On Productive Mineral Assemblages of the Kochkar Gold-Arsenic Deposit (Southern Urals)

V. M. Yanovskii, L. V. Mikhailova (TsNIGRI)

Questions concerning the mineralogy, geochemistry, and genesis of the ores of the Kochkar deposit have been considered in the works of M. N. Al’bov, N. I. Borodaevskii, S. D. Sher, S. S. Borshchanskaya, G. M. Virovlyanskii, N. G. Zakrzhevskaya, N. K. Vysotskii, N. V. Petrovskaya, L. I. Bulatovskaya, M. I. Romanova, P. V. Pokrovskii, E. A. Savari, I. I. Chupilika, G. N. Shavkin, and others.

In the sulfide–quartz ore veins of the deposit, about 100 ore and vein minerals are known. The most widespread are: quartz, which makes up as much as 95% of most ore veins; carbonates; arsenopyrite; pyrite; pyrite–marcasite; chalcopyrite; sphalerite; pyrrhotite; galena; tetrahedrite; tetradymite; bismuthinite; jamesonite.

The ore veins of Kochkar contain, as a rule, 5–7% sulfides, being a typical representative of the formation of low-sulfide veins (N. V. Petrovskaya), but at the same time, in some parts of the ore field the sulfide content increases to 40–50%. Judging from the mineral-thermometric determinations of L. I. Koltun, the mineral complexes of the ore stage formed in the temperature interval from 270 to 170°. The textures and structures of the mineral complexes indicate the formation of the greater part of the minerals and vein quartz by replacement; the manifestation of filling of rupture cavities is extremely limited (breccia, cockade, and crustification textures). Replacement structures are widespread (looped and threadlike), as are the structure of mutual boundaries, skeletal and zonal replacement structures, and fractured structure; emulsion-like and lamellar structures of solid-solution decomposition are encountered rarely.

The hydrothermal process is divided into three stages of mineral formation.

The pre-ore stage is represented by quartz–feldspar and quartz veins, accompanied by metasomatites of epidote–amphibole composition along tabashka dikes and of greisen type along granitoids.

The ore stage is represented by sulfide–quartz veins, accompanied by carbonatization and sericitization of tabashka dikes and by beresitization of granitoids.

In the post-ore stage, quartz and quartz–carbonate veins are formed.

Characteristics of the Mineral Associations of the Ore Stage

The mineralogical composition of most veins of the Kochkar deposit is comparatively uniform: these are pyrite–quartz and arsenopyrite–quartz low-sulfide ores with varying contents of gold and sulfides of copper, zinc, and lead; tetrahedrite, jamesonite, tetradymite, boulangerite, and bournonite form substantial concentrations in individual veins. The sulfides have a superimposed, epigenetic character with respect to the ore quartz.

The character of fracturing, the presence of inclusions of the host rocks, and the forms of the contacts of the quartz veins determine the variety of ore textures. Veinlet and veinlet-disseminated textures are developed everywhere. Banded textures are observed more rarely; bands of pyrite or arsenopyrite are oriented parallel to the contacts of the veins.

Dissemination in the ore quartz is usually represented by single grains or by clusters of idiomorphic grains of pyrite or arsenopyrite.

Intergrowths of minerals of late associations (pyrrhotite, pyrite–marcasite, galena, tetradymite, etc.) form nests of irregular shape and various sizes, or short veinlet-like segregations in quartz.

Some veins, predominantly arsenopyrite-bearing ones, are characterized by stockwork textures. Massive and brecciated textures have a limited distribution; cockade and crustification textures are very rare.

Thus, the presence of textures indicating normal opening of fracture cavities and the filling of voids is extremely limited. Cases of metasomatic formation of sulfides and of a considerable part of the ore quartz predominate.

Mineralogical mapping of the deep levels of the deposit made it possible to distinguish five paragenetic mineral associations: 1 — arsenopyrite–quartz, 2 — quartz–pyrite; 3 — chalcopyrite–sphalerite–pyrrhotite (with pyrite–marcasite, pale ore, gold), 4 — gold–tetradymite (with galena, boulangerite, bournonite, bismuthinite), 5 — late-ore quartz–carbonate–galena. These associations are partly separated spatially, divided by intra-ore movements noted over the entire area of the ore field, and partly occupy different structural positions (the minerals of the late-ore association sometimes fill their own systems of fractures); they differ in the composition of the solutions that deposited them and, above all, in the contents of the principal components of the ores: gold, arsenic, bismuth, tellurium, etc. Except for the first two, each of these associations corresponds to separate intervals of the ore-forming process, which may be regarded as substages of the ore stage of mineral formation. The quartz–pyrite and arsenopyrite–quartz associations are linked by a gradual transition; they are distinguished by the quantitative ratio of ore minerals and, despite their spatial separation and differences in composition, are considered together in the arsenopyrite–pyrite substage of the ore-forming process. In ore areas of mixed arsenopyrite–pyrite–quartz composition, arsenopyrite is replaced and corroded by pyrite. Replacement of arsenopyrite by later sulfides is widely developed. Probably, the properties of arsenopyrite favored the deposition within it, along fine fractures, of subsequent pyrite, pyrrhotite, chalcopyrite, pyrite–marcasite, and gold. In the form of anhedral inclusions, gold in arsenopyrite is encountered less often than in pyrite, but it often cements crushed grains of arsenopyrite.

The main minerals of the quartz–pyrite association are ore quartz and fine-grained pyrite (cubic crystals with edge sizes up to 0.5–0.6 cm, more commonly 0.1–0.2 cm). The high arsenic content of the ore pyrite (according to the results of semiquantitative spectral analysis, the arsenic content in monomineralic pyrite samples is 0.1–1.0%) is one of the typomorphic features of pyrite of the ore stage at the Kochkar deposit.

The relationships of pyrite with pyrrhotite, chalcopyrite, tetradymite, galena, and other later minerals indicate the superposition of subsequent minerals; pyrite crystals are often filled with anhedral inclusions of late minerals, including gold.

The chalcopyrite–sphalerite–pyrrhotite association includes pyrrhotite, chalcopyrite, pyrite–marcasite, fahlores, sphalerite, arsenopyrite, calcite, ankerite, and gold, forming disseminations, irregularly shaped nests, or differently oriented veinlets in ore quartz. The amount of minerals of the chalcopyrite–sphalerite–pyrrhotite association usually does not exceed 1–2% of the mass of the ore bodies. The presence of breccia textures (Zh. Zelenaya) and crosscutting textures, where early sulfides are cut by minerals of the chalcopyrite–sphalerite–pyrrhotite association, indicates their later deposition.

Intergrowths of pyrrhotite, chalcopyrite, galena, and gold form anhedral inclusions in pyrite and, more rarely, in arsenopyrite. Sometimes gold forms small idiomorphic crystals in fahlore and chalcopyrite (Zh. Zelenaya, Sev. Aleksandrovskaya). The relationships between gold and chalcopyrite may indicate their nearly simultaneous formation.

The main minerals of the gold–tetradymite association are tetradymite, galena, native bismuth, and bismuthinite, while jamesonite, boulangerite, and bournonite are less widespread.

Intergrowths of minerals of this association form fine disseminations in ore quartz. The most common intergrowths are gold with tetradymite, gold with bismuthinite, galena with tetradymite and bismuthinite, and native bismuth with tetradymite. Bismuth minerals are characterized by the formation of graphic replacement structures with minerals of the chalcopyrite–sphalerite–pyrrhotite association.

In the late-ore substage, veins of simple composition were formed—quartz and quartz–calcite veins with an insignificant content of sulfides: galena, chalcopyrite, fahlores, gold, electrum, polybasite, pyrargyrite, argentite, cinnabar, silver, and sternbergite (?). Judging from the very rare occurrences of this mineral complex, it is of no substantial importance for the industrial gold content of the ores.

The proposed scheme of the sequence of mineral formation in the ore stage of the hydrothermal process differs from preceding schemes not only in the characteristic mineral complexes distinguished, but also in the position of gold. According to our data, gold is associated with three successive mineral associations: 1) quartz–pyrite, 2) chalcopyrite–sphalerite–pyrrhotite, 3) gold–tetradymite.

These associations are productive for gold. The arsenopyrite–quartz association determines the productivity of ore veins for arsenic.

Spatial position of productive mineral associations.

Comparing the relative distribution of productive associations over the area of the ore field (300 and 200 m levels), it is possible to obtain approximate quantitative ratios of the associations for

of a certain level of mineralization: about 30% of the total length of the ore bodies is accounted for by the arsenopyrite–quartz assemblage, about 75% is occupied by the quartz–pyrite assemblage, 15–20% by the chalcopyrite–sphalerite–pyrrhotite assemblage, and 3–5% by the gold–tetradymite assemblage. Although these figures give only a very approximate idea of the quantitative ratio of the assemblages, it can be seen that the main background of mineralization is determined by the first two assemblages.

The area of distribution of the arsenopyrite–quartz assemblage approximately corresponds to the upper tier of the plagiogranite massif enclosing the mineralization—the zone of development of banded plagiogranites and granite-gneisses with interbeds and lenses of crystalline schists, gneisses, and amphibolites.

Gold occurs in arsenopyrite in the form of veinlets and anhedral inclusions, usually intergrown with pyrrhotite, chalcopyrite, and tetradymite. Comparing the contents of gold and arsenic in areas of development of the arsenopyrite–quartz assemblage for veins in different parts of the deposit, we came to the conclusion that there is no relationship between these ore components. Areas of elevated gold concentrations and areas of predominant development of the arsenopyrite–quartz assemblage are spatially separated. The correlation relationships of gold and arsenic, analyzed by V. I. Pokusaev and Yu. V. Ovsyannikov for the principal veins of the ore field, also confirm the conclusion that the arsenopyrite–quartz assemblage is practically non-auriferous. Stable correlation relationships were noted for such pairs of elements as gold–bismuth, bismuth–tellurium, and gold–tellurium; high correlation coefficients (more than 0.8) were obtained for veins in whose ores we established a substantial distribution of minerals of the gold–tetradymite assemblage.

Most of the ore bodies within the central part of the ore field are composed predominantly of minerals of the quartz–pyrite assemblage. On the flanks of the field the quartz–pyrite assemblage “overlaps” the arsenopyrite–quartz assemblage; a gradual transition is observed from pyrite–quartz ores to ores of mixed, arsenopyrite–pyrite–quartz composition, and the quantitative ratios between pyrite and arsenopyrite in these areas are variable. Drilling data confirm that the quartz–pyrite assemblage is widely developed on the 600–900 m levels, and its qualitative composition does not change with depth.

In most cases the relationships of gold with pyrite indicate a later, superimposed character of the gold. Judging from the technological test (L. P. Bergelson), the presence of dispersed gold in pyrite is possible, but not more than 8–10% of the total amount of gold in ores of this composition. Comparison of the map of the gold content of ore bodies and the distribution scheme of productive mineral assemblages of the level makes it possible to assign the quartz–pyrite assemblage to the auriferous ones; and although the gold contents in ores of this type are low, this assemblage determines the economic character of most ore bodies.

The chalcopyrite–sphalerite–pyrrhotite assemblage has a considerably smaller areal distribution than the assemblages of early sulfides and is usually located within the contours of the latter; spatially it tends toward the zone of development of quartz–pyrite ores, and less toward arsenopyrite–quartz ores. Near the surface, the presence of pyrrhotite and chalcopyrite has been noted by many researchers. Below the 300 m level this assemblage was observed by us in a number of deep boreholes. Characteristic is the formation by minerals of the chalcopyrite–sphalerite–pyrrhotite assemblage of fine dissemination in the enclosing rocks. Halos

development of rare disseminations and small veinlets of pyrrhotite and chalcopyrite near the ore zones are also noted at the 300 m level.

High gold contents are not characteristic of the chalcopyrite–sphalerite–pyrrhotite association. Ores that include chalcopyrite, sphalerite, fahlores, and pyrrhotite have a gold content not exceeding 10–15 g/t.

The minerals of the following gold–tetradymite association take part in the composition of separate, short ore lenses. In many deep boreholes, minerals of the gold–tetradymite association occur at depths of about 500–900 m. In the total mass of ores, the gold–tetradymite association accounts for several percent, but this association is connected with the formation of areas with high gold concentrations. Enriched portions of ore bodies (2–3 times relative to the average background) often coincide with the outlines of the distribution of the gold–tetradymite association.

Large areas of high gold concentrations, on the scale of groups of production blocks, ore lenses, etc., are controlled, both with respect to their spatial arrangement and with respect to their form, by structural elements.

From several lower levels and deep boreholes intersecting sulfide–quartz veins at depths from 500 to 700 m, a regular relationship is established between the position of areas of distribution of the gold–tetradymite association and a series of steeply dipping submeridional pre-ore faults. Against the background of the widespread development of quartz–pyrite mineralization, the areas of distribution of minerals of the gold–tetradymite association have the form of elongated vertical zones confined to submeridional pre-ore fractures that delimit a system of tectonic blocks.

Cases of the manifestation of such control in the placement of the gold–tetradymite association within the block structure of the Kochkar deposit may be regarded as a regular phenomenon. At the nodes of intersections of VSV and latitudinal ore-hosting faults with pre-ore meridional fractures, this association appears and enrichment of the ore bodies in gold takes place.

In conclusion, it should be emphasized that, during the ore stage of formation of the Kochkar deposit, five associations are formed successively, among which three successive ones—quartz–pyrite, chalcopyrite–sphalerite–pyrrhotite, and gold–tetradymite—are productive for gold. The degree of gold content of these associations increases from the early to the late one. The quartz–pyrite association determines the “background” industrial gold content of most of the ore bodies of the deposit, while the highest gold concentrations are associated with the chalcopyrite–sphalerite–pyrrhotite and gold–tetradymite associations, whose spatial position is clearly controlled by faults delimiting a group of tectonic blocks.