ON THE PARAGENESIS OF METASOMATIC FORMATIONS IN GORNAYA SHORIA (KUZNETSK ALATAU)

V. A. SARAEV

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

This article presents the results of work carried out by the author during 1960–1968 with the aim of studying certain deposits of secondary quartzites, quartzolites, and skarns, as well as the Riphean–Cambrian volcanism of Gornaya Shoria.

As is known, the region under consideration as a whole is characterized by: a fragmentary block–mass structure of the Earth’s crust; a linear elongation of the main tectonic elements (meridional direction); the broad development of Riphean–Cambrian volcanism; sharply differentiated gravitational and magnetic fields, etc. The features of its geological structure characterize Gornaya Shoria as a region of prolonged, chiefly Salairian, tectogenesis and as a Riphean–Cambrian geosyncline.

The evolution of magmatism and the processes of hydrothermal metamorphism associated with it in Gornaya Shoria have certain distinctive features. Four Riphean–Cambrian volcanic complexes are distinguished: 1) Riphean, 2) Ust-Anzas, 3) Mrass, 4) Mundybash.

In Riphean time, volcanism was manifested in the form of separate volcanic centers that bordered the Biya and Shor uplifts, composed of Riphean–Cambrian, predominantly carbonate, deposits. The basic volcanics of the Riphean complex (diabase formation) differ from similar Cambrian formations by their increased basicity, titaniferous character, and sodium specialization of the alkalies. Owing to the insignificant differentiation of magma, rather poor areas of hydrothermally altered rocks arise in the zones of volcanic structures (quartz–sericite, feldspar–quartz–sericite, quartz, barite, and propylitized rocks).

In Ust-Anzas time \((Cm_1^1)\), the extension of linear zones was already fairly considerable, and was compensated by massive eruptions of basaltic lava under submarine conditions. The volcanics of the complex (spilite–diabase formation) are characterized by the broad development of spilites and by a lower content of potassium and titanium in comparison with Riphean volcanics. Formations of hydrothermal metamorphism have not been detected.

In Mrass time \((Cm_1^2)\), the differentiation of magma reaches the level of rhyolites. In the basic volcanics, an increased amount of potassium is recorded (in comparison with the volcanics of the first two complexes), and decreased amounts of iron and titanium. In near-vent and vent zones there arise quartz–sericite, feldspar–quartz–sericite, quartz, ba-

ritic, quartz–kaolinite–hydromica with pyrite, kaolinite–quartz–illite with tourmaline, sericite–chlorite–stilpnomelane (?) and propylitized rocks.

The volcanics of the Mundybash complex ($Cm_2$), in terms of chemistry, belong to the orthophyre group ($K_2O/Na_2O = 1.9$), with a certain rhyolitic tendency. Their iron and titanium contents decrease sharply ($TiO_2 = 0.36\%$; the arithmetic mean of the chemical-analysis results for 30 samples). Deposits of aluminous-type secondary quartzites are spatially and genetically associated with the volcanics of the complex: quartz, topaz, andalusite, diaspore, zunyite, pyrophyllite, and sericite mineral types.

Associated with the volcanics of the first three complexes are the plutonites of the Bazass gabbro–diorite–diabase ($Cm_1$), the Atalyk gabbro–pyroxenite–dunite ($Cm_{1-2}$), and the Lebed tonalite–granodiorite–plagiogranite ($Cm_{2-3}$) complexes; with the volcanics of the Mundybash complex are associated the plutonites of the Shalym gabbro–syenite ($Cm_{2-3}$) complex and the epil leucite complex of Mount Sokol ($Cm_{2-3}$).

Skarns, skarnoids, hornfelses, and quartzolites are the most characteristic post-intrusive formations of the plutonic complexes; moreover, the predominant part of the metasomatites is spatially and genetically connected with the intrusive massifs of the Lebed and Shalym complexes.

The secondary quartzites, together with the propylitized rocks surrounding them, are associated with volcanic areas and are localized within the Tashelga–Kondoma mobile zone, which separates the Biya and Shoria massifs, i.e., where processes of magma differentiation were intensively manifested, where subaerial volcanism took place. Deposits of secondary quartzites of the siliceous type are confined to faults (weakened zones) and are localized in ancient, deeply eroded volcanoes (Mount Uronnik). Such secondary quartzites formed in the concluding stages of formation of the Riphean and Mrass volcanic complexes.

Deposits of secondary quartzites of the aluminous type are associated with explosion breccias and extrusive rocks of the Mundybash complex (Mount Kubes, Mount Chernichnaya, Mount Tatuya, and the Shalym mine).

As can be seen, the intensity of hydrothermal metamorphism increased with increasing differentiation of magmatic centers, as volatiles and alkalis—especially potassium—increased. The formation of high-alumina secondary quartzites marked the end of the Riphean–Cambrian volcanic cycle, which, in turn, completed the geosynclinal development of the region.

At the present erosional level, a close spatial connection is often observed between secondary quartzites and skarns—the most characteristic formations, on the one hand, of geosynclinal volcanism, and on the other, of intrusive magmatism. This connection is established especially clearly in the Shalym volcano-plutonic zone (the Kondoma iron-ore district). Deposits of secondary quartzites and skarns in this zone are located from one another at distances of 0.3–3 km.

High-alumina secondary quartzites formed in vent zones at the expense of explosion breccias and extrusive rocks of the Mundybash complex ($Cm_2$), while magnetite-bearing skarns formed in the near-contact parts of a syenite intrusion (the Shalym gabbro–syenite complex—$Cm_{2-3}$).

Phenomena of recrystallization of the secondary quartzites, as well as the presence in them of veinlets of pyroxene and epidote, and features of the geological structure indicate that the mineral types of secondary quartzites formed earlier than the skarns.

It is evident that the features of the distribution of skarns and secondary quartzites in the volcano-plutonic zone, and their mutual relationships, are governed by a paragenetic association.

The paragenesis of metasomatic formations in volcano-plutonic zones is determined by the commonality of the magmatic source, which is the source both of volcanogenic and plutonic rocks and of those magmatogenic solutions that form both hydrothermally altered rocks (secondary quartzites, propylites, argillisites) and contact-metasomatic formations (skarns, skarnoids, quartzolites).