Morphology of Accessory Magnetite Crystals and Features of the Distribution of Impurity Elements in It from Granites of the Turochak Massif and Diorites of the Ulmen Massif (Gorny Altai)

A. I. Bazhenov, T. I. Poluektova

(Presented by Professor A. M. Kuzmin)

Magnetite, being the most widespread accessory mineral of igneous rocks, is at present attracting the attention of an ever increasing number of researchers [1, 2, 3, 4, 5, 6, 7]. The features of crystal morphology, as well as the behavior of impurity elements in magnetite, may serve as one of the additional criteria for genetic subdivision of intrusive formations—for example, granitoids [3]—and also for solving a number of problems in petrology and geochemistry.

The authors of the article set themselves the goal, by studying the features of the morphology of magnetite crystals and the character of the behavior of impurity elements in it from the diorites of the Ulmen massif, of clarifying the influence on the latter of the younger Turochak granitoid intrusion. The rocks described are located in the middle course of the Ulmen River, a right tributary of the Biya River.

For the study of accessory minerals, in particular magnetite, samples were taken from granites of the endocontact part of the massif in the direction away from the contact toward the center of the body, and from diorites of the Ulmen massif in the direction away from the contact with the granite pluton. The distance between sampling points on average did not exceed 1 km. The weight of the samples was 2.5 kg. Processing of the material was carried out according to the usual method.

The Turochak granite massif has the form of a large batholith. Its northeastern contact dips gently to the northeast, as evidenced by the comparatively great width of the contact aureole. The age of the granites is pre-Devonian. The Ulmen diorite massif is a relatively large stock elongated in a sublatitudinal direction. In the west it is cut off by the Turochak granite intrusion; to the east it is overlain by effusive-sedimentary formations of the Lower and Middle Cambrian. The age of the diorites of the Ulmen massif is dated as Salairian.

The granites are a coarse-grained rock consisting of plagioclase, microcline, quartz, and biotite. Among the accessory minerals there are apatite, sphene, zircon, magnetite, and others. The average quantitative-mineralogical composition of the rock is given in Table 1.

The amount of plagioclase in the granites of the endocontact part of the massif is variable. The range of fluctuation of this mineral in the rock increases with approach to the contact with the diorites and is 19.5–40.8%. The composition of the plagioclase changes from oligoclase to andesine. The presen—

as a rule, in xenomorphic grains of rectangular section; twinning is polysynthetic, and wedging-out of twins is characteristic. Andesine has the usual zonal structure of the crystals. The zoning is normal, continuous; the boundaries of individual zones are sharp, and the composition changes on passing from one zone to another from No. 40 to No. 32. Secondary alterations are very weakly expressed. With approach to the contact of the granites with the diorites, the plagioclases are distinguished by an exceptional freshness of the grains, with clearly discernible cleavage directed perpendicular to the twinning.

Table 1

Content of minerals in volumes, %

The content of microcline in the granites varies from 28.4 to 37.7%. At the contact with the diorites its amount increases to 50%. The mineral occurs in xenomorphic, more rarely idiomorphic, grains. The microcline lattice is in most cases well expressed; individuals without a lattice are sometimes encountered. Secondary alterations of the mineral are expressed in strong pelitization; at the contact with the diorites, microcline occurs in fresh, unaltered grains.

The amount of biotite in the marginal zone of the intrusive varies from 0.5 to 3.9%. The total ferruginosity of this dark-colored mineral increases with approach to the contact with the diorites, where it amounts to 63%. It is observed in the form of rims around magnetite and veinlets in microcline.

The quartz content ranges from 21 to 31%. The mineral has the cloudy extinction characteristic of it.

The diorites of the Ulmen Massif are a medium-grained, equigranular rock consisting of hornblende, plagioclase, orthoclase, diopside, and accessory minerals (Table 1). Among the accessories, magnetite, apatite, sphene, and zircon are present.

The amount of plagioclase in the diorites fluctuates within wide limits, from 30 to 85%. Its composition changes from No. 27 at the contact with the granites to No. 80—85 with increasing distance from it. The mineral is represented by hypidiomorphic grains, almost entirely replaced by zoisite.

The content of hornblende in the diorites decreases from 49% at the contact with the granites of the Turochak Massif to 13% at the point most distant from the contact. The refractive index of the mineral varies from 1.65 to 1.70 for \(N_g\). Hornblende is present in two generations, which are manifested especially clearly at the contact with the granites: the later green hornblende, along cleavage cracks, replaces brown amphibole and diopside.

The amount of diopside in the diorites increases with approach to the contact with the granites, where it is almost entirely replaced by chlorite.

Microcline appears, as a rule, near the contact with the granites of the Turochak Massif.

The content of accessory magnetite in the granites of the Turochak Massif, in its central part, varies from 228 g/t to 1172 g/t. The arithmetic mean is 739 g/t. The coefficient of variation is 73%. The granites of the marginal zone contain magnetite in considerably

in a smaller amount; the range of its fluctuation is 28–480 g/t; the arithmetic mean is 177.5 g/t. The coefficient of variation is 115%.

Magnetite in the granites of the Turochak massif occurs as octahedral crystals, as well as in irregular segregations and the smallest inclusions in quartz, feldspar, and biotite. Along with well-bounded octahedral crystals possessing lustrous faces, crystals of magnetite of distorted shape are not infrequently encountered, chiefly in samples from the contact with diorites. The surface of imperfect crystals is covered with plate-like encrustations, which impart a peculiar ribbing to the crystals. V. V. Lyakhovich [5] explains such sculpture of the faces by the process of dissolution of the crystals.

The size of magnetite crystals in the granites is approximately uniform and varies from 0.14 to 0.27 mm.

The amount of the mineral described in the diorites of the Ulmen massif fluctuates within wide limits, reaching a maximum value (3792 g/t) at the contact with the Turochak granites, and a minimum (20 g/t) at a considerable distance from it. Its arithmetic mean content in the intrusive is 1720 g/t. The coefficient of variation is 102%.

Magnetite in the diorites occurs as irregular, rounded, rounded-octahedral, and more rarely octahedral grains.

Spherical formations are most widely distributed in the diorites from the contact with the granite. Their size is 0.1–0.2 mm. The grain surface is even and lustrous. With increasing distance from the contact with the granites, rounded individuals of magnetic iron ore play a subordinate role; well-bounded octahedral crystals become predominant. At the most distant points, octahedra of magnetite 0.10–0.15 mm in size prevail.

Spectral semiquantitative analysis of the magnetites was carried out in the problem laboratory of TPI by analyst L. V. Simakhina. The following impurity elements were established: cobalt, nickel, chromium, vanadium, tin, copper, zinc, silver, and beryllium. The samples contain much manganese and titanium; we shall not dwell on their characteristics.

Cobalt in magnetite from granites was established only in the marginal zone of the intrusive, where its amount varies from 0.001 to 0.006%. It is characteristic that in the sample from the contact with diorite the element described is not detected. The content of cobalt in magnetite from diorites varies from 0.001% in magnetic iron ore from the contact with the granites to 0.01% at points distant from the contact.

The closeness of the ionic radii of divalent cobalt and iron \((R \mathrm{Co}^{2+} = 0.82;\ R \mathrm{Fe}^{2+} = 0.83)\), as well as the tendency of cobalt to concentrate in the residual magmatic melt and, finally, the similarity of their atomic structures, caused the substitution of divalent iron by cobalt in the crystal lattice of magnetite.

Nickel and chromium are sporadically encountered in the magnetites of the intrusives studied. With an increase in nickel content, the amount of chromium also increases; when the role of nickel decreases, the concentration of chromium correspondingly decreases. In the granites, the amount of these two impurity elements in magnetite regularly increases with increasing distance from the contact with the diorites. In magnetite from the diorites, the amount of nickel and chromium decreases with approach to the contact with the granites (Fig. 1).

The ionic radii of nickel and chromium \((R_i \mathrm{Ni}^{2+} = 0.78,\ R \mathrm{Cr}^{3+} = 0.65)\) are close to the ionic radius of divalent iron; therefore they enter isomorphously into the magnetite lattice.

Vanadium in magnetite from granites, in an elevated amount (0.03%), was noted in samples from the central zone of the massif. Along the periphery ...

Content of impurity elements, %

Distance from contact with granites, km

1 — nickel
2 — vanadium
3 — copper

Fig. 1. Variation diagram of the content of impurity elements in the diorites of the Ulmen Massif: 1 — nickel, 2 — vanadium, 3 — copper.

vanadium has been recorded in samples in insignificant amounts (0.003%). In the diorites, the concentration of this element in magnetite increases with distance from the contact with the granites (Fig. 1). Vanadium, as is known, accumulates in later formations and enters the crystal lattice of magnetite owing to the possible substitution of $\mathrm{Fe}^{3+}$ $(R_i = 0.67)$ by $\mathrm{V}^{3+}$ $(R_i = 0.65)$.

Tin was established only in one sample of magnetite from the endocontact granites, and in one sample from the diorites at a considerable distance from the contact. Its appearance in the mineral is evidently associated with superimposed hydrothermal processes, and it is present in the form of molecular dispersion.

Copper in magnetite from the granites was noted in elevated concentration in samples from the marginal zone of the intrusion, spatially tending toward the contact of the body with the diorites. The content of this element in magnetite from the diorites varies from ten-thousandths to hundredths of a percent, decreasing toward the contact with the granites (Fig. 1). Copper readily enters the crystal lattice of magnetite, replacing divalent iron, owing to the closeness of ionic radii $(R_i\ \mathrm{Cu}^{2+} — 0.71,\ R_i\ \mathrm{Fe}^{2+} — 0.83)$.

Zinc is contained in the described mineral from the granites of all samples in a constant amount and is 0.001%. No definite tendency in the behavior of zinc in the magnetite of the diorites is established. It can only be noted that its highest concentration (0.01%) was recorded in the sample from the contact with the granites. Zinc, as a rule, accumulates in the residual melt and readily replaces divalent iron in the crystal structure of magnetite.

Beryllium was established in magnetite from the granites of the central zone of the intrusion. In magnetic iron ore from the diorites it was found in samples from the contact with granite. This element probably enters the crystal lattice of magnetite in the form of molecular dispersion.

Silver was recorded in one sample of magnetite from the endocontact granites and in one sample from the diorites situated at a considerable distance from the contact with the granitic pluton. It is also present in the crystal structure of magnetite in the form of molecular dispersion.

Conclusions

The material presented makes it possible to draw the following conclusions concerning the influence of the granites of the Turochak intrusion on the diorites of the Ulmen massif.

1. The material composition of the diorites changes regularly with approach to the contact with the granites: the rock-forming minerals and accessory magnetite are characterized by an extremely uneven distribution; as the contact with the granites is approached, two generations of hornblende appear.

2. Magnetite crystals in contact with the granites bear traces of dissolution.

3. The content of impurity elements in magnetite from the diorites changes regularly with approach to the contact with the granites: the amount of nickel, copper, vanadium, and cobalt in the magnetic iron ore decreases as the contact with the granites is approached; in contact with the granites, beryllium appears in the magnetite, indicating superimposed processes.

4. The material composition of the granites changes regularly with approach to the periphery of the intrusion: the rock-forming minerals are distributed unevenly in the endocontact granites.

1. The content of magnetite in granites decreases as the contact with diorite is approached.

2. Magnetite crystals in granites tending toward the contact with diorite bear weakly expressed traces of dissolution.

3. The distribution of impurity elements in magnetite from granites changes regularly as the periphery of the intrusive body is approached: cobalt has been established only in the granites of the endocontact; the concentration of vanadium in magnetite increases in the central part of the body; the copper content in the mineral increases as the contact with diorites is approached.

LITERATURE

1. N. I. Belyashov. Principal regularities in the distribution of impurity elements in magnetites from the ores of the Kacharskoe deposit (Turgai trough). Izv. AN Kaz. SSR, Ser. Geol. Nauk, issue 6(57), 1963.

2. B. A. Vakhrushev. Problems of mineralogy, geochemistry, and genesis of the iron ores of the Kondoma district of Gornaya Shoria (Western Siberia). Izd. SO AN SSSR, 1959.

3. V. A. Vakhrushev. Impurity elements of accessory magnetite as criteria for the genetic subdivision of granitoids of the Altai–Sayan region. DAN SSSR, vol. 147, No. 3, 1962.

4. V. A. Vakhrushev. Impurity elements in magnetite from iron-ore deposits of the Altai–Sayan region. In: Collected Materials on Genetic and Experimental Mineralogy. Izd. SO AN SSSR, 1963.

5. V. V. Lyakhovich. Some data on the composition of accessory magnetite. Tr. Instituta Miner., Geokh. i Kristallokhim. Red. El-tov, issue 3, 1959.

6. D. S. Shteinberg, V. G. Fominykh. On the distribution of impurity elements in titanomagnetites of the Urals. DAN SSSR, 139, No. 6, 1961.