Genesis of the Gabbro-Syenite Complex of the Kondoma District (Mountain Shoria)

G. M. Ivanova

(Presented by the scientific seminar of the Department of Petrography)

The territory of the Kondoma group of iron-ore deposits is situated in the southwestern part of the Kuznetsk Alatau, within the Tashelgino–Kondoma mobile zone. The deposits of this group are genetically associated with a gabbro-syenite complex, represented by five syenite massifs and by spatially associated stock-like bodies of gabbroid composition.

The studies we have carried out made it possible to prove the comagmatic nature of the syenitoid and gabbroid rocks in the district and to unite them into a single gabbro-syenite complex, establishing its connection with the iron mineralization in the district. The formation of the complex proceeded in three successive stages, each of which was associated with the intrusion of separate portions of basaltic magma differentiated to varying degrees. The direction of differentiation of the parent magma was trachytic. Differentiates of such magmas are usually encountered in consolidated parts of folded regions that have already lost their activity. The Kondoma district belongs precisely to such a post-folded area, where the formation of this complex took place under semiplatform conditions.

The differentiation of the magma in the “trachytic” direction was manifested in the uniform accumulation of alkalis and a decrease in feldspar lime. The accumulation of silica, alkalis, and volatiles (OH, H, CO₂, haloids, etc.) occurred in the final, concluding stage of magma differentiation in the upper parts of the magmatic chamber, as is indicated by the characteristic bend of the variation curve in the upper right part of the petrochemical diagram for the composition of the rocks of this complex in the district. Such a feature of the curve, reflecting magma differentiation, is characteristic of the rocks of Pantelleria Island and other regions and is termed pantelleritic.

Moreover, among the alkalis, potassium accumulates more rapidly than sodium. Thus, the process of albitization, associated with sodium metasomatism, lags behind the potassium process and is superimposed upon already crystallized alkaline syenites of the potassium series and syenitized rocks, in the formation of which the leading role belongs to potassium and volatiles. The upper parts of the variation curve correspond to the extreme alkaline differentiates of the upper part of the magmatic melt. Upon reaching the surface, these differentiates would produce alkaline effusives. Even earlier, R. O. Daly, in his works, noted the possible gaseous melting of the roof and the upward advance of magma to the surface, accompanied by the quiet opening of volcanic vents without explosions.

Owing to enrichment in its upper part with volatiles and alkalis, the melt here becomes very active and gives rise to the rather widely developed phenomena of magmatic replacement, as well as to intensive activation of postmagmatic ore-forming processes. Syenites formed as a result of the separation of an alkaline solution-melt, with the manifestation of magmatic replacement, have a leucocratic composition and belong to the alkaline syenites. Within the massif they form upper selvages with gradual transitions into lime-alkaline syenites, which compose the cores of the massifs.

Simultaneously with the crystallization of the alkaline syenite magma, in the course of differentiation, postmagmatic ore-bearing solutions separate off; they migrate upward along the contacts and, under favorable conditions, produce an industrial localization of iron ores.

Moreover, phenomena of magmatic replacement also played a significant role in this process. The connection between the phenomena of magmatic replacement and ore-forming processes is defined as paragenetic, owing its origin to their common deep-seated magmatic source. In this case the source of iron is the ferrous-magnesian silicates of previously formed gabbroid rocks or of the enclosing effusives of basic composition. Iron is removed from these rocks during their magmatic replacement, as a result of which solutions rich in Fe and Mg appear; during their migration into the zone of the cold contact they may form skarns. Skarn solutions are formed in place, but they tend to migrate upward along the contact and to become localized along steep fracture zones. In areas of more prolonged heating these solutions produce partial replacement and recrystallization of the syenites.

Thus, the alkaline syenites formed during magmatic replacement, as well as the skarns and ore-magnetite formations, are closely related paragenetic formations.

The thickness of iron-ore formation is to a significant degree determined by the thickness of the sequence that underwent alkaline metasomatism and magmatic replacement, i.e., by the thickness of the effusive sequence. There is no doubt that the scale of magmatic replacement in the vertical direction is incomparably greater (by hundreds of times) than its horizontal extent, owing to the predominant vertical migration of alkaline magmatic solution-melts.

After the formation of the iron-ore deposits in the district, the active activity of this magmatic source began to die out. Its last connection with them was manifested in the emplacement of dike rocks.