and pyroxene arose.

Chemical analyses of the banded hornfels are given below. From the analyses it is evident that in the olivine hornfels (analysis 1) and in the pyroxene bands replaced by tremolite and magnetite (analysis 2), the total contents of ferric and ferrous iron are approximately the same, but the quantitative ratios of these oxides are reversed: in the olivine hornfels ferrous iron predominates (5.13%), whereas in the black tremolite–magnetite band there is approximately the same amount (5.93%) of ferric iron. On the basis of these analyses it is thus not difficult to see that the process of metasomatic replacement of the pyroxene bands consists in the introduction of calcium, the oxidation of iron, and the consequent formation of tremolite, which completely replaced the pyroxene veinlets or partially developed between the latter in the olivine hornfels. The analyses presented record small amounts of alumina and alkalis, which are not characteristic of magmatogenic hyperbasites and serpentinites of the area studied by us even in these small amounts. From comparison of the analyses with one another it is evident that, in place of the replaced pyroxene ba-

Table 1

Analysis 1—gray band—olivine hornfels; analysis 2—black band—tremolite-magnetite aggregate that has replaced pyroxene bands. The analyses were performed at the Department of Silicate Technology, TPI.

in the bands alumina and alkalis are contained in larger amounts than in olivine hornfelses that have been partially subjected to metasomatism. This fact gives grounds for discerning an introduction, along with calcium, of small amounts of alkalis and alumina.

The change of mineral associations in the course of replacement rather definitely indicates the behavior of iron, which, as in this case it is not difficult to ascertain, behaved immobily during the replacement of pyroxene by tremolite. Therefore, whether we see in the olivine grains inclusions in the form of small grains of magnetite, or a finer magnetite “dust,” up to the appearance of a brownish pelitomorphic substance, all this may be regarded as the result of the forsteritization of olivine of early generations at a later stage of the metasomatic process, under conditions in which olivine relatively rich in iron was unstable, while the iron released from its crystal lattice, replaced by an equivalent amount of introduced magnesium, was chemically inert and little, or almost not at all, mobile.

REFERENCES

1. N. L. Bowen and O. F. Tuttle. Collection Problems of Physical Chemistry in Mineralogy and Petrography, IL, 1950.
2. H. Williams, F. D. Turner, C. M. Gilbert. Petrography, IL, 1957.
3. B. M. Merenkov. Relict-pseudomorphic structures of ultrabasic rocks and the products of their alteration. Proceedings of the All-Union Conference on the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, issue 17, Publishing House of the Academy of Sciences of the USSR, 1957.
4. A. Poldervaart. The Am. Mineral., 35, Nos. 11, 12, 1950.