Explosion Formations in the Area of the Yulia Mine

(Batenyov Ridge)

D. I. Tsarev

(Presented by the scientific seminar of the Departments of Mineralogy, Petrography, and Mineral Resources)

In recent years, works on explosion breccias composing cross-cutting bodies—dikes, pipes, branching veins, etc. [4, 8, 10, 16, 17, 24, 29]—have begun to appear frequently in the geological literature. A large proportion of such breccias spatially coincides with ore fields or directly with ore-bearing intrusive bodies [4, 8, 9, 16], and therefore is of special interest. P. F. Ivankin [8] cites several existing views on the genesis of breccias accompanying magmatic bodies. We shall not consider these views here, but shall confine ourselves to presenting our own point of view, obtained in studying the geological structure of the area of the Yulia mine in the Batenyov Ridge.

A characteristic feature of the area of the Yulia mine, as of the entire Batenyov Ridge, is the presence of isolated small areas of volcanogenic rocks, lying with sharp angular unconformity on an Upper Proterozoic–Lower Paleozoic basement. By analogy with the volcanic formations of the framing of the Minusinsk depression, these rocks have been assigned to the Cambrian and Devonian ($D_{1-2}$). It is interesting to note that in most cases small intrusive bodies of the syenite–granodiorite series are spatially associated with areas of these volcanic rocks (near the village of Katyushkina, near Lake Volchye, east of the Khakass game-breeding sovkhoz, near the Bagrad bridge, at the Karasuk deposit, etc.). In studying these areas it became clear that the connection between the surface volcanic formations and the intrusive bodies is genetic. In confirmation of this connection we shall dwell on the characteristics of some of the above-mentioned areas of volcano-plutonic formations.

The area of elevation 808.5 (near the village of Katyushkina) is a synclinal fold, the limbs of which are composed of carbonate rocks, and the hinge part of volcanic rocks of the upper part of the Lower Cambrian and the lower part of the Middle Cambrian. These rocks are cut by small bodies of quartz and quartz-free syenites and syenite-porphyries, by a mass of dikes, and by explosion breccias. The latter, in addition to cross-cutting bodies (dikes and pipes), together with tuffs and ignimbrite-like rocks form cover formations that have been preserved as patches on the Cambrian effusives.

The explosion breccias consist of unsorted fragments of slag-like plagioclase porphyrites, syenites, ortho-

phyres, marbles, quartzites, cemented by fine-fragmental material of the same rocks mixed with epidote and calcite, or by felsite, which bends around the fragments in fluidal streams. Among the angular rock fragments there occur rounded forms that arose as a result of melting or of the chipping of corners during movement along the volcanic conduit. The size of the fragments of syenites and porphyrites reaches 40 cm across, and individual blocks of marble up to 3 meters. The syenites in the fragments are leucocratic, quartz-bearing or quartz-free, and have textures from fine- to coarse-grained; metasomatically overgrown porphyroblasts of orthoclase are observed. Their petrographic and petrochemical compositions are close to those of the syenites cutting Cambrian effusives on the southeastern slope of elevation 808.5 and at the Yulia lead deposit. The size of fragments in the upper horizons of the explosion breccias decreases, the number of fragments of plagioclase porphyrites and syenites decreases, fragments of marbles and quartzites disappear completely, and the explosion breccias are replaced by red tuffs and welded tuffs with numerous mutually parallel lenticular inclusions of brown felsite (fiamme) [14]. The general color tones of the welded tuffs range from brown to dark brown; in detail the coloration is mottled because of the varied composition of the rock fragments. The fragments are predominantly felsites and orthophyres. Plagioclase porphyrites account for as much as 5%. Syenites occur in individual fragments. The shape of the rock fragments is angular and oval, sometimes pear-shaped, which indicates the plastic state of some of them during ejection from the volcanic conduit. The size of the fragments varies widely: from fractions of a mm to 2–3 cm in some varieties of welded tuffs and up to 10–15 cm in others. There is more xenogenic material (plagioclase porphyrites, leucocratic syenites, etc.) in the lower horizons of the welded tuffs, which are gradually replaced by the explosion breccias underlying them.

The fiamme are generally lenticular in form, in places having bizarre outlines with split ends; their amount is up to 50% of the total volume of the rock; at the upper limit their dimensions reach 40 cm in length, 20 cm in width, and 6 cm in thickness, while the most common are 1–10 cm in length. Large fiamme along the periphery have a dark-brown color, which toward the middle is replaced by pinkish-brown. In the central part along the length there runs a band about 2 mm wide, which in the middle likewise has a dark-brown color and along the margins is light pink. This band branches according to the branching of the fiamme, but never reaches its end. At the point of branching the band widens, forming a dark-brown triangular field with concave sides. Some fiamme contain fragments of feldspar crystals and epidote phenocrysts. In all probability, the middle band is the boundary at which two energy levels meet; these arose during heating, as a result of exothermic processes, of a plastic piece of alkaline lava located among a mass of incandescent rock fragments ejected from the volcanic conduit at the moment of an explosive eruption. As many authors of works devoted to the study of welded tuffs (ignimbrites) indicate, the latter were formed as a result of deposits of burning clouds (nuées ardentes) in a Pelean type of eruption [8, 11, 14, 15, 28]. Large masses of pyroclastic material deposited in this way retain within themselves for a long time a high temperature, which is maintained by heat formed during the degassing and crystallization of these deposits. According to the results of experiments carried out by F. R. Boyd [3], the minimum

the sintering temperature of rhyolitic glass is 600°C. This temperature depends on the pressure, time, and H₂O content in the glass.

Thus, the above-mentioned piece of plastic, partly solidified lava, under the influence of the high temperature and pressure of the overlying red-hot pyroclasts, was flattened and spread out; its periphery and central part were enriched with hematite dust. This circumstance testifies to physicochemical processes in the piece of lava that promoted the extrusion of iron. The iron accumulated at the place where the fiamme branched, since three energy levels met here. Thus, the process of iron extrusion from the piece of lava was reflected in the darker coloration of the surface of the fiamme, of its midline, and of the triangular areas at the branching point. The appearance of numerous oddly shaped offshoots and split ends of the fiamme indicates that, under the action of pressure, the molten material penetrated between solid fragments. On the surface of the fiamme one can often observe transverse cracks similar to desiccation cracks. These cracks do not pass deeper than the dark-colored layer. In small varieties of fiamme, however, which are completely colored dark brown, the cracks sometimes cut across them. This circumstance confirms the data that pieces of partly solidified lava, after their deposition, were partially or completely melted, and upon solidification their volume decreased and transverse cracks appeared.

Microscopic study of the sintered tuffs confirms their formation from deposits of glowing clouds that carried red-hot solid fragments of rocks mixed with shreds of lava. After deposition, the small fragments were sintered and, together with the lava shreds, cemented the fragmental mass, forming a pseudofluidal texture of the mesostasis. The large fragments underwent less sintering. The degree of sintering of these tuffs distinguishes them from the concept of “ignimbrite” now widespread in the geological literature, which presupposes sintering of glass fragments into an almost homogeneous mass [14, 19]. But the term “ignimbrite” itself (fiery shower), from the genetic point of view, is quite acceptable for the sintered tuffs of elevation 808.5.

Under the microscope, the fiamme show felsitic, cryptofelsitic, or microspherulitic structures of the groundmass, in which rare grains and glomerocrysts of albite-oligoclase, crystals and fragments of crystals of perthitized orthoclase \((2V = 68—72^\circ)\) are situated. Some of the plagioclase grains are completely replaced by epidote and sericite with a small admixture of ore dust. With a microspherulitic structure of the groundmass, the fiamme contain a large number of micromiandellets filled with green chlorite. The micromiandellets are arranged chaotically or in the form of fluidal bands, but as a rule they are confined to the center of the largest microspherulites. The microspherulites are undoubtedly of autopneumatolytic origin and indicate a high degree of gas saturation of the magma that produced this eruption. Autopneumatolytic formations should also, in all probability, include the radiating mineral aggregates that form a comb-like structure on the surface of some feldspar crystal fragments. R. K. Martin regarded similar mineral aggregates as a combination of silica and feldspar [15]. It should be noted that reddish carbonates commonly develop along the fiamme and the mesostasis.

In this article we shall not dwell on the characteristics of all varieties of explosion formations of the area of elevation 808.5; we shall note only that the dike-like bodies composed of them (fracture

volcanic conduits) are usually accompanied by dikes of orthophyres, microsyenites, and albitophyres. The dikes either cut the explosion breccias along strike, or are situated nearby, parallel to their contact with the enclosing rocks. In petrographic composition the dike rocks are identical to the greater part of the fragments of the explosion breccias. In places, transitions are observed from fragmental dikes into ordinary orthophyre dikes. These circumstances indicate a genetic connection between the dikes and the explosion breccias.

Fig. 1. Diagram of the genetic series of magmatic rocks of the Yulia mine area (the numbers correspond to the rocks listed in Table 2)

Chemical silicate analyses of fiamme from welded tuffs and of syenite fragments from explosion breccias showed a great similarity in their compositions (Table 1).

In the diagram (Fig. 1, Table 2), constructed by the method of G. S. Momdzhi [18], where the quantity

\[ M=\frac{100\,Mg}{Na+K+Ca+Mg}\%, \]

is plotted on the abscissa axis, and on the ordinate axis

\[ H=\frac{100\,(Na+K)}{Na+K+Ca+Mg}\% \quad \text{and} \quad C=\frac{100\,Ca}{Na+K+Ca+Mg}\% \]

(the contents of the elements are taken recalculated to atomic quantities), two series of points arise, one of which corresponds to the values \(MC\) (marked by circles), the second to \(MH\). The series of points lying on smooth curves, close to a hyperbola and a parabola, correspond to genetic

Table 1

Table 2

…of a petrographic series of rocks formed as a result of crystallization differentiation from a single magmatic source. In our case, one genetic series includes the basalt porphyrites of elevation 808.5, the intrusive and subvolcanic bodies of syenitic composition of elevation 808.5, Yulia, and others, fragments of syenites in explosion breccias, and fiamme from welded tuffs. The similarity in the contents of trace elements, according to the results of semiquantitative spectral analysis in rocks of the syenite group, and the similarity of the optical constants of plagioclases and orthoclases in syenites, orthophyres, explosion breccias, and welded tuffs also indicate the genetic relationship of these rocks (Tables 3, 4).

In the area of elevation 808.5, as a result of prospecting and exploration work carried out by staff of the Krasnoyarsk Geological Administration, three types of sulfide mineralization were found: 1) chalcopyrite and pyrite mineralization in skarned limestones and pyroxene-plagioclase porphyrites; 2) disseminated chalcopyrite, galena, and pyrite mineralization in silicified or epidotized effusives, in explosion breccias, tuffs, and limestones; and 3) chalcopyrite and galena mineralization in quartz veins.

All three types of mineralization are associated with postmagmatic hydrothermal activity as the concluding stage of a volcanic process caused by the intrusion into the near-surface zone of the Earth’s crust of a gas-rich alkaline differentiate of basaltic magma.

In the area of the Bagrad Bridge, on the right and left sides of the Sukhaya Yerba River, a volcanogenic sequence lies with a sharp angular unconformity upon Lower–Middle Cambrian carbonate and terrigenous-carbonate rocks; at its base it is represented chiefly by andesite and trachyandesite porphyrites, and above by tuffs of orthophyres, quartz porphyries, albitophyres, and banded felsites. By analogy with the Devonian volcanic formations of the framing of the Minusinsk Basin, this sequence is assigned to the Devonian \((D_{1-2})\).

The porphyrites and Cambrian rocks, especially the terrigenous ones, are cut by numerous dikes of orthophyres, microsyenites, syenite-porphyries, plagioporphyries, porphyrites, and so on. Some of the dikes of orthophyres, syenite-porphyries, and microsyenites at the present-day surface are surrounded by halos of explosion breccias, in places occupying areas of up to several tens of thousands of square meters. In vertical and steeply inclined exposures one can observe how the breccias encase the dikes like a sheath and pinch out with depth. The pinching-out follows the dip of the dike. The composition and structures of most fragments of the explosion breccias are identical to the compositions and structures of the dikes that they surround. To a lesser extent there occur fragments of the same composition as the dike, but with a greater degree of recrystallization, indicating that the rocks composing these fragments formed at a considerable (relative) depth. In addition, the explosion breccias not infrequently contain fragments of sedimentary rocks brought up from depth: limestones, quartzites, and so on. The cement of the breccias is of various kinds: hydrochemical, hydrothermal (epidote-chlorite-calcite), magmatic (felsitic); in places welded fragments occur.

During microscopic study of the dikes associated with explosion breccias, in some of them, in addition to xenoliths of quartzites, a mass of xenogenic grains of plagioclase (albite-oligoclase), more rarely orthoclase, is observed in the form of glomerocrysts, cataclased fragments of crystals, often resorbed, fused grains of quartz, placed in an allotriomorphic-granular or felsitic groundmass,

Table 3

Note: the data were obtained from the results of 30 spectral analyses.

Table 4

Note: the data were obtained from numerous measurements on the Fedorov stage in 24 thin sections (12 thin sections from each group).

having in places a fluidal texture. The quartz grains include embayments and isolated areas of the felsitic groundmass. The structures and mineral inclusions in such rocks indicate the capture by syenite magma of its own fragments, shattered by previous explosions.

Near the Khakassian animal-breeding state farm, a linearly elongated intrusive body of porphyritic syenites, intruded into Devonian plagioclase porphyrites, is cut by dikes of microsyenites and syenite porphyries. In the cirque-like exposure one can observe how some of these dikes, cutting through the roof of the intrusive body—the plagioclase porphyrites—are accompanied, on reaching the surface, by explosion breccias, like sleeves, composed of fragments of microsyenites and syenite porphyries.

As another example one may cite the Karasuk polymetallic deposit. Here an isometric intrusive body, composed of felsite porphyry and syenite porphyry, is surrounded by a half-ring of volcanic breccia. The breccia consists of fragments of porphyrites, felsites, felsite porphyries, orthophyres, and limestones, cemented by felsite, orthophyre, and a chlorite–epidote–calcite mixture, and represents a neck that pierced Cambrian limestones [12].

From our point of view, this breccia is an explosion breccia, formed during the intrusion of gas-rich ore-bearing magma of syenitic composition. Erosion-denudation processes have now exposed this intrusion, which has appeared to view in the above-mentioned form.

The examples given in this article are in good agreement with data obtained by other researchers in various regions of the Soviet Union, and convincingly testify to the genetic connection of welded tuffs (ignimbrites) and explosion breccias with hypabyssal intrusive and subvolcanic bodies.

The mechanism of explosive processes may be set forth from the standpoint of a thermally retrograde increase in vapor pressure in a magmatic chamber. Goranson (1937), Kennedy (1955), and later Ritman (1960) pointed out that the water content in unsaturated magma is distributed in such a way that the vapor pressure in it is everywhere the same. In this case water must accumulate in colder magmatic masses that are under lower pressures. An equilibrium distribution of gases occurs when, in gas-saturated pyromagma, the hydrostatic pressure is equal to the vapor pressure, and when the vapor pressure in unsaturated magma is everywhere the same [23]. In the upper part of the magmatic column, during cooling and crystallization of the magma, water vapors accumulate. If the vapor pressure exceeds the pressure of the roof of the magmatic chamber, the equilibrium is disturbed, a gas phase separates from the magma, the magma turns into active pyromagma and, having blown up the roof of the chamber, bursts out onto the surface. During the explosion, fragments of the enclosing rocks, the crystallized apical part of the magmatic column, scraps of lava, and a large quantity of gases will be ejected.

On the basis of the foregoing, the following conclusions should be drawn.

1. The development of volcanism in the area of the Yulia mine in Cambrian and Devonian time began with the outpouring of lavas of basic composition and proceeded toward increasing alkalinity and acidity in accordance with the differentiation of basaltic magma.

2. Gas-rich alkaline and acidic differentiates of the magma produced a mass of intrusive bodies of the syenite–granosyenite series. With the formation—

by these intrusive bodies under near-surface conditions are genetically associated with explosion formations (breccias and welded tuffs).

1. Explosion formations are mainly confined to sites of polymetallic mineralization and sometimes themselves contain disseminations of copper, lead, and iron sulfides.

2. Polymetallic mineralization is associated with the postmagmatic hydrothermal stage of volcanism.

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