ON CRYSTALLINE-ZONAL AND COLLOMORPHIC STRUCTURES IN THE PRIMARY ORES OF THE GAI COPPER-PYRITE DEPOSIT

B. M. LEVANOV

(Presented by the scientific seminar of the Department of Mineralogy and Crystallography, the Department of Petrography, and the Department of Mineral Deposits and Prospecting for Ores of Rare and Radioactive Elements)

As is known, the physicochemical conditions of ore formation are reflected in their textural and structural features. Therefore, in each particular case, a detailed analysis of such features will help clarify the conditions under which the accumulation of ore matter proceeded. The significance of a careful study of textural and structural details in ores for deciphering the processes of mineral formation and the course of subsequent changes during the geological history of a deposit has been repeatedly pointed out in their works by A. G. Betekhtin [2], S. A. Vakhromeev [3], D. P. Grigor’ev [4], F. N. Shakhov [17], S. A. Yushko [18], and others.

Among the entire variety of structural formations observed in the primary ores of sulfide deposits, crystalline-zonal and collomorphic structures are of great interest. This is due to the fact that such structures are considered indications of the primary internal fabric in the ores of deposits [2, 12, 17]. A considerable number of works concerning the formation and development of crystalline-zonal and collomorphic structures in primary ores have been written on the basis of materials from the study of pyrite deposits of the Urals. Among them one should mention the works of S. N. Ivanov [6], T. N. Shadlun [14, 15, 16], P. Ya. Yarosh [19], and others.

The content of the present article is a description of crystalline-zonal and collomorphic structures in the primary ores of the Gai copper-pyrite deposit, located in the Southern Urals. There is no need to dwell on questions of the geological structure and material composition of the ores, since they have been covered sufficiently fully in the relevant literature [7, 8, 9]. We shall note only that the deposit is of metasomatic genesis and has a mineralogical composition, typical in its general features, characteristic of pyrite deposits of the Southern Urals.

During a detailed study of polished sections under the microscope, using electrolytic structural etching by the method,

according to the scheme proposed by V. I. Bazhenov [1], a broad development is revealed of pyrite grains with a crystalline-zonal structure. They are common in pyrite-chalcopyrite ores of massive and banded texture. The content of zonal pyrite in such ores reaches 20% of the entire mass of the observed pyrite. In ores composed of bands predominantly of pyrite-chalcopyrite and pyrite-sphalerite composition, a dependence is established between the distribution of crystalline-zonal pyrite and the mineralogical composition of the bands. Zonal pyrite develops mainly in pyrite-chalcopyrite bands, where its amount ranges within 10–15% of the total mass of pyrite grains, and only very rarely, in isolated cases, is it encountered in pyrite-sphalerite bands. In bornite-bearing ores, pyrite with a crystalline-zonal structure is observed very rarely.

The degree of manifestation and the forms of development of crystalline-zonal structures in pyrite are quite diverse, but on the whole they are characterized by the following features. Zoning develops both in individual grains (Fig. 1) and in pyrite aggregates (Fig. 2), with the clearest zonal structure being observed in its relatively coarse-grained formations. The form of zonal figures is for the most part polygonal; rhombic and square forms are encountered in smaller amounts, and irregular triangles very rarely. The described difference in the pattern of zoning in pyrite grains apparently depends on the position of the plane of section [19]. The outlines of zonal figures usually do not coincide with the outlines of the grains themselves. It should be noted that in a number of cases the zonal figures have an open character, and sometimes the zoning in its pattern corresponds to the one-sided growth of one or two faces of a crystal (Fig. 2). In the relatively rarely encountered pyrite grains with complete and clear development of zoning over the entire area of the grain, a simplification of the zonal pattern is observed from the center of the grain toward its peripheral parts. The polygonal form of the zoning in the center of the grain, as the zones build up, generally assumes square or rhombic outlines in its peripheral parts. This is evidently caused by the simplification of the crystallographic forms of pyrite grains in the process of their growth [19]. Usually, however, in the peripheral parts of grains the zoning disappears, or only indistinct outlines remain of it; moreover, the zoning preserved in the central parts of the grains, as a rule, occupies an insignificant area in comparison with the non-zonal border. During cataclasis phenomena that occurred at the deposit in individual areas, zonal pyrites also underwent partial crushing. This led to the formation in them of numerous, irregularly intersecting fine cracks. The shape of the grains is preserved in most cases, but displacements of individual parts of grains relative to one another are also not rare. The spaces between fragments of grains are filled predominantly with quartz, and also, considerably more rarely, with chalcopyrite and fahlore. The zonal structure of pyrite grains in such cases is preserved, and very rarely one can observe disappearance of growth zones having the character of gradual fading of the zones as they approach the cutting crack. In some cases, mechanical deformations lead not to crushing of zonal pyrite, but to disruption of the continuity of grains along the boundaries between individual zones. In this case such disruptions are manifested mainly in the marginal parts of grains and are filled with quartz, chalcopyrite, and fahlore (Fig. 3). Structural etching reveals the zonal structure also of the internal parts of the grains described, but disruptions between zones and development along them of the above-mentioned minerals are not observed. The phenomenon under consideration is apparently due to the fact that the boundary surfaces

Fig. 1. Character of the zonal structure of pyrite grains. Polished section. Etched. Magnification 270×.

Fig. 2. Character of zonality in pyrite aggregates. Polished section. Etched. Magnification 270×.

of individual zones are weakened directions, and under external pressure the stresses arising in the grains found release in the disruption of the continuity of the grains along these directions. Moreover, these stresses were most readily relieved in the peripheral parts of the grains. The cracks formed in this way became favorable sites for the deposition of mineral matter from later solutions.

The replacement corrosion structures of zonal pyrites by minerals deposited later, established in the ores of the deposit; the filling of cracks in its grains by quartz, chalcopyrite, and bright ore; and the development of these minerals along the boundaries between zones in pyrite indicate

Fig. 3. Development of chalcopyrite (light gray) and quartz (dark) along cracks in a cataclased pyrite grain (white). Polished section. Magnification 270.

the crystallization of pyrite in the initial stages of formation of the deposit. The zonality of the pyrite grains is due, according to the opinion of A. G. Betekhtin [2], V. K. Cherepnin [12], and F. N. Shakhov [17], to their crystallization from true solutions under conditions of gradual change in the physicochemical state of the latter. The factual material presented above testifies to the preservation of zonality in pyrite grains throughout the entire geological history of ore formation in the deposit, which is characteristic, according to observations by S. N. Ivanov [6] and other researchers, of ores that have not undergone metamorphic effects. The observed disappearance of growth zones in the peripheral parts of grains is connected, in the opinion of P. Ya. Yarosh [19], with the action of later ore solutions of a different composition. This is also confirmed by the work of E. V. Frantsova [10] on the study of the internal structure of pyrites in one of the gold-ore deposits of Eastern Transbaikalia.

Along with crystalline-zonal pyrite, collomorphic formations of it have been encountered in the ores of the deposit. Collomorphic structures in pyrite aggregates are observed mainly in pyrite–chalcopyrite ores of massive and banded texture, both in areas where pyrite grains with a crystalline-zonal structure are developed and entirely separately. In comparison with crystalline-zonal pyrite, collomorphic pyrite occurs considerably more rarely. In bornite-bearing ores

very rarely can one observe relicts of the collomorphic structure of pyrite aggregates.

In studying polished sections of pyrite–chalcopyrite ores without the use of structural etching, individual pyrite aggregates reveal the development of peculiar replacement figures, formed mainly by chalcopyrite and, rarely, by quartz (Fig. 4). They are represented by very unevenly distributed spots of chalcopyrite and quartz within the pyrite segregations; these spots have rounded outlines and differ only slightly from one another in size. Within these spots one can observe relicts of pyrite in the form of

Fig. 4. Concentric replacement figures of pyrite (white) by chalcopyrite (light gray) and quartz (black). Polished section. Magn. 160.

concentric bands of various widths. Structural etching reveals an extremely interesting structure of pyrite aggregates with the replacement figures under consideration. They are composed mainly of concentric-zonal pyrite nodules having different diameters. The results of etching, shown in Figs. 5 and 6, indicate that the central parts of the pyrite nodules were subjected to replacement. The structure of these parts is strongly obscured by the processes of replacement, but in a number of cases their finely banded character can be observed. Farther outward, with increasing distance from the center of the nodule, zones of various widths develop with a radial-fibrous and cryptocrystalline structure. These zones, in turn, gradually pass into an unevenly developed, almost finely banded aggregate around the circumference, with an indistinctly expressed structure. The peripheral parts of the nodules in all cases are composed of a distinctly crystalline aggregate, in which a crystalline-zonal structure is manifested everywhere. The growth zones in it form broken lines that bend around the pyrite nodule and are composed of a large number of open polygons. The crystalline-zonal zones of individual nodules merge with one another, and, as a whole, a series of such nodules forms a pyrite aggregate. The intervals between the crystalline-zonal zones of the nodules and the places where these zones merge are represented by a fine-grained aggregate with a crystalline-zonal structure of the grains. At the same time, the zonal figures in such areas have a very irregular arrangement. Replacement of the central parts of the nodules by chalcopyrite and quartz is accompanied—

Fig. 5. Concentric-zonal structure of pyrite buds and their overgrowth by a crystalline-zonal aggregate. The black in the centers of the buds is chalcopyrite replaced by limonite. The black between the buds is chalcopyrite and quartz. Polished section. Etched. Magnification 270×.

Fig. 6. Structure of concentric-zonal pyrite buds. The black in the upper right and lower left corners is chalcopyrite replaced by limonite. The black between the buds is quartz. Polished section. Etched. Magnification 270×.

replacement is also observed along the boundaries between spherules in places where a fine-grained aggregate develops. It should be noted that cases of overgrowth of pyrite spherules by a crystalline-zonal aggregate were noted by G. F. Chervyakovskii in the ores of pyrite deposits of the Urals and were taken by him as clear evidence of the formation of the latter after the former [11].

Along with aggregates of pyrite that have been completely preserved and only slightly affected by replacement, relics of the collomorphic structure of the latter are established in cases of intense replacement by chalcopyrite. They are represented by concentric bands of varying width and quite often have a ring-like character. Structural etching reveals the fine-grained structure both of the concentric bands of pyrite and of the chalcopyrite aggregates. When chalcopyrite does not participate in the replacement of collomorphic pyrite, a relative increase is observed in the size of the grains composing the chalcopyrite aggregate.

Very rarely, relics of the collomorphic structure of pyrite aggregates can be encountered in bornite-bearing ores. Here the collomorphic aggregates of pyrite are replaced by bornite and, as a whole, form a pattern consisting of alternating bands of pyrite and bornite of varying width. Moreover, the pyrite bands, as a rule, are poorly preserved and sometimes consist of a chain of small segregations separated from one another. Figure 7 shows the result of replacement of a pyrite aggregate by bornite. Structural etching reveals the fine-grained structure of pyrite and bornite.

In addition to collomorphic formations of pyrite, a single case of the development of collomorphic bornite was encountered in the ores of the deposit. The aggregate is represented by two concenters of relatively small diameter.

Fig. 7. Relics of the collomorphic structure of pyrite (light gray), replaced by bornite (gray). Polished section. Magnification 270.

The concenters have strictly rounded outlines and a concentric-zonal internal structure, emphasized by the development, along concentric zones, of replacing chalcopyrite. That chalcopyrite in this case replaces bornite can be judged from radial veinlets of chalcopyrite…

chalcopyrite, which cuts bornite (Fig. 8). The chalcopyrite that most intensively replaces it develops in the central parts of the bornite aggregates. It is not difficult to see that replacement does not proceed from all sides, but is confined to definite directions in the concentric zones. This feature may have two causes. Either the replacement made use of weakened areas in the concentric zones, or the replacing material entered mainly from one side. The second supposition seems more correct, since such a strictly uniform arrangement of areas favorable for replacement is unlikely. Thus, the facts set forth testify to the independent formation of collomorphic aggregates of bornite, which were later replaced by chalcopyrite.

Fig. 8. Collomorphic bornite—rounded formations in the center of the figure. Polished section. Magnification 270.

There is at present no unified opinion on the causes of the formation of collomorphic aggregates of minerals in endogenous deposits and, in particular, of the collomorphic formations of pyrite and bornite considered here. Most geologists engaged with these questions regard them as evidence of the participation in ore formation of colloidal solutions formed under conditions of sharp supersaturation and characterized by a high concentration of dispersed particles [2, 14]. At the same time, a number of researchers, in particular D. P. Grigor'ev [4], regard them as intergrowths of separate, simpler individuals and argue against the participation of colloids in ore formation. Metacolloidal formations, in the opinion of these researchers, cannot serve as proof of the presence of ore matter in solution in colloidal form, since such formations may be produced by direct crystallization from a truly saturated solution. The supposition that mineral matter may precipitate in the form of a gel from true saturated solutions under certain physicochemical conditions seems more probable. For example, this may occur when hydrothermal solutions mix with acid surface waters [3, 17]. It is known that such conditions occur in the case of the formation of a deposit at a comparatively shallow depth.

Thus, the factual material presented above makes it possible to draw the following conclusions:

1. At the deposit there existed conditions under which crystalline-zonal and collomorphic structures arose in the ores.

2. Collomorphic and crystalline-zonal pyrite formed almost simultaneously, but the former evidently formed somewhat earlier, since crystalline-zonal pyrite, as a rule, overgrows collomorphic pyrite.

3. The physicochemical conditions of ore formation at the deposit are characterized by strong variability. Evidence of this is provided by the close spatial interrelations of collomorphic and crystalline-zonal structures in pyrite aggregates. Such conditions are characteristic of deposits formed at shallow depth.

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