DISTRIBUTION FEATURES OF ACCESSORY MAGNETITE IN GRANITES OF THE KAMENSKY MASSIF (NW Altai)

T. I. Poluektova

(Presented by Prof. A. M. Kuzmin)

Accessory magnetite from the granites of the Kamensky massif attracts attention because in the named rocks it occurs in comparatively large quantities. The present work examines the features of variation in the content of this mineral in the rocks of the intrusive body, as well as the principal regularities in the distribution within it of trace elements.

The Kamensky granitoid massif is located in the northwestern part of the Irtysh folded zone. Its area is about 200 sq. km. Geologically, the intrusive body is a laccolith, gently plunging to the southeast [2]. From the southeast and northeast the granitoids are in contact with Devonian greenstone schists. The southwestern and northwestern contacts of the massif are overlain by a thick sequence of Quaternary deposits. The age of the pluton is referred to the Permian.

In terms of petrographic composition, rocks of five intrusive phases are distinguished in the structure of the massif (in order of formation): coarse-grained porphyritic biotite granites; medium-grained two-mica, aplite-like granites; aplites; pegmatites. The rocks of the first phase are most widespread; among them, granites of the central and endocontact parts of the massif are distinguished.

Magnetite—a mineral constantly present in the rocks of the massif—is usually observed as inclusions in all rock-forming minerals and is distributed extremely unevenly in the granites. Thus, in the granites of the first phase of the central part of the massif, its content ranges from 3750 to 25200 g/t; the average is 9400 g/t. In the endocontact granites the amount of magnetite decreases to 6650 g/t. It should be noted that in a sample from the contact with the enclosing rocks its content is 4350 g/t, while at 1750 m from the contact toward the center of the body the concentration of this mineral reaches 18280 g/t. In medium-grained two-mica granites (phase II) the average magnetite content is 77224 g/t. In rocks of the subsequent intrusive phases it decreases to 5848 g/t in aplite-like granites, to 3370 g/t in aplites, and to 4410 g/t in pegmatites. Thus, on the whole, the magnetite content decreases from rocks of the early intrusive phases to the later ones. This conclusion is confirmed by studies by a number of authors [3].

Magnetite occurs as crystals of octahedral form. The surface of their faces is usually pitted and sculptured. Quite often the crystals are elongated in the direction \([111]\) or, conversely, flat-

etched in the (111) plane. Along with well-formed crystals, curved-faced and spheroidal magnetite formations are encountered, similar to those described for diamond [4].

The sizes of magnetite grains range from 0.14 to 0.33 mm. Mass measurements of crystal size made it possible to note that, in granites of the first phase in the central part of the massif, the most frequent crystal sizes lie within 0.20–0.24 mm. In intensely kalispatized varieties of the rocks, magnetite is present almost exclusively as rounded grains, or grains close to them, with a size of 0.15–0.20 mm.

In the rock, magnetite usually occupies intergranular areas; it is often found included in biotite, which in individual cases is replaced by chlorite, in orthoclase, plagioclase, and quartz. In granites of the second phase, it is also observed in the form of a fine sprinkling.

When the mineral is studied in polished sections*) in reflected light, after etching in concentrated hydrochloric acid, the decomposition structures of the solid solution

\[ n\mathrm{FeFe}_{2}\mathrm{O}_{4}\ (100-n)\ \mathrm{FeTiO}_{3}, \]

which represent the segregation of titanate in magnetite in the form of ilmenite, are revealed quite clearly. The form of the segregations proves to be very diverse. Thus, in sections parallel to the octahedral face, ilmenite plates form a distinctive network with symmetry corresponding to a third-order axis of symmetry (Fig. 1). More often, ilmenite plates

Fig. 1a.

are arranged in magnetite in the form of randomly dotted, irregularly nodular (Fig. 2), and parallel linear segregations (Fig. 3). In the latter case, linear-dotted, linear-dashed, linear-curtain-like, and some other forms can be distinguished, crossing magnetite grains from one edge to the other. Sometimes dendrite-like ilmenite formations are observed along the edges of the magnetite grain. The described types of ilmenite segregations may be characteristic of one or another grain, but no less often they occur in combination within one and the same individual. At high magnifications (600–800X), it is sometimes possible to observe a very fine lattice structure, which probably corresponds to ulvöspinel structures, and very small black inclusions—spinels.

*) To obtain a preparation from fine-grained magnetite concentrate, a method was developed for cementing it with the use of etacryl powder (Akr-15) and etacrylic liquid. After hardening, the resulting specimen is polished like an ordinary polished section.

Magnetite grains are often subject to oxidation, with the formation, along cracks in the mineral and other defects, of platy hematite crystals (Fig. 4). The latter sometimes completely replaces magnetite. In this case, the resulting Debyegram shows lines

Fig. 1b.

Fig. 2.

Fig. 3.

Fig. 4.

\((d/n)\) 2.199(2), 1.688(10). 1.307(5), 1.137(4), and 1.101(6), which are typical only of hematite.

The chemical composition of magnetite was studied from data of spectral semiquantitative and quantitative analyses, as well as from determination of its radioactivity. The following impurity elements were established in the mineral: Si, Ca, Mg, Al, Na, K, P, Ti, Mn, Ba, Sr, Cr, V, Ni, Co, Zr, Nb, Li, La, Ga, Sn, Cu, Pb, Zn, Cd, Ag, As. The presence of such impurity elements as Si, Ca, Al, Na, K is explained by the presence of inclusions of rock-forming minerals [1], and therefore we shall not dwell on the distribution characteristics of these elements. With regard to Mg, it may be noted that its presence is partly due to inclusions of biotite; partly it may isomorphically replace \(Fe^{2+}\) ions and form a series of minerals of the spinel group included in magnetite (spinel, magnesioferrite, magnocobside, etc.).

Titanium is constantly present in the magnetite of the intrusive rocks.

Its content in samples of granites of the first phase from the central part of the massif ranges from 0.06 to 0.2%; the most frequent are samples with a titanium content of 0.1%. In magnetite from granites of the endocontact facies, the concentration of this element increases and lies within the range 0.3–0.5%. In the rocks of later differentiates, the amount of titanium in magnetite decreases to 0.06%; only in isolated samples from granites of the second phase does it amount to 0.1%.

Vanadium in magnetite from granites of the first phase lies within the range from 0.0017 to 0.01%; in the endocontact zone of the intrusive body it decreases, so that its maximum content is no higher than 0.006%. In rocks of the second, third, fourth, and fifth phases it is not recorded in all samples, and its amount usually does not exceed 0.001–0.003%.

Manganese is a constant element of all rocks of the massif. Its amount in them is determined by figures of 0.1–0.3%.

Nickel is established in magnetite only in granites of the first phase from the central part of the massif, in an amount of 0.0003–0.002%.

The content of zinc in magnetite of all rocks of the massif is fairly constant and lies within the range 0.002–0.04%.

Tin is recorded in samples comparatively rarely, and its concentration varies over wide limits: 0.0003–0.3%. With some approximation, an increase in the content of this element in magnetite of the late differentiates may be noted.

The amount of lead ranges within 0.001–0.003%; its content decreases toward the contact of the massif with the enclosing rocks. In samples from rocks of subsequent intrusive phases, the concentration of the element increases to 0.006%.

Copper, with few exceptions, was recorded in all samples of magnetite from granites of the first phase in the central part of the massif; in the endocontact of the intrusive body, copper was recorded only in one sample. Its content in magnetites of all rocks ranges from 0.001 to 0.003%.

Zirconium is recorded in magnetite in amounts from 0.003 to 0.02%. No definite regularity in the behavior of zirconium in magnetite of rocks of one or another intrusive phase has been observed.

Gallium, barium, and strontium are constantly present in the mineral described, in amounts from thousandths to hundredths of a percent. Other elements—phosphorus, niobium, lithium, yttrium, ytterbium, boron, and arsenic—were detected in individual samples.

Regarding the form of entry of the impurity elements characterized above into the crystalline structure of magnetite, it may be noted that Ti³⁺ (0.76), Ti⁴⁺ (0.68), V³⁺ (0.74), V⁴⁺ (0.63), Cr³⁺ (0.63), Mn²⁺ (0.80), Mn³⁺ (0.66), Zn²⁺ (0.74), Sn⁴⁺ (0.71), Ni²⁺ (0.69), Pb⁴⁺ (0.84), Zr⁴⁺ (0.79), Nb⁴⁺ (0.74), Ga³⁺ (0.62), Li⁺ (0.68), Cu²⁺ (0.72)*)—owing to the closeness of their ionic radii, as well as the similarity of atomic structure—substitute isomorphously in the crystal lattice of magnetite for Fe²⁺ (0.82) or Fe³⁺ (0.67) ions. The presence of Y³⁺ (0.92), Yb³⁺ (0.85), La³⁺ (1.14), Cu⁺ (0.96) is evidently explained by their occurrence in the form of molecular dispersion. Ba²⁺ and Sr²⁺, as impurity elements in magnetites, are often noted in the literature [3]. However, the nature of their entry into the crystalline structure is not clear.

The figures of quantitative spectral analysis for elements of the iron group—Mn, V, Cr, and Ti—are given in Table 1.

*) The figures in parentheses next to the ion of an element correspond to its effective radius.

As follows from the table, the general tendency in the content of manganese, vanadium, chromium, and titanium toward a decrease in their amount from differentiates of the first phase to later ones, as well as according to the data of semiquantitative spectral analysis, is preserved. A certain deviation from this rule is indicated in the content of vanadium and chromium in the magnetite of rocks of the second phase.

Table 1

The results of the study of the β-activity of magnetites from rocks of different intrusive phases are presented in Table 2.

Table 2

β-activity of magnetite from rocks of the Kamensky massif

β-activity of the mineral is characteristic of all rocks of the massif. In samples from granites of the first phase of the central part of the massif it varies from 0.3 to 3.7 arbitrary units; the average activity is 1.4 arbitrary units. In the granites of the endocontact it decreases to 0.8 arbitrary units, with variations from 0.2 to 1.5 arbitrary units; the β-activity of magnetite in rocks of later intrusive phases increases.

Thus, as a result of the study of magnetite from the granites of the Kamensky massif, the following conclusions may be drawn.

1. Magnetite is a constantly present accessory mineral in the granites of the massif and occurs in high amounts.

2. Magnetite is distributed unevenly in the rocks: its content in granites of the first phase of the central part of the massif varies from 3750 to 25200 g/t, with an average of 9400 g/t; in the granites of the endocontact the average amount of magnetite decreases to 6650 g/t.

3. The content of magnetite decreases from rocks of the early phases to the late ones.

4. In composition, magnetite is heterogeneous and represents a mixture of a solid solution of ilmenite and magnetite, which underwent exsolution as a result of a decrease in temperature.

1. The following trace elements have been identified in the mineral: Si, Ca, Mg, K, Al, Na, P, Ti, Mn, Ba, Sr, Cr, V, Ni, Co, Zr, Nb, Li, La, Ga, Sn, Cu, Pb, Zn, Cd, Ag, As:

a) trace elements in magnetite, both from rocks of one and the same composition and from different intrusive phases, are distributed unevenly;

b) the contents of Mn, Ti, V, and Cr in magnetite decrease from the rocks of early differentiates to the later ones, whereas the amounts of Sn and Pb increase in this direction.

1. The magnetites of the rocks of the massif are characterized by radioactivity, which generally increases from the rocks of the first intrusive phase to the rocks of the subsequent phases.

LITERATURE

1. V. A. Vakhrushev. Trace elements of accessory magnetite as criteria for the genetic subdivision of granitoids of the Altai-Sayan region. Dokl. AN SSSR, vol. 147, 1963.

2. V. S. Kuzebnyi. Features of age relationships and certain problems of the petrology of gabbro–granodiorite–plagiogranite intrusions in the northwestern part of Rudny Altai. In: Magmatism, Geochemistry, and Metallogeny of Rudny Altai. Tr. IGN AN Kaz. SSR, vol. 17, 1966.

3. B. K. L’vov. Petrology, mineralogy, and geochemistry of the granitoids of the Kochkarsky district. Publishing House of Leningrad State University, 1965.

4. I. I. Shafranovskii. On the genesis of rounded diamonds. In: Collection on the Crystallography of Rounded Diamonds. Publishing House of Leningrad State University, 1948.