Abstract

The assemblage of Nb-Ta-Ti-Sn oxide minerals in some zoned Devonian granitic pegmatites at the La Calandria mine, Cañada del Puerto, shows complex intergrowths of achalaite, ferrotitanowodginite, Ta-rich rutile, columbite-(Mn), ixiolite, and fluorcalciomicrolite. The chemical composition of these phases is variable, but in general the minerals show slight predominance of Nb over Ta and a clearly defined predominance of Fe over Mn, with minor participation of Ti, Sn, and W. The minerals in this assemblage are not in equilibrium and represent magmatic and subsolidus phases that are distinguished texturally and chemically. The primary magmatic stage of mineralization possibly crystallized ixiolite + Ta-rich rutile I in the outer zones of the pegmatite and, less commonly, local columbite-(Mn) in the inner part. Subsolidus unmixing of ixiolite produced achalaite and ferrotitanowodginite I + Ta-rich rutile II. Contemporaneously, Ta-rich rutile I locally exsolved ferrotitanowodginite/achalaite II + cassiterite. A localized Ca-F-rich hydrothermal overprinting event transformed Ta-rich rutile I to Ta-rich rutile III + ferrotitanowodginite/achalaite II + fluorcalciomicrolite. Furthermore, the hydrothermal overprint produced peripheral transformation of columbite-(Mn) to possibly achalaite III and widespread distribution of fluorcalciomicrolite throughout the assemblage.

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