DISTRIBUTION OF URANIUM AND THORIUM IN VOLCANOGENIC-INTRUSIVE ROCKS OF THE NORTH MINUSINSK DEPRESSION

(BASED ON THE EXAMPLE OF ONE CENTRAL-TYPE PALEOVOLCANO)

Yu. A. Fomin

(Presented by the scientific seminar of the Departments of Mineralogy, Petrography, and Mineral Resources)

Published information concerning the distribution of radioactive elements in related magmatic rocks of identical composition but formed under different facies conditions is sparse and contradictory. In the literature, the contents of U and Th in intrusive and effusive formations are compared chiefly. According to some researchers [2, 4, and others], the contents of U and Th in effusives are somewhat higher than in analogous intrusive rocks. This is contradicted by the materials of V. I. Rekharskii, O. V. Krutetskaya [1], and A. Ewart and J. Stipp [5]. In particular, V. I. Rekharskii and O. V. Krutetskaya point to the fact that intrusive rocks of acidic, intermediate, and basic composition in the Northern Tien Shan are distinguished by elevated uranium contents in comparison with effusives.

In the present article, data are given on the distribution of U and Th in the rocks of a complex volcanogenic-intrusive series of the northeastern mountain framing of the North Minusinsk Depression, using as an example one of the central-type paleovolcanoes. In the structure of the paleovolcano [3] there participate formations of the effusive proper and vent facies of volcanites, as well as facies of hypabyssal intrusions. In composition, the effusives composing the edifice belong to two independent associations: liparitic and basaltoid. Quantitatively, the former sharply predominate. Their effusive facies proper is represented by earlier pyroclastics (ash, crystal- and litho-crystal-clastic tuffs) and later lavas with subordinate development of tuffs. The lower parts of the essentially lava portion of the section are composed of liparitic and trachyliparitic porphyries and their tuffs; the upper parts—of felsite-porphyries and their tuffs.

The vent formations are spatially confined to the pyroclastics. They include liparitic porphyries (quartz albitophyres) and younger quartz porphyries. According to petrographic and petrochemical features, the former correspond to the sheetlike liparitic porphyries, the latter to felsite-porphyries. In addition, a dike-like body of eruptive tuff breccias is associated with the contact of one of the necks of quartz porphyries.

Effusives of the basaltoid series are represented by basaltic and andesitic porphyrites, forming a group of sheets that separates the early pyroclastics from the later lavas of acidic composition. The basaltoid sheets alternate with sedimentary deposits, testifying—

Table 1

Distribution of U and Th in volcanogenic-intrusive rocks of the paleovolcano
(the denominator indicates the number of determinations)

indicating a certain waning of volcanic activity at that time. Effusive formations within the edifice are cut by intrusive dikes, whose position is determined predominantly by ring fractures. This fact indicates a somewhat younger age of the dikes in comparison with the volcanics. The composition of the dikes is acidic (quartz microsyenites and syenite-porphyries) or intermediate-basic (diorite-porphyrites, syenite-diorites). Petrochemically, the former are identical to liparitoids, and the latter to basaltoids.

The distribution patterns of U and Th in the effusive and intrusive rocks listed above were clarified on the basis of determinations of the partial contents of U in 144 samples and Th in 63 samples. Uranium was analyzed by the luminescence-pearl method; thorium was determined photocolorimetrically with the dye “Arsenazo III.” The analyses were performed in the laboratory of Geochemistry of Endogenic Deposits of the IGiG, Siberian Branch, USSR Academy of Sciences (analysts G. M. Melgunova and Z. V. Malyasova). Data on the distribution of radioactive elements in the rocks of the paleovolcano are summarized in Table 1.

The highest concentrations of U and Th are observed in volcanogenic rocks of acidic composition (the average U content is \(6.0 \cdot 10^{-4}\%\), Th—\(31.3 \cdot 10^{-4}\%\), Th/U—5.2). Moreover, the contents of U and Th in the early pyroclasts and in the later, essentially lava products of volcanism (including vent formations) are almost equal and very close to the average values. Within the pyroclasts, ash tuffs are enriched in uranium and thorium, whereas their crystallo- and lithocrystalloclastic varieties are depleted—evidently owing to reduced concentrations of these elements in lithoclasts and crystal clasts of feldspars. At the same time, the thorium-uranium ratio changes little. In lava products, elevated contents of U and Th are observed in vent formations. The same regularity is revealed when comparing, as a whole, the rocks of the effusive proper and vent facies. In the former, the concentrations of U and Th, as well as the Th/U value, are somewhat lower than the average values and amount, respectively, to \(5.8 \cdot 10^{-4}\%\); \(30.1 \cdot 10^{-4}\%\); 5.2. In rocks of the vent facies these values are higher: \(6.3 \cdot 10^{-4}\%\); \(35.0 \cdot 10^{-4}\%\); 5.6.

In addition, a regularity is observed in the distribution of U and Th in lavas of liparitic composition through time, namely: enrichment of the later formations (both sheet and vent) in radioactive elements. Simultaneously, the thorium-uranium ratio increases.

Intrusive rocks of acidic composition (quartz microsyenites and syenite-porphyries) are characterized by reduced concentrations of U (\(4.3 \cdot 10^{-4}\%\)) and especially Th (\(14.4 \cdot 10^{-4}\%\)). The thorium-uranium ratio decreases to 3.4.

Rocks of the basaltoid series are distinguished by a comparatively low content of radioactive elements. The amount of uranium in basaltic porphyrites averages \(1.2 \cdot 10^{-4}\%\). In andesitic porphyrites this value is somewhat higher (\(2.6 \cdot 10^{-4}\%\)). The average Th content in basaltoids is \(4.2 \cdot 10^{-4}\%\). Th/U for them averages 1.8. In dike diorite-porphyrites and syenite-diorites the contents of U and Th are markedly lower (respectively \(1.5 \cdot 10^{-4}\%\) and \(2.3 \cdot 10^{-4}\%\); Th/U—1.5).

Thus, for related rocks of the liparitic and basaltoid series within the studied edifice, the concentrations of radioactive elements are higher in volcanics, whereas intrusive formations are depleted in them. The latter are also characterized by reduced values of the thorium-uranium ratio.

In volcanogenic formations (of acidic composition), U and Th are distributed unevenly. The vent facies of effusives are enriched in these elements. In the early pyroclasts, variations in U and Th contents are associ-

...probably with their textural-structural features. Overall, this sequence, composed of almost undifferentiated material, is characterized by U and Th concentrations close to average values. The distribution of radioactive elements in the subsequent lavas is governed by differentiation of the melt, with enrichment in uranium and, especially, thorium in the younger products.

REFERENCES

1. V. I. Rekharskii, O. V. Krutetskaya. Uranium in the rocks of the southwestern spurs of the Northern Tien Shan. Izv. AN SSSR, ser. geol., No. 7, 1961.

2. A. A. Smyslov. Radioactive elements in the erupted rocks of Northern Kazakhstan. Geokhimiya, 1958, No. 3.

3. Yu. A. Fomin, V. G. Krylov, V. A. Gavrilenko. On an ancient volcanic apparatus in the southeastern spurs of the Solgon Ridge. Proceedings of the scientific conference of young scientists of Tomsk higher educational institutions, vol. I, section of natural sciences, Tomsk State University Press, Tomsk, 1968.

4. Adams J. S. A., Osmond J. K., Rogers J. J. W. The geochemistry of thorium and uranium. Phys. and Chem. of the Earth, 3, 1959.

5. Ewart A., Stipp J. J. Geochim. et cosmochim. acta, 32, No. 7, 1968.