SOME DATA ON THE DISTRIBUTION AND QUANTITATIVE RATIOS OF ROCKS AND GOLD CONTENT IN MOUNTAIN SHORIA

V. A. SARAEV

(Presented by the scientific seminar of the Department of Petrography)

In this article the author presents the results of measuring the areas of outcrops of rocks of various composition and different ages, and gives a quantitative characterization of the relationship of gold content with magmatism, tectonics, and manifestations of hydrothermal metamorphism in the territory of Mountain Shoria.

Our calculations and the subsequent derivation of coefficients are based on the following assumptions: 1) the areas of outcrops of magmatic rocks in the region reflect the volumes of erupted masses, and consequently the intensity of magmatic processes; 2) the degree of folding of the Riphean–Devonian rocks developed in the region is conventionally taken as identical. At the same time, it must be noted that the degree of folding of the Riphean and Cambrian formations in the region is almost the same; the post-Cambrian formations occupy an insignificant area and occur mainly within the territory of the Telbess synclinorium and the Uimen–Lebed trough. Further, the erosional level of the Devonian and Cambrian intrusions is more or less the same, since almost all intrusive massifs are exposed at the level of skarn formation (more rarely, in the apical part).

A number of researchers (A. M. Kuzmin, V. I. Fominsky, V. P. Studenikin, Yu. G. Shcherbakov, M. Ya. Shlain) have distinguished the following structural-facies zones in the territory of Mountain Shoria: the Biya horst, the Shor massif, the Abakan synclinorium, the Telbess synclinorium, the Uimen–Lebed trough, and the Tashelgino–Kondoma mobile zone. Naturally, only separate parts of these zones, developed in the basin of the upper reaches of the Mrassu, Kondoma, and Lebed rivers, entered into the area under consideration (~9800 sq. km).

The areas of rock outcrops were determined with a planimeter on a medium-scale geological map. Individual small intrusive bodies were determined with the aid of a special grid on a large-scale geological map. All determinations were carried out for each structural-facies zone separately, with calculation of the areas of volcanogenic, plutonogenic, carbonate, and clastic (terrigenous) rocks. In calculating the magmatic formations, their age and composition were taken into account.

Because the areas of gold-bearing rocks (skarns, secondary quartzites, quartzolites, etc.) have not been sufficiently studied, it was impossible to measure them; therefore the author adopted the number of gold manifestations—

...bearing rocks, expressed as percentages. The quantity of gold is also given as percentages, reflecting the known reserves in the placer and primary deposits of Mountain Shoria. The data from measurements and recalculations are summarized in Table 1 and used for constructing the diagrams (Fig. 1).

Fig. 1. A. Areal distribution of rocks in Mountain Shoria.
B. Distribution of rocks by geological structures.
C. Dependence of gold content on $Kef$, $Km$, $Kmg$ and on the number of occurrences of gold-bearing rocks by geological structures.

1. Clastic rocks. 2. Carbonate rocks. 3. Volcanic rocks of the diabase and spilite-diabase formations. 4. Volcanic rocks of the porphyry (andesite-basalt) formation. 5. Volcanic rocks of the orthophyre formation. 6. Volcanic rocks of acidic composition. 7. Intrusive rocks of basic composition. 8. Intrusive rocks of intermediate composition. 9. Intrusive rocks of acidic composition. 10. Variegated unit. 11. Volcanic rocks. 12. Plutonites. 13. Gold-bearing hydrothermalites and metasomatites.
   I — Biya horst + Telbes synclinorium.
   II — Tashelga-Kondoma zone.
   III — Abakan synclinorium.
   IV — Shoria massif.
   V — Uimen-Lebed trough.

For correlation and evaluation of magmatism, tectonics, and gold content (metalliferousness) of the geological structures, the author proposes using the coefficients he derived: effusivity — $Kef$, mobility — $Km$, and magmatism — $Kmg$.

\[ Kef = \frac{Sv}{Sp}, \]
where $Sv$ is the area of volcanogenic rocks,

\[ Sp \]
is the area of intrusive rocks.

\[ Km = \frac{Sm}{S_o}, \]
where $Sm$ is the area of magmatic rocks, $S_o$ is the area of sedimentary rocks.

\[ Kmg=\frac{Sm}{S}\quad \text{or}\quad Kmg=\frac{Sm}{S}\cdot 100\%, \]

where \(S\) is the area of the structure (district).

The coefficient of magmatism is a quantity showing what share of the upper part of the Earth’s crust is made up of magmatic rocks.

The coefficient of mobility (or movability) is a quantity showing the relative mobility of a segment of the Earth’s crust and reflecting the intensity of magmatic rock formation relative to lithogenesis.

The coefficient of effusiveness is a quantity determining how many times the amount of volcanogenic material is greater or less than that of intrusive magmatic material. This quantity reflects the degree of gas saturation of the magmatic chamber and its energy, and the degree of differentiation of magmatic matter into volcanites and plutonites.

The coefficient of magmatism may take values from 0 to 1 (0–100%), while the coefficients of mobility and effusiveness may take values from 0 to 1, 2, 3, etc. Where possible, of course, areas should expediently be replaced by volumes (masses). These indicators, as the author sees it, may be used in metallogenic constructions.

Calculations show that the main mass of gold and occurrences of gold-bearing rocks in Mountain Shoria is located in structures with high mobility (\(Km=3.7—5.25\)), with high values of the coefficient of magmatism (\(Kmg=0.78—0.84\)) and with values of \(Kef=1.67—1.90\). Thus, in the examined parts of the Tashelgino-Kondoma mobile zone and the Abakan synclinorium, 93.6% of all gold reserves are enclosed, with a 78% share in the occurrence of gold-bearing rocks. It is evident from the table that the predominant number of gold-ore deposits was formed in the Salair tectono-magmatic cycle. The greatest gold content is shown by the Riphean-Cambrian volcanogenic formations of the diabase, spilite-diabase, and porphyrite formations, intruded mainly by intrusive massifs of the Lebed tonalite-granodiorite-plagiogranite complex (\(Cm_{2-3}\)).

Indicative for assessing gold content is the coefficient of effusiveness (Fig. 1 v). Thus, values of \(Kef=1.67—1.90\) are characteristic of highly gold-bearing structures. Calculations show that structures composed mainly of volcanites have low gold content. Thus, in the northeastern part of the Uymen-Lebed trough, composed chiefly of Devonian volcanites (\(Kef=13.7\)), occurrences of gold mineralization and gold-bearing placers have not yet been established. However, as studies in volcanic zones (1–4) show, significant potential reserves of one or another element are possible, occurring in dispersed form in volcanites predominantly of basic and intermediate composition, or associated with volcanogenic hydrothermally altered rocks.

Calculations also show that gold content depends first of all on the intensity and duration of the volcanic process (72% of all volcanites are located in zones II and III), and then on processes associated with intrusive magmatism (magmatic replacement, skarn formation, etc.). The processes of intrusive magmatism mainly mobilize the gold contained in volcanites and, in individual areas, contribute to its concentration. Gold mineralization in specific cases depends on the tectonic conditions of the area.

At present it is impossible to determine the productivity of individual ore-bearing formations in Mountain Shoria, since many deposits—

Table 1

Notes:
*) 354 — area of igneous rocks in sq. km;
19.9 — area of igneous rocks, expressed as % of the structure’s area;
3.6 — area of igneous rocks, expressed as % of the entire area of Mountain Shoria.

occurrences of skarns, secondary quartzites, quartzolites, and quartz veins have not been evaluated for gold. In addition, these gold-bearing formations in the region are paragenetically related. This relationship is established especially clearly in the Kondoma iron-ore district, where deposits of skarns and secondary quartzites are located at distances of 0.3–3 km from one another.

The paragenesis of ore-bearing formations is evidently explained by the common magmatic source, which was the source both of volcanic and plutonic rocks and of those magmatogenic solutions that formed both hydrothermally altered rocks (propylites, secondary quartzites, argillizites) and contact-metasomatic formations (skarns, skarnoids, quartzolites, etc.).

The practice of prospecting, survey, and exploration work shows that the most gold-bearing formations in Mountain Shoria are ferruginous skarns, secondary quartzites, and quartzolites with sulfide mineralization and signs of prolonged deformation.

REFERENCES

1. G. M. Vlasov, M. M. Vasilevsky. Hydrothermally Altered Rocks of Central Kamchatka, Their Ore Potential and Patterns of Spatial Distribution. Izd. “Nedra,” 1964.

2. V. N. Kotlyar. The problem of the ore potential of volcanogenic formations. Sovetskaya geologiya, 11, 1967.

3. N. I. Nakovnik. Secondary Quartzites of the USSR. Izd. “Nedra,” 1964.

4. A. A. Onosovskaya. Nonferrous metals (copper, lead, zinc) in volcanogenic-sedimentary complexes of the Altai-Sayan folded region. In: Geology and Metallogeny of Effusive-Sedimentary Formations of Siberia. Tr. SNIIGGiMS, issue 35, “Nedra,” 1964.