FEATURES OF HYDROTHERMAL METAMORPHISM OF BASIC EFFUSIVE ROCKS OF THE TALANOVSKY GRABEN

(NORTHERN SPURS OF THE KUZNETSK ALATAU)

V. G. Kryukov, V. Z. Mustafin, V. K. Cherepnin

(Presented by Professor A. M. Kuzmin)

Recently, a considerable body of material has been published on alterations of acidic and subalkaline volcanogenic formations; to a lesser extent—on intermediate and basic ones, which are characterized by the distinctive nature of the products of hydrothermal metamorphism, caused by the similarity of the newly formed rock to acidic or subalkaline varieties of effusive rocks. Such facts are very clearly manifested within the area of the Talanovsky graben. Study of one of the sectors of this structure makes it possible to consider certain features of the hydrothermal alteration of volcanites of basic composition.

The sector is composed of labradorite and labradorite–pyroxene porphyries and their clastolavas. The strike of the rocks described is nearly meridional, with monoclinal eastward dip at angles of 40–70° [3]. The porphyries are characterized by dark-gray to black coloration and by massive, amygdaloidal, and weakly fluidal textures. Their mineralogical composition is simple: plagioclase No. 40–60, rarely No. 70–76 (60–80%); pyroxene, represented by augite (10–30%); and magnetite (10–20%). In addition, rare grains of apatite and pyrrhotite are noted.

The effusive rocks of basic composition are intruded by a large dike-like body of basalt porphyries with numerous apophyses. The strike of this body is submeridional, with an eastward dip at angles of 60–70°. The basalt porphyries are distinguished by a dark-gray color with a greenish tint and by a considerable degree of recrystallization. In their composition, plagioclases of medium basicity predominate (No. 30–50, less commonly No. 50–60). Femic minerals (augite–omphacite and olivine) play a subordinate role.

The effusive and subvolcanic varieties of porphyries are cut by dikes of syenite porphyries, diorite porphyries, and spessartites. All of them have a northwestern strike and a steep (70–80°) southwestern dip. Their length is tens, sometimes hundreds, of meters, with thicknesses from 0.5–1.5 to 10–20 m. The youngest are the dikes of diorite porphyries and spessartites.

The syenite porphyries are fine- and cryptocrystalline rocks. Exclusively potassium-feldspar porphyritic segregations constitute 5–15% of the total volume. The prismatic-granular groundmass of the rock consists predominantly of potassium feldspar, as well as of dark-colored minerals, almost completely replaced by chlorite and sericite. Quantitatively, feldspars and mafic minerals occur in ratios from 2:1 to 4:1.

Diorite porphyrites are characterized by a large number of porphyritic phenocrysts of plagioclase No. 50–55 and augite, often titaniferous. Their ophitic groundmass consists of plagioclase No. 30–50 (about 40–60%) and augite (30–40%). Some amounts of hornblende, apatite, magnetite, ilmenite, and pyrite are also identified.

Spessartites have a porphyritic texture ranging from fine- to coarse-grained. Their principal minerals are: ordinary hornblende (40–60%), plagioclase No. 30–50 (about 30–40%), and augite (up to 5–10%). In addition, individual grains of biotite, apatite, ilmenite, magnetite, and pyrite are recorded.

The structural features of the area are determined by the monoclinal dip of the effusives and by tectonic dislocations of submeridional, northwestern, and sublatitudinal trends (Fig. 1). In terms of character

Fig. 1. Geological map of the area (compiled by V. G. Kryukov and V. Z. Mustafin): 1 — labradorite, labrador-pyroxene porphyrites and their clastolavas; 2 — basalt porphyrites; 3 — shonkinite-porphyries; 4 — spessartites; 5 — diorite porphyrites; 6 — skarn-like formations; 7 — medium-temperature propylites; 8 — low-temperature propylites; 9 — garnet and epidote-garnet metasomatites; 10 — ore bodies; 11 — pyroxene “skarns”; 12 — pyroxene-garnet “skarns”; 13 — sampling line for silicate analysis; 14 — disjunctive faults.

The disjunctives correspond to faults and strike-slip faults. The spatial distribution of the products of hydrothermal metamorphism is controlled by northwestern and partly submeridional structures.

Within the area studied, on the basis of the investigation of newly formed minerals, three types of altered rocks can be distinguished, associated with the action of hydrothermal solutions of different stages of mineralization. The first stage corresponds to skarn-like formations; the second stage gave rise to propylites; and the third stage to quartz–sericite metasomatites.

Skarn-like formations (in the sense of D. S. Korzhinskii, 1945) develop chiefly along the contact of basalt porphyrites with basic effusives. The width of the zone of newly formed minerals arising in subvolcanic rocks reaches 50–90 m. In volcanites the alterations are much more local, forming zones about 5–10 m wide. Only in areas with numerous apophyses of basalt porphyrites are swellings of the zones of skarn-like formations observed, acquiring an extremely irregular form.

The newly formed rocks are characterized by a dark greenish-gray coloration. The metasomatic minerals in them occur as a fine-grained aggregate of massive texture, and also form spots, veinlets, small druses of recrystallization, and inclusions of large metacrysts. Quantitatively, the newly formed minerals constitute from 20–40 to 60–80% of the rock mass.

With a certain degree of convention, taking into account the different composition of the original rocks, the structural position, and the mineralogical composition of the newly formed minerals, two mineral facies are distinguished: 1) pyroxene, 2) pyroxene–garnet.

The complex of hydrothermally metamorphosed rocks that makes up the pyroxene facies is developed along the contacts of subvolcanic bodies with effusives. In this case the alterations are expressed most clearly in basalt porphyrites and in spessartites.

The mineralogical composition of this facies is characterized by the predominance of dark-colored minerals, among which pyroxene constitutes 40–70%. The amount of andesine reaches 20–40%. In addition, amphiboles are noted, as well as epidote, chlorite, calcite, quartz, and magnetite. The “endoskarns” and “exoskarns” distinguished for the facies described differ somewhat in mineralogical composition. The former are characterized by the presence of salite, hornblende, and ferroactinolite; in the “exoskarns,” along with salite, ferrosalite is observed.

In studying the mineralogical composition it was found that in the contact zones of effusives and subvolcanic rocks the grain size of salite reaches 2–4 mm, whereas with increasing distance from the contact the crystal size decreases to 0.2–0.5 mm. Newly formed pyroxenes show weak pleochroism from light bluish-green along the \(Np\) axis to pale green along the \(Nq\) axis. The angle \(c \wedge Nq\) is up to 46°, birefringence is about 0.030, and the refractive indices along \(N'g\): for salite—about 1.725, and for ferrosalite—1.735.

Hornblende composes the central parts of microdruses of recrystallization, where the size of its crystals reaches 0.5–1.5 mm. It is characterized by distinct pleochroism from greenish-yellow along the \(Np\) axis to bluish-green along \(Nq\), with the absorption scheme \(Np < Nm < Nq\). The angle \(c \wedge Nq\) is about 23°, and the refractive indices are \(N'g = 1.638\) and \(N'm = 1.625\). All this indicates [1] an amphibole rich in magnesium (60%).

Ferroactinolite, as a rule, forms a rim around grains of hornblende, the size of its crystals reaching only 0.3–0.5 mm. Pleochroism is sharp: along \(Nq\)—bluish-greenish, along \(Np\)—yellowish-gre...

novated. The angle with $\Delta N'g$ is 12–14°; birefringence is about 0.017–0.020, and the refractive index along $N'_q = 1.700$.

Chlorite is represented by fibrous and radial-radiating aggregates of pennine with weak pleochroism from light yellow-green along the $Nq$ axis to bluish green along the $Np$ axis. The absorption scheme is $Np \approx Nm \approx Nq$; refractive indices along $N'_q = 1.570$, and along $N'_p$—slightly less than 1.570.

Calcite is characterized by carbonate cleavage, pseudoabsorption, high birefringence, and refractive indices: $n'_o$ about 1.660 and $n'_e = 1.485$. Its grains reach 1–5 mm.

The pyroxene-garnet facies is far less widespread, gravitating toward the contacts of subvolcanic bodies with effusives. In essence, it represents the result of further development of the process, when the amount of garnet increases so much that it becomes possible to distinguish an independent facies. The quantitative ratio of pyroxene to garnet corresponds to 3:1. In addition to widely developed ferrosalite and garnet, andesine No. 30–35, epidote, calcite, quartz, magnetite, and, more rarely, actinolite, albite, sericite, sphene, and apatite are also noted. In individual areas the development of an iron-rich variety of pyroxene, close to hedenbergite, is established.

The typical minerals of skarn-like formations are pyroxenes, garnet, and hornblende, which indicate only partial mobility of Na$_2$O and K$_2$O. The mobility of H$_2$O and CO$_2$ is beyond doubt. Analysis of the mineral composition of these formations also indicates the practically complete absence of noticeable movements of MgO, iron oxides, CaO, P$_2$O$_5$, SiO$_2$, and Al$_2$O$_3$.

Propylites 4,7 are extremely widespread, appearing especially clearly in effusives and syenite-porphyries. With the “skarns” they have both crosscutting relationships and gradual transitions. Propylites differ from skarn-like formations in the following features: 1) coloration in greenish tones; 2) the presence of breccia-like textures; 3) the absence of garnets and pyroxenes; 4) broad development of typical minerals, namely epidote, actinolite, chlorite, albite, and pyrite, with their total amount not exceeding 20–30%.

In the propylites, corrosion of pyroxene and garnet grains by epidote is observed. The latter, together with pyrite, often replaces magnetite. Along with the typical minerals that define the character of the new formations of the stage, orthoclase, sericite, and quartz are present in small amounts. It is interesting that sericite, as a rule, is separated from quartz by albite, which, in turn, is separated from calcite by an aggregate of quartz crystals. In this case epidote is characterized by: grain size 0.1–0.3 mm, weak pleochroism from colorless along $Np$ to yellowish green along $Nq$, with the absorption scheme $Np < Ng < Nm$. Its birefringence is about 0.020, and the refractive index along $N'_q = 1.770$.

The grains of actinolite, forming platy crystals and radial-radiating aggregates, reach 0.3–0.5 mm. Unlike ferroactinolite, the refractive index of the mineral described along the axis is 1.670.

Among the chlorites, pennine and clinochlore are noted, the former being associated with epidote, whereas clinochlore is associated with albite and calcite. Clinochlore is characterized, in contrast to pennine, by a higher interference color and a refractive index along $N_q$ of about 1.585.

It should be noted that, even macroscopically, among the propylites it is possible to distinguish facies with epidote (medium-temperature propylites) and epidote-free facies (low-temperature propylites). The latter, being more locally developed, usually tend toward the north-

to western structures. The width of the zone of low-temperature propylites ranges from 20–40 m to 100–200 m.

Study of the facies composition of hydrothermal alterations of the propylite type (epidote, actinolite, chlorites, albite, pyrite) makes it possible to note that the increasing intensity of the postmagmatic process leads to a certain mobility of the oxides of magnesium and iron.

Quartz-sericite metasomatites are, in terms of abundance, sharply subordinate to propylites. Appearing as nests, “columns,” and lenses with a depth, they rather quickly pinch out. At the surface the width of their outcrop is 10–15 m, more rarely up to 20–25 m, while already at depths of 60–90 m from the ground surface they are represented by zones up to 0.5–1.0 m thick. The quartz-sericite metasomatites are confined to the same northwestern structures with the most intense fracturing. They are characterized by certain features that make it possible to distinguish them from propylites and from “skarns”: 1) reddish-greenish to light greenish-gray colors; 2) banded, spotted, massive, breccia textures; 3) complete absence of garnets, pyroxenes, amphiboles, and epidote; 4) wide development of sericite and quartz, as well as orthoclase, albite, carbonates, and chlorite; 5) intensive manifestation of sulfide mineralization, as well as fluorite; 6) quantitatively, the new formations make up as much as 80–90% of the entire rock mass.

The width of the zone transitional from propylites to quartz-sericite metasomatites is measured in the first meters. Within it, together with relicts of epidote, actinolite, and the original effusive rock, the following new formations are established: calcite, albite, pennine, and sericite. At the same time, the role of albite increases with depth. Rock intervals directly adjoining disjunctive structures have a more complex composition. For them, the predominant role of quartz and sericite is recorded. At the same time, dolomite, orthoclase, albite, clinochlore, prochlorite, and fluorite are fairly widely developed. The ore minerals are represented by chalcopyrite, galena, sphalerite, pyrite, arsenopyrite, and hematite.

It should be noted that sericite, occurring practically throughout the entire metasomatic column, plays an independent role only in low-temperature formations. This mineral develops intensively in mineralized intervals, where its amount reaches 25–35% of the rock mass. In this case it often forms, together with adularia or quartz, microdruses of recrystallization.

Within the mineralized intervals, in addition to sericite, such characteristic minerals as dolomite (high refractive indices \(n^{o} = 1.685\) and \(n'_{e}\) about 1.505), as well as clinochlore and prochlorite (sharp pleochroism from blue-green along the \(Np\) axis to yellowish-green along the \(Ng\) axis; refractive index along \(Ng\) about 1.640), are observed.

Ore minerals are usually confined to accumulations of sericite and chlorite, forming disseminations, spots, microveinlets, and the cement of breccias. In this case the vein minerals, as a rule, form equilibrium associations with the ore minerals, which is emphasized by approximately equal idiomorphism of both.

On the basis of the mineralogical composition (quartz, sericite, chlorite), the following sequence of mobility of components is established: H\(_2\)O, CO\(_2\), S, O\(_2\), K\(_2\)O, Na\(_2\)O.

The process of formation of the various types of altered rocks, judging from the relationships of the newly formed minerals, is generally continuous, expressing a gradual decrease in temperature. The latter indicates the manifestation of regressive hydrothermal metamorphism, in the final stage of which ore mineralization was formed.

The metasomatic transformation of the effusive rocks is reflected in Fig. 2 and Table 1, which show the change in the chemical composition of labrador-pyroxene porphyrites. It should be noted that the use of Barth’s oxygen method in determining the gain and loss of chemical components during hydrothermal metamorphism, in the case of slight fluctuations in porosity, does not provoke substantial objections among researchers [5, 6].

Analysis of the diagram shows that the initial stages of the process are distinguished by a considerable basicity of the solutions and by the participation of sodium. In contact metamorphism, no substantial introduction of any components is observed. However, already at the stage of formation of medium-temperature propylites, an introduction of trivalent iron, calcium, and sodium is noted; these are consumed mainly in the formation of epidote, albite, calcite, and actinolite. Sodium also plays a leading role in the low-temperature propylites. Quartz-sericite metasomatites are characterized by an increasing role of potassium, as well as aluminum, trivalent iron, and water. This accounts for the fact that sericite, ferruginous chlorite, and orthoclase are widely developed.

Fig. 2. Diagram of the gain and loss of chemical components during the hydrothermal metamorphism of basic effusive rocks. The symbols are the same as in Fig. 1.

Thus, the material presented shows that postmagmatic activity was manifested in the formation of solutions that led to the appearance of skarn-like formations, propylites, and quartz-sericite metasomatites; the order of their spatial distribution emphasizes the decrease in the temperature of the process, which usually occurs in the case of regressive metamorphism.

Table 1

Change in chemical composition in labrador-pyroxene porphyrites during hydrothermal metamorphism

Continuation of the table

The metasomatic transformation of the effusives at the initial stages was effected by basic solutions. Gradually they became neutral (propylites), and then acidic (quartz–sericite metasomatites).

And, finally, the close spatial association of ore mineralization with quartz–sericite metasomatites makes it possible to distinguish the latter as a near-ore halo of altered rocks.

REFERENCES

1. A. N. Winchell. Optical Mineralogy. IL Publishing House, 1956.
2. D. S. Korzhinskii. Formation of contact deposits. Izvestiya AN SSSR, Geological Series, No. 3, 1945.
3. V. Z. Mustafin et al. Main features of the geological structure of the Talanovsky graben (northwestern slope of the Kuznetsk Alatau). Izv. TPI, vol. 151, 1966.
4. N. I. Nakovnik. Propylitized rocks, their mineral facies, genesis, and practical significance. Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, part 83, No. 2, 1954.
5. N. I. Nakovnik. Determination of quantitative change in matter during hydrothermal metamorphism. Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, part 87, issue 4, 1958.
6. V. A. Rudnik. Methods for determining quantitative change in matter during metamorphism. In: Metasomatic Changes of Wall Rocks and Their Role in Ore Formation. Nedra Publishing House, 1966.
7. F. N. Shakhov. Geology of Vein Deposits. Nauka Publishing House, 1964.