Morphogenetic Features of the Olkhovskoye Ore Field

V. A. Bulynnikov (SNIIGGiMS)

The Olkhovskoye ore field is one of the largest, thoroughly explored and well-studied gold-ore objects of Western Siberia; at present it has been almost completely mined out. A. Ya. Bulynnikov, D. A. Timofeevsky, A. M. Khazagarov, G. P. Dokukin, and many others took part in its study. The published and archival works of these researchers provide a detailed geological and mineralogical-petrographic characterization of the field, of individual areas, and of ore bodies.

The ore field is located in the near-contact zone of the northwestern part of the Shindinsky intrusive massif, composed predominantly of plagiogranites and adamellites, more rarely diorites. The rocks enclosing the intrusion—marbleized and dolomitized limestones and hornfelsed effusive-sedimentary deposits of Cambrian age—are gathered into complex folds and cut by numerous disjunctive disturbances. The plicative and principal fault structures of the area were formed mainly in pre-intrusive time and exerted a major influence on the morphology of the intrusion. During emplacement and after the consolidation of the granitoid massif, these structures were considerably complicated. Anticlinal and synclinal folds were deformed, with changes in the strike and dip conditions of their limbs, and were broken into separate blocks displaced relative to one another. Disjunctive disturbances were repeatedly renewed, and new faults, zones of crumpling, and foliation also formed.

Dike formations are widely developed in the area. According to A. M. Khazagarov [5], they are subdivided into “pre-granite” dikes—diorites, microdiorites—and “post-granite” dikes—quartz diorites, spessartites, diorite porphyrites (pre-ore), and granite porphyries, felsite porphyries, and diabases (post-ore).

Mineralization is spatially confined to a comparatively large intrusive protrusion developed in the sector where the sublatitudinal anticline branches into folds of northwestern and southwestern directions, and where the rocks of the continent are cut by a large number of disjunctive disturbances. The latter form a complex tectonic framework within which the main mass of ore bodies and most of the pre-ore and post-ore dikes are localized (Fig. 1). According to the data of A. Ya. Bulynnikov [1] and D. A. Timofeevsky [4], the Olkhovskoye ore field

is a characteristic example of high-temperature hydrothermal-metasomatic sulfide and quartz-sulfide deposits.

The process of ore deposition under identical structural conditions developed differently. In limestones and at their contact with intrusive rocks, lens-shaped, nest-shaped, vein, and pipe-shaped bodies of massive and disseminated sulfide ores were localized, whereas in intrusive rocks

Fig. 1. Schematic structure of the Olkhovskoye ore field and its relation to the intrusive contact: 1 — limestones, 2 — effusive-sedimentary deposits, 3 — granitic massif projection, 4 — sulfide ore bodies, 5 — quartz-sulfide veins, 6 — disjunctive disturbances: I — Main zone, II — Southern zone, III — zone of the Golden Ridge; contours of the ore field: 7 — at the erosion level, 8 — at horizon XI; 9 — position of the intrusive contact at horizon XI.

rocks and in hornfelsed effusive-sedimentary deposits, quartz and quartz-sulfide veins were formed. The indicated ore types were formed by unified primary solutions, are closely related to one another, and often form various kinds of mutual transitions. D. A. Timofeevsky [4] distinguishes the following stages of mineralization: 1) pyrrhotite (pyrrhotite, pyrite, magnetite); 2) quartz-pyrite (quartz, pyrite, marcasite, arsenopyrite, and iron carbonates); 3) gold-polymetallic (fine-grained pyrrhotite, pyrite, chalcopyrite, sphalerite, native bismuth and gold, and others); 4) carbonate (calcite, chalcedony-like quartz).

The numerous sulfide and quartz-sulfide bodies, as a rule, are grouped into separate large ore zones. The following are distinguished: a) the main ore zone, comprising sulfide bodies developed within a sublatitudinal zone of folding, dipping south and southwest at angles of 50–65°; b) the near-contact zone, which includes sulfide and quartz-sulfide bodies confined to the steeply dipping contact of the intrusive projection; c) the central zone, comprising a series of transverse ore zones and bodies of northeastern strike with a dip to

southeast at angles of 35–60°; d) the southern zone with a relatively small number of quartz and quartz–sulfide veins, confined to a system of bedding-plane and crosscutting faults of the Southern crush zone, dipping northward at angles of 25–40°. The quartz–sulfide veins of the Zolotoy Konek ridge, developed along the continuation of the intrusive salient and having predominantly sublatitudinal strike and gentle northward dip, belong mainly to the near-contact ore zone (Fig. 1).

In the morphogenetic analysis of the ore field and of the ore zones included in it, a number of new and important regularities are revealed in the structure of the field, in ore localization, and in the behavior of mineralization at depth. The sulfide ore bodies and many quartz–sulfide veins and pre-ore dikes of dioritic rocks often dip conformably to the intrusive contact or at some angle to it, with a tendency to converge. The area of contraction of mineralization and fan-shaped joining of ore bodies and dikes into a single node gradually plunges in a southeasterly direction. In all ore zones the ore bodies incline in easterly and southeasterly directions, with a considerable decrease in the areas over which mineralization is distributed and with a general tendency of ores to pinch out on the lower levels. With depth, the principal parameters of the ore field and zones (length, width, area), the number of ore bodies and dikes, and also the total volume of ores (Fig. 2, Table 1) decrease regularly. The small, areally restricted root part of the ore field is located at a depth of about 350 m from the collar of shaft No. 2. Within it there is a convergence of all ore zones, and small dikes and gold-poor ore bodies with vertical dip are developed.

Fig. 2. Figure of the Olkhovskoye ore field in various horizontal sections to a depth of 350 m. a — Olkhovsky bundle, b — Southern branch.

It is also highly characteristic that above the root part of the field there appears a large, vertically dipping, pipe-like body of massive sulfide ores.

The structure of the Olkhovskoye field and the features of ore localization indicate that the ore bodies and dikes of dioritic rocks form a complex ore–magmatic bundle, converging at depth, which may be represented as an irregular pyramid gently plunging to the southeast. In the erosion section, the frontal part of the bundle has an oval form, elongated in the latitudinal direction along the Main and partly the Near-Contact zones. Beginning with the III–IV levels and deeper, its horizontal section assumes a triangular form, determined by ore zones of sublatitudinal, northwestern, and northeastern trends. At a depth of 100–250 m this figure is complicated by an additional Southern ore branch, gently inclined to the southeast, i.e., toward the apex of the pyramid (Fig. 2).

The main Olkhovskoye bundle includes all industrially valuable sulfide and quartz-sulfide ore bodies, which are confined to the Near-Contact, Main, and Central zones and the structures accompanying them, as well as the overwhelming majority of pre-ore dikes. The Southern branch unites a small number of mostly gold-poor quartz and quartz-sulfide veins and isolated dikes. They are developed in the area where the Southern fold zone emerges from the intrusive apophysis, near the junction of this zone with the “Golden Horse” fault, and are poorly explored. The main parameters characterizing the Olkhovskoye ore–magmatic bundle and its relative ore content in different horizontal sections are given in Table 1.

Table 1

Analysis of the spatial distribution of the ore bodies and their morphology shows that the ore fluids entered the interconnected ore-localizing structures from the root area of the ore–magmatic system. Initially they moved along the Near-Contact and Main zones, penetrating in places into transverse faults of the Central zone and into other subparallel and branching fractures. Geological and thermodynamic conditions did not allow the ore solutions to spread uniformly along the principal and accompanying structures. Therefore, the process of ore deposition in some parts of one or another zone repeatedly died out and resumed its development in other areas, usually where they are joined by ore-controlling faults of a different strike. The following directions of movement of the ore flows predominated: a) root part—Near-Contact—Central—Main or Near-Contact zones; b) root part—Main—Central—Near-Contact or Main zones; c) Main or Near-Contact—Central—Near-Contact or Main zones, etc. Into the Southern zone

ore solutions entered from an independent root zone or, possibly, from the Olkhovka bundle along the Southern shear zone. Since the framework of the ore-conducting and ore-hosting structures of the field was a single interconnected one, the directions of movement of individual ore streams often coincided or were combined with one another. In general, as the ore solutions advanced toward the frontal part of the field, they spread along numerous ore-feeding and ore-hosting structures, i.e., the flow expanded upward along the rise (Fig. 3). This led to an increase in the size of the ore field in the upper levels, in comparison with the near-root part, by almost a factor of 7, and to an increase in the number of sulfide ore bodies and their total area, respectively, by 5–7 and by several tens of times (Table 1). The general northwestern direction of movement of the ore solutions coincided with the strike of the intrusive protrusion and, apparently, corresponded to the thermodynamic and structural conditions prevailing at that time.

Fig. 3. Diagram of the main ore streams along the rise of the Olkhovka bundle; 1 — Main zone, 2 — Central zone, 3 — Near-contact zone, 4 — Southern zone, 5 — places of junction of ore streams (the arrows correspond to the axes of ore bodies).

Some deviations from the general regularities of ore localization noted above are due to details in the structure of individual parts of the ore field and zones. Thus, for example, the contraction in mineralization at the VI level occurred mainly because of the wedging-out, down-dip and up-dip, of the ore structures of the Central zone (VIII and XV–XXXVI sectors) and the development, at the contact with the intrusion in the southeastern part of the field, of hornfelses. In addition, at the level of the VI horizon there is a change in the direction of dip of the intrusive contact to the opposite one, i.e., toward the intrusion. This, apparently, contributed to the emergence of local compression zones that negatively affected the process of ore deposition in the Near-contact and other zones. The presence of few, but

large ore bodies in the upper horizons of the field is explained by the development of a thick oxidation zone, where individual ore bodies were mechanically combined, changed their outlines and dimensions during the destruction of the enclosing limestones.

The Olkhovskoye ore-magmatic bundle, in essence, has no clear bounding structures. Only in the north and partly in the southwest do its boundaries in places pass along the Main zone and the contact of the intrusive protrusion. Therefore, it may be assumed that the wedging-out of mineralization along strike and dip was caused mainly by the cessation, at a certain distance, of injections of ore solutions in connection with the natural decrease of their internal pressure.

From the external contours of the distribution of mineralization, distinguished by projecting the ore bodies onto longitudinal sections, it is possible to measure the angles of divergence of the ore flow. For the field as a whole and for individual ore zones they varied from 35–55° in the root part to 80–90° and more upward along the flow. Gradual, smooth increases in the angles are characteristic of ore zones occurring in a homogeneous thickness of limestones (the Main and Central zones), whereas the Near-Contact zone, developed among rocks of different lithological composition, is characterized by abrupt changes in the flow angles. Quantitative data characterizing the structure of the Olkhovskoye ore field, taking into account its eroded part, are given in Table 2.

Table 2

Taking into account the insignificant degree of erosion of the ore field, it may be assumed that its total extent down the dip was about 2.0 km. According to the classification of P. F. Ivankin, the Olkhovskoye ore-magmatic bundle belongs to the type of combined one-sided, gently inclined bundles of medium extent.

The actual root part of the ore-magmatic system is located in the relatively deep (unobserved) near-contact parts of the Shindinsky granodiorite massif. It apparently represented a zone of disintegration of the ore-generating magma into constituent magmatic and hydrothermal siliceous-sulfide ore formations. The ores were deposited after consolidation of the upper parts of the granitoid massif, following the stage of dike intrusion. Judging from the composition of the ore bodies and accompanying ore dikes and from the nature of the near-ore alteration of the enclosing rocks, the initial magma was to a considerable extent enriched in iron, sulfur, silicon, magnesium, calcium, and other elements. Therefore, in composition it most likely corresponded to hybrid (?) diorites, sharply enriched in mineralizers and gold.

In conclusion, it should be noted that the study of endogenous ore fields as volumetric three-dimensional systems makes it possible to determine not only their general form, but also to reveal many new regularities of ore localiza-

localization and the behavior of mineralization at depth. This, in turn, may contribute to the improvement of our knowledge in the field of ore genesis, the systematics of ore fields, and the prediction of mineralization at depth.

REFERENCES

1. Buly nnikov A. Ya. Petrology and ore potential of the Olkhovskoye gold-ore deposit. Proceedings of Tomsk State University, 1940.

2. Ivankin P. F. Problems of bundles, small intrusions, and hydrothermal jets in pyrite–polymetallic fields of Rudny Altai. Reports of the USSR Academy of Sciences, vol. 138, no. 4, 1961.

3. Ivankin P. F. On forms of association of sulfide mineralization with magmatic rocks. In: Problems of the Geology of Ore Deposits of Transbaikalia. Chita, 1963.

4. Timofeevskii D. A. Gold-ore deposits of the Eastern Sayans. Proceedings of NIGRIzoloto, vol. I, 1950.

5. Khazagarov A. M. Some features of the localization of gold mineralization in the Olkhovskoye ore field (Eastern Sayans). Geology of Ore Deposits, vol. 5, no. 3, 1963.