ON METASOMATIC AMPHIBOLES FROM THE AREA OF THE KOMMUNAR MINE (KHAKASSIA)

A. F. KOROBEINIKOV

(Presented by Professor A. M. Kuzmin)

In studying the contact-metasomatic formations of the Kaliostrovsky sector, located in the exocontact of a large granodiorite intrusion, and hydrothermally metamorphosed dikes of fine-grained diorites of Podlunnyi Golets, linear zones of albitized rocks were discovered. Zones of albitization with a thickness of up to 0.2–2.1 m and an extent of up to several tens of meters are traced both in the porphyrites and in the dike diorites. These essentially albitic formations almost everywhere contain hornblende veinlets from fractions of a centimeter to 1–2 cm thick. Albitization usually proceeds along zones of increased fracturing of the rocks, gradually spreading to the sides, and ends with complete replacement of the enclosing porphyrites and diorites by a quartz-albite aggregate with veinlets of dark-green hornblende. The mineral composition of the metasomatic rock is represented by albite No. 1–3 (up to 10–20%), green hornblende (up to 10–18%), sphene and epidote (up to 2% of the total rock volume). A very interesting circumstance is that in the subsequent ore stage the zones of albitized rocks underwent silicification and gold mineralization. Vein quartz, together with gold, following along the selvages of amphibole veinlets, replaces and cements individual grains of hornblende or, in the form of small lenses and clustered segregations, impregnates the quartz-albite metasomatic aggregate. Usually the central parts of the metasomatic zones, containing the largest amphibole veinlets, prove to be enriched in vein quartz and gold.

In the diorite dike bodies, albitization also spreads along zones of maximum fracturing confined to the endocontacts of the dikes. The front of metasomatic replacement follows along the contact of the dikes, spreading into the dike bodies along diagonal and longitudinal shear fractures for a distance of 0.5–2 m, with dike thicknesses from 12 to 18 m. As a result, metasomatic zones arise with uneven, embayment-like margins on the side of the enclosing porphyritic gabbro-diorites. The mineral composition of these zones is analogous to that described above in the porphyrites.

The structure of the hornblende veinlets is distinctive. As a rule, they are linear and relatively thin (up to 1 cm), with sharp contacts. Veinlets with a subparallel or fan-shaped arrangement of amphibole aggregates are not uncommon, sometimes penetrating into the near-selvage zones.

parts of veinlets. In other cases hornblende is separated out in the form of individual small lenses or irregularly shaped nests, but it is always accompanied by rims of metasomatic albite, which, in turn, indicates the syngenetic character of the minerals described. During the hydrothermal ore stage, zones of albitized rocks undergo silicification and gold mineralization. In this process, the central parts of the metasomatic zones, usually containing the largest amphibole veinlets, prove to be enriched in vein quartz and gold. The noted relationships between the metasomatic and later vein mineral associations evidently reflect the inheritance of the circulation paths of both the early metasomatizing and the later gold-bearing solutions.

Table 1 gives the compositions of unaltered dike diorites, apodioritic metasomatites, and the calculation of the material balance during metasomatism by the method of N. I. Nakovnik.

Table 1

Change in composition and material balance during albitization of fine-grained diorites
(in grams per 100 cubic cm of rock)

1—unaltered dike fine-grained diorite of the Podlunnyi area; 2—albitized fine-grained diorite with amphibole veinlets of the Podlunnyi area; 3—quartz-albite aggregate formed at the expense of metasomatic diorite with rare amphibole veinlets of the Podlunnyi area.

As is evident from the table, in zones of silica-sodium metasomatism there is high mobility of silicon, sodium, calcium, magnesium, and iron.

During metasomatism of the diorites, sodium and silicon were introduced, and some redistribution of matter occurred with local movement of siderophile elements within the zones of postmagmatic albitization themselves. Evidently, the formation of amphiboles proceeded mainly at the expense of equalization of the compositions of the enclosing medium—solution, and the iron-

iron, calcium, aluminum, and magnesium were supplied from the country rocks. The alkaline solutions penetrating along fractures into the diorites and porphyrites apparently initially decomposed the dark-colored components of the rocks. The liberated iron and magnesium ions accumulated in the solution and then went into the construction of metasomatic amphiboles, forming separate small lenses and nodules rimmed by albite. The number of such small lenses gradually increased as the process of silica-alkali metasomatism developed, and they coalesced into separate bands and then into entire veinlets.

Gold grains spatially associated with these veinlets are usually situated between individual grains of hornblende and vein quartz, the latter corroding the amphibole. Along fractures they replace the green hornblende, sometimes forming interesting gold grains of elongate-prismatic and columnar form by complete replacement of individual amphibole crystals. Much more rarely, gold grains appear within the albite aggregate.

Green Hornblende from Albitized Rocks

Hornblende-1 usually occurs as tabular and hexagonal grains or fibrous aggregates. Individual individuals of dark-green amphibole, up to 1 cm long and 0.3–0.4 cm wide, are arranged transverse or diagonal to the contacts of the veinlets.

The chemical composition, optical properties, specific gravity, composition of trace elements, and interplanar spacings of hornblende-1 are given in Tables 2 and 3.

Calculation of the chemical analysis by the method of I. D. Borneman-Starynkevich [3] on the basis of \((x+y=13615):13=1047\) gives the following crystallochemical formula of the amphibole:

\[ (\mathrm{Na}_{0.13}\mathrm{K}_{0.06}\mathrm{H}_{3}\mathrm{O}_{0.73})_{1.0} \mathrm{Ca}_{2.03} (\mathrm{Mg}_{1.38}\mathrm{Fe}^{+2}_{2.72}\mathrm{Mn}_{0.07}\mathrm{Fe}^{+3}_{0.48}\mathrm{Ti}_{0.03}\mathrm{Al}_{0.32})_{5.0} \cdot [\mathrm{Si}_{7.44}\mathrm{Al}_{0.56}]_{8.0}\mathrm{O}_{22} [(\mathrm{OH}_{1.37}\mathrm{Cl}_{0.03})_{1.40}\mathrm{O}_{0.6}]_{2.0}. \tag{1} \]

It has been shown theoretically [3] and experimentally [6] that \(\mathrm{H}_{3}\mathrm{O}\) enters the structure of amphiboles, which makes it possible to write formula (1). In the formulas obtained there is a small excess of Ca (0.03), which may be disregarded, since it amounts to only 0.179 wt. % CaO. This excess calcium is most probably due to a certain mechanical contamination of the amphibole by calcite or to isomorphous substitution by sodium, which may occupy the eighth cationic position in the structure of hornblende [2, 12]. In all other respects, an almost ideal distribution of cations among the structural positions, corresponding to the theoretical one, is established.

Proceeding from a comparison of formula (1) with the formula of tremolite by the method of I. V. Ginzburg [5, 7], the optical properties, and the type of debyegram (Table 3), the following isomorphous substitutions may be outlined for blue-green hornblende-1:

The substitution of silicon by aluminum, as indicated by V. S. Sobolev [12], must be accompanied by the simultaneous addition of sodium \(\mathrm{Si}^{+4} \to \mathrm{Al}^{+3}, \mathrm{Na}^{+1}\) and by the obligatory substitution \(\mathrm{MgSi} \to \mathrm{Al}_{2}\), as is noted in augites. Thus, the chemical composition, structural type of the crystal lattice, optical properties, and predominance of the isomorphism \(\mathrm{Si}^{+4} \to \mathrm{Al}^{+3}\), \(\mathrm{Mg}^{+2} \to \mathrm{Fe}^{+2}\), \(\mathrm{Mg}^{+2} \to \mathrm{Fe}^{+3}\), \(\mathrm{Al}^{+3}\) make it possible to assign this amphibole to green hornblende, transitional to ferrotremolite [8].

Amphibole from skarn–magnetite deposits

Amphibole-II is observed in exoskarns in the form of nest-like aggregates of radiating and fibrous structure among diopside and magnetite. In the latter case the amphibole appears in the form of fibrous rims at the contacts of vein gold-bearing quartz cutting the magnetite iron ore. Sometimes it forms small amphibole–magnetite veinlets, lenses, irregular accumulations, and stringers among uralitized gabbro-diorites and effusives of the exocontact of the granodiorite pluton.

Table 2

Chemical composition and properties of metasomatic amphiboles of the Kommunar ore field

*) The analysis is recalculated owing to exclusion of 6.97% CaO at the expense of finely disseminated calcite in the amphibole (on the basis of X-ray structural and microscopic study). Chemical analyses were carried out in the Central Laboratory of the Krasnoyarsk Geological Administration.

The size of the amphibole segregations does not exceed several centimeters in cross section, with a length of 5–18 cm, while individual individuals average 0.5–1.5 × 0.2–0.5 cm. As a rule, the amphibole described is dark green or gray-green in color and in outward appearance resembles aggregates of actinolite. Green amphibole often replaces diopside grains from the margins or along cleavage, or fills the spaces between the prisms of the latter. Sometimes at the contacts of magnetite aggregates and vein quartz, fan-shaped conta-

tovo-reaction rims of fibrous amphibole. It is more probable that these amphiboles formed at the expense of the decomposition of diopside and magnetite, when the incoming gold-bearing solutions, in dissolving the earlier minerals, borrowed iron and magnesium ions. All this gives grounds for speaking of the metasomatic origin of amphibole-II during the period of hydrothermal metamorphism of the skarn–magnetite formations.

Under the microscope, amphibole-II appears in the form of elongate prismatic and tabular grains or parallel-sheaf and fibrous aggregates. It is colored dark green or light green.

Table 3

Interplanar spacings in amphiboles
Tube BSV-1, Fe—anticathode, without filter, \(D = 57.3\) mm, exposure 3 hours

170 — green hornblende-II from quartz–albite metasomatites of the Kaliostrovsky area (Northern lens);

166 — fibrous amphibole-II from the contact of quartz veinlets with metasomatic magnetite of the Kaliostrovsky area (Southern lens).

The analyses were carried out in the Central Laboratory of the West Siberian Geological Administration by B. G. Erenburg.

The chemical composition, optical properties, specific gravity, composition of impurity elements, and interplanar spacings of amphibole-II are given in Tables 2 and 3. Recalculation of the chemical analysis by the method of I. D. Borneman-Starynkevich [3] on the basis \((x+y=13536):\ 13=1041\) gave the following crystal-chemical formula for the amphibole:

\[ (\mathrm{Ca}_{2.12}\mathrm{H}_{3}\mathrm{O}_{0.88})_{3.0} (\mathrm{Mg}_{1.33}\mathrm{Fe}^{+2}_{3.18}\mathrm{Fe}^{+3}_{0.21}\mathrm{Ti}_{0.01}\mathrm{Al}_{0.23})_{5.0} [\mathrm{Si}_{7.92}\mathrm{Al}_{0.08}]_{8.0} \times \]

\[ \times \mathrm{O}_{22}[(\mathrm{OH})_{1.33}\mathrm{Cl}_{0.03}\mathrm{F}_{0.03})_{1.39}\mathrm{O}_{0.61}]_{2.0}. \tag{II} \]

As in amphibole-I, in amphibole-II there appears a slight excess of calcium (0.12), amounting to no more than 0.77 wt.% CaO, which is probably due to mechanical impurities of finely disseminated calcite. In contrast to green hornblende-I, amphibole-II does not contain Na and K in its composition, which is probably due to the very low alkali content in the surrounding skarns during the period of amphibole deposition and to the specific composition of the metamorphosing solutions, or to inaccuracy in the analysis performed. In general, the crystal-chemical formula (II) is very close to the formula of hornblende-I, as are the optical properties (see Table 2). Only a lower content is observed in amphibole-II, which apparently is explained by a somewhat reduced partial pressure of oxygen (or an increase in the pH of the solutions) during its formation than during the deposition of hornblende-I, as was experimentally shown by Ernst [6]. If, however, one compares the content of oxygen replacing OH in amphiboles-I and II, which may serve as an indicator of the oxygen saturation of the system during their crystallization, it turns out that it should be approximately the same for both. In general, these systems should be characterized by a somewhat reduced partial pressure of oxygen. It is quite possible that fluorine could also have played some role in this, the content of which (in the OH group) in amphibole-II is twice as high as in hornblende-I, and the latter to a certain extent could have affected some optical properties of hornblende-II. Thus, for example, the refractive indices of amphiboles-II are somewhat lower in comparison with those of hornblende-I, although the degree of ferruginosity in them is approximately the same \((\mathrm{fmI}=80.7,\ \mathrm{fmII}=80.2)\).

When formula (II) is compared with the formula of tremolite by the method of I. V. Ginzburg [8, 10], the optical properties and X-ray patterns (Table 3) indicate the following isomorphous substitutions for amphibole-II: \(\mathrm{Mg}^{+2} \to \mathrm{Al}^{+3},\ \mathrm{Fe}^{+3}\) and \(\mathrm{Mg}^{+2} \to \mathrm{Fe}^{+2}\) stronger than \(\mathrm{Si}^{+4} \to \mathrm{Al}^{+3}\) and \(\mathrm{OH}^{-1} \to \mathrm{Cl}^{-1},\ \mathrm{F}^{-1},\ \mathrm{O}^{-2}\), which is characteristic of all amphibole groups [7].

Thus, here too the chemical composition, the structural type of the crystal lattice, optical properties, pleochroism in green-bluish tones, and the predominance of isomorphism, highly characteristic of amphiboles, make it possible to assign this amphibole to ferrotremolite [8].

Hornblende-I was subjected to thermal analysis. Its differential heating curve, shown in Fig. 1-1, is generally similar to the thermal curves of amphiboles published in the literature [1, 10, 11], and differs from them by a somewhat unusual configuration and weak manifestation of endothermic effects. Three temperature arrests are clearly established. Up to a temperature of \(120\text{—}130^\circ\), the main amount of adsorbed water is released, which is recorded by the smooth character of the heating curves (Fig. 1-1) and dehydration (Fig. 1-2). The first endothermic effect, unlike those known in the literature [11], is shifted considerably to the right (appearing at a temperature of \(610^\circ\) instead of \(450\text{—}520^\circ\)) and evidently corresponds to the beginning of dehydration of hornblende, when constitutive ...

... onno-zeolitic water, constituting up to 50% of all the water in the mineral [10]. The release of constitutional water at the relatively low temperature of 500° without destruction of the crystal lattice, as V. S. Sobolev [12] indicates, is due to the oxidation of ferrous iron to ferric iron, when an isomorphous replacement of \(OH^{-1}\) ions by \(O^{-2}\) takes place. According to the investigations of D. S. Belyankin and E. V. Donskaya [1], at a temperature of 600° the greater part of the iron in amphiboles is already oxidized.

The second endothermic effect at a temperature of 1020–1060° marks the complete destruction of the amphibole lattice and leads to the release of residual constitutional water with the formation of clinopyroxene and hematite [10].

These two endothermic effects correspond to sharp breaks on the weight-loss curve (Fig. 1-2). Up to a temperature of 100° the weight loss occurs gradually and is marked by a smooth curve characteristic of the release of adsorption water, and the weight of hornblende-I decreases by 1.2%. Then, at a temperature of 500–530°, constitutional water is released, which is recorded on the dehydration curve (Fig. 1-2), and at a temperature of 625–675° a second “jump” appears, which coincides with the first endothermic effect (Fig. 1-1), after which the sample again acquires a constant weight.

The next sharp decrease in weight, corresponding to the complete release of constitutional water, is observed at a temperature of 870–920°; in this case the weight of the mineral decreases by 2.3% of the original. The latter agrees with the data of chemical analysis (Table 1). This decrease in weight

Fig. 1. Thermal curves of hornblende-I: 1—differential heating curve of hornblende-I; 2—weight-loss curve of hornblende-I during heating in air

does not correspond to the second endothermic effect on the heating curve (Fig. 1-1), since, as is known, the reaction does not keep pace with the recording of temperatures on the pyrometer [10]. Investigations by many authors show that the weight loss during heating of amphiboles in air does not correspond to the loss of water, since in this case, along with dehydration, the process of oxidation of divalent iron to trivalent iron proceeds simultaneously, with the addition of oxygen from the air and a change in the optical properties of the mineral [1, 10, 11].

Some conclusions

1. The metasomatic processes with which the formation of blue-green hornblendes is connected belong to the late alkaline stage of near-fracture metasomatism according to the classification of D. S. Korzhinskii [14]. The high mobility of silicon and sodium and the low mobility of aluminum and iron in the zones of albitization evidently characterize the low-temperature stage of metasomatism.

2. From an analysis of the formulas of the hornblendes, a close connection between their composition and the composition of the medium in which they form follows quite obviously. This has been repeatedly pointed out for amphiboles by many investigators [7, 12, 13, 14, 15, 16]. It is sufficient to recall that the considerable iron content of our amphiboles is apparently due to the elevated content, in the composition of the previously formed metasomatic rocks, of silica and sodium. The ratio MgO : FeO : Fe₂O₃ (I—1.5:5.1:1.0 and II—3.3:13.7:1.0) is an especially sensitive indicator of alkalinity at a given temperature, since MgO is a stronger base than FeO, and all the more so than Fe₂O₃. Accordingly, the very high content of divalent iron in the amphiboles studied may to some extent serve as an indicator of increased alkalinity of the medium, which is also confirmed by field observations of the mineral parageneses. The lowered content of alumina in the amphiboles studied, in comparison with the metasomatic amphiboles described in the literature [14], is evidently due both to the inert behavior of aluminum in the metasomatic process and to its insignificant content in the solutions that formed the amphiboles. Apparently, at the initial stage of metasomatism almost all the aluminum extracted from the country rocks went into the construction of albite, and by the period of deposition of the hornblendes its concentration in the solutions had sharply decreased.

3. A general feature of the amphiboles studied is blue-green pleochroism and lowered refractive indices at a high content of iron and hydroxyl. According to the total iron content (fm % = 80.7 and 80.2), both amphiboles fall on the variation line constructed for blue-green hornblendes from crystalline schists and hornfelses, and do not correlate with metasomatic blue-green hornblendes from other regions [14]. At the same time, they are quite distinctly separated from high-temperature hornblendes [13]. The total iron content obtained on the basis of optical data for amphiboles I and II, according to curve 1 constructed for metasomatic hornblendes [14], is 33–39 molecular percent. Such a sharp deviation from the chemical-analysis data (more than twofold) does not permit use of the indicated graph. Evidently, all this establishes the fact that the formation of the described blue-green hornblendes of the metasomatic facies took place under specific conditions.

The somewhat lowered birefringence of both amphiboles and the decrease in the angles 2V and cNg (2V to 56° and cNg to 14–16°), apparently, are explained by significant substitutions of Mg²⁺ and Al³⁺ by Fe²⁺ and Fe³⁺ occurring in the lattice, and are not so much connected with internal twinning of the cells [4] or the appearance of microblockiness [7]. As V. S. Sobolev [12] indicates, such substitution entails a considerable change in 2V and even in the orientation of the ellipsoid. Thus, the data we obtained once again confirm V. S. Sobolev’s conclusion that, in amphiboles, an increase in iron content regularly decreases the negative angle of the optical axes.

The appearance of blue coloration in thin sections of the amphiboles described is probably also connected with an increase in the content of ferrous and ferric iron, more precisely, with their particular position in the structure of the minerals in the presence of hydroxyl and, in part, sodium [12].

1. The not infrequent spatial association of gold with ferruginous amphiboles and the occurrence of the latter in zones of hydrothermal metamorphism of rocks may serve as one of the prospecting criteria for ore gold in the district.

LITERATURE

1. D. S. Belyankin and E. V. Donskaya. A thermo-optical study of the mineral actinolite. Izv. AN SSSR, geol. ser., No. 1, 1939.

2. G. B. Bokiy. Crystal Chemistry. Moscow State University Press, 1962.

3. I. D. Borneman-Starynkevich. The chemical formula of minerals, vol. IV, Amphiboles. Zap. Vses. Min. ob-va, part 89, issue 2, 1960.

4. I. V. Ginzburg. Hastingsite of the zone of alkali-granite metasomatism and isomorphism in monoclinic amphiboles. Tr. Mineral. muzeya, issue 10, 1961.

5. I. A. Ginzburg, G. A. Sidorenko, D. L. Rogachev. On the relationship between the principal isomorphic substitutions and certain parameters of the crystal structure of amphiboles. Tr. Mineral. muzeya, issue 11, 1961.

6. I. V. Ginzburg, G. V. Yukhnevich. On the hydroxonium ion in amphiboles. Geochemistry, No. 1, 1962.

7. I. V. Ginzburg. On three unusual hornblendes of granitic rocks. Tr. Mineral. muzeya, issue 13, 1962.

8. W. A. Deer et al. Rock-Forming Minerals. Mir Publishers, 1966.

9. D. S. Korzhinskii. Metasomatic zoning during near-fissure metamorphism and veins. Zap. Vses. min. ob-va, part 85, issue 4, 1947.

10. L. N. Ovchinnikov. Contact-metasomatic deposits of the Middle and Northern Urals. Tr. Gorno-geol. in-ta UFAN SSSR, issue 39, 1960.

11. L. N. Ovchinnikov, A. S. Shur, N. T. El’kina. A thermoanalytical study of amphiboles of certain skarn zones of the Urals. Tr. Pervogo soveshchaniya po termografii. Moscow, USSR Academy of Sciences Press, 1953.

12. V. S. Sobolev. Introduction to the Mineralogy of Silicates. Lviv State University Press, 1949.

13. V. S. Sobolev. The significance of the iron content of femic minerals and auxiliary diagrams for determining the compositions of biotites, hornblendes, and rhombic pyroxenes. Mineral. sb. L’vov. geol. ob-va, No. 4, 1950.

14. L. I. Shabynina. Once more on the significance of the iron content of femic minerals. Mineral. sb. L’vov. geol. ob-va, No. 12, 1958.

15. G. W. de Vore. The association of stringly polarizing cations with weakly polarizing cations influence in element distribution, mineral composition and crystal growth. J. Geol., No. 2, 1957.

16. R. Kretz. The distribution on certain elements among coexisting, calcic pyroxenes, calcic amphiboles and biotites in skarns. Geoch. Cosm. Acta, No. 3–4, 1960.