FEATURES OF THE DISTRIBUTION OF ACCESSORY APATITE IN THE GRANITOIDS OF THE MOKHNATUKHINSKY AND KAMENSKY MASSIFS

(NORTHWESTERN PART OF RUDNY ALTAI)

T. I. POLUEKTOVA

(Presented by Professor A. M. Kuzmin)

Information is appearing increasingly often in the literature indicating that, for petrogenetic purposes, the morphological features of apatite are no less significant than those of zircon \[1, 2, 5\].

The present work gives the results of a study of the nature of the distribution of accessory apatite in the granitoids of two intrusive bodies that are similar in petrographic and petrochemical composition but occupy different structural positions. The level of erosional truncation of both intrusions is not great.

The Mokhnatukhinsky and Kamensky granitoid massifs are located within the Irtysh uplift zone, which in the study area, according to its geological-structural features, is subdivided into southwestern and axial subzones. The first of these occurs among a metamorphosed sandy-shaly sequence of Upper Devonian—Lower Carboniferous age in the southwestern subzone; the second intrudes the Middle Devonian greenstone schists of the axial subzone. According to determinations by F. S. Zakirova, the granitoid bodies are Permian in age (220–260 million years), and they are considered as part of the Kolbino intrusive complex. Both massifs are composed of coarse-grained porphyritic biotite granites, which are intruded (in order of formation) by low-thickness sheet-like bodies of medium-grained two-mica granites, veins of aplite-like granites, aplites, and pegmatites. In chemical composition, the biotite granites belong to the class of rocks of normal composition.

The rocks are enriched in silica and alkalis; potassium predominates over sodium in them, while the contents of iron oxides, magnesium, and feldspathic lime are characterized by low values. The petrographic composition of the coarse-grained porphyritic biotite granites is ordinary: plagioclase, alkali feldspar, quartz, biotite, secondary minerals (sericite, chlorite), and accessory minerals. Plagioclase is present in two generations. Plagioclase I forms porphyritic segregations in the form of tabular crystals having a zonal structure. The composition of the zones changes from the center to the periphery of the crystal from No. 25–22 to No. 13–6. Plagioclase II is part of the groundmass of the rock in the form of small tabular individuals bearing polysynthetic twinning. The composition of these plagioclases is No. 13–5. Alkali feldspar forms large metacrysts and also occurs as xenomorphic grains in the groundmass of the rock. By optical properties, the alkali feldsparisummaa

feldspar is heterogeneous; in its crystals and grains there are areas with extinction corresponding to orthoclase and areas of triclinic alkali feldspar. The angle \(2V\) corresponds to typical microclines and is \(-80\)—\(-86^\circ\). However, the microcline lattice is not always distinct. Quartz is observed as angular, rounded grains in the groundmass of the rock, and also as poikilitic formations in phenocrysts of plagioclase and in metacrysts of alkali feldspar. Its extinction is undulatory.

Biotite is distributed unevenly in the granites, occurring both as individual flakes and as clustered accumulations. Pleochroism scheme: \(N_g\)—dark red to deep red, \(N_p\)—pink, light yellow. The refractive index varies from 1.653 to 1.685.

The granitoids of both intrusive bodies differ, chiefly, in the quantitative ratios of both petrochemical oxides and rock-forming minerals. For example, the content and sum of alkalis \(K_2O+Na_2O\) in the granites of the Mokhnatukhinsky massif are higher than in those of the Kamensky massif. The amount of plagioclase and biotite in the granitoids of the Mokhnatukhinsky massif is greater than in the Kamensky massif, whereas the content of quartz and microcline, on the contrary, predominates in the latter.

Among the accessory minerals of the Mokhnatukhinsky intrusion are zircon, apatite, monazite, xenotime, garnet, ilmenite, sphene, pyrite, leucoxene, anatase, brookite, rutile, molybdenite, wolframite, and fluorite. In the granites of the Kamensky massif the last seven minerals were not identified, while an accessory mineral such as magnetite is constantly present and in large amounts. In addition, tourmaline is found sporadically in them.

Apatite—one of the consistently occurring accessory minerals in the rocks of the compared intrusive bodies—is distributed in them extremely unevenly. The limits of variation in apatite content, as well as its averaged value in the rocks of the intrusive bodies, are given in Table 1.

As can be seen from the table, the amount of apatite in the rocks of the Mokhnatukhinsky massif is higher than in those of the Kamensky massif. Overall, the character of apatite distribution in the granitic rocks of the first massif differs somewhat from that of the second. Thus, in the granites of the marginal zone of the Mokhnatukhinsky massif the calcium phosphate content is higher than in the center of the intrusion (44 g/t and 16.65 g/t, respectively). It is interesting to note that in a granite sample taken from the contact with biotite schists, the content of this mineral is minimal (0.06 g/t), whereas at 50 m from the boundary with the enclosing rocks the amount of apatite in the granites is maximal (234 g/t). In the rocks of the late differentiates the content of calcium phosphate decreases regularly. In the biotite granites of the Kamensky massif the apatite content in its endocontact facies is lower than in the central facies. In the medium-grained two-mica granites this mineral was recorded in only one sample; in the vein rocks it was not established at all.

Apatite is present in the granitoids of the massifs under consideration in the form of prismatic crystals. Moreover, polyhedra from the rocks of the Mokhnatukhinsky massif are characterized by a greater variety of simple forms in comparison with crystals from the second intrusion (Table 2). In the granites of the marginal facies, faces of the dipyramid \((10\overline{1}1)\) appear on the crystals. In the rocks of the late differentiates, faces of the pinacoid \((0001)\) show subordinate development or are completely displaced by the growing faces \((10\overline{1}1)\). In addition, in the vein rocks, faces of an additional form develop on the crystals of calcium phosphate—

Table 1

Apatite content in the rocks of the Mokhnatukhinsky and Kamensky massifs (g/t)

1 — leucocratic porphyritic biotite granites of the central part of the massif; 2 — medium-grained biotite granites of the endocontact facies; 3 — medium-grained biotite granites; 4 — aplite granites; 5 — aplites; 6 — pegmatites.

Morphology of apatite crystals in the rocks of the Mokhnatukhinsky and Kamensky massifs

Table 2

prisms (1120). The surface of the crystal faces is uneven, rough; the edges are often smoothed, and the apices are not infrequently blunted, so that the crystal acquires a rounded or nearly rounded shape. Rounded apatite grains usually occur in granites that have undergone metasomatic alteration. Thus, in greisenized granites, apatite crystals are rare; its rounded grains predominate. In a sample of intensely greisenized granites taken at the contact with a tungsten-bearing quartz vein, apatite disappears. A number of authors also indicate the good solubility of apatite during postmagmatic processes [2, 8].

In biotite granites, chiefly in the central part of the massif, parallel intergrowths of two or three crystals are often encountered. On the prism faces, small accretions are observed in the form of ilmenite grains and small flakes of biotite. The sizes of the crystals vary from 0.48×0.24 mm to 0.12×0.1 mm, with an average of 0.27×0.14 mm; in the marginal zone of the massif the size of the individuals decreases to 0.36×0.14—0.12×0.01 mm, averaging 0.20×0.7 mm. In the rocks of vein differentiates, elongated crystals predominate, measuring 0.36×0.4—0.49×0.07 mm.

The color of the mineral is turbid gray, bluish, pale and rusty yellow. The latter is due to a thin film of iron hydroxides covering the crystals, which is easily removed by dissolution in HCl. The turbid-gray color is imparted to the polyhedra by abundant gas-liquid inclusions. In addition to these inclusions, apatites contain crystals of ilmenite and biotite (Fig. 1), as well as accumulations of small black grains. The latter are often so abundant that they give the crystal a black color. Such inclusions are oriented parallel to the vertical axis and not infrequently emphasize the zonal structure of the crystal: its central part is black, the next zone smoky gray, and the outer zone colorless. There are suggestions in the literature that such inclusions may represent finely dispersed graphite, which fills cavities arranged parallel to the vertical axis of the crystal [3, 4].

The main mass of apatite in the rocks of the intrusions studied is closely associated with biotite, in whose flakes its crystals extend in chains along cleavage cracks. Very rarely, calcium phosphate is observed enclosed in orthoclase. Very characteristic are elongated hexagonal individuals that cut and replace the rock-forming minerals of the granites. Such apatites are usually broken by a system of cracks parallel to the face (0001) (Fig. 2).

By X-ray structural analysis, in apatite from biotite grani-

Fig. 1. Biotite inclusions in apatite crystals. Magnification ×30

In the rocks of the Mokhnatukhinsky massif, Zr, Y, Pb, Th, and U have been established. The last two elements were not detected in the rocks of the late differentiates. In apatites from the Kamensky intrusive, only Zr and Y were established as trace elements.

Fig. 2. Prismatic apatite crystal penetrating all rock-forming minerals. The crystal is broken by cracks parallel to the faces of the pinacoid

Thus, two granitoid intrusives similar in petrographic and petrochemical composition are characterized by differences in the behavior of accessory apatite. These differences are as follows:

1. The amount of calcium phosphate in the rocks of the Mokhnatukhinsky massif is higher than in those of the Kamensky massif. In the first intrusive, apatite is recorded in all differentiates; in the second, it has been established only in biotite granites.
2. Apatite crystals from the rocks of the Mokhnatukhinsky massif are richer in simple forms.
3. Apatite of the Mokhnatukhinsky intrusive contains a larger set of trace elements.

REFERENCES

1. Anikeeva N. F. The significance of accessory apatite in the study of the Upper Mesozoic volcanogenic formation of the Karkaralinsky district. In: Accessory minerals and elements as a criterion of comagmaticity and metallogenic specialization of magmatic complexes. “Nauka,” 1965.

2. Bukanov V. V. Apatite from Alpine-type veins at the Prinolyarny ...

Urals. ZVMO, issue 90, no. 5, 1963.

1. Lyakhovich V. V., Chervinskaya A. D. On the nature of the distribution of accessory minerals in granite massifs. Tr. IMGRE, issue 4, 1960.
2. Matkovsky O. I. Accessory minerals of the granitoids of the Osnitsky complex of Volhynia. 1956.
3. Hoppe Günter. Die Formen des akzessorischen Apatits. Berichte der Geologischen Gesellschaft. H. 2. 1962.