Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

New data on minerals with anomalous compositions of LREE in high-alkaline pegmatites

Ermolaeva V.N.*, Pekov I.V.*^,**, Chukanov N.V.***

* GEOKHI RAS, Moskow; ** MSU, Moskow; *** IPCP RAS, Chernogolovka;

cvera@mail.ru

As a rule, minerals-concentrators of light lanthanides (LREE) in magmatic rocks (including peralkaline ones), as well as in parageneses formed on early stages of pegmatite formation, are characterized by the prevalence of Ce over other rare earth elements. It is caused by relative abundance of LREE in nature. In case of Khibiny-Lovozero alkaline complex on Kola Peninsula, the examples are loparite, minerals of apatite and pyrochlore groups, rinkite, steenstrupine. A more complex situation takes place in late parageneses of high-alkaline and peralkaline  pegmatites and related hydrothermalites. In LREE-rich minerals from these associations, quite often La or Nd are predominant REE. As an example of such minerals with anomalous compositions of LREE, for Khibiny-Lovozero alkaline complex, different La-dominant phosphates, carbonates and silicates were noted, including: belovite-(La), ankilite-(La), remondite-(La), kukharenkoite-(La), nordite-(La), ferronordite-(La), members of the olgite series (Pekov, 2005).

Recently a number of other minerals with anomalous compositions of LREE (including Nd-dominant ones) has been discovered by us in pegmatites of Khibiny-Lovozero alkaline complex (table 1). Among them, there are Ti-Th silicates formed on hydrothermal stage (pegmatite 71, Mt. Malyi Punkaruaiv, Lovozero; pegmatite on Mt. Khibinpakhchorr, Khibiny), belovite-(La) as a component of bituminous rim around belovite-(Ce) crystal (pegmatite body «Shomiokitovoe», Mt. Alluaiv, Lovozero), vitusite-(La), rhabdophane-(La), rhabdophane-(Nd), «abenakiite-(Nd)» as components of pseudomorphs after steenstrupine-(Ce) («Shkatulka» pegmatite, Mt. Alluaiv, Lovozero) and silicate of  Th and REE (pegmatite on Mt. Koashva, Khibiny), detected as inclusions in solid bituminous substance.

The depletion of late minerals by Ce can occur owing to its oxidation to Ce⁴⁺, if oxidation potential of the system essentially increases. It is obvious that just this mechanism takes place at the replacement of steenstrupine-(Ce) by La- и Nd-dominant silicates and phosphates (analyses 5-8 in table 1). However the main factor influencing REE separation has crystal-chemical origin: it is structure affinity of different minerals to different REE, first of all depending on their ionic radii. Another factor favouring farther fractionation of REE between different phases is low temperature of crystallization: just minerals of late hydrothermal associations are characterized by the most unusual and contrast compositions of REE. It is caused by the following: aqueous environment at a temperatures ≤150-200°С is optimum for the formation of alkaline-REE complex compounds, in a part of which individual REE and their groups show maximum contrast of properties. Thus La³⁺ is the largest of REE³⁺ cations, and it shows the most effective fractionation (Pekov, 2005). In the case of close associations of REE minerals with organic compounds (analyses 9, 10 in table 1), another chemical factor can play a role, which is connected with various properties on different LREE at their interaction with molecules of organic compounds.

Table 1. Chemical composition of late REE-containing minerals from high-alkaline pegmatites of Khibiny-Lovozero complex.

Reference

Pekov I.V. Genetic mineralogy and crystallochemistry of rare earth elements in high-alkaline postmagmatic systems. Doctoral thesis in geology and mineralogy. Moscow: MSU, 2005. 652 p.