SOME GEOLOGICAL FEATURES, STRUCTURAL CONDITIONS OF LOCALIZATION, AND GENESIS OF GOLD ORE DEPOSITS IN ONE AREA OF THE EASTERN SAYAN

G. A. FEOFILAKTOV (BSU)

The gold-ore district is located in the southeastern part of the Eastern Sayan. Although small placer gold deposits in the district have been known for a long time, the first primary deposits were discovered and explored only in the last decade. This article has been written on the basis of a synthesis of the large body of accumulated material from prospecting and exploration work carried out at these deposits by geologists V. A. Globa, A. I. Verkhozin, G. A. Feofilaktov, V. V. Levitsky, V. A. Lyubov, V. A. Ananin, and others.

Geology and Structure of the District

According to the tectonic scheme of V. P. Arsent’ev [2], the district is situated at the junction of Caledonian and Baikalian folded structures adjoining the Siberian Platform from the south. The regional structural elements of the district are the Gargan–Butugol anticlinal zone and the Oka and Il’chir synclinoria adjoining it from the northwest and southeast (Fig. 1). The anticlinal zone is elongated in a northeastern direction. Its southwestern part is represented by the Gargan Archean block, and its northeastern part by Paleozoic and Upper Proterozoic granitic intrusions. The Oka and Il’chir synclinoria are composed of carbonate and schistose sequences of Lower and Upper Proterozoic age. Within the junctions of the Proterozoic synclinoria with the anticlinal zone, Cambrian deposits are widely developed—conglomerates, sandstones, dolomites, and schists—formed in narrow graben-synclinal troughs. In turn, the anticlinal uplift is separated from the adjoining synclinoria and troughs by deep faults, along which bodies of ultrabasic rocks of Lower Cambrian age are emplaced.

Most of the gold-ore deposits of the district are confined to the exocontact belt of the Paleozoic granitic intrusion with the Gargan block. The rocks of the block within this belt are represented mainly by biotite gneiss-granites.

In the marginal parts of the block, Archean gneiss-granites are overlain, with angular unconformity, by limestones of the Mongosha Formation of the Upper Proterozoic. Limestones are also observed within the block in trough-like depressions or in graben structures.

Fig. 1. Geological-structural map of the ore district.
1 — Upper Cambrian deposits: conglomerates, shales, sandstones, dolomites;
2 — Lower Cambrian deposits: limestones, dolomites, sandstones;
3 — suite of Upper Proterozoic limestones;
4–5 — shale and carbonate suites of Lower and Middle Proterozoic age;
6 — Archean blocks, unconformably overlain by intrusions;
7 — plagiogranites;
8 — dunites and peridotites;
9 — gabbro and gabbro-norites;
10 — Upper Proterozoic granites;
11 — lines of major [[unclear: crypto-something]] thrusts;
12 — geological contacts (a) and petrographic unconformities (b);
13 — deposits and their numbers;
14 — gold occurrences with polymetallic mineralization (a) and white ores (b);
15 — outlines of the Central (I), Western (II), and Eastern (III) ore fields.

Fig. 2. Geological-structural map of the Central ore field.
(a) — geological positions; (b) — crystalline schists; 1 — schists; 2 — crystalline limestones; 3 — dikes of leukocratic microdiorites, lamprophyres, and granite-aplites; 4 — dikes of microdiorites, lamprophyres, and diorite porphyrites; 5 — biotite medium- and coarse-grained granites; 6 — biotite-hornblende diorites and granodiorites; 7 — hornblende metadiabase diorites; 8 — hornblende metadiabase diorites; 9 — serpentinitized dunites and dunites of the Nizhneilozoisk age; 10 — Archean gneiss-granites; 11 — quartzites; 12 — granitoid mylonites of various fault zones: (a) reliable, (b) inferred; 13 — fault lines: (a) reliable, (b) inferred; 14 — tectonic blocks of known structures; 15 — sections of development of carbonate rocks; 16 — breccia zones; 17 — undeformed and stratigraphic contacts: (a) and (b); 18 — elements of bedding of tectonic schistosity: (a) and gneissosity (b); 19 — ore-mineral metasomatic rocks; 20 — gold occurrences with quartz–polymetallic (a) and pyrite (b) formations.

The Paleozoic granite intrusion forms large massifs that weld the Gargan block together with the synclinoria adjoining it. All these granite massifs, in their marginal parts, especially in contact with limestones and serpentinites, have a rim composed of hybrid rocks: diorites, gabbro, and hornblendites. This rim is intruded by biotite medium-grained granodiorites and granites, which make up the main part of the massifs. The granite intrusion belongs to the fissure type and has a post-orogenic character [1]. In the marginal parts of the granite massifs, large bodies of leucocratic biotite granites, representing the second phase of intrusion, are often traceable.

Fig. 3. Sections for the geologic-structural map of the Central ore field (symbols, see Fig. 2)

The granite intrusion is accompanied by a wide range of dike formations, divided by age into three groups. The first includes dikes of altered microdiorites, gabbro-diorites, diorite porphyries, and lamprophyres, developed mainly among the rocks of the block, but absent within the granite intrusion. The dikes are confined to steeply dipping fractures with northwestern (310–340°), sublatitudinal (270–290°), and meridional (5–10°) strikes. The length of the dikes varies from 0.2 to 2 km, with thicknesses from 1 to 50 m. The second age group includes dike- and stock-like large bodies of leucocratic granite-aplites, granite-porphyries, and felsites, structurally confined to northwestern-trending faults. Veinlet-disseminated quartz-molybdenum mineralization is associated with the granite-aplites. The third age group of dikes is represented by lamprophyres (spessartites and kersantites), which cut the granite-aplites. Up to the present time many researchers (L. S. Vol-

Kov et al.) united the dikes into an independent intrusive complex. However, the grouping of the described intrusive formations into a single complex is motivated by the following facts.

1. All these formations are spatially drawn toward one another. Earlier dikes of dioritic rocks are arranged around the granite massifs and occupy a supra-intrusive position. The post-granite dikes are observed in the marginal parts of the intrusion, only rarely extending beyond its limits.

2. All rocks of the intrusive complex are petrochemically and geochemically related to one another. They are characterized by a predominance of plagioclase over potassium feldspars, and the ratio of K₂O to Na₂O ranges from 1 : 2 to 1 : 8 [1]. According to mineralogical and spectral analyses, both in the rocks of the granite intrusion and in the dike formations one and the same association of accessory minerals and trace elements is observed. From the diagram presented for the distribution of trace elements by rock type, it is evident that, in the behavior of these admixtures, characteristic associations are inherited from the early intrusive phases to the late ones (Fig. 4).

3. The noted indications of a genetic connection among all formations of the complex are supported by the closeness of the absolute age of their formation, determined by the potassium-argon method: for the granite massifs and diorite dikes, 482.4 million years, and for the granite-aplite dikes, 412 million years (F. P. Mitrofonov, 1964; G. A. Feofilaktov, 1967).

Turning to the description of the structure of the district, it should be noted that plicative tectonics within the anticlinorial uplift is difficult to decipher because of the fragmentation of the blocks of sedimentary rocks. The determining factor in the localization of gold mineralization is faulting. The largest of these are deep-seated faults traced along the junction of the anticlinal zone with the northern and southern Proterozoic synclinoria. Between the deep faults there is a complex cascade of attendant disturbances, divided by age and orientation into two groups:

1. Pre-intrusive (with respect to the Paleozoic granites) Upper Proterozoic tectonic disturbances. They are most strongly developed along the contours of the Archean block. Among them the most ancient are thrusts of Proterozoic limestones over Archean gneiss-granites, developed in the marginal parts of the block. The interformational thrusts are distinctly cut by major normal faults of northeastern and meridional strike, controlled by zones of mylonitization or by zones of coarse-clastic breccias. In the marginal parts of the block, the zones of northeastern-striking normal faults are conjugate with deep-seated faults, are traced parallel to the latter, and form graben structures.

2. Post-intrusive rupture zones of northwestern strike, directly attendant upon the deep faults. The largest of such structures, called the Main Fault, is traced diagonally between the Northern and Southern deep faults at the junction of the Paleozoic granite intrusion with the Archean block. In the central part it is healed by a massif of ultramafic rocks (the Middle Fault), while on the flanks it is represented by thick zones of crumpling with a graben-like structure in cross section.

Parallel to the main structure, smaller analogous tectonic zones are traced. By genetic features, most of them belong to upthrust-strike-slip faults and strike-slip faults (according to the orientation of striations on slickenside surfaces). The vertical amplitudes of displacement of blocks along these zones reach 400 m, as is observed from their displacement of the plane of the gently dipping contact of limestones with gneiss-granites.

a) Hornblendites and melanocratic diorites (Phase I)
(34 samples)

b) Dikes of intermediate composition
(28 samples)

c) Granodiorites and granites (Phase II)
(35 samples)

d) Leucocratic granite-aplites
(30 samples)

e) Quartz–polymetallic gold-ore veins
(81 samples)

Left vertical axis: Average content in points
Right vertical axis: Frequency of occurrence in percent

Elements: Ni Co Cr V Cu Pb Zn Mo Sn Ag Bi Ba Sr Zr As Sb Cd Sc Se Y

Fig. 4. Diagrams of the distribution of trace elements in rocks of the Kholba intrusive body; corresponds to: 1—0.001; 2—0.001–0.003; 3—0.003–0.001; 4—0.01–0.033; 5—0.033–0.1%.

Types of Gold-Ore Deposits and Conditions of Their Localization

In analyzing the conditions governing the distribution of gold mineralization in the area, it has been noted that it is regularly situated relative to the Paleozoic granite intrusion and its dike formations, being controlled by structural elements. This regularity is expressed in the spatial confinement of the mineralization to this intrusive complex and in the manifestation of endogenous zonality of the mineralization depending on its degree of remoteness from the intrusion. The spatial association of the mineralization with the intrusion is emphasized by the occurrence of gold mineralization in the exo- and endocontact parts of the granite intrusion, and also beyond its limits at distances of up to 3 km. All deposits and ore occurrences occupy a supra-intrusive position. Most of the ore bodies occur in dike formations or are formed along their contact with the enclosing rocks.

The manifestation of endogenous zonality in the change of mineralization types depending on the distance of the mineralization from the granite intrusion makes it possible to divide all deposits and ore occurrences clearly into three principal types:

1st type—deposits and ore occurrences with a quartz-polymetallic ore formation, located in the endo- and exocontact parts of the ore-generating granite intrusion;

2nd type—deposits and ore occurrences with a quartz-pyrite-tellurobismuth ore formation, located in the Archean structural stage at a distance from the intrusion;

3rd type—deposits and ore occurrences with a quartz-galena-tetrahedrite ore formation, located at a considerable distance from the intrusion in Proterozoic and Cambrian, predominantly carbonate, sequences.

The noted regularities in the distribution of gold mineralization and its mineralization types relative to granite intrusions undoubtedly indicate their genetic connection. This is also indicated by features of the geochemical kinship of the rocks of the intrusive complex with the various ore formations. According to spectral-analysis data, both are characterized by an identical assemblage of trace elements (Fig. 3).

The first type of deposits and ore occurrences is controlled by the Main fault zone. In turn, according to the spatial clustering of deposits within this zone, three ore fields are distinguished—Central (I), Western (II), and Eastern (III) (Fig. 1). The largest ore field—the Central (Figs. 2, 3)—is confined to the Main zone between the Northern and Middle deep faults and includes deposits Nos. 1 and 2. The ore-controlling structure of this ore field, in the form of a broad belt of folding, is traced for 14–15 km. In combination with more ancient disturbances of meridional and northeastern trends, it divides the ore field into tectonic blocks, imparting to it a complex mosaic structure. The eastern part of the folded belt, in cross section, has a clearly expressed graben-like structure with a maximally downthrown central block. This block is composed of intensely deformed limestones, squeezed between Archean gneiss-granites and a granite intrusion. The width of the block varies from 20 to 100 m, and its total length reaches 3 km. On both sides the limestone block is separated from the enclosing rocks by thick zones of mylonites, to which widely developed granite-aplite dikes are confined. The western part of the fault is represented by a more complex folded belt, arranged en echelon adjoining the eastern part.

in the form of a wedge-shaped block. It is predominantly developed among gneiss-granites, including the endocontact part of the granitic intrusion.

The ore bodies of deposits No. 1 (the eastern part of the ore field) and No. 2 (the western part of the field) are represented by quartz-sulfide-polymetallic veins occurring in steeply dipping zones of foliation of northwestern strike. At deposit No. 1 they are confined to the footwall side of the micrograben, lying along the contact of dikes of granite-aplites with foliated and graphitized limestones. Among them the largest are Sulfidnaya, Dorzhi-Banzarovskaya, Perevalnaya, and Dorozhnye (ore clusters 1 and 2, Fig. 2). At deposit No. 2 the veins occur in foliation zones among gneiss-granites, being localized in places where these zones bend or intersect with transverse zones. More than 50 veins are known here; of 12 explored ones, five have commercial parameters (ore clusters 3, 4, 5, 6, 8). The veins of both deposits are characterized by frequent lenticular swelling, abrupt bulges, and transitions along strike into veined and stockwork zones. The extent of the ore bodies ranges from 70 to 150 m, not exceeding 220 m; their thickness ranges from 0.1 to 4.5 m, averaging from 0.5 to 1.6 m. All gold-ore veins are composed of quartz of several generations [10], among which quartz with disseminated impregnations and nests of sulfides from 3 to 30% of the ore mass predominates. Among the latter, in all veins the greatest development is shown by pyrite of three generations, galena, sphalerite, and more rarely pyrrhotite and chalcopyrite. Visible dissemination of native gold is noted occasionally; electrum, gold telluride, and rarely brannerite and pale ores are usually associated with it. The principal veins of deposits No. 1 and 2 have been explored to a depth of 250 m; however, the observed vertical interval over which the ore bodies are exposed by relief indicates a depth of mineralization exceeding 400–500 m. The distribution of gold in the veins by concentration classes has a columnar and nest-like character. The commercial outlines of the ore bodies almost coincide with the outlines of the veins: both, in projection onto the vertical plane, have the form of ore columns, considerably elongated down dip, with a steep plunge.

The western ore field includes deposits No. 3 and 4 and ore occurrence No. 5, situated directly along the Northern deep-seated fault (Fig. 1). The quartz-sulfide veins of deposit No. 3 are confined to normal faults and normal-strike-slip faults of latitudinal strike, conjugate with the deep-seated fault and developed among limestones gently overlying gneiss-granites. The veins are arranged one after another, with insignificant interruptions, in a single zone with a total extent of up to 1 km. The thickness of the veins ranges from 0.1 to 1.5 m; sulfide and gold mineralization is distributed in them extremely unevenly. The depth of distribution of gold mineralization is controlled by the contact of the limestones with the gneiss-granites, and within the latter the commercial mineralization completely dies out. Deposit No. 4, located 4 km west of deposit No. 3, is analogous to deposit No. 2 in the conditions of localization of the ore bodies and in their characteristics.

The eastern ore field includes deposits No. 6 and 7 and ore occurrence No. 8, which are situated directly in the marginal part of the Archean block in the exocontact of the granitic intrusion. The quartz-sulfide veins of deposit No. 6 occur among gneiss-granites in zones of northwestern strike at the places where they intersect disturbances of meridional and northeastern strikes. The veins of ore occurrence No. 8 are confined to cleavage fractures developed in dikes of granite-aplites. Deposit No. 7 is located on the very eastern flank of the Main Zone and is confined to one of the zones of mylonitization,

splaying the southern branch of the deep fault. The principal ore body of the deposit is vein No. 1, occurring in the contact zone between granodiorites and schistose ap serpentinite talc-carbonate rocks. The vein is 115 m long; its thickness, unlike the veins of other deposits, varies from 0.5 to 5.5 m, averaging 2–3 m. The southern flank of the vein is overlain by basalts, while to the north it pinches out, forming a “horse-tail” type structure. The morphology of the vein is complicated by geniculate bends and abrupt swellings. The vein is composed of white quartz with sparse disseminated sulfides; in the hanging wall of the vein, at the contact with listvenites, abundant sulfide mineralization is observed. Together with sulfides there occur the thinnest veinlets of gold, native silver, electrum, and acanthite.

The second type includes deposits Nos. 9 and 10, situated within the northeastern termination of the Archean block near the Southern fault. The ore bodies of the deposits are represented by small quartz-pyrite veins confined to steeply dipping zones of northwestern strike near the intersection of the latter with thrusts of limestones onto gneiss-granites. The morphology of the quartz-pyrite veins is analogous to that of the veins of deposits of the first type. Industrial mineralization in the veins is usually observed in sectors where a late association of minerals is developed: native gold, tetradymite, calaverite, krennerite, hessite, and rickardite. According to exploration data, these deposits are characterized by an insignificant depth of distribution of industrial mineralization (not more than 100 m from the surface). The reasons for this phenomenon have still been poorly studied. For deposits Nos. 10 and 9, as well as for deposits Nos. 2, 4, and 6, occurring in Archean gneiss-granites, the shielding role of the overlying limestone sequence is established; mineralization is absent among these limestones. Apparently this is explained by the weak development of ruptural disturbances among the limestones, owing to their increased plasticity in comparison with the underlying brittle gneiss-granite basement. In addition, the main part of the ore-controlling fractures among the gneiss-granites is of strike-slip genesis. Such fractures split the gneiss-granite basement but do not penetrate into the gently lying roof of limestones. Therefore all quartz-sulfide veins confined to fractures of strike-slip genesis pinch out before the line of thrusting of limestones onto gneiss-granites. However, the penetration of ore-bearing solutions into the limestone sequence occurs near deep faults along steep ruptural disturbances of normal-fault or reverse-fault genesis, and the ore bodies here are formed in the accompanying shear or tension fractures. Localization of gold mineralization among the limestones is usually noted in the upper horizons, near their contact with the overlying schists. It is precisely in such a structural setting that the deposits of the third type are found—the Dinamitnoe deposit and the Zelenoe ore occurrence, with mineralization of telluride ores. All the gold deposits and ore occurrences of this group are considerably removed from the granitic ore-generating intrusion, which may perhaps be associated with the geochemical capacity of antimony compounds to be transported to greater distances from the parent intrusions [10]. Gold-ore mineralization in these sectors is associated with quartz veins bearing abundant disseminated telluride ores with admixtures of galena, pyrite, chalcopyrite, and sphalerite. For ore occurrences of the third type, the shielding role of schist sequences overlying the ore-hosting limestones or sandstones is emphasized. In general, analysis of the conditions of localization of the deposits of the district and of the ore bodies yields the following conclusions.

1. All deposits are situated in the endo- and exocontact of the Paleozoic granite intrusion and most often occupy a supra-intrusive position. The ore bodies occur in all structural-lithological levels of the roof that encloses the intrusion. Ultramafic rocks are practically non-ore-hosting. The role of contacts between different geological environments and stratigraphic-lithological levels as shielding elements of mineralization is emphasized by the universal absence of cases in which ore bodies cut across these boundaries.

2. The veins are usually grouped into distinctive clusters, forming a “swarm-like” arrangement. Such clusters form in ore-controlling zones, as a rule at sites where these zones are intersected by transverse ruptural disturbances or by zones of fracturing, often of concealed character, and also near the junctions of ore-controlling zones with deep-seated faults. All vein clusters are accompanied by the widespread development of dike rocks. The dikes and the ore constitute a single ore-magmatic system, are formed from a single magmatic center, and make use of the same structural elements. The process of formation of the ore-bearing complex proceeded according to the following scheme: hornblendites and diorites (as hybrid rocks) and dikes of dioritic rocks—granite intrusion—dikes of leucocratic rocks and molybdenum mineralization—dikes of lamprophyres—gold mineralization.

3. The principal ore bodies of the main deposits are directly confined to the most powerful tectonic zones of mylonitization of northwestern strike, or to associated large cleavage fractures close to them. In such zones the ore veins occur as linearly elongated, lenticular bodies localized at bends in the zones or where they intersect transverse disjunctives. In small fractures subsidiary to the large zones, small quartz-sulfide veins are localized, only rarely constituting ore bodies with a high concentration of metal.

4. In their geological-structural conditions of localization and genetic features, the gold deposits of the area described are in many respects similar to the deposits of the Olkhovsko-Chibizhek and Martai gold-ore districts, situated within the Altai-Sayan folded region.

REFERENCES

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