GEOLOGICAL STRUCTURE AND AGE RELATIONSHIPS OF INTRUSIVE ROCKS IN THE SARALA DISTRICT

V. I. Bazhenov, V. I. Grigoriev

(Presented by the seminar of the Department of Geology and Exploration of Mineral Deposits)

The Sarala ore field is located in the zone of a deep fault which, according to data from P. F. Ivankin, L. V. Alabin, and others [4], is one of the branches of the Kuznetsk-Alatau deep fault. This fault is emphasized by a thick belt of dike rocks extending, in accordance with its general strike, in a north-northwesterly direction. The same fault also controls the localization of intrusive massifs, of which the largest is the Ararat massif, situated north of the Sarala ore field and extending in a submeridional direction northward to the Levaya Sarala River; farther north the intrusive rocks are traced as a chain of outcrops confined to the zone of the above-mentioned fault.

The deep fault underwent a long and complex history of development. Having been initiated already in the pre-intrusive epoch, it was repeatedly renewed, as indicated by the abundance of dike rocks and the multistage character of their emplacement. At the same time, the oldest dikes are apparently pre-intrusive, whereas the youngest cut the conglomerates of the Byskar Series of the Lower Devonian. The youngest of them, in composition and chemistry, resemble the young basalts of the Minusinsk depression and probably also have an Upper Paleozoic or Lower Mesozoic age.

Such a prolonged history of formation of the fault also left its imprint on the development of intrusive magmatism, for which a multistage character is likewise typical. The Ararat intrusion is the largest intrusive body of the Sarala district. In the past, this intrusion was studied repeatedly by various authors. A. Ya. Bulynnikov [1], among the intrusive formations, distinguished an intrusion of quartz albitites, which he considered not only the oldest among the magmatic rocks but also older than the surrounding rocks of the effusive-sedimentary sequence. He regarded the intrusion of gabbro and diorites of Mt. B. Ararat as related to the granodiorites of Mt. M. Ararat. G. V. Voitkevich and V. I. Zimnukhova [2] considered the gabbro-diorite intrusion of Mt. B. Ararat to be the oldest, and attributed a metasomatic origin to the plagiogranites (quartz albitites according to A. Ya. Bulynnikov) [2, 3].

Geological mapping carried out by us in the northern part of the Sarala gold-bearing district yielded new factual material that makes it possible to illuminate somewhat differently the history of development of intrusive magmatism in the district described. On the basis of the stud—

...studying the petrography of the intrusion and the mutual relationships of the individual petrographic varieties, we distinguish within the area studied two intrusive complexes of different ages: 1) diorite–plagiogranite and 2) granite–granodiorite.

The rocks of the diorite–plagiogranite intrusive complex make up the southern part of the Ararat intrusion. Two phases are clearly distinguished in the formation of the complex.

The first phase of intrusive activity is associated with the emplacement of the diorites of Mt. B. Ararat. This intrusion has, in plan, an oval form elongated in a sublatitudinal direction, composing Mt. B. Ararat (Fig. 1). The small intrusive bodies of Mt. Zolotoy Rog and Kaskadny Kar also belong to this same phase. The southern contact of the intrusion dips northward. The intrusion consists for the most part of medium-grained diorites composed of intermediate plagioclase–andesine and poikilitic hornblende. In places, especially near the contacts with the enclosing rocks, the diorites grade into hornblende gabbro. In separate areas there occur pegmatitic varieties of diorites, consisting of plagioclase and large crystals of hornblende up to 5–7 cm in size. They usually occur as small vein-like bodies on the southern slope of Mt. B. Ararat, and are also very characteristic of the Kaskadny Kar intrusion. The spatial proximity of these formations to the contact of the plagiogranites permits the assumption of their metasomatic origin under the influence of hydrothermal solutions associated with this younger intrusion.

The second phase of the diorite–plagiogranite complex consists of plagiogranites. This body has a clearly linear form and apparently belongs to the type of fissure intrusions. The plagiogranite intrusion is situated on the southern slope of Mt. B. Ararat, extending along the southern contact of the diorite—

Fig. 1. Diagram of the geological structure of the northern part of the Sarala district (compiled by V. I. Bazhenov and V. I. Grigor’ev). 1 — eluvial–deluvial and alluvial Quaternary deposits; 2 — conglomerates of the Byskarskaya Series of the Lower Devonian; 3 — marbles, calcareous–siliceous schists and tuffs of the Glavstan Formation; 4 — felsites, sandstones and schists of the Andreevskaya Formation; 5 — granites, granodiorites and diorites of the granite–granodiorite intrusive complex; 6 — plagiogranites of the diorite–plagiogranite intrusive complex; 7 — diorites and gabbro-diorites of the diorite–plagiogranite intrusive complex; 8 — hornfelsed rocks; 9 — faults.

of the intrusion from the Teplaya River to the summit of Mt. Nenastnaya. Its western continuation is a body of plagiogranites, isometric in plan, exposed on the summit of Mt. Stolovaya. In its petrographic composition, the intrusive body described consists of fine- and medium-grained plagiogranites composed of albite and quartz. Biotite and potassium feldspar occur as accessory constituents in individual areas.

Plagiogranites exposed on the summit and eastern slope of Mt. M. Ararat also belong to this same phase. They are close to the rocks described above, but differ from them in a somewhat higher biotite content and in the occurrence of gneiss-like varieties.

The younger age of the plagiogranites relative to the diorites is demonstrated quite clearly. Along with the fact that the plagiogranites cut the diorite intrusion, xenoliths of diorites are frequently encountered in the plagiogranites, often having undergone intense reworking. Near contacts with diorites, eruptive breccias with fragments of dioritic composition develop in the plagiogranites. In the endocontact zone of the diorite intrusion, abundant injections of small veinlets of plagiogranitic composition, up to 10–20 cm thick, are observed.

Rocks of the granite–granodiorite intrusive complex have been established in the northern part of the Sarala district. They compose the summit and western slope of Mt. M. Ararat. In the form of small bodies they are also found on its eastern slope. Farther north, the rocks of this complex extend in a meridional direction as a chain of outcrops, emphasizing the continuation of a deep-seated fault. A small outcrop of these rocks has been established on the eastern slope of Mt. B. Ararat.

The rocks of the complex consist predominantly of gray, sometimes flesh-red granites, which usually occupy the central part of the massif. Closer to the periphery they pass into granodiorites, while the near-contact parts of the intrusion are represented by gray diorites. The rocks have a medium-grained texture. Their mineralogical composition is fairly uniform. The principal minerals of the rock are plagioclase of oligoclase-andesine composition, orthoclase often with perthitic intergrowths of plagioclase, and quartz. Dark-colored minerals are represented in the granites by biotite, and in the more basic varieties by common hornblende.

The relationships of the rocks of the granite–granodiorite complex with the other igneous rocks described above in the area under discussion are quite clearly expressed. Their relationships with the diorite intrusion of Mt. B. Ararat are established unambiguously. A small outcrop of biotite granites on the eastern slope of Mt. B. Ararat was noted above. Here the diorites are clearly cut by the granites, and in the exocontact of the granite intrusion the diorites are subjected to contact effects expressed in their epidotization and partial hornfelsing. In addition, on the northeastern margin of the massif the diorites near the contact with the granite intrusion are subjected to significant silicification, up to the formation in places of a true stockwork of thin quartz veinlets.

The relationships of the granite–granodiorite complex with the plagiogranites are established no less clearly. They were studied by us on Mt. M. Ararat. The plagiogranites of Mt. M. Ararat described above apparently represent remnants of the roof of the granite intrusion. The plagiogranites experience noticeable contact metamorphism. They are subjected to evident hornfelsing. In this process, the hypidiomorphic-granular textures of the unaltered plagiogranites, during metamorphism, pass into granoblastic textures characteristic of hornfels. In places, in areas of the most intense contact influence, typical pyroxene–plagioclase hornfels develop at the expense of the plagiogranites.

veins. On the eastern slope of Mt. M. Ararat, bodies of garnet, garnet-pyroxene, and pyroxene skarns were encountered, formed as a result of the skarnification of plagiogranites in the exocontact of the granite intrusion. At the same time, eruptive breccias are often observed within the granites; their fragments consist of plagiogranites, while the cement has a granodioritic composition.

The data presented indicate a significant hiatus in the formation of the two distinguished intrusive complexes. Comparison of them with the intrusive complexes of the Kuznetsk Alatau makes it possible to parallelize the ancient diorite-plagiogranite complex of Sarala with the Kundustuyul gabbro-plagiogranite complex of the Mariinsk taiga, with which, in the opinion of M. P. Kortusov [5], the gold content of a number of districts of the Mariinsk taiga is genetically associated. The granite-granodiorite complex of the Sarala district is close in petrographic composition to the granitoid intrusions of the Martaiga magmatic complex, widely developed in the Kuznetsk Alatau.

The age of the intrusive complexes of the Sarala district is difficult to determine. In any case, one may speak of their Lower Paleozoic age, since rocks of both complexes cut bituminous limestones with Newlandia, and at the same time pebbles of these rocks occur in Lower Devonian conglomerates. This is fully consistent with the data of M. P. Kortusov [5] on the Middle Cambrian age of the formations of the Kundustuyul complex and on the Ordovician-Silurian age of the granite-granodiorite intrusion of the Martaiga complex.

LITERATURE

1. A. Ya. Bulynnikov. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountain system. Tomsk, Tomsk State University Press, 1948.

2. G. V. Voytkevich, V. I. Zimnukhova. Intrusive complexes of the Sarala district. In: Intrusive Complexes of Gold-Ore Districts of the South of Krasnoyarsk Krai. Novosibirsk, “Nauka,” 1967.

3. V. I. Zimnukhova, O. I. Shokhina. On the gold-bearing plagiogranites of the Ararat massif (Kuznetsk Alatau). Materials on Metallogeny and Mineral Resources of Krasnoyarsk Krai. Krasnoyarsk Book Publishing House, 1968.

4. P. F. Ivankin, L. V. Alabin, S. S. Dolgushin, B. N. Zimoglyadov, N. V. Rogov, Z. P. Senkevich. Regularities in the distribution of the most important gold-ore districts in the Altai-Sayan folded region. In the collection: Geological, Geochemical, and Morphological Features of Magmatogenic Ore Deposits of the Altai-Sayan Region. Proceedings of SNIIGGiMS, issue 104, 1969.

5. M. P. Kortusov. Paleozoic intrusive complexes of the Mariinsk taiga (Kuznetsk Alatau). Vol. 1. Lower Paleozoic intrusive complexes. Tomsk, Tomsk State University Press, 1967.