DISTRIBUTION OF INDICATOR ELEMENTS IN GOLD-QUARTZ DEPOSITS OF THE KINDAKAN-IROKINDA ORE CLUSTER

(NORTHERN BURYATIA)

L. F. Mitrofanov

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

The gold-ore quartz veins of the deposits of the Kindikan-Irokinda ore cluster are located within the area of development of metamorphic rocks composing a tectonic uplift of the Archean basement, known as the South Muya block [8]. The geological structure of this block includes gneisses of various composition (garnet-pyroxene, garnet-amphibole, amphibole-pyroxene paragneisses and garnet-mica gneisses), calciphyres, metamorphosed limestones, and amphibolites. The rocks of the metamorphic complex are intruded by numerous dikes of Archean plagiogranites and Mesozoic dikes of diorites and felsite-porphyries [7]. The gold-ore mineralization of the district is genetically associated with the latter (N. P. Andreev, A. A. Gamchan, 1959). Structurally, all ore bodies are localized in tectonic fractures of northeastern, northwestern, and sublatitudinal (70–80°) strike, branching from the Irokinda fault, which borders the South Muya block on the west, separating it from the Proterozoic frame [6].

The ore bodies are composed of quartz by 90–95% (in some areas, by 100%). Among other vein minerals, calcite and ankerite are present. Sericite and chlorite (delessite) are noted in the composition of some quartz veins. Ore minerals are represented mainly by sulfides (their content usually does not exceed 1%): galena, pyrite, sphalerite, chalcopyrite, chalcocite, and arsenopyrite. In significant quantities in the upper levels of some industrial ore bodies, fahlores are present. Native minerals are represented by gold and silver [4]. According to the indicated mineral composition, the gold-ore deposits of the ore cluster described are classified as low-sulfide [6, 7].

In the material composition of the ores, copper, zinc, lead, cobalt, nickel, chromium, zirconium, barium, and strontium occur as impurities. In the upper levels of individual veins, antimony and arsenic are present. Tungsten, molybdenum, and silver sometimes appear. Among all these elements accompanying gold mineralization, it was necessary to identify indicator elements. For this purpose, metallometric sampling was carried out (sample weight 150–200 g) in the form of dashed grooves, separately along the ore bodies and separately along the enclosing hydrothermally altered and unaltered rocks. All metallometric samples were subjected to approximate quantitative spectral analysis for 32 elements. The analyses were performed by spectroscopic engineers

TsLG of the Ural Geological Administration, G. M. Podzhivalova, V. G. Bessonova, and N. F. Babikova.

As A. A. Saukov and other researchers [4, 9] indicate, determination of the natural background or local normal field of those elements being prospected in an area is of great importance for identifying anomalies and dispersion halos. Yanishevsky et al. [3, 4, 10] termed the natural background or local normal field the geochemical background. Below the term geochemical background (abbreviated GB) is used; to determine it, metallometric samples were taken from all varieties of rocks composing the territory of the ore cluster (40–50 samples for each variety). The indicator elements for the Kindikan-Irokinda ore cluster are zinc, lead, and copper, whose distribution in the rocks of the cluster follows a normal, less often a lognormal, law. The distribution law was determined mainly graphically, using Razumovsky’s probability template [1]. The dispersion halos of zinc and copper are the clearest and very narrow; those of lead are somewhat wider, but also very distinct. The geochemical backgrounds (Table 1) of the indicated elements were calculated as the arithmetic mean and determined from Razumovsky’s cumulate [1].

Table

Geochemical backgrounds of copper, lead, and zinc in the rocks of the Kindikan-Irokinda ore cluster

Below is a brief description of the dispersion halos of the indicator elements.

Zinc around the gold-ore quartz veins of the Kindikan-Irokinda ore cluster has a very narrow dispersion halo, which, as a rule, is confined directly to the ore bodies and to zones within them with the highest sulfide content, especially sphalerite. In individual cases, a slight increase (1.5–2 GB) is observed around small zones of disturbed rocks (intensely laminated, often with clay gouge); moreover, they are more often observed in the footwalls of the zones (Fig. 2). It should be noted that the zinc content in altered rocks on the hanging-wall side of ore bodies decreases with distance from the latter in comparison with the GB for the corresponding unaltered rocks (Figs. 1, 2), and in individual cases is completely absent (sample 8, Yurasovskaya vein, Fig. 1).

The distribution of the average zinc content along verticals, both in the rocks hosting the studied veins (Serebryakovskaya, Yurasovskaya, and Petrovskaya) and directly in the ore bodies, is variable. For example, along the Serebryakovskaya vein (Figs. 3, 4), the zinc content in the host rocks at the surface is close to the GB; at the hori-

![Graphical profiles of trace-element distribution near veins]

Fig. 1. Graphs of the distribution of trace elements near the Yurasovskaya vein (adit 8).
1. Beresites. 2. Garnet-pyroxene paragneisses. 3. Garnet-micaceous rocks. 4. Amphibole and amphibole-pyroxene paragneisses. 5. Disturbance zone. 6. Ore body.

Legend: Zn in g/t, 0.002%; Pb in g/t, 0.001%; Cu in g/t, 0.002%; Co in g/t, 0.0005%; Ag in g/t, 0.001%.

![Graphical profiles of trace-element distribution near vein]

Legend: Zn in g/t, 0.002%; Cu in g/t, 0.002%; Pb in g/t, 0.001%; Ag in g/t, 0.001%; Co in g/t, 0.0005%; Sb in g/t, 0.005%; Au in g/t, 0.0005%.

Fig. 2. Graphs of the distribution of trace elements near the Serebryakovskaya vein (adit 11). Symbols as in Fig. 1.

At the level of adit 10 it is much lower than the GF; farther down, with depth, its content increases noticeably and at the level of adit 12 approaches the GF for amphibole and amphibole-pyroxene paragneisses. The average zinc content in the enclosing rocks on the footwall side of the vein at the surface and at the level of adit 12 is somewhat higher than in the rocks of its hanging wall, whereas at the levels of adits 10 and 11, on the contrary, it is higher in the rocks of the hanging wall. Further, if in the rocks on the footwall side, at the middle levels, a significant decrease in the average zinc content is observed, and at the surface and at the lower level (adit 12) an increase, then in the rocks of the hanging wall it practically does not change, although a slight decrease with depth is also noted. In the ore body of this vein the average content decreases markedly with depth and at the last level is equal to 0.5 GF. In the hydrothermally altered rocks enclosing the Petrovskaya and Yurasovskaya veins, a clear decrease in the average zinc content with depth is also observed. Directly along the ore body of the Yurasov—

in Serebryakovskaya (Fig. 5) a sharp decrease in zinc contents with depth is noted, while along the Petrovskaya vein (Fig. 6), at the level of adit 1, the average content of this metal is approximately 4 GF; at the level of adit 2 it is slightly more than GF; at the level of adit 3, 0.5 GF; and at the lowest level—adit 5—it again increases and reaches 1.5 GF.

Thus, in the distribution of zinc in the vertical section a certain regularity is observed which, as is clear from the above, differs for all three ore bodies studied.

Lead, in comparison with zinc, has a broader dispersion halo. Its geochemical background in the rocks of the ore cluster, as can be seen from Table 1, fluctuates within the Clarke value, while in the dispersion halos around the Yurasovskaya and Serebryakovskaya veins (Figs. 1, 2) the lead content is much higher than the GF and usually reaches 4–5 GF, and in individual cases 10–15 GF (Serebryakovskaya vein, Fig. 2). The maximum contents of zinc and lead usually coincide, but the reverse also occurs; for example, in the beresitized rocks enclosing the Serebryakovskaya vein, a very high lead content is noted, while zinc is appreciably below the GF (Fig. 2).

The distribution of lead in vertical sections through the ore bodies studied is varied. For example, in the rocks enclosing the Serebryakovskaya vein (Fig. 3), its content at the surface reaches 5–6 GF; at the level of adit 10 it drops sharply to 1–1.5 GF; and from the level of adit 11 it rises sharply, reaching 11 GF at the level of adit 12. The distribution of lead in the enclosing rocks on the footwall and hanging-wall sides is approximately the same at all levels. It is true that at the upper levels beneath the vein its content is somewhat higher than above it, while at the level of adit 12 the opposite relation is observed (Fig. 3). Along the Yurasovskaya II vein the distribution of lead is more uniform (3.5–5.1 GF). Along the Petrovskaya vein, in the three upper levels, no change in lead content is observed in the enclosing rocks, and it corresponds to the GF for garnet–pyroxene paragneisses. At the lowest level—adit 5—it increases to the GF (Fig. 6).

Fig. 3. Graphs of the change with depth in average contents of indicator elements in halos and in the ore body (Serebryakovskaya vein). 1. In the rocks of the footwall side. 2. Hanging-wall side. 3. In the ore body. 4. Overall for the enclosing rocks. 1. Lead. 2. Zinc. 3. Copper.

In the distribution of lead within the Serebryakovskaya and Yurasovskaya veins themselves, a tendency is observed, vertically, toward a decrease in its average content with depth. In the Serebryakovskaya vein a peak is detected at the adit 10 level, approximately equal to 10 GF, while by the adit 12 level the lead content decreases to 1 GF (Fig. 3). In the Yurasovskaya vein the decrease in the content of this metal with depth is insignificant, reaching approximately 4 GF at the adit 8 level, as against 6 GF at the adit 6 level (Fig. 5).

In addition to the above-described dispersion halos of the indicator elements lead and zinc, their indicator ratio was also studied for the ore bodies investigated [2]. To calculate the latter, the average contents of these elements at the corresponding levels were used. In the Serebryakovskaya vein, the indicator ratio of lead to zinc contained in the enclosing rocks distinctly increases with depth. If at the surface this ratio is equal to 0.865, then already at the following levels it is greater than unity and is, respectively, 1.115 at adit 10, 1.231 at adit 11, and 2.294 at adit 12. Such a steady increase in the indicator ratio is characteristic of the rocks enclosing the vein as a whole. At the same time, for the rocks of the footwall, a tendency is also established for the indicator ratio to increase with depth, but its graph (Fig. 4) is characterized by the presence of

Fig. 4. Graphs of variation with depth. 1 — sums of indicator elements, II — indicator ratio of lead to zinc (Serebryakovskaya vein). 1. In footwall rocks. 2. In hanging-wall rocks. 3. Along the ore body. 4. Overall for the enclosing rocks.

large peaks. This influence is determined by the fact that zinc, probably distributed uniformly along the vertical in the under-ore rocks, was leached from them, whereas lead was introduced.

This judgment is based on a comparison of the geochemical backgrounds of zinc and lead (respectively 0.008 and 0.0012—0.002) with the average contents of these metals in the over-ore hydrothermally altered rocks, respectively equal to 0.004—0.006% and 0.003—0.014%. Along the ore body there is a distinct, steady decrease in the indicator ratio with depth, from 1.818 at the surface to 0.487 at the adit 12 level (Fig. 4). Such a clear decrease of this ratio along pa-

of the Serebryakovskaya vein is explained by the fact that in this same direction there is a more intense decrease in lead contents in comparison with zinc. At the same time, the total content of both elements also decreases with depth.

A completely different picture is observed for the Yurasovskaya and Petrovskaya veins (Figs. 5, 7). In the rocks enclosing the Yurasovskaya vein,

Fig. 5. Graphs of variation with depth (Yurasovskaya vein): 1. Indicator elements by ore body. 2. Their sums. 3. By enclosing rocks of the footwall and hanging wall. 4. Indicator ratios of lead to zinc. Symbols as in Figs. 3 and 4.

as a whole, and also separately in the hanging-wall and footwall rocks, there is a sharp increase in the indicator ratio with depth, which is expressed on the graphs (Fig. 5) by almost straight lines. Directly within the ore body, however, the indicator ratio decreases with depth from 4.133 at the level of adit 6 to 2.05–2.263, respectively, at the levels of adits 7 and 8 (Fig. 5). For the Petrovskaya vein, as well as for its enclosing hydrothermally altered rocks, a general tendency toward an increase with depth in the indicator ratio of lead to zinc is characteristic, but the graphs (Fig. 7) are more broken than for the Yurasovskaya vein. The regularity revealed for the Yurasovskaya vein is explained by a significant decrease with depth in the average zinc content and an insignificant decrease in lead; for the Petrovskaya vein, by a decrease in zinc contents and a sharp increase in the amount of lead at the very lowest level.

Copper. The dispersion halo of copper, like that of zinc, is very narrow and is localized in the immediate vicinity of the ore body, usually confined to the hydrothermally altered rocks of the footwall. The maxi-

Fig. 6. Graphs of variation with depth of indicator elements (Petrovskaya vein). I — by ore body; II — by enclosing rocks. Symbols as in Fig. 3.

Fig. 7. Graphs of the variation with depth of the indicator ratio of lead to zinc along the Petrovskaya vein. Symbols as in Fig. 4.

minimal contents of it, as a rule, coincide with the maxima of zinc and lead. In the vertical distribution of copper no significant fluctuations are observed. Thus, along the Serebryakovskaya and Petrovskaya veins the copper content in the enclosing rocks, both as a whole and by the rocks of the hanging and footwall sides, is somewhat higher on the lower horizons (Figs. 3, 6), whereas along the Yurasovskaya vein it is higher on the upper horizons (Fig. 5). Directly in the ore bodies studied, however, there is a clear decrease in the average copper contents with depth.

From the materials presented, several conclusions may be drawn.

1. The dispersion halos for the indicator elements differ: for zinc and copper they are narrow and are confined to sections in the ore bodies with a significant concentration of sulfides; for lead they are broader. The maximum concentrations of these elements, as a rule, coincide.

2. The average content of the indicator elements down dip in the Yurasovskaya and Serebryakovskaya veins tends to decrease; moreover, the contents of zinc and copper in the indicated veins are similar, while lead in the Yurasovskaya vein is considerably higher than in the Serebryakovskaya vein on all horizons. In the Petrovskaya vein, on the three upper horizons, an analogous redistribution of these elements is observed, while on the fourth horizon their average content increases sharply.

3. Along the Petrovskaya and Yurasovskaya veins the graphs of the indicator ratios of lead to zinc have a broken character, but with a clear tendency for their value to increase on the lower horizons; along the Serebryakovskaya vein it decreases sharply and steadily.

4. The revealed zonality in the distribution of the indicator elements and the increase in the indicator ratio of lead to zinc on the lower horizons may indicate the prospectivity with depth of the Petrovskaya vein and, partly, the Yurasovskaya vein, and the absence of such prospectivity for the Serebryakovskaya vein.

LITERATURE

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2. I. I. Ginzburg, G. I. Rossman, K. M. Mukanov, I. V. Borisevich, Yu. V. Goltsman, I. B. Ivanov, I. V. Chernyshev. Application of Geochemical Methods in Metallogenic Studies of Ore Districts. Moscow, “Nedra,” 1966.

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