Nb/Ta, Zr/Hf and REE fractionation in exotic pegmatite from the Keivy province, NW Russia, with implications for rare-metal mineralization in alkali feldspar granite systems

ORE GEOLOGY REVIEWS(2022)

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Abstract
The White Tundra pegmatites are associated with the Keivy alkali feldspar granite complex in the Kola Peninsula, NW Russia. The host granites are aegirine-arfvedsonite, biotite-arfvedsonite and biotite-ferrohastingsite varieties, ranging compositionally from peralkaline through metaluminous to peraluminous. The pegmatites are remarkable for the diversity of the rare-metal mineralization. The inferred crystallization sequence of the main REE, HFSE and Ti minerals in the pegmatite is: zircon - fergusonite-(Y) - monazite-(Ce) - gadolinite-hingganite series - britholite-(Y) - astrophyllite - titanite - allanite-(Ce) - chevkinite-(Ce) - ilmenite - kainosite-(Y) - REE carbonates. The pegmatites show unusual parageneses indicating a transition of the mineral associations from agpaitic to miaskitic in the same body (e.g. zirconosilicate -> zircon; astrophyllite -> titanite). A number of minerals have anomalous minor element contents. Astrophyllite and some titanites have low (even < 1) Nb/Ta and Zr/Hf ratios that contrast with the overall geochemical environment (ratios >> 1); titanite with very high Y and HREE contents; elevated Sn, W and V contents in some titanosilicates; allanite-(Ce) with very high Ti contents coupled with low (less than stoichiometric) values of Al, suggesting a Ti -> Al substitution. Whole-rock compositions of the host granite show a steady decrease of Nb/Ta and Zr/Hf ratios in peralkaline through metaluminous to peraluminous types. Compositions of Ti minerals suggest that further fractionation of Nb-Ta, Zr-Hf and REE in the White Tundra exotic pegmatite and the crystallization of Ti minerals with anomalous compositions are due to a combination of several factors: assimilation by the primary magma (enriched in Zr, Nb, REE) of upper crustal lithologies enriched in Ta, Hf, LREE, W, Sn; consumption of significant amounts of Nb and Zr by early-crystallized pegmatitic fergusonite and zircon with Nb/Ta and Zr/Hf >> 1; transition of HFSE mineral assemblages in pegmatite from agpaitic to miaskitic with consequent lowering of Nb/Ta and Zr/Hf; crystallization of minerals in a low-temperature (hydrothermal) environment with changing F contents, that promoted a sporadic further decrease of Nb/Ta and Zr/Hf (to values < 1) and increase of Y + HREE. The implications of the fractionation mechanisms for ore-forming processes related to rare-metal granites are discussed.
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Key words
Astrophyllite,Titanite,Ferriallanite,Pegmatite,Alkali feldspar granite,REE, Nb/Ta, Zr/Hf fractionation,Rare-metal deposits,Keivy,NW Russia
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