On the Hypsometric and Structural Position of Quartz–Gold Ore Veins of the Kindikan–Irokinda Ore Cluster

A. D. Minkov, L. F. Mitrofanov, V. A. Rubanov

(Presented by the scientific seminar of the Department of Mineral Deposits)

The Kindikan and Irokinda ore fields (combined into an ore cluster of the same name) are located in the central part of the South Muya Range of the Olekma–Vitim mountain region, within a large tectonic uplift of the Archean basement known as the South Muya block.

The geological structure of the ore cluster includes metamorphic rocks of the Kindikan suite of Archean age. They are represented by various varieties of gneisses (garnet–pyroxene, garnet–amphibole, amphibole–pyroxene paragneisses, etc.), skarnified and marbleized limestones, and amphibolites. The distinguished rock varieties occur as alternating beds and members with thicknesses from 100–200 to 1000 m. Intrusive rocks are represented by Archean granites forming dike-like bodies, 0.1–0.2 to 15–20 m thick and from tens to 500–700 m long along strike. Ultramafic rocks of the same age form lens-shaped bodies completely altered to serpentinites. In addition, within the area described, dikes of diorites, diorite porphyries, spessartites, and felsite porphyries belonging to the Mesozoic intrusive complex are fairly widespread. The gold-ore mineralization of the region is associated with the latter. Comparatively large areas consist of shadow migmatites and granite-gneisses formed during ultrametamorphism in Archean time. The metamorphic rocks are gathered into tight, usually linearly elongated folds with steeply dipping limbs (50–80°), complicated by folds of higher orders. Small flexures, schistosity, and corrugation 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-ore 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 Proterozoic framing on the west and east, respectively. Between these faults there developed a network of feathering disturbances of various orientations. The western part of the South Muya block, adjacent from the east to the Irokinda fault and here termed by us the Kindikan–Irokinda ore cluster, has been studied in greatest detail at present. In this part of the block, a rather dense network is established of disturbances feathering the Irokinda fault—

…among which one may distinguish: 1. Northeast-trending ruptures, as a rule confined to the axes of anticlinal folds. In disturbances of this group, auriferous quartz veins are sometimes hosted (Sluchainaya, Yubileinaya, etc.). 2. Northwest-trending disturbances feathering the main faults of the Kilyan tectonic zone. They are often filled by auriferous quartz veins (Tuluinskaya, Petrovskaya, Khrebtovaya, etc.). 3. Sublatitudinal (70—80°) disturbances are feathering structures of the northeastern ruptures. They also serve as ore-hosting structures for some gold-quartz veins (Serebryakovskaya, Berezovaya, etc.).

To clarify the regularities in the distribution of quartz veins with a productive mineral association within the ore cluster, a diagram was constructed of the hypsometric and structural position of the gold-quartz veins known within the Kindikan-Irokinda ore cluster.

On the ordinate axis of this diagram, hypsometric levels from 900 to 1800 m in absolute elevations are plotted; on the abscissa axis the relative distance from the Irokinda fault is shown in km. (Conventionally, the Irokinda fault is taken as a vertical line coinciding with the ordinate axis, but for convenience in constructing the diagram it is shifted from its true position 1000 m to the left.)

In constructing it, all quartz veins known within the Kindikan-Irokinda ore cluster were taken into account, provided that reliable information exists for them on the elements of occurrence. The veins are plotted on a vertical plane normal to the plane of the Irokinda fault, taking into account the distance from the fault and the hypsometric position of their outcrops at the surface. For each vein, the available data on the composition of ore mineralization are shown (the size of the symbol determines the content of the ore mineral). The contents of gold and silver are shown analogously, and for explored veins additional data are given on the average contents of gold and silver and the average thickness at each exploration level.

Analysis of the resulting diagram (Fig. 1) within the Kindikan-Irokinda ore cluster makes it possible to identify a number of features in the distribution of quartz veins and the gold mineralization within them. It is clearly seen that all quartz veins dip toward the Irokinda fault.

Most veins with an industrial concentration of gold and a higher content of ore minerals are located in the immediate vicinity of the Irokinda fault (the Verkhnaya, No. 30, Yurasovskaya II, Petrovskaya, and other veins). An exception is the Serebryakovskaya vein, situated at a considerable distance (about 7 km) from the fault but containing one rich ore shoot. The ore-feeding channel in this area was probably the Serebryakovskoye main disturbance.

In the vertical plane (depending on the hypsometric position of the ore bodies), the available data (Fig. 1) indicate a dependence of the concentration of the useful component on the hypsometric level at which the vein is situated. Three hypsometric zones may be distinguished, characterized by different contents of the useful component and ore minerals.

The first is situated above the 1600 m mark and includes ore bodies with a high content of sulfides and gold (Verkhnaya, Petrovskaya, Khrebtovaya). The second, in the interval 1500—1600 m, is characterized by the fact that the veins of these hypsometric levels carry comparatively poor sulfide and gold-silver mineralization. In the Petrovskaya industrial vein, in this interval, a decrease in the average content of the useful component is observed. The third zone (the lower interval)

Fig. 1. Structural–hypsometric scheme of the position of quartz veins: 1 — gold, 2 — silver, 3 — pyrite, 4 — chalcopyrite, 5 — galena, 6 — sphalerite, 7 — fahlore, 8 — arsenopyrite, 9 — calcite.

Axes in the figure:
Vertical axis — absolute elevations, in meters.
Horizontal axis — distance from the Irokinda fault, in km.

Inset labels: surface; valley floors; probable workings; underground mine workings of the Skvoznaya adit.

Visible vein labels in the plot include: v.-21, v.-8, v.-7, v.-20, v.-29, v.-30, v.-3, v.-11, v.-6, v.-10, v.-9, v.-5, v.-4, v.-28, v.-32, v.-1, v.-2, v.-24, v.-23, v.-25, v.-22, v.-13, v.-19, v.-15, v.-26, v.-27, v.-18, v.-16, v.-17, v.-34, v.-35.

below the 1500 m level (Fig. 1) includes veins rich in ore minerals and gold (the Tuluyinskaya, Yurasovskaya II, No. 30 veins). Moreover, at this level the indicated veins tend toward the Irokinda fault. At a considerable distance from the latter lie the Pasmurnaya, Sluchainaya, and other veins, with a low average content of the useful component.

When considering the distribution of mineralization by groups of veins of different spatial orientation, it is established that, in the group of veins of northeastern strike, ore bodies with a high gold content are situated in immediate proximity to the fault.

The group of veins of sublatitudinal orientation is characterized by an elevated gold content in veins of a high hypsometric level (more than 1600 m) and by a reduced amount of it in veins located below the indicated level. The distance of the veins of this group from the Irokinda fault is approximately the same and evidently does not play an important role here.

The distribution of gold and sulfides in veins of northwestern strike is analogous to the distribution in all ore bodies of the Kindikan–Irokinda ore cluster as a whole.

In the ore cluster, zoning is manifested, caused by a change in the composition of the enclosing rocks, which consists in the fact that the most favorable rocks for the localization of industrial ore bodies and ore shoots are garnet–pyroxene paragneisses and calc-silicate rocks, while unfavorable rocks are garnet–mica rocks, amphibolites, and shadow migmatites. In the case where a vein is situated in favorable rocks, it has a comparatively great thickness (up to 1 m and more), is usually represented by all four stages of hypogene mineral formation, and has an industrial gold content. In unfavorable rocks it sharply loses thickness, and the content of gold and sulfides decreases. A regularity of this type is well expressed, for example, in the Verkhnyaya and Yurasovskaya II veins (Rubanov, Gribanov, 1966), although it has been studied mainly along the strike of the ore bodies. The absence of a sufficient number of intersections extending beyond the limits of the near-ore altered rocks and the absence of raises do not yet make it possible to trace such changes down the dip of the ore veins.

Analysis of the gold content of quartz veins in plan and vertically within the ore cluster made it possible to establish that the sections of them richest in gold occur in the formations of the lower subformation of the Kindikan Formation, which consists mainly of rocks favorable in physicomechanical properties—garnet–pyroxene paragneisses. Rocks unfavorable in physicomechanical properties for the localization of gold mineralization include garnet–mica paragneisses (Rubanov, Gribanov, 1966), which compose to a considerable degree the middle subformation of the Kindikan Formation.

Thus, for example, the comparatively well-studied veins of the Yurasovskaya II and Tuluyinskaya ore cluster are localized on all exploration levels in rocks of the lower subformation and are characterized by the presence of a stable ore-mineral association with a high content of the useful component. The upper levels of vein No. 30 pass along the boundary between the lower and middle subformations and are characterized by poor contents of ore minerals and gold. When the vein passes into rocks of the lower subformation, the concentration of gold and sulfides increases rather sharply.

In the distribution of ore minerals in quartz veins for the ore cluster as a whole, regularities are established that are analogous to their distribution in individual ore bodies. In veins with high hypsometric—

At a low level (1600–1750 m), along with sulfides, a comparatively very high content of pale ores is established (the Petrovskaya, Khrebtovaya, and other veins), decreasing at deeper levels. Such minerals as galena and pyrite are persistent in the veins of the Kindikan–Irokinda ore cluster. We find their elevated content in veins situated at different hypsometric levels.

The study of horizontal mineralogical zonality within the ore cluster makes it possible to establish an approximately analogous picture in the distribution of ore minerals with increasing distance from the Irokinda fault (Fig. 1). Closer to the fault are veins containing, in addition to pyrite, galena, chalcopyrite, and sphalerite, pale ore. In the group of veins most distant from the fault, pale ore is absent. Pyrite and galena have the broadest distribution in the ore bodies. We observe comparatively high concentrations of them both in veins directly at the fault (Tuluninskaya vein, etc.) and at a considerable distance (Serebryakovskaya, Yubileinaya veins, etc.). Sphalerite and chalcopyrite are distributed similarly, although their content is significantly lower in the more distant ore bodies (Fig. 1).

From the foregoing it follows that the Irokinda fault played the role of an ore-conducting channel during the formation of the quartz veins of the Kindikan–Irokinda ore cluster.

Taking into account the established regularities in the distribution of quartz veins and their commercial gold content—their confinement to disjunctives feathering the Irokinda fault at distances from the fault of up to 5 km, and the presence of favorable enclosing rocks—it may be assumed that veins occurring in rocks of the lower sub-suite and situated in the area adjoining the indicated fault are prospective. In this connection, the presence in the vein of minerals of the productive stage of mineralization should be taken into account.

One cannot rule out (for example, the Serebryakovskaya vein) the possibility of encountering commercial veins at a considerable distance from the Irokinda fault if they are confined to major rupture disturbances (such as the main Serebryakovskoe disturbance), along which ore-bearing solutions could have penetrated to considerable distances from the Irokinda fault. Although the absence of other large zones of the type of the main Serebryakovskoe disturbance is confirmed over a considerable area of the South Muya block by geological mapping carried out in recent years, the hypsometric criterion is not an important criterion for evaluating quartz veins. The role of pre-ore structures in the localization of gold mineralization in the South Muya block was undoubtedly decisive.