Relationships of Dike Bodies with Gold-Bearing Quartz Veins in the Berikul Ore Field

I. V. Kucherenko, A. P. Gribanov (TPI)

The study of dike bodies and of their relationships with mineralization occupies an important place in the complex of investigations of ore fields. In many cases it makes it possible to obtain valuable data in determining the place of mineralization in the general evolution of ore-bearing magmatic centers. Analysis of the relationships of dikes with one another and with ores is necessary for determining the sequence—if not of their inception, then in any case of the development of those structural elements that are marked by dikes and ores, and consequently for interpreting the structural conditions of formation of ore fields. The comparatively broad development of dikes of varied composition and different ages in the Berikul ore field, which play an essential role in the structure of the latter, determines the formulation of the problem.

Mineralization in the Berikul ore field is confined to the northeastern limb of a large anticlinal fold composed of marbled limestones and Cambrian effusive rocks. In the eastern part of the ore field the latter are intruded by comparatively small bodies of diorites, gabbro-diorites, and gabbro, apparently coalescing at depth into a single massif. The ore-hosting rocks proper are the aforementioned effusive rocks, represented by aphanitic varieties that tend mainly toward the contact with the marbled limestones, and by plagioclase porphyrites. Pyroxene porphyrites, volcanic breccias, and volcanic glasses are distributed in the sequence in a sharply subordinate manner. The ore-hosting sequence occurs in the form of a wedge that narrows with depth and toward the northwest and sharply widens toward the southeast. This form is due, on the one hand, to the northeastward dip of the limestone contact, onto which the effusive rocks overlie, and on the other hand to the configuration of the intrusive bodies. The thickness of the ore-hosting sequence in the central part of the ore field, within its present boundaries, reaches 900–1000 m at the surface, decreasing to 500 m at the 510 m level.

Ore material fills two systems of shear fractures and one system of rupture fractures that feather a major tectonic disturbance known as the Main fault. The latter cuts the wedge of ore-hosting effusive rocks approximately in its central part, strikes almost concordantly with the strike of the folded structure in a northwestern direction, and dips to the southwest at steep angles of 75–85°. Per-

The first system of shear fractures has a northeastern strike, the second a nearly meridional one; fractures of both systems dip to the northwest and west-northwest at angles of 40–60°. Rupture fractures of the third system have an approximately latitudinal strike and dip gently to the south, southwest, and southeast at angles generally of 5–20°. The enumerated vein fractures are broken into blocks by numerous shear fractures, which have varied, predominantly northwestern orientations and also formed in connection with the development of the Main Fault.

Abundant factual material indicates that the gold-ore veins pinch out sharply at the contact with marbled limestones and comparatively rapidly in intrusive rocks, and in the general case with increasing distance from the Main Fault.

The dikes of igneous rocks established in the ore field and in its environs may be combined into five groups:

1) dikes of acid rocks—aplites, aplite-pegmatites, and pegmatites;
2) dikes of intermediate rocks—diorites and diorite porphyries;
3) dikes of lamprophyres—spessartites;
4) dikes of basic rocks—microdiabases, diabases, and diabase porphyries;
5) dikes of alkaline rocks—quartz sölvsbergites, syenite porphyries, camptonites, bostonites.

Of the varieties listed, only dikes of acid and basic rocks are widely developed; the others occur episodically (diorites, diorite porphyries) or are exceptionally rare (quartz sölvsbergites, spessartites). Dikes of syenite porphyries, camptonites, and bostonites are developed to the northeast of the ore field and were described by V. A. Zavaritskii (Zavaritskii, 1945); their position and role in the formation of the ore field are at present unclear. They are considered to be Devonian in age.

The emplacement of all dikes took place after the hornfelsing and skarn formation of the effusive sequence—processes connected with the formation of intrusive bodies of intermediate and basic composition. Practically all dikes, against the background of hornfelsed and skarned host rocks, bear no traces of these processes. True, in several cases skarned rocks were encountered that are close in appearance and composition to diabases, but the dike form of these rocks has not been reliably established. Numerous examples occur in which dikes of acid and basic rocks cut zones of skarn formation. In this connection, the conclusion is undoubted that the formation of the dike complex took place after folding and the emplacement of the intrusive bodies mentioned.

The process of formation of the most widespread dikes may be divided into two stages: pre-ore and post-ore. In the pre-ore stage there were emplaced (beginning with the more ancient): diorites and diorite porphyries; aplites, aplite-pegmatites, and pegmatites; diabases of two generations (including facial varieties—microdiabases and diabase porphyries). In the post-ore stage, diabases of the third generation formed.

Below is given a brief description of the listed dikes and their relationships with the ores.

Diorites in the form of dikes up to 1.5–2 m thick occur near the contact with intrusive bodies of basic rocks. In their central parts they are composed of normal medium-grained diorites; closer to the contacts, owing to an increase in the quantity of dark-colored components and a decrease in grain size, they acquire a dark-gray color. The thickness of these peculiar chilled zones amounts to several tens of centimeters. Scant material indicates that they heal even fractures extending for many tens of meters. The two dikes known at a depth of 450 m from the surface have one azi-

dip azimuths of \(303^\circ \angle 36^\circ\), the other \(312^\circ \angle 33^\circ\). On the basis of a number of features it may be assumed that these dikes are apophyses of intrusive bodies.

Quartz veins either cut the diorite dikes or occur concordantly with them on the hanging or footwall side; in all cases the dikes are hydrothermally altered at contacts with the veins and are undoubtedly pre-ore.

Diorite porphyrites occur episodically and are noted in the upper levels on the northwestern flank of the ore field and in the deep levels on the southeastern flank of the latter. These are greenish-gray, dense rocks with a porphyritic texture, caused by the presence of relatively large porphyritic segregations of labradorite in a recrystallized mass of andesine and hornblende. In all known cases, dikes of diorite porphyrites are cut by quartz veins and are hydrothermally altered around them; on this basis their pre-ore age likewise admits of no doubt. The form of the dikes indicates that the magmatic material filled rupture fractures.

Aplites, aplite-pegmatites, and pegmatites occur in the form of veins with a thickness of up to several tens of centimeters, exceptionally rarely more than 1 m. A nest-like form of the bodies is not uncommon. Along strike and dip the veins extend for several meters or several tens of meters, often splitting and pinching out at a distance of 1–2 m. Their spatial orientation is varied. From the form of the bodies and the character of their contacts with the enclosing rocks, it may be judged that these dikes mark rupture fractures.

The rocks described show considerable fluctuations in composition. Thus, the quartz content in aplites ranges from 18.6 to 32.7%, and that of acid plagioclases from 6.2% to 30.6%. In all cases potassium feldspars predominate, and their content reaches 50–60%. Apatite, zircon, sphene, ore minerals, and others are noted as accessory minerals. Pegmatites are composed of quartz and microperthitic potassium feldspar, pelitized to varying degrees. Both minerals are usually in micrographic intergrowth. Acid plagioclases are present in insignificant amounts in the form of small idiomorphic crystals. A little biotite, sphene, and ore mineral are also noted.

In a number of cases the dikes are composed of zones of pegmatite and aplite and are called aplite-pegmatites. Such a spatial association of aplites and pegmatites, and the similarity of their composition, testify to a close genetic relationship of these formations, as was pointed out by A. N. Zavaritsky (Zavaritsky, 1956).

The dikes described either are cut in a transverse direction by quartz veins or enclose the latter. In all cases, near the veins, aplites and pegmatites have been transformed into beresites and are pre-ore.

Microdiabases, diabases, and diabase porphyrites are distributed unevenly within the ore field and form “nodes”—areas of concentration, as, for example, established in the central part of the ore field at the 134 m level, in the southeastern part at deep levels (in particular, at the 510 m level), and in other places. They are characterized by an extent of many tens to the first hundreds of meters, with a thickness of up to 2–2.5 m at maximum. The orientation of the dikes is varied. The morphological features of the dikes indicate that the magmatic material filled rupture fractures. Apparently, the formation of dikes of basic rocks was preceded by more intense structural preparation than the formation of dikes of intermediate and acid rocks.

All the varieties listed occur within a single dike. The most recrystallized diabases are noted in the central parts of the thickest dikes. These are fine-grained, black,

dark-gray or dark-brown massive rocks with an ophitic structure of an aggregate composed of basic andesine and labradorite, augite with an admixture of enstatite, and, occasionally, olivine and brown hornblende. Diabase porphyrites are distinguished by a porphyritic structure with an ophitic structure of the groundmass; the porphyritic segregations are composed of plagioclases and pyroxenes. Microdiabases usually make up the peripheral zones of thick dikes, the so-called chilled zones, 1–2 cm thick, or, in the case of small thickness (several centimeters) of the latter, the entire dike. Microdiabases differ from diabases and diabase porphyrites only in their finer-grained composition and amygdaloidal structure.

All rocks have undergone secondary alteration to one degree or another.

Among the pre-ore diabase dikes, two generations have been distinguished. Diabases of the second generation cut diabases of the first generation, with the formation of clearly expressed chilled zones.

Of particular interest is the study of the relationships between diabase dikes and ores. Researchers who have dealt with this question (Monich, 1937, 1938; Timofeevsky et al., 1952) pointed only to cases of quartz veins being cut by diabases and, on this basis, regarded the diabases as post-ore formations connected with Devonian or even younger magmatic activity. Meanwhile, the extensive factual material studied by us indicates that these relationships are more diverse.

In some cases the pre-ore diabases are cut by quartz veins without any noticeable decrease in thickness; in other cases (namely, where thick diabase bodies are present) the quartz veins rapidly pinch out in the dikes, or else bluntly abut against them, only here and there showing a tendency to penetrate into the dikes in the form of thin quartz-carbonate veinlets with sulfides. Sometimes only weak brecciation of the pre-ore diabases in vein fractures can be observed.

Some large quartz veins (for example, veins No. 49, No. 56, and others) occur concordantly with pre-ore diabase dikes throughout the entire extent exposed by mine workings, i.e., over a distance of the first hundreds of meters along strike and many tens of meters down dip. In some sectors one and the same vein occurs in the footwall of the dike, in others—in the hanging wall or in the dike itself. Such “migration” is explained by the fact that quartz veins, filling shear fractures, are more rectilinear than diabase dikes, for which angular forms are not uncommon. The spatial connection of small quartz veins with dikes is especially clearly manifested in the deep levels on the southeastern flank of the ore field, near a large stock of gabbro-diorites. Here, along 500 meters, quartz veins have been exposed by the Kvershlag, occurring almost exclusively either in the hanging wall or in the footwall of diabase dikes. Apparently, the gabbro-diorites, as on the eastern flank of the ore field, under conditions of remoteness from the Main Fault, were an environment unfavorable for the formation of large subsidiary fractures and had a negative influence on the development of these fractures in the porphyrites adjoining the stock. Here the subsidiary fractures developed weakly only at the contacts of two media sharply different in physicomechanical properties (Table 1)—diabases and porphyrites.

Differences in the physicomechanical properties of diabases and the enclosing porphyrites played an essential role also in cases of formation of large vein fractures concordant with diabase bodies (veins No. 49, 56, etc.).

In all cases the pre-ore diabases are hydrothermally altered in contacts with quartz veins, with the formation of a thin selvage of lightened-

and intensely carbonatized rocks. Hydrothermal alteration of the diabase rocks consists in the decomposition and replacement of plagioclases by colorless micas, complete replacement of pyroxenes by carbonates and chlorite, and decomposition of hornblende with the formation of a large amount of leucoxene. A small amount of secondary quartz and pyrite is noted. With increasing distance from the quartz vein the intensity of alteration gradually diminishes, and at a distance of 10–20 cm from the vein the wall-rock alterations are manifested only in an increased content of carbonates in the rock.

Table 1

Results of determinations of the physico-mechanical properties of diabases and porphyrites*)

*) The determinations were carried out by G. D. Del at the Department of Strength of Materials of the Tomsk Polytechnic Institute.

The insignificant thickness of the zones of near-ore alteration of the diabase dikes in comparison with the enclosing porphyrites is probably due to the fact that the conditions for the penetration of hydrothermal solutions in the diabases, owing to their low effective porosity (Table 2), were less favorable than in the porphyrites, which are characterized by higher values of effective porosity.

Table 2

Values of effective porosity of diabases and porphyrites (average of 5 specimens)*)

*) The analyses were carried out in the laboratory of oil-reservoir physics of the Department of Fossil Fuels of the Tomsk Polytechnic Institute.

Diabases of the third generation (post-ore) are most widely developed in the upper horizons of the ore field. They also fill rupture fractures and cut gold-bearing veins. However, the opposite picture can often be observed—the vein fractures, although weakly expressed, continue into the post-ore dikes, especially those of small thickness; in connection with this, resolving the question of the age of these dikes relative to mineralization is difficult. In such cases, the complete absence of signs of near-ore alteration in the diabases at contacts with the ores indicates the post-ore age of the dikes. Where dikes intersect ores with a breccia texture, the facts that dikes occur undisturbed in the brecciated ores, and that bodies of dikes cut fragments of enclosing rocks enclosed in the ores, contribute to an unambiguous resolution of the question, so that two

parts of one and the same fragment are located on opposite sides of the dike, etc.

The material presented makes it possible to draw a number of conclusions.

1. The formation of the dike complex of the Berikul ore field took place over a long period of time, both before mineralization and after it.

2. The overwhelming majority of dikes fill rupture fractures.

3. A comparison, in historical sequence, of the scales and features of the fractures healed by various dikes and ores indicates a gradual increase in tectonic stresses by the time of ore deposition.

4. In individual parts of the ore field there exists a definite spatial relationship between quartz gold-ore veins and dikes of pre-ore diabases; this relationship is due to the preferential development of vein fractures in the contacts between rocks that differ in their physicomechanical properties—diabases and the enclosing porphyrites.

5. The criterion of mutual intersections, in interpreting the relationships between diabase dikes and quartz veins, in a number of cases cannot be taken as a basis for judging the relative age of either. In this connection it is necessary to make a careful study of signs of the influence of some formations upon others—hydrothermal alteration of diabases in contacts with quartz veins, metamorphism of ores near dikes, etc.

REFERENCES

Zavaritskii A. N. Igneous rocks. Publishing House of the Academy of Sciences of the USSR, 1956.

Zavaritskii V. A. Alkaline vein rocks from the vicinity of the Berikul gold-ore deposit. Izvestiya of the Academy of Sciences of the USSR, Geological Series, No. 6, 1945.

Monich V. K. Geological sketch of the Berikul gold-mining district. Transactions of the Zolotorazvedka Trust and NIGRIzoloto, issue 7, 1937.

Monich V. K. On the problem of cobalt-nickel ores of Siberia. Vestnik ZSGRT, No. 1, 1938.

Timofeevskii D. A., Shcheglov P. I., Borishanskaya S. S. The Berikul gold-ore deposit. In: Gold-ore deposits of the Kuznetsk Alatau, 1952.