THE ROLE OF THE CONTACT SURFACE BETWEEN LIMESTONES AND PORPHYRITES IN THE LOCALIZATION OF GOLD MINERALIZATION IN STEEPLY DIPPING VEINS OF THE BERIKUL DEPOSIT

V. F. BIRYUKOV, V. M. TOKAREV, A. I. KUCHIN (Tula OEF TsNIGRI)

It is known that one of the principal factors determining the localization of metasomatic sulfide bodies at the Staro-Berikul deposit is the lithological factor (V. K. Monich, 1937; D. A. Timofeevskii, 1952). This factor, in combination with favorable structural conditions, determines the exceptional confinement of metasomatic bodies to the contact of Lower Cambrian limestones and Middle Cambrian porphyrites—rocks with sharply different composition and physico-mechanical properties.

The role of the contact surface between limestones and porphyrites is not limited to this. Its influence is also reflected in the localization of commercial gold mineralization in steeply dipping veins. This influence, over the deposit as a whole, is rather complex and, in our opinion, very significant.

In analyzing, in this connection, extensive factual material, we establish the following interesting regularity. In places where the contact surface has a moderate dip angle (60–70°), commercial mineralization in steeply dipping veins is concentrated mainly in sections tending toward the limestones. Conversely, in places where the contact dips very steeply (80–85°), the outline of commercial mineralization in steeply dipping veins seems to retreat from the limestones, although the ore-hosting fractures can be traced as far as the contact surface. This regularity is clearly established within the commercial contour of the Staro-Berikul deposit, the central and northwestern parts of which adjoin directly the limestone contact, which here has a moderate dip. Incidentally, near-contact metasomatic deposits are also localized here. On the southeastern flank of the deposit the limestone contact sharply turns to a steep dip, and the contour of commercial mineralization moves away from the limestones toward the southeast. Farther south, according to borehole data, the contact becomes very steep, and therefore, immediately near it, mineralization in the quartz-sulfide veins intersected by boreholes is noncommercial. Still farther south,

within the Novo-Berikul deposit, the limestone contact again becomes flatter, and the mineralization in the steeply dipping veins explored here becomes commercial, while its contour (according to mining data) is situated directly along the contact surface.

From what has been considered it is evident that the influence of the contact surface of the limestones and porphyrites on the distribution of commercial mineralization in the steeply dipping veins of the Berikul ore field is very great and, therefore, there naturally arises the need to explain the causes that determined the manifestation of this regularity.

In our opinion, the spatial relationship of commercial mineralization in steeply dipping veins to the contact surface of the limestones and porphyrites could have depended on the position of tectonic stresses at the moment of ore deposition. Indeed, according to the established deformation plan in the late pre-ore and intra-ore stages (I. V. Kucherenko, 1967; V. M. Tokarev et al., 1968), the release of compressive forces of meridional direction, expressed in the manifestation of intense fracturing (“transverse” relative to the steeply dipping fractures, according to V. N. Ponomarev and N. V. Roslyakova, 1965), took place along the line of maximum extension, the direction of which in the general case is perpendicular to the limestone contact. Taking into account that the line of maximum extension has an eastward dip (40–50°), it may be expected that the manifestation of “transverse” fracturing will be most intense near the contact surface that dips more gently, toward which the tensile forces will be directed at a more acute angle. This is in fact observed in the northwestern and central parts of the deposit, where, in addition to the widespread development of fracturing in the effusives, intense crushing is noted directly along the contact surface, which here has a moderately steep dip (60–70°). On the other hand, in areas where the limestone contact dips more steeply, the indicated tectonic forces, being directed at an almost right angle to the contact, will be extinguished by the elastic counteraction of the limestones, and their release will most likely occur at some distance from the contact surface. Such a structural feature is characteristic of the southeastern part of the deposit, where “transverse” fracturing is manifested at some distance from the limestone contact. Directly near the contact surface, no tectonic preparation of any significance is established.

For the final determination of the role of the contact surface of the limestones and porphyrites in the distribution of mineralization, it is necessary to clarify the significance of “transverse” fracturing in the formation of ore bodies. In the opinion of N. V. Roslyakova (1966), this fracturing promoted more intensive crushing of the vein quartz during the process of ore formation, which made possible the abundant penetration into the ore-hosting fractures of subsequent portions of ore-bearing solutions. In addition, this fracturing disturbed the equilibrium state of the hydrothermal solutions filling the vein fractures, contributing to the concentration of sulfides and gold in the areas where it was manifested.

This, in our view, explains the fact that, although the steeply dipping veins can be traced to the contact surface of the limestones and porphyrites, commercial mineralization within them is concentrated in the areas where “transverse” fracturing is developed; these areas are located at varying distances from the contact, depending on the dip angle of the latter.

Thus, the contact surface between the limestones and porphyrites, being a structural element of folded tectonics, to some extent influenced the localization and spatial distribution of gold mineralization in the steeply dipping veins of the Staro-Berikul deposit. All this indicates that the folded structures were not neutral, but took part in the formation of the deposit.