A New Type of Carbonatites

S. S. Il’enok

(Presented by the Department of Petrography)

One of the most interesting questions in petrology, one that is increasingly attracting attention, is the origin of carbonatites and especially the connection of useful minerals with them. By the present time a number of basic features of their occurrence have been established. First of all, these include the confinement of carbonatites to the marginal parts of platforms. At the same time, individual carbonatite deposits have been found within geosynclinal zones or areas of completed folding (Eastern Sayan, Kaiserstuhl in the FRG). Their confinement to large fracture zones (the great African fractures) is also noted.

Most researchers consider the confinement and genetic connection of carbonatites with massifs of the alkaline–ultrabasic series to be characteristic and, in most cases, exclusive. Among the enclosing rocks, a sharply expressed manifestation of alkaline metasomatism has been revealed. In addition to fenitization, L. S. Borodin (1957, 1960) includes here the formation of ijolites through nephelinization of pyroxenites. He establishes a relationship in the formation of alkaline rocks and carbonatites. During nephelinization, aegirinization of pyroxene occurs and the latter is replaced by garnet (schorlomite) and phlogopite. The evolution of the solutions is characterized by enrichment in calcium, magnesium, and iron at the expense of the rock being replaced, in the form of pyroxenite, with the formation, at significant stages of the metasomatic process, of carbonate rocks enriched in rare-earth minerals.

By form of occurrence among carbonatites, central cores of pipe-like bodies of concentrically zonal structure, linear zones, as well as veins and lenses are distinguished. L. S. Borodin relates differences in the features of carbonatite occurrence to the degree of depth of the erosional section in primarily differentiated and undifferentiated massifs and volcanic vents.

In individual cases, the closeness of carbonatites to kimberlites is established (the Chadobets uplift, Krasnoyarsk Territory).

Thus, there is a widely manifested connection of carbonatites with ultrabasic rocks—chiefly olivinites, peridotites, and pyroxenites—which are accompanied by various alkaline rocks. However, as we shall see below, carbonatites also occur in another, namely gabbroid, complex of rocks. In this case, here

they are closely associated with the formation of a group of alkaline rocks, from feldspathic melteigites to nepheline syenites and urtites.

The complex under consideration is developed in Gornaya Shoriya, gravitating toward the boundary with the Minusinsk intermontane trough. The gabbroid magmatic complex is confined to fault zones formed during the development of the Devonian depression of the Minusinsk trough. Along one of the fault zones of northwestern strike, branching off from the main meridional fault zone that separates this trough from the Kuznetsk Alatau, there is a chain of gabbroid massifs. A zone of carbonatites is associated with one of these massifs; it is situated along the periphery of the latter. The massif has an isometric, concentrically zoned structure and consists chiefly of olivine and olivine-free gabbros, titanomagnetite gabbros, forellensteins, and thin horizons of pyroxenites. In the marginal zone there is a ringlike, dike-shaped body of coarse-grained pyroxenites and gabbro-pyroxenites. The enclosing rocks are a siliceous-carbonate folded sequence of Sinian age.

In the southern marginal part of the massif there is a zone of carbonatites, with an exposed length of 2.7 km and a width of 0.3 km. Judging from separate rock outcrops, its length reaches 4 km. To the north the zone borders on gabbro, as well as on the pyroxenites of the ring body. To the south they pass into genetically related melteigites and ijupirangites. Among the carbonatites, calcite coarse-grained varieties with forsterite, wollastonite, titanaugite, and salite are widespread. Dolomite varieties with chondrodite, which is often replaced by serpentine, occur as separate bands. In certain areas fluorite, celestine, and barite are developed. Cerium- and niobium-bearing minerals are encountered—perovskite, sphene, ilmenite. Rarely, pale-brown parisite is observed.

Among the carbonatites there are small, nestlike or nodule-shaped bodies (Il’enok, 1960, 1962), composed of melilite, nepheline, grossular, and, in the marginal parts, wollastonite. Veins 0.2–0.5 m thick of melilite-nepheline-garnet composition occur. In these, nepheline is segregated in the central part, and garnet in the marginal part of the bodies. Often, in the nodules and veins, there is a fine and very dense dissemination of sulfides: pyrrhotite, alabandite, pyrite, and more rarely pentlandite, mellerite, and molybdenite. Within the gabbro there are veins of melilitite, and near the contact a vein of monticellite rock with an admixture of spinel has been found.

In the marginal zone of the carbonatites, at the boundary with the gabbro, veins of nepheline-albite-sodalite composition with phlogopite, aegirine, and magnetite have been encountered.

In the marginal zone of the massif the gabbro has an alkaline selvage of monzonites and syenites. Its thickness is about 50 m. Dikes of sölvsbergites are confined to this zone.

It is characteristic that in the melteigites and ijupirangites nepheline forms small segregations or nests, which are surrounded by a pyroxene aggregate of radial structure. The pyroxene grains entering into the nepheline aggregate are well bounded and are larger in size. All this indicates not a later manifestation of nepheline, but rather its simultaneous formation together with pyroxene. The latter is represented by titan-augite.

The features of the occurrence of the carbonatites indicate that the carbonate material was derived from the carbonate substrate of the enclosing rocks in situ. This sharply distinguishes them from carbonatites of the ultrabasic family, where calcium accumulation is linked with the process of nephelinization. The connection of the indicated formations with a typically gabbroid complex and

the particular features of their occurrence make it possible to distinguish them as a new, distinctive type of carbonatites. The appearance of these rocks in an area of completed folding, in connection with faults during formation of the Minusinsk trough, brings them closer to the carbonatites of the Kaiserstuhl in the FRG, which are associated with the formation of the Rhine graben. At the same time, magmatic rocks of alkaline-gabbroid composition occurred there—essexites, theralites, dikes of phonolites and monchiquites, and extrusions of tephrites. The similarity of the two complexes is also manifested in the near-surface, subvolcanic conditions of their formation.

It should be noted that to the east, in the direction of the Minusinsk trough, in the marginal zones of Devonian gabbro massifs, nepheline syenites occur, and alkaline syenites and nordmarkites are also widely developed. Lateral transitions from nepheline syenites into weakly developed carbonatites are observed. All this indicates a genetic link in the manifestation of these and other rocks, which is substantiated by the sequence of their formation.

The origin of the indicated group of rocks is associated with the differential mobility of the components of gabbroic magma during its cooling, with the accumulation of a melt enriched in silica and alkalis. A major role here is played by the increase in the chemical activity of alkalis in zones of contact with carbonate rocks. This is confirmed by the presence of alkaline selvages in the gabbro massifs studied. In the presence of favorable tectonic conditions, such as zones of crushing among the enclosing rocks, mobilization of this mobile melt and circulation in the medium represented by carbonate rocks are observed. Here, in deeper zones, nepheline syenites are formed, accompanied both by melanocratic rocks—melteigites—and by leucocratic ones—urtites, which are very weakly developed in the massifs under consideration. In higher zones (the Patyn massif), at early stages they produce the formation of melteigites and jacupirangites, and at later stages—carbonatites. In the latter, iron minerals are already weakly developed. The sharp desilicification of the solutions caused early precipitation of wollastonite. A considerable enrichment of the late products of crystallization in rare-earth elements is observed.

REFERENCES

1. L. S. Borodin. On types of carbonatite deposits and their relation to massifs of ultrabasic alkaline rocks. Izv. AN SSSR, Ser. Geol., No. 5, 1957.

2. L. S. Borodin. Genesis of carbonatite deposits and their geological relation to massifs of ultrabasic alkaline rocks. International Geological Congress, 21st Session, Reports of Soviet Geologists, Moscow, 1960.

3. S. S. Il’enok. Principal features of the petrology of the Patyn massif. Geologiya i Geofizika, Siberian Branch, Academy of Sciences of the USSR, No. 4, 1960.

4. S. S. Il’enok. Alkaline rocks of Mount Patyn. In: Nepheline Rocks of the Kuznetsk Alatau, Materials on the Geology of Western Siberia, Vol. 63, 1962.