Proceedings of the Tomsk Order of the October Revolution
and Order of the Red Banner of Labor
Polytechnic Institute named after S. M. Kirov

Vol. 289 1976

SOME FEATURES OF THE BEHAVIOR OF URANIUM AND THORIUM IN VOLCANOGENIC FORMATIONS OF THE NORTHEASTERN MOUNTAIN FRAMING OF THE MINUSINSK DEPRESSION

Yu. A. Fomish

(Presented by Professor A. M. Kuzmin)

At present, considerable attention is being paid to the geochemistry of uranium and thorium in the magmatic process. At the same time, the problem of the distribution of these elements in erupted rocks characterized by different conditions of formation and different composition remains topical. The overwhelming majority of concrete examples in this field relate to manifestations of plutonic magmatism, chiefly granitoid, to a lesser extent alkaline, and in rare cases of basic and ultrabasic composition. Information on the distribution of radioactive elements in volcanogenic formations, however, is comparatively sparse and often fragmentary in character [1—5, 7—10, 13—15, 19—21].

For the products of Devonian volcanism of the Minusinsk intermontane depressions, studies on this question have appeared only recently. Among them one should note the investigations of V. P. Kovalev [7, 8], concerning the effusives of the South Minusinsk depression, for which an affiliation mainly with the alkaline-earth series and, to a much lesser degree, with the alkaline series has been established. In the former, enrichment in uranium and thorium occurs in the youngest and most acidic varieties of rocks; moreover, uranium accumulates in them preferentially over thorium. For the effusives of the alkaline series, a predominant accumulation of thorium is observed.

In the present article, features of the behavior of uranium and thorium in volcanogenic formations of the northeastern mountain framing of the North Minusinsk depression (right bank of the Yenisei River) are analyzed.

Many works have been devoted to the study of the volcanogenic Devonian of the Minusinsk depressions and their mountain framing. It was carried out most fully and systematically by I. V. Luchitskii [11], A. A. Mossakovskii [12], and several others. Of interest, in our opinion, is A. A. Mossakovskii’s conclusion regarding the wide development, within the mountain uplifts bounding the depressions, alongside basic—alkaline effusives, of acidic varieties of rocks, which, in his view, form a genetically independent formation.

The alternation of volcanic products varied in composition, represented, on the one hand, by basic alkaline and, on the other hand, by acidic lavas and tuffs, is a characteristic feature of the area studied. In general, the section of the effusive sequence here is as follows. Its lower part is characterized by the development of basaltic, andesite-ba-

of basaltic and, in rare cases, andesitic porphyrites, forming sheet-like bodies or flows. Higher in the section there occur ignimbrites, ash and lithic-crystal-clastic tuffs, and more rarely lavas of dacitic, liparite-dacitic, and liparitic composition. The acid effusives, in turn, are overlain by basaltic and andesite-basaltic porphyrites and their tuffs, everywhere alternating with horizons of tuffaceous-sedimentary deposits (the latter sometimes quantitatively predominate). Up the section the basaltoids are replaced by lavas and tuffs of liparitic, liparite-dacitic, and, characteristically, trachy-liparitic composition. Spatially associated with the late basaltoids are phonolite-porphyries and alkaline trachytic porphyries, which have a limited areal distribution. They also complete the section of the sequence.

For the basaltoids, whose formation is apparently connected with a fissure type of volcanism, identification of rocks of subvolcanic facies is extremely difficult. On the other hand, the formation of liparite-dacitic porphyries and their tuffs is to a considerable extent due to the activity of central-type volcanoes, the remnants of which are detected comparatively easily. Also not uncommon are subvolcanic bodies of alkaline effusives.

Analysis of the chemical composition of the effusives in this region makes it possible, following A. A. Mossakovskii, to distinguish among them two genetically independent associations: basalt-phonolitic and dacite-liparite-trachyliparitic. The first association is characterized chiefly by the basaltoid composition of the lavas and the close connection with them of alkaline rocks. These principal features were noted earlier by I. V. Luchitskii [11]. Petrochemically, the rocks of the formation are characterized by a normal composition, undersaturation with silica, and a clearly expressed sodium specialization. In contrast to them, as is evident from Table 1, the effusives of the liparitoid association belong to the plumasitic series, are always supersaturated with silica, and among the alkalis potassium predominates in them.

The differences in the chemistry of the two volcanic associations are clearly visible from the ternary variation diagram (Fig. 1), which also makes it possible to judge the course and character of the differentiation of two initial magmas: basaltoid (tholeiitic) and granitoid. The latter, according to Yu. M. Sheinman [18], is of crustal origin. In both cases the evolution of the magma leads to accumulation of alkalis, but if in the first case there is a simultaneous increase in sodium and potassium, in the second only the sodium content increases.

The main regularities in the distribution of uranium and thorium in them correspond to the petrochemical features of the effusives of both associations. It follows from Table 2 that the silica-supersaturated effusives of the potassium series are enriched in radioactive elements in comparison with basaltoids and the alkaline rocks associated with them. Another characteristic feature of the acid effusives, following from comparison of the values of “standard deviations” from the average contents of uranium and thorium, is the relatively uneven distribution of both elements in them.

The behavior of the elements studied during the evolution of both magmas is somewhat different.

For rocks of the basaltoid series, accumulation of uranium and thorium during differentiation of the melt is characteristic. At the same time, judging by the change in Th/u at the early stages of the process, uranium proved to be more active. And only in the extreme alkaline differentiates—phonolite-porphyries—is preferential accumulation of thorium observed. In these formations there is noted, on the one hand, a negative correlation of uranium and thorium with calcium and magnesium and, on the other hand, a distinct positive correlation of them

Fig. 1. Variation diagram of the volcanogenic rocks of the district. Solid lines show changes in the contents of \((0.9F_2O_3+FeO+MnO)\), MgO, and \((Na_2O+0.7K_2O)\); dashed lines show changes in the contents of \(K_2O\), \(Na_2O\), and CaO. Filled circles show rocks derived from basaltic magma: 1. Basalt porphyrites (17 analyses). 2. Andesite-basalt porphyrites (9 analyses). 3. Andesite porphyrites (4 analyses). 4. Trachyte porphyrites (4 analyses). 5. Phonolite porphyries (4 analyses). Open circles show rocks derived from granitoid magma. 6. Dacite porphyries (7 analyses). 7. Liparite porphyries (10 analyses). 8. Liparite-dacite porphyries of increased alkalinity (36 analyses). 9. Trachyliparite porphyries (6 analyses).

with alkalis; moreover, apparently, uranium is more closely associated with potassium, and thorium with sodium, as has already been pointed out more than once for magmatic rocks [6, et al.]. A comparison of the curves for \(SiO_2\) and Th/u also confirms the presence of a close connection of silica primarily with uranium, since, as long as enrichment of the rocks in \(SiO_2\) took place, uranium accumulated in them to a large extent. The relationship between the radioactivity of the rocks of this association and their ferruginosity is distinctive. In basaltic and andesitic porphyrites, uranium and thorium are linked with total iron by a distinct negative correlation, whereas in alkaline effusives this correlation is positive.

Calculation of the uranium contents in varieties of phonolite porphyries enriched and depleted in femic minerals confirmed this conclusion: in melanocratic phonolite porphyries the uranium concentration is higher \((5.54\cdot10^{-4}\%\) from 5 determinations) than in their leucocratic varieties \((4.34\cdot10^{-4}\%\) from 9 determinations).

In effusives of the liparite series there is also an increase in the contents of uranium and thorium from their early formations to later ones, which have somewhat increased alkalinity, mainly due to sodium. At the same time, the fact of a certain increase in Th/U is of interest, indicating a more intensive accumulation of thorium in the rocks.

According to available data [4], porphyritic segregations of various

Table 1

Mean contents (\(\overline{X}\)) and standard deviations (\(S\))
of rock-forming oxides (wt. %) in Devonian volcanogenic rocks*

The determinations were made at the Central Chemical Laboratory of ZSGU (Novokuznetsk).

* In Tables 1 and 2 read as follows:

1. Early basaltoids.
2. Early ignimbrites, tuffs, and lavas of liparite–dacite porphyries.
3. Late basaltoids.
4. Late liparite–dacite and trachyliparite porphyries and their tuffs.
5. Alkaline trachyte porphyries, phonolite porphyries.

The number of analyses is indicated in parentheses.

Table 2

Mean contents and standard deviations of uranium and thorium
in Devonian volcanogenic rocks

Note. Uranium was analyzed by the luminescence method; thorium was determined by the photocolorimetric method using the “Arsenazo III” dye. The determinations were made in the laboratory of the Institute of Geology and Geophysics, Siberian Branch, USSR Academy of Sciences.

minerals in basaltoids differ by reduced uranium contents compared with the same minerals of the groundmass. Our data (Table 3) confirm this conclusion both for basaltoids (porphyritic segregations of plagioclase and monoclinic pyroxene) and

Table 3

Content of uranium (\(n \cdot 10^{-4}\%\)) and thorium (\(n \cdot 10^{-3}\%\)) in effusive rocks,
enriched (1) and depleted (2) in porphyritic segregations

for liparite porphyries (phenocrysts of orthoclase, albite). Thus, a large part of the uranium and thorium in the effusives is concentrated in the groundmass of the rocks.

The results of analysis of uranium contents in products of volcanism, depending on their facies affiliation, are given in Table 4.

Table 4

Distribution of uranium in effusives depending on their facies affiliation

In particular, there are data on the uranium content in rocks of various facies of one of the paleovolcanoes of liparitic composition, described by us earlier [17]. Judging from the figures obtained, the effusives of the vent and sheet facies practically do not differ in this respect. The pyroclasts of the eroded ancient cone are characterized by a somewhat reduced concentration of the element.

A different picture is observed for alkaline effusives. The distribution of uranium here was traced on the example of a well-exposed laccolith-like body of phonolite-porphyries, representing a transition from rocks of the proper subvolcanic facies to sheet formations. Comparison of uranium contents in the former and the latter shows enrichment of the marginal parts of the body in it. Naturally, the data given in Table 4 characterize particular cases; on the whole, however, the problem remains open.

Conclusions

1. In the area studied there is an alternation of effusives of two genetically independent associations: basalt-phonolite and dacite-liparite-trachyliparite, which are products of two parental magmas.

2. Acid effusives are characterized by elevated contents of radioactive elements. On average, their uranium content is \(5.50 \cdot 10^{-4}\%\), thorium \(3.35 \cdot 10^{-3}\%\), Th/U — 6.1, whereas for rocks of the basaltoid series these figures are lower: for basalt and andesite porphyrites, respectively, \(2.07 \cdot 10^{-4}\) and \(0.96 \cdot 10^{-3}\), Th/U — 4.6, and for alkaline formations — \(4.22 \cdot 10^{-4}\) and \(1.45 \cdot 10^{-3}\), Th/U — 3.4. Another distinctive feature of liparitoids is the relatively uneven distribution in them of both elements.

1. In rocks of both associations, the accumulation of U and Th occurs in the final products of melt differentiation. For rocks of the basaltoid series, uranium is more active in the early stages of the process, and only in phonolite-porphyries does thorium accumulate more intensively. In liparite porphyries, thorium is relatively more active.

2. A negative correlation of U and Th in the rocks with calcium and magnesium is observed, and a positive correlation with the alkalis; moreover, uranium is more closely associated with potassium, and thorium with sodium. Uranium is more closely associated with silicic acid (positive correlation).

3. The observed relationship between the structure of the effusives and the content of radioactive elements in them consists in an increase in the concentrations of U and Th in rock varieties depleted in porphyritic phenocrysts.

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