Fissure Volcanic Edifices in the Area of the Yuliya Mine

(Batenevsky Ridge)

D. I. Tsarev

(Presented by Professor K. V. Radugin)

Finds of Devonian volcanic vents of the central type in the framing of the Minusinsk depressions are frequent [11], but very little is known about the fissure type of eruption. This is not because fissure vents are encountered more rarely, but, probably, because diagnosing them is more difficult.

Usually we see fissure volcanic edifices in outcrops in the form of ordinary or clastic dikes [4, 9, 10, 12]. It is difficult to judge whether an ordinary dike communicated with the surface or not if we do not see its direct transition into a surface outpouring. This question is comparatively easier to resolve with clastic dikes, but first of all it is necessary to have evidence that we are dealing with an endogenous formation and not with another kind of clastic dike [3, 6].

On the Batenevsky Ridge, in the area of the Yuliya mine, on an Upper Proterozoic–Lower Paleozoic basement, in grabens and Middle Paleozoic synclines, separate small areas of Cambrian and Devonian volcanogenic rocks have been preserved (near the village of Katyushkina, by the Bagrad bridge, near the Khakassian animal-breeding state farm). Among these rocks the author recorded and studied several fissure volcanic edifices which, because of their small dimensions (width 1–5 m, length up to 200 m), should be regarded as secondary. The principal vents, in all probability, are concealed beneath volcanic deposits or are so deeply eroded that they appear to our observation as hypabyssal intrusive bodies.

In the present article we shall confine ourselves to a general characterization of fissure volcanic conduits near the village of Katyushkina. Here, on the northeastern slope of elevation 808.5, among Cambrian pyroxene-plagioclase porphyrites, there can be traced in a NW direction a series of red felsite, orthophyre, microsyenite dikes and dike-like cutting bodies filled with fragmental material. The latter have a thickness of 1–5 m, and in swellings up to 50 m. The dike-like bodies, or, as we shall call them henceforth, clastic dikes, locally pass into ordinary dikes or are cut by them lengthwise. The enclosing porphyrites are altered (epidotized) in contacts with all types of dikes. Usually, in association with the clastic dikes, explosion breccias are exposed, consisting of fragments of leucocratic syenites, marbles, quartzites, porphyrites, and orthophyres. Fragments of syenites and marbles reach 40–50 cm in diameter.

The composition of the material of the fragments making up the clastic dikes is predominantly felsite-orthophyric, with or without an admixture of plagioclase and pyroxene-plagioclase porphyrites, leucocratic syenites, identical in composition and structure to the syenites exposed at the foot of the SW slope of elevation 808.5. According to the predominant composition, color, and structure, the fragmental material may be divided into three rock types: 1) a type of red felsites, 2) pink orthophyres, 3) brown felsitic orthophyres with a fluidal cementing mass. All three types locally grade into one another.

For the first type, a characteristic feature is abundant epidotization and carbonatization, which indicate a process of pneumatolysis that arose during the blowing of heated volcanic gases through the fragmental mass.

Under the microscope, in felsitic fragments one observes felsitic aphyric and porphyritic structures with a felsitic or trachytic groundmass. In rare porphyritic segregations there is untwinned albite No. 7 or albite with simple twins. In the groundmass, grains of albite and orthoclase have a patchy structure or occur as microlites with corroded margins. Among the groundmass are grains of quartz, calcite, and flakes of green chlorite. The calcite is white or reddish brown and contains a fine dusting of ore mineral. The pink color of the felsite probably arose through the finely dispersed dissemination of ferric oxides. The mass cementing the fragments is an epidote-calcite mixture. The epidote grains are prismatic, idiomorphic; between them is xenomorphic calcite.

Determination of alkalies by flame photometry in three samples of red felsites showed the following contents: Na from 2.62 to 3.19%, K from 1.80 to 3.24%. Spectral semiquantitative analysis of the same rocks indicates a somewhat elevated content, for leucocratic rocks, of vanadium, manganese, and chromium.

The second type of clastic formations is characterized by the fact that its fragments are so strongly welded together that they form an ignimbrite-like rock with a texture close to homogeneous.

Under the microscope it is found that the structure of the fragments is transitional from felsitic to orthophyric and trachytic, with poorly outlined microlites of potassium feldspar. The outlines of the fragments are indistinct. But where fragmental orthophyres pass into a homogeneous orthophyric dike, a binding mass of felsitic-orthophyric structure is observed under the microscope between the fragments. The fragments differ from the groundmass in a comparatively more advanced degree of recrystallization. Rare, rather large grains of feldspars occur both as phenocrysts and in individual fragments of crystals. The optical constants indicate that these are orthoclase ($2v$ from $-68^\circ$ to $-72^\circ$) and albite-oligoclase (No. 12—14).

A distinctive feature of the third type of fragmental material is the fluidal texture of the felsitic groundmass cementing the fragments. In outward appearance these rocks resemble so-called tufolavas [2, 16] and are a volcanic breccia consisting mainly of fragments of brown and pink felsite, orthophyre, and crystals of orthoclase and plagioclase, smoothly enveloped by fluidal streams of brown and pink felsite. The nature of these fluidal streams is twofold: 1) fragments of orthophyre and felsite softened and flattened under the action of high temperature; 2) pieces stretched by the flow of lava, reaching 1 m in length at a thickness of 4 cm. Among the fragments there are leucocratic syenites and lilac slag-like plagioclase porphyrites. Fluidality,

clearly observed in polished specimens, suggests a once laminar movement, under high pressure, of a mixture of rock fragments and lava tightly compressed between the walls of a fracture. The possible movement of this mixture under great pressure is indicated by the lens-shaped form of the solid rock fragments and their transverse fracturing. It is noticeable that, as the fragments soften, they lengthen, are stretched in the direction of mass movement, and turn into fluidal bands. The rock contains sparse disseminations of oxidized pyrite.

Under the microscope, the fluidal bands reveal a felsitic, cryptofelsitic structure with hairlike crystallites, and a fluctuation texture. The hairlike crystallites are elongated in the direction of the fluidal streams and, when the microscope stage is rotated, go extinct simultaneously throughout the entire band. The feldspar crystals, being more refractory, have sharply angular outlines and are usually cataclased. Orthoclase in crystal fragments is perthitized \((2v = -68)\). Orthophyre in fragments has a structure varying from orthophyric to pilotaxitic and trachytic, with serrated outlines of microlites of potassium feldspar.

The structural and textural features of the rocks described above, composing dike-like bodies, their epidotization and carbonatization, the disseminations of sulfides in them, the epidotization and reddening of the exocontacts, and also the spatial association with them of dikes of orthophyre, felsite, and microsyenite—all this indicates that in the present case we are dealing with formations connected with endogenous processes. The deposits of volcanic rocks occurring nearby (tuffs and welded tuffs), identical in composition to the fragmental rocks described above, give grounds for considering the latter to be formations of fissure volcanic conduits [14, 17].

The connection of dikes of acidic and alkaline rocks with fissure volcanic apparatuses is undoubtedly genetic; this is indicated by transitions observed in some places from fragmental rocks into dikes of orthophyres. In all probability, these dikes produced eruptions of orthophyres.

Fissure volcanic conduits similar to those described above are also found in the areas of the Bagrad Bridge and the Khakassian game-breeding state farm.

On the basis of the study of volcanic apparatuses, we draw the following conclusions:

1. In the area of the Yuliya mine, fissure acid-alkaline volcanism took place in the Middle Cambrian and Devonian.

2. The presence, among the fragmental material of fissure volcanic conduits, of fragments of syenites and orthophyres indicates a possible genetic connection of the surface volcanism of the area with hypabyssal intrusive bodies [1, 13].

3. The distribution of volcanic apparatuses spatially coincides with sites of chalcopyrite and galena mineralization [Mount 808.5, Bagrad Bridge, game-breeding state farm].

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