ملخص: Two mineralogically di.erent rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin- Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assod
e e–Issalane Terrane. The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W–Nb–Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories. The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with ‘‘snow ball’’ quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP). The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK¼1.10–1.29; ASI¼1.17–1.31), sodolithic and .uorine rich with high SiO2, Al2O3, Na2O ? K2O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO2, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P2O5 content (0.03–0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin- Amzi granite is W–Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta–Sn bearing with insigni.cant W content. At Tin-Amzi the W–Nb–Ta minerals de.ne a sequence formed by W-columbite tantalite followed by wolframoixiolite and .nally wolframite showing the e.ect of hydrothermal overprinting with an extremeWenrichment of the .uids. At Ebelekan, the Sn–Nb–Ta oxides follow a Mn sequence: manganocolumbite ! manganotantalite ! wodginite ? titanowodginite ! cassiterite that represents a trend of primary crystallisation resulting from progressive substitution Fe ! Mn and Nb ! Ta during the magmatic fractionation.
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لغة: إنجليزية