GEOCHEMICAL FEATURES OF THE GABBRO–SYENITE COMPLEX IN THE TASHTAGOL MINE AREA

G. M. Ivanova

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

In the Kondoma district, within the Tashelgino–Kondoma tectonic zone, there are exposures of syenite massifs arranged in a chain trending northeast; the contact-metasomatic iron-ore deposits of the district are closely associated with them. In this connection, a large amount of work has been carried out on the study of the syenites and, in particular, of their petrochemical composition; many chemical analyses have been made, but the conditions of their formation have so far been poorly studied.

Spatially closely associated with the syenites are exposures of pyroxene and pyroxene–hornblende porphyrites, established in the Tashtagol, Shalym, and Sheregesh ore fields. In 1963, in the Chernichny area of the Shalym ore field, a stock-shaped body of gabbro was intersected by core-drilling boreholes.

The gabbro-porphyrites and gabbro are cut by syenites and have, with the latter, in some cases sharp and in others indistinct assimilation contacts.

The close spatial relationship of gabbro-porphyrites and gabbro with syenites suggests a genetic relationship between them.

Clarification of the genetic relationship between the basic and alkaline rocks of the complex is of great importance for establishing the conditions of formation of the syenite massifs and the ore-forming processes associated with them, since recently in the district facts have been established of alkaline replacement of the enclosing rocks, manifested in the form of potassium metasomatism passing into magmatic replacement. The iron released during the decomposition of silicates and oxides, as S. S. Il’enok (1960) pointed out, probably goes into the formation of metasomatic ore occurrences. In the present article the author considers related geochemical features of the gabbro and syenite groups of rocks, confirming their genetic relationship.

To characterize the behavior of rare and dispersed elements in groups of rocks of basic and alkaline composition, 180 spectral analyses were carried out, the results of which are plotted in Fig. 1a, b, c. In both groups of rocks the same elements are present: Ba, Be, V, Ga, Co, Mn, Cu, Ni, Sc, Sr, Ti, Pb, except for Zn and Zr, which are absent in rocks of basic composition. Certain features of the behavior of these elements in each group of rocks should be noted.

In the gabbro-porphyrites, in comparison with the syenites, an increased content of Be, V, Co, Mn, Ni, Sc is observed. Co and Ni serve as indica-

... the early stage of crystallization of the rocks. Having ionic radii similar in magnitude to Mg and Fe, they enter as isomorphous admixtures into the composition of iron–magnesian silicates, which also explains their higher content in the gabbro-porphyrites, although they are present here in amounts below the Clarke values. In the syenites, however, the content of these elements remains high, exceeding the Clarke values for average rocks (after A. N. Vinogradov).

The contents of V and Mn in rocks of the gabbro group correspond to the average values for rocks of this composition. Their higher content in the latter, as compared with the syenites, is explained by the fact that Mn, by the magnitude of its ionic radius, is related to Fe and enters

Fig. 1.

Fig. 1, a. Diagram of the distribution of trace elements in the gabbro-porphyrites of the Tashtagol massif (the average content of elements is given from 10 analyses): 1—0.02%, 2—0.04%, 3—0.06%, 4—0.08%, etc., for the large circle. The radius of the small circle corresponds to a content of 0.002%.

b. Diagram of the distribution of trace elements in lime-alkaline syenites of the Tashtagol massif (the average content of elements is given from 110 analyses): 1—0.02%, 2—0.04%, 3—0.06, 4—0.08%, 5—0.10%, etc. The radius of the small circle corresponds to a content of 0.002%.

c. Diagram of the distribution of trace elements in alkaline syenites of the Tashtagol massif (the average content of elements is given from 50 analyses): 1—0.02%, 2—0.04%, 3—0.06%, 4—0.08%, 5—0.10%, etc. The radius of the small circle corresponds to a content of 0.002%.

as an isomorphous admixture into the composition of iron-bearing minerals. Vanadium is also present as an isomorphous admixture in iron–magnesian silicates. The content of these elements in the syenites is somewhat below the Clarke values.

The somewhat elevated content of Sc in the basic rocks, as compared with the alkaline rocks, is likewise explained by its affinity to divalent iron in the magnitude of its ionic radius (P. L. Mitchell, 1948). A high content of this element is retained in the syenites. At the same time, it is above the Clarke values in both groups of rocks.

Barium is present in the gabbro-porphyrites, lime-alkaline syenites, and alkaline syenites in amounts above the Clarke values; its relative content is highest in the first variety, lower in the second, and still lower in the third.

Lead is contained in the syenites in greater quantity than in the gabbro-porphyrites, which is explained by its accumulation in the late stages of magma crystallization and by the similarity of its ionic radius to that of potassium. Its content in the gabbro-porphyrites is above the Clarke values, and in the syenites it is lower. Gallium, copper, strontium, and titanium are present in approximately equal amounts in both groups of rocks, although the Clarkes of two of these elements—copper and titanium—differ sharply: for gabbro-porphyrites they are respectively 0.01 and 0.9, and for syenites 0.002 and 0.23.

Proceeding from the foregoing, the following brief conclusions may be drawn:

1. Both groups of rocks are similar in the presence of the same elements, except for Zn and Zr, which are noted only in the syenites. The same elements are also absent from the gabbro-porphyrites and syenites, such as W, Mo, Bi, Hb, Ge, In, Cd, As, Nb, Sn, Hg, Sb, Ta, Te, Ce, Pt, Li, and others.

2. In the rocks of both groups, an elevated content of the same elements is observed relative to others: Ba, V, Mn, Ti, Sr (Fig. 1a, b, c).

3. For the syenites, an elevated content of Co and Ni relative to the Clarke value is characteristic, whereas in the gabbro-porphyrites their content is below the Clarke value (with a higher content relative to the syenites). If one takes into account that these elements are characteristic of rocks of the early stages of crystallization, then the above-mentioned relationship between their quantitative contents and the Clarke values indicates a genetic connection between the rock groups under consideration.

4. An approximately equal quantitative content of such elements as copper and titanium has been noted in the alkaline and basic rocks, despite a substantial difference in the magnitude of their Clarke values for these rocks.

The conclusions listed above show a close geochemical connection between the rocks of the gabbro and syenite groups in the area of the Tashtagol mine, which makes it possible, with considerable confidence, to unite them into a single magmatic complex. At the same time, the first phase of intrusion includes the gabbros and gabbro-porphyrites, while the second is associated with the formation of syenites of various varieties.