ON THE QUESTION OF THE PALINGENETIC ORIGIN OF THE ORE-BEARING VOLCANO-PLUTONIC COMPLEX IN TRANSBAIKALIA

Yu. P. Mironov (TsNIGRI)

We studied Mesozoic magmatism of the Nerchinsk–Kuenga structural zone of Eastern Transbaikalia in the area of Vershina Darasuna, Bukachachi, and Kykera.

In these areas, the most widespread of the Mesozoic formations is a volcano-plutonic complex of average granodioritic composition. The complex is subdivided into a plutonic series of Triassic–Middle Jurassic age (230–160 million years), roughly corresponding to the Amanan complex and part of the Amudzhikan complex distinguished in the region; an extrusive series of Middle Jurassic age (180–150 million years), corresponding to the greater part of the Amudzhikan complex; and an effusive series of Middle–Upper Jurassic age (170–150 million years).

The plutonic series is represented by gabbro and diorites (1st phase), granodiorites and granites (2nd phase), leucocratic granites, granosyenites, and syenites (3rd phase). The plutonic series ends with the formation of porphyritic granodiorites and granodiorite-porphyries.

The extrusive series is represented mainly by granodiorite-porphyries, plagiogranite-porphyries, granite-porphyries, and a group of hybrid porphyritic rocks.

For the effusive series, dacites and rhyodacites are most characteristic, and rhyolites to a lesser degree.

The different series are combined by us into a single volcano-plutonic complex (Amanan–Amudzhikan).

Volcano-plutonic character of the complex

A number of facts testify in favor of the comagmatic nature of the different series, for example: a regular change in composition from older to younger formations in the sequence of plutonic and extrusive-effusive rocks; the phase-facies character of the relationships between each preceding and each subsequent group of rocks; confinement to common structural trends and blocks of the different series; and certain petrographic features of the rocks. For example, analysis of the distribution and character of mono- and polymineral feldspar ovoids in the rocks leads to the conclusion that the series are comagmatic. They are absent in the rocks of both older and younger complexes, while ...

sovs and are observed exclusively in granitoids of the Mesozoic volcano-plutonic complex.

The petrochemical and geochemical features of the rocks of the different series do not contradict the conclusion that they are comagmatic. Thus, for example, both the plutonic and the extrusive series are characterized by a weakly expressed pantelleritic tendency; the character of magmatism is close to the San Francisco type; the typomorphic accessory mineral is sphene; rocks of the different series are enriched in Co, Ni, Ti, Mo, Pb; and gold-molybdenum mineralization is associated with rocks of both the plutonic and the extrusive series. Analysis of the relative age of the rocks of the complex shows that all of them are younger than the Middle Paleozoic granitoids and are cut by granites of the Upper Jurassic Nerchugan complex; the absolute-age figures range from the Triassic to the Upper Jurassic (229–150 million years). The comagmatic nature of the rocks is also indicated by some of their mineralogical features, for example the presence in all rocks of moderately acidic and basic composition of amphiboles of the hastingsite type \(((-) 2V = 34^\circ,\ cNg = 19^\circ)\), and magnesiohastingsite \(((-) 2V = 52^\circ–54^\circ,\ cNg = 25^\circ–39^\circ)\). The same conclusion is reached from analysis of the structural and textural features of the rocks. Margination and corroded structures occur in almost all rocks; there are piterlites, absent both in more ancient and in younger rock associations; the rocks of the volcano-plutonic complex are sharply distinguished by their textural features. Proterozoic-Paleozoic granitoids are characterized by gneissic, banded textures; Mesozoic granitoids by directive, massive textures; and the younger Nerchugan complex by massive, miarolitic textures. Rocks of the plutonic and extrusive series are characterized by the same set of inclusions, among which the most common are hornfelsed or recrystallized, to the stage of amphibole formation, andesite-basalts.

The Amanan-Amudzhikan complex is distinguished among the others by close values of gamma activity and magnetism of the rocks. In addition, there are certain interesting particular details. For example, schisolitic rocks of the extrusive series have chilled zones at contacts with all rocks of Proterozoic and Paleozoic age, but not a single similar case has been encountered at contacts with plutonic-series phases close in age.

Two Types of Rocks and Massifs within the Plutonic Series

During work of a revision character and partial mapping of areas where Mesozoic complexes are developed, it became clear that the rocks of the plutonic series are heterogeneous and are broadly divided into two groups, each of which is characterized by its own features. We have at our disposal two sets of criteria for distinguishing the rocks. First, there are considerations of a general geological character; second, those of a genetic character. The former make it possible to show on the map the position of massifs of different rock types; the latter, to reveal the reason for the existence of different rock types. The criteria of a general geological character for distinguishing rocks and massifs of different types are summarized in Table 1.

As a result of comparing granitoid rocks of the two types, the author came to the conclusion that the rocks of the first group are of metasomatic origin, and those of the second are magmatic; moreover, the massifs of the first group are composed predominantly of rocks of the first phases of the plutonic series, whereas those of the second are composed of rocks of the last phases.

Any proof of the palingenetic origin of rocks formed in situ must consist of two parts: proof of me...

metasomatic character of the products of the initial stage of the process and the magmatic nature of the products of the final stage. The development of the volcano-plutonic complex ends with the formation of extrusive and effusive rocks, which are known to have a magmatic origin; therefore, in order to demonstrate the palingenetic character of the complex, it is sufficient to show the metasomatic nature of at least the first two phases of the plutonic series.

Table 1

The metasomatic origin is most clearly recorded for gabbros, diorites, and granodiorites of the first phases of the plutonic series; therefore in this paper we shall pay special attention to these groups of rocks. Because of the limited scope of the article, comparatively little attention will have to be given to the features indicating a magmatic origin for part of the rocks of the plutonic series.

Metasomatic rocks

Gabbros and diorites. In the rocks of the first phases of the plutonic series, shadow structures and features of replacement have been noted, especially clearly visible in the contact parts of the massifs. In a number of cases, in contacts with diorites and granosyenites, more ancient amphibolites have been found, alternating as bands with gabbro-amphibolites. The banded structure of the enclosing rocks continues and can be traced in the diorites formed from them.

A process has been described in which amphibolites with microfolding, schistosity, and swirl structures are reworked by blastitic aggregates of hornblende. In place of metamorphic rocks, gabbroids arise with amphibolite relict structures developed to varying degrees. In these same gabbros and diorites, numerous metacrysts of hornblende

hornblendes are “shadow” crystals, i.e., they consist of a mass of identically oriented individuals, between which the enclosing rock is located.

Microscopic study of the rocks showed that the composition of the minerals varies sharply. In the gabbroid rocks five types of amphiboles are distinguished, of which three are most interesting: 1) ordinary green hornblende, having \((-)\,2V = 75^\circ—80^\circ\), \(cNg =\) up to \(25^\circ\); 2) weakly alkaline hornblende, having \((-)\,2V = 70^\circ—77^\circ\), \(cNg = 17^\circ—39^\circ\); 3) hastingsite, magnesiohastingsite, with the following optical constants: \((-)\,2V = 16^\circ—58^\circ\), \(cNg = 11^\circ—26^\circ\). Such diversity of mineral types is difficult to explain if the rock formed during crystallization from magma.

During oxidation the hornblendes in them become more magnesian, and not more ferruginous, as would have been expected (magnesiohastingsite develops, with the following optical constants: \((-)\,2V = 52^\circ—60^\circ\), \(cNg = 25^\circ—35^\circ\); an amphibole of the eckermannite type appears, having \((-)\,2V = 16^\circ—34^\circ\), \(cNg = 22^\circ—35^\circ\)). This phenomenon is also difficult to explain on the basis of the assumption of the existence of eutectic crystallization.

Such an interesting phenomenon has been described as the presence of a connection between the composition of a rock and the orientation in space of its directive textures. In the areas where work was carried out, Paleozoic rocks, in particular granodiorites, are characterized by a northeasterly strike of banding; for diorites of Mesozoic age, by a northwesterly one. As the granodiorites are recrystallized to the stage of the appearance of diorites, with the introduction of \(CaO\), \(MgO\), \(FeO\), the orientation of the banding gradually changes from northeasterly to northwesterly. The gradual change in the orientation of banding can be explained by the phenomenon of anisotropy of mineral growth under the new tectonic conditions characteristic of the Mesozoic stage.

Gabbroids are extremely characterized by blastic structures: granoblastic, poikiloblastic, lepidoblastic, and others. In complexly built massifs (in the rocks there are layers with alternating predominance of plagioclase and hornblende) alternating idiomorphism of the minerals is noted. The frequent change of the structural features of the rocks is striking. In the gabbroid rocks of borehole 283 of the V. Darasun ore field, in the interval \(0—110\) m, the structure of the rocks changes 33 times (alternating idiomorphism of hornblende, plagioclase, and other features).

In conclusion to this section it is necessary to dwell on such an original phenomenon as the appearance of contour crystals and even “contour” rocks, consisting of contour crystals. In such rocks, hornblende of the hastingsite type \(((-)\,2V = 34^\circ,\ cNg = 11^\circ—19^\circ)\) consists of ring-shaped single crystals with enclosing rock inside them. Rocks almost wholly composed of contour crystals were discovered by us in the gabbroids of the contact parts of the Ikilon massif (V. Darasun district) and in the ore field of Vershiny Darasuna. The metasomatic nature of such crystals, i.e., also of rocks with contour crystals, was shown by V. D. Nikitin (1955), and therefore this question is not discussed specially here.

Granodiorites and granites. In the Akima massif of the Kyker district, granodiorites have been found in which potassium feldspar (microcline with the following optical constants \((-)\,2V = 80^\circ\), \(Ng:1(010) = 16^\circ\)) occurs exclusively as phenocrysts; in another part of the rocks an unusually great diversity of types of phenocrysts, both monomineral and polymineral, is noted.

In the granodiorites of the Ikilyun massif, over an area of about 2 m one can find phenocrysts of plagioclase of composition $\mathrm{an}_{30}\mathrm{an}_{10-20}$, microcline $((-)\,2V = 82^\circ,\ \mathrm{Ng}:I(010) = 16^\circ)$, orthoclase $((-)\,2V = 69^\circ,\ \mathrm{Ng}:I(010) = 89^\circ)$, anorthoclase $((-)\,2V = 60^\circ,\ \mathrm{Ng}:I(010) = 81^\circ)$, complex ovoids consisting of anorthoclase $((-)\,2V = 62^\circ,\ \mathrm{Ng}:I(010) = 10^\circ)$ and grid microcline, which, as an ordinary metasomatic mineral, replaces anorthoclase from the margins. Often the anorthoclase–microcline ovoids are surrounded by an oligoclase $(\mathrm{an}_{10-20})$ shell. The phenomenon of the joint occurrence of anorthoclase and microcline is explicable if one adheres to the point of view of R. B. Ferguson, R. J. Trail, W. H. Taylor, G. Donnay, J. Wyart, and others (collected volume Physics of Minerals, 1954), who see the cause of structural transformations in feldspars in replacement processes, in the influence of sodium atoms, the catalytic action of water, etc., and not in submicroscopic twinning of minerals.

In the eastern part of the Ikilyun massif, granodiorites of the second phase with a double directive texture have been described; this texture is manifested owing to the mutually perpendicular arrangement of elongated grains of magnesiohastingsite $((-)\,2V = 56^\circ,\ cNg = 15^\circ)$. The grains are oriented in northeastern and northwestern directions in accordance with the systems of fracturing in the enclosing Paleozoic granodiorites. The arrangement of minerals in accordance with the fracture systems of the enclosing rocks may testify to their metasomatic origin.

The composition of the minerals of the granodiorites varies sharply. The hornblendes of these rocks are analogous to the amphiboles of gabbroids and differ only in smaller size; the composition of plagioclase varies from $\mathrm{an}_5$ to $\mathrm{an}_{50}$. No less diverse are the potassium feldspars of the granodiorites. Literally in single rock samples, microcline is recorded $((-)\,2V = 80^\circ,\ \mathrm{Ng}:I(010) = 15^\circ—21^\circ)$, isomicrocline $((+)\,2V = 28^\circ—82^\circ)$, orthoclase $((-)\,2V = 69^\circ—71^\circ,\ \mathrm{Ng}:I(010) = 89^\circ—90^\circ)$; even potassium feldspar with a sharply reduced angle of optical axes, down to $(-)\,2V = 16^\circ$, has been found. Thus, the principal rock-forming minerals are characterized by sharp fluctuations in composition, which are not explained by the magmatic concept (conditions during crystallization in a magma chamber would have to be too heterogeneous).

Analogously to the gabbroids, in this group of rocks contour formations have also been described. In the Akiminsk and Saundinsk massifs of the Kyker district there are granodiorites and syenite-diorites in which potassium feldspar (grid microcline) occurs exclusively in the form of contour crystals, which are an indication of metasomatic processes.

According to published data (V. I. Kirilyuk, E. M. Leifman, and others, 1964) and in part the author’s own data, granodiorites with approximately identical characteristics but with different amounts of porphyroblasts were selected by the author. It turned out that there is a certain dependence between the amount of porphyroblasts in the rock and its absolute age. Varieties of granodiorites without porphyroblastic material have an absolute age of 160–229 million years; granodiorites with an enormous number of porphyroblasts, 130–140 million years. Such a dependence can be explained if one assumes that the formation of the plutonic-series massifs is a prolonged process and is connected with the introduction of components of salic minerals. Then, naturally, in areas with a large introduction of $\mathrm{Na_2O}$ and $\mathrm{K_2O}$ (with a large number of porphyroblasts), the rocks present there were rejuvenated to a greater degree.

Magmatic rocks

In the first section of the article it was mentioned that the development of the plutonic series ends with the formation of porphyritic granodiorites and granodiorite-porphyries. These rocks contain the greatest number of phenocrysts; their absolute age in our area ranges from 180 to 160 million years, while in more eastern areas, owing to the general rejuvenation of the age of the complex, their absolute age ranges from 160 to 130 million years (Kirilyuk, Leifman).

It is precisely for these rocks that features of magmatic origin are noted. For them we have described a gradual-discontinuous transition to effusive rocks of moderately acid composition (rhyodacites) and to granodiorite-porphyries of the extrusive series; weakly expressed chilled zones appear in the marginal parts of the massifs. A relationship is noted between the size of the massifs and the grain size of the rocks composing them; the degree of idiomorphism of the minerals of the rocks is directly dependent on the order of crystallization; the number of phenocrysts and their size depend on the degree of distance of the rock from the contacts; there is a relationship between the grain size of the groundmass and the size of the phenocrysts of the rocks; and a satisfactory correlation is observed between the quantity of femic and salic minerals in the rocks. The rocks of the extrusive and effusive series may also be assigned to this same group without special proof.

Discussion of the material

1. After the features of metasomatic origin have been presented, the reason for the division of the massifs into two types can be satisfactorily explained. It is not surprising that massifs of predominantly metasomatic origin (type I) are characterized by repeated changes of facies with different structural-textural features and different quantities of inclusions. The “pseudostratified” structure of parts of the massifs can also be explained by their metamorphogenic nature.

The formation of the metasomatites of the complex ends with the development of leucocratic granites together with aplites and pegmatites; therefore, in massifs of metasomatic origin they occur in large quantities. The process of magma formation somewhat lags in time relative to the leucocratic granites and pegmatites; therefore the latter are absent from massifs of predominantly magmatic origin.

1. Having data on the absolute age of the rocks, geological observations on the age relationships of the rocks, and their genetic characteristics, one can trace the history of development of the volcano-plutonic complex. Judging from our data, in the Triassic (230 million years ago) purely metasomatic processes took place—apparently migration of alkalis—which led to the appearance of gabbroids (the front of basification) and granitoids (the front of granitization). The development of the plutonic series ends at the end of the Middle Jurassic (160 million years ago) with the formation of porphyritic and porphyry granites of magmatic origin. The beginning of palingenesis can be dated rather precisely by the time of formation of the first extrusive rocks (180 million years ago); the period of active magma formation evidently occurred in the Middle Jurassic (180–150 million years ago), judging from the mass outpourings at this time of moderately acid effusive rocks.

2. The ore metasomatites of Vershina Darasuna, according to I. N. Tomson, have an absolute age of 174 million years, which corresponds in time to the beginning of the process of magma formation, i.e., to the stage of mobilization of granitoid rocks. The rocks of the volcano-plutonic complex.

can be divided into three groups of different origin: metasomatic, magmatic, and rocks of an intermediate character. The latter correspond to the stage of mobilization of the rocks, i.e., in time they are closest to the gold mineralization. The age proximity of the gold mineralization and the genetically intermediate rocks is supplemented by data on their almost complete spatial coincidence.

Fig. 1. 1. Deposits of depressions of Upper Jurassic–Lower Cretaceous age. 2. Magmatic rocks of the volcano-plutonic complex. 3. Rocks of the volcano-plutonic complex of an intermediate character between magmatic and metasomatic. 4. Metasomatic rocks of the volcano-plutonic complex. 5. Points of gold ore occurrences. 6. Faults of regional significance.

Figure 1 shows a diagram of the spatial distribution of rocks of the three genetic groups and of gold ore occurrences. As is evident from the diagram, the spatial distribution of rocks of different genesis is sharply uneven. The southeastern zone proved to be saturated mainly with metasomatic rocks, the northwestern zone with magmatic rocks. 93% of the ore points proved to lie within the zone with predominant development of pred—

ore-bearing metasomatites; moreover, 88% of the ore occurrences are located in zones of development of genetically intermediate rocks, from which it follows that the principal concentrations of gold are associated precisely with them. The frequently noted association of gold mineralization with dikes does not contradict the conclusion made above, for the concentration and localization of gold are not equivalent concepts. Within the territory described there are two areas saturated with dikes—the Darasun and Bukachacha areas; moreover, in the Bukachacha area there are far more dikes than in the Darasun area, yet no gold mineralization associated with them is observed. It follows from this that, in prospecting for gold mineralization, attention must be paid not only to extrusive series but also to plutonic series, which may provide no less information on the presence or absence of mineralization.

1. Genetic analysis may be useful for refining certain criteria in prospecting for gold mineralization. Let us list some of these criteria: a) in prospecting for gold mineralization, one should take into account the presence of its association with the genetically intermediate group of rocks of ore-bearing complexes; b) dikes located in the metasomatites of ore-bearing complexes are the most promising objects for prospecting for gold mineralization; c) in the Darasun, Kyker, and Bukachacha areas, the so-called “hybrid porphyries” and dike belts, which are traditional prospecting criteria, have received almost no development. It is precisely in such areas that genetic analysis of ore-bearing complexes must be carried out; d) the extrusive series of the volcano-plutonic complex may be roughly divided into two parts: dikes of an undifferentiated series (granodiorite porphyries, plagiogranite porphyries) and dikes of a differentiated series (granite porphyries, diorite porphyrites). The dikes of the undifferentiated series are the first products of the process of magma formation; they have the closest links with genetically intermediate rocks, and therefore their presence in metasomatites is the most favorable condition for the localization of gold mineralization (an example is the plagiogranite-porphyry dike of V. Darasun); e) ring structures filled with Mesozoic granitoids are characteristic of the Nerchinsk-Kuenga structural zone. On the diagram (Fig. 1), two such structures are visible: the Bukachacha structure, situated in the north in the area where the zone of magmatic rocks widens, and the Darasun structure, situated in the southwestern part of the district. The first of these structures is composed predominantly of magmatic rocks with which gold mineralization is not associated; the second is composed predominantly of metasomatic rocks. Gold mineralization is located within the southern structure; therefore, its association with predominantly metasomatic ring structures may be a general rule.

2. In conclusion, a few words should be said about the possible causes of palingenesis. In the Nerchinsk-Kuenga zone, the synchronous intrusion of basaltoids (predominantly effusives) and granitoids of the volcano-plutonic complex is recorded. The main volume of basites in time lies between the metasomatic and magmatic parts of the complex, i.e., it falls at the stage of rock mobilization; for this reason, the cause of palingenesis may be seen in the rise of basaltic magmas into the upper structural levels of the Earth’s crust.

REFERENCES

1. V. P. Kirilyuk, E. M. Leifman, et al. New data on determining the absolute age of certain geological formations of the Amazar-Shilka interfluve (Eastern Transbaikalia). Geochemistry, 1954, No. 12.

2. V. D. Nikitin. Peculiarities of the processes of formation of minerals during metasomatic phenomena. In: Crystallography, No. 4, Leningrad State University Press, 1955.

3. Collected volume Physics of Minerals. Mir Publishers, Moscow, 1964.