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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

   

Rare metal potencial of alkaline magmas

Kogarko L.N.

Vernadsky Institute of Geochemistry and Analytical Chemistry Russian Academy of Sciences

kogarko@geokhi.ru

Moscow, Russia

Among the igneous formations of the world alkaline rocks including carbonatites have the highest ore potential for rare-metals. About 95% of the magmatic strontium, cerium, niobium, tantalum, zirconium and hafnium are connected with the various formations of alkaline rocks and carbonatites. For example  contemporary international niobium market is controlled by carbonatites deposits.

Russia is one of the first places in the world on the prevalence of alkaline and carbonatite magmatism and related rare-metal mineralization. Many agpaitic alkaline intrusions are characterized by the magmatic layering which is related with rare metal ore horizons (massif Ilimaussaq with eudialyte and steenstrupine layers which are the sources of zirconium, hafnium and uranium; Lovozero massif where loparite ore constitute the ore horizons, eudialyte concentration confined to the layered eudialyteferous complex). To reveal the rare metal ores formation mechanism we studied the main ore minerals (loparite and eudialyte) composition of Lovozero superlarge deposit in vertical section. Stratigraphically upward in the section loparite is enriched with Sr, Nb, Ta and Th and depleted with Ca, Ti, REE. The highest contents of niobium and tantalum in loparite ores are found in the upper zone of Lovozero massif, which makes this area the most perspective for niobium and tantalum mineralization. The highest content of rare earths elements, titanium and calcium is revealed in the lower part of the section, which makes this area the most promising on titanium and rare earth mineralization. In the vertical section of eudialyte complex an increasing of iron and chlorine was found in eudiallyte crystals , the same pattern was observed for eudialytes of Ilimaussaq massif. The presence of hidden stratification in the ore minerals of Lovozero deposit indicates the magmatic genesis of rare metal ores similar to other layered intrusions, such as Bushveld, Stillwater and others. Detailed studies of the petrography and mineralogy of super large Lovozero rare-metal deposit showed that the change in the form and time of crystallization of loparites allocation which is the main mineral- for niobium, tantalum and rare earth elements are new geochemical criteria for rare element ore-bearing of alkaline magmas. Rare metal ore-bearing zones of giant Lovozero intrusions are only those that contain early idiomorphic loparite (the upper zone of the differentiated intrusion up to 1350 m). The lowermost zone of Lovozero intrusion (with a thickness of about 870 m), which is characterized by late xenomorphic loparites is not perspective for rare-metal ores. Thus a necessary condition for the appearance of magmatic cumulative rare-metal deposits is early cotectic ore mineral melt saturation. In this case there is an isomorphism of ore minerals. If the ore component concentration is much lower than cotectic the ore mineral crystallisation will be implemented at later stages in a small volume of interstitial melt, when the phenomenon of convective-gravitational differentiation and segregation of mineral phases are difficult, resulting to dispersion of ore components in the form of xenomorphic secretions of accessory minerals. Thus we can also conclude that the world's largest alkaline coeval intrusion Khibiny is not perspective for loparite ore because the original alkaline magma of Khibina complex was not saturated with respect to loparite at the early stages, loparite occurs mostly in pegmatites and less often in the interstitial phases form. The presence of hidden stratification in respect to loparite suggests the closed nature of ultra-alkaline magmas evolution in a huge magma chamber. These results are in good agreement with our isotopic data (Kogarko et al, 2010), which showed a purely mantle sources of giant alkaline Kola Peninsula intrusions without crustal contamination processes.

According to our researches, alkaline magmatism and related superlarge rare-metal ore deposits as well as carbonatites formed at the interval of 2,5 - 2,8 Ga and during the Earth evolution process increasing of their activity were permament. (Fig.) Kimberlites, as well as potassium-alkaline rocks appeared on the Earth much later, at the interval of 2000-1400 Ma, their intensity also increased over geological time. (Fig )

   The appearance of alkaline rocks and carbonatites on the border of Archean Proterozoic period coincided with a number of major events on the Earth. Most of the authors associate changing in geodynamic regime of our planet when plume tectonics was joined by plate tectonics with the same time border. At this stage of  the Earth's evolution oxygen atmosphere appeared, there was oxidation of oceanic sediments. As the geochemical consequences of subducted materials global degassing and releasing of the oxidized fluid (water and carbon dioxide) mantle oxidation occurred and large-scale mantle metasomatism began, leading to the formation of enriched with rare and ore elements cameras, alkaline rocks, carbonatites and kimberlites later. The later origin of the deepest alkaline rocks, kimberlites, in Earth's history is likely connected with the formation of continental cratons and mature thickened lithosphere (the necessary conditions for the kimberlites generation) at the later stages of our planets evolution.

   One of the interesting problems is the fact that the intensity of kimberlite magmatism was increasing in geologic time but their diamond-bearing was decreasing. (Fig) We are developing a model of continuous mantle oxidation in Earth's history that suggests that an increasing in oxidative capacity of the mantle substrate led to diamong phase instability, they "burned out" and diamond-bearing of kimberlites decreased.

Now the model of the kimbirlites genesis as the result of metasomatic interaction with the mantle potassium enriched alkaline fluids is accepted. During the rising of a mantle diapir from the depth about 660km Ca-perovskite phase becomes unstable and as a result of its reaction with magnesia perovskite and ferriperiklaze majorite, ringwoodite and after a further pressure drop vadsleit appears. During this process, only a portion of potassium becomes majorite, since the distribution coefficient of potassium in the Ca-perovskite is 26 times greater than this value for majorite. The rest of potassium appears to remain outside the crystal lattices of minerals comprising this zone of the mantle. On the basis of the ratio of distribution coefficients of potassium in the Ca-perovskite and majorite, we can definitely conclude that the thermodynamic activity of K2O in the system increases more than an order during the transition from association of magnesium and calcium perovskite-ferripericlaze to paragenesis of majorite-ringwoodite. Thus it creates conditions for the potassium transition to the melt or fluid at the boundary between the lower and upper mantle (about 660 km). Separated fluids will migrate to the upper structure layers of the mantle and produce metasomatic elaboration of host rocks and may lead to the kimberlites generation.

 

References:

Kogarko l.N. Problems of the genesis of Kola Peninsulas giant rare metal deposits // Russian Arctica: Geological History, Minerageny, Geoecology. St. Petersburg: 2002. P. 773-788.

Kogarko L.N., Lahaye Y., Brey G. Plume-related source of superlarge rare metal deposits from Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics// Min Petrol. 2010. V. 98. P. 197-208.