DIKE ROCK COMPLEX OF THE LYUBAVINSKAYA ORE ZONE

(Central Transbaikalia)

G. V. Shubin, V. M. Volkov

(Presented by Professor A. M. Kuzmin)

In the southwestern part of Central Transbaikalia, a dike complex of rocks of varied composition is widely developed. Taking into account the circumstance that, in this area, dike rocks are one of the principal prospecting criteria for gold mineralization, the question arises of the need for a detailed petrological study of the great diversity of these rocks, with consideration of the geological history of their formation. At present, the dike rocks with which the mineralization is associated are distinguished as an independent Middle–Upper Jurassic complex, individual rock varieties of which possess features of petrochemical and geochemical affinity with the volcano-plutonic formation [3]. Familiarization with the materials on this question has shown that definite criteria have not yet been developed for separating the great diversity of dike bodies into the emerging magmatic or volcano-plutonic complexes of Central Transbaikalia. This question requires independent study and is not considered here.

In the present work, certain aspects are touched upon of the petrochemical characteristics of dike rocks that are widely developed within the Lyubavinskaya ore zone and that are accepted by geologists as standards of the Middle–Upper Jurassic dike complex of the southwestern part of Central Transbaikalia.

The dike rocks of the Lyubavinskaya ore zone, in terms of their composition, conditions of occurrence, and relation to gold mineralization, may be divided into the group of granite-porphyries, quartz porphyries, felsites, albitophyres, and the group of rocks of the lamprophyre series. This entire complex

Fig. 1. Layout scheme of igneous rocks of the Lyubavinskaya ore zone.
1 — stocks of granitoids; dikes: 2 — granite-porphyries, 3 — albitophyres, 4 — lamprophyres; 5 — tectonic disturbances

is distinctly confined to a sublatitudinal tectonic zone of increased fracturing (Fig. 1).

Granite-porphyries, quartz porphyries, and felsites form a broad belt elongated in the latitudinal direction, dipping southward at an angle of 65–70°, and are represented by a series of closely spaced, often mutually connected dike bodies of varying thickness. All of them have a similar petrographic composition and differ only in structural features. Macroscopically these are medium- or fine-grained rocks of light-yellow or pinkish-gray color, with porphyritic phenocrysts of quartz, plagioclase (No. 28–30), and orthoclase ranging in size from 1 to 5 mm. The porphyritic phenocrysts make up 35% of the rock volume. On the basis of the composition of the porphyritic phenocrysts among the rocks described, the following varieties may be distinguished: quartz-plagioclase and quartz-orthoclase. Around the porphyritic phenocrysts one can often observe a comb-like quartz–feldspar rim, which may indicate that “the porphyritic crystals of the granite-porphyries began to form in a deeper part of the chamber, and then, as the magma was squeezed out, it carried them into higher, sharply hypabyssal conditions, where they were relatively quickly overgrown by a polycrystalline rim while the uneven-grained groundmass was forming” [2]. The groundmass consists of small grains of orthoclase, quartz, more rarely plagioclase, small flakes of muscovite and biotite. In the central parts of large dikes, a microgranitic and granophyric structure of the groundmass is characteristic, whereas in endocontact zones and small dikes a spherulitic structure appears. In a number of cases, structures of a transitional type are noted.

Albitophyres are represented by thick (5–30 m) and extensive steeply dipping dike-like bodies of southeastern strike. Macroscopically, these are leucocratic light-gray or yellowish-gray, massive rocks with a characteristic holocrystalline porphyritic texture. The porphyritic phenocrysts (20–50%) are composed essentially of albite (No. 3–5). The groundmass consists of small albite grains with an admixture of quartz and sericite. In the form of inclusions, the phenocrysts contain rare, rather large flakes of sericite, grains of zoisite and sphene, and plates of chlorite and muscovite developing after biotite. The presence of elongated forms of zoisite–chlorite aggregates gives grounds to suppose that they arose as pseudomorphs after hornblende.

Rocks of the lamprophyre series are represented by a series of scattered, steeply dipping dikes 50–70 cm thick, of northeastern strike. Their extent usually amounts to the first hundreds of meters. On the basis of petrographic features, quartz porphyrites, microdiorites, porphyrites, and spessartites are distinguished among them.

Quartz porphyrites are massive, ash-gray, sometimes with a greenish tint, holocrystalline porphyritic rocks. Phenocrysts of plagioclase (No. 25) and, more rarely, quartz, 2–3 mm in size, make up 10–15% of the rock volume. Hornblende occurs very rarely as phenocrysts. The groundmass is hypidiomorphic-granular and is represented by plagioclase (45–50%), hornblende (15–25%), and potassium feldspar (10–15%); quartz is present as an admixture. Among secondary minerals there is comparatively much chlorite and carbonates, and a little sericite. Among accessory minerals sphene, zircon, and ore minerals occur.

Microdiorites. Macroscopically these are gray, with a greenish tint, more rarely dark- or yellowish-gray, massive fine-crystalline rocks. Under the microscope, deep autometasomatic and partly postmagmatic alteration of the rocks is established, corresponding to the stage of greenschist alteration. Primary minerals have almost not been preserved. The nature of the primary rock is indicated by relict pseudomorphic textures and secondary minerals. Plagioclases and pseudomorphs after them make up 60–70%, chlorite 15–20%, calcite 10–12%; quartz is present as an admixture, and more rarely potassium feldspar. Among accessory minerals there is comparatively much sphene and ore minerals; apatite occurs rarely. Plagioclase is almost entirely replaced by calcite, epidote, and sericite. Along the periphery of plagioclase grains a narrow pelitized rim of albite (No. 3), evidently secondary, is usually visible. Chlorite sometimes forms relatively large, almost monocrystalline flakes that arose, judging by their form, after individuals of hornblende and, more rarely, after biotite.

Porphyrites are massive, mottled-gray rocks with a holocrystalline porphyritic texture. Phenocrysts of zonal plagioclase (No. 28–37) make up about 50% of the rock volume. Biotite and isolated chlorite–carbonate pseudomorphs, developing after hornblende phenocrysts, are present as an admixture in the composition of the porphyritic segregations. The groundmass consists of the finest grains of feldspar and flakes of sericite. Accessory minerals are zircon, apatite, sphene, and ore minerals. In the case of intense processes of autometamorphism and partial influence of postmagmatic solutions, carbonate (50–60%), chlorite (15–20%), albite (10–15%), and quartz (10–20%) develop. The primary minerals of the groundmass and the phenocrysts are represented essentially by relicts of plagioclase. As a result, an apoporphyritic quartz–chlorite–carbonate rock is formed.

Spessartites. These are dark-gray, massive rocks with a fine-crystalline, sparkling fracture. Under the microscope a relict pseudomorphic texture is manifested. The composition, form of the pseudomorphs, and structural features make it possible to conclude that the original rock was holocrystalline-porphyritic, with a large number of hornblende segregations and isolated plagioclase phenocrysts. At present the hornblende grains have been replaced by chlorite, carbonate, and ore minerals, and the plagioclase grains by carbonate. The groundmass has also undergone alteration and now consists of carbonate, chlorite, and quartz; sericite, biotite, and albite are present as admixtures. Among accessory minerals, sphene, zircon, apatite, and a large quantity of ore minerals occur in small amounts.

Table 1. Chemical composition of dike rocks

Brief petrochemical characterization of the rocks

Comparison of the chemical composition of the rocks of the dike complex (Table 1) makes it possible to reveal certain features that are clearly expressed on the vector diagram (Fig. 2).

Fig. 2. Vector diagram of the chemical compositions of dike rocks of the Lyubavinskaya ore zone (for vector numbers see Table 1). Variation lines of the evolution of compositions of magmatic calc-alkaline series of types: Л—Lassen Peak, И—Yellowstone Park.

In general, the group of rocks of the acidic series differs from the quartz porphyries of Dely, which are close to them in chemistry, by a reduced content of free silica ($Q = 27.9$ versus 33.4 for Dely), increased alkalinity ($a : c$ equals 9.8 versus 7.9 for Dely), reduced magnesium content, and increased iron content.

Among the rocks of the acidic series, two subgroups may be distinguished by their petrochemical features: rocks of the normal series (granite-porphyries, quartz porphyries, felsites) and rocks supersaturated with aluminum (albitophyres). The latter differ from the granite-porphyries by an increased content of femic components (parameter “в”), a sharply reduced calcium content, increased alkalinity ($a : c$ equals 12.7 versus 5.5), reduced iron content, and increased magnesium content.

The group of rocks of intermediate composition is, in general, similar to diorites in

Table 2

Contents of trace elements in dike rocks

NOTE: per unit content of rock \(1 \cdot 10^{-3}\%\);
\(M_o\) — modal content;
\(M_{\mathrm{sr}}\) — arithmetic mean content, in parentheses the number of analyses is indicated.

Dells, differing in a somewhat elevated content of femic components and magnesium, and a reduced content of iron.

In general, for the entire complex of dike rocks of the Lyubavinskaya ore zone, what is characteristic is—in spite of some fluctuation in the sum of alkalis—the predominance of sodium over potassium, an elevated magnesium content, and a reduced iron content.

Differences between the distinguished groups of rocks in trace elements are insignificant and are directly dependent on their composition (Table 2). Thus, rocks of intermediate composition are characterized by elevated contents of Ni, Nb, V, Cu, Zn, Co, Cr, Y, Yb, and Sc, whereas in granite porphyries and albitophyres elevated contents of Pb, W, Ga, and Li are observed.

Taking into account the general geological-tectonic and petrochemical features of development of the rocks described, one may conclude that all the diversity of dikes of the Lyubavinskaya ore belt is related. Geological observations make it possible to say that the earliest formations of the dike complex are granite porphyries; these were followed by the formation of albitophyres, and dikes of the lamprophyre series completed this process. Such an evolution of magmatic formations is explained by processes of differentiation of the parent magma and resembles (Fig. 2) the evolution of alkaline-earth series of the Lassen Peak and Yellowstone Park type [1].

Genetically, the rocks under consideration evidently have a close connection with the Middle–Upper Jurassic magmatic center that produced the volcano-plutonic series of rocks of Central Transbaikalia [3]. At the same time, the volcanic formations and the comagmatic dike rocks have a submeridional strike, being confined to the Onon-Tura and Byrtsa-Olenga tectonic zones. The dike rocks considered and the gold mineralization paragenetically associated with them, as a rule, are concentrated in sublatitudinal tectonic zones. The different character of their spatial distribution is connected with a sharp change in the tectonic regime during the period of development of the South Daurian uplift and magmatism.

Taking into account the remarks set forth above, it is evident that one may speak of a two-stage development of the volcano-plutonic series. The first stage was expressed in the formation of the volcanic rocks proper and of dike bodies comagmatic with them. The second stage was characterized by intrusive activity proper, with the formation of dike bodies, with the lamprophyre series of which gold mineralization is paragenetically associated. This circumstance, taking into account their close spatial relationship, has practical significance as a prospecting criterion, irrespective of the presence of dikes of acid composition.

REFERENCES

1. A. N. Zavaritskii. Introduction to the Petrochemistry of Igneous Rocks. Publishing House of the Academy of Sciences of the USSR, 1944.

2. A. M. Kuz’min. The role of concentration flows at the stage of crystallization differentiation of magma. Izv. TPI, vol. 121, 1963.

3. V. V. Starchenko. Geology of the Middle–Upper Jurassic Volcano-Plutonic Formations of the Southeastern Part of Central Transbaikalia and the Main Features of Their Metallogeny. Abstract of a dissertation for the degree of Candidate of Geological-Mineralogical Sciences, L’vov, 1968.

4. G. V. Shubin, S. A. Chubarov, A. V. Matsyushevskii, A. I. Volkova. Structure of the ore field and genesis of ores of the Lyubavinskoye deposit (Transbaikalia). In: Geology of Gold-Ore Deposits of Siberia. Nauka Publishing House, 1970.