INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
THE TaNbSnTi OXIDE-MINERAL PARAGENESIS FROM LA VIQUITA,
GALLISKI, M.A.; MÁRQUEZ-ZAVALÍA, MARÍA FLORENCIA; CERNÝ, P.; MARTÍNEZ, V.; CHAPMAN, R.
Mineralogical Association of Canada
Año: 2008 vol. 46 p. 491 - 491
The paragenesis and chemical composition of the primary TaNbSnTi oxide minerals in the La Viquita spodumene-bearing granitic pegmatite, San Luis, Sierras Pampeanas, Argentina, evolved from the border zone inward. Early phases consist of minor ferrocolumbite in the border and wall zones. In the intermediate zones, formed by a coarse-grained association of Kfs + Qtz + Ms + Ab with inwardly increasing proportions of spodumene, homogeneous or oscillatory-zoned ferrocolumbite crystals occur in the outer parts. They show a fractionation trend to ferrotantalite with Ta/(Ta + Nb) ranging from 0.35 to 0.56 at an almost constant Mn/(Mn + Fe2+ + Fe3+) of ~0.33, and locally to rare manganocolumbite. In the middle and inner intermediate zones, the dominant phases are cassiterite, wodginite with elevated Zr contents and ferrowodginite. All members of the wodginite group are structurally ordered and carry some signs of exsolution-induced lamellae of ferrotapiolite. Ferrotapiolite is dominant in small Ms + Ab + Qtz units of the core-margin association, with intergrowths of ferrotitanowodginite, inclusions of hafnian zircon, and late-stage cassiterite. From the wall zone to the core-margin assemblage, the suite of TaNbSnTi minerals shows a fractionation path with a non-linear increase in Ta/(Ta + Nb), basically in three stages marked by sequential dominance of columbite-group minerals, wodginite-group minerals and ferrotapiolite + cassiterite. The phase changes and compositional variations are attributed mainly to the combined effects of increasing proportion of Sn, Fe and Ti (the latter two possibly introduced from the host rocks), and of increasing f(O2) in the melt. Keywords: wodginite-group minerals, columbite-group minerals, ferrotapiolite, cassiterite, granitic pegmatite, San Luis, Argentina.