Criteria for the Depth of Gold Mineralization: A Case Study of the Berikul Ore Field in the Kuznetsk Alatau

Yu. G. Shcherbakov, N. V. Roslyakova (Institute of Geology and Geophysics, Siberian Branch, USSR Academy of Sciences)

The Berikul ore field is located among porphyries in the exocontact zone of the Dudet granitoid massif. The host rocks, and the source rocks for the massif, are volcanogenic-sedimentary rocks of the Cambrian. As has been established, these strata are distinguished by an elevated local Clarke of gold, and therefore most of the genetically diverse gold deposits of the Kuznetsk Alatau are associated with them [4]. In Precambrian gneiss-schist complexes with a reduced local Clarke of gold, deposits of this element are unknown. Hence, one of the criteria for the probable downward extent of mineralization is the position of the footwall of the gold-bearing geochemical level. In the area of the Berikul ore field, the vertical thickness of the volcanic Cambrian preserved from erosion is no less than three kilometers.

Another criterion for the possible depth of a deposit is the down-dip persistence of the ore-controlling structures. The structure of the Berikul ore field is determined by the large, gently dipping Fourth vein, dipping (10–20°) to the southeast, and by a system of thin (0.2–0.5 m) quartz veins branching off from it, dipping at angles of 40–60° in the opposite direction, i.e., to the northwest (Fig. 1). Along the contact of the gently dipping Fourth vein with limestones, metasomatic sulfide bodies with gold, or the so-called Khotim bodies [1], can be traced. They are confined mainly to bends of this contact. The Fourth vein does not constitute a single continuous body. It is interrupted by barren zones of rock crushing, breccias of dragging, and shear zones. It has long been observed that many steep veins, on approaching the gently dipping one, flatten out or split into apophyses (Fig. 1). At the junctions with steep veins, the thickness of the gently dipping vein increases from approximately half a meter to 5–6 m. Its strike gradually changes from NE 70° on the upper levels to NE 10° on the lower ones. Throughout the ore field, the steep veins change their strike simultaneously with the gently dipping vein. All the foregoing and, in addition, a certain displacement of the rocks of the hanging wall of the gently dipping vein to the northwest led us to the conclusion that the structure of the vein field is controlled by a complex thrust. The gently dipping vein represents its principal, but not its only, plane. The amplitude of the thrust does not exceed several tens of meters. Tensional fractures feathering the thrust plane controlled the formation of the steep veins. The thrust

the diorite stock, which served as a rigid buttress for tangential forces, bends around. In connection with the thrust, several systems of fracturing arose in the rocks enclosing the veins. The most widespread are tension fractures, approximately parallel to the steep veins and always changing their strike in the same way as they do. Another system of fractures, parallel to the gently dipping vein, becomes more intense with increasing distance from it.

The fractures of this system are persistent in strike and dip, have smooth walls, sometimes with slickensides, which indicates their shear character. Both of the named systems of fractures, as well as other, less widespread fractures of various directions and extents, show no visible connection with the intensity of mineralization. An entirely special role belongs to fractures cutting at angles close to a right angle both the gently dipping vein and the steep ones. These fractures have a shear character. Their concentration is accompanied by an increase in the gold content in the steep veins. This circumstance was first noted by V. N. Ponomarev. The position of the named system of fractures, like the direction of thrusting, is connected with the moment of rotation around the diorite massif. At a distance of 100–150 m downward from the gently dipping vein, this fracturing gradually dies out, and at the same time the gold content in the veins decreases. At deeper levels the density of these fractures sometimes increases again (more than 10 per meter), and this is accompanied by the appearance of new ore shoots in the veins. In the hanging wall of the Fourth vein, this fracture system is traced at places where it is intersected by steep veins, for example, Tatarskaya and Seventeenth. On the whole, the fracturing of the rocks of the hanging wall of the gently dipping vein has been poorly studied.

For judging the possible persistence of gold mineralization to depth, the established multi-tiered character of the thrust structure acquires special significance. Gently dipping fractures and veins are now known in the center of the ore field above the main thrust plane and at deep levels in its southeastern part. Their persistence along strike and the scale of the mineralization associated with them are not yet entirely clear; however, in our opinion, the very fact of the recurrence of ore-controlling structures associated with the imbrication of a complex thrust is extremely favorable. Thus, the structure of the ore field does not in principle limit the prospects for the extension of mineralization to depth. What is evident is only the need for its further refinement.

An equally important criterion for the possible extension of ore mineralization to depth is the magnitude of the interval of favorable physicochemical conditions for the deposition of gold in the veins. The multi-stage formation of veins at the deposit has long been known [1, 2]. Quantitatively, they are dominated by quartz of the first pre-ore generation. It contains inclusions of chlorite, rutile, and, rarely, hematite; it replaces porphyrites. In the second stage, quartz with pyrrhotite and a small amount of pyrite and arsenopyrite was deposited. In this same stage, according to D. A. Timofeevsky and S. S. Borishanskaya [3], native bismuth and bismuthinite precipitated from the solutions. The minerals of the third generation, in addition to quartz, include pyrite, arsenopyrite, and parankerite. In the fourth, carbonate–sulfide stage, chalcopyrite, sphalerite, galena, tennantite, small amounts of pyrrhotite, rarely with pentlandite, pyrite, arsenopyrite, chabazite, and some other minerals were deposited. In this same stage the bulk of the gold and silver was deposited. The fifth, barren generation consists of quartz, calcite, chlorite, and sericite and completes the formation of the ore veins.

BLOCK DIAGRAM
OF THE STAROBERIKUL GOLD-ORE DEPOSIT
based on mine materials

Overthrust block

Overthrust wing

Labels on the diagram: Rosypnushka, Prokop’evskaya, Magistral’naya, Andreevskaya, Tsentralka, Inter-2a.

Legend:

• Limestones, \(Cfm_1\)
• Porphyrites, \(Cfm_2\)
• Diorites, \(Cm\text{-}O\)
• Diabases
• Gold-bearing quartz veins
• Gold-bearing metasomatic schists
• Gold-bearing sulfide bodies at the contacts of the gently dipping vein with limestones
• Projection of the mean surface of steep veins onto the surface

Fig. 1

The total sulfide content in the ore columns of the veins ranges from 5 to 20% of the vein mass. No stable changes in ore composition with depth could be noted.

Determination of the thermoluminescent properties of vein quartz made it possible to establish a decrease in the glow intensity of its increasingly later generations from \(12 \cdot 10^{-2}\) mA to \(3 \cdot 10^{-2}\) mA (Fig. 2). The independence of thermoluminescence intensity from the depth of ore occurrence down to the 700 m level was also revealed, as was its independence from affiliation

Fig. 2.

with one or another fragment of the ore field. Only early quartz, in places where it comes into contact with quartz of subsequent generations, acquires the thermoluminescent properties characteristic of them.

The study of more than two hundred gas–liquid inclusions in vein quartz revealed the specificity of the physicochemical conditions of deposition of its different generations (Fig. 3). Pre-ore quartz contains a mass of the finest inclusions with different ratios of gas and liquid phases. Their homogenization temperatures into the liquid phase at different hypsometric levels vary from 400°C to 600°C. Most inclusions burst at temperatures of about 600°C without reaching homogenization. Inclusions in second-stage quartz at normal temperatures contain more liquid than gas and homogenize into liquid, likewise independently of the sampling depth, at temperatures from 315°C to 390°C. In a similar temperature interval, from 300°C to 380°C, but more often into the gas phase, inclusions of the third stage homogenize. Quartz of the fourth, so-called productive generation, which carries the main mass of gold at all levels of the deposit, is characterized by

homogenization temperatures of inclusions from 170°C to 235°C. Inclusions in lustrous quartz of the fifth generation homogenize at temperatures of about 100°C.

Thus, the results of thermoluminescence and thermometric analyses definitely indicate the stability, with depth, of the thermodynamic conditions of ore deposition.

In conclusion, it remains to note that the position of the Berikul ore field within a major gold-bearing geochemical

Fig. 3. Homogenization temperatures of gas–liquid inclusions in vein quartz from the Berikul deposit.

(In the figure: vertical axis—depth, in meters; horizontal axis—temperature, °C; dots—measurements; I, II–III, IV, V—generations of vein quartz.)

stage, the highly imbricated character of the thrust, which contributed to the development of the ore-controlling structure proper, and, finally, the undoubtedly very broad temperature interval of ore formation extending to depth—all this should be taken into account as arguments for a favorable prognostic evaluation of the ore field to at least two kilometers from the present surface.

REFERENCES

1. Budynnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai–Sayan mountain region. Proceedings of Tomsk University, 1948.

2. Dembo T. M. Geological structure and gold mineralization of the northern part of the Kuznetsk Alatau. Proceedings of NIGRIZoloto, 1952, issue 19.

3. Timofeevskii D. A., Borishanskaya S. S. Gold–cobalt mineralization of Martaiga (the Berikul deposit). Proceedings of NIGRIZoloto, 1951, issue 18.

4. Shcherbakov Yu. G. Distribution and conditions of concentration of gold in ore provinces. “Nauka,” 1967.