STRUCTURE OF A GOLD-ORE DEPOSIT IN THE AMUR REGION

V. D. MELNIKOV, I. I. FATYANOV
(Far Eastern Branch, Siberian Branch, Academy of Sciences of the USSR)

The deposit is situated near the junction of the Mongol-Okhotsk and East Asian folded systems, in the area where the Aldan Shield and the Bureya Massif converge. This position determined the complex fold-block structure of the ore field and caused the intensive manifestation of ruptural disturbances.

The mineralization, represented by a system of gold-quartz veins, is localized in sedimentary rocks of Paleozoic (?) age at the intersection of major disjunctive disturbances of sublatitudinal and near-meridional trends (Fig. 1). The ore field is elongated along the southern limb of an anticlinal fold of west-northwestern strike, with a southwestward dip of the rocks at angles of 20–50°. In the core of the fold there occur predominantly massive sandstones of the Tokur Formation; on the limbs, finely interbedded aleuro-argillites of the Ekimchan Formation. The sedimentary rocks within the deposit area are intruded by dikes of diabase porphyrites and stocks of quartz syenite-diorites of Cretaceous age.

The upper part of the Tokur Formation, 300–350 m thick and directly enclosing the mineralization, is represented by an alternation of beds of massive sandstones with strata of interbedded sandstones, siltstones, and argillites. Characteristic features of the formation within the deposit area are: a) the presence of horizons of sedimentation breccias; b) a gradual upward increase, along the section, in the thickness of the beds of interbedded rocks; c) a quantitative decrease from west to east in the role of psammitic material in the composition of the formation; d) the presence of layers of carbonate sandstones within the beds of interbedded rocks; e) the presence of carbonaceous matter in the composition of the rocks of the formation.

However, the principal feature of the upper part of the Tokur Formation is the sharp physico-mechanical heterogeneity of the rocks composing it. During the manifestation of intense tectonic deformations, the heterogeneity of the formation’s structure led to the development in the rocks of a large number of interstratal and oblique, relative to the bedding, cleavage-type fractures, which later hosted the gold mineralization.

Resting concordantly on the rocks of the Tokur Formation are dense, finely interbedded aleuro-argillites of the Ekimchan Formation, characterized by a low degree of permeability and high plasticity. In this connection, the rocks of the Ekimchan Formation serve as a distinctive screen for the mineralization (Fig. 2), and the contact of the formations should be regarded as an independent—

Fig. 2. Shielding effect of the thinly interbedded siltstone–argillites of the Ekiman Formation. Transverse section through the central part of the deposit.
1 — Ekiman Formation; 2 — Tokur Formation; 3 — contact between formations; 4 — gold–quartz veins; 5 — tectonic faults; 6 — mine workings.

Visible labels in the section: Aéroflot, Monterosso, Monterosso, Adit 18, Crosscut 75; level marks 600, 700, 800, 900; vein marks Zh. 86, Zh. 208, Zh. 220, Zh. 228, Zh. 260.

Fig. 1. Geological-structural scheme of the central part of the deposit at the 700 m level.
1 — Tokur Formation. Predominantly massive sandstones; 2, 3 — upper part of the Tokur Formation. Alternation of beds of massive sandstones with strata of interbedded sandstones and aleuro-argillites; 4 — Ekimchan Formation. Dense, finely interbedded aleuro-argillites; 5 — horizons of sedimentation breccias; 6 — dikes of diabase porphyrites; 7 — stocks of quartz syenite-diorites; 8 — zones of metasomatic silicification along the Main Fault; 9 — gold-quartz veins; 10 — tectonic faults; 11 — elements of bedding attitude of rocks; 12 — mine workings.

Visible map labels: Diorite; Pankov; Main Fault; Tokur Fault.

Scale: 0, 50, 100, 200.

a lithological-structural element controlling the upward vertical extent of mineralization.

In the northern part of the deposit there extends a regional sublatitudinal fault, possibly of thrust type, called the Main fault; its dip is 30–40° to the south. The fault is accompanied by the development of a large zone of weakly auriferous metasomatic quartzites 20–30 m thick, as well as by intense crushing and silicification of the enclosing rocks. The presence of such a thick zone of metasomatic quartzites characterizes the Main fault as a structure favorable for the circulation of mineralizing solutions. However, the influence of the fault on the formation of gold-bearing quartz veins is still unclear, since the quartz veins are formations later than the quartzites.

The overwhelming majority of the gold-ore bodies are located in the hanging wall of the fault. North of it, only an insignificant number of quartz veins without commercial mineralization are known. Thus, the Main fault is a structure limiting the downward vertical extent of gold mineralization. At the same time, because the Main fault along its strike cuts, at angles of 25–30°, the upper part of the Tokur Formation, which hosts the mineralization, a narrowing of the ore field to the west and its widening to the east are observed.

Within the deposit area a series of steeply dipping faults of submeridional direction has been revealed: the Tokur, Dike, Diorite, and Barandzha faults. The faults are filled with gouge, whose thickness reaches 0.5–2.5 m, and are accompanied by zones of fractured rocks.

In the plane of slickensides developed along the faults, two sets of striations dipping to the south are observed: at angles of 10–12° and at angles of 45–50°. Striations of the second direction, which obscure the first, noticeably predominate, indicating their later origin.

Along the faults there occurred one-sided normal strike-slip displacements with amplitudes of the horizontal component equal to: 1) for the Barandzha fault, 300–400 m; 2) for the Diorite fault, 150 m; 3) for the Dike fault, 100 m; and 4) for the Tokur fault, 50 m. The amplitude of displacement regularly increases from east to west.

The rocks of the Tokur and Ekimchan formations, as well as the metasomatic quartzites of the Main fault, were displaced by the indicated distances. At the same time, the displacement amplitude of the vein zones and stocks of syenite-diorites does not exceed 10–12 m.

The facts cited unequivocally indicate that: 1) the submeridional faults are long-lived, having been initiated still in pre-ore time; 2) the main movements along them also occurred in the pre-ore period; and 3) movements established on the basis of the widespread development of steeply dipping striations on slickensides, as well as the small displacement amplitudes of vein zones and stocks of syenite-diorites, also occurred in the post-ore period.

One-sided displacements along the faults led to the formation of a number of step-like blocks, shifting the rocks of the upper part of the Tokur Formation, which localize the mineralization, in a southwesterly direction. This accordingly determines the inclination of the gold mineralization in the same direction.

Gold mineralization at the deposit is represented by a system of gold-quartz veins confined to interstratal and obliquely cutting, relative to bedding, cleavage-type fractures. Two principal directions of ore bodies are noted: northwestern and nearly latitudinal. Veins of both directions in most cases dip at angles of 25–40°;

however, in places their flattening is observed. Sometimes the veins acquire a steep dip, following along connecting fractures between the main ore-hosting fractures. The thickness of the veins ranges from the first centimeters to 1.5–2.0 m. In places they pass into barren zones of crushing and grinding of rocks.

The direction of the gold-quartz veins in the ore field changes regularly from east to west. On the eastern flank of the ore field the veins have a distinct northwestern strike, following fractures close to the strike of the rock bedding. On the western flank of the deposit, with approach to the Main Fault, they are usually confined to associated sublatitudinal fractures, cutting the bedding of the rocks at angles of 20–25°. In the central part of the ore field, between the Dyke and Diorite faults, veins of both directions are widely developed. Here there is a considerable complication of the structure of the ore field, as well as of the morphology of the ore bodies. The number of veins of comparatively small extent in the area sharply increases. Blocks of rock contoured by veins often acquire in plan the form of rhombs or parallelograms. As a whole, the junction node of gold-quartz veins of both directions represents a gigantic stockwork with a steep dip to the south-southwest.

Most of the gold-ore bodies of the deposit are grouped into a number of fairly stable vein systems, several hundred meters long. Vein systems usually consist of a series of closely spaced, parallel or en echelon ore bodies. Much more rarely, individual extended veins are encountered, persistent in strike and dip.

At the level with an absolute elevation of 700 m, three such systems are clearly distinguished: a) the system of veins 286—184—289, b) the system of the Oktyabr’skaya I—Pervaya veins, and c) the system of the Sentyabr’skaya vein. A characteristic feature of all three vein systems is their confinement at a comparatively large distance to the horizons of sedimentation breccias.

The ore veins of the deposit have a parallel-banded or breccia-like texture. The parallel-banded texture develops as a result of replacement of rocks by quartz from a series of closely spaced fractures. During replacement, complex finely banded quartz veins with ribbons-xenoliths of shales arise. The number of bands in thick veins may exceed ten. Where parallel planes of cleavage are sharply expressed, complexly oriented branching veinlets develop, usually bounded by distinct lateral fractures. With crowding of such veinlets and isolation of xenoliths of the enclosing rocks detached by them, breccia-like textures arise.

Within the deposit area, dikes of diabase porphyrites and stocks of quartz syenite-diorites are widely developed. Both groups of small intrusions are concentrated within a large belt crossing the ore field in a submeridional direction.

Dikes of diabase porphyrites are confined to steeply dipping (75–90°) tension fractures of north-northeastern and submeridional strike. Within the ore field they form several zones of crowding. The dikes distinctly cut the gold-quartz veins, not infrequently contain xenoliths of ore quartz, and sometimes follow fractures that displace the ore bodies. In turn, the dikes are displaced along fractures, renewed in the course of later movements, passing in the vein salbands. The length of individual dikes is very considerable and reaches 300–500 m; their thickness ranges from 0.1 to 2.5–3.0 m. Along the salbands of the dikes, late movements, expressed by slickensides, are often observed.

Stocks of quartz syenite-diorites, in almost rectilinear chains elongated in a submeridional direction, cross the ore ...

field. Their emplacement probably occurred along narrow, tectonically weakened zones. The stocks are persistent pipe-shaped bodies with an oval or round cross-section, from 15 to 70 m across, dipping to the southwest at angles of 60–70° to the horizon. They cut metasomatic quartzites, gold-quartz veins, and dikes of diabase porphyrites.

In the rocks of the Tokur Formation within the deposit area, vertical and steeply dipping transverse-separation fractures of northwestern and sublatitudinal strike are widespread. They are confined to sandstone interbeds in the strata of interbedded rocks and are completely absent from the siltstone-argillite interbeds. The fractures are constantly filled with quartz-carbonate material, forming a dense series of veinlets (Fig. 3), in places almost completely replacing the sandstone interbeds.

Fig. 3. Transverse-separation fractures in sandstone interbeds, filled with quartz-carbonate material. Polished hand specimen. Natural size

To characterize the various fracture systems within the ore field, a diagram is given of the orientation of tectonic fractures, processed according to the method of A. V. Korolev (1), based on 476 measurements (Fig. 4).

On the diagram, maxima I and Ia stand out clearly; they characterize steeply dipping fracturing of submeridional and north-northeastern direction, accompanying regional faults that break the ore field into a number of stepped blocks, as well as zones of concentration of diabase-porphyrite dikes and chains of syenite-diorite stocks.

Maximum II corresponds to fractures of west-northwestern and near-latitudinal strike, dipping south and southwest at angles of 20–40°. The maximum corresponds to a large sublatitudinal Main fault, as well as sublatitudinal and west-northwestern gold-quartz veins and late disturbances in the selvages of the veins.

Possibly both the sublatitudinal and near-meridional faults, as well as the tectonically weakened zones with chains of stocks and concentrations of dikes, recorded on the fracture diagram by maxima I, Ia, and II, are a reflection of major basement fractures, since they are persistent in the district and do not depend on folded structures.

Maximum III corresponds to gold-quartz veins of northwestern strike on the eastern flank of the ore field, and maximum IV to transverse-separation fractures in sandstone interbeds, filled with quartz-carbonate material.

Fig. 4. Orientation diagram of tectonic fractures in rocks of the upper part of the Tokur Formation

The formation of the deposit structure proceeded under the influence not only of disjunctive, but also of plicative deformations. The intense folding deformations of the sedimentary rocks of the ore field are evidenced by the results of microstructural analysis carried out by G. I. Neronskii [2]. At the same time G. I. Neronskii established that the gold-quartz veins also underwent significant plastic deformations. Microstructural diagrams in the sandstones enclosing the gold-quartz veins show a girdle structure. Microstructural diagrams compiled for the veins also have a girdle structure, with a decrease in the density of the girdles from the early generations of quartz to the later ones.

The general monoclinal structure of the southern limb of the anticline is complicated by folding of higher orders. Thus, in the southeastern and central parts of the ore field, a series of small second-order folds with wing spans of 40–80 m, elongated in a northwestern direction, has been recorded. Microfolding with fold amplitudes of several meters is also widely manifested.

In beds of interlayered sandstones and aleuro-argillites, under the influence of deformational stresses, the phenomenon of boudinage often develops. Brittle sandstone layers become boudinaged, lose continuity, and form a chain of separated lenses that retain the general direction of the layer (Fig. 5). The phenomenon of boudinage is especially widespread in rock beds containing layers of carbonate sandstones.

Fig. 5. Boudinage of layers of carbonate sandstones in rocks of the upper part of the Tokur Formation. Polished hand specimen. Natural size.

In conclusion, let us note that the position of the deposit in the general structure of the district is determined by the influence of the following principal lithologic-structural elements: 1) the deposit is located at the intersection of large zones of regional disturbances of sublatitudinal and near-meridional trends; 2) the gold mineralization is localized in rocks, heterogeneous in their physicomechanical properties, that compose the upper part of the Tokur Formation; 3) upward along the vertical, the mineralization is screened by dense, thinly interlayered aleuro-argillites of the Ekimchan Formation; 4) downward along the vertical, the mineralization is chiefly bounded by the sublatitudinal Main Fault, accompanied by a zone of metasomatic quartzites; 5) the gold-ore bodies are controlled by a system of interstratal and obliquely crosscutting, relative to bedding, fractures of northwestern and sublatitudinal trends.

LITERATURE

1. Korolev A. V., Shekhtman P. A. Structural conditions of formation of postmagmatic ores. Nedra, 1965.

2. Neronskii G. I. The nature of dynamometamorphism of rocks and ores of the Verkhne-Selemdzha district. In: Eighth Conference of Young Scientists of the Far East, 1965.

3. Ralkevich E. A., Moiseenko V. G., Molchanov P. Ya., Melnikov V. D., Fatyanov I. I. Deposit of a low-sulfide gold-quartz formation (Priamurye). In press.