Main Structural Elements of the Lyubavinskoye Gold Ore Deposit in Central Transbaikalia

G. V. Shubin, S. A. Chubarov (TPI)

The Lyubavinskoye gold ore deposit is located within the Kyra District of Chita Oblast and lies on the southeastern flank of the large Upper Mesozoic Altan–Kyra–Byrchinskaya depression, being confined to an intraregional zone of increased fracturing of sublatitudinal strike, which appeared at the surface as the expression of a deep basement fault.

The geological structure of the deposit includes: 1) granitoid stocks of Triassic age, having a complex morphology with an overall elongation in a sublatitudinal direction; 2) dike rocks represented by albitophyres, granite-porphyries, quartz porphyries and porphyrites, microdiorites, porphyrites, and spessartites. These rocks form a dike belt of complex morphology with an overall sublatitudinal strike.

The above-mentioned magmatic formations cut through the Permian sandy-shale sequence, which within the deposit composes the limb of a second-order anticlinal fold, striking 260° and dipping south at 50–70°.

Study of the mineralization of the ore bodies made it possible to distinguish six mineral associations at the deposit: arsenopyrite-pyrite, quartz, quartz-sulfide, quartz-ankerite-gold-sulfide, quartz-pyrite, and quartz-calcite-sulfide. Analysis of genetic features [3] permits the conclusion that the quartz-gold mineralization is of medium- to high-temperature type, and that formation of the Lyubavinskoye deposit took place at depths ranging from 1 to 2 km; it belongs to the group of hydrothermal deposits of the hypabyssal facies.

The area of the deposit has undergone repeated tectonic deformations, which brought about the development of an entire series of disjunctive disturbances, among which it is possible to distinguish pre-dike, pre-ore, intra-ore, and post-ore disturbances, differing in scale and degree of manifestation (Fig. 1).

Near-latitudinal pre-dike structures dipping south at 65–70° are zones and belts of increased fracturing, in a number of cases filled by rocks of the dike complex or by a chain of

Fig. 1. Schematic plan of the principal fault structures in the central part of the Lyubavinskoye deposit.
D — pre-ore tectonic faults; K — quartz veins; V — intra-ore tectonic faults; P — post-ore tectonic faults.

Fig. 2. Diagram of the distribution of magmatic formations and the position of controlling structures

1 — granite stocks; 2 — belt of granite-porphyry dikes; 3 — disjunctive disturbances; 4 — the “Northern” fault

of individual, irregularly shaped stocks elongated in a latitudinal direction (Fig. 2), as well as with products of hydrothermal activity after repeated renewals. The formation of the above-mentioned structures, which influenced not only the distribution but also the morphology of the magmatic bodies, was closely connected with the manifestation, within the deposit, of a deep-seated fault.

The character, form, and spatial position of the pre-ore, intra-ore, and post-ore disturbances were strongly influenced by the sharply expressed mechanical heterogeneity of the deposit area by that time, caused by the widespread intrusion of granitoid stocks and rocks of the dike complex.

The formation of pre-ore disturbances, which were mainly of shear nature and had a reverse-fault–strike-slip character, took place both through renewal of ancient structures and through the emergence of new fracture systems. The different character of the fractures that arose in pre-ore time left its definite imprint on the morphological features of the veins. Thus, the filling of shear fractures with vein material led to the formation of morphologically complex veins, often with swellings, pinches, conduits, and en échelon veinlets and lenses (Fig. 3, d). The filling of rupture fractures, by contrast, was expressed in the formation of fairly persistent quartz veins (Fig. 3, a, b). Cases of the formation of feathering systems of the “horse-tail” type are not uncommon. The morphological features of the veins were also influenced by the character of the enclosing rocks; thus, in granitoid stocks, branching of quartz veins is often observed, and typical stockwork zones are formed both within granitoid bodies and in large zones of crushing and mylonitization in rocks of the sandy-shaly sequence (the Kopyshnye veins). Flexural bends of veins are observed where differently oriented fractures are filled (Fig. 3, e).

Among the intra-ore disturbances, on the basis of their structural position, time of manifestation, and character of mineral filling, early and late ones can be distinguished.

The early intra-ore disturbances, being by nature tension fractures with subsequent shear phenomena, are characterized by even, well-sustained contacts, constant and relatively small thickness, and, as a rule, gentle dip angles with meridional, northwestern, and northeastern...

Fig. 3. Morphology of veins

1 — A — Kseniya vein — 2 (eastern drift, adit — 5); B — Grishevskaia vein (drift — 2, Ivanovskaia adit); V — Annenskaia vein (western drift, Annenskaia shaft); G — segment of a quartz vein, 1 — granites, 2 — quartz fractures, 2 — zone of meridional early intra-ore tectonic disturbances, Mokraia Yama area; 3–4 — Evgrafovskaia vein; D — Seniia vein (raise — 3); E — Annenskaia vein (raise — 15).

strike. In the course of their development these systems of fractures distinctly displaced the pre-ore disturbances (Fig. 3, e).

As for the late intra-ore disturbances, they are characterized by steeply, more rarely gently dipping zones of crushing and mylonitization, having predominantly northeastern or approximately meridional strike. Analyzing the character and degree of displacement along these structures, one may assume that, from the time of their formation onward, block displacements became especially clearly expressed; these also continued in post-ore time, which caused the emergence of a whole series of small post-ore conjugate fractures of a shear character.

In carrying out underground mine workings, one often has to deal with the displacement of quartz veins by post-ore disturbances. As observations have shown, displacements of quartz veins obey the following regularity: a) for steeply dipping displacers (more than 45°), the block of the vein in the hanging wall of the displacer is displaced, relative to the block in the footwall, in the direction of dip of the vein; b) for gently dipping displacers (less than 45°), the block of the vein in the hanging wall of the displacer is displaced, relative to the block in the footwall, in the direction opposite to the dip of the vein. Partial deviations from this regularity are noted for veins occurring in intrusive bodies and, more rarely, in sandstones. It should be noted here that, in the practical application of this regularity, one must strictly distinguish whether the given disturbance is pre-ore or post-ore. In the case of encountering a pre-ore disturbance and an associated quartz vein, one may observe an apparent displacement of the vein, the search for which should be conducted toward the obtuse angle formed by the vein with the pre-ore disturbance.

It must be said that in none of the late intra-ore disturbances did we encounter gold-quartz-sulfide mineralization characteristic of the early stages of ore formation. In these disturbances there occurred hydrothermal, predominantly carbonate mineralization of a low-temperature series, completing the ore process. Moreover, as the results of geochemical studies of monomineral fractions from the disturbances described have shown, the latter lack accessory elements characteristic of the productive stages of the ore process.

When considering level plans of the central part of the Lyubavinskoye deposit (Fig. 1), attention is drawn to the degree of intensity of manifestation of different types of structures, with distinct regularities in their interrelations. Thus, on the eastern flank we see a relative predominance of pre-ore disturbances; farther to the west the role of intra-ore disturbances increases; and, finally, directly on the western flank of the ore field the role of post-ore disturbances sharply increases. This circumstance is not accidental and was caused by the successive development and differing intensity of manifestation of particular structures within the Lyubavinsky fault during its development from east to west. This circumstance was favored by the presence, on the eastern flank of the fault zone, of the enormous Khamara-Tyrinsky granitoid massif, which acted as a reaction to those dynamic stresses that arose from the western flank during the development of the depression.

Comparing the geological structure of different parts of the deposit, one may note their general similarity: in each area there are granitoid stocks and a single dike belt, occurring in rocks of sandstone-shale composition; and, finally, the systems of ore-controlling shear fractures are everywhere the same (Fig. 4). The similarity of the fracture-

Fig. 4. Diagram of the attitude elements of quartz veins.

1 — Evgrafovskii area; II — Geologicheskii area; III — Malo-Fedorovskii area; IV — Bol’she-Fedorovskii area. 4 — tension fractures; 5 — shear fractures. A — diagram of the position of latitudinal ore-hosting structures (2) relative to granitoid stocks (1) and the dike belt (3) in the Evgrafovskii and Geologicheskii areas; B — diagram of the possible position of latitudinal ore-hosting structures relative to granitoid stocks and the dike belt in the Bol’she-Fedorovskii and Malo-Fedorovskii areas.

…systems in different areas of a single deposit may indicate that the plan of deformation was the same everywhere and, if it changed over time, did so synchronously in all fragments of the deposit. As can be seen from Fig. 4, in the Bol’she-Fedorovskii and Malo-Fedorovskii areas systems of tension fractures are absent, which may be explained by their being screened by dike bodies, which in plan extend north of the granitoid stocks of these areas and overlap the zone where this fracture system might be detected at the surface. The conclusion following from the similarity and unity of the deformation plan during the formation of fracture structures, and from the presence of tension fractures to which quartz veins are confined in other areas, makes it quite realistic to suppose the existence of similar tension structures also in the ar—

at the Malo-Fedorovsky and Bol'she-Fedorovsky sites. Here there is a possibility of discovering fractures of this system, to which, by analogy with nearby areas, blind ore bodies screened by a belt of dike rocks should be confined. It is characteristic that the forecast areas of the Malo- and Bol'she-Fedorovsky sites are, along strike, a direct continuation of areas with already known veins of the eastern flank of the deposit.

Consideration of the relationship between structural elements and ore-formation processes makes it possible, first of all, to express a number of considerations concerning the phenomena of zoning [1]. Within the deposit the following morphological types of zoning, according to D. O. Ontoev [2], are observed: a) longitudinal horizontal zoning along the main ore-controlling fault, and b) vertical zoning in individual ore veins. Longitudinal horizontal zoning at the deposit is manifested in the replacement of quartz–arsenopyrite ores (intermittently productive association) in the eastern flank by quartz–polysulfide ores (continuously productive association) toward the west. Vertical zoning is emphasized by the different hypsometric spatial distribution of mineral associations, which is expressed at the deposit in the replacement of quartz–arsenopyrite ores by quartz–polysulfide ores at the middle and lower levels of the gold-bearing veins.

The successive development of ore-hosting fracture structures sometimes leads to spatial separation of the mineral associations under consideration, up to the formation of independent ore bodies. At the same time, the possibility is not excluded that mineral associations of different ages may coincide at one hypsometric level. An example of such coincidence may be considered the Sekushchaya vein with the Kolonkovaya and Kopyshnye veins.

In view of the insufficient exposure at depth of the ore bodies by underground mine workings, at the present stage of study of the deposit we cannot speak about regularities in the distribution of pro—

Fig. 5. Character of the distribution of mineralization in the plane of the Annenskaya vein (A) and the Geologicheskaya—13 vein (B)

65/24 — the numerator is the average thickness in cm; the denominator is the average gold content in arbitrary units within the contour of the ore shoot (1)

of industrial mineralization (converted from grade to thickness) in the plane of the vein. At present it is advisable to use the regularities in the behavior of the general mineralization (the continuously productive mineral association). If we consider, in the plane of vein 13 (Fig. 5), a segment of the geological character of the behavior of the mineralization, we shall see that it gives a contour with a definite inclination (of the order of 45° to the southeast). Within the latter, constancy is observed in the average gold grade and the thickness of the vein. Thus, on the upper level we have an average vein thickness of 21.6 cm with an average grade of 24.6 conventional units. On the middle level, respectively, we have an average thickness of 23.6 cm with a grade of 24.4 conventional units. This circumstance is of great importance in determining the economics of exploration and exploitation work. A similar character of the behavior of mineralization is also observed in other veins of nearly latitudinal strike in this sector. As for veins of another strike (for example, nearly meridional ones), here there appears to be a northwestern inclination, likewise at an angle of about 45° (Fig. 5). It is possible that different veins, depending on the position of ore-feeding channels (pre-ore and early intra-ore fractures), will be characterized by a definite type of inclination of the mineralization. Numerous early fractures, which played a screening role for productive mineral associations, also had a considerable role in the concentration of industrial mineralization in the plane of the veins.

The concentration of industrial mineralization also took place in so-called “structural traps,” which may include: a) places where two pre-ore systems of fractures intersect; b) structures of the “diedre” type; c) places where veins or ore-bearing solutions were screened by an intra-ore or pre-ore fracture, and in sectors of flexural bending of the quartz vein.

Taking into account the factors set forth above makes it possible to approach in a different way the evaluation of the deposit in general and, in the first place, of its deep levels, for it must be agreed—as previous investigators have repeatedly pointed out—that the future of the deposit lies in the exploration and exploitation of the deep levels.

The above-noted facts concerning a possible inclination, within the veins, of mineralization with consistent grade and vein thickness; the accounting for “structural traps” in the formation of shoots and nests; and the presence of horizontal and vertical zoning, indicating the prospects of the eastern flanks of the deposit, make it possible to approach quite differently the question of the distribution and behavior of mineralization as a whole and on the deep levels in particular. Exploration work of recent years eloquently confirms this.

In light of these questions, one should dwell on one more important circumstance. The materials available to us make it possible to regard the Lyubavinskoye deposit as a zone between a major fault (with some structures complicating it), situated north of the deposit, and the principal belt of dike rocks, situated to the south, with the general dip of the Lyubavinskaya ore zone to the south—in the same direction as the structural elements enclosing it (Fig. 2). The fault, which we have called the “Northern” fault and which is situated north of the deposit, has, according to V. M. Kvyatkovskii (1961), a gentle (30–35°) dip to the south; the structures complicating it also dip southward at a steeper angle (65–80°). It seems to us that this fault is genetically similar to the well-known Sergievskii fault in the district

Khazergi and is its continuation within the Lyubavinskoye deposit. The main ore-localizing faults, situated within the Lyubavinskaya ore zone and represented by steeply dipping (60–65°) northward quartz–gold-ore veins, dip toward these structures. In turn, these quartz veins are screened by the main dike belt, which dips steeply (65°) to the south and bounds the deposit on the south. Judging from the structural pattern described above, the Lyubavinskaya ore zone is characterized by the development of ore bodies confined to steeply dipping feathering fractures located on the hanging-wall side of the major Northern fault.

Analysis of these materials makes it possible to draw certain conclusions concerning the depth of possible extension of mineralization and possible (especially blind) ore bodies within the Lyubavinskoye deposit, taking into account the erosional truncation and the different depths of subsidence of the ore-controlling structure (the Northern fault) relative to the present-day surface.

As a first approximation, the depth of possible extension of the ore bodies is determined by the following figures. In the area of the sites in the central part of the deposit this depth will be approximately 700–800 m. Toward the western flank of the deposit, the depth of extension of the ore bodies gradually decreases, reaching 200–300 m, whereas toward the eastern flank one may expect an increase in the prospects for searching for ore bodies at deep levels.

REFERENCES

1. Vasil'ev B. D., Shubin G. V. General features of the zonality of gold mineralization, as exemplified by some districts of the Kuznetsk Alatau and Central Transbaikalia. Problems of the Geology of Gold in Siberia, Tomsk State University Press, Tomsk, 1968.

2. Ontoev D. O. On the relationship between various types of hypogene zonality in some rare-metal deposits of Eastern Transbaikalia. In: Conference. Problems of Postmagmatic Ore Formation, vol. I, Publishing House of the Czechoslovak Academy of Sciences, Prague, 1963.

3. Shubin G. V., Matsyushevskii A. V. Genetic features of the Lyubavinskoye gold-ore deposit. Problems of the Geology of Gold in Siberia, Tomsk State University Press, Tomsk, 1968.