GEOCHEMICAL FEATURES OF THE MAINSKY AND OLKHOVSKY GABBRO-PLAGIOGRANITE COMPLEXES

S. S. Kumeev

(Presented by the scientific seminar of the departments of mineralogy, petrography, and mineral deposits)

The gabbro-plagiogranite type of magmatic formations is very widely distributed both within the Western and the Eastern Sayan. At the same time, the Mainsky complex, distinguished on the northern slope of the Western Sayan, is spatially and genetically associated with spilites and keratophyres of the Lower Cambrian Nizhnemonok Formation and, accordingly, is of Lower Cambrian age [3]. In the zone of the Kizir-Kazyr synclinorium of the Eastern Sayan, the Olkhovsky gabbro-plagiogranite complex is described; it is comagmatic with the effusives of the Upper Cambrian Kizir Formation and has an Upper Cambrian–Ordovician age [2, 4].

Despite differences in the age and position of these structural-facies zones, both magmatic complexes are characterized by a uniform trend in the change of their petrographic composition and chemistry, forming identical series of rock associations: gabbro—quartz gabbro—quartz diorite—granodiorite—tonalite—plagiogranite—adamellite—granite. The principal differences between the formations of the Mainsky and Olkhovsky complexes lie in the features of metallogenic specialization characteristic of each complex: the Mainsky is accompanied by copper mineralization, whereas the Olkhovsky is accompanied by iron-ore and gold mineralization, manifested in some exocontact zones. In addition, minor differences are established in the scales of distribution of intermediate rock types—quartz diorites, granodiorites, and tonalites—which are more widely manifested among the formations of the Olkhovsky complex; in certain features of chemistry—a higher content of free silica in the rocks of the Mainsky complex; and, finally, in the greater diversity of acidic members of the gabbro-plagiogranite series in the Olkhovsky rocks, where, in addition to plagiogranites and adamellites, the frequent presence of biotite, hornblende, and alaskite granites is noted.

The geochemical characterization of the complexes is given mainly for two large massifs—the Malokandatsky massif (Mainsky complex), located in the area of the confluence of the Maly and Bolshoi Kandat rivers, and the Chetsky massif (Olkhovsky complex), located in the basin of the Chet River, a tributary of the Kazyr River.

Table 1 presents data on the distribution of average contents of accessory elements among the rocks of both massifs, based on the results of semiquantitative spectral analysis. For clarity of comparison, the rocks of the complexes are combined into three main groups. The first

group comprises basic rocks, the second intermediate, and the third acidic. The variations in the contents of individual elements within these groups, i.e., their distribution among individualized petrographic varieties, are insignificant, which makes such a grouping possible. The number of analyses of individual elements by rock groups ranges from 40 to 150. For comparison, the table also gives the clarkes according to A. P. Vinogradov, which makes it possible to establish the features of the concentrations of these elements in the rocks of gabbro-plagiogranite complexes.

Table 1

Distribution of accessory elements among rocks of the Mainsky and Olkhovsky gabbro-plagiogranite complexes (in weight %)

As is evident from the table, the distributions of the average contents of the principal trace elements in all three groups of rocks of the Malokandatsky massif are similar. The greatest difference in contents is established for barium, zirconium, copper, and chromium. At the same time, the basic rocks are characterized by concentrations of vanadium, copper, zinc, and chromium, whereas the acidic rocks are characterized by zirconium.

A similar character of the distribution of accessory elements is also found in the rocks of the Chetsky massif, where the basic rocks concentrate vanadium, chromium, and nickel; the intermediate rocks—barium, zinc, and chromium; and the acidic rocks—barium, zirconium, and lead.

In comparison with the clarke contents of the corresponding rock types, the gabbroids of both massifs show a reduced content

of copper, zinc, titanium, chromium, cobalt, nickel, manganese, zirconium, and strontium. The contents of the remaining elements are close to, or equal to, Clarke values. In the group of intermediate rocks, contents below Clarke values are established for zinc, gallium, titanium, chromium, nickel, manganese, zirconium, and strontium. The group of acidic rocks is characterized by reduced contents of zinc, gallium, titanium, vanadium, barium, manganese, yttrium, and strontium. Only the contents of zirconium and copper in the granitoids of the Mainsky complex and molybdenum in the granitoids of the Olkhovsky complex exceed Clarke values. The remaining elements are contained in the acidic rocks of both complexes in amounts close to Clarke values.

A comparison of the distribution of trace elements in the corresponding rock varieties makes it possible to draw the following conclusions:

1. The basic rocks of the Mainsky complex preferentially concentrate copper and zinc, whereas the rocks of the Olkhovsky complex concentrate barium, lead, and nickel.

2. The intermediate rocks of the Mainsky complex contain a greater amount of zinc and chromium; the analogous group of rocks of the Olkhovsky complex contains manganese, barium, and lead.

3. In the Olkhovsky granitoids, higher concentrations of lead and barium are noted, whereas in the Mainsky granitoids higher concentrations of copper, zinc, zirconium, and vanadium are noted.

Table 2

Distribution of accessory elements in the principal rock-forming minerals of the Mainsky and Olkhovsky complexes (in wt. %, average of 2–4 analyses)

Thus, in the rocks of both gabbro-plagiogranite massifs, a uniform pattern is observed in the distribution of the principal elements, which form fairly stable concentrations both relative to Clarke contents and relative to their occurrence in the rock-

...in the corresponding petrographic groups. The individual geochemical features of the complexes consist in comparatively high contents of copper, zinc, and vanadium in the rocks of the Mainsky complex, and of lead and barium in the rocks of the Olkhovsky complex.

A comparison of the concentrations of accessory elements in the corresponding minerals of the acidic rocks of the complexes shows that the plagioclases of the granitoids of the Malokandatsky massif are characterized by smaller amounts of copper, zinc, barium, and zirconium, and by elevated contents of titanium, vanadium, and manganese. The hornblendes of these granitoids, in comparison with the hornblendes of the acidic rocks of the Chetsky massif, concentrate copper, zinc, chromium, cobalt, and zirconium, and contain less lead, titanium, barium, manganese, and scandium. In the potassium feldspars of the Olkhovsky granites the presence of beryllium and a considerable amount of lead is established. Scandium and niobium are noted in the biotites.

From the features of the distribution of the elements it may be concluded that the geochemical similarity of the rocks of the massifs is due to more or less similar amounts of these elements in the principal rock-forming minerals. The elevated contents of copper and zinc in the rocks of the Mainsky complex are associated with their relative enrichment in hornblende, and vanadium with plagioclase. In turn, in the formations of the Olkhovsky complex the following are more readily concentrated: lead in plagioclase; barium in hornblende and potassium feldspar; while molybdenum is associated with accessory molybdenite.

The data presented on the contents of trace elements in the rocks of the Malokandatsky and Chetsky massifs make it possible to conclude not only that there is continuity and kinship in the geochemical features of the rocks composing each gabbro–plagiogranite series, but also that the character of variation and the comparability of the contents of these elements in identical groups of rocks of each massif are of the same type, i.e., that they are geochemically analogous. The latter circumstance, to a certain degree, indicates similar paths of formation of both gabbro–plagiogranite series and the identical character of the processes that determined their origin.

REFERENCES

1. A. P. Vinogradov. Average content of chemical elements in the principal types of igneous rocks of the Earth’s crust. Geokhimiya, No. 7, 1962.

2. G. V. Polyakov, G. S. Fedoseev, A. E. Teleshev, S. M. Nikolaev. The Tsipdinsky pluton of the Olkhovsky granitoid complex (Eastern Sayan). In: Magmatic Formations of the Altai–Sayan Folded Region, Moscow, “Nauka,” 1965.

3. V. N. Smyshlyaev. The plagiogranite intrusive complex of the northern slope of the Western Sayan. In: Magmatic Complexes of the Altai–Sayan Folded Region. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1963.

4. A. D. Shelkovnikov. Principal features of the petrology and metallogeny of the Olkhovsky granodiorite–plagiogranite complex. In: New Data on the Geology of the South of Krasnoyarsk Krai, Krasnoyarsk, 1964.