ON THE MINERALOGY OF MAGNESIAN–FERRUGINOUS CHLORITES FROM ONE OF THE TITANOMAGNETITE DEPOSITS OF THE EASTERN SAYAN

O. M. Glazunov

(Presented by Professor A. M. Kuzmin)

In studying one of the titanomagnetite deposits of the Lysanskaya group in the Eastern Sayan, an exceptionally wide distribution of magnesian–ferruginous chlorites was established within its ore field. Among other silicates of layered structure, chlorites occupy one of the foremost places both in area of distribution and in the number of varieties encountered. They may be seen around grains of titanomagnetite in serpentinized peridotites; they replace pyroxene and amphiboles in gabbro and pyroxenites and, finally, compose the selvages of ore bodies and veins that cut, without exception, all rocks of the intrusive complex. In addition, chlorites are situated at the boundary between various lithological formations and form linear zones and aureoles in the apical part of ore-bearing massifs not yet exposed by erosion.

In association with chlorites, minerals such as titanomagnetite, magnetite, ilmenite, more rarely rutile, pyrite, quartz, albite, epidote, biotite, dolomite, and garnet are especially often encountered.

Taking into account the spatial and genetic connection of chlorites with ore-forming minerals, and consequently their significant participation in the process of ore deposition, on the advice of Professor A. M. Kuzmin we undertook a study of this mineral group, important for the given deposit and almost uninvestigated. It seems to us that sites of chloritization under certain geological conditions may be used in searching for blind titanomagnetite deposits in the western part of the Eastern Sayan, since they stand out quite clearly against the background of chloritization of rocks caused by regional metamorphism.

All the chlorites of the deposit differ both in composition and in optical properties, and particular types of chlorites are closely connected with definite mineralogical parageneses and geological settings. According to the classification of D. P. Serdyuchenko, supplemented by the studies of V. P. Ivanova and V. I. Mikheev, the following series of chlorites are distinguished at the deposit: 1) the pennine–clinochlore series, 2) the clinochlore series, 3) the prochlorite series.

Pennine–clinochlore series. This series includes green chlorites with refractive index \(Nm\), varying from 1.577 to 1.590. The habit of the crystals is scaly; cleavage is noticeable only in ud-

lineated flakes. In crossed nicols it is almost isotropic and uniaxial. In some specimens it is biaxial with \(2v = (-)5\text{—}10\); it shows anomalous interference colors and extinction close to straight. The mineral aggregates are dense, very fine-scaly or rosette-like. They rim titanomagnetite and sometimes, together with pyrite and ilmenite, fill fractures in pyroxenites.

Fig. 1. Thermal curves of chlorites I, Ia, 3—clinochlore from the Eastern Sayan. (Ia—analysis by V. P. Ivanova, 1949) 5, 5a, 5b—ripidolites from the Eastern Sayan, (5b—analysis by A. F. Korzhinskii, 1959).

Minerals of this series are characterized in general by thermic curves similar in configuration (Fig. 1), with a first pronounced endothermic effect at a temperature of 640—650°C and a second weak effect at 825—830°C, caused by the release of water from the brucite and mica layers. A sharp exothermic peak at 850—855°C indicates complete destruction of the chlorite structure with formation of magnesium silicates in its place [13]. In the character of the indicated effects, the chlorites described are close to the magnesian chlorite from the amphibolites of the Ukrainian crystalline massif. Some difference between them in the character of the heating curve is apparently explained by the unequal degree of comminution of the compared samples and, possibly, by a difference in the content of bound water in them.

One of the most clearly expressed features of the chlorites of this series, in addition to those indicated above, in contrast to the chlorites described here, is their good crystallinity (confirmed macroscopically and by Debye films) and the absence in them along the cleavage of the finest inclusions of ilmenite. The chemical composition of one representative of this series is expressed by the following formula (Table 1, analysis 3):

\[ (\mathrm{Si},\ \mathrm{Al})_{4.00}\{(\mathrm{Al},\ \mathrm{Ti},\ \mathrm{Fe}^{3+})_{0.96}(\mathrm{Fe}^{2+},\ \mathrm{Mg},\ \mathrm{Ca})_{4.88}\}_{5.84}(\mathrm{OH})_{4.10}\mathrm{O}_{11.30}. \]

As is evident, this chlorite, in composition and optical properties, is almost analogous to pennine from Pfitsch (Tyrol), described by Eskola [13], but at the same time differs from the compared one by a somewhat higher content of aluminum entering the silicon-oxygen radical, and by a smaller proportion of magnesium in the octahedral layer (Table 1).

2. The clinochlore series. This series comprises chlorites occurring among serpentinites and pyroxenites, and also forming the outer part of the endocontact selvage of the massifs. In appearance the chlorites are dark green; they form fine-scaly or worm-like aggregates and almost always include numerous, well-formed octahedra of magnetite. The refractive indices of the minerals for Nm vary from 1.593 to 1.617. On the heating curves, the first endothermic effect, caused by the release of the main portion of constitutional water, coincides with 600—650°; the second effect—with 800°, the second effect being three times smaller in magnitude than the first (Fig. 1). It is interesting to note that on

Table 1

Results of chemical analysis of chlorites¹)

1. Pennine from veinlets in serpentinite. Nm = 1.583. Sample, borehole 16–34. Eastern Sayan.
2. Pennine from Pfitsch, Tyrol (Tschermak, 1917) [13].
3. Clinochlore from the contact of gabbro and pyroxenite. Nm = 1.594. Sample rk — 11–pᵃ. Eastern Sayan.
4. Dark-green clinochlore. Nannobor Hill, Western Australia (after Simpson, 1936) [13].
5. Ripidolite from a quartz–carbonate vein. Nm = 1.606. Sample 110. Eastern Sayan.
6. Ripidolite from a biotite–dolomite vein. Nm = 1.621, sample, borehole 17—9ᵇ. Eastern Sayan.
7. Ripidolite from chlorite schists. Nm = Np. Androtra, Madagascar (after Orcel, 1927).
   Below, in all the tables presented, a single numbering has been adopted for chlorites from the Eastern Sayan.

¹) Analyses by the chemical laboratory of the Krasnoyarsk Geological Administration. Head of the laboratory: N. G. Rubanik.

of the thermal curve of one of the chlorites of this series, in addition to endothermic effects at 600–650 and 800°C, a distinct exothermic peak at 400° is indicated, absent from the thermograms of other chlorites of this series. This effect is apparently connected with the oxidation of ferrous iron in the chlorites [15]. According to the thermal determination, this chlorite occupies an intermediate position between the pennine-clinochlore subgroup and prochlorite-corundophilite according to Ivanova’s classification [3]. Calculation of the crystallochemical formula of clinochlore (Table 1, analysis 3), sampled from the surface of the contact of gabbro with schists, shows that chlorites of this series have an almost equal amount of magnesium and ferrous iron, but prove to be richer than those described above in the ferric component and somewhat depleted in magnesium. The formula of this chlorite is given below.

\[ (\mathrm{Si},\ \mathrm{Al})_{4.00}\,[(\mathrm{Al},\ \mathrm{Ti},\ \mathrm{Fe}^{3+})_{1.41}\,(\mathrm{Fe}^{2+},\ \mathrm{Mg},\ \mathrm{Ca})_{4.96}]_{6.37}\,(\mathrm{OH})_{7.60}\,\mathrm{O}_{10.05}. \]

These chlorites are similar to metachlorite from the Malka River (Caucasus) or to shuchardite from the Kempirsai massif [7, 13]. The difference between our chlorite and the Malka chlorite consists in the unequal contents of Si, Al, Mg in them and in the absence of nickel in our chlorite. The Lysansk clinochlores are brought closer to shuchardite by similar roentgenometric data; however, one cannot speak of their complete similarity, since in the Debyegrams of our chlorites, along with the lines of shuchardite, there are intense lines characteristic of the standard thuringite [9]. Comparison of the tables of interplanar spacings of the chlorite with reference data shows that the strongest diffraction lines (intensity 8–10 points) correspond in this sample (Table 2, analysis 3) to the lines of clinochlore. In chemical composition this chlorite (Table 1, analysis 3) is close to clinochlore from Nainabar Hill, Western Australia (Table 1). On D. P. Serdyuchenko’s classification diagram (Fig. 2), both analyses fall in the field of clinochlore.

Fig. 2. D. S. Serdyuchenko’s diagram for the composition of chlorites.
1, 2, 5, 6—from the Eastern Sayan; 2, 4, 7—given for comparison.

Prochlorite series. This includes chlorites that separated later in time than those described above. The conditions of their occurrence differ somewhat from the conditions of distribution of the preceding chlorites. Prochlorites are genetically connected with quartz, quartz-carbonate, calcite-biotite, and epidote veins, which heal joints and longitudinal tectonic zones in various intrusive massifs. In specifying the relative age position of the prochlorites in the general complex of associated minerals, we shall note that they fill cavities in epidote and cut calcite and biotite. Pyrite and apatite are often found together with the prochlorites.

All chlorites belonging to this series have a dense green or greenish-violet, more rarely whitish-green color; they form

Table 2

Table of interplanar spacings in chlorites

3 — clinochlore from the Eastern Sayan. \(4^{a}\) — the same from the Akhmetovskaya mine (Mikheev, 1957). 5 — ripidolite from the Eastern Sayan. \(6^{a}\) — ripidolite (after Lazarenko, cited in Mikheev, 1957). Chlorites 3 and 5 were analyzed by B. G. Erenburg (ZSGU). Recording conditions: Fe — anticathode; \(2R = 57,3\); \(d = 0,7\), exposure 4 hours, NaCl correction.

fibrous-platy and spheroidal-platy aggregates. The refractive index varies from 1.606 to 1.620.

Extinction with respect to cleavage cracks is straight. The sign of elongation is negative, \(2v=(+)10—20\). The chemical composition of the prochlorites varies within rather broad limits, as may be seen by comparing the formulas (in Table 1, analyses 5 and 6).

\[ (\mathrm{Si},\ \mathrm{Al})_{4.00}\{(\mathrm{Al},\ \mathrm{Ti},\ \mathrm{Fe}^{3+})_{1.35}(\mathrm{Fe}^{2+},\ \mathrm{Mg},\ \mathrm{Ca})_{4.53}\}_{5.88}(\mathrm{OH})_{8.00}\ \mathrm{O}_{9.93}. \tag{5} \]

\[ (\mathrm{Si},\ \mathrm{Al},\ \mathrm{Fe}^{3+},\ \mathrm{Ti})_{4.00}\{(\mathrm{Fe}^{2+},\ \mathrm{Mg},\ \mathrm{Ca},\ \mathrm{Ti})\}_{3.74}(\mathrm{OH})_{6.26}\ \mathrm{O}_{10.85}. \tag{6} \]

In their thermal characteristics the prochlorites studied approach ripidolites, or occupy a position intermediate between ripidolite and kemmererite [4]. The thermal curves of the prochlorites differ from the curves of previously characterized chlorites by a first endothermic effect that is very elongated vertically and by the absence of a second endoeffect (Fig. 1). Similar chlorites correspond to ripidolites from the chlorite schists of Androtra, Madagascar (Orcel, 1927), (Table 1, analysis 7), to chlorites from albitized chlorite rocks of Krivoi Rog (Polovinkina and Ivanova, 1953), and to ripidolite from cracks in granite-gneisses of the Arzhant’er-Izer massif (Orcel, 1927, cited after Serdyuchenko). On the composition diagram our chlorites (analyses 5 and 6) occupy the upper part of the prochlorite band, lying near the \(Sp—At\) line. Close to them is the analysis of chlorite from Androtra. Comparison of the prochlorites with chlorite from the quartz plateau of Kvarkush in the Northern Urals, recently described by N. P. Starkov, makes it possible to verify their identity from the results of chemical and thermal analysis [14]. The chlorite from quartz-carbonate veins from Mt. Lysan is close to those cited (Table 1, 5). However, the prochlorite from Lysan is characterized by a certain distinctiveness. In particular, it has an exothermic peak at a temperature of 850°C that is more sharply expressed than that of the chlorite from Kvarkush. All the bound water in the prochlorite is released in a single stage at a temperature of 550—650°C. Since in the composition of the chlorites under consideration magnesium considerably predominates over iron, they may be assigned to the magnesian-ferruginous group.

In addition to this, we shall characterize chlorites which, judging from their high refractive indices, show a marked deviation toward ferruginous varieties. One of these chlorites is genetically associated with stringers of shorlomite cutting the ore-bearing serpentinites. The chlorite is scaly, dark green in color, and contains no extraneous inclusions; \(Nm = 1.624\). The mineral separates later than garnet, as is shown by the fact that the garnet is cut by chlorite stringers. Because it was impossible to select suitable weighed portions, the chlorite was subjected neither to thermal nor to spectral and chemical analyses. Therefore its composition can be judged only from optical data. The second chlorite forms petal-like, unevenly distributed segregations and streaks confined to fractures of crushing and schistosity in the albitites. The chlorite localizations have diffuse boundaries. Areas of complete replacement of albitite by chlorite are observed. As a result, the rock acquires an external resemblance to gneiss-like gabbro. The color of the mineral is green; the aggregates are fine-scaly, \(Nm = 1.633\). According to spectral-analysis data, in it, in addition to iron and magnesium prevailing over it, Al, Ti, Zn, Mg, Sr, Zr, Be, Ba, and Co were established—that is, precisely that complex of elements which is not characteristic of our other chlorites (Table 3, analysis 11). This peculiarity in the behavior of certain elements in this chlorite is explained by the distinctive conditions of its genesis, as will be discussed below.

As the study shows, chlorites at the titanomagnetite deposit are extremely widespread. In composition, the majority of the chlorites do not go beyond the limits of the magnesian-ferruginous subgroup; chlorites deviating toward ferruginous varieties are rarely found. Within the subgroup, the composition of the chlorites changes from pennines to prochlorites.

The genetic sequence of the chlorites appears as follows. The earliest chlorites, associated with titanomagnetite and ilmenite, or replacing hornblende, have an essentially magnesian composition; the later ones, connected with the manifestation of the proper hydrothermal stage of the intrusion, have a magnesian-ferruginous character. And, finally, the final stage in the development of the intrusion is marked by the separation of magnesian-ferruginous chlorites with an elevated iron content.

Table 3

Results of spectral analysis of chlorites

1—pennine; 2,3—clinochlore; 5,5^6—ripidolite; 8, 9, 10, 11—chlorites.

Note: 0,0 n—hundredths of a percent, 0,n—tenths of a percent.

In terms of the content of trace elements, our chlorites, with the exception of only one of the chlorites with \(Nm—1.633\), cannot be differentiated. All of them are characterized by the presence of Ti, Ni, Co, V, Cu, Zn and by the sporadic occurrence of Zr, Be, Ba, Sr (Table 3). Of particular interest is the high content of Ni and Co in the chlorites and the insignificant content of V and Cu. This distinguishes them somewhat from chlorites of deposits of another genetic type [17]. A general feature of our chlorites is the constant presence of Zn in them. The known data do not yet explain how and where zinc finds a place in chlorites. It probably enters into the composition of the octahedral layer, replacing Mg or Fe owing to the closeness of the ionic radii. In any case, the presence of zinc in chlorites, in the opinion of D. P. Serdyuchenko, makes it possible, with appropriate study, to distinguish independent zinc-bearing chlorites [13].

With the transition from pennines to prochlorites, the amount of iron in the minerals, and consequently also the refractive indices, increases. Their thermal properties also change in connection with the different water content in the chlorites. If in pennine—clinochlores constitutional water is released in two stages in the temperature interval from 600° (bru-

... layer) up to 825° (a mica-like layer), then in prochlorites all the water is released in a single stage. At the same time, the temperature interval of water release in prochlorites narrows to 640—650°. The rapid and complete release of water is evidently explained by the fact that, with an increase in the hydroxyl group, its bond with the chlorite molecule weakens. Prochlorites are also distinguished by a more rapid destruction of the structure upon heating and by the formation, in their place, of magnesium orthosilicates.

A change in the composition of chlorites is accompanied by a change in the dimensions of the unit cell (the parameters \(b\) and \(c \sin \beta\)). This change is explained by the phenomena of polymorphism (Serdyuchenko, 1953) and isomorphism (Mikheev, 1957).

According to calculations by B. G. Erenburg, who indexed some of our X-ray powder photographs, the parameter \(b\) increases noticeably in passing from clinochlores to prochlorites, that is, as the minerals become enriched in the ferruginous component and as the proportion of magnesium in their composition decreases (Table 1).

For illustration of what has been said, all the data are summarized in Table 4.

Table 4

1, 2 — clinochlore and 4 — ripidolite from the Eastern Sayan; 3 — clinochlore and 5 — ripidolite according to Mikheev (1957).

It is necessary to dwell briefly on the question of the genesis of the chlorites. As the material presented shows, most of the chlorites associated with titanomagnetite deposits were formed as a result of magnesian metasomatism, which probably took place at a relatively low oxygen potential and low temperature. How active the metasomatism was is shown at least by the fact of the formation of magnesian chlorites at the expense of albitites, in which the magnesium content (0.5—0.6%) is far from sufficient for the crystallization from them of chlorites of the magnesian-ferruginous series.

In order to clarify the conditions of formation of our chlorites, along with paragenetic analysis, calculations were made of the oxidation potential [8] \(\left(\dfrac{\mathrm{Fe}^{3+}}{\mathrm{Fe}^{2+}+\mathrm{Fe}^{3+}}\right)\). The results were compared with the oxidation potential of magnetite, which, according to the data of D. O. Ontoev and A. G. Betekhtin, is often taken as a kind of indicator of the oxygen regime of a system: 2.8. The magnitude of the potential is equal to 0.86 for pennines, -0.1 for clinochlores, and -0.68 for ripidolites, and in general proves to be considerably lower than the corresponding data for ferruginous chlorites and magnetite. As is known, magnetite forms in a reducing environment [2]. Consequently, it may be assumed that our chlorites crystallized in an even more sharply expressed reducing environment, at a relatively low oxygen potential.

The difference in the values of the oxidation potential indicates a change in the partial pressure of oxygen (the oxygen regime) during the process of mineral formation. If the earliest pennines in time of formation are associated with titanomagnetite and have an elevated oxygen potential, then in the subsequent prochlorites it...

decreases noticeably. As a result, instead of magnetite, in the presence of sulfur, such sulfides as pyrite and pyrrhotite, as well as carbonates, are precipitated together with the chlorites. Probably, in the present case the mineral-forming solutions were undersaturated with sulfur, since in all pyrites a large deficiency of sulfur relative to iron is established.

Among the volatile components in the composition of the postmagmatic solutions, besides H₂O and S, there were undoubtedly F and CO₂, as is proved by the occurrence of carbonates and apatite together with the chlorites. At the same time, the crystallization of individual chlorites apparently took place at a sufficiently high temperature and in an alkaline medium, for the restructuring of the structure of individual chlorites, consisting in the entry of titanium into the silicic-oxygen radical, according to available data, could have occurred only under these physicochemical conditions [12].

In conclusion, let us note the following.

Among the chlorites distributed within the limits of the titanomagnetite deposit studied, chlorites of magnesian-ferruginous composition predominate, chiefly clinochlores and prochlorites. Magnesian and ferruginous chlorites are of subordinate importance and are not characteristic of these deposits. This conclusion evidently has general significance.

The close spatial and genetic association of chlorites with such ore minerals as titanomagnetite and ilmenite makes it possible, along with other indicators (serpentinization, carbonatization), to regard chlorites as one of the reliable criteria in the search for new titanium deposits under the conditions of the western slope of the Eastern Sayan.

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