ACCESSORY MINERALS OF APLITE-LIKE GRANITES OF THE BUZURGAT AREA OF THE ULEN-TUIM PLUTON

(Khakassia)

E. V. VLADIMIROVA

(Presented by Professor A. M. Kuzmin)

This article reports the results of a study of accessory minerals of the aplite-like granites of the Buzurgat area of the Ulen-Tuim pluton, revealing the general mineralogical-geochemical features of the rocks and the regularities in the spatial distribution of accessories within them. The data obtained may be used for purposes of correlating the intrusive rocks of the region.

Brief geological-petrographic characterization of the aplite-like granites

The Ulen-Tuim granitoid pluton is situated on the eastern slope of the Kuznetsk Alatau, in the area of the middle course of the Bely Iyus River. The pluton is composed of rocks varying in composition from diorites to granite-aplites. The entire complex assemblage of rocks is commonly regarded as having formed in three phases of emplacement of an intrusion of Salairian age.

The Buzurgat area is located in the upper reaches of the stream of the same name, which flows from the left into the Kara-Tege River. Here medium-grained gray granodiorites of the first phase and granosyenites of the second phase of the above-mentioned pluton are exposed, cut by a small (about 12 km²) stock-like body of aplite-like granites and by dikes of aplite and pegmatite. Both the aplite-like granites and the aplite and pegmatite dikes developed in the area are acidic differentiates of the granitoid magma of the Ulen-Tuim pluton and are assigned by a number of investigators to its third phase of emplacement.

The aplite-like granites are pink in color, homogeneous in texture, and consistent in composition. The principal minerals of the aplite-like granites are: K-feldspars (30–40%), plagioclases (20–30%), quartz (25–40%), and biotite (0.2–1%). The K-feldspar is represented mostly by perthite of exsolution, more rarely by untwinned orthoclase or orthoclase with simple twinning. The plagioclase is idiomorphic; its composition is No. 20–25. Quartz forms sharply xenomorphic, irregular, somewhat elongated grains. The biotite is strongly altered. The rock texture is granitic, more rarely micropegmatitic.

The granites include extremely rare, low-thickness (1–2 m) pegmatoid segregations, sometimes lens-shaped. Autometamorphic and autometasomatic processes in the area were expressed in phenomena of albitization (thin fine-grained rims around grains

of plagioclase, or small nest-like accumulations and veinlets of it in K-feldspars); greisenization (the appearance of muscovite, tourmaline, topaz, fluorite) and other lower-temperature postmagmatic alterations.

Accessory Minerals

For the purpose of studying the impurity minerals of the rocks under consideration, the author processed 12 crushed samples weighing from 2.0 to 2.5 kg, 32 heavy-mineral samples weighing from 30.0 to 100.0 kg, and carried out a study of polished sections.

The accessory minerals of the aplite-like granites amount in total, by weight, to no more than 0.3–0.4%, and in the pegmatoid segregations to as much as 0.5%. Their contents, calculated in grams per ton with allowance for the loss coefficient, are given in Table 1. Among the accessory iron minerals, magnetite, martite, and hematite are widespread. The maximum content of magnetite has been established in the pegmatoid segregations, whereas martite and hematite are characteristic of the aplite-like granites. Among the titanium minerals there are ilmenite, sphene, rutile, and anatase. The greatest frequency of occurrence is noted for ilmenite, although its quantitative contents are insignificant. Rutile has a frequency of occurrence of 66%; its contents range from isolated traces to 170.0 g/t. Of the other oxides, the most widespread is zircon; corundum is found rarely. Of the phosphates, monazite and apatite are constantly present. The contents of monazite are considerably greater than those of apatite. Among the rare-earth minerals in the aplite-like granites and pegmatoid segregations, in addition to the monazite noted above, there is orthite; among the tantaloniobates—columbite; pyrochlore, eschynite, topaz, fluorite, and tourmaline—products of postmagmatic processes—occur very rarely (frequency of occurrence 33%); among the sulfides—pyrite; among the native elements—lead; among the exogenous minerals—cerussite and limonite. A characterization of some of them is given below.

Magnetite is distributed unevenly. Its content increases noticeably in the central and southern parts of the stock-like body (in areas saturated with pegmatoid segregations), whereas in the northern part it is almost absent. This is probably explained by the uneven manifestation of postmagmatic processes. The mineral crystallizes with the formation of octahedra 0.2–0.3 mm in size. The crystals bear traces of dissolution. Magnetite from the pegmatoid segregations has grain sizes up to 0.5 mm.

It is interesting to note that, in addition to magnetite, a substantial part of the magnetic fraction (up to 2000.0 g/t) is sometimes made up of magnetic balls 0.05 to 0.6 mm in size. These unusual formations are easily crushed. They consist of two parts: a core and an outer shell. The core is metallic, spherical in shape, has a smooth shiny surface of steel-gray color, is strongly magnetic, and is comparatively easily flattened and scratched with a needle. In a Cu₂SO₄ solution it quickly becomes coated with metallic copper. Semiquantitative spectral analysis showed the presence in it of iron as the principal element and, in addition, Cr—0.01%, Ni—0.003–0.01%, Cu—0.003%. The outer shell of the balls consists of a brittle black homogeneous mass, nonmagnetic, which, according to spectral-analysis data, contains Mn > 1.0%, Ba—0.001%, and traces of Pb, As, Ca. This mass is not transparent in transmitted light and readily dissolves in HCl, coloring the solution greenish-yellow. The origin of the magnetic balls found in the crushed samples from the aplite-like granites of the Buzurgat area has not yet been clarified. It should be noted that similar formations have so far not been found either in specimens, in polished sections, or in the material of heavy-mineral samples.

Table 1

Distribution of accessory minerals in the rocks of the Buzurgat area

Note: the sign + indicates mineral content in single grains; the sign ++ indicates mineral content in rare grains.

clarified. It should be noted that neither in the specimens, nor in the concentrates, nor in the material of the concentrate samples have such formations yet been found.

Martite occurs mainly as octahedra 0.3–0.5 mm in size. The color of the crystal surfaces is brownish black; in powder it is dark cherry-red. It is weakly magnetic.

Hematite occurs as irregularly shaped grains of bright cherry-red color. It contains small crystals of monazite, apatite, and a light, slightly greenish mica (?).

Ilmenite is present in most samples in single grains. Its grains have the form of thick black plates, often covered with a gray film of leucoxene.

Sphene is a rare mineral. It forms small (up to 0.1 mm) crystals of envelope-like habit with smooth faces; it is transparent, with a light-yellow coloration.

Anatase occurs as single crystals of dipyramidal appearance with fine transverse striation on the faces. The color is light blue; the luster is adamantine.

Rutile is present in insignificant amounts. Its crystals, 0.2–0.5 mm in size, have a hexagonal-prismatic habit with distinct fine striation along the main axis. Usually transparent, light-yellow crystals sometimes contain black dense inclusions, concentrated either at one end of the crystal or perpendicular to its long axis in the form of a ring. The inclusions are opaque and probably consist of a fine-grained aggregate of ilmenite. Rutile contains small crystals of monazite of the second variety.

Monazite is a very characteristic mineral that is constantly encountered in the rocks under consideration. The morphology of the crystals, their relationships with the rock-forming and other accessory minerals, and also the character of the separation of brown oxidation products of TR made it possible to distinguish two varieties of monazite, probably corresponding to two generations of the rare-earth phosphate. The first variety of monazite appears as elongated crystals of prismatic or tabular habit with dipyramids at the ends. The crystal size ranges from 0.1 to 1.2 mm, and the ratio of width to length from 1:2 to 1:5. The crystals are more often encountered within grains of potash feldspars or strongly altered biotite, less often in plagioclase. A characteristic feature of this variety is the formation around monazite crystals of pleochroic halos. Sometimes, as a result of secondary alterations, only small relict areas of transparent monazite remain in the general brown mass. The strong degree of secondary alteration of monazite of this variety gives grounds to speak of its early generation.

Monazite of the second variety forms small (not more than 0.1 mm), isometric crystals, transparent, with a clearly expressed zonal structure. They occur as individual crystals or as clusters of 2–5 crystals in quartz grains, inside or on the boundary of rutile crystals, at the boundary of quartz and potash-feldspar grains, or more often in a fine-grained aggregate of albite. Secondary alteration of these crystals is weak. The mineral association and the absence of alteration products of monazites indicate a later stage of formation of the rare-earth phosphorite of this variety.

In the material of the concentrate samples, monazite crystals are tabular or prismatic. The planes of the faces are uneven, pitted; the edges of the crystals are smoothed. The coloration is uneven—from light yellow to reddish yellow. Specific gravity 5.20. The mineral is radioactive. Radiochemical analysis established in the monazite: Th—6.07%, U—0.259%, Pb—

0.106%. Semiquantitative spectral analysis showed that monazite contains Ce, La, and Y in amounts of more than 1.0%, Mn—0.01%; Ti—0.01%, and traces of Be, Ba, and As.

Columbite occurs rarely. Its black, dark-red translucent, rectilinearly outlined crystals have been found in perthitized grains of K-feldspars. In the material from crushed samples, prismatic flat columbite crystals 0.2–0.5 mm in size have pointed ends. The surface of the crystals is smooth, with a semimetallic luster. Spectral analysis established the following principal elements in the mineral: Mn, Nb, Ta; Ti > 1.0%, W > 1.0%, Be—0.1%.

A mineral of the euxenite type occurs comparatively often in the heavy-mineral fractions. Its grains are isometric in form, up to 0.7 mm in size (it has not yet been found as crystals). At the surface it is dark brown, dull, and in fragments it is translucent brown; the fracture is conchoidal, with a strong resinous luster on fresh breaks. In thin sections, around the dark-brown, weakly translucent mineral, a fine-aggregate, mica-like rim of secondary alteration products is visible. The mineral of the euxenite type, like columbite, is found in crystals of K-feldspars. Spectral analysis reveals high contents of Ca, Th, Ce, and Nb in it.

Zircon is a constant accessory mineral of the rocks of the area under consideration, although its contents are insignificant. By the form and size of the crystals and by the presence of impurity elements, four varieties of zircon are distinguished: the first variety (small, transparent), the second variety (large, transparent), malacon, and cyrtolite. In aplite-like granites, zircon of the first type and malacon are widespread; in pegmatoid segregations, predominantly cyrtolite and zircon of the second type occur, while the other two varieties (the first type and malacon) are present in subordinate amounts. Zircon of the first type is represented by small, slightly elongated crystals 0.05–0.1 mm in size, with pyramidal terminations. The elongation is 1:1.5 and 1:2. The surface of the faces is smooth, with small cracks inside. The color is pale yellowish; it sometimes contains rare, fine inclusions of a dark-green mineral (hornblende?). In thin sections it is found in grains of K-feldspars.

Zircon of the second type has isometric crystals with development of the faces {100}, {110}, {111}; their size is up to 1.5 mm, and their color is pale pinkish. According to spectral-analysis data, the first two varieties of zircon do not contain noticeable impurities of extraneous elements.

Malacon is noted only in isolated grains. Its crystals, 0.2–0.4 mm in size, have an octahedral form. The color of malacon is light brown. In transmitted light the mineral is amorphous. Spectral analysis established the presence of Si, Pb, Mn, U, Th, and Be. The fourth type of zircon, cyrtolite (like the large transparent zircon of the second type), is characteristic only of pegmatoid segregations, in which it makes up to 80% of the total amount of the mineral in the sample. Crystals of this zircon type have the form of strongly elongated tetragonal prisms with an axial ratio of 1:10. Maximum dimensions are up to 2.5 mm. Usually cyrtolite crystals are surrounded by pleochroic halos, and inside they have an amorphous mass with small relics of crystalline structure. The crystals are opaque, light brown in color, and their surface is covered with rusty-brown spots (sometimes the spots merge and form a film). Internally the crystals have a heterogeneous structure. For the most part this is a dense glassy mass with a greasy luster on fresh fracture, in which areas of loose structure appear. Both the dense and the loose formations are thickly impregnated with point inclusions of rusty-brown color. According to semiquantitative data—

of quantitative spectral analysis, this zircon contains admixtures of Th, Hf, Y, Yb.

For the purpose of comparing the compositions and quantitative ratios of accessory minerals in the aplite-like granites and other groups of rocks of the Buzurgat area of the Ulen-Tuim pluton, we compiled Table 2.

Table 2

Distribution of accessory minerals in the rocks of the Ulen-Tuim pluton in the Buzurgat area

Note: the sign + denotes content in unit occurrences;
the sign ++ denotes content in rare occurrences.

From consideration of this table it is evident that the average composition of the accessory minerals of the aplite-like granites differs from the composition of the impurity minerals of rocks of the earlier phases of one and the same pluton and almost completely coincides with its late dike formations of acid composition. For the early stages in the formation of the rocks of the area, magnetite, sphene, zircon, apatite, sulfides, and native zinc are characteristic. In the granodiorites and granosyenites, rare-metal minerals and minerals of volatile components were not found. In rocks of more acid composition, a decrease is observed in the contents of iron and titanium minerals (magnetite and sphene) and a noticeable enrichment in rare-metal mineralization. A similar species composition of heavy impurity minerals and their quantitative ratios are also preserved for the dike rocks.

The action of postmagmatic processes in the stock-like body of aplite-like granites is accompanied by martitization of magnetite, leucoxenization of ilmenite, and the formation of secondary minerals after a number of other accessories. The phenomenon of martitization is widespread in nature and has been noted by many investigators. In particular, R. V. Masgutov [3], considering the products of martitization of accessory minerals among granites of one of the massifs in Northern Kazakhstan, comes to the conclusion that the processes of alteration of magnetite and its transition into hematite are characteristic of granites that have undergone intensive sodium metaso-

matosis. The author emphasizes that such phenomena are closely connected with endogenous processes.

Of considerable interest is the general decrease in the rock, under the superposition of secondary processes, of magnetite, apatite, and ilmenite, and the increase in the amount of rare-earth minerals. As noted by V. V. Lyakhovich and B. I. Zolotarev [2], and also more recently for the rocks of the Kalba complex by N. N. Amshinsky et al. [1], the reason for the decrease in the contents of magnetite, ilmenite, and apatite is intensive postmagmatic processes, such as potassium feldspathization, albitization, and greisenization.

From the foregoing, the following principal conclusions may be drawn.

1. The aplite-like granites of the Buzurgat area contain a diverse complex of accessory minerals, among which rare-metal and rare-earth minerals occur, containing such elements as Th, Tr, Hf, Ta, Nb, U, Be, W, which can probably be used as an additional criterion for distinguishing an independent intrusive phase in which the studied stock-like body of aplite-like granites was formed.

2. Attention is drawn to the great similarity in the composition and quantitative ratios of the accessories of the aplite-like granites and dike aplite-pegmatites, which may indicate a very close connection of their formation within the general cycle of development of the granitoid intrusion.

3. The most widespread accessory mineral of the rocks studied is monazite.

REFERENCES

1. N. N. Amshinsky, I. V. Marinich [et al.]. Accessories of the Granitoids of Altai and Methods for Their Study. State Geological Committee of the USSR, SNIIGGiMS. Nedra Publishing House, Moscow, 1964.

2. V. V. Lyakhovich, V. I. Zolotarev. Accessory minerals in the granitoids of Gorny Altai. Proceedings of IMGRE, issue 2, Publishing House of the Academy of Sciences of the USSR, 1959.

3. R. V. Masgutov. Some data on the question of martitization. Publishing House of the Academy of Sciences of the Kazakh SSR, ser. geol., issue 1, 1957.

4. L. V. Tauson, Z. V. Studenikova. Patterns in the distribution of lead, zinc, and molybdenum in igneous rocks. Publishing House of the Academy of Sciences of the USSR, 1959.