TRACE ELEMENTS IN THE CAMBRIAN–PROTEROZOIC SPILITE–DIABASE FORMATION OF THE SOUTHWESTERN MARGIN OF THE KUZNETSK ALATAU

L. V. PESHEKHONOV

(Presented by the Interdepartmental Scientific Seminar)

In the southwestern spurs of the Kuznetsk Alatau, in the area of the settlements of Kamzas, Talon, and Taimet, over a considerable area there are developed volcanogenic formations that we assign to the Cambrian–Proterozoic spilite–diabase formation. The spilite–diabase formation is characterized by a variegated composition; along with volcanogenic rocks, sedimentary deposits are also developed in it. Among the sedimentary rocks, siliceous shales are widespread, usually replacing lava formations along strike. Carbonate and terrigenous sediments are less developed; in sections along the rivers Aryk, Kamzas, Kommunza, and Malaya Khairyuza they alternate with volcanogenic rocks, which indicates their formation during stages of relative waning of volcanism.

The volcanogenic formations proper were manifested mainly as covers and flows of lava of basic composition, the eruptive products of which are represented by a number of petrographic varieties of porphyrites. Among them, albitized basaltic and diabase porphyrites are typical; spilites are of minor development. Pyroclastic formations, represented by tuffs, tuff-breccias, and lava breccias, are of substantial importance in the composition of the formation; alternating with the lava products of volcanism, on the watershed of the Bolshaya Khairyuza and Kommunza rivers they become predominant.

This article discusses certain features of the distribution of trace elements in the volcanogenic formations of the spilite–diabase formation. Determination of trace elements was carried out by the method of semiquantitative spectral analysis in the spectral laboratory of the Complex Thematic Expedition of the West Siberian Geological Administration and in the spectral laboratory of the Geological Prospecting Faculty of the Tomsk Polytechnic Institute. All the below-presented re—

The conclusions and constructions were made on the basis of the arithmetic mean contents and an assessment of the frequency of occurrence of trace elements from analyses of 207 samples. In order to reveal the features of the distribution of trace elements among the volcanogenic rocks of the formation, we compiled selections of their contents in the principal types of volcanogenic formations that are most widely developed.

In the first group of volcanogenic rocks we included albitized diabases and diabase porphyrites; in the second—albitized pyroxene–plagioclase and plagioclase–pyroxene basaltic porphyrites; and in the third—rocks of pyroclastic origin. It was assumed here that, owing to the observed regional character of albitization, it was reflected to an equal degree in changes in the material composition of the identified principal rock types and in the concentrations of trace elements in them. Therefore, the ratio of the contents of trace elements in the albitized volcanogenic formations reflects the primary ratio of trace-element contents in the non-albitized effusive and pyroclastic rocks of the formation.

Before proceeding to consider the features of the distribution and content of trace elements, it should be noted that such elements as Sn, Ag, Pb, As, and Mo occur in single samples; we attribute them to superimposed, later hydrothermal processes and do not consider them further.

The determination of arithmetic mean contents and the assessment of the frequency of occurrence of trace elements in albitized diabases and diabase porphyrites were carried out from analyses of 62 samples. In these rocks the trace elements, in decreasing order of abundance, are arranged in the following series: Ni, Co, Cu, V, Mn, Ti, Ga (100); Cr, Zr (93.5); Zn (98.1); Sr (85.7); Ba (47.8)*.

In the albitized basaltic porphyrites, on the basis of analyses of 85 samples, the trace elements, in decreasing order of abundance, are arranged in the following series: Ni, Co, Cu, Ga, Cr, V, Mn, Ti, Zr (100); Sr, Zn (82.2); Ba (49.3).

The pyroclastic formations of the formation do not differ from the effusive rocks in their set of trace elements. According to analyses of 80 samples, the trace elements in them, in decreasing order of abundance, are arranged in the following series: Ni, Co, Cu, Ga, Cr, V, Mn, Ti; Zr (100); Zn (88.6); Sr (84.3); Ba (62.0).

The values of the average contents of Ni, Co, Cu, Ga, Cr, V, Mn, Ti, Zr, Sr, Zn, and Ba are shown on the distribution graph of trace elements in the principal rock types of the spilite–diabase formation (Fig. 1) and in Table 1. From the graph and the table, substantial fluctuations are evident in the values of the average content of a number of elements determined for each of the identified groups of volcanogenic rocks. Thus, albitized basaltic porphyrites are characterized by a significant decrease in the contents of Ga, Co, Zr, Ni, Cu, Zn, Cr, Ba, Sr, and Mn in comparison with albitized diabases, diabase porphyrites, and pyroclastic formations. At the same time, the copper content in the albitized basaltic porphyrites is less by a factor of 2, nickel by a factor of 3.5, and chromium by a factor of 6.

The titanium content increases from the albitized diabase varieties to the pyroclastic rocks almost twofold, and in the albitized basaltic porphyrites it occupies an intermediate value. Only the vanadium content does not undergo substantial deviations from the mean value in all types of volcanogenic rocks. At the same time

* Here and below, the figures in parentheses denote the number of samples in which a given element was detected, expressed as a percentage of the total number of samples.

Values of the average contents of minor elements in the principal rock types of the spilite–diabase formation

It should be noted that, despite the close values of the average contents of trace elements in albitized diabase varieties and pyroclastic rocks, the latter show the highest contents of Ti, Zn, Cu, and Zr. These are the main features in the distribution of trace elements in the distinguished groups of rocks of the spilite–diabase formation; among them, the features requiring explanation are the regularly low concentrations of most trace elements in albitized basalt porphyrites and the elevated contents of a number of elements in the pyroclastic formations of the formation under description.

The observed change in the concentrations of trace elements in the rocks of the formation may, in principle, be determined either by processes of regional metamorphism (albitization), or by the different material composition of the distinguished groups of rocks, or by the different nature of the facies conditions at the time of solidification of the effusive rocks and accumulation of the pyroclastic products. We have already discussed above the influence of the albitization process and noted that, because of its regional character, all the distinguished rock varieties were affected by albitization processes to the same degree. Therefore, if there was any change in the primary concentrations of trace elements, it should be assumed that the ratio of their contents in the different rocks did not change substantially. In this connection, we are not inclined to explain the low concentrations of almost all elements in albitized basalt porphyrites, as compared with their concentrations in albitized diabases and diabase porphyrites, by the secondary process of albitization. The uneven distribution of trace elements in the rocks of the formation in this case also cannot be explained by the different material composition of the rocks, since albitized basalt porphyrites and albitized diabase porphyrites are related in material composition and differ only in structural features caused by the facies conditions of their formation.

On the basis of the foregoing, we are inclined to explain the distribution and concentration of trace elements in the rocks of the formation by the different character of the facies conditions that existed at the time of formation of the formation, assigning them the leading significance. The elevated content of most elements in pyroclastic and diabase rocks, compared with albitized basalt porphyrites, is probably explained by essentially submarine conditions of the volcanogenic process, under which the primary basaltic rock varieties, being in direct contact with water, lost insignificant amounts of Ga, Co, Zr, Mn, Sr, Zn and, to a greater degree, Ni, Cu, Cr, Ba. The diabase varieties of rocks of the formation, composing the central parts of flows and sills, thereby had a powerful screen above them. Owing to these features, they almost completely preserved the primary content of trace elements.

Low concentrations of trace elements in basalt porphyrites are also observed in those cases when they formed under subaerial conditions, as evidenced by their interbedding with pyroclastic formations. The latter sometimes even become predominant, which, as is known, is not characteristic of the submarine form of volcanic activity. In this case, the decrease in the content of trace elements in the basaltic varieties is evidently explained by their removal by residual solutions formed as a result of the cooling and solidification of magma under surface conditions. At the same time, in the pyroclastic formations underlying and overlying the basalt porphyrites, as a rule, a considerable increase in the content of elements is observed. The enrichment of pyroclastic products with trace elements, in our opinion, may be explained by the following.

![Graph: Trace-element content, in %, plotted for rock types I–III. Elements labeled include Ti, Mn, V, Cr, Zn, Ni, Cu, Zr, Co, Ga, Ba, Sr, and others.]

Fig. 1. Distribution of trace elements in the principal rock types of the spilite-diabase formation.
I. Albitized diabases and diabase porphyrites.
II. Albitized pyroxene-plagioclase and plagioclase-pyroxene basaltic porphyrites and spilites.
III. Pyroclastic varieties of volcanogenic rocks: tuffs, tuff breccias, lava breccias.

Thus, pyroclastic formations, by virtue of the conditions of their formation, are “dry” products of volcanism; therefore they have lost almost none of the dispersed elements contained in them. Moreover, owing to their porous structure they tend to capture an additional quantity of dispersed elements carried by solutions from the surface parts of cooling lava flows, which explains the observed elevated concentration of minor elements in them.

Conclusions

1. The study we carried out of minor elements in the ancient spilite-diabase formation of the southwestern spurs of the Kuznetsk Alatau showed their uneven distribution in different rocks.

2. The content of minor elements depends only slightly on variations in the initial composition of the basaltic magma, and is determined mainly by the facies conditions of rock formation and by their textural features.

3. Minor elements in volcanogenic rocks may be used as indicators for determining the types of volcanogenic rocks.

Literature

1. A. I. Almukhamedova. Behavior of titanium in the processes of differentiation of basaltic magma. Geochemistry, No. 1, 1967.

2. A. P. Vinogradov. Average content of chemical elements in the principal types of igneous rocks. Geochemistry, No. 7, 1962.

3. M. I. Vlasova. Regularities in the distribution of rare and dispersed elements in the effusive rocks of the southern Karamazar. In: Problems of Mineralogy, Geochemistry, and Technology of Mineral Raw Materials. “Fan” Publishing House, Tashkent, 1966.