Some Features of the Localization of Ore Shoots at the Darasun Deposit

V. G. Zvyagin (Darasun, Zabaikalzoloto Trust)

The ore field of the Darasun deposit is composed of igneous-metamorphic rocks of various ages and compositions. The veins are confined to extended joint fractures, predominantly of northwestern and northeastern strike. The formation and development of the fractures is associated with normal-fault–strike-slip movements.

In the process of formation of the deposit, seven stages of mineralization are distinguished, separated by movements: quartz-tourmaline, quartz-pyrite, arsenopyrite, sphalerite-galena, chalcopyrite, sulfoantimonite, and carbonate. The main mass of gold is associated with the chalcopyrite stage. An insignificant amount of it was also deposited in the arsenopyrite stage.

Mineralization in the plane of the veins has a discontinuous character: ore shoots alternate with pinches. Ore shoots are understood as “...accumulations of ore minerals more or less sharply separated from the enclosing poor vein filling, as well as gold-enriched areas with sufficiently distinct boundaries and a certain elongation in one direction” (Lindgren, 1934). The dimensions of ore shoots vary and depend on specific geological-structural features. Their size along strike ranges from 30–50 m to 250–400 m, and down dip from 50–100 m to 600 m or more. Ore shoots are usually elongated down the dip of the veins, but isometric ones are also not uncommon.

The most important factor determining the distribution of mineralization in ore bodies is structural. The joint fractures enclosing the veins are wavelike curved surfaces. During displacement in the plane of the fracture, some segments are compressed, while in others gaping zones are formed, favorable for the circulation of ore-bearing solutions and the deposition of useful components. It is in such segments that ore shoots are formed. This type of ore shoot is especially characteristic of gold, tin, and tungsten deposits and has been described many times both in Soviet literature and abroad. This type of ore shoot is also widely developed at the Darasun deposit.

Here, during prospecting-exploration and mining operations, it was noted that mineralization is confined to segments of veins with quite definite elements of occurrence (Fig. 1). Significant changes in the elements of occurrence, as a rule, indicate either the presence of ...

Fig. 1. Dependence of the attitude elements of the vein and the contents of gold and silver
(sketch of block 10, level 310, Sentyabrskaya vein)

1 — gold content, 2 — silver content, 3 — attitude elements of the vein

...the end of the ore shoot and the disappearance of ore minerals or even vein minerals, or its beginning. Moreover, changes both in strike angles and in dip angles exert an equally successful influence on the intensity of mineralization. Obviously, analysis of the combined influence of changes in the dip and strike of a vein should ultimately reflect the dependence of mineralization on the character of the vein plane.

To clarify this, we constructed the relief of the plane for a number of veins of the Darasun deposit. The methodology of all the constructions is simple and consists of the following. On composite plans of mine workings, an ideal plane is constructed, roughly parallel to a given vein. Then, over a grid of \(25 \times 25\) m, the distance from this plane to the vein is measured. From the obtained marks, the relief of the vein is constructed in contour lines, with subsequent plotting on it of sampling data in grade contours. In this process it is established that:

1. The relief of the veins of the Darasun deposit is characterized by a complex surface. According to the work of M. V. Gzovsky, in isotropic rocks this waviness may be caused by two reasons:

a) the nonuniform orientation, in the material being destroyed, of the principal stress axes from the very beginning of deformation;

b) in an initially homogeneous stress field, the appearance of small ruptures generates local inhomogeneities, which cause the ruptures to grow in directions that do not coincide with their initial orientation. With a further increase in stresses, small curved disturbances gradually expand on the flanks and merge into a single, complexly built seam having sinusoidal, garland-like waviness.

1. The position of the ore shoots is clearly controlled by the relief of the ore-bearing cavity. This regularity, established for practically all veins of the deposit, is further emphasized by the fact that the very shape of the ore shoots and their orientation strictly coincide with details of the fracture relief. Thus, for example, in the V-Elektricheskaya vein the ore shoot is, in general outline, isometric and is confined to a “depression” that is likewise isometric in shape. In the Novo-Kuznetsovskaya vein, the ore shoots have a form elongated down the dip, which is also determined by the corresponding forms of relief of the vein walls. The ore shoot of the Sen-

Fig. 2. Ore shoot along the Sentyabrskaya vein

1. Isolines of the vein relief. Gold contents in conventional units: 2—up to 2 conventional units; 3—from 2 to 5 conventional units; 4—from 5 to 10 conventional units; 5—from 10 to 30 conventional units; 6—more than 30 conventional units; 7—line of junction with the Novo-Elektricheskaya vein.

The Sentyabrskaya has a gentle southeasterly plunge. Similarly, the long axis of the “depression” containing the ore shoot plunges to the southeast (Fig. 2). Similar phenomena could be illustrated by examples from the Medvedevskaya, Lagunovskaya, Yuzhno-Kuznetsovskaya, Futbolnaya, and other veins.

1. The fact is noteworthy that parts of the ore shoot with the maximum concentration of gold—streams, nests, lenses—are located in places of maximum gradients of the vein-plane relief. At the same time, the greatest dispersions of the gradient are also observed here. In other words, all else being equal, the more complex and “capricious” the morphology of the vein, the higher its gold content. Among the important causes of ore deposition we shall indicate the following:

2. Changes in the physicochemical conditions of solution circulation: temperature, pressure in the system, concentration of substances, acidity–alkalinity of the solutions, and so forth.

3. The degree of brecciation of the vein material deposited by early, pre-gold-bearing solutions, since crushing increases the permeability of the medium and raises its chemical activity.

4. Gold deposition is promoted by sulfide material of the early stages—quartz-pyrite, pyrite-arsenopyrite, sphalerite-galena.

In light of the factual material presented, it is possible, in a different plane than was previously assumed, to resolve a number of such questions concerning the genesis of the deposit as primary zoning, control of mineralization, and the depth of distribution of commercial mineralization.

Foreign practice shows that at many gold-ore deposits the depth of commercial mineralization is 1.5–2 km, and at the well-known Kolar deposit the depth of mining reached more than 3000 m without substantial changes in grade. Meanwhile, all researchers of Darasun set the maximum depth of commercial mineralization at 600–800 m. These figures are determined rather by the attained depth of vein mining and by the existing level of mining technology.

It seems tempting to consider the question of the possible depth of commercial mineralization, at least hypothetically, from the standpoint of the connection between the ore shoots of the deposit and the relief of the veins. The wave-like character of the cleavage fractures at the Darasun deposit is a regular phenomenon. When, for example, the plan of the Novo-Kuznetsovskaya vein is examined, several such waves are established, along with the corresponding number of ore shoots separated by pinches. And if this is so, it should be assumed that the same waviness of structure will persist in the vertical direction as well, and that favorable ore-hosting zones may appear somewhere at deep levels. But the mineralization will likewise have a discontinuous character, determined by the internal morphology of the fractures; ore shoots should also be separated vertically by pinch zones. The fundamental possibility of such a phenomenon can be shown by the example of such veins as Medvedevskaya and Yuzhno-Kuznetsovskaya. In both veins, two ore shoots have been mined, separated vertically by “pinches.” The distribution of gold in the plane of the veins is directly dependent on their relief (Fig. 3).

As a first approximation, one may even approach a quantitative estimate of the “vertical” pinches. If the magnitude of the “horizontal” pinches in the general case is comparable with the dimensions of the ore shoots and is equal to the length of half-waves, then the “vertical” pinches should correspond to the length of the ore shoots along the dip of the veins and amount, for example, for the Novo-Kuznetsovskaya vein, to 500–700 m. The considerations set forth make it possible to believe that the depth of commercial mineralization at the deposit may reach 1–1.5 km and more.

At present, intensive exploration is being carried out at the deposit for “blind” ore shoots localized in NW structures at the place of their junction with major NE disturbances. In their exploration, it is extremely important to study the nature of the wedging-out of ore shoots in space and the vertical zoning.

Without touching on the geochemical and mineralogical aspects of the question, we shall show some regularities in the placement of ore shoots and the areas of their wedging-out, obtained through the simplest statistical processing of sampling data.

It is natural to assume that the conditions of ore formation in the central parts of ore bodies and on their periphery are not the same. Therefore, the statistical characteristics of samples from different parts of the ore body should also differ. This is indeed the case, as can be shown by the example of the Sentyabrskaya vein. Mathematical processing of sampling data (2500 samples) from different levels of the vein shows that within the ore shoot there is a stable direct correlation between the gold and silver contents and the maximum values of their variances. Conversely, in the zones where the vein wedges out, the relationship between gold and silver is disrupted, and the value of the variances sharply decreases.

The change in the correlation coefficient is apparently connected with the different migrational capacity of gold and silver, although the maximum

Fig. 3. Ore shoots along the Medvedevskaya vein

1 — contour lines of the vein relief. Gold contents in arbitrary units: 2 — up to 4 arbitrary units; 3 — from 4 to 10 arbitrary units; 4 — more than 10 arbitrary units; 5 — line of junction with the Glavnaya vein.

their concentrations coincide, but silver varies more smoothly, and its relatively elevated amounts extend over a larger area. Hence, in the pinch-out areas, the association between gold and silver is weak. It is possible that the weakening of the association between gold and silver on the flanks of the vein is due to the fact that here the principal amounts of these elements were deposited during different stages. In places where ores of arsenopyrite composition are predominantly developed—the upper levels of the vein—a significant correlation is established between Au and As \((r = 0.37 + 0.92)\). Here, too, the relationship between gold content and thickness increases.

Attention is drawn to the regular change, down the dip of the ore bodies, in the Au/Ag ratio, which in the pinch-out zones is less than 1, while in rich ores it increases to 2–3. This ratio may be used to estimate predicted reserves for veins explored at different levels by a sparse network of workings. Calculations show that between vein productivity and the Au/Ag ratio there is a linear relationship with a correlation coefficient

\(r = 0.863\). This dependence is expressed by the formula: \(P = 1.46K - 0.94\), where \(P\) is productivity in kg per linear meter of vein, and \(K\) is the Au/Ag ratio.

For two veins, this formula was used to calculate predicted gold reserves. The difference from the actual reserves was 11% and 40%.

Analysis of the structural conditions of localization of ore shoots, and the study of zonality with mathematical processing of sampling data, make it possible to develop criteria for the prospective evaluation of ore bodies and to carry out prospecting and exploration work more effectively.

REFERENCES

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2. Wolfson F. I. Problems in the study of hydrothermal deposits. Publishing House of the Academy of Sciences of the USSR, Moscow, 1953.

3. Gzovskii M. V. The mechanism of formation of complex tectonic ruptures. Razvedka i okhrana nedr, No. 7, 1956.

4. Kh’yulin K. D. Structural conditions of ore deposition. Transactions of MGRI, vol. XI, 1938.

5. Shakhov F. N. Geology of vein deposits. “Nauka,” 1964.