2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Zinc in ilmenite-group minerals from Dmitrovka metasomatites, Azov region, Ukraine

Sharygin V.V.*, Kryvdik S.G.**

^* V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia; ^** N.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation NAS of Ukraine, Kyiv, Ukraine

sharygin@igm.nsc.ru

In the Dmitrovka open pit (north-east exocontact of the Oktyabrsky Massif) alkali metasomatites form a zone with north-east strike in the Anadol complex granites. The thickness of individual metasomatic bodies within this zone may be up some meters (Shcherbak et al., 1994). The ilmenite-group minerals (ilmenite, pyrophanite, ecandrewsite) were observed as accessory phases in four samples of leucocratic aegirine albitites, which are variable in the suite and amounts of mafic minerals (aegirine, arfvedsonite, fluorphlogopite, annite, astrophyllite-kupletskite) (Kryvdik et al., 2010; 2011; Sharygin, Kryvdik, 2010). They commonly form large individual grains (up to 200-300 μm) sometimes rimmed by leucoxene aggregate and rarely occur as inclusions (20-40 μm) in aegirine, zircon and K-feldspar. Crystal inclusions in their grains are represented by zircon, aegirine, pyrochlore, albite, basnaesite -(Ce) and monazite-(Ce). Leucoxene is submicron-size aggregate, in which individual/relics of ilmenite-pyrophanite-ecandrewsite and new-formed Fe-niobates (columbite ?) are sometimes visible. Rutile with variable Nb and Fe are commonly localized in outer zone of leucoxene on the contact with other minerals. The phase relationships in the Dmitrovka metasomatites are evidences that the ilmenite-group minerals are products of early crystallization whereas formation of leucoxene is related to later processes. It should be noted that Zn-containing ilmenite and pyrophanite as well as ecandrewsite are common constituents of metasomatized granites, high-grade metapelites and latest rocks of peralkaline complexes (Sakoma, Martin, 2002; Mitchell, Liferovich, 2004 and references herein; Rao et al., 2008; Prochazka et al., 2010).

Microprobe and scanning microscope have shown that majority of grains are zoned and have broad variations in Fe, Mn and Zn at approximately constant content of Nb (0.5-2.0 wt.% Nb₂O₅). All metasomatite samples contain Mn-ilmenite with the predominance of Fe over Mn. Both Fe-pyrophanite and Mn-ilmenite occur only in sample DM-7 (Table 1, Fig. 1). Ilmenite from two samples (Dm-8, DM-Astr) is characterized by low content of ZnO (0.1-2 wt.%). Zoning in large individual grains is expressed in gradual increase of ZnO from core to rim (from 1-3 up to 10-16, sometimes up to 23 wt.%). The outermost parts with maximal content of Zn already belong to Fe-Mn-ecandrewsite (Fig. 1). In general, zoning character in ilmenite and pyrophanite is slightly different. Isomorphism Fe->Zn predominates in ilmenite, whereas (Fe,Mn)->Zn is common of pyrophanite. Small individuals (1-5 μm) of new-formed ecandrewsite are sometimes localized on the boundary of ilmenite/pyrophanite and leucoxene aggregate.

Table 1. Representative analyses of the ilmenite-group minerals from the Dmitrovka apogranite metasomatites.

c, m, r, or - core, middle, rim and outermost part of grain. ZrO₂ and Al₂O₃ are below detection limits.

Leucoxene also strongly varies in composition (in wt.%): SiO₂ - 0.5-1.1; TiO₂ - 58.3-65.8; Nb₂O₅ - 0.6-2.1; Al₂O₃ - 0-0.3; FeO_t - 11.6-24.6; MnO - 0.7-2.1; ZnO - 1.8-19.9. In general, it conventionally corresponds to Zn-rich compositions from pseudorutile Fe³⁺₂Ti₃O₉ to Fe³⁺-rich “ferropseudobrookite” (Fe²⁺_0.5Fe³⁺_0.5)(Ti_1.5Fe³⁺_0.5)O₅. Low sums (92-97 wt.%) suggest the presence of kleberite Fe³⁺_2-xTi₃O_9-3x(OH)_3x, hydrated species of pseudorutile (Grey et al., 1994). Composition of leucoxene strongly depends on composition of primary ilmenite-group mineral. Low content of MnO and presence of SiO₂ are common of all leucoxenes, even for aggregate replacing pyrophanite. Ilmenite is more stable phase than pyrophanite for replacement by leucoxene. Zn-enriched leucoxene was previously observed in partly metasomatized granitoids from Nigeria and Czech Republic (Sakoma, Martin, 2002; Prochazka et al., 2010). Its appearance around primary ilmenite is related to the H₂O presence and fO₂ increasing during metasomatism.

The melanocratic minerals in the Dmitrovka metasomatites also contain Zn, but its concentration in them is essentially lower (in wt.%): astrophyllite - 0.4-1.0; kupletskite - 1.4-1.8; arfvedsonite - 0.3-1.3; annite - 0.5-0.7; fluorphlogopite - 1.7-1.9; aegirine - 0.05-0.15 (Krivdik et al., 2010; 2011; Khomenko, Vyshnevskii, 2010; Sharygin, Kryvdik, 2010). Thus, the ilmenite-group minerals and their alteration products are main careers for Zn in the Dmitrovka leucocratic metasomatites. In addition to leucocratic species veined melanocratic metasomatites enriched in aegirine, arfvedsonite and molybdenite occur in the Dmirovka open pit. These rocks contain remarkable concentrations of Zn (90-540 ppm) (Mikhailov, Shun’ko, 2002). Sphalerite has been identified in them by optical method, and this mineral seems to be a sole career for Zn.

Previous studies for dike peralkaline phonolites of the Oktyabrsky Massif have shown that kupletskite and micas are the main phases to accumulate Zn (Sharygin, 2009; Sharygin et al., 2009). The Dmitrovka metasomatites are characterized by another type of Zn mineralization: in the lack of sulfides it is localized in early titanates, and on the late stages - in small amounts in phyllosilicates (micas, kupletskite-astrophyllite) and arfvedsonite. In sulfur-rich environments sphalerite is the main phase to accumulate Zn.

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