Minerals of the ilmenite-pyrophanite series in alumina-rich fenites of the Khibiny alkaline complex (Kola Peninsula, Russia) and their indicator importance

Yakovleva O.S.*, Pekov I.V.*^,**, Bryzgalov I.A.*

*Faculty of Geology, Moscow State University, Moscow, Russia; **Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia

Accessory minerals of the ilmenite-pyrophanite series, Fe²⁺TiO₃– Mn²⁺TiO₃, are typical in fenitized xenoliths of the Khibiny alkaline complex (Kola Peninsula, Russia). They occur in fine-grained fenites of different compositions at mountains Kaskasnyunchorr, Kukisvumchorr, Eveslogchorr, Yukspor, Partomchorr, Poachvumchorr, Marchenko peak. Usually their grains are 0.n mm, rarely up to several mm in size.

Ilmenite is very common mineral of these rocks. In some cases it plays a rock-forming role in specific “ore” fenites. Typical minerals associated with ilmenite in different types of fenites are: titanite, pyrrhotite, rutile; nepheline, biotite, Ti-enriched magnetite, pyrrhotite, albite, potassic feldspar, ferrosilite, aegerine, Nb-rich rutile, aenigmatite; albite, potassic feldspar, biotite, hercynite, corundum; albite, sillimanite, sekaninaite, hercynite, native iron.

Pyrophanite is rare mineral in fenitized xenoliths within the Khibiny complex that was not earlier found there. It occurs in feldspar-rich rocks in associations with: nepheline, aegirine, aenigmatite, biotite, pyrrhotite, titanite, ilmenite; biotite, hercynite, pyrrhotite, almandine, ilmenorutile; nepheline, biotite, Ti-enriched magnetite, hedenbergite, ferrosilite, pyrrhotite. Pyrophanite is found only in pyrrhotite-rich rocks. In come cases, pyrophanite is associated with ilmenite that significantly prevails. In a pyrrhotite-bearing, mostly feldspar rock pyrophanite crysttalized later than ilmenite.

Typical admixtures in ilmenite are Mn (0.01-0.42 apfu) and Nb (0.01-0.05 apfu). Concentrations of other components are low (apfu): Mg (typically < 0.02; 0.09 in one case), Cr (up to 0.02), V (up to 0.01) and Al (up to 0.02). Pyrophanite contains 0.17-0.45 apfu Fe, distribution of other admixtures is the same as in ilmenite. Thus, in the apoxenolithic Al-rich fenites of the Khibiny complex, almost full the ilmenite-pyrophanite isomorphous series occurs, with prevailing Fe-dominant members. These minerals do not show the hematite scheme of isomorphous substitutions, (Fe,Mn)²⁺TiO₃ → Fe³⁺Fe³⁺O₃. The main substitution schemes are: Fe²⁺→ Mn²⁺ and 3Ti⁴⁺→ (Fe,Mn)²⁺+ 2Nb⁵⁺.

Minerals of the ilmenite-pyrophanite series prove to be important sensitive indicators of the chemical conditions of mineral-forming media in the alumina-rich fenites of Khibiny. Content of Mn in these minerals indicates the activity of S2-. In spite of high Fe/Mn ratio (from 15 to 100, mainly > 20, according to our data), high-manganese varieties of ilmenite, up to Mn-dominant member of the series, pyrophanite, occur there. It is the consequence of higher affinity to S2- of Fe than Mn: the increase of the S2- activity causes concentratrion of Fe in sulfides, mainly pyrrhotite. When pyrophanite crystallized later than ilmenite in the same rock, then it shows that the activity of S2- increased with time. Further increase of the S2- activity results in replacement of pyrophanite by alabandine, MnS (Kaskasnyunchorr, Yukspor). The absence of the hematite isomorphous scheme in ilmenite-pyrophanite series minerals indicates reducing conditions of mineral formation. Another important chemical feature of these minerals in apoxenolithic fenits of Khibiny is low magnesium content. It correlates with general scarcity of Mg of these rocks that is caused by nature of their protholite, the pre-Cambrian alumina-rich apopelitic metamorphic rocks.

This work was supported by grant of President of Russian Federation no. NSh-3848.2010.5.