The Ta-Nb-Sn-Ti oxide-mineral paragenesis in La Viquita, a spodumene-bearing rare-element granitic pegmatite from San Luis, Argentina.

GALLISKI MIGUEL ANGEL; MÁRQUEZ-ZAVALÍA MARÍA FLORENCIA; CERNÝ PETR; MARTÍNEZ VIVIANA; CHAPMAN RON

Universidad de Buenos Aires

Congreso; 8º Congreso de Mineralogia y Metalogenia; 2006

Asociación Mineralogica Argentina

The paragenesis and chemical composition of the primary Ta-Nb-Sn-Ti oxide minerals in La Viquita spodumene-bearing granitic pegmatite, Pampean Ranges, evolve 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 inward-increasing proportion of spodumene, homogeneous or oscillatory-zoned ferrocolumbite crystals occur in the outer parts. They show a fractionation trend to ferrotantalite with Ta/(Ta+Nb) 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 zirconium contents, and ferrowodginite. All members of the wodginite group are structurally ordered and carry some exsolutions 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 Ta-Nb-Sn-Ti minerals shows a fractionation path of  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 are attributed mainly to the combined effects of increasing proportion of Sn, Fe and Ti (the latter two probably introduced from host rocks), and of increasing  f(O2), at low values of ìFLi in the melt. Keywords: wodginite-group minerals, ferrotapiolite, cassiterite, granitic pegmatite, Argentina.