MAIN PATTERNS OF ORE LOCALIZATION IN THE “OLKHOVSKOYE” AND “MEDVEZHYE” AREAS

(EASTERN SAYAN)

D. G. Gogiberidze

(Submitted by Professor A. M. Kuzmin)

The Olkhovskoye ore field, which includes the gold-ore deposits Olkhovskoye (with the Olkhovskoye proper and Medvezhiy areas) and Konstantinovskoye, is situated within a narrow belt of metamorphosed rocks extending along the contact of a granodiorite intrusion. The ore field is composed of limestones of the Chibizhek Formation \((Cm_1)\), effusive-sedimentary rocks of the Osinovka Formation \((Cm_{1-2})\), a Lower Paleozoic granodiorite intrusion, and numerous dikes, predominantly of basic composition and of pre-granite age. Under the influence of contact metamorphism, the Chibizhek limestones have been transformed into medium-grained spotted and banded marbles, and the rocks of the Osinovka Formation into various hornfelses. Skarnification is widely developed, occurring almost exclusively in rocks of the Osinovka Formation. Chloritization and sericitization are characteristic of the volcanic rocks; silicification and carbonatization are observed in all rocks.

Structurally, the ore field is confined to the southern limb of a sublatitudinal anticlinal fold, in the core of which limestones of the Chibizhek Formation are exposed. According to V. A. Bulynnikov and V. G. Senkevich, the principal anticlinal structure of the Olkhovsko-Chibizhek ore district can be traced from the Olkhovskoye deposit in a northwestern direction to the headwaters of the Dzheb River. In the area of the Olkhovskoye deposit the main anticline splits into two branches, northern and southern. The northern anticline has a sublatitudinal strike and extends along the northern contact of the granodiorite massif to the Konstantinovskoye deposit and farther east and southeast. The southern anticline branches off from the main one to the southwest, passes through the “Medvezhiy” area, borders the northwestern contact of the intrusion, and then, repeating the bend of the massif, gradually turns and continues along the contact of the massif in a southeastern direction. The limbs of the northern and southern anticlines adjoining the intrusion form a synclinal fold of generally west-northwest strike, closing in the area of the bend of the main anticline and cut in its core by the granodiorite intrusion, which is emplaced mainly among rocks of the Osinovka Formation and only occasionally includes the Chibizhek limestones. The boundaries of the intrusion are often located near the contact between the sedimentary units and generally repeat the bends of this contact. Protrusions and apophyses that complicate the form

of the massif, are confined to areas of development of large tectonic fractures and crushing zones and reflect their principal directions.

Diorite-porphyrite, microdiorite, lamprophyre, and similar dikes form bodies ranging in thickness from several centimeters to tens of meters and up to 500 m in length. The sharply predominant strike of the dikes is WNW, with steep dip to the NNE. Among other directions, a submeridional strike is distinguished, with gentle and steep dips to the east. Usually the dikes are grouped into belts consisting of several closely spaced bodies, often connected with one another by branches, and occur among the limestones of the Chibizhek Formation; much more rarely they are encountered in rocks of the Osinovka Formation. Among the numerous disjunctive disturbances that divide the area into a series of tectonic blocks, two large tectonic crushing zones of sublatitudinal and NNE strike are distinguished; these are traced, respectively, along the northern (Osinovka zone) and western (“Pologaya” zone) contacts of the granodiorite massif and dip toward it. These zones are also conformable, respectively, with the northern and western limbs of the synclinal fold and develop near the contacts between the Osinovka and Chibizhek formations.

The “Olkhovskoye” deposit is situated at the northwestern end of the granodiorite intrusion, near the place where the main anticlinal structure of the area virgates; the Olkhovsky sector is confined to the southern limb of the northern anticline, and the Medvezhy sector to the western limb of the southern anticline (Fig. 1). Industrial mineralization is localized near the intrusive contact (no farther than 500 m), predominantly in Chibizhek limestones, and is represented by sulfide, quartz-sulfide, and, in the near-surface part, oxidized ores. Of the total volume of ore minerals in the primary ores, about 90% is accounted for by pyrrhotite and pyrite and about 10% by marcasite and chalcopyrite; arsenopyrite, sphalerite, and magnetite are sporadically noted in appreciable amounts. Gangue minerals are represented by quartz and calcite; siderite, ankerite, chlorite, sericite, serpentine, and graphite are encountered. The oxidized ores are composed mainly of hydrates of iron oxides and quartz. Sulfide ores are of principal importance, constituting about 70% of the total weight of the commercial ore of the deposits; according to the degree of saturation with ore minerals, they are subdivided into massive (ore minerals more than 70%) and disseminated, connected by mutual transitions. Quartz-sulfide ores differ from sulfide ores in their elevated quartz content, averaging not less than 50%.

The ore bodies of the deposit are characterized by great diversity and complexity of form; changes in direction and dip, lateral branches, pinches and swells are typical of them. In silicate rocks the ore bodies have the form of simple or branching veins, as well as stockworks. In limestones, vein-like, lens-like, and columnar ore bodies are widespread. As a rule, one of the sides of the latter, more often the hanging wall adjacent to the ore-controlling fracture, proves to be even, straight, or with smooth bends, whereas the other sides have very capricious sinuous outlines with abrupt changes in direction.

It has long been established at the Olkhovskoye deposit that there are three main features in the distribution of mineralization: 1—direct connection of the ore bodies with tectonic fractures and zones, 2—localization of mineralization near the contact of the granodiorite intrusion in areas where the latter forms protrusions into the enclosing rocks, 3—confinement of sulfide ores to limestones, and of quartz-sulfide ores to silicate rocks.

Tectonic fractures and zones are very widespread. Morphologically they are expressed in different ways: from barely noticeable seams that quickly disappear when traced, to thick zones of crushing and mylonitization with well-defined slip planes and slickenside clay, extending for tens and hundreds of meters. Particularly intensive development of rupture disturbances is observed near the contact between the Osinovskaya and Chibizhek suites, where the “Pologaya,” “Osnovnaya,” and “Yuzhnaya” tectonic zones pass. The orientation of the fractures is highly varied, but fractures whose dip directions lie in the NE and SW quadrants predominate. Gently dipping and steeply dipping fractures are clearly distinguished.

The orientation of the ore bodies is in full agreement with the orientation of the tectonic fractures; moreover, not only the directions coincide, but, as a rule, the quantitative ratios of these directions are maintained. This regularity has been demonstrated in the Medvezhye area by comparing the degree of development of systems of ore bodies and systems of tectonic fractures. In particular, about 73% of the ore and 78% of the gold in this area are associated with ore bodies of NW strike, which are also characteristic of the most widespread fractures. It should be emphasized that the direct dependence of the degree of development of systems of ore bodies on the degree of development of the corresponding systems of tectonic fractures is applicable only to the area as a whole (at the scale of individual ore bodies it may not be fully manifested because of the peculiarities of the localization of the latter), and only to those systems of tectonic fractures that correspond to the directions of dikes, tongues, and projections of granites, and to bends of major tectonic zones. Tectonic fractures, even if developed to a sufficient degree but differing in their orientation from the orientation of the main structural elements of the area, prove to be practically barren.

Consequently, only those fracture directions proved favorable for mineralization which, by virtue of certain displacements of tectonic blocks, underwent repeated and most intense pressing and opening and existed throughout the entire period of development of the deposit. Characteristically, an analogous regularity is also manifested in the distribution of gold in the ores. Enriched sections of ore bodies coincide with places where stages of mineralization that preceded the deposition of gold were superimposed upon one another. The favorability of such sections for the concentration of gold evidently lies in their heightened and persistent fracturing, which developed chiefly along the boundaries of mineral grains and aggregates and was renewed from stage to stage.

In the Medvezhye area, ore bodies form at places of junction and intersection of different systems of tectonic fractures, either having different strike directions or the same strike but different dip angles. The latter situation applies only to the most widespread fractures of NW strike, since in the other fractures mineralization develops exclusively at places of junction with fractures of other directions, with obligatory participation of fractures of NW strike. An indispensable condition for the localization of ore bodies is the combination of gently dipping and steeply dipping fractures. In individual cases, ore bodies also arise at places of junction of steeply dipping fractures dipping in opposite directions, being situated in the lower acute angle formed by these fractures. Of several intersecting fracture systems, as a rule only one is ore-controlling and determines the elements of attitude of the ore body, whereas the others, even if numerous,

control offshoots and small apophyses of the main body, or do not contain mineralization at all.

The ore-hosting properties of systems of tectonic fractures are determined by the features of their structure and distribution; these properties are usually possessed by complex tectonic zones formed by the merging and uniting of numerous small fractures, sometimes belonging to adjacent systems. Often, owing to continuity in the development of deposit structures, such zones are formed along contacts of different rocks; moreover, if two different contacts intersect (for example, a gently dipping contact of limestones with granites or porphyry dikes), the ore-hosting zones may pass either along one of them or along both together. All of the above accounts for the wavy structure of the ore bodies, their sharp bends down dip, and their frequent confinement to contacts of different rocks. Most often, ore bodies in the Medvezhye area are observed at the contact of limestones with granites, porphyry dikes, and hornfelses; they also occur along contacts of hornfelses with granites and of porphyry dikes with granites.

The localization of individual ore bodies, the conditions of which we have just examined, is subordinate to the general regularities of the distribution of mineralization in the area. First of all, mineralization develops only near the “Gently Dipping” tectonic zone, and at a certain limiting distance from it it ceases without any changes in the geological setting. Since the “Gently Dipping” zone has a general NNW strike and a gentle (10–40°) dip to the ENE, it controls the vertical distribution of mineralization and determines its inclination in an easterly direction. In horizontal projection, all ore bodies of the area, regardless of their conditions of occurrence, are grouped into two intersecting belts of WNW and NNE trends, coinciding in their position and orientation with zones of tectonic fractures, and the east-northeastern belt, in addition, with the outcrop of the granodiorite massif. These zones control the horizontal distribution of mineralization; moreover, as in the case of the “Gently Dipping” tectonic zone, the termination of mineralization is not connected with the presence or absence of visible geological factors that could have hindered its development. In general, in the Medvezhye area the ratio of the vertical extent of mineralization to its horizontal extent is 1:3.

The role of the principal ore-controlling zones in the localization of mineralization is not the same. Comparatively few ore bodies are confined directly to the “Gently Dipping” tectonic zone, and only in places where ore-controlling zones of WNW and NNE trends are coupled with it. At the same time, on the footwall side of the “Gently Dipping” zone is concentrated the overwhelming majority of the ore bodies of the area, and the gold-enriched parts of the ore bodies are situated closer to the zone, whereas on the hanging-wall side they are farther away; this quite definitely indicates the shielding influence of this zone in the process of ore formation. Along the ore-controlling zone of NNE trend, which, although clearly expressed and traceable for a considerable distance along the contact of limestones with granites, ore bodies are observed, as a rule, in places of coupling with fractures of WNW strike and have the same orientation. The ore-controlling zone of WNW trend is morphologically expressed much more weakly than the first two and is in fact represented by numerous tectonic fractures. However, it is precisely this zone, like all fractures of WNW strike, that is leading in terms of the amount of mineralization associated with them.

The Olkhovskoye deposit proper is localized within

a block of rocks bounded by the “Main” and “Southern” tectonic zones and elongated in a WNW direction. In terms of the conditions of localization of mineralization at the Olkhovskoye deposit, several sectors are clearly distinguished: the “Central” sector, the “Main” zone, the “Intrusive Contact,” the “Western,” and the “Southeastern” sectors.

The ore bodies of the Central sector are controlled by a series of tectonic fractures united into a zone which, in its position and attitude elements, corresponds to the WNW continuation of the eastern branch of the “Main” zone, characterized by ENE strike and a dip to the SSE at an angle of 40–50°. The immediate continuation of the “Main” zone is only the eastern group of fractures, whereas the western fractures, as a result of a sharp turn of the “Main” zone to the WNW, are joined to the latter at an acute angle, giving the impression of feathering fractures. When traced in a WNW direction, the fractures approach the intrusive contact but nowhere intersect it. In addition to the indicated ore-hosting zone, which determines the attitude elements of the ore bodies, gently dipping fractures in a NNW direction are widely developed in the sector; these themselves do not contain significant mineralization.

The sectors of the “Main” zone and of the intrusive contact are, in the character of their ore-controlling structures, of the same type: mineralization develops in the plane of rupture disturbances of WNW strike (the middle part of the “Main” zone and tectonic fractures passing along the contact of granites with limestones) at places where they are joined by the ore-hosting zone of the Central sector. In accordance with the strike and dip angle of the latter, the mineralization of the sectors dips to the WSW at an angle of 30–40°. In the granites, within the contact zone, gently dipping NNW fractures are widely developed; here they are often accompanied by low-thickness quartz-sulfide veinlets.

The southeastern sector of the deposit, in its elongation in a WNW direction and in the confinement of the predominant part of the mineralization to the contact of limestones with granites, resembles the intrusive-contact sector. However, unlike it, the development of contact ore bodies here is connected not with the ore-controlling zone of the Central sector, which passes considerably lower, but with a zone of gently dipping NNW fractures. In the exocontact of the granodiorite intrusion, vein-like ore bodies are confined to these fractures; along their strike they approach the intrusive contact and, at the place where they join it, give rise to contact ore bodies. The apparent reason why the gently dipping NNW fractures acquired ore-controlling properties (in the Central and Intrusive Contact sectors, as noted above, ore bodies with similar attitude elements are absent) is the unusually abundant distribution in this part of the Olkhovskoye deposit of porphyritic dikes, whose attitude elements coincide with the attitude elements of the ore-controlling fractures. The position of the western sector corresponds to the continuation, along strike, of the ore-controlling zone of the Central sector, with some displacement to the south. In contrast to the other sectors, whose ore bodies occur chiefly in limestones, the mineralization of this sector develops predominantly in hornfelses and granites and is represented by gently, rarely steeply, north-dipping veins. The distribution of ore bodies to the west is bounded by the “Main” tectonic zone, which here has a NE strike. The mineralized zone itself contains no ore; however, at places where the veins intersect the tectonic fracture parallel to the “Main” zone, a sharp increase in vein thickness is observed.

Thus, just as in the Medvezhye area, the localization of mineralization at the Olkhovskoye deposit is determined by the intersection or conjugation of tectonic zones of several trends, the leading role among which belongs to steeply dipping fractures of ENE and WNW strike and to gently dipping fractures dipping northward. The ratio of the vertical extent of mineralization to the horizontal extent of mineralization at the Olkhovskoye deposit proper is also 1 : 3.

In addition to the structural factors of ore localization considered above, at the Olkhovskoye deposit a certain role in ore formation is also played by the lithological composition of the enclosing rocks, which exerts a substantial influence on the mineralogical composition and structure of the ore bodies. However, if one compares the direct influence of fault tectonics and of the lithological composition of the rocks on the localization of mineralization, the influence of the latter proves to be insignificant. An excellent example in this respect is the western part of the Olkhovskoye deposit, where ore bodies occurring in silicate rocks pinch out upon entering limestones, whereas in other parts industrial mineralization is observed exclusively in limestones. In general, however, other conditions being equal, mineralization develops first of all in limestones and only subsequently in granodiorites, hornfelses, and dikes.

Let us now consider one of the possible causes of the spatial association of mineralization with the near-contact zone of the granodiorite intrusion. It was noted above that the contour of the granite massif, as a rule, passes near the contact of the Osinovskaya and Chibizhekskaya formations and often follows the widespread trends of tectonic fractures. It therefore seems natural that, since the spatial position of the contact of the granodiorite intrusion with the enclosing rocks and the localization of mineralization are controlled by one and the same factor—namely, by ruptural dislocations that tend in their development toward the contact between sedimentary sequences—these two features (the intrusive contact and the mineralization) should closely coincide in space. The chemical aspect of the ore-forming process also plays an important role here, but this is a subject for special studies.