Magnesian Metasomatites from the Contacts of the Granodiorite Intrusion of the Kommunar Ore Field (Khakassia)

A. F. Korobeinikov

(Presented by Prof. A. M. Kuzmin)

The Kommunar gold-ore field is situated in the northeastern exocontact zone of the Solton granodiorite intrusion. In its northeastern part this massif cuts through and metamorphoses rocks of a diabase-porphyrite and volcanogenic-sedimentary sequence, presumably of Middle Cambrian [9] or even Precambrian age [7]. The absolute age of the quartz biotite diorites of the massif, determined in the laboratory of the West Siberian Geological Administration, is 476 million years, which gives grounds for considering the age of the intrusion to be Upper Cambrian–Lower Ordovician, i.e., Salairian. The intrusion is confined to the arch part of a large anticlinal structure and has a subconcordant attitude with the enclosing rocks on the side of the eastern contact, and a crosscutting one in the area of its northern closure. The contact line is complexly sinuous, with numerous embayments and apophyses into the rocks of the continent.

Rocks of three intrusive phases, separated in time by deformation breaks, take part in the structure of the intrusion. The first comprises gabbro-diorites, augite diorites and meladiorites, hybrid pyroxenites, etc.; the second comprises quartz leucodiorites and granodiorites; the third comprises fine-grained granites, aplites, and syenites. The most widespread are formations of the second intrusive phase, which occupy the central part of the massif.

In studying the rocks of the endocontact aureole of the Solton granodiorite intrusion, distinctive, essentially pyroxene and hornblende rocks were encountered; in the form of a narrow tongue they extend for 250–400 m from the contact along the northeastern margin of the massif. Study of these rocks made it possible to establish that they arose in the magmatic stage through metasomatic reworking of the roof of the massif, composed of hornfelsed diabase porphyrites and pyroxene-plagioclase hornfelses, and of diorites of the apical part of the intrusion. Directly within the studied area, diorites and granodiorites take part in the structure of the intrusion.

The metasomatic rocks, located within the granodiorite intrusion, have the form of an elongated tongue-shaped body up to 200 m long and 37–60 m wide, with curvilinear outlines and gradual or rather distinct linear contacts with syenite-diorites and diorites. Their internal structure is clearly zonal: in the central part, plagioclase-pyroxene-olivine and pyroxene rocks predominate, which toward the periphery are replaced by essentially hornblende varieties and, finally, at the contact with the enclosing diorites,

(or monzonites), coarse-grained plagioclase–hornblende gabbros and gabbro-diorites of pegmatitic aspect are sometimes observed (Fig. 1). In other cases, the amphibole formations are spatially confined to the central part of the metasomatic bodies and are fringed by plagioclase–hornblende coarse-grained rocks, or else are replaced directly by intrusive rocks.

Metasomatic rocks, according to the nature of the material being replaced, are subdivided into two groups: 1) metasomatites formed at the expense of enclosing effusive rocks of basic composition, and 2) metasomatites formed by replacement of diorites in the apical part of the intrusion. In general, the following sequence of formation of the metasomatic column has been established: 1) hornfelsed diabase porphyrite or apoporphyritic plagioclase–pyroxene hornfels (initial rocks), 2) plagioclase–pyroxene–olivine metasomatite, 3) pyroxene metasomatite (pyroxenite), 4) plagioclase–hornblende rock, 5) essentially hornblende metasomatite (hornblendite); and diorite or quartz monzonite (initial rock), coarse-grained plagioclase–hornblende gabbro of pegmatitic aspect. Two temperature facies of magnesian metasomatism are clearly distinguished: an early, essentially pyroxene facies, formed with increasing temperature of the process (the magmatic stage proper), and a late, essentially amphibole facies, formed with decreasing temperature of the process (early postmagmatic stage).

The zone of metasomatic rocks is characterized by an abrupt spatial replacement of essentially pyroxene formations by hornblende ones, with the segregation of plagioclases of the early plagioclase–pyroxene facies into separate nests and clustered accumulations among the hornblende rocks that replace them (Fig. 1).

The essentially hornblende (hornblendite) bodies are lenticular or irregularly oval in plan; their dimensions range from 2–3 to 20–80 m in length, with a width of 1–20 m. Boundaries with the plagioclase–pyroxene rocks of the inner part of the metasomatic column are more often gradual, whereas with the lateral intrusive rocks they are more distinct (Fig. 1). Very often, relics of the pyroxene–plagioclase–olivine associa-

Fig. 1. Diagram of the zonal structure of a metasomatic body located at the contact of the Solgon granodiorite intrusion (in plan).
1 — pyroxene–plagioclase–olivine metasomatites; 2 — essentially hornblende metasomatites (hornblendites); 3 — coarse-grained gabbros of pegmatitic aspect; 4 — quartz monzonite.

... occur as nests among metasomatic hornblendite, giving the rock a spotted appearance.

Let us characterize the named metasomatites in more detail.

Plagioclase–pyroxene–olivine metasomatic formations are dark-gray, dense, coarse-grained rocks (with pyroxene and plagioclase grains up to 5–6 mm), not infrequently of spotted texture, caused by the uneven distribution of plagioclase and rare relicts of fine-grained plagioclase–pyroxene hornfelses. The rocks have a sharply variable mineral composition: for example, over a distance of 1–2 m, plagioclase–pyroxene varieties may several times be replaced by substantially pyroxene varieties. They are composed of plagioclase (up to 20–40%),¹ pyroxene (up to 60–80%), olivine (up to 10–17%), brown and green hornblendes (up to 5–7%), apatite, titanite, and rare magnetite (up to 1%). In thin sections a vari-grained structure of the rock is established.

Plagioclase No. 47–63 is very often distinctly zoned and is usually xenomorphic with respect to the mafic components. The more basic varieties, No. 58–63, in the centers of crystals are replaced toward the periphery by more acidic plagioclase, No. 47–53. Secondary alteration of plagioclase (saussuritization) increases sharply in the amphibolized varieties of the rocks.

Pyroxene, according to its optical properties ($2V = +48^\circ$, $c \ Ng = 38—41^\circ$; $Ng = 1.693$, $Np = 1.668 \pm 0.002$, $Ng - Np = 0.025$), belongs to diopside-augite with a ferrisilicate molecule content of up to 5% [3]. Semiquantitative spectral analysis established in it trace elements: manganese, titanium, vanadium—up to 0.03%, chromium and cobalt—up to 0.001, nickel—up to 0.003, and traces of copper. Pyroxene is overgrown and replaced by brown and green hornblendes, the replacement process beginning from the periphery of its individual grains, taking over ever larger areas of the pyroxene aggregate. Green hornblende in turn develops over brown hornblende, and then chlorite develops as well.

Olivine forms small (up to 0.3 mm) grains, which are concentrated around large crystals (up to 2–5 mm) of diopside-augite and, according to optical properties ($2V = -83^\circ$; $Ng = 1.712$, $Np = 1.675 \pm 0.003$, $Ng - Np = 0.037$), belongs to chrysolite containing up to 23% of the fayalite molecule [3]. It is replaced by pale-green serpentine or dark-yellow bowlingite.

Dark-gray coarse-grained pyroxene–plagioclase metasomatites (pyroxenites) consist of pyroxene (up to 85–90%), belonging to diopside-augite, and plagioclase No. 57–63 (up to 3–10%). Olivine and brown hornblende make up as much as 5–11%. Among accessory minerals, apatite and magnetite are widespread. The size of pyroxene crystals is up to 5–6 mm and sometimes more. The texture of the formations described is massive, more rarely spotted. In thin sections, a panidiomorphic-granular structure of the rock is clearly established, while in varieties containing plagioclase, a sharp xenomorphism of the latter with respect to pyroxene is observed. The optical properties of the named minerals are analogous to those of the plagioclase–pyroxene–olivine metasomatites.

Hornblende metasomatites (hornblendites) are dark greenish-gray, almost black, medium- and coarse-grained (up to 5–8 mm and more) rocks of massive and spotted texture. The latter is most often manifested in plagioclase–amphibole formations of the marginal parts of the metasomatic zone and is caused by the uneven distribution of plagioclase. In this case

the rock changes over a short distance (1–2 m) from essentially amphibole varieties (up to 80% hornblendes) to plagioclase–hornblende varieties containing 50–60% amphibole. Mineralogical composition of the rocks: brown and green hornblendes (50–80%), plagioclase No. 57–63 (10–40%), biotite (5–7%), and relics of pyroxene. Among the accessory minerals apatite and abundant magnetite were found. Transitions between pyroxene and amphibole rocks are often quite gradual, owing to varying degrees of replacement of pyroxene and olivine by hornblendes. In this connection, an entire series of transitional varieties is observed, from pyroxene to hornblende rocks. The latter, in their composition and appearance, approach typical hornblendites.

In thin sections the hornblende rock shows a hypidiomorphic-granular texture. The principal rock-forming mineral is brown hornblende ($2V = -76^\circ$ with $Ng = 13^\circ$, $Np = 1.668$, $Np = 1.650 \pm 0.002$, $Ng - Np = 0.018$; pleochroism in yellow-brown tones: $Ng$—brown, $Nm$—light reddish-brown, $Nm$—light yellow). The optical properties given permit it to be assigned to a moderately ferruginous variety [3]. Spectral analyses of this mineral showed an elevated content of manganese (0.06%), vanadium (0.03%), chromium (0.01%), and titanium (0.003%).

Green hornblende, making up as much as 10–30% of the rock volume, corrodes the brown hornblende and relic pyroxene. It is characterized by the following optical properties: in thin sections it has a light-green color and pleochroism, $Ng$—light green, $Nm$ and $Np$—colorless and, rarely, pale greenish; $2V = -56^\circ$ with $Ng = 15^\circ$, $Ng = 1.658$, $Np = 1.636 \pm 0.002$, $Ng - Np = 0.022$. According to the optical properties cited, the amphibole belongs to a low-iron variety [3]. Sometimes this hornblende is replaced by chlorite.

Plagioclase No. 57–63 is almost always xenomorphic with respect to brown hornblende; as in essentially pyroxene rocks, it not infrequently has a zonal structure. The mineral is intensively replaced by zoisite and sericite, especially in the central parts of the crystals, where a saussuritic aggregate arises. Sometimes corrosion of plagioclase by brown and green hornblendes is observed.

Biotite replaces brown hornblende and, by its optical properties ($Ng = 1.631$, $Np = 1.583$, $Ng - Np = 0.048$; pleochroism in light-brown tones: $Ng$—light brown, $Np$—pale reddish and colorless; $2V = -5^\circ$), belongs to a variety containing up to 50% of the phlogopite molecule [3]. It probably arose at a later stage of alteration of the hornblende rocks. In turn, this mineral is replaced by chlorite.

Apatite forms columnar (up to 1–1.5 mm) crystals and is distributed extremely unevenly in the rocks, sometimes in the form of considerable accumulations. Most often it occurs as poikilitic intergrowths in amphiboles and, more rarely, in plagioclases.

Magnetite is of two generations: an early, coarse-crystalline one (0.3–1 mm), in association with brown hornblende and relic pyroxene; and a finely disseminated one in the serpentine–chlorite aggregate that replaces pyroxene, olivine, and, more rarely, brown hornblende. Coarse-crystalline magnetite is more often situated between grains of dark-colored components and more rarely as inclusions in the latter. In this case, individual grains of it not infrequently have a skeletal structure.

Coarse-grained gabbros of pegmatitic appearance are dense rocks of spotted texture. The spotting

here is due to the clustered segregation of amphiboles within the predominant plagioclase mass. The rock is composed of 60–75% plagioclase No. 47–62 and 30–35% brown and green hornblende and rare biotite. Among the amphiboles, brown hornblende predominates (up to 25–40%). Of the accessory minerals, apatite, sphene, and magnetite have been identified. These rocks are already characterized by an equal idiomorphism of plagioclases and brown hornblende. The optical properties of the rock-forming minerals do not differ from those of analogous minerals of the hornfels.

In order to study the chemistry of the process of formation of the described assemblages, analyses of the initial and altered rocks were carried out, and the chemical analyses were recalculated by the method of N. I. Nakovnik (Table 1). The results of the recalculation are presented in Fig. 2, which illustrates the balance of matter during magnesian metasomatism. The data presented show that within the metasomatic zone there is a regular change in the composition of the metasomatic assemblages. With increasing distance from the intrusive and from the initial rocks toward the center of the metasomatic zone, the content of alumina, silica, and alkalis in the rocks decreases (Table 1, Fig. 2). The amount of magnesium, calcium, and ferrous iron

Fig. 2. Graph of addition and removal of matter during magnesian metasomatism in the magmatic stage of hornfelsed porphyrites (according to the chemical analyses of rocks given in Table 1).

increases sharply in the same direction, i.e., during the formation of magnesian metasomatites increasingly remote from the contacts (in the middle part of the zones), the role of magnesium, calcium, and iron sharply increases, whereas the proportion of alumina, silica, and alkalis decreases. At the same time, amphibolization is accompanied by an increase in the basicity of plagioclase from andesine (No. 47–55) in the hornfels to labradorite (No. 60–63) in the plagioclase–hornblende metasomatites. Evidently, during magnesian metasomatism, along with the introduction of magnesium, there occurs a local accumulation of calcium (owing to its inertness), i.e., a general basification of the rock is effected (Zharikov, 1959). The MgO content regularly increases from the hornfels to the apohornfels pyroxene and hornblende rocks. The increase in magnesium concentration at the contact of the intrusive with the country rocks (diabase porphyrites) was most likely due both to its mobilization from the enclosing porphyrites and especially to introduction by solutions from the deep magmatic chamber that formed the granodiorite pluton. The solutions, penetrating along weakened tectonic zones that arose in the api-

Table 1

Changes in composition and balance of substances during magnesian metasomatism at the contact of orthopyroxene diabase porphyrites and monzonites (in grams per 100 cubic cm of rock)

1—orthopyroxene diabase porphyrite (sample V-12-24); 2—pyroxene-plagioclase-olivine metasomatite (sample 596); 3—hornblende metasomatite (hornblendite) (sample 1412); 4—plagioclase-hornblende gabbro of pegmatitic appearance (sample 3-19-13); 5—syenite-diorite (quartz monzonite) of the Sologon intrusion (sample 357). Chemical analyses were performed at the Central Laboratory of the Krasnoyarsk Geological Administration.

of the apical part of the forming intrusion, under favorable thermodynamic conditions (in the case of contacts between rocks differing in composition and density) removed excess aluminum, silicon, and alkalies and deposited magnesian minerals.

The formation, in the marginal parts of the metasomatic zone, of coarse-grained gabbro and gabbro-diorites of pegmatitic aspect could have proceeded through the reaction interaction of metamorphosing solutions with the enclosing diorites and monzonites during the period of formation of hornblendites. The essence of reaction contact metasomatism, as D. S. Korzhinskii (1955) points out, consists in the equalization of the chemical potentials or activities of the components of the solution. As a result, in the peripheral parts of the zone there occurred a local redistribution of petrogenic elements, primarily calcium, silicon, and partly magnesium and iron, with the formation, at the expense of the original diorites and monzonites, of rocks of pegmatitic aspect. This was evidently promoted by considerable heating of the contact area. In the near-contact pegmatitic formations that arose at the expense of intrusive rocks of dioritic composition, a sharp desilication is observed (Table 1, Fig. 2), which is due to the higher mobility of silica diffusing farther toward the lateral intrusive rocks, similar to what was noted by A. I. Bazhenov [2] for rocks of the endocontacts of dikes in one of the regions of Altai.

Figure 3 gives a composition–paragenesis diagram for rocks of the formation of magnesian metasomatites that arose at the expense of hornblende porphyrites (1) and diorites of the apical part of the granodiorite massif. The paragenesis shown in the diagram includes pyroxene + plagioclase + olivine and + hornblende. Among the described para-

Fig. 3. Composition–paragenesis diagram of the formation of magnesian metasomatites of the magmatic stage. The triangles correspond to the compositions of the initial (1) and metasomatic rocks (2, 3, 4) according to the data of chemical analyses in Table 1. The arrow shows the direction of the metasomatic process.

genesis two age groups are clearly distinguished: 1) pyroxene + plagioclase + olivine + brown hornblende (as an impurity) and 2) brown hornblende + green hornblende + plagioclase + pyroxene (as a relict). Ultimately, an association arises that corresponds to metasomatic hornblendites. On the composition–paragenesis diagram all the rocks fall within the field of the pyroxene—plagioclase—hornblende triangle. At the same time, the rock varieties that are the extreme products of magnesian metasomatism (pyroxenites and hornblendites) approach the pyroxene—hornblende tie line, whereas the pegmatitic gabbros, which are products of the reaction interaction of solutions with the forming metasomatites and the enclosing diorites, on the contrary approach the tie line of plagioclase rocks.

Thus, the material presented indicates that the formation of rocks of pyroxene–plagioclase–olivine and hornblende composition, approaching pyroxenites and hornblendites respectively, in the apical part of the granodiorite intrusion was due to processes of high-temperature magnesian metasomatism. According to data from many investigators [1, 2, 6, 10, etc.], the time of manifestation of magnesian metasomatism belongs to the magmatic and early (alkaline) postmagmatic stages (according to D. S. Korzhinskii).

In our case the following facts support this: 1) the confinement of the magnesian metasomatites to the apical part of the endocontact of the granodiorite intrusion; 2) the high-temperature character of the mineral associations encountered; 3) the zonal structure of the metasomatic bodies and their relationship with the rocks of the intrusion; 4) the same composition of accessory elements characteristic of the magmatic stage proper (chromium, manganese, titanium, nickel, vanadium, cobalt, zinc, molybdenum) in the rocks and minerals of the massif and the metasomatites. The development of zoning in the magnesian metasomatites and the relative stability of the chemistry of minerals of variable composition in all zones of metasomatism make it possible to assign the metasomatic column to the infiltration type [6]. Undoubtedly, diffusion phenomena also took a substantial part in the formation of the metasomatic rocks.

The source of the solutions was apparently the magmatic melt (which formed the granodiorite pluton), during whose differentiation magnesian solutions separated at a certain stage [1]. During interaction of the metamorphosing solutions with porphyrites and hornfelses, mobilization of magnesium could also have occurred in the apical part of the intrusion. In the opinion of some investigators [1, etc.], the denser rocks of the roof of the intrusion could have served as a barrier to the metamorphosing solutions. Owing to the different porosity at the boundary of the two media, a “conjugation” of some components took place, whose filtration coefficient is small; i.e., in the formation of the magnesian metasomatites the filtration effect played a significant role [4, 5, 6, 8, etc.].

Thus, the formation of magnesian metasomatites in the contact zone of the granodiorite intrusion proceeded in two successive stages: an early one (with an increase in the temperature of the process) and a late one (with a decrease in temperature). The staged character of the contact process and the different temperature character of the medium and of the solutions (an increase and decrease of the temperature gradient respectively in the magmatic and early postmagmatic stages) caused the appearance in the metasomatic bodies of two mineral associations of different ages.

associations (plagioclase–pyroxene and hornblende) and zonality.

REFERENCES

1. A. I. Bazhenov. On the role of assimilation and metasomatism in the formation of rocks of the Iedygem intrusive massif (Gorny Altai). Izv. Tomsk. Politekhn. In-ta, vol. 120, 1962.

2. A. I. Bazhenov. Features of contact phenomena associated with dike rocks of basic, alkaline, intermediate, and acidic compositions (on the example of the Elekmonar intrusive massif in Gorny Altai). Izv. Tomsk. Politekhn. In-ta, vol. 127, issue 1, 1964.

3. A. N. Winchell and G. Winchell. Optical Mineralogy. Foreign Literature Publishing House, 1953.

4. V. A. Zharikov et al. On the different rates of filtration of anions and cations during the percolation of solutions through finely porous filters. Dokl. AN SSSR, vol. 14, No. 1, 1961.

5. D. S. Korzhinsky. The filtration effect in solutions and its significance for geology. Izv. AN SSSR, Ser. Geol., No. 2, 1947.

6. D. S. Korzhinsky. Theory of Infiltration Metasomatic Zoning. Publishing House of the Academy of Sciences of the USSR, 1954.

7. A. A. Mossakovsky. Tectonic Development of the Minusinsk Depressions and Their Mountain Framing in the Precambrian and Paleozoic. Gosgeoltekhizdat, 1963.

8. L. N. Ovchinnikov and A. S. Shur. On the filtration effect during the percolation of solutions through mineral filters. Proceedings of the Fourth Meeting on Experimental Mineralogy and Petrography, issue II, Publishing House of the Academy of Sciences of the USSR, 1953.

9. N. A. Fogelman and A. E. Shabalevsky. Conditions of localization of stockwork deposits within the Kommunar ore field, on the eastern slope of the Kuznetsk Alatau. Tr. NIGRIzolota, No. 21, 1956.

10. A. I. Shabynin. On certain features of the formation of ore-bearing skarns in dolomitic contacts. Geol. Rudn. M-nii, No. 1, 1961.

1. The mineral composition of the rock is given in volume percent. ↩