ON THE GEOLOGY AND PETROGRAPHY OF THE PALEOVOLCANO OF MOUNT URONNIK (Gornaya Shoria)

V. A. Saraev

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

The Lower Cambrian volcano of Mt. Uronnik is located in the interfluve of the Chanysh and Kaurchak, right tributaries of the Lebed River. The study of the geology and petrography of the paleovolcano is of practical interest, since gold-ore mineralization, with elevated contents of silver, zinc, lead, bismuth, arsenic, tellurium, tin, nickel, cobalt, and mercury, is genetically associated with the hydrothermally altered rocks of the volcano.

The geology of the area on a small scale has been studied by many researchers. The first serious work on the geology was the investigations of A. M. Kuzmin [2]. Later the area was visited by A. P. Lebedev [3], V. A. Kuznetsov, and G. P. Nagorskii, who provided a summary of the magmatism and stratigraphy of the entire Lebed region. In 1952–60 E. S. Shinkarev, A. M. Shinkareva, V. I. Fominskii, V. N. Konovalov, and V. I. Zinov'ev carried out geological mapping work. The latter identified in the area of Mt. Uronnik secondary quartzites of quartz–sericite composition, formed, in the opinion of V. I. Zinov'ev, at the expense of felsites and felsite porphyries. In 1963 M. V. Krapivin carried out detailed geophysical work on the watershed of the Maiskii–Talonskii springs, while N. F. Stolbova and V. I. Yarygin, working there in parallel, assigned the volcanogenic rocks to formations of an explosion pipe and noted the zonality of the secondary quartzites. From 1962 to 1965 the Shalym expedition of ZSGU (D. A. Zaitsev, A. A. Lagutin, A. I. Perepelitsyn, V. A. Saraev) carried out prospecting and mapping work, as a result of which auriferous hydrothermally altered rocks of the type of secondary quartzites and propylites were discovered.

The investigations carried out in 1962–1966 yielded materials that allow us to assign the volcanogenic rocks of the area to formations of activity of a central polygenic volcano of mixed type, according to the classification of A. Rittmann [5].

The evidence for the presence of the Uronnik volcano is as follows: the presence of lavas and pyroclastics ranging from basic to acidic composition; the local development of agglomeratic xenotuffs and agglutinate; sharp facies variability; a diversity of rock types over a rather small area; the occurrence of hydrothermally altered rocks of the type of argillizites, secondary quartzites, and propylites; and the geological structure (Fig. 1).

Fig. 1. Geological map of the area of the paleovolcano of Mount Uronnik (schematized).
Compiled by V. A. Saraev.

1 — weathering crust; 2 — Devonian red-colored clastic rocks; 3 — volcanomictic sandstones and conglomerates; 4 — marbles; 5 — tuffs and clastolavas of acidic composition; 6 — agglomeratic xenotuff; 7 — agglutinate; 8 — blocky agglomeratic lava; 9 — mixed tuffs of basaltic composition; 10 — lava of basaltic composition; 11 — porphyritic granites; 12 — monolithic rocks of contact relations, according to VSEGEI data; 13 — gold-bearing secondary quartzites; 14 — secondary quartzites of quartz-sericite composition; 15 — propylitization; 16 — argillization; 17 — gold-bearing placers; 18 — disjunctives; 19 — lithological boundaries; 20 — bedding.

The paleovolcano of Mt. Uronnik is situated, in the structural plan, within the Tashelgino-Kondoma mobile zone, which borders the Shoria massif on the west. The volcanogenic formations constitute part of the southwestern limb of the Upper Kondoma anticline. The volcanic edifice is located in a tectonically weakened zone at the intersection of the main disjunctive faults of submeridional trend and additional ones of NW strike.

The volcanogenic rocks composing the paleovolcano are assigned to the formations of the Mrasskaya Formation, formed in the upper Lower Cambrian. In neighboring areas the rocks of the Mrasskaya Formation, of extremely variegated composition (locally forming sequences of the “layer-cake” type), rest with angular unconformity on the spilite-diabase formation of the lower Lower Cambrian (Ust-Anzas Formation).

The rocks of the Mrasskaya Formation are intruded and metamorphosed by intrusions of the Lebed tonalite—granodiorite—plagiogranite complex of Middle Cambrian age. Within the area studied, the volcanogenic rocks are cut by the Chanysh intrusive massif. In plan it has a lens-like form, elongated concordantly in a NW direction, with a cross-section of about 2 km and an area of about 20 sq. km. The contacts dip subparallelly to the NE at an angle of 70° (azimuth of strike 300°).

The petrographic varieties of the intrusion (6) are represented by three groups of rocks: 1) hornblendites, gabbro-diorites, and quartz monzonites; 2) porphyritic biotite-hornblende granites and their varieties; 3) aplites, malchites. The contact effect of the intrusion was expressed in the transformation of quartz-sericite secondary quartzites into coarser-grained quartz-muscovite secondary quartzites and in the formation of thin lens- and nest-like bodies of garnet-pyroxene skarns, zones of skarnification within the development of blocky agglomerate lava of basaltic composition, at the contacts of marbles and volcanomictic sandstones.

On the eroded surface of the volcanogenic formations lie variegated clastic rocks of the Andobinskaya sequence of Middle Devonian age. As a result of block tectonic movements, the variegated deposits, together with the volcanites underlying them, were dropped to a considerable depth, forming the Andobinsk graben.

The petrographic varieties of the volcanogenic formations of the Mt. Uronnik area, in accordance with the classification of E. F. Maleev [4], are represented by the following groups of rocks: 1) vesicular lavas—amygdaloidal porphyrites, plagioclase and plagioclase-pyroxene porphyrites; 2) lavoclastic rocks—blocky agglomerate lava and clastolava of andesite-basalt composition, clastolava of acidic composition; 3) pyroclastic rocks—agglutinate, mixed tuffs of basaltic composition, ash tuff of acidic composition, agglomeratic xenotuff; 4) pyroclastic-sedimentary—tuff conglomerate; 5) volcanoterrigenous—volcanomictic sandstones and conglomerates. A characterization of the lavo- and pyroclastic rocks is given below.

Vesicular lavas—amygdaloidal porphyrites have a limited distribution and are observed on the western and northern slopes of Mt. Uronnik (Fig. 1). These are dark-green dense rocks with a large number of amygdules (up to 25–60%) of rounded form, ranging in size from 1 mm to 10 mm, filled chiefly with epidote, partly with calcite, quartz, and chlorite. Under the microscope the structure of the groundmass is intersertal, with newly formed lepidoblastic texture.

Block agglomerate lava of basaltic composition is exposed on the eastern slope of Mt. Uronnik between clastolava of acid composition and agglutinate. This is a dense, lithoid, dark-green and greenish-gray rock, consisting of fragments of plagioclase-pyroxene porphyrites more than 200 mm in size, cemented by a mass of small fragments (up to 1 mm) with traces of melting. The fine-fragmental material is composed of porphyrites, plagioclases, and pyroxene replaced by green hornblende. These rocks are amphibolized and skarnified. Garnets of brown tones are scattered over a considerable area in the form of separate inclusions of ovoid shapes. In places, garnet-pyroxene skarns are observed in the form of small nests. Skarnification is manifested in the exocontact part of the Chanysh massif. The green hornblende replacing the pyroxene of the porphyritic segregations has the following optical constants: Ng = 1.666; Np = 1.645; Ng Λc = 19°.

Clastolava of andesite-basalt composition is localized in the watershed part of the Maiskii-Talonskii streams. The color of the rock is brownish-green, yellowish-green; in places a variegated coloration in bright tones appears (a combination of red, white, and greenish colors). The rock is characterized by the fact that, within the lava mass of andesite-basalt composition, there is a noticeable quantity of fragments of analogous composition with traces of melting (up to 10 mm across). The clastolava is intensely altered (which accounts for the variegated coloration): it is argillized, propylitized, and in places transformed into secondary quartzites of quartz-kaolinite, quartz-pyrophyllite, and quartz-hydromica composition.

Clastolava of acid composition is traceable from the Kaurchak River in a NW direction for more than 3 km, with a width of up to 500 m. In color and dense lithoid structure it differs sharply from the other rock types of the area. The color of the rock is light gray, pinkish, light greenish, and lilac. The structure is fine-grained, the texture massive. Very rarely, within the light-colored fine-grained mass, angular fragments of plagioclase porphyrites are observed (up to 30 mm across); as a usual phenomenon there are inclusions of fine-grained structure, dark-gray and greenish-gray in color, rounded and lenticular in form, up to 10 mm in size. The clastolava is intensely altered and has for the most part been transformed into secondary quartzites of quartz-sericite composition, more rarely monoquartz and quartz-muscovite composition (near the intrusion). Under the microscope, relicts of sintered ash particles are detected; characteristic is the presence of small lenses and bands formed as a result of their sintering; in a number of samples the original fluidal texture and spherulitic, microfelsitic structure are manifested.

Agglutinate is mapped as rock outcrops on the summit of Mt. Uronnik and on the right side of the Kaurchak River. These are dark-green and greenish-gray porous rocks of conglomeratic appearance. The agglutinate is composed of irregular, pancake-shaped pieces of lava of basaltic composition ejected from the volcano.

Mixed tuffs of basaltic composition are widespread south of the rocks described above, where in the area of the Kaurchak—Lebed interfluve (Lebed gold-skarn zone) they are replaced by plagioclase and plagioclase-pyroxene porphyrites. These are green-colored dense rocks of massive, sometimes banded, texture. Vitrolithic-crystal-clastic tuffs are noted. The fragment sizes range from 0.1 to 10 mm, more rarely from 0.02 to 0.1 mm. The form of the fragments is angular, irregular, round, with traces of melting. The fragmental material makes up 30–85% of the rock and is represented by plagioclases,

plagioclase-pyroxene porphyrites, aphanites, plagioclase, and pyroxene replaced by green hornblende. Mineralogical composition: plagioclase No. 55; green hornblende after pyroxene with optical constants: \(Ng = 1.661\), \(Np = 1.642\), \(Ng \angle c = 19^\circ\); relicts of monoclinic pyroxene; actinolite; epidote with refractive indices: \(Ng = 1.730\), \(Np = 1.715\); zoisite; chlorite; sericite; albite; quartz; biotite; sossurite; pyrite; chalcopyrite; limonite; ilmenite. The cementing mass is composed of a chlorite-actinolite aggregate.

It should be noted that the lavas and pyroclastics of basaltic composition are intensely altered; the process of actinolitization, and in some areas epidotization, is especially widespread. The finer-grained varieties of tuffs, occurring as rare thin interbeds, contain a noticeable amount of pyrite and, more rarely, chalcopyrite, in the form of finely disseminated inclusions. In general, the pyroclastics and lavas of basaltic composition are characterized by the mineralogical composition given for the mixed tuffs.

Agglomerate xenotuffs are widespread on the watershed of the Maysky–Talonsky streams. These are variegated rocks: greenish-gray, ash-gray, sometimes pinkish-gray in color, consisting of fragments and the mass cementing them. The clastic material of the agglomerate xenotuffs is represented by bombs and lapilli of spindle-shaped, lens-shaped, ellipsoidal, and rounded forms, up to 30 cm in size.

Fig. 2. Diagram of the contents of the principal components of hydrothermally altered rocks of the paleovolcano of Mount Uronnik: 1, 5, 7 — argillized basic-composition clastolava; 8, 9 — ferruginized secondary quartzites of kaolinite-quartz composition; 10, 11 — argillized agglomerate xenotuff; 2, 3, 4, 6 — propylitized basic-composition clastolava; 12–15 — secondary quartzites after acid-composition clastolava.

They are composed of lavas of basaltic and andesitic composition, microfelsite, and felsite-porphyry. The amount of clastic material locally reaches 40% of the entire rock. The cementing mass has the character of tuff-lava of andesitic and andesite-dacitic composition. Around individual bombs a distinct fluidality and a violet baking rim are observed. Under the microscope a vitro-lithocrystalloclastic structure and fluidal, fluidal-banded textures are found. Mineralogical composition: feldspar, quartz, sericite, pyrophyllite, biotite, epidote, chlorite, hydromicas, kaolinite, tourmaline, hematite, pyrite, limonite. From the diagram (Fig. 2) it can be seen that the agglomeratic xenotuffs (without fragments of acidic composition), in their chemical composition, belong to the andesite group of rocks. The xenotuffs are intensely altered: argillized, hematitized, baritized, and pyritized.

Hydrothermally altered rocks of the paleovolcano are widely distributed. Among them are developed secondary quartzites, propylitized and argillized rocks, which formed in the near-vent zone as a result of the hydrothermal activity of the paleovolcano. The secondary quartzites belong to formations of the following mineralogical facies: quartz-sericite, quartz-hydromica, quartz-kaolinite, quartz-pyrophyllite, and monoquartz. From the diagram (Fig. 2) it is evident that the hydrothermally altered rocks of the paleovolcano consist chiefly of silica, alumina, ferric and ferrous iron (80–93%). The propylitized rocks differ from the other types by a noticeable content of CaO and Na₂O. The argillized rocks, in comparison with the secondary quartzites, contain a larger amount of alumina (up to 26.7%) and less K₂O.

As a result of denudation and deep-seated fault disturbances, the volcanic edifice underwent profound changes, so that under present-day conditions we can observe only the relics of the volcano and judge its morphology and geological structure from the qualitative changes in the magma, the type of eruptions, and the hydrothermal activity.

The evolution of the volcanic activity appears to us as follows. After the formation of the spilite-diabase formation in the lower Lower Cambrian, occupying a considerable area of Gornaya Shoria, local uplifts arose in the upper Lower Cambrian along the periphery of the Shoria massif, accompanied by vigorous volcanic activity. At this time an entire series of volcanoes arose, which formed the sedimentary-volcanogenic sequence of the Mrasskaya Formation.

The volcanism of the area began with the outpouring of enormous masses of basaltic lavas and ended with a powerful eruptive process that produced agglomeratic xenotuffs, and then with the eruption of clastolavas and tuffs of acidic composition. Thus, the Uronnik volcano is characterized by a continuous evolution of magma from basic to acidic. According to the concepts of A. Rittmann [5], the style of its edifice should change from a lava volcano through a stratovolcano to a volcano of loose products.

Within the investigated area the volcanogenic formations were formed under subaerial conditions in the near-vent, partly intermediate, and distal zones of the volcanic edifice, and they belong to the andesite-basalt formation of the basalt-andesite-liparite group of formations according to the classification of Yu. A. Kuznetsov [1].

References

1. Yu. A. Kuznetsov. The Main Types of Magmatic Formations. Nedra Publishing House, 1964.

2. A. I. Kuzmin. A brief report on geological investigations in the upper reaches of the Lebed River, a right tributary of the Biya River. Proceedings of the Siberian Branch of the Geological Committee, 1924.

3. A. P. Lebedev. On the petrography of northeastern Altai. Proceedings of the Petrographic Institute, Academy of Sciences of the USSR, issue 13, 1938.

4. E. F. Maleev. Volcaniclastic Rocks. Gosgeoltekhizdat, 1963.

5. A. Ritman. Volcanoes and Their Activity. Mir Publishing House, 1964.

6. V. I. Yarygin. Some questions of the petrography of the Chanysh massif (Gorny Altai). Proceedings of TPI, vol. 135, 1965.