THE Ta-Nb-Sn-Ti OXIDE-MINERAL PARAGENESIS FROM LA VIQUITA, A SPODUMENE-BEARING RARE-ELEMENT GRANITIC PEGMATITE, SAN LUIS, ARGENTINA

Abstract

The paragenesis and chemical composition of the primary Ta–Nb–Sn–Ti 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 columbite-(Fe) 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 columbite-(Fe) crystals occur in the outer parts. They show a fractionation trend to tantalite-(Fe) 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 columbite-(Mn). 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 Ta–Nb–Sn–Ti 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.