On the Dolomites of the Alguy Talc Deposit

A. I. Shevelev

(Presented by Professor A. M. Kuzmina)

Dolomites, quartzites, tremolite rocks, as well as dike formations of diabases take part in the geological structure of the deposit. Among the enclosing rocks, dolomites are the most widespread; this article is devoted to describing some of their properties and features.

In the study, thin sections were used, taken rather uniformly from prospect pits near the talc deposit, and also from core-drilling boreholes drilled in the dolomites along a line intersecting the central part of the deposit and located in direct contact with the ore body.

The dolomites occur as thick beds of almost meridional strike in the western and southeastern parts of the deposit.

In appearance, the dolomite is fine-grained, dark gray in color, dense, massive, with an uneven fracture. Near contacts with the talc deposit, the dolomite contains inclusions of quartz and calcite, and is also characterized by increased fracturing and contains areas of breccia-like structure.

Under the microscope, the rock consists of small grains of dolomite (usually within 0.02 mm) of polygonal or rhomboid form. The grains have a brownish-gray color.

In some thin sections, banding is observed in the dolomite, caused by the parallel alternation of bands of organic matter. The presence of organic matter, which occurs both in the form of bands and as separate spots, is indicated by the fact that, upon calcination, the dark-gray areas of the rock acquire a white color as a result of the burning out of the organic matter.

In the dolomite, recrystallized areas with larger grains are observed. These areas are confined to fractures, or are arranged as discontinuous bands, or in the form of irregular segregations.

In zones of disturbance there are recrystallized dolomite grains of elongated form, oriented along the fracture, as a result of tectonic movements.

Dolomite grains in recrystallized areas are characterized by larger sizes (up to 0.1–0.3 mm, rarely up to 0.7–0.8 mm) than the grains of the main mass (up to 0.02 mm). Their form is polygonal or rhomboid; their color is lighter than in the main mass,

which is evidently due to purification from organic matter during recrystallization; owing to the different color of the main mass and the recrystallized areas, the rock assumes a spotted appearance under the microscope.

The rock contains veinlets of dolomite of hydrothermal origin, which occur either as discontinuous formations or as bands of uniform thickness along the entire length of the thin section. Their thickness varies from fractions of a millimeter to 8–9 mm. The veinlets cut the banding and the recrystallized areas in the dolomite. They are composed of grains of light-colored dolomite, have mainly sharp contacts, and therefore stand out clearly within the main mass of the dolomite (Photo 1). Large dolomite veinlets up to 8–9 mm thick are composed of dolomite grains up to 1.0 mm in size, of rhombohedral habit; among them there are relics of fine-grained dolomite of the main mass. From these large veinlets, thinner ones branch off along cracks, likewise composed of light-colored dolomite grains.

In the veins, together with dolomite, calcite is present. An example is the dolomite from pit 424: the rock is almost completely recrystallized (grain size 0.06–0.08 mm); within it, primary unrecrystallized areas with a grain size of 0.02 mm have been preserved as individual spots. The veinlets cutting the rock are filled with light-colored dolomite and calcite. Calcite occurs either as irregular segregations, or fills the spaces between dolomite grains, and also completely fills individual parts of cracks. Among the calcite segregations there are rhombohedral grains of dolomite in the form of the smallest relics (Photo 2).

Near the contact with the talc deposit, inclusions of quartz appear in the dolomite in the form of veinlets along cracks. In disrupted

Fig. 1. Veinlet of hydrothermal dolomite (white) in fine-grained dark-gray dolomite.
Magn. 55. Nicols — (borehole 5, depth 95.0 m).

Fig. 2. Veinlet of calcite (light) in dolomite. Among the calcite are rhombohedral segregations of light-colored dolomite (marked +).
Magn. 120, nicols — (pit 424)

areas of the dolomite the veinlets widen, forming irregular segregations, among which dolomite that has undergone recrystallization occurs as inclusions. The size of the quartz grains ranges from 0.03 to 0.2 mm and is, for the most part, the same for each veinlet. In large veinlets the quartz grains have different sizes. Thus, in borehole 5 at a depth of 61 m (thin section 5–3), at the contact with dolomite the quartz has the form of elongated grains oriented along the contact, whereas in the direc-

toward the central part the grain size decreases, and they acquire a polygonal shape.

Along large cracks there is a process of replacement of dolomite grains by quartz grains. These quartz grains, in turn, along cracks begin to be replaced by fine-grained dolomite (photo 3).

At the contacts of quartz veinlets with dolomite, a process of dedolomitization of the dolomite is observed, which usually takes place at the boundary between quartz and dolomite grains. Some dolomite grains are almost entirely replaced by calcite, and the primary rock occurs in the form of the finest inclusions.

In the dolomite at the eastern contact with the talc body, in borehole 4 at a depth of 80 m, there are segregations of brucite. The grains are acicular in form; their length reaches 1.0–1.3 mm, with a thickness of up to 0.01 mm. Brucite has the characteristic low interference colors, negative elongation, and occurs as individual needles among fine-grained dolomite. Calcite develops along later cracks in the rock.

Fig. 3. Replacement of quartz (light) by fine-grained dolomite (gray) along cracks.
Magn. 55. Nicols crossed—(borehole 5, depth 61.2 m)

In some thin sections, in fairly large (0.4–0.6 mm) dolomite grains, glide twins develop, consisting of an alternation of bands parallel to the rhombohedron and behaving differently with respect to polarized light under crossed nicols. This is seen quite well in photo 4. Glide twins were found in veinlets along tectonic zones.

Fig. 4. Glide twins in dolomite grains.
Magn. 120. Nicols—(borehole 5, depth 72.0 m)

The chemical composition of the dolomites at the deposit can be judged from the data in Table 1, which gives chemical analyses of samples taken from the faces of prospecting pits. The analyses are grouped by exploration lines from the northern part of the deposit to the southern one, and within the lines they are arranged in the direction from the lower to the higher horizons of the dolomite sequence. As can be seen from Table 1, the most significant impurity is SiO₂, which is represented in the rock by quartz; the latter, judging from thin sections, usually occurs in the form of veinlets gravitating toward disturbed zones. An increased quartz content is also noted in dolomites lying in direct contact with the talc deposit, which can be traced

through borehole 5 (western contact), which entered from the ore body into the dolomite at a depth of 58 m and passed through it to a depth of 165 m.

Curves of variation in the content, along the borehole, of dolomite and calcite (according to thermal-analysis data), as well as of SiO₂, Fe₂O₃, and insoluble

Table 1

Chemical composition of dolomites

residue (by chemical analysis) are given in Fig. 5, where it is evident that in the immediate vicinity of the talc deposit the quartz content is rather high (up to 12%); then it gradually decreases and, from a depth of approximately 90 m, remains within the range up to 0.5%. Since the borehole was not drilled perpendicular to the strike of the bed, the true thickness of the silicification zone is determined as 10–15 m. From approximately a depth of 140 m, the quartz content begins to increase. The curve of the content of insoluble residue almost completely repeats the curve of SiO₂ content, which also confirms the presence of quartz as the principal impurity in the rock. The graph indicates an inverse relationship between the contents of dolomite and calcite in the rock. Thus, beginning at a depth of 145 m, the dolomite content decreases to 7.7% (at a depth of 160 m), while the amount of calcite increases to 86.2%, and farther with depth there is again an increase in dolomite. In the transitional zone from essentially dolomitic rock to essentially calcitic rock, according to thermal-analysis data, ankerite and pyrite appear. To this zone is confined

also an increase in quartz content. The current absence of thin sections from this interval and of the results of chemical analyses does not make it possible to interpret these changes with certainty, but it may be assumed that they were caused by hydrothermal processes that led to the dedolomitization of the dolomites, with the formation of a substantially calcitic rock and the enrichment of the hydrothermal solutions with magnesium; these solutions served as the source for the deposition of hydrothermal dolomite along fractures.

Fig. 5. Graph of variation with depth in the contents of $\mathrm{CaMg(CO_3)_2}$ and $\mathrm{CaCO_3}$ (differential-thermal analysis data), $\mathrm{SiO_2}$, $\mathrm{Fe_2O_3}$, and insoluble residue (chemical-analysis data) in dolomites from borehole 5.