MORPHOLOGY OF GOLD GRAINS FROM THE AREA OF THE KOMMUNAROVSKY MINE (KHAKASSIA)

A. F. KOROBEINIKOV, A. M. KUZMIN

The Kommunarovsky gold-bearing district has been known for its placer and primary gold deposits since the second half of the last century. The primary deposits are represented there by three structural-morphological types of gold mineralization: vein, stockwork, and skarn-magnetite with gold mineralization superimposed on it. No researcher had undertaken a special study of the morphology of native gold and its aggregates from both the primary and placer deposits of this gold-bearing district. For this reason, the authors undertook the task of studying the morphology of gold grains from the area of the Kommunarovsky mine, since solving this problem has not only scientific but also practical significance—for example, in determining the source of supply of alluvial placers.

For the purpose of studying the morphology of gold grains from the primary deposits of Kommunar, samples with visible gold were taken mainly from the stockwork ores of Podlunnyi Golets, the Northern Zone, the Maslovsky and Solgonsky sites, and the skarn-magnetite lenses of the Kaliostrovsky deposit. In this sampling from each deposit was carried out from different parts both across the area and along the dip of the ore body¹). At the same time, samples were taken from the Saktychul and Nizhne-Solgonsky alluvial placers, and the source of supply of the latter had not been reliably established: it was assumed that, by studying the morphology of gold grains from the Nizhne-Solgonsky placer, it would be possible to determine which primary deposits had taken part in its formation.

Until recently, systematic study of the morphology of gold aggregates was carried out mainly on gold grains and nuggets from placers [4, 7, 8, 18, 21, 28, and others]. Considerable attention was also paid to the investigation of the morphological features of gold grains from primary ores, especially when the gold grains occurred either in leached ores of sulfide gold-bearing deposits [13, 19, 27, 31], or in cavities of quartz veins. This group probably includes the cited

¹) In 1959–1961, samples with visible gold were collected by one of the authors of the article, A. F. Korobeinikov, with the participation of mine geologists of the Kommunarovsky ore occurrence V. I. Yablonsky, A. S. Sukhikh, and mining foreman Yu. M. Nesterov. Taking this opportunity, the authors express their sincere gratitude to them for the assistance rendered. The authors express their sincere gratitude to Associate Professor A. I. Bazhenov for reviewing and critical comments on this article.

Examples given by V. I. Vernadsky [1] include the morphology of intergrowths of gold crystals from Sysert, Oregon, Transylvania, and other localities. Or, finally, a description is given of hexagonal forms of gold that formed on zinc shavings at concentrating mills [11]. Many investigators have described various features of gold morphology, from the general forms of gold grains [1, 9, 11, 15, and others] to fine details of surface sculpture, imprints, etc. [16, 17, 20, 22, and others], and especially the microstructure of gold in polished sections [7, 18, and others]. In most cases, however, the authors do not dwell on the technique of preparing gold grains. At the same time, one of the chief reasons that hamper the study of the morphology of gold aggregates is the fact that extracting gold grains, for example from quartz veins, without mechanically damaging them is very difficult.

To liberate the gold grains from samples of the Kommunarovsky ores, the authors used a method of etching vein quartz with hydrofluoric acid and calcite with acetic acid. For this purpose, hand specimens from nests with visible gold were selected from the ores and studied under a binocular microscope. The samples were then immersed for no more than 3–5 days, respectively, in hydrofluoric and acetic acids, until the vein quartz and calcite had completely dissolved. As a result, aggregate intergrowths of native gold free from the enclosing rock were obtained; these were then studied under binocular and polarizing microscopes at magnifications of up to 250×. No noticeable effect of the acids was detected either on the native gold or on the accompanying ore minerals (pyrite, arsenopyrite, pyrrhotite, chalcopyrite, galena, magnetite, tennantite, and others).

In studying the gold grains, the main attention was focused on microscopic examination of the morphological features of aggregates of native gold and on measuring the sizes of individual gold grains (the grains were measured along their elongation) in samples from each deposit. The number of gold grains in one sample ranged from 84 to 3000. The measurement results were processed using variation statistics [26, 29] and are presented in the form of diagrams (Fig. 1). In constructing the diagram, the frequency of occurrence of gold grains of particular classes was plotted on the abscissa, and their sizes on the ordinate. As can be seen from the diagram, each deposit studied is characterized by gold grains of a size range specific to it. Moreover, even within one and the same deposit, in different parts of it, depending on the conditions of localization of the gold mineralization, aggregates of different sizes are often formed. According to statistical data, most gold grains from the vein ores fall, by size, into five classes: the first class includes grains measuring 5.0–1.0 mm; the second, 1.0–0.1 mm; the third, 0.1–0.05 mm; the fourth, 0.05–0.01 mm; and the fifth, 0.01–0.005 mm. However, the percentage ratio of gold grains of these classes differs at each deposit (Fig. 1). Thus, the gold of the Podlunnyi Golets deposit (Fig. 1—II) forms grains that differ in size and shape (Fig. 3 a–zh). The main mass of gold grains does not exceed 0.02 and 0.5 mm in size. Large intergrowths of gold, up to 1–5 mm and more in size, occur sporadically, as clusters.

At the Maslovsky area, the sizes of individual gold grains most often fluctuate within the limits of 0.03 and 0.5 mm; they rarely reach 2–3 mm, and still more rarely do they fall below 0.01 mm (Fig. 1—II). Gold grains intergrown with sulfides occur exceptionally rarely.

The predominant portion of gold grains in the ores of the “Severnaya Zona” deposit does not exceed 0.02 and 0.5 mm in size (Fig. 1—III). In intergrowths with sulfides they are fairly rare (no more than 2–3%).

The average size of gold grains from the stockwork of the Solgon area (Fig. 1—V) is 0.01 mm; only exceptionally rarely do its aggregates reach 0.1–0.3 mm. We did not find gold grains intergrown with sulfides in this area.

The ores of the Kaliostrovskoye deposit, unlike those mentioned, are characterized by the presence of larger gold grains, 0.1–0.7 mm in size (Fig. 1—IV). Gold grains smaller than 0.01 mm are exceptionally rare, i.e., grains of the fourth and

Fig. 1. Diagram of variation curves for the distribution of gold grains of different sizes in deposits of the Kommunarovsky gold-ore district: I — Maslovskoye deposit, 1000 gold grains; II — Podlunnyi Golets, 1600 gold grains; III — Northern zone, 3000 gold grains; IV — Kaliostrovskoye deposit, 160 gold grains; V — Solgon ore occurrence, 84 gold grains; VI — Nizhne-Solgonskaya placer, 2500 gold grains.

fifth classes, which are very characteristic of the stockwork ores of other deposits, are almost entirely absent.

In the Maslovskoye and Podlunnyi areas, among veins and veinlets composed of coarse-grained quartz, often of drusy texture and with not infrequent cavities, aggregates of native gold are in general considerably larger (from 0.08 to 2 mm, with an average value of 0.5 mm) than in the stockwork ores of the same deposits, composed of fine-grained vein quartz.

It is interesting that both the gold content and the size of the gold grains gradually decrease with depth, which is especially clearly expressed in the vein type of deposits. Evidently, this indicates the specific character of ore deposition in large fracture zones.

When gold grains from the Kommunarovsky district and from a number of Siberian deposits [12] are compared by size, two principal groups are clearly distinguished in each of them: coarse gold, 0.5–0.1 mm in size, and fine gold, 0.05–0.01 mm in size; moreover, the second group of grains in most gold-ore deposits is, as a rule, predomina-

giving. Possibly this is explained by differing conditions of gold deposition at the initial and final stages of ore formation.

The morphological features of native gold from the deposits described are also different. In ores of the vein and stockwork type, the forms of gold grains are extremely diverse and depend, to one degree or another, on the character of the space they occupy; this allows us to distinguish four principal morphogenetic groups of gold grains: 1) gold grains formed by the filling of fissure cavities in vein quartz and in the enclosing rocks; 2) gold grains formed

Fig. 2. Diagram of the distribution of gold grains of different morphogenetic groups in deposits of the Kommunarovsky gold-ore district: I—Maslovsky site—stockwork type; II—Polunny Golets—stockwork type; III—Northern zone—stockwork type; IV—Solgonskoe ore occurrence—stockwork type; V—vein deposit; VI—Kaliostrovskoe skarn-magnetite deposit. 1—gold grains formed by the filling of fissure cavities in vein quartz and in the enclosing rocks; 2—gold grains formed by filling intergranular voids of quartz veins and enclosing rocks; 3—gold grains formed by filling both fissure cavities and adjacent intergranular voids; 4—gold grains formed by replacement of previously deposited minerals.

by filling intergranular voids of quartz veins and wall rocks; 3) gold grains formed by filling both fissure cavities and adjacent intergranular voids; 4) gold grains formed by replacement of previously deposited minerals. Of all the identified groups of gold grains in the ores of Kommunar, the first and third are the most widespread (Fig. 2). In accordance with the pecu-

Fig. 3. Native gold from the Poludenny Goltsy deposit:
a) a gold grain of complex combination form: 1—acicular form formed in the intergranular space of a quartz veinlet, 2—fringed form formed in complex systems of microcracks in vein quartz;
b) a gold grain with combination striation and a thin-platy overgrowth on octahedral faces;
c) a combination form of a gold grain caused both by the filling of intergranular voids, with the formation of a drusy aggregate, and by a complex system of microcracks in quartz (fringed aggregate);
d) a complex-patterned form of a gold grain with negative impressions of individual crystals of vein quartz;
e) overgrowth of pyrite by a complex dendritic-leafy and thin-platy aggregate of gold;
f) a gold grain of flattened corrugated form with thin-leafy and threadlike fringes, developed along fine cracks in vein quartz;
g) an intergrowth of gold of irregular form, formed in the intergranular space of vein quartz.

...features of the morphological outlines, up to 11 varieties of gold particles are distinguished in each group (platy, dendritic-leaflike, fringed, scaly, petaloid, grain-like and drop-shaped, lump-like and spongy, acicular, columnar, pyramid-like, ribbed-platy, stepped-platy, and a number of other forms). Gold in well-bounded crystals occurs extremely rarely and, as a rule, in the forms {111}, {100}, {110}, and still more rarely in combinations {111} and {110}. Most crystals occur in intergrowths of aggregates of gold particles, which in size do not exceed 0.05 mm and are found mainly among grains of the fourth and fifth classes; therefore they could not be extracted from the gold aggregate without damaging the crystal itself. For this reason such crystals and their intergrowths were studied at 250-fold magnification, and only in isolated cases was it possible to carry out studies of the crystals on a two-circle goniometer GD-1. Gold crystals, as a rule, have a somewhat distorted appearance, approaching curved-faced forms [23]. The surface of imperfect crystals is covered with platy overgrowths, which in a number of cases, like combination striation on the faces {111} and {100}, gold crystals often assume the character of skeletal formations (Fig. 3, в; 7, г) and often prove to be platy-layered (Fig. 4, б, в; 5 а). Thus, in the ores of the Kaliostrovskoye deposit an interesting skeletal crystal of octahedral form (4 mm in size) was found, the faces of which are pressed inward, and on the depressed surface there are cube-like projections (Fig. 7, г). In general, skeletal forms of gold in the Kommunar ores, following the classification of Shafranovsky and Mokievsky [24], can be combined into two groups: dendritic and funnel-shaped forms.

In the general case, the form of gold particles, usually representing intergrowths of a large number of very small gold crystals, depends on the morphology of that space which, to one degree or another, is filled by metal-bearing solutions penetrating into a fine system of fractures. Hence, naturally, the morphology itself of the gold particles may be not only varied, but also very complex in structure. In the stockwork ores of the Podlunny Golets, Maslovsky, Northern Zone deposits and the Solgonskoye ore occurrence, as a rule, gold particles of the first group are widespread (Fig. 2) (thin-platy, grain-like, lump-like, fringed, dendritic-leaflike, and petaloid forms, Figs. 3, 4, 5, 6). The appearance of gold particles of flattened aspect may, perhaps, depend on the easier and more rapid penetration of gold-bearing solutions along fractures developed in quartz veinlets and near-salband parts of the enclosing rocks [25].

In veins composed of coarse-grained, sometimes drusy quartz of early generations, formation could have proceeded in intergranular spaces. The form of the gold particles in these veins is mainly determined by the presence of cavities, which are filled with gold partly or completely. In the case of partial filling of cavities, a gold particle may be either a single crystal or an intergrowth of crystals (Fig. 4, б; 5, а, б; 7, г). When cavities were filled with gold solutions entirely, the form of the gold particles, more often representing a polycrystalline aggregate, will correspond to the character of the cavity (Fig. 3, а; 4, а; 5, г; 6, а, б).

Conversely, in the ores of the skarn–magnetite lenses of the Kaliostrovskoye deposit, the second morphogenetic group of gold particles is mainly widespread (Fig. 2). Here the increased porosity of the ore-bearing skarns already exerted a great influence. Voids between large crystals of skarn minerals and magnetite, communicating with ore-feeding fractures, were filled with vein material and gold...

Fig. 4. Native gold from the Maslovsky sector: a) fringe-like or intricately patterned form of a gold grain with the complexity of its main part; b) imperfect gold crystal in the form of 111, with thin plates growing on the 111 faces; c) gold grain with uneven, stepwise, platy overgrowth of the 111 faces.

Fig. 5. Native gold from a deposit of the Northern zone: a) stepwise-platy intergrowth of prismatic-looking crystals of cubic gold; b) intergrowth of cubic gold; c) platy form of a gold grain with platy-stepwise overgrowths on one of the crystal faces; d) encrustation of galena by a dendritic-leafy aggregate of gold.

Fig. 6. Native gold from the Solgonskoe ore occurrence:
a) a platy form of a gold grain with traces of impressions of the uneven surface of a fracture filled in vein quartz;
b) a gold grain of the same origin, of lump-like form.

Fig. 7. Native gold from the Kaliostrovskoe deposit:
a) a thick-platy gold grain of complex structure, 1 — gold, 2 — magnetite;
b) cubic form of a gold crystal intergrown with a platy one;
c) crystal of octahedral form intergrown with a platy aggregate;
d) skeletal crystal of octahedral form with indented faces.

Apparently, this was one of the causes that produced gold particles of relatively large size and, as a rule, of thick-tabular, pyramid-like, short-columnar, and lump-like form. It should be noted that in the skarn–magnetite lenses the gold is situated either directly in the quartz veinlets, or among magnetite and skarn minerals but close to quartz stringers. In the first case the gold particles quite often acquire hexagonal or tabular forms because, in their growth, they follow along quartz grains, conforming to the comb-like texture of the vein quartz.

Growth figures are often observed on the gold particles (Figs. 3, б; 4, б; 5, а, в), emphasized by combinational striation and by layer-by-layer overgrowth of faces [14]. Following the classification of S. Sh. Gendelev [2], among the whole variety of striations developed on the faces of gold crystals and aggregates, several types may be distinguished: 1) monoribbed stepped, 2) monoribbed stepped combined, 3) feathery stepped combined, and, more rarely, 4) trigonal stepped. Uneven overgrowth of individual faces often causes the formation of grains of ribbed-platy and platy-stepped form (Figs. 4, в; 5, а). Evidently, these morphological features of the gold particles indicate their growth under relatively free conditions, before the cavities were completely filled by aggregates of gold.

In addition, impressions of crystals of vein quartz, actinolite, magnetite, and more rarely pyrite, arsenopyrite, and galena (Figs. 3, д; 5, г), or of the surfaces of mineral aggregates that bounded the fissures being filled (Figs. 6, а; 4, а), are very often observed on the faces of the largest gold grains.

The investigations carried out on the Kommunar ores show that the deposition of gold occurred in the late stages of the hydrothermal process, after deformation had been imposed on the previously formed vein quartz, following the precipitation of the main mass of sulfides and, most probably, following the deposition of galena and tetradymite. The precipitation of gold in the final stages of hydrothermal mineralization at lower temperatures has also been noted for a number of other deposits [5, 6, 16, 17, 2, 30, 32, and others].

As our observations show, aggregates of native gold are spatially connected mainly with light-gray, medium-grained vein quartz of early generations. In general, at the Kommunar deposits one or two generations of gold are distinguished, associated with the quartz–pyrite–chalcopyrite and, less commonly, quartz–pyrite–pyrrhotite stages. At the same time, no distinct association of gold with sulfides is observed. Quartz veins and stringers enriched in sulfides are often poorer in gold than veins devoid of sulfides. All this indicates a certain spatial independence in the distribution of sulfides and gold.

In large quartz veins the distribution of native gold is, as a rule, nest-like in character, whereas in stockworks it is columnar. It is clearly observed that in thin stringers and in zones where veins branch sharply into separate small apophyses, the metal content generally increases and becomes more uniform. In general, its distribution in ore bodies is directly dependent on the character and degree of fracturing of the enclosing rocks and ores, caused by pre-ore and intra-ore deformations. Among quartz veins and stringers, two systems of fractures predominate: longitudinal and transverse (relative to the strike of the veinlets). The first of these predominates, and the largest aggregates of native metal tend toward them. For the most part, large gold particles tend toward the selvages of quartz stringers, toward zones where fractures intersect, or occur as intergrowths and separate—

scaly accumulations appears on the planes of fractures in vein quartz and the enclosing rocks. In this case, the small fractures parallel to the planes of quartz veinlets (Fig. 8) prove to be more enriched in gold; in them one sometimes encounters gold smeared over the surfaces of slickensides by subsequent tectonic movements. Occasionally, in vein quartz and along cracks along the salbands of quartz veinlets, accumulations of thin-scaly gold are observed, resembling at first glance ordinary coatings of ochres and representing an aggregate of very thin (down to 0.008–0.001 mm) flakes of native gold¹. As we see it, part of such thin-scaly and finely dispersed gold in the vein quartz could have formed after crystallization of the main mass of quartz, when its deposition was not yet complete. In this case, owing to the more rapid precipitation of gold, it was captured by the growing quartz. A number of researchers have pointed to such crystallization in hydrothermal solutions [3, 23].

Fig. 8. Filling by native gold (1) of longitudinal cracks in a quartz veinlet (3); (2) — pyrite (Northern zone).

In stockwork ores, individual gold particles are not infrequently observed in the form of irregular subisometric (Fig. 6, b) or elongated dendritic forms (Fig. 3, a, v, g, d), representing intergrowths of imperfect octahedral crystals of gold, on separate parts of which an aggregate of very fine, complexly fringed and petal-like gold has grown. Most likely such intergrowths arose through the deposition of gold both in the intergranular space and in the systems of complex microcracks adjoining it in the vein quartz; as observations show, the latter very often proves, even before the deposition of gold, to be deformed and crushed to varying degrees.

In isolated cases replacement of galena and pyrite of early generations by gold was observed; the replacement of galena proceeds mainly along cleavage cracks, while pyrite is replaced along systems of microcracks.

The presence on slickenside surfaces of films of smeared gold indicates that, following the formation of the ore veinlets, the latter underwent repeated crushing and abrasion. The newly arising fracture cavities thereby promoted the movement of new portions of solutions and the deposition of ore matter. Accordingly, all this affected the character of the distribution of gold within the ore bodies and its morphology.

On the basis of all that has been set forth, it may be assumed that the deposition of gold in the ore bodies of the Kommunarovka deposits took place under complex conditions of repeated tectonic movements and renewed influxes of metal-bearing solutions.

In studying the gold of the Nizhne-Solgon placer it is established that the size of the gold particles on average does not exceed 0.02 mm (Fig. 1-IV). Placer gold is divided by size into 3–4 classes. The larger grains of the first class in most cases have the form of rounded

thin plates. Individual gold particles are sometimes characterized by fanciful outlines with smoothed edges. Gold particles of irregular shape, often completely unrounded, are encountered rarely. In this case they are usually spongy and have a rough surface. Exceptionally rarely, individual spiral-shaped grains are found. The size of gold particles of this class is approximately 0.09—0.01 mm. The degree of roundness of most grains ranges from medium to good. Gold particles of the second class (0.010—0.005 mm in size) have rounded outlines and, more rarely, an irregular lumpy form; grains in the form of needles are often encountered. The degree of roundness of the gold particles ranges from weak to good. Gold of the third class, up to 0.003 mm in size and smaller, generally resembles very small grains. Individual gold particles are well rounded.

In comparing the morphological features of native gold from lode deposits and from the Nizhne-Solgon alluvial placer, a connection between the placer gold and the Solgon stockwork ore occurrence is clearly established. The sizes and morphology of the gold particles of the placer (Fig. 1—IV) and of the primary ores of the area (Fig. 1—V) are very similar, although in the placer they are somewhat smaller. In contrast to the Nizhne-Solgon placer, the gold particles of the Saktychul alluvial placer, whose principal source of supply is the Kaliostrov deposit, have, for the most part, a thick-platy and subisometric lumpy appearance and larger sizes (0.050—0.5 mm and more), which corresponds to the character of the gold particles of the primary sources. Evidently, gold particles of dendritic and fringed forms, which are the most widespread in ores of the stockwork type, upon entering deluvial-alluvial conditions are readily destroyed, with the formation of fine and thin-scaly gold. At the same time, some compaction of it occurred in the process of rounding. Gold particles of thick-platy, lumpy, and columnar forms, developed in skarn-magnetite ores, are almost not destroyed, but only become somewhat compacted. Dendrite-like, thin-platy, and leaf-like aggregates of the primary deposits, extracted during the appropriate processing of the ores, even when handled carefully, break up into separate scales and plates.

Thus, a comprehensive study of gold particles from primary and placer deposits permits the conclusion that the most probable source of the placer gold of the Nizhne-Solgon placer is the Solgon ore occurrences of the stockwork type. Such a conclusion is supported by such facts as identical fineness values of the gold from the primary sources and the Nizhne-Solgon placer (the assays fluctuate within the range 900—950), the size and morphological features of the gold particles, and the not infrequent occurrence of almost unrounded large gold particles and nuggets weighing up to 1—5 g. A significant contribution of such coarse gold from the Kaliostrov deposit, which lies 5—6 km from the placer described, is unlikely. Moreover, between the Saktychul placer, located 2.5 km upstream from the Nizhne-Solgon placer, there is a section of relatively poor placers. As for the presence of abundant magnetite in both placers, which until now has been the chief argument in establishing the sources of supply of the indicated placers, it may be noted that magnetite entered the Nizhne-Solgon placer, as a rule, through the destruction of barren skarn-magnetite lenses of the area, and was not brought together with gold only from the Kaliostrov deposit. Undoubtedly, some insignificant part of fine-grained magnetite and native gold also came from the Kaliostrov deposit; however, this material was not decisive in determining the main concentration of gold particles in the formation of the Nizhne-Solgon placer.

Thus, as a result of studying the morphology of gold from the area of the Kommunarovsky mine, it is established that particular forms of gold particles are characteristic of different types of gold-ore deposits, namely: 1) in stockwork ores, gold particles of platy and dendrite-leaflike forms predominate; 2) in drusy quartz veins—lump-like, thick-platy and columnar forms, and druses of crystals; and 3) in skarn-magnetite lenses with superposed mineralization—thick-platy, thick-columnar, and lump-like forms, druses and individual crystals.

The study of the morphological features of gold from primary deposits, together with investigation of the textural-structural features of the enclosing rocks and ores by the method of etching with hydrofluoric acid, provides additional material for elucidating the conditions of ore formation; and the morphology of placer gold particles makes it possible to refine and search for the possible sources feeding the latter.

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1. In such areas the appearance of iron hydroxides often completely masks the presence of thin-scaly gold. ↩