Khibina (Khibiny)

The Khibina alkaline complex with an area of 1327 km2 is second only to the Guli complex (Meimecha Kotui No 2) in terms of size. It forms an isolated mountain massif between Lake Imandra in the west and Lake Umbozero to the east, the mountains rising above the ancient surrounding peneplain to a height of 700-1000 m. The complex is emplaced into Archaean granite gneisses and Proterozoic volcanic-sedimentary rocks along steep outer contacts, which have been traced to a depth of 7 km by geophysical methods. Adjacent to the outer contacts albite-aegirine fenites and hornfelses are extensively developed. The intrusion has a concentric, zonal structure in which primary igneous layering is very well developed. Galakhov (1975) distinguished several zones in the complex, which correspond to distinct ring and conical intrusions formed as a result of successive phases of intrusion. The earliest intrusions are alkaline and nepheline trachytes and rhomb and nepheline porphyries which form a steeply inclined body 0.5 km in thickness in the western part of the massif and xenoliths in the peripheral zones. The zones of the complex, from the periphery to the centre, are as follows (see Table). (1) alkaline syenite (umptekite) and nepheline syenite (0.3 km thick). (2) and (3) massive and trachytic khibinites (about 5.5km thick). (4) rischorrite (biotite-nepheline syenite) - ijolite - urtite - apatite-nepheline rocks (2-3 km). (5) melteigite, ijolite and urtite. (6) and (7) heterogeneous nepheline syenite and foyaite (3.5-4 km). (8) carbonatite. The khibinites of zone (3) are trachytic and in the deepest parts of the exposed section are stratified in the form of alternating sequences of leucocratic nepheline syenite and melanocratic ijolitic rocks. The rischorrites of zone (4) comprise a complex ring-like intrusive body the rocks of which are characterised by poikilitic textures and the occurrence of dactylotypic and micropegmatitic intergrowths of alkali feldspar and nepheline. Metasomatic proceeses may have played an important role in the generation of these rocks. The melteigite-ijolite-urtite series that comprises the fifth zone develops a striking layered complex within which is the well known apatite-nepheline ore deposit (described below) In the northwestern part of the complex within the khibinites are a high density of xenoliths of peridotite, perovskite clinopyroxenite and ijolite which are similar in composition to the alkaline ultramafic rocks of the Afrikanda and Kovdor complexes. Throughout the intrusion there are alkaline pegmatites and numerous zones of albitization. In the eastern part, near to the bodies of rischorrite and urtite, there is a carbonatite stockwork with a diameter of about 800 m (Fig. 19), which lies at the focus of the multiple layered complex. Drilling has indicated that the carbonatite extends to a depth of at least 1.7 km. The carbonatites are considered to be younger than the principal layered units of the complex and the dykes of tinguaite, alkaline trachyte and alkaline lamprophyres (monchiquite and damkjernite). Dykes are widespread in this part of the complex, where they cut the principal alkaline rocks with the formation of typical eruptive-explosive breccias which have lamprophyric, tinguaitic, orthoclasitic and chalcedonic cements. As well as the dykes in the eastern part of the complex, there are numerous pipes of eruptive breccia that have diameters of 50-100 m and contain fragments which include olivinite, clinopyroxenite, phoscorite, urtite and nepheline syenite set in a matrix which may be tinguaitic or of phlogopite picrite. The carbonatite stock has a complicated structure. Multistage carbonatite breccias are cut by a stockwork of carbonatites which extend through the central part of the stock. Vertical zoning is evident in the stock with in the upper part magnesian varieties of carbonatite and in the lower part aegirine-biotite-calcite carbonatites with apatite. Zones of carbonatisation of the host-rocks, including foyaites and tinguaites, are related to the carbonatites. There are a number of formational stages of differing composition amongst the carbonatites including (1) biotite-, aegirine-biotite- and albite-calcite carbonatites, (2) manganiferous calcite-ankerite and siderite carbonatites and (3) manganiferous siderite and ankerite carbonatites with significant natrolite and dawsonite (Dudkin, 1991). Noteworthy chemical features of the carbonatites are a predominance of Na over K and high contents of Sr, Ba, Mn, F, S and rare earth elements. More than 80 minerals have been identified in the carbonatites, and closely associated rocks, including dawsonite, nahcolite, fluorite, cryolite, synchisite, parisite, burbankite and edingtonite (Dudkin, 1991). The carbonatites have been described in detail by Dudkin et al. (1984) and Dudkin (1991). The mineralogy of Khibina is described in the two volume monograph by Kostyleva-Labuntsova et al. (1978) and a range of unstable and water soluble minerals from the complex is described by Khomyakov (1987). The geological structure of the massif is outlined in the maps and papers of Eliseev et al. (1939) and Zak et al (1972), while Galakhov (1975) has described the petrology of the complex in detail. There is a brief account in Gerasimovsky et al. (1974).