GEOLOGICAL STRUCTURE AND LOCALIZATION FEATURES OF ORE BODIES IN THE IROKINDA-KINDIKAN ORE FIELD

V. A. Rubanov, L. F. Mitrofanov, A. D. Mikov (TPI)

The Irokinda-Kindikan ore field is located in the central part of the South Muya Range of the Olekma-Vitim mountainous region, within a large tectonic uplift of the Archean basement [3], known as the South Muya block.

The geological structure of the ore field includes metamorphic rocks of the Kindikan suite of Archean age. They are represented by various types of gneisses (garnet-pyroxene, garnet-amphibole, amphibole-pyroxene paragneisses, and garnet-mica gneisses), skarned and marmorized limestones, and amphibolites. The distinguished rock varieties occur as alternating beds and packets with thicknesses ranging from 100–200 to 1000 m.

Intrusive rocks are represented by Archean granites forming dike-like bodies with thicknesses from 0.1–0.2 to 15–20 m and strike lengths from tens of meters to 500–700 m. Archean ultrabasic rocks form small lens-shaped bodies, completely transformed into serpentinites. In addition, dikes of diorites, diorite porphyries, spessartites, and felsite porphyries, assigned to the Mesozoic intrusive complex, are relatively widespread in the area described. Gold-ore mineralization of the district is genetically associated with the latter (N. P. Andreev, A. A. Gamyan, 1959). It is also necessary to note such formations as shadow migmatites, shadow granites, and granito-gneisses of varying composition, formed in the process of ultrametamorphism [1] in Archean time and composing fairly large areas within the studied territory.

The metamorphic rocks are gathered into tight, usually linearly elongated folds with steeply dipping limbs (50°–80°), complicated by folds of higher orders, often having the form of brachyfolds. Small flexures, foliation, and gneissic banding are developed everywhere. The axes of the linear folds are elongated in a submeridional (10°–20°) direction. The determining factor in the localization of gold-bearing quartz veins is faulting. The largest of these are regional deep-seated faults—the Kilyan and Tuldun faults—which bound the Archean uplift from its ...

of Proterozoic framing from the west and east, respectively. Between these faults there is a dense network of subsidiary dislocations with varying orientations. Among them the following may be distinguished:

1. Northeastern-trending ruptures, as a rule confined to the axes of anticlinal folds. Gold-bearing quartz veins sometimes occur within dislocations of this group (Sluchainaya, Yubileinaya, etc.). The type of movement of this group of dislocations and the displacement amplitude along them are unknown.

2. Sublatitudinal (70–80°) dislocations, which in turn are subsidiary to the northeastern ruptures. They also host ore for some gold-quartz veins (Serebryakovskaya, Berezovaya, etc.). The dislocations of this group are presumably related to strike-slip ruptures. The displacement amplitudes have not been established.

3. Northwestern dislocations, subsidiary to the principal faults of the Kilyan tectonic zone. They are often filled with gold-bearing quartz veins (Tuluinskaya, Petrovskaya, Khrebtovaya, etc.) and belong to the normal-fault type, with displacement amplitude within 30–50 m.

Gold-bearing quartz veins have a strike length from 200 m (veins No. 12, Serebryakovskaya, Berezovaya, etc.) to 400–800 m (Yurasovskaya, No. 9, Verkhnaya veins). They have been traced down dip to 300–450 m. The average thickness of the veins varies within 0.2–1.0 m, reaching 3–3.5 m in swellings. The change in thickness within swellings occurs rather gradually. Swellings and pinches in the veins are distributed relatively evenly. Thus, in the Yurasovskaya vein the swellings recur, on average, every 20 m along strike. In other veins they occur much more rarely. Along with swellings, the ore bodies are characterized by significant pinches. Sections of complete wedging-out between swellings of a vein are traced along strike for 0.5 to 30 m and more. In them a considerable number of short veinlets and lenses of quartz are often observed, as if cementing the shattered enclosing rocks.

The shape of the veins is usually simple, plate-like (veins No. 12, 9, etc.), but it may be sharply (vein No. 13) or weakly expressed as bead-like (Yurasovskaya, Serebryakovskaya, and others). Many veins (Yurasovskaya, Serebryakovskaya, Berezovaya, and others) are accompanied by a considerable number of apophyses with thickness from 0.1–0.2 to 1 m and a length usually within the first meters and rarely the first tens of meters. In low-thickness ore bodies (veins Nos. 9, 11, 12, 13), apophyses are absent.

The wedging-out of ore bodies is usually characterized by a gradual decrease in thickness. Sometimes a vein ends in short swellings (vein No. 9). In all cases, the flanks of veins end in veinlets and lenses of quartz. Rarely are veins cut by ruptural dislocations (vein No. 12).

At the deposit, a relationship has been established between the thickness of quartz veins and the composition of the enclosing rocks. Such lithological control, as statistical processing of the materials has shown (Fig. 1), consists in the fact that the maximum thickness of veins is observed in garnet-pyroxene and amphibole-pyroxene paragneisses, and the minimum—in garnet-mica gneisses, where the veins practically wedge out.

The ore bodies of the deposits are represented by quartz veins with an insignificant quantity of sulfides (1–3%), and therefore they may be assigned to the formation of low-sulfide ores according to the classification

Fig. 1. Diagram of the dependence of gold content (a) and thickness (b) along the Verkhnyaya vein on the lithological composition of the host rocks: 1) in garnet–pyroxene paragneisses; 2) in limestones; 3) in amphibolites.

Vertical axis: Class frequency (%)
Horizontal axes: Thickness, m; Content, conventional units.

N. V. Petrovskaya [2]. The principal vein mineral in the ore bodies is quartz. In addition, calcite and ankerite are present in small amounts (up to 2–4%). Among the ore minerals, pyrite, galena, and pale ore are common. Sphalerite, chalcopyrite, arsenopyrite, and gold occur comparatively less often. The ore minerals usually form fine disseminations, and sometimes nests up to 5–10 cm in size. Gold is present in varying amounts in many hydrothermal quartz veins, and its segregations can almost everywhere be observed with the naked eye. Gold particles occur in association with sulfides, and sometimes separately in quartz. Comparatively coarse gold is often found in leaching cavities at the outcrops of ore veins. Gold can also often be seen in limonite. The size of the gold particles ranges from tenths of fractions to 1–2 mm, usually less. In some veins, in milky-white quartz, gold particles occur that reach 5 mm and fill fine cracks in it.

In the plane of the veins, sulfides and gold are distributed rather unevenly and often form ore shoots. Their number and dimensions vary in different veins. Thus, in the Yurasovskaya vein, explored by adits on three levels, five ore shoots have been identified. Their dimensions along strike are from 10 to 30 m, and down dip up to 450 m. Ore shoots of considerable size have also been encountered in other veins of the ore field (Petrovskaya, Serebryakovskaya, etc.). In general, such ore shoots are not sufficiently persistent. They are accompanied by swellings and pinchings and are traced down dip and along strike for tens, and more rarely hundreds, of meters. In many veins only small ore nests are observed (Yubileinaya, Sluchainaya, and others), which have no industrial significance.

Lithological control of gold mineralization is established; it is manifested in the fact that the most favorable rocks for the formation of gold-bearing ores are garnet-pyroxene, amphibole-pyroxene, and amphibole paragneisses and skarnized marbles. Sections of veins at intersections with these rocks prove to be the most enriched in gold and ore minerals. Thus, sections of vein No. 9 at its intersection with garnet-pyroxene paragneisses, and of the Verkhnaya vein at its intersection with paragneisses and marbles, are characterized by an increased gold content (Fig. 1). This relationship also holds for the Yurasovskaya vein, which lies mainly in garnet-pyroxene paragneisses. On the northeastern flank, however, where the vein enters garnet-mica crystalline schists, it rapidly wedges out and the gold content drops sharply.

REFERENCES

1. Eliseev I. D. Metamorphism. Nedra Publishing House, 1963.
2. Petrovskaya N. V. Mineral associations in gold-ore deposits of the Soviet Union. Transactions of TsNIGRI, issue 76, 1967.
3. Salop L. I. Geology of the Baikal Mountain Region. Vol. II, Magmatism, Tectonics, History of Geological Development. Nedra Publishing House, 1967.