ON ALBITE–NEPHELINE PEGMATOID FORMATIONS ASSOCIATED WITH ALKALINE INTRUSIONS OF NORTHEASTERN TUVA

A. D. NOZHKIN, A. G. MIRONOV

(Presented by the scientific seminar of the Department of Mineral Deposits)

In Northeastern Tuva, in the zone of junction of two large geologic-tectonic structures—the East Sayan anticlinorium and the Tuvan synclinorium—a whole series of massifs (outcrops) of alkaline rocks is developed. Some researchers [5, 8] assign these to the Middle Paleozoic complex of granitoids and alkaline rocks of Eastern Tuva. On the basis of geological-structural study, L. V. Dmitriev [3] regards them as a single independent intrusion.

The host rocks of the intrusion are chiefly carbonate metamorphic sequences of Precambrian and Lower Paleozoic age, and gabbroids and diorites of the Tonnuol intrusive complex. Magmatic rocks of increased alkalinity, diverse in composition, are represented by nepheline syenites, alkaline syenites and granites, as well as calcareous-alkaline syenites and granites. A definite dependence [3] is established between the petrographic composition of the magmatic rocks and the composition of the host environment and the depth of the erosional level: nepheline syenites are confined to the most uplifted tectonic blocks composed of dolomites; alkaline syenites and granites are widespread among the enclosing gabbroids and diorites, with granites occurring, as a rule, in the most downthrown blocks.

At present the composition and petrographic and geochemical features of the alkaline magmatic rocks have been studied rather fully [3, 5, 6, 8, and others]. However, the pegmatites, vein and metasomatic formations developed in association with them, often with rare-metal mineralization, have been studied insufficiently. Below are given certain features of the composition and conditions of formation of comparatively rare albite–nepheline veins of pegmatioid appearance, occurring not in nepheline syenites, as is usually the case, but in hornblende syenites.

Structure and composition of the veins

Albite–nepheline veins were found on the watershed of the Kizhi-Khem and Sorug rivers among outcrops of hornblende syenites, exposed at the present erosional level to an insignificant depth, as indicated by the abundance among them of xenoliths of the host rocks.

The syenites are medium- and coarse-grained, massive, with a hypidiomorphic-granular texture. They consist of microcline-perthite (65–75%), plagioclase No. 1–15 (10–15%), hornblende, more rarely biotite (10—

15%), the accessory minerals are sphene and magnetite. In petrochemical terms these are leucocratic, silica-saturated ($Q \sim 4$) rocks of the normal series, rich in alkalies ($a : c = 9$–18), in whose composition sodium noticeably predominates ($n = 60$–65).

For the veins, a distinct control is established by rupture disturbances of predominantly northwestern strike, which are feathering structures with respect to the regional sublatitudinal fault. The dimensions of the veins along strike are 10–15, more rarely 50 m, with thicknesses of 0.5–1.5, sometimes up to 10 m. Their dip is steep—60–90°. The form is lenticular or bead-like.

The veins are composed chiefly of nepheline and feldspar. Amphibole, biotite, and also accessory and secondary minerals are of subordinate importance.

Most veins are characterized by zoning, caused, as will be shown below, by the stadial character of their formation.

The central parts of the veins are made up of a coarse-grained aggregate consisting mainly of nepheline (50–60%) and saccharoidal albite (25–30%), as well as amphibole (5–10%), biotite (up to 5%), and a small amount of potassium feldspar, eudialyte, and zircon. Rather often in this zone there are lenticular and nest-like segregations that do not contain nepheline and consist of an aggregate of fine-crystalline saccharoidal albite with biotite, zircon, and fluorite.

Toward the contact of the veins, the nepheline content decreases (to 30–40%); along with saccharoidal albite, lath-like albite is present (10–15%), as well as potassium feldspar (15–25%). The content of mafic minerals, especially biotite, increases somewhat. Among the accessory minerals, in addition to eudialyte and zircon, ilmenite is noted. Here the size of the grains composing the mineral zones and the degree of their idiomorphism noticeably decrease.

The marginal parts of the veins are represented by a fine-grained aggregate of microcline (30–35%), lath-like albite (25–35%), nepheline (10–15%), biotite (15–20%), amphibole (5–10%), and accessory minerals—ilmenite, zircon, eudialyte.

It is not difficult to see that, from the inner toward the outer zones, the role of nepheline, saccharoidal albite, and eudialyte decreases, while, correspondingly, the content of potassium feldspar, biotite, lath-like albite, and ilmenite increases.

In the contact aureole of the feldspar-nepheline veins, the fenites are albitized and biotitized. Moreover, only lath-like albite is developed here. The transition from unaltered fenites to the rocks of the contact aureole of the feldspar-nepheline bodies and further to the coarse-grained aggregate of essentially nepheline composition is completely gradual. Relatively sharp boundaries are observed only at lenticular segregations of saccharoidal albite with an elevated zircon content. In connection with this, the albite-nepheline formations can essentially be assigned not so much to veins as to vein-like bodies.

The indicated zones also differ noticeably in the quantitative content of trace elements.

As follows from Table 1, toward the inner zones of the body the content of rare and radioactive elements—U, Th, La, Y, Yb, Zr, Li, Sn—increases noticeably; their elevated concentration is chiefly associated with late albitization, and, conversely, the content of lead, which enters the structure of potassium feldspar, as well as Ba and Sr, which are evidently geochemically related to mafic minerals, decreases.

Let us consider the principal features and interrelations of the main and accessory minerals of the veins.

One of the most characteristic minerals of the bodies described is nepheline, represented in the inner zone by large (up to 4 cm) crystals of light-green color, and in the outer zone by finer grains of pinkish-red color. The latter are almost entirely replaced by a fine-scaly aggregate of liebenerite, more rarely by cancrinite. Large crystals are replaced by secondary products only along the periphery. Along with the usual optically negative sign, some large crystals display a positive, weakly anomalous biaxial conoscopic figure, which is evidently caused by some enrichment of nepheline in the anorthite molecule [1].

Albite is represented by two clearly distinguished varieties. The first of these is widespread in the near-selvage parts of the bodies, as well as in their near-contact aureole. It is represented by polysynthetically twinned laths 1–5 mm in size, corresponding in composition to albite No. 8–10 ($Np = 1.532$, $Nm = 1.535$, $Ng = 1.541$). It develops predominantly after potassium feldspar, which is at first replaced by chessboard albite, and the latter by lath-shaped albite. This sequence is especially clearly visible in altered rocks of the contact aureole. In turn, lath-shaped albite is replaced by nepheline, occurring in it as relics. The second variety of albite is distinguished above all by its saccharoidal appearance, the isometric character of small (0.1–1 mm), untwinned grains. It is distributed throughout the entire thickness of the bodies, but predominates in their inner zones, where it is observed not only in association with nepheline but also in separate lens-like accumulations together with biotite, zircon, and fluorite. Small xenomorphic grains of this variety of albite readily replace potassium feldspar, nepheline, and even amphibole. In composition it corresponds to albite No. 4–6 ($Ng = 1.539$, $Nm = 1.533$, $Np = 1.530$). In the marginal parts of the veins, microcline is rather widely developed, where it is formed by small (1–5 mm) grains with an indistinctly expressed twin lattice and numerous perthitic intergrowths. In the inner zones it is represented exclusively by relics from replacement by albite and nepheline.

Dark-colored minerals are of sharply subordinate importance. Among them, biotite and amphibole are noted. In the central parts of the vein bodies, in association with nepheline and albite, biotite is represented by rare large (up to 1.5 cm) platy segregations. The near-contact zones are enriched in fine-scaly biotite. Amphiboles are distributed more or less uniformly throughout the entire vein. But whereas the peripheral parts are characterized by short-prismatic segregations of common hornblende ($Ng = 1.670$, $Np = 1.650$, $cNg = 15–17^\circ$, $2V—78^\circ$), in the central parts of the bodies described, in close association with nepheline, an alkaline amphibole is developed as fine inclusions (0.05–0.1 mm) in it and as larger xenomorphic segregations, corresponding to an intermediate member of the magnesio-riebeckite–barkevikite series ($Ng = 1.699$, $Np = 1672$, $cNg \sim 28$, $Ng > Nm > Np$). In transmitted light the development of biotite after hornblende is observed quite clearly, while the alkaline amphibole replaces both of these minerals.

Among the accessory minerals, which are represented by zircon, eudialyte, rinkolite (?), ilmenite, and fluorite, the first three are of some interest. Zircon as the most widespread mineral, and eudialyte and rinkolite (?) as first finds in the vein bodies of the syenite massif.

Zircon is distributed relatively uniformly in the veins; however, the main part of it nevertheless tends toward areas of late saccharoidal albite associated with biotite. Small (0.1–2 mm) bipyramidal zircon crystals predominate, with development of faces of pi-

Note:
8055 — hornblende schist,
8065 — biotitized and albitized schist,
8065б — near-contact calcite–albite–nepheline rock.

pyramids 111 and, more rarely, prisms 110, 010, yellow, brown, more rarely water-transparent in color. A characteristic feature of them is their skeletal structure and zoning, emphasized by the density of coloration. Moreover, the inner zones are usually more densely colored. A distinctly expressed skeletal structure is more often found in the peripheral zones of single crystals, although relics of untwinned albite are occasionally also observed in the inner zones.

Adhering to the views of S. A. Rudenko [9], who has dealt specifically with the study of zircon metacrystals, it is apparently possible to explain the similar structure of the mineral under study by its formation through metasomatic replacement of albite.

Spectral analysis showed an elevated content in zircon (in %) of uranium 0.3—1, thorium, Yb — up to 0.3, Y — 0.1, as well as the presence of Be — 0.01 and Mn — 0.003.

Eudialyte, in contrast to zircon, does not form noticeable accumulations, occurring chiefly as rare long-prismatic (3—15 mm) crystals of pink and brown coloration in paragenesis with nepheline. In transmitted light it is colorless, weakly brownish. Pleochroism and cleavage are absent. Elongation is negative. Optically uniaxial, positive; Ne — 1.608, No — 1.604.

In some grains an inhomogeneous structure is established. In this case the inner zones in their properties correspond to the usual composition of eudialyte, while the outer zones are composed of products of its hypergene alteration, from cream to brown in color, often isotropic, with characteristic branching transverse fracturing. They most likely correspond to intermediate varieties between eudialyte and zirfesite (N — 1.615) [4].

As regards the relationships of eudialyte with other minerals, it is necessary to note the presence in some of its crystals of relics of polysynthetically twinned albite, optically oriented analogously to the albite segregations surrounding the eudialyte. At the same time, in a number of cases eudialyte is clearly corroded by later untwinned fine-grained albite of the second variety, often with the formation at the contact of small zircon crystals.

In thin sections, in isolated very small (0.02—0.1 mm) grains, a yellow mineral was encountered. Its relief is high, birefringence 0.002—0.004. Pleochroism and cleavage are not expressed. Optically biaxial, positive. In the surrounding grains of untwinned albite, radial-rayed fracturing is observed, evidently caused by,

Table 1

Impurities in rocks (in %)

8065 в — a substantially nepheline rock,
8065 г — a substantially albitic rock with zircon and biotite.
U and Th were determined by chemical analysis; the contents of the remaining elements are given from the results of semiquantitative spectral analysis.

by the phenomenon of metamict breakdown of the mineral. In these properties (and only these could be determined) the mineral most closely resembles uranium-bearing rinkolite.

Of the other accessory minerals, ilmenite and fluorite are present. Thick-tabular crystals of ilmenite occur in close association with nepheline. Rare small (0.2—0.8 mm) grains of light-violet fluorite occur exclusively with late albite.

Genetic features of the veins

Two modes of formation of the above-described veins are most probable: either by crystallization of injected material in fracture cavities, or by replacement and recrystallization of the enclosing syenites. The factual evidence, in terms of the structure and mineralogical features of the pegmatoid formations, largely indicates their metasomatic origin. Indeed, the veinlike bodies occur only in syenites and are unknown in the enclosing rocks. All bodies show completely gradual transitions from syenites, through fine-grained albitized rocks, to coarse-crystalline feldspar–nepheline formations. Furthermore, the most characteristic textures are replacement textures. Thus, microcline occurs only as relics, being replaced by albite and nepheline; biotite is coordinated with hornblende and, in turn, is replaced by alkaline amphibole; nepheline is replaced by saccharoidal albite. Metacrysts of nepheline, zircon, and eudialyte, containing inclusions of the surrounding minerals, are also very widely developed. Finally, underdeveloped skeletal forms occur together with well-formed crystals of the same mineral, in particular zircon and others.

The appearance of metasomatic albite–nepheline pegmatoid formations among hornblende syenites is not accidental. As already noted, in the structure of the alkaline complex of Northeastern Tuva a significant role belongs to nepheline syenites, the formation of which is linked [3, 5, 6, 10] with the process of interaction of alkaline granitoid magma with carbonate rocks. Such interaction caused desilication and an increase in the alkalinity of the magmatic melt; as a result, following the formation of the bodies of lime–alkaline granites and syenites of the first phase, intrusion of nepheline and alkaline syenites of the second phase took place.

The formation of feldspar–nepheline bodies is caused by the activi-

with sodium desilicated fluids released already during the crystallization of the alkaline nepheline-syenite intrusion.

Naturally, the action of these fluids on the syenites causes, first of all, their albitization. Potassium feldspar is replaced by chessboard and then by platy albite. In connection with a further decrease in the acidity potential, growth of nepheline crystals begins. Alkaline-earth hornblende is replaced by the more alkaline amphibole. The most sodic member of the series eudialyte–mesodialyte–eucolite–eudialyte is formed.

After a certain break, when the acidity of the solutions again increased markedly, saccharoidal albite and fluorite formed. The formation of albite proceeded chiefly by replacement of nepheline. The appearance of the main mass of zircon, as well as rinkolite (?), is associated with this same stage. However, as was shown above, they were precipitated from the solution not simultaneously with albite, but somewhat later.

The noted sequence of metasomatic processes is connected with a change over time in the composition of the solutions and in their acid–alkaline properties. In the present case, at first there is an increase in the alkalinity of the solutions, then a decrease, which in general does not contradict D. S. Korzhinskii’s theoretical conclusions on this question.

However, the change in the composition and acid–alkaline character of the solutions occurred not only in time but also in space. Studying the character of the distribution of minerals in veins, their interrelations, and using diagrams constructed from the results of recalculations of the chemical compositions of mineral aggregates from different vein zones, it is not difficult to come to the conclusion that simultaneously with sodium metasomatism a potassium metasomatism also developed, somewhat separated from the former in space.

The potassium released during replacement of K-feldspar by albite, and then by nepheline, causes in the surrounding rocks biotitization of ordinary hornblende and recrystallization of K-feldspar in the near-contact parts of the bodies being formed. Moreover, biotitization and growth of microcline crystals in the near-contact zones of pegmatoid veins are manifested quite distinctly; however, within the bodies themselves microcline is observed only as relicts, and in their central parts it is practically absent altogether.

Consequently, there are no grounds for speaking of an independent stage of potassium metasomatism with introduction of potassium by metasomatic solutions, since, given a broad front of such K-feldspathization, even with an intensely manifested subsequent sodium metasomatism, relict areas composed essentially of microcline could have been observed in individual veins. However, such areas have not been established.

Confirmation of these conclusions can also be found in the analysis of diagrams reflecting the behavior of elements during metasomatism. In particular, the first diagram (Fig. 1) gives a general idea of the mobility of elements in the final products of metasomatism by comparing their chemical compositions from different parts of a vein with the unchanged host rock. On the basis of this diagram one can only conclude that, in the course of development of metasomatic replacement, sodium and alumina were introduced, while potassium, magnesium, iron, and calcium were removed.

A more graphic idea of the course of the metasomatic process at individual stages of metasomatism is given by the second diagram (Fig. 2), in which the rock formed as a result of the preceding stage is taken as the initial one. It is clear that in metasomatic bodies in general, and in albite–nepheline formations in particular, there are no areas in which mineral associations corresponding to individual stages of metasomatism would be spatially isolated. Usually, at...

Table 2

Change in the chemical composition of syenites in the process of their metasomatic alteration
(method with “allowance for mobility”)

Note. Silicate analyses were performed in the Central Laboratory of the Ural Geological Administration (city of Sverdlovsk).

Fig. 1. Diagram of changes in the chemical composition of syenites during their metasomatic transformation. The curves are given for the near-contact selvage of the albite–nepheline (1) zone and for the internal, predominantly nepheline (2) and albite (3) zones.

Fig. 2. Diagram of changes in the chemical composition of the outer zones of the albite–nepheline body during their replacement by the inner zones. The curves are given for the near-contact selvage of the albite–nepheline (1) zone and for the internal, predominantly nepheline (2) and albite (3) zones.

there is an overlap of these associations. However, the presence of zoning elements in the bodies and the distinctness of the original rocks and final products of metasomatism make such comparisons quite representative.

Analysis of the second diagram shows that, in the course of development of the metasomatic process, an intensive redistribution of silica, alumina, potassium, sodium, and calcium takes place. Moreover, whereas throughout the entire process of formation of the albite–nepheline bodies K, Ca, and Fe⁺³ were removed from the inner zones toward the outer zones and the enclosing syenites, and Na was introduced, the behavior of Si, Al, Fe⁺², Mg, as well as M and Ti, shows considerable changes. In the initial stage of mineral formation there is removal of Si, Fe⁺², Mg, and Ti and introduction of Al. In the subsequent stage of the process, in connection with a decrease in the alkalinity potential, by contrast, some removal of Al and introduction of the other elements, including silica, are established. However, the silica content in the solution was insufficient for it to precipitate in a free state.

Thus, the metasomatic albite–nepheline pegmatoid formations are products of a single, essentially sodium metasomatic stage, represented most clearly by two stages: a stage of increasing alkalinity (formation of bladed albite, nepheline, eudialyte) and a stage of decreasing alkalinity—increasing acidity (precipitation of saccharoidal albite, fluorite, zircon).

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