On Paragenetic Mineral Associations in Gold-Bearing Quartz Veins of the Mariinsk Taiga and Khakassia

ON PARAGENETIC MINERAL ASSOCIATIONS IN GOLD-ORE QUARTZ VEINS OF THE MARIINSK TAIGA AND KHAKASSIA

V. I. Bazhenov

(Presented by the seminar of the Department of Geology and Exploration of Mineral Deposits, Geodesy, and Mine Surveying)

The study of paragenetic mineral associations is of great importance for a correct understanding of the process of ore formation, as well as for the selection and substantiation of criteria for the detailed classification of ore deposits and their comparison within metallogenic provinces.

By the term “mineral association,” in agreement with N. V. Petrovskaya [3, 4] and A. G. Betekhtin [1], we understand assemblages of minerals formed simultaneously, arising during one stage of the ore-forming process.

The study of mineral paragenetic associations was carried out by us personally and partly from literature sources in the Sarala, Central, Komsomolsk, and partly in the Berikul ore fields. All these deposits are represented by quartz veins occurring in host rocks of different composition. Thus, the veins of the Central deposit occur in rocks of a granodiorite intrusion; the veins of the Komsomolsk deposit—in diorites; the veins of Berikul—in intermediate and basic effusives; while in the Sarala ore field the quartz veins are situated in a complex effusive-sedimentary sequence. In the opinion of most researchers, all of them are genetically connected with granodiorite intrusions of moderate acidity of Lower Paleozoic age. According to the conditions of formation, they are medium-temperature, formed within the temperature interval from 320° to 200°C [2].

The process of formation of the quartz veins was discontinuous, associated with the successive pulsating influx of ore-bearing solutions. On the basis of the criterion of intersection of mineral associations of different ages, observed in mine workings or in hand specimens, several stages of mineralization are distinguished in the ore fields, characterizing the ore process. Table 1 gives the sequence of ore formation in the various ore fields. It shows that, on the whole, the process of ore formation in all the ore fields studied is of the same type, and the general sequence of the main stages of mineralization is everywhere maintained. True, in individual ore fields (Central, Berikul) the appearance of new stages is noted, complicating the process of ore formation.

The early quartz stage of mineralization is extremely widespread in all ore fields. However, in pure form it is observed only in barren veins, and also in separate areas—

Table

Mineral associations in the gold-ore quartz veins of the Mariinsk Taiga and Khakassia

stage of industrial veins that do not carry sulfide and commercial gold mineralization. The corresponding mineral association is usually composed of gray, dark-gray, in places almost black, quartz. In a number of cases, as a result of later recrystallization, lightening to light gray is observed. A spotted texture of the vein quartz is often established. Under the microscope, together with quartz, an admixture of sericite, chlorite, and rutile is found; these constitute a complex of relict minerals preserved as a result of replacement of the original rock. It should also be noted that these minerals, especially sericite, are often concentrated in the form of spots that preserve the outlines of formerly existing fragments of the enclosing rocks, which later underwent dissolution and replacement by vein quartz. In the Sarala ore field, where the enclosing rocks for many veins are carbonaceous and carbonaceous-argillaceous schists, a finely dispersed carbonaceous substance appears as a relict admixture, imparting a black color to the quartz. There is no doubt that the formation of this variety of vein quartz took place with substantial participation of the process of metasomatic replacement of the enclosing rocks, with the formation of its fine-grained aggregate. Subsequently, recrystallization of the quartz occurred, with the formation of medium- and coarse-grained aggregates. No sulfide mineralization is associated with this mineral association. Only in extremely rare cases, in some barren veins, is a very sparse dissemination of pyrite found.

The quartz–pyrite–arsenopyrite stage of mineralization is also widespread, though somewhat less so than the preceding one. Thus, the corresponding mineral association is entirely absent from many barren veins.

The formation of this mineral association begins with the deposition of light-gray or white, medium- or coarse-grained quartz. The formation of sulfide mineralization takes place after deposition of the main mass of quartz. Ore minerals are represented mainly by pyrite, arsenopyrite, and gold. Cobaltite appears rarely and in insignificant amounts (Tsentral’nyi). The quantitative ratios of sulfides and quartz, and on the other hand the ratios between pyrite and arsenopyrite, vary within wide limits. The most abundant pyrite–arsenopyrite mineralization is found in the Tsentral’nyi and Berikul’ ore fields. It is manifested to a lesser degree in the Sarala and Komsomol’skii ore fields. Considerable variations are also observed within individual ore fields. The most intense sulfide mineralization, in the form of massive sulfide ores, is observed in many industrial veins of Tsentral’nyi, Berikul’, and, rarely, Sarala. In weakly gold-bearing veins, the sulfides form a more or less dense dissemination in the vein quartz. In most industrial veins, pyrite quantitatively predominates over arsenopyrite. However, in some veins of the Tsentral’nyi ore field (Tysyachnaya, Sibzolotovskaya 1, Nezavidnaya III, etc.) and of Berikul’, arsenopyrite is the predominant mineral.

Under the microscope, a fine-grained texture of pyrite and arsenopyrite is established. A characteristic growth zoning is often manifested both in pyrite crystals and in arsenopyrite. Sometimes an insignificant admixture of scheelite (Sarala) and molybdenite (Tsentral’nyi) is noted in the ores.

Gold content associated with the mineral association described is manifested frequently. It may be assumed that the degree of gold content is directly dependent on the amount of sulfides. A fairly large number of examples are known of gold-bearing veins in which later sulfide mineralization is not found. This proves...

does, in our opinion, connect gold with the quartz–pyrite–arsenopyrite stage of mineralization. The sizes of individual gold particles also depend on the amount of sulfides in the ores. In ores with abundant arsenopyrite mineralization, gold forms extremely fine, submicroscopic segregations in arsenopyrite (Tsentralnyi, Berikul). As the sulfide content in the ores decreases, the size of the gold segregations increases; and in the low-sulfide veins of Saraly (Kazenno-Verkinskaya, Staro-Tumannaya, etc.), coarse gold, often occurring in quartz, plays a significant role. The distribution of such coarse gold is considerably more uneven than that of fine, finely dispersed gold.

Thus, the quartz–pyrite–arsenopyrite mineral association is an early productive mineral association, with which industrial gold content is not infrequently associated.

The quartz–polymetallic stage of mineralization is even less widespread than the quartz–pyrite–arsenopyrite stage. It is most clearly manifested in the richest veins of Saraly (Andreevskaya, Kaskadnaya, Ivanovskaya, etc.), Tsentralnyi (Tsentralnaya, Kavkazskaya, Altaiskaya, Lotereynaya, Dmitrievskaya, etc.), Komsomolsk (Komsomolskaya, Pionerskaya, No. 35, etc.), and Berikul. Mineralization corresponding to this stage is recorded either in the form of independent ore bodies localized in the hanging or footwall side of ore bodies of the preceding stages of mineralization, or spatially superposed on earlier products. As in the early stages of mineralization, at the beginning of the quartz–polymetallic stage a significant quantity of quartz is deposited, followed by the formation of diverse sulfide mineralization.

In this stage, three mineral paragenetic associations are clearly distinguished, corresponding to different degrees of mineral equilibrium and usually separated in space: 1—pyrite–sphalerite–galena, 2—pyrrhotite–sphalerite–galena, 3—galena.

Pyrite–sphalerite–galena mineralization is the most widespread and is known in all the ore fields studied. The principal minerals of this mineral association, along with pyrite II, sphalerite, and galena, are also chalcopyrite, pale ore, more rarely bismuthinite, argentite, pyrargyrite, native gold, silver, bismuth, and some other minerals. Microscopic study of the ores reveals phenomena of weak corrosion of some minerals by others, as well as local intersections, which reflect the successive deposition of minerals and, in the opinion of N. V. Petrovskaya [4], testify to the existence of physicochemical equilibrium.

The main mass of gold in the industrial veins is associated with this mineral association. The gold is usually fine and in its distribution is closely associated with the sulfides. Thus, it is the principal productive mineral association, and its presence in veins may serve as a criterion for a positive preliminary evaluation of the ore. It should be noted that the highest gold contents are observed in those cases where the sulfides form a fine-grained aggregate. In those cases where the sulfides form a coarse-crystalline aggregate (the Kavkazskaya and Dmitrievskaya veins of the Tsentralnyi ore field), the gold content in the ore drops sharply. Such sectors are usually small in size and are confined to those parts of the vein fissures that are characterized by persistent elements of occurrence, the absence of transverse disturbances, apophyses, etc. This makes it possible to suppose that such a phenomenon is not connected with the process of later recrystallization of the ores, but undoubtedly is primary.

The reason for the weak gold content of ores with coarse-crystalline aggregates of sphalerite and galena apparently consists in the fact that the formation of fine-grained and coarse-crystalline sulfides took place under different physicochemical conditions of the medium. The former form under complex physicochemical conditions characterized by frequent disturbances of equilibrium. In this case an enormous number of crystallization centers arose, which led to the massive deposition of matter, including gold, from solutions. Conversely, coarse-crystalline aggregates formed in a calmer setting, when the number of crystallization centers that arose was relatively small, and therefore there was the possibility for the growth of comparatively large crystalline individuals.

The pyrrhotite–sphalerite–galena mineral association was noted by us in studying the mineralogy of the ores of the Central Vein. At various operating levels it composes individual segments of the vein, its apophyses, and also small parallel veins. Thus, in space it is separated from the preceding association. Its principal minerals are pyrrhotite, sphalerite, galena, chalcopyrite, gold, bismuthinite, and bismuth. Under the microscope the absence of intense corrosion of these minerals is established, which indicates the existence of equilibrium.

The ore minerals compose an allotriomorphic-granular, evenly grained aggregate of pyrrhotite, in which the remaining sulfides fill the interstices of pyrrhotite and quartz grains.

This mineral association is also productive. Gold forms small droplet-shaped segregations in sulfides: chalcopyrite, galena, pyrrhotite, and bismuthinite.

The galena mineral association is noted in a number of quartz veins of Sarala (Karovaya, Galenitovaya, Ozernaya, and others). Ores composed of this mineral association consist of white or light-gray vein quartz containing disseminations of fine-grained galena. No other ore minerals are deposited here. According to sampling data, gold contents are very low.

The late quartz stage of mineralization is established in the Central ore field. It is manifested as rather thick quartz veins composed of sulfide-free milky-white quartz. They often contain corroded fragments and inclusions of sulfides of earlier mineral associations that are in various stages of dissolution. Gold is not at all associated with the described mineral association. Therefore, its superposition on earlier productive associations leads to the impoverishment of the latter.

Late quartz–carbonate mineral associations are rather widely distributed in all the ore fields studied. They are represented, for the most part, by quartz–carbonate and carbonate veins. Among the carbonates, calcite occurs most often, more rarely dolomite and ankerite. Sometimes (Sarala, Berikul) such veins contain poor sulfide mineralization represented by sparse disseminations of pyrite, low-iron sphalerite, and galena. The confinement of sulfide mineralization in quartz–carbonate veins of the Sarala ore field to those areas that occur in contact with quartz veins rich in sulfides permits the conclusion that the source of the noted sulfides of the carbonate veins was the earlier mineral associations, and that the formation of sulfide mineralization in the carbonate veins is connected with the process of dissolution and redeposition of earlier sulfides. Gold is not observed in this mineral association.

The late quartz–carbonate and carbo-

...mineral associations of the Berikul ore field. Here the occurrence has been established of carbonate veins with cobalt–nickel mineralization, localized in the form of small veins along transverse ruptural disturbances. The ore minerals in them are represented by gersdorffite, nickeline, rammelsbergite, and some others. The mineral association is distinguished by its gold content, which often has industrial significance. The gold, usually fine, occurs as thin dissemination in sulfides and arsenides. There are views that the indicated cobalt–nickel mineralization is genetically connected with a later epoch of mineralization and is only spatially superposed on the gold-ore veins. But according to the data of I. V. Kucherenko, it represents one of the concluding stages of the general process of formation of gold-ore quartz veins.

In addition, according to the data of I. V. Kucherenko, an antimonite mineral association is distinguished as one of the latest. A quartz–carbonate vein with antimonite is confined to a transverse disturbance cutting the gold-ore quartz vein of the early stages of mineralization.

The described cobalt–nickel and stibnite mineralization is unusual for gold-ore deposits. Therefore, apparently, they represent “alien” mineral associations.

The metamorphogenic stage was clearly manifested in the Sarala ore field. In contact with post-ore dikes of basic composition, the ores of the early mineral associations undergo contact metamorphism, with the formation of a pyrrhotite–sphalerite–galena mineral association. The aureole of contact metamorphism depends on the thickness of the dike and usually does not exceed several meters. Under the microscope it is established that the ores of this mineral association are a granoblastic aggregate of pyrrhotite, ferruginous sphalerite, galena, chalcopyrite, and a number of other minerals. Contact metamorphism is expressed in the dissociation of pyrite and the formation of pyrrhotite, the decomposition of fahlores with the formation of a fine-grained aggregate of chalcopyrite and arsenopyrite, and also in the recrystallization of hydrothermal sphalerite and galena. Gold occurs as small droplet-like segregations in various sulfides. At the same time, no noticeable enrichment or impoverishment of contents within the contact aureole is observed.

Thus, the replacement through time of mineral associations emphasizes the staged character of the ore-forming process. The formation of each mineral association begins with a wave of acid leaching, recorded by the deposition of quartz; then, as the alkalinity of the solutions increases, sulfides are precipitated. Deposition of the main mass of gold is connected with the concluding periods of the second and third stages of mineralization. Insignificant amounts of it were deposited in the concluding carbonate stages of mineralization. The possibility is not excluded that some sulfides—pyrite, sphalerite, galena—as well as part of the gold are connected with the process of regeneration of metals from earlier mineral associations.

REFERENCES

1. A. G. Betekhtin, A. D. Genkin, A. A. Filimonova, T. N. Shadlun. Textures and Structures of Ores. Publishing House of the USSR Academy of Sciences, 1958.

2. A. F. Korobeynikov, V. I. Bazhenov, I. V. Kucherenko, A. G. Osipov. Hydrothermally altered rocks of the principal gold-ore fields of the Sayan–Altai folded region.—In: Criteria of Ore Potential of Metasomatites. Materials for the Symposium. Alma-Ata, 1969.

3. N. V. Petrovskaya. On the systematics of mineral associations arising during hydrothermal ore formation. Geology of Ore Deposits, No. 1, 1965.

4. N. V. Petrovskaya. Mineral associations in gold-ore deposits of the Soviet Union. Transactions of TsNIGRI, 1967, issue 76.