INFLUENCE OF HOST ROCKS ON THE LOCALIZATION OF MINERALIZATION IN GOLD-ORE DEPOSITS OF THE EASTERN SAYAN

A. M. Khazagarov (KGU)

Lithological conditions of ore formation, along with structural ones, are undoubtedly the principal factors that determine the site of concentration, the character, and the scale of mineralization. It is therefore no accident that this question has received considerable attention in the literature [1, 2, 3, etc.], especially in recent times. In most published works, questions concerning the influence of host rocks on ore formation are considered in terms of the favorability of one or another environment for ore deposition.

Observational materials from several gold-ore deposits of the Olkhovsko-Chibizhek district show that the host rocks determine not only particular conditions for the concentration of ores, but that the character of ore mineralization, ore types, and the morphological features of ore bodies depend to a considerable degree on their composition and properties. Frequently, the distribution of different grades of ore and the zonality in the distribution of mineral associations are also governed by the characteristics of the host environment.

Features of the Geological Structure of the Olkhovsko-Chibizhek District

This district is characterized mainly by the development of metasomatic sulfide mineralization, which is manifested with particular intensity at the Olkhovskoe, Konstantinovskoe, and Medvezh’e deposits. The geological structure of the district is rather complex; it includes a broad assemblage of rocks differing in lithological and chemical composition. The most widely developed are the carbonate rocks of the Chibizhek Formation (limestones, dolomites), effusive and sandy-shaly rocks of the Osinovskaya Formation, and intrusive formations that cut the effusive-sedimentary sequence of the Lower and Middle Cambrian. These formations are represented by ancient dikes of diorite and diabase porphyrites and by rocks of the Olkhovka intrusive complex—diorites, quartz diorites, granodiorites, plagiogranites, and granites. The granitoid intrusion is accompanied by vein derivatives—aplites, pegmatites, and quartz veins. Lithological and structural factors are of decisive importance in the localization and distribution of mineralization in the ore fields.

The principal structural element of the district is an anticline of sublatitudinal strike, in the core of which carbonate rocks occur—

of the Chibizhek Formation, while the limbs are composed of rocks of the Osinovka Formation. The folded structure is strongly complicated by numerous fault dislocations. The largest of these is a fault whose strike approximately coincides with the axis of the anticline and which dips southward at angles from 60° to 75°. In its hanging wall there is a system of attendant, steeply dipping faults of NE and NW strike. The configuration of the northern contact of the granitoid intrusion with the rocks of the Chibizhek and Osinovka formations is, to a considerable extent, controlled by the above-mentioned faults. At the same time, the most disturbed areas are characterized by complex, sinuous outlines of the intrusive contact and by a high concentration of ore formations. Gold-ore deposits and ore occurrences are confined to such deformed areas, which in plan are expressed as protrusions of the intrusive body, embayments of the enclosing rocks into the intrusive massif, or complex blocks.

Types of ores and the character of ore mineralization under different lithological conditions

Depending on the lithological conditions of formation and the mineral composition of the ores, the following types are distinguished in the deposits of the district: 1. Ores in carbonate rocks: a) carbonate-hematite; b) carbonate-quartz-sulfide; c) carbonate-sulfide ores of the early stage of mineralization; d) essentially sulfide ores formed in the course of multistage mineralization.

1. Ores in hornfelsed effusive and sandstone-shale rocks: a) quartz ores poor in sulfides and low-sulfide; b) quartz ores moderately sulfide-bearing;

2. Ores in intrusive rocks—granites, granodiorites, and diorites: a) quartz low-sulfide; b) quartz moderately sulfide-bearing; c) veinlet-disseminated ores in zones of crushing and mylonitization; d) veinlet ores in stockwork zones.

3. Ores in carbonate rocks: a) carbonate-hematite ores are distributed mainly along tectonic zones in rocks of the Chibizhek Formation (the Konstantinovskoye deposit, the Chazan, Georgievka, Neozhidannyi, and other areas). In these zones, crushed and folded rocks are cemented by dolomite, ankerite, early calcite, and bear veinlets and disseminations of hematite. In individual intervals of these zones, especially near the intrusion, superimposed sulfide mineralization is developed (pyrite, pyrrhotite, chalcopyrite) and calcite II, with which gold is associated. The thickness of such zones ranges from 0.5 to 2 m. At the Konstantinovskoye deposit these ores are distinguished by a high gold content (Fig. 1);

b) carbonate-quartz sulfide ores occur more rarely; they compose the “Dislerovskaya” vein, which, according to A. Ya. Bulynnikov [2], lies in skarned limestones near the contact with the intrusion and contains, along with pyrite, galena, sphalerite, and gold;

c) carbonate-sulfide ores of the early stage are most often encountered on the lower levels of the Olkhovskoye and Medvezhye deposits. In recent years ores of this type have been established at the Kulemnyi area. They are composed of early calcite, dolomite, coarse-grained pyrrhotite, and pyrite of the first stage, and are characterized by weak gold content. Morphologically, these ores form nests, lenses, and large irregular bodies of massive and disseminated ores;

d) essentially sulfide ores are the most widespread and are characterized by high gold content. Their mineral composition is very complex; they consist mainly of sulfides, carbonates, and quartz of the early and middle stages. The latter include the second and third

stages of mineral formation, considered “productive” for gold, during which the following were deposited: among ore minerals—arsenopyrite, pyrite II, III, pyrrhotite II, marcasite, chalcopyrite, sphalerite, galena, and tetradymite; among vein minerals—quartz II, III, and calcite II. Studies by A. Ya. Bulynnikova [2] and D. A. Timofeevsky [11] have established that the bulk of the gold was deposited together with tellurides at the end of the productive stage. This type constitutes the principal part of the ores of the Olkhovskoe, Konstantinovskoe, and Medvezh’e deposits.

Fig. 1. Schematic distribution of different ore types at the Konstantinovskoe deposit (plan).
1 — limestones; 2 — dolomitized limestones; 3 — quartz diorites; 4 — zones with carbonate–hematite ores; 5 — areas of calcite–hematite–sulfide ores; 6 — areas of substantially sulfide ores; 7 — areas of quartz moderately sulfide ores; 8 — quartz low-sulfide veins; 9 — tectonic disturbance.

Spatially, they are more extensively developed in the nearest exocontact zone, within 50–200 m, or are confined to the contact surfaces of limestones with silicate rocks—granites, diorites, and hornfelses. The ore bodies are distinguished by complex and varied forms; in the exocontact, nests, lenses, and irregular veins predominate, which is determined by the confinement of the ore bodies to fractures, zones, and to their intersections and junctions; contact ore bodies have the form of irregular deposits. The dimensions of the ore bodies are: in the exocontact, 30–50 m along strike with a thickness of 0.5–1.5 m; in the contact, up to 100–150 m along strike, with a thickness of 5–10 m.

Bodies of massive sulfide ores are, as a rule, accompanied by a halo of disseminated ores, as a result of which the outlines of the ore bodies become diffuse.

2. Ores in hornfelsed effusive and sandy-shaly rocks are represented by quartz and quartz-sulfide veins and pro-

veinlets. The latter are most highly developed in the flank sectors of the Ol’khovskoe deposit, “Zolotoi Konek” and “Yugo-Zapadnyi.” Here they form a suite of subparallel veins confined to shear and tension fractures. The dimensions of the veins along strike and dip do not exceed 100–250 m, and their thickness is 0.5–1.5 m. The sulfide mineralization is represented chiefly by pyrrhotite II, chalcopyrite, and, more rarely, pyrite. In terms of sulfide content, according to N. V. Petrovskaya, all these veins may be subdivided into poor-sulfide (up to 0.5%), low-sulfide (0.5–5%), and moderately sulfide. The degree of “sulfidation” in most veins changes in the vertical direction: in the upper horizons the ores are predominantly moderately sulfide, in the middle horizons—low-sulfide, and at depth (250–350 m)—poor-sulfide. This type is characterized by distinct and sharp contacts between the ores and the host rocks.

3. Ores in intrusive rocks—granites, granodiorites, diorites, and, more rarely, in ancient dikes of diorite-porphyries—are represented by quartz and quartz-sulfide veins, zones of sulfide mineralization, and quartz veinlet impregnation. They are developed mainly within the limits of the ore fields and are confined to an endocontact zone up to 200–300 m wide; in the depth of the massif they occur only sporadically (Veselaya vein, vein at elevation 830).

Along strike and dip the veins can be traced for 200–300 m, rarely up to 500 m; vein thickness ranges from 0.15 to 1.5 m.

Characteristic features of the veins in granites are: a) the development of wall-rock alterations in the form of beresitization zones extending up to 0.2–0.5 m away from the vein; b) the sulfide mineralization is represented chiefly by pyrite and partly by chalcopyrite. In contrast to the veins in hornfelses, pyrrhotite is little developed in the granites.

Recently, zones of crushing in granites and diorites, accompanied by intense sulfide mineralization and quartz veinlet impregnation, have acquired important industrial significance. Such ore zones are observed at the Medvezh’e and Konstantinovskoe deposits. At the former, in a crushing zone up to 2–4 m thick, the granites have been transformed into mylonites, strongly altered by hydrothermal processes. Chloritization, sericitization, silicification, and sulfide mineralization are developed in them. The latter is expressed as dense impregnation or continuous accumulations of fine-grained pyrite. Gold in the ore zone is associated with them. In contrast to simple veins formed by vein filling of cavities, in the mineralization of this zone an important role was played, on the one hand, by hydrothermal alteration of the rocks at the early stages of the ore stage, which caused a sharp increase in their porosity, and, on the other, by processes of metasomatic ore deposition in already altered rocks.

At the Konstantinovskoe deposit there is intense development of quartz veinlet impregnation confined to a fracture zone in quartz diorites. A similar picture was observed at the “Zolotoi Konek” sector, where a diorite dike was penetrated by a large number of frequently mutually intersecting quartz veinlets, together forming a stockwork type of mineralization. In contrast to the ore zone of Medvezh’e, here the processes of sericitization and chloritization were much more weakly developed, and the mineralization was formed mainly by the filling of fine fractures in diorites with quartz.

From the foregoing it is evident that: 1) in carbonate rocks there are developed predominantly carbonate-hematite, carbonate-sulfide, and essentially sulfide ores, with calcite, dolomite, hematite, and sulfides—pyrrhotite, pyrite, chalcopyrite—playing the leading role in mineralization. The importance of quartz in the mineralization is very limited. Ore bodies are formed in the course of multistage metasomatic transformation of the enclosing carbonate rocks and early hydrothermal

carbonates and sulfides; 2) for terrigenous–volcanogenic rocks, essentially quartz ores are characteristic, with limited development of sulfides (up to 30%). They occur in the form of veins and vein zones. Mineralization takes place by the filling of fracture cavities with vein material; 3) in granites and diorites, under ordinary conditions, mainly quartz-vein mineralization develops, which, under appropriate structural conditions, may pass into a stockwork. Along tectonic zones in granites there are sometimes formed veinlet-disseminated quartz-sulfide ores, in the formation of which metasomatism plays the leading role. Such mineralization is preceded by intense hydrothermal alteration of the rocks. As studies by I. N. Kigai and S. V. Nikolaev [5] have shown, pre-ore hydrothermal-metasomatic processes increase the porosity of rocks; in connection with this, the conditions for migration of solutions and metasomatic ore deposition are improved.

Change in the Character of Mineralization in Connection with a Change in the Composition of the Host Environment

For the deposits of the Olkhovsko-Chibizhek district, a change in the mineral composition of the ores, and sometimes in the morphology of the ore body, within a single ore zone or fracture is characteristic in connection with a change in the composition of the host environment.

As early as the 1930s A. Ya. Bulynnikov noted that gently dipping quartz veins occurring at the surface and in granites pass into a sulfide deposit where, down dip, it enters limestones. Subsequently this regularity was confirmed by observations of mine geologists at different sites and different depths of the Olkhovskoe and neighboring deposits. At the Olkhovskoe deposit it is quite often observed that systems of fractures branching off from the hanging wall of the “main” sublatitudinal zone, along the rise (at angles from 15 to 60°), intersect various lithological environments—limestones, porphyrite dikes, diorites, a massif of granites and diorites, as well as blocks of hornfels of the Osinovskaya suite.

In this connection, ore solutions ascending along fractures and zones of crumpled rocks pass through their course under conditions of frequent change of lithological media, which react differently in interaction with the solutions.

Thus, in a limestone area within an exocontact zone of 50–200 m, carbonate-sulfide or sulfide ores are formed in the form of veins confined to fractures, or nests and lenses at intersections and junctions of differently oriented fractures. When the ore fracture approaches or intersects the contact of limestones with silicate rocks, at the surface of the contact bodies of essentially sulfide ores are formed (Fig. 2). In these ores quartz appears, often reaching as much as 15–20%; the content of copper, tellurides, and gold increases. With the emergence of the fracture into aluminosilicate rocks, the composition of the ores and the morphology of the ore bodies change sharply. The contact sulfide deposit passes into a quartz-sulfide vein and then into an essentially quartz vein.

At the Konstantinovskoe deposit, carbonate-hematite ores, as the ore zone approaches the granite contact, become enriched in sulfides and turn into essentially sulfide ores (Fig. 1).

At the Medvezhii site, along a gently dipping zone cutting different lithological environments, veinlet-disseminated quartz-sulfide ores in altered and crushed granites are replaced by purely sulfide ores in limestones, and then, in unaltered granites, pass into quartz veins (Fig. 3). The veinlets and small veins of poor sulfide quartz that crop out at the surface are, in this case, indicators of concealed sulfide mineralization at depth.

Thus, in a single common channel of circulation of ore solutions, under similar conditions of pressure, temperature, and identical initial composition of the solutions, but in host rocks differing in composition, different types of ores are formed.

Fig. 2. Change in the character of mineralization depending on the composition of the host rocks at the “Medvezh’e” deposit (section).
1 — limestones, dolomites; 2 — hornfelsed sandstones, shales, effusive rocks with limestone lenses; 3 — granites, granodiorites; 4 — pre-granite dikes of diorite porphyries; 5 — predominantly sulfide ores; 6 — veinlet-disseminated quartz-sulfide ores; 7 — quartz veins, low-sulfide; 8 — quartz veins, moderately sulfide; 9 — zones of hydrothermal alteration; 10 — tectonic disturbances.

The difference in the physicochemical properties of the rocks through which the solutions circulate determines the complexity of the process of their interaction. On the one hand, it determines the selective precipitation of ore components and, on the other, the transfer into solution of some portion of the components of the host rocks. This naturally leads to a change in the initial composition of the solution, chiefly through changes in the content of silica, carbon dioxide, iron, and sulfur. This is especially noticeable in ore veins that cut first limestones at a steep angle and then granites. In the deeper levels, early pyrrhotite and carbonates predominate in them; at intermediate levels and closer to the contact, the role of late sulfides and quartz increases. With the transition into granites, the vein becomes quartz-sulfide, and in the upper levels it is predominantly quartz. Consequently, in granites the solutions become more acidic, and with approach to the surface the acidity increases still further, while the contents of sulfur and iron decrease.

The Role of Screening in the Localization of Sulfide Mineralization

At the deposits of the district, as indicated above, sulfide ore bodies occurring at the contact of carbonate rocks with aluminosilicate rocks—granites, diorites, hornfels, and ancient dikes—are fairly widespread. These bodies are distinguished as “contact” bodies, in contrast to ore bodies situated away from the contact. They are characterized by relatively larger dimensions, development

Fig. 3. Diagram of the formation of contact ore bodies at the intersection of a ore fissure with contacts of carbonate rocks: A—with granites and dikes; B—with hornfels, at the Ol’khovskoe deposit (section).
1—limestones and dolomites; 2—hornfels, hornfelsed effusive–schistose rocks; 3—granites, granodiorites; 4—pre-granite dikes of diorite-porphyrites; 5—relicts of carbonate–pyrrhotite ores of the first stage of mineralization; 6—massive sulfide ores (pyrrhotite, pyrite, chalcopyrite, marcasite, etc.); 7—quartz and quartz–sulfide veins; 8—disseminated sulfide ores; 9—tectonic fractures; 10—mine workings; 11—inferred direction of movement of ore-bearing solutions.

multistage mineralization and elevated gold content. At the Olkhovskoe deposit they are for the most part confined to areas of steep contact of the granites at the intersection of, and approach to, this contact by ore fractures from the side of the “main” fault. They are not infrequently formed at the contact of limestones with hornfelses or dikes, but, as a rule, where these are intersected by ore fractures (Fig. 3).

At the Medvezh’e deposit, fairly large accumulations of sulfide ores are formed in limestones beneath the gently dipping surface of hornfelses of the Osinovskaya suite, or in contact with ancient dikes.

Study of the conditions and causes of localization of such ores shows that they formed as a result of the damming of ore streams at the surface of poorly permeable or impermeable aluminosilicate rocks. F. I. Wolfson [3] considers that such “rocks are damming horizons, which do not let the solvent pass through, or let through only the solvent, while retaining dissolved substances.” In this connection, the flow of ore solutions spreads out along the surface of the screen through the more permeable carbonate rocks. Owing to the high activity of these rocks, and also to prolonged interaction with ore solutions, favorable conditions are created for metasomatic ore deposition. In this process the largest bodies are formed in areas subjected to deformation at the pre-ore or intra-ore stage, which is probably related to increased permeability of the deformed rocks.

D. S. Korzhinskii [4] points out that the rate of percolation of solutions is strongly influenced by the filtration effect—the unequal permeability of rocks to different components of the solutions percolating through them. In this case the solvent percolates more rapidly than the dissolved substance. Evidently, this explains the silicification of near-ore zones in granites and diorites.

Thus, in the process of formation of the gold-ore deposits of this district, screening is of very great importance. It is associated with the formation of contact ore bodies in which a significant part of the commercial ores of the Olkhovskoe and Chibizhekskoe deposits was concentrated.

Conclusions

1. The gold-ore deposits of the Olkhovsko–Chibizhekskii district are characterized by a great diversity of types of gold-bearing ores, which is to a considerable extent due to the influence of the composition of the enclosing rocks. At the same time, the position, form of localization, and mineral composition of the ores depend in many respects on the physicochemical and mechanical properties of the rocks, in combination with their structural factors.

2. In carbonate rocks, predominantly sulfide and carbonate-hematite ores are developed, formed in the process of metasomatic replacement of the rocks. Quartz-vein mineralization and the quartz content in the ores are minor. Silicate rocks are characterized by essentially quartz ores, formed in the process of vein filling of fracture cavities. The sulfide content in the ores is limited.

3. In the localization of metasomatic sulfide ores, at the contact of carbonate and silicate rocks, a very important role is played by the screening of ore solutions by the surface of impermeable or slightly permeable silicate rocks.

4. The examples considered illustrate the manifestation of several ore formations “of stable mineral associations similar in composition, formed under similar geological conditions” [6] within the limits of a single ore field. In connection with the fact that the formation of mineral

associations is often determined by the composition of the host rocks, the concept of an “ore formation” in specific cases must be refined by indicating the lithological conditions under which a given association is formed.

REFERENCES

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