CONTACT-METASOMATIC AND HYDROTHERMAL FORMATIONS OF THE KOMMUNAR GOLD ORE FIELD

A. F. Korobeynikov (TPI)

The Kommunar gold ore district has been known for its placer and primary occurrences since the second half of the last century and has long attracted the attention of many researchers (A. M. Zaitsev, P. P. Gudkov, V. A. Obruchev, Ya. S. Edelshtein, A. Ya. Bulynnikov, D. I. Kalinnikov, A. A. Argunova, V. G. Yankelevich, P. S. Bernstein, T. M. Kaikova, N. A. Fogelman, A. E. Shabalovskii, E. P. Mironov, N. I. Popov, S. V. Mainagashev, L. G. Osinov, S. S. Ilenok, N. A. Okhapkin, A. V. Bozin, V. M. Yaroshevich, and others). Nevertheless, many questions of the geology—and especially of metamorphism, metasomatism, and ore formation—remained insufficiently studied, which adversely affected the assessment of the ore field’s prospects and the orientation of prospecting and exploration work. This circumstance prompted the author to carry out detailed geological-mineralogical studies of the contact-metasomatic and hydrothermal complexes, their genesis, conditions of localization, and their relationship to intrusive magmatism.

The area described is situated on the eastern slope of the Kuznetsk Alatau, south of the boundary of the latter with the Chebakov Depression of the Minusinsk intermontane basin, on the left bank of the Belyi Iyus River and its tributaries. The geological features of the area are determined by its position within the Iyus–Syya structural-facies zone, which is characterized by a great thickness (16–18 km) of Upper Proterozoic, Lower and Middle Cambrian deposits of geosynclinal type [1, 8]. These formations are gathered into large folds of submeridional and NE strike, complicated by folds of the second, third, and higher orders. The principal plicative structure of the area is the Kommunar anticline, which to the east-northeast is replaced by the Syya–Efremkino syncline [1, 8]. The ore field is located on the eastern limb of the named anticline, near its axial line, and is composed of rocks of a diabase-porphyrite and volcanogenic-sedimentary sequence of Lower Paleozoic [10] or, possibly, even Precambrian [3, 8] age. The rocks of both sequences are cut by small subconcordant bodies of amphibolized gabbro-diorites (Pt₃?), by the large Solgon granodiorite massif (Cm₃—O) (an intrusion of quartz monzonites, according to A. Ya. Bulynnikov), and by small stock- and dike-like bodies of olivine gabbro and gabbro-porphyrites, genetically related, most likely, to the Devonian gabbro-syenite intrusion.

During the emplacement of the sublayered bodies of the gabbro-diorite complex, thin zones of hornfels and autometamorphic amphibolites arose.

...lites. The author has not established hydrothermal formations in connection with the formation of the gabbro-diorite bodies. However, A. Ya. Bulynnikov had earlier associated stockwork gold-quartz mineralization with this intrusion.

The emplacement of the granodiorite complex was accompanied by the formation of a series of contact-metasomatic and hydrothermal formations, beginning with hornfelses, magnesian metasomatites of the magmatic stage, alkaline metasomatites, skarns and post-skarn metasomatites, and ending with diverse hydrothermal veins, including gold-quartz veins (Table 1).

The formation of small stock- and dike-like bodies of the gabbro-syenite complex was limited to weak endo- and exocontact transformations, expressed in partial hornfelsing, lime-alkaline metasomatism, and the occurrence of isolated quartz-calcite veins.

On the whole, within the ore field only the contact-metasomatic and hydrothermal rocks that arose during the period of formation of the granodiorite pluton and its dike series underwent substantial development. In this connection, we shall henceforth dwell only on their characteristics.

Magnesian metasomatites include essentially pyroxene and hornblende rocks in the apical part of the granodiorite massif and hornfelsed porphyrites of the roof, at the expense of which they were formed. In contrast to hybrid rocks, they are characterized by confinement to tectonic zones, a zonal structure (plagioclase-pyroxene-olivine rocks predominate in the centers of the bodies, changing toward the periphery into essentially hornblende rocks), and the replacement of essentially pyroxene assemblages by hornblende assemblages under conditions of an increasing and then decreasing temperature gradient. Analysis of the material balance during metasomatism showed that the origin of the described complexes is due to processes of magnesian metasomatism of aluminosilicate rocks [6], and that the metasomatic column belongs to the infiltration type (according to D. S. Korzhinskii [4]). The metasomatites are characterized by equalization of the chemical compositions of themselves and of the rocks they replace, leading to general basification [6].

In addition, in the diorites of the endocontact of the intrusive body, isolated xenoliths of metasomatized limestones—dolomite-forsterite calc-silicate rocks—have been encountered. Study of the material balance during metasomatism indicates the reactional character of the process of transformation of limestone xenoliths at their contact with the diorites of the apical part of the intrusion, still during the magmatic stage.

Alkaline metasomatites include quartz-albite and scapolite rocks, composing linear bodies of sublatitudinal and submeridional strike, respectively. The rocks of the diabase-porphyrite sequence and, in part, the diorites of the parent intrusion underwent albitization and scapolitization. The formation of quartz-albite, and subsequently also scapolite, rocks is connected, respectively, with processes of infiltration silica-alkaline and lime-sodium metasomatism (Table 2) in the early alkaline stage of postmagmatic mineral formation (according to the classification of D. S. Korzhinskii). The origin of albite and scapolite rocks is determined by variations in the concentrations of calcium, sodium, chlorine, and carbon dioxide in the solutions, in accordance with the physicochemical conditions of the environment.

Skarns are formed at the expense of carbonate and aluminosilicate rocks in zones of infiltration of lime-magnesian solutions. The entire diversity of skarns depends on the degree of heating of the replaced medium, the composition of the solutions and of the original rocks: in calcium-rich and less heated media, more ferruginous skarn minerals arise than...

Scheme of classification of contact-metasomatic and hydrothermal formations from the halo of the Solonets granodiorite intrusion of the Kommunar ore field

Table 1

in aluminosilicate rocks. Study of the chemistry of skarn formation (Table 3) makes it possible to speak of removal of iron from the underlying aluminosilicate rocks. The iron released from the metamorphosed effusives was partly used in the construction of pyroxene-andradite skarns, and, with further development of the metasomatic process, in magnetite mineralization.

Table 2

Change in composition and material balance during alkaline metasomatism of porphyrites and fine-grained diorites (in grams per 100 cu. cm of rock)

1 — unaltered porphyrite from the Kaliostrovsky area; 1—A — unaltered dike fine-grained diorite from the Podlunny area; 3a — quartz-albite metasomatite with rare amphibole veinlets, developed at the expense of fine-grained diorite (Podlunny area); 4 — scapolite metasomatite at the expense of hornfelsed porphyrite (Kaliostrovsky area).

All chemical analyses were carried out in the laboratories of the Krasnoyarsk and West Siberian Geological Administrations.

Post-skarn metasomatites (magnetite ores) are developed in the exocontact aureole of the Solgon intrusion. The pulsational character of the post-skarn period of metasomatic alteration, the nature of the enclosing medium, the evolution of the compositions of the metamorphosing solutions, and the change in the thermodynamic setting determined the facies (zonal) structure of the metasomatic zones in plan: 1) magnetite ores, 2) epidosites, 3) tremolite-actinolite, 4) chlorite, and 5) calcite-quartz-chlorite rocks. The material balance during formation of the named rocks is given in Table 3.

Gold-quartz ores. The hydrothermal period, in relation to the contact-metasomatic one, was the next stage of the unified postmagmatic process and was separated in time by a major deformational break. All known gold-ore occurrences are located in the contact zone of the Solgon intrusion and are confined-

confined mainly to small bodies of amphibolized gabbro-diorites, and also to skarn-magnetite lenses. More rarely, ore veins also occur among volcanogenic-sedimentary rocks or, finally, in the exocontact of the granodiorite massif. The author has established the following types of mineral associations, listed in the order of their formation: 1) quartz-pyrrhotite, 2) quartz-gold, 3) quartz-chalcopyrite-tellurobismuthite, 4) quartz-sphalerite-galena, 5) calcite-quartz-chlorite, 6) quartz-carbonate, 7) quartz-prehnite, and 8) barite. Gold mineralization is associated with the first three and the fifth associations, while the last three are post-ore. The first, second, fourth, and fifth mineral associations may be assigned to the type of gold ores with a low content of sulfides and their sporadic distribution in veins, and the fourth—to the type of gold ores with a moderate content of sulfides (according to the classification of N. V. Petrovskaya [9]). Usually each mineral complex forms independent bodies, but in places superposition is manifested. The formation of ore complexes was determined mainly by the potentials of metals, sulfur, and carbon dioxide in the solutions; moreover, in the first period of ore deposition a sulfur deficit is evident, which led to the formation of quartz veins with rare pyrrhotite. Subsequently the potential of sulfur and of a number of heavy metals increased, which led to the appearance of quartz-sulfide concentrations. In the final period of ore formation, on the contrary, the potential of carbon dioxide rose sharply and the potential of heavy metals decreased, as a result of which calcite-quartz-chlorite gold-bearing complexes, almost devoid of sulfides, were formed.

Wall-rock alterations. Wall-rock alterations were manifested very unevenly and were expressed in the formation of epidotized, actinolitized, beresitized, carbonatized, and prehnitized rocks. Wall-rock metasomatism, as a rule, preceded the main period of vein filling. An analysis of the material balance during metasomatism shows (Table 4) that, in the formation of wall-rock altered rocks, the composition and properties of the hydrothermal solutions were of leading importance, along with the phenomenon of redistribution of matter during replacement. The formation of gold ores and wall-rock altered rocks proceeded in a stage-pulsational manner, which led to the appearance of associations of different temperature and to phenomena of superposition, and to the absence of clearly expressed horizontal zoning.

Considering the influence of fracturing on the character of the distribution of dike and postmagmatic formations of the ore field, we establish the following. All age groups of dikes are situated in fractures of the same type, which probably indicates dike shaping of the main fracture structures of the ore field. Contact-metasomatic formations are controlled by longitudinal faults with associated fine fracturing. The different character of the manifestation of fracture tectonics in the rocks of the ore field caused the formation of a definite type of gold mineralization: along linear gabbro-diorite massifs, longitudinal faults arose with systems of feathering fractures, in which quartz-veinlet ores with poor sulfide mineralization took shape. Bedding-parallel shear fractures in the rocks of the volcanogenic-sedimentary sequence predetermined the localization of quartz-sulfide veins. In skarn-magnetite lenses, quartz-gold mineralization was concentrated in those cases where intramineralization fracturing developed in them.

According to data from aqueous extracts of gas-liquid inclusions in minerals, from the temperatures of their decrepitation and homogenization, and from thermodynamic calculations of certain metasomatic reactions, it has been possible to establish the features of changes in the composition and properties of the mineral-forming solutions.

Table 3

Changes in composition and balance of matter during skarnization and subsequent metasomatism of porphyrites and carbonate rocks
(in grams per 100 cu. cm of rock)

1—hornblende porphyrite of the Malo-Berezovsky area; 1-B—dark-gray marbled limestone of the Soltonsky area; 2—aluminous salite-andradite exoskarn of the Soltonsky area; 3—almandine-pyroxene skarn of the Malo-Berezovsky area; 4—alunophyrite, garnet skarn of the Malo-Berezovsky area; 5—alunophyrite, diopside-garnet skarn of the Malo-Berezovsky area; 6—tremolite metasomatite; alunophyrite from the Soltonsky area; 7—epidosite; aloskarn from the Soltonsky area; 8—chlorite metasomatite from the Kalinostrovsky area; 9—quartz-calcite-chlorite metasomatic rock from the Kalinostrovsky area.

Table 4

Changes in composition and balance of matter during the formation of near-ore metasomatites
(g per 100 cubic cm of rock)

1 — uralitized porphyritic gabbro-diorite (average of five analyses); 2 — epidotized and silicified gabbro-diorite at the contact with quartz vein (Tayshinsky area); 3 — amphibolitized gabbro-diorite at the contact with quartz vein (Podlunny adit); 4 — albitized gabbro-diorite at the contact with quartz vein (Podlunny adit); 5 — berezitized gabbro-diorite at the contact with a gold-bearing quartz vein (Maysky area); 6 — berezitized gabbro-diorite at the contact with quartz-calcite vein (Maysky area); 7 — carbonatized gabbro-diorite at the contact with quartz vein through which fluids passed (Podlunny adit); 8 — prenitized gabbro-diorite from the contact of a quartz vein (Podlunny adit).

On the whole, a weakly alkaline and alkaline character of the solutions is recorded, with local increases in the potential of oxygen, chlorine, sulfur, and carbon dioxide in them (in time and space). In the metasomatic stage the solutions were characterized by a chloride–magnesian–sodium composition and had temperatures from 630 to 270°C, while in the quartz–gold-ore stage they were hydrogen-sulfide–hydrocarbonate–sodium, at temperatures from 360 to 80°C. Such a change in the physicochemical and thermodynamic properties of the mineral-forming solutions was evidently caused, on the one hand, by the stage–pulsation character of the influx of hydrotherms and, on the other, by their evolutionary development in the process of ore formation.

On the basis of paragenetic and physicochemical analysis of the mineral associations, an attempt has been made to consider the energy balance of the processes of metasomatism and vein filling on the basis of the isobaric potentials of formation of certain minerals. From the thermodynamic point of view, the material obtained explains the stage–pulsation character of the course of the named processes, as well as the sequence of mineral formation.

On the basis of numerous new data, with the inclusion of results from determining the absolute age of certain intrusive rocks and ores, the genetic connection of the gold mineralization with the granodiorite complex is confirmed and additionally demonstrated. The processes of contact metasomatism and subsequent gold mineralization are apparently connected with a single magmatic center. Commercial mineralization of all types is paragenetically connected with the formation of the dike complex of the diorite–lamprophyre series, derived from the Solgon granodiorite intrusion of Salair age (476 million years).

Considering the structural conditions of the deposition of gold from solutions, we arrive at the following conclusions. The formation of gold-ore deposits was favored by the following structural conditions:

1. The presence of extended fracture cavities. The precipitation of gold in them in the form of relatively large aggregates occurred in a comparatively quiet tectonic setting. In this case the distribution of the ore component acquired a nest-like character.

2. The formation of veins in longitudinal shear fractures at the contacts of carbonaceous–siliceous shales with horizons of tuffaceous and effusive rocks occurred by means of intermittent filling by hydrotherms of the opening parts of fractures, as a result of which complexly constructed quartz–sulfide bodies arose. In this case the deposition of gold was influenced both by structural factors and by the composition of the enclosing medium (carbonaceous–argillaceous matter, early sulfides), and therefore gold particles are associated with quartz and sulfides. During crystallization of the vein material in a tectonically stressed setting, local differentiation of the solutions could take place owing to: a) hydrothermal differentiation (as a result of their interaction with the enclosing rocks) and b) the mechanical action of tectonic processes in the form of squeezing out and seepage of the residual parts of gold-bearing solutions into the near-salband parts of veins and veinlets, favoring the appearance of gold in them. Conversely, in large closed cavities, owing to crystallization of part of the vein material, under undersaturation and in a calmer dynamic setting, no substantial “separation of matter” took place in the solutions [5].

3. The presence of systems of fractures adjoining major controlling faults, which contributed to the formation of stockwork ores with a relatively uniform distribution of the metal. Here, both structural and physicochemical factors of the medium undoubtedly influenced the deposition of gold. Skarn–magnetite lenses

and the quartz veins, owing to their increased porosity and fracturing, proved favorable for the penetration of gold-bearing portions of the solutions.

In considering the physicochemical conditions of gold deposition from solutions in contact-metasomatic and hydrothermal-vein formations, we arrive at the following conclusions. 1. It is assumed that heavy metals were transported by hydrothermal solutions in the form of complex compounds of the type $\mathrm{Na[AuCl_4]}$ in the first stage of ore formation (I and II associations) and $\mathrm{Na_3[AuS_3]}$ in the quartz-sulfide (III) stage. 2. The deposition of gold from solutions was influenced, in addition to structural factors, by the physicochemical and electrochemical properties of the medium and of the metal-bearing solutions. 3. During the formation of stockworks and small veins, amphiboles, widely developed in the selvages of veins and veinlets, played the role of precipitants of gold from solutions. The chief influence on the deposition of gold was exerted by processes of interaction between ions of divalent iron, which entered the solution during the decomposition of amphiboles, and gold complexes [7]. During the decomposition of amphiboles, in addition to iron, magnesium also entered the metal-bearing solutions; being a strongly polarizing ion, it promoted the destruction of complex gold ions with the precipitation of gold from the solutions in native form. It is quite possible that all these causes explain the usual association of gold with rocks enriched in iron and magnesium. In the quartz-sulfide veins formed in sub-bedding cleavage fractures of the volcanogenic-sedimentary sequence, the chief influence on gold deposition was exerted by the physicochemical properties of the mineral-forming solutions (a drop in temperature, metal concentration, change in acidity—alkalinity, pulsatory character of supply, etc.), the presence of carbonaceous-clayey matter in the enclosing rocks, and the specifically high magnitude of the difference in electrical potentials of previously deposited vein quartz [2].

Within the skarn-magnetite deposits, the principal cause of gold deposition from hydrothermal solutions was electrolysis and hydrolysis of gold ions in solutions under conditions of natural electrical fields; processes of interaction of divalent iron ions with gold complexes played a secondary role [7]. The gold released during electrolysis was deposited on magnetite and other previously formed minerals; i.e., in this case its deposition may be regarded as occurring in a closed galvanic cell.

The material obtained may be used to solve a number of practical problems, including the development of prospecting criteria and indicators for gold.

1. Spatially, gold mineralization is confined to areas of development of the volcanogenic-sedimentary sequence with sub-bedded bodies of amphibolized gabbro-diorites (upper structural level). The latter may be regarded as productive if they are located in the contact aureole of a granodiorite intrusion in a favorable geological-structural setting.

2. The most favorable places for localization of gold mineralization proved to be the junctions of major longitudinal faults with nodes of concentration of fine fracturing. Therefore, the presence in gabbro-diorites of zones of increased fracturing of NE strike with steep dips to the NW and SE, adjoining controlling faults, may serve as a very important prospecting criterion for gold mineralization.

3. The presence of sub-bedding cleavage fractures in rocks of the volcanogenic-sedimentary sequence at contacts of carbonaceous-siliceous shales with tuffs and effusives may serve as a criterion for prospecting quartz-sulfide gold-bearing veins.

1. Skarn–magnetite lenses located near major zones of longitudinal faults may prove promising for superimposed gold.

2. Areas of sharp bends and complications of sublayered bodies of gabbro-diorites, sharp reductions in their thickness, complication by faults, as well as flexural bends of the strata of volcanogenic-sedimentary rocks above productive horizons (gabbro-diorites, skarn–magnetite lenses) could have acted as peculiar “traps” for gold-bearing hydrothermal solutions; in this connection they serve as one of the prospecting criteria for gold.

3. Ore deposition was preceded by actinolitization, beresitization, and carbonatization of the wall rocks, which is a reliable prospecting indicator.

4. Study of the material composition of the ores made it possible to establish four types of paragenetic associations of gold: 1) quartz–pyrrhotite, 2) quartz–gold, 3) quartz–chalcopyrite–tellurobismuthite, and 4) calcite–quartz–chlorite. Therefore, the discovery of these mineral concentrations should serve as an undoubted prospecting indicator for gold.

5. The lower horizons of the ore field, to a depth of 500–800 m, down to the contact with the lower diabase-porphyrite sequence and the Solgonskii granodiorite intrusion (at the present mining level of 100–300 m), should be regarded as the most promising for the search for new ore bodies and deposits.

REFERENCES

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4. D. S. Korzhinskii. The distinction between infiltration and diffusion metasomatic zonality with respect to minerals of variable composition. DAN SSSR, vol. 86, No. 3, 1952.

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