On the Role of Polymerization of Silicic Acid in Hydrothermal Solutions in the Formation of Morphological Varieties of Vein Quartz

V. M. GRISHIN, Z. G. ZLOMANOVA (IGN, Academy of Sciences of the Kazakh SSR).

In Northern Kazakhstan, quartz is one of the widespread minerals in deposits represented by quartz gold-bearing veins. Typical examples of such objects are the deposits we studied: Bestyube, Dzhalambet, Stepnyak, Zhanatyube, Aksu. Confined to sedimentary and volcanogenic-sedimentary Ordovician deposits, the veins are located in systems of fissure and rupture disturbances developing in areas where Stepnyak intrusions [3] of gabbro-diorite composition are distributed. The quartz composing such veins is distinguished by diverse primary textures. Visually, cryptocrystalline massive quartz, fine-grained sugary quartz, medium- and coarse-grained quartz, comb-like faceted quartz, transparent and translucent crystal-like quartz, and also typical rock crystal are distinguished.

Cryptocrystalline quartzes with massive and not infrequently banded texture make up the principal mass of industrial gold-bearing veins. Less developed are medium- and coarse-grained faceted, sometimes comb-like, milky-white, light-gray transparent and translucent quartz. The latter varieties compose the central parts of veins or form, at the surface, dumps of once-eroded upper parts of veins of gold-ore deposits. Granular and comb-like quartzes form areas within one another with indistinct transition boundaries. It is often possible to see gradual transitions from fine-grained to coarse-grained and comb-like varieties of quartz. In this case, the finer-grained aggregates are almost always located closer to the selvages of the vein, whereas toward the center (middle of the vein) an increase in grain size is often observed, with gradual transitions successively from fine-, medium-grained to coarse-grained and comb-like varieties. The mechanism and conditions of their formation are conceived as the result of comparatively calm crystallization of vein quartz according to the principle of geometric selection on prepared seeds [4]. The considered varieties of quartz segregations are often separated by tectonic movements and belong to different stages in relative age. Usually in such cases transparent and crystal-like quartzes represent the latest formations. The primary interrelations of the varieties and their appearance

are complicated in the course of repeated tectonic deformations, redeposition, and leaching by later hydrothermal solutions.

Electron-microscopic studies of the ecology of the mineral by the one-stage replica method [6] on a UEMB-100 microscope at magnifications of 20–40 thousand times showed that grains of different appearance in the fracture possess certain, different internal structures.

Fig. 1. Internal structures of fine- and cryptocrystalline massive and sometimes banded quartz: a) typical structure of a metacolloid; b) cryptocrystalline groundmass with indistinctly separated individuals; c) cryptocrystalline groundmass with indistinctly separated individuals with local dislocations.

structures. Among cryptocrystalline and fine-grained quartzes with allotriomorphic structures, several varieties are distinguished. The most typical for them are varieties characterized by a metacolloidal structure (Fig. 1, a). Sometimes these are structures with a poorly expressed hummocky surface (Fig. 1, b), caused by individuals of indistinct form immersed in an unstructured groundmass. In other cases, in analogous and related structural varieties, a series of small “blind” associated dislocations is developed, possibly connected with tectonic deformations of the gel during its recrystallization (Fig. 1, c).

In fine-, medium-, and more rarely coarse-grained quartzes, other internal structures are established, caused by the development of quartz individuals with definite crystallographic outlines. In individual cases, as a rough approximation, they correspond to quartz crystals of dipyramidal form (Fig. 2, a) with somewhat convex faces. In other cases these are quartz crystals with a clearly expressed prism and pyramid (Fig. 2, b). Gradual transitions are observed to varieties of quartz in which the structures are almost not manifested, because of the indistinct boundaries of the individuals.

Faceted milky-white, semitransparent varieties of quartz, sometimes approaching rock crystal, have a different internal structure. In some of them extremely small individuals are established. Their outlines are not revealed at the magnifications used. Usually, however, they are distinguished by a rather homogeneous structure with a conchoidal fracture typical of glasses. Growth accessories are widely developed. In rock-crystal varieties, zonally arranged foreign inclusions are often observed (Fig. 3, a), negative and

Fig. 2. Internal structures in quartz with a poorly expressed granular texture:
a) quartz with a structure caused by individuals of dipyramidal form; b) individuals with prism and pyramid faces.

positive crystals (Fig. 3, б), as well as structures (Fig. 3, в) usually explained in connection with dislocations of the crystal lattice [1, 2, 13].

The internal structures revealed are typical of quartz veins from all the deposits we have studied. Moreover, in quartz veins with a primary banded texture, caused by successive openings of cracks and their filling with quartz, estab—

Fig. 3. Internal structures in the vein quartz of a rock-crystal occurrence:
а) zonal arrangement of foreign inclusions. Negative crystals are visible;
б) crystal neogrowths immersed in the host crystal;
в) structures of dislocation of the crystal lattice, emphasized by mineral neogrowths.

...a different degree of manifestation of internal structures and their different character. It should be noted that they are significantly complicated by tectonic deformations. In such areas quartz is characterized by a peculiar cleavage [4], at first glance typical of calcite or barite (Fig. 4, a). Other structures appear in quartzes that have undergone leaching (Fig. 4, b). Their variations and, probably, the degree of leaching depend on the primary internal structures of the quartz. In a number of cases it was observed how struc-

Fig. 4. Secondary internal structures:
a) clearly expressed cleavage in deformed quartz;
b) internal structure of quartz that has undergone a leaching process;
c) new formations of crystals (1) of barium on the surface of leached quartz.

leaching patterns are complicated by the replacement of etching by deposition, with the formation of individuals growing on the corroded quartz (Fig. 4, b).

The internal structures of artificial quartz differ quite clearly from those established by us and indicate different conditions of recrystallization and precipitation of silicic acid [8, 10, 14]. At the same time, some varieties of the internal structures of natural quartzes show similarity to the structures of other mineral species that arise in a distinctive experimental setting, when crystal growth occurs at the expense of supercooled minute droplets settling on a substrate. The surface structure changes depending on the velocity of the gas flow and, consequently, on the duration of droplet formation. Internal structures of analogous appearance arise under the influence of radiation [9] on solutions.

The data obtained unequivocally indicate that the formation of morphological varieties of quartz is preceded by the development in the solution of the smallest particles—colloids of silicic acid [11]. Studies of the process of dissolution and precipitation of silicic acid from solution [1] show that a process of polymerization occurs in this case. In the final stages of formation of quartz gold-bearing veins, transparent and translucent rock-crystal quartz is formed. Judging from the absence of individuals in them, at this stage quartz is formed by molecular addition from “spent” solutions at sites belonging to active centers of molecular attachment, since at insignificant supersaturations growth is possible only at growth points associated with defects.

In a natural setting, the “straight” sequence in the polymerization of silicic acid considered here does not always take place; this is expressed primarily in a change in the sequence of formation of morphological varieties of quartz. The frequently observed combination of individuals differing in degree of development indicates that in the hydrothermal solution there are individuals that were in different conditions of formation or transport. It is not excluded that these phenomena are connected with the mobilization of silicic acid from the host rocks (which is confirmed by data on the decrease in their silicic-acid content), but renewal of the solution at the expense of deep sources is also not excluded.

The data obtained on the internal structures of quartz show the necessity of their detailed study in order to establish the mechanism of formation of quartz varieties, to study the state of silicic acid in hydrothermal solutions, and, possibly, the depths of solution formation, the separation of sources of ore-bearing solutions, and the relation of silicic acid to ore components.

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